TCThe CryosphereTCThe Cryosphere1994-0424Copernicus PublicationsGöttingen, Germany10.5194/tc-10-941-2016Evidence of recent changes in the ice regime of lakes in the Canadian High
Arctic from spaceborne satellite observationsSurduCristina M.cristina.surdu@esa.intDuguayClaude R.crduguay@uwaterloo.cahttps://orcid.org/0000-0002-1044-5850Fernández PrietoDiegoEarth Observation Science, Applications and Future Technologies Department, European Space Agency (ESA),
European Space Research Institute (ESRIN), Frascati (Rome), ItalyDepartment of Geography and Environmental Management and Interdisciplinary Centre on Climate Change,
University of Waterloo, Waterloo, Ontario, CanadaCristina M. Surdu (cristina.surdu@esa.int) and Claude R.
Duguay (crduguay@uwaterloo.ca)10May201610394196025September201517November201524March201617April2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://tc.copernicus.org/articles/10/941/2016/tc-10-941-2016.htmlThe full text article is available as a PDF file from https://tc.copernicus.org/articles/10/941/2016/tc-10-941-2016.pdf
Arctic lakes, through their ice cover phenology, are a key indicator of
climatic changes that the high-latitude environment is experiencing. In the
case of lakes in the Canadian Arctic Archipelago (CAA), many of which are ice
covered more than 10 months per year, warmer temperatures could result in ice
regime shifts. Within the dominant polar-desert environment, small local
warmer areas have been identified. These relatively small regions – polar
oases – with longer growing seasons and greater biological productivity and
diversity are secluded from the
surrounding barren polar desert. The ice regimes of 11 lakes located in both
polar-desert and polar-oasis environments, with surface areas between 4 and
542 km2, many of unknown bathymetry, were documented. In order to
investigate the response of ice cover of lakes in the CAA to climate
conditions during recent years, a 15-year time series (1997–2011) of
RADARSAT-1/2 ScanSAR Wide Swath, ASAR Wide Swath, and Landsat acquisitions
were analyzed. Results show that melt onset occurred earlier for all observed
lakes. With the exception of Lower Murray Lake, all lakes experienced earlier
summer ice minimum and water-clear-of-ice (WCI) dates, with greater changes
being observed for polar-oasis lakes (9–24 days earlier WCI dates for lakes
located in polar oases and 2–20 days earlier WCI dates for polar-desert
lakes). Additionally, results suggest that some lakes may be transitioning
from a perennial/multiyear to a seasonal ice regime, with only a few lakes
maintaining a multiyear ice cover on occasional years. Aside Lake Hazen and
Murray Lakes, which preserved their ice cover during the summer of 2009, no
residual ice was observed on any of the other lakes from 2007 to 2011.
Introduction
In a rapidly changing climate (Zdanowicz et al., 2012; IPCC,
2013; Lenaerts et al., 2013; Woo and Young, 2014), with each of the last 3
decades being successively warmer than any preceding decade (Derksen et
al., 2012) and as a result of complex energy exchanges between atmosphere,
ocean, and land, the Arctic cryosphere is possibly transitioning towards a
new state. As a major feature of the Arctic landscape, lakes, through their
ice cover phenology (timing of ice formation, onset of melt, and end of
break-up), are a key indicator of climatic changes (Heron and Woo, 1994;
Duguay et al., 2006; Williamson et al., 2008) that the high-latitude
environment is experiencing. Lake ice phenology is dependent on several
factors, including meteorological conditions (e.g., air temperature, lack or
presence of snow, snow depth and density, wind speed) and lakes' physical
characteristics (e.g., surface area, bathymetry, elevation). However,
long-term analysis indicates that lake ice phenology is primarily responsive
to air temperature (Palecki and Barry, 1986; Jeffries et al., 1996; Duguay et
al., 2006). Increasing air temperatures in the Canadian Arctic in all
seasons, with an almost total absence of negative temperature anomalies over
the past 4 decades, has altered the ice regimes of many Arctic lakes (e.g.,
timing of ice formation and decay, maximum ice thickness), with later
freeze-up and earlier break-up dates for lakes in this region (Derksen at
al., 2012).
The lake ice season starts with ice formation on the lake surface or freeze
onset, which is the first day of the year on which the presence of ice is
detected in a pixel. As the ice season progresses, a solid ice cover forms
and the lake becomes completely frozen over. In spring, lake ice melt onset
(MO) is considered as the first day of the year on which melt is detected in
a pixel, observed as ice-free patch(es) on the otherwise ice-covered lake
surface and marks the beginning of the break-up season. Gradually, more ice
starts to melt until the lake becomes ice free or water clear of ice (WCI).
The first day with no ice on the lake surface is considered the end of the
break-up season. Consequently, the break-up season extends from MO to WCI.
The minimum lake ice cover extent prior to complete melt at the end of
summer, also referred to as the summer ice minimum, is generally observed to
occur a few days before the lakes become ice free or, in the case of lakes
that maintain a multiyear ice cover on occasional years, a few days prior to
ice refreezing in the fall. Changes in the ice phenology (i.e., timing of
ice-on and ice-off dates) of Arctic lakes have major implications for the
physical and biogeochemical processes and the aquatic primary production and
fauna, as they are strongly dependent on the presence of ice (Smol and
Douglas, 2007; Veillette et al., 2010; Michelutti et al., 2013). Recent
studies of Arctic lakes indicate thinner ice covers and less lakes freezing
to bed on the North Slope of Alaska during the winter (Surdu et al., 2014)
and reduced summer ice cover, leading to the loss of perennial ice of lakes
on Northern Ellesmere Island, Nunavut (Mueller et al., 2009), with a rapid
decline observed since 2008, following persistent warm summer temperatures
(Paquette et al., 2015).
The Canadian Arctic Archipelago (CAA), the study area of this investigation,
extends approximately between 60–83∘ N and 60–123∘ W,
from the Low Arctic to the High Arctic, and covers a territory of
approximately 1 425 000 km2, including over 35 000 islands. This
region is projected to experience the greatest annual temperature increase in
the North American Arctic during the course of the next 8 decades (ACIA,
2005). Projected climate conditions (2041–2070) employing two similar
Canadian Regional Climate Model (CRCM) scenarios (Brown and Duguay, 2011)
show that, consequent to loss of perennial ice, lakes on Ellesmere Island
will experience maximum ice regime changes (e.g., shorter ice cover duration,
thinner ice covers, loss of perennial/multiyear ice). As a result, lakes are
projected to break-up earlier by over 30 days, with lakes in the CAA being
expected to experience shorter ice seasons by 25–40 days. It is hypothesized
that lakes located in polar-oasis environments across the CAA may experience
greater changes than those within the areas with typical polar-desert
climate.
Polar or High Arctic oases are fairly small regions (with surface areas
ranging from 10-2 to 102km2) (Woo and Young, 1997) of
relatively great biological production and diversity, with warmer soil and
longer growing season (Courtin and Labine, 1977), discretely localized from
the surrounding arid landscape of polar deserts (Svoboda and Freedman, 1981).
Polar oases cover 6 % of the High Arctic landscape (Bliss, 1977). Several
polar oases have been identified across the High Arctic and, in general,
these oases are located in shrub zones (Edlund and Alt, 1989) or in sedge
meadows and polar semi-desert areas (Bliss, 1977). Polar oases can be
separated into thermal and biological oases. Thermal polar oases have an
atypical warmer microclimate than the surrounding polar desert (i.e., the
vicinity of Lake Hazen, including Craig Lake, Buchanan Lake near Eureka, and
several areas on Devon Island). Unlike polar thermal oases, the biological
oases have a climate similar to polar deserts but are characterized by
increased biological productivity and diversity. Polar Bear Pass, a wetland
considered to be a critical area for migratory birds, caribou, and muskox
(Young and Labine, 2010), is a typical polar biological oasis.
Monitoring changes in the lake ice cover and the rate at which they occur in
the High Arctic is limited by the sparse and inconsistent observations and
the short observational period in these remote regions. Accurate and
consistent monitoring of small Arctic lakes requires a complex combination of
spaceborne observations, model simulations, and where available, in situ
measurements. Opportunities exist to monitor small lakes across the Arctic by
exploiting the existing observations from heritage C-band synthetic aperture
radar (SAR) missions (i.e., ERS-1/2, ENVISAT's Advanced Synthetic Aperture
Radar (ASAR), RADARSAT-1/2, and Sentinel-1A). These missions, complemented by
data from optical sensors (i.e., Landsat), improve detection of ice cover
conditions of Arctic lakes.
Previous knowledge about past ice conditions for small High Arctic lakes is
limited to a few lakes located on Northern Ellesmere Island (Belzile et
al., 2001; Jeffries et al., 2005; Mueller et al., 2009; Cook and Bradley,
2010) and Colour Lake located on Axel Heiberg Island (Adams et al., 1989;
Doran et al., 1996). Latifovic and Pouliot (2007) have provided records of
ice conditions for larger High Arctic lakes, results obtained from the
Advanced Very High Resolution Radiometer (AVHRR), at 1.1km×1.1km spatial resolution. Lakes that have been previously studied
and are also analyzed in the current study include Upper and Lower Murray
Lakes, previously monitored between 1997 and 2007 (Cook and Bradley, 2010),
Lake Hazen, and Stanwell Fletcher Lake between 1985 and 2004 (Latifovic and
Pouliot, 2007). However, changes that lakes have undergone over the last 2
decades and the current state of ice conditions for most High Arctic lakes
remain unknown.
In order to identify possible ice regime changes for 11 lakes in the central
and eastern Canadian High Arctic from 1997 to 2011 using available SAR and
optical spaceborne satellite observations, the main objectives of this study
are (1) to analyze and report the annual rate of change in the timing of ice
decay onset, summer ice minimum, and end of ice break-up; (2) to comparatively
investigate the changes in the ice regimes of polar-desert lakes versus those
of polar-oasis lakes; and (3) to continue observation records of some lakes
that have been previously studied and to set the baseline data of a long-term
monitoring record for High Arctic lakes. Additionally, this work aims to
relate changes in the ice regimes of coastal High Arctic lakes to ongoing
changes within other components of the cryosphere.
Study area
The High Arctic is in its majority a polar-desert area with mostly barren
land surfaces, intense and persistent coldness, and low amounts of
precipitation (Woo and Young, 2006). Mean annual temperature (1950–2011) at
Alert, Nunavut (82∘30′ N, 62∘20′ W), Canada, is
-19 ∘C; at Eureka, Nunavut (79∘59′ N,
85∘56′ W), is -18 ∘C; and at Resolute, Nunavut
(74∘41′ N, 94∘49′ W), is -15 ∘C. For the
same period, mean total precipitation is 146 mm (16 mm rain and
130 mm snow) at Alert, 67 mm (23 mm rain and 44 mm snow) at
Eureka, and 128 mm (47 mm rain and 81 mm snow) at Resolute
(Environment Canada, 2011).
Positive temperatures are registered only during July and August and
occasionally in June and September, and most precipitation falls between July
and October, typically as snow (Environment Canada, 2011). The summer melt
periods are shortest (∼ 3 weeks) for the northern coast, while they last
∼ 8 weeks near Alert and ∼ 10 weeks at Lake Hazen (Keatley et
al., 2007). This study indicates that melt periods of lakes on Northern
Ellesmere Island last ∼ 6 weeks, melt periods of Lake Hazen last
∼ 6.6 weeks, while all other lakes have melt periods lasting
∼ 4 weeks.
The current study focuses on 11 lakes mainly located in the central and
eastern Canadian High Arctic, mostly small lakes with surface areas between 4
and 16 km2, with the exception of lake L11 on Baffin Island
(64 km2), Stanwell Fletcher Lake (339 km2), and Lake Hazen
(542 km2) – the largest lake within the Arctic Circle (Fig. 1).
Selection of lakes considered three main aspects: (1) lakes had to be large
enough so that break-up can be captured with SAR; (2) lakes had to be located
in both arid (polar desert) and semi-desert (polar oasis) environments and
thus allow a comparison between the two; and (3) lakes with previous ice
records were selected in order to ensure continuity of observations. Six of
the investigated lakes are located in polar-desert environments. For the
purpose of assigning the lakes to a specific polar environment, the other
five lakes have been categorized as polar-oasis lakes, with the note that
unlike the thermal-oasis climate around Lake Hazen, Craig Lake, Buchanan
Lake,
and L4 on Devon Island, Hunting Camp Lake on Bathurst Island is a biological
oasis with typical polar-desert climate.
Location of monitored lakes in the Canadian Arctic Archipelago.
Distribution of polar semi-deserts and sedge meadows is also shown (Woo and
Young, 1997, after Bliss, 1977). Inset shows location of the Canadian Arctic
Archipelago within the North American Arctic.
Location and basic characteristics of study lakes.
A summary of each lake's basic characteristics is shown in Table 1. Maximum
ice thickness of lakes on Northern Ellesmere at the beginning of winter
ranges from 1.1 to 2 m (Jeffries and Krouse, 1985; Belzile et
al., 2001; Mueller et al., 2009). In situ observations on Upper and Lower
Murray Lakes at the start of the melt season in early June 2005 indicated an
ice thickness of 1.5–2.2 m (Cook and Bradley, 2010). Existing snow
observations indicate average end-of-winter snow depths of approximately
20 cm near Resolute Bay, Cornwallis Island, in 1976–1977 (Woo and
Marsh, 1978), 26 cm on the plateau in Hot Weather Creek, Ellesmere
Island, between 1989 and 1992 (Young et al., 1997), and less than 8 cm
on the plateau of Polar Bear Pass, Bathurst Island, between 2008 and 2009
(Assini and Young, 2012). However, in situ snow depth measurements across the
Canadian High Arctic are sparse/unavailable and inconsistent.
Polar oases are characterized by a milder microclimate (Woo and Young, 1997)
that is mainly attributed to higher incoming radiation, given the fact
that most frequently they develop in relatively flat coastal lowlands and are
being protected by topography, the exception being the thermal oasis surrounding
Lake Hazen (France, 1993). Lake Hazen is situated in a trough. Lake Hazen is
sheltered from the cold Arctic Ocean air by the Grant Land Mountains
(> 2000 m) in the north and a plateau (400–900 m) in the
south. Similar to the Lake Hazen basin, sheltered by the neighboring
mountains, the polar oasis on Fosheim Peninsula on Ellesmere Island (approx.
80∘08′ N) experiences a greater amount of solar radiation,
particularly during the month of June. Consequently, snowmelt in the area
often occurs about a month early (Woo and Young, 1996). Additionally,
vegetation growth is favored as a result of extended thaw seasons (Edlund
and Alt, 1989). The biological oasis on Polar Bear Pass (75∘40′ N,
98∘30′ W), covers an area of approximately 100 km2,
extends from one end of the island to the other, has a typical polar-desert
climate, and is characterized by long winters and cool, moist summers. Within
the area lie two large lakes, several small lakes and a multitude of small
ponds (Woo and Young, 1996).
Ellesmere Island contains some of the largest polar oases in the Queen
Elizabeth Islands of Arctic Canada, including Fosheim Peninsula, Tanquary
Fiord, and Lake Hazen (Edlund and Alt 1989). Other High Arctic oases have been
identified on Devon Island (Bliss, 1977), Alexandra Fiord on Ellesmere Island
(Freedman et al., 1994), Polar Bear Pass (Bathurst Island), and at Sherard Bay
on Melville Island (Aiken et al., 1999). Given that the current analysis is
limited by the availability of low spatial resolution SAR imagery
(150 m for ASAR, and 100 m for RADARSAT-1/2), Lake Hazen,
Craig Lake, Buchanan Lake, lake L4 on Devon Island, and Hunting Camp Lake
(biological oasis with polar-desert climate) on Bathurst Island were the only
investigated lakes located in polar-oases environments.
Data and methodsSatellite acquisitions
The ability of spaceborne sensors to monitor and detect changes in the ice
cover of high-latitude lakes has been previously demonstrated from both an
optical (Latifovic and Pouliot, 2007; Arp et al., 2013) and a SAR approach
(Morris et al., 1995; Duguay et al., 2002; Surdu et al., 2014) or a
combination of optical and SAR (Cook and Bradley, 2010).
Due to frequent revisits at high northern latitudes and their ability to
acquire data during polar darkness and through cloud cover, spaceborne SAR
sensors are suitable for monitoring changes in the ice cover of High Arctic
lakes. In C-band (∼ 5.3 GHz) SAR imagery, the high contrast
between ice and open water, representing the amount of radar signal or
backscatter (σ∘) returned to the sensor, allows detection of
the timing of summer ice minimum and WCI (Morris et al., 1995;
Duguay et al., 2002; Geldsetzer et al., 2010). Robust determination of the
timing of lake freeze-up using SAR is limited by the low σ∘
contrast between the open water and the newly formed floating ice (Cook and
Bradley, 2010) and also because the C-band co-polarized backscatter from
water is not only sensitive to wind speed but also to wind direction
(Geldsetzer and Van Der Sanden, 2013). Additionally, backscatter intensity is
also dependent on local radar incidence angle (Duguay et al., 2002, Surdu et
al., 2015). Considering the limitations that freeze-up detection pose with
SAR, particularly at VV polarizations and to a lesser degree for HH-polarized
images, this study focuses on monitoring the break-up period of High Arctic
lakes in the central and eastern Canadian High Arctic. The relatively coarse
spatial resolution of SAR images used in this study, limited MO detection,
particularly for small lakes (Cook and Bradley, 2010). To minimize this
constraint, Landsat data were also used to identify the small areas of open
water, otherwise uncertain in SAR imagery.
Given that the current study includes 11 lakes that were monitored for a
period of 15 years, the number of satellite observations employed in the
analysis was considerable: ∼ 27 000 SAR acquisitions (RADARSAT-1/2 and
ASAR) and over 2000 Landsat images, with a mean frequency of image
acquisition ranging from 2 to 9 days (Table 2); ∼ 1600 SAR images were
segmented to derive ice/open-water fractions.
The ASAR images were provided by the European Space Agency (ESA) as a Wide
Swath Mode Medium Resolution Image (ASA_WSM_1P) product. The ASAR
instrument, on board ESA's ENVISAT, uses the ScanSAR technique when in
wide-swath mode (the same as RADARSAT-1/2), provides a spatial resolution
adapted for regional monitoring (approx. 150 m, with a pixel spacing
of 75 m). The combination of HH- and VV-polarized images was acquired
at incidence angles ranging from 17 to 42∘. The time lapse between
repeat passes (or revisit time) of ENVISAT is 35 days. In order to increase
the frequency of observations, data from different tracks, descending and
ascending orbits, were used.
RADARSAT-1/2 data, with a spatial resolution of 100 m and a pixel
spacing of 50 m, 2×2 block averaged to 100 m
(obtained from the Canadian Ice Service), acquired at incidence angles of
19–49∘ (RADARSAT-1) and 20–46∘ (RADARSAT-2), are a ScanSAR
Wide mode product. The single-polarized (HH) RADARSAT-1 and single- and
dual-polarized (HH + HV) RADARSAT-1/2 images were acquired approximately
every 2–3 days during the break-up season of each year of study.
In addition to SAR data, archived Landsat 4 Thematic Mapper (TM) and
Landsat 7 Enhanced Thematic Mapper Plus (ETM+) imagery, with a spatial
resolution of 30 m, was also used. Because of the data gaps in the
Landsat imagery from 1997 to 2003 and the limited number of images during
spring melt after 2003 for some of the lakes included in this study, the
Landsat images were not used for calculating ice/open-water fractions.
Instead, the Landsat imagery was utilized to complement and evaluate the SAR
observations and thus build a reliable record from the beginning to the end
of the ice season during the 15 years of record.
Image processing and analysis
A total of ∼ 1600 SAR images acquired from the beginning of the melt
season until the water was clear of ice (WCI or break-up end or 100 % ice
free), or, in cases when multiyear ice was identified, until the beginning of
freeze-up, were selected. The selected images were segmented using the most
common clustering method, the unsupervised K-means classification
algorithm. This algorithm has proved to be a suitable method to discriminate
between ice and open water and thus monitor the lake ice break-up using SAR
data (Sobiech and Dierking 2013). Keeping in mind the large number of images
analyzed in this study, the K-means algorithm was preferred over a fixed
threshold method as it is flexible to changing ice conditions (Sobiech and
Dierking, 2013) during the melt season. The unsupervised K-means
classification is an iterative process in which image intensity values are
divided into “k” classes or clusters. Throughout the 20 iterations
performed for each segmentation, the K-means classification assigned each
intensity value to the class with the nearest arithmetic mean
(minimum-distance technique).
In order to reduce the inherent speckle present in SAR images, a Lee filter
(Lee, 1980) with a kernel size of 3×3 was applied to all geocoded
images. After the speckle was removed, regions of interest (ROIs) covering
the lake areas were selected. Following ROI designation (i.e., vector file of
selected lake), image segmentation of each ROI was performed. The
classification only included the pixels inside the ROI; all other pixels
outside the lake boundaries were excluded from analysis. In order to account
for the different ice classes, the segmentation was set to five clusters. To
discriminate between ice and open-water clusters in the resulting
segmentation maps, each segmentation output was visually assessed against the
original SAR image. To additionally evaluate the class-merging accuracy, when
available, segmentation results were assessed against optical images
(Landsat) acquired on the same date. When ancillary optical data were not
available, the backscatter threshold values of the original SAR acquisition
were used to verify the segmentation results prior to cluster merging. Once
clusters belonging to either the ice or open-water class have been
identified, the resulting five classes were further merged into two classes:
one ice and one open-water class in the ENvironment for Visualizing Images
(ENVI) software, using the post-classification function. Following merging, a
two-class map was generated for each segmented SAR image (Fig. 2). Text files
showing the percentage (%) or fraction of open water and ice were
extracted for each ROI of the classified maps in order to quantify the amount
of ice present on lakes from the start of ice decay until the end of the
break-up season.
SAR-image segmentation processing steps: (a) Landsat image
of Lake Hazen, 19 July 2010; (b) original ASAR image of Lake Hazen acquired
on 19 July 2010; (c)K-means classified image (five clusters);
(d) two-class map of ice (light blue) and open water (dark blue).
The white line in the original SAR image represents the lake polygon that was
used for defining the ROIs covering the lake.
In order to estimate the magnitude and significance of changes during the
15-year period, a Mann–Kendall test using Sen's slope (Sen, 1968) was
performed. This non-parametric statistical test, widely used for detecting
monotonic trends in hydrological long-term time series (Hirsch et al., 1982;
Zhang et al., 2001), was deemed to be one of the most powerful trend tests
(Hess et al., 2001) as it can deal with data that are not normally distributed
and has minimum sensitivity to data gaps related to inhomogeneous time series
(Tabari et al., 2011) or values below a detection limit. This method has
been successfully used previously for detecting the presence of trends in
long-term observation of river and lake ice (Smith, 2000; Futter, 2003;
Duguay et al., 2006). However, caution should be used in interpretation of
the statistical significance values considering that the trend analysis was
performed on a relatively short-term time series of 15 years. The observed
changes in ice regimes are shown as number of days, change being reported
relative to the 1997–2011 calculated mean (days of change) for each
individual lake observed from spaceborne acquisitions.
Summary of yearly number and frequency of satellite images used for
the ice cover monitoring of the investigated High Arctic lakes during the
break-up season from 1997 to 2011.
Year ofTotal number of images used observations(ASAR, RADARSAT-1/2, Landsat) HazenCraigBuchananL6HuntingUpper/EleanorL9StanwellL11Frequency(Devon I.)CampLower(Somerset I.)Fletcher(Baffin I.)of imageMurrayacquisition(days)19971099774589534235804–13199839392301741184181101982082–10199930342271981263098113111873–112000273220118313723991231061532–1220014150436435307382513012782982–720022530285344263252272892562482–920031313265340272162202652412592–1020041025254327241182022762502822–1120052321250326272312122672332581–1120061211103453202451532012612462450–620071311243283512941522252842592850–6200812812570664251816343953450212430–520091761084604023282182933162632150–62010170523212982361811891831423320–720113162881057970326685847251–8Total/mean12601060451044933485142428583413296743182–9Climate data
Climate records of air temperature, including daily, monthly, and seasonal
averages from 1997 to 2011, were collected in support of the analysis of
satellite-derived ice phenology parameters. Given that the majority of
weather stations in the CAA with longer climate records are situated at a
significant distance from most lakes included in the current study, with
distances ranging from 60 to 255 km, a combination of weather station
and surface air temperature reanalysis data was used to assess the observed
changes in lake ice regimes. Additionally, Moderate Resolution Imaging
Spectroradiometer (MODIS) thermal data were used to show differences in
surface skin air temperature between warmer and colder years and thus capture
the interannual temperature variability.
Weather station records
Meteorological station data from Environment Canada's National Climate Data
and Information Archive were used for post-analysis of the spaceborne
observations. These records include mean temperature data from 1 January 1997
to 31 December 2011, for three permanent weather stations: Alert, Eureka, and
Resolute, Nunavut. Air temperature anomalies from 1997 to 2011 based on the
available weather station annual mean temperature records are shown in
Fig. 3. These anomalies are to be used as a reference in interpretation of
lake ice events during the same period. The weather station records,
complemented by ERA-Interim reanalysis data, were used for assessment of the
relation between air temperature and ice phenology of lakes situated within a
0–120 km range from the weather station. Temperature records for lakes
located further from the weather stations were based exclusively on
ERA-Interim reanalysis data. Previous evaluations of ERA-Interim data over
the Arctic shows that near-surface temperature estimates from reanalysis
agree well with sparsely sampled observations from conventional
climatological data sets and are coherent across the Arctic (Simmons et
al., 2010; Simmons and Poli, 2014).
Air temperature anomalies for (a) Alert,
(b) Eureka, and (c) Resolute relative to the 1997–2011 mean annual temperature.
ERA-Interim reanalysis data
The ERA-Interim is the largest global atmospheric reanalysis product of the
European Centre for Medium-Range Weather Forecasts (ECMWF). The
full-resolution (∼ 0.75∘×0.75∘) gridded product
is derived from data assimilation from a variety of sources: radiances from
the Special Sensor Microwave Imager (SSM/I), radiosonde temperature,
scatterometer ocean surface wind data including recalibrated data from the
European Remote Sensing (ERS-1/2) satellites and, until 2009, from QuickSCAT
(Dee et al., 2011). This reanalysis product provides global coverage since 1979. For the purpose of this study, the 2 m near-surface temperature
computed with a sequential data assimilation scheme, advancing forward in
time using 12-hourly analysis cycles, was utilized.
MODIS surface temperature
MODIS Aqua and Terra MOD 11-L2 data, at a spatial resolution of 1 km,
were used to derive the MODIS UW-L3 land surface temperature product
(Kheyrollah Pour et al., 2014). In order to derive the monthly average land surface temperature (LST),
daily averages were calculated first. For the daily-averaged UW-L3 product,
observations are separated into either a daytime bin (from 06:00 to
18:00) or a nighttime bin (from 18:00 to 06:00 of the next day). For
the geographical region of interest, two sets of data are produced, one
containing the average of all daytime observations and the other containing
those of all nighttime observations. Then, the intermediate sum of all MODIS
Aqua/Terra daytime/nighttime observations for each pixel is calculated. These
values are averaged together to produce the final monthly surface temperature
average with equal weighting between daytime and nighttime values.
Results
Satellite observations of the ice cover on 11 lakes in the central and
eastern Canadian High Arctic from 1997 to 2011 reveal great variability in
the timing of ice MO, summer ice minimum, and WCI dates, with a noticeable
direction toward earlier ice-off dates and frequent loss of the multiyear ice
cover. In light of the relatively short period of this study, the current
results are indicative of a recent direction rather than a long-term trend
observed in the ice phenology of the investigated High Arctic lakes.
Additionally, these results could also be reflective of a recent cyclical
behavior change of lake ice in response to changes in air temperatures
during the 15-year period of the study.
Melt onset
MO (start of break-up) was considered as the first date when surface melt or
patches of open water were noticed in satellite observations through image
segmentation for the SAR acquisitions and visual assessment of the Landsat
images. At the start of the break-up season, pooling water was observed atop
the ice cover of lakes on Ellesmere Island (Cook and Bradley, 2010).
Considering the similar backscatter characteristics of pooling water on the
ice surface and open water (Hall, 1998) at the beginning of lake ice
break-up, discriminating between ice and open water in SAR images poses
certain challenges. In order to improve the accuracy of MO detection, the
Landsat imagery provided a valuable complement to the SAR observations. As
such, during the few years with larger temporal gaps for the available SAR
acquisitions, the complementary optical images reduced the data gaps to less
than 5 days between spaceborne acquisitions, thus considerably reducing the
uncertainty in estimation of the MO date. Spaceborne observations were
available for most lakes during MO from 1997 to 2011 (Table 3). Table 3
displays the range of observed MO dates (shown as DOYs) during this period,
the mean MO date for each lake calculated based on 1997–2011 mean derived
from spaceborne observations, and the total days, representing the calculated
earlier/later MO days using the Mann–Kendall statistical test, relative to
the 1997–2011 mean.
Melt onset dates shown as day of the year (DOY) for the studied
lakes from 1997 to 2011. Missing values (NA) indicate the lack of available
satellite imagery. Total days for each individual lake refer to the total
number of days change during the 1997–2011 period and are reported to the
mean melt onset DOY of the same period. The statistical significance is
indicated by the α values.
Year ofHazenCraigBuchananUnnamedHuntingUpperLowerEleanorUnnamedStanwellUnnamedobservations(Devon)CampMurrayMurray(Somerset)Fletcher(Baffin)1997205NA20923220019419420818316918219981681681931841711841841931701651691999177184189217171176176194183180178200017116517219119216516419818617718520011671671882001771671692001841781852002173167195196175NANA2031841791972003NANA19219118017117319118618119420041621661992331861711642001861882102005171170191210174182182200181175189200619718318621317316816819917918119220071681621851831671661651851781721792008160166185184167166164186179167180200917916519119417016616219418417918720101591631801751691721741751781711792011174172172172164NANA1731731681741997–2011 mean1741691881981761731721931811751851997–2011 total days-14-3-15-39-20-13-20-23-7-3-3α> 0.1> 0.10.050.050.010.10.050.05> 0.1> 0.1> 0.1
Changes – shown as number of days – in the melt onset date of
investigated lakes in the central and eastern Canadian High Arctic
(1997–2011). Number of days change is reported relative to the 1997–2011
mean melt onset day derived from spaceborne observations during this period.
Lakes in polar-oasis environments are shown as blue bars and lakes in
polar-desert environments are shown as grey bars. The red line indicates the
1997–2011 mean number of days change for melt onset.
During the 15-year period with available satellite acquisitions, advanced MO
(compared to the 1997–2011 observation period mean) was observed for all
11 lakes, with earlier MO by a total of 39 days for lake L4 on Devon Island
(α=0.05), earlier by 20 days for Hunting Camp Lake (significant at
the 0.01 level), by 15 days for Buchanan Lake at the 0.05 level, and by
3 days for Craig Lake, Stanwell Fletcher Lake, and lake L11 on Baffin Island
(α=> 0.1). Mean MO dates for lakes on Northern Ellesmere Island
(i.e., Craig Lake, Upper and Lower Murray Lakes, and Lake Hazen) ranges
between 18 June (DOY169) and 24 June (DOY175). MO for lake L4 on Devon
Island, Eleanor Lake, and Buchanan Lake, was observed to start between 7 July
(DOY188) and 17 July (DOY198). Overall, the greatest changes in timing of MO
dates and statistical more significant – despite not being a long-term trend
and rather a measure of change detection during a 15-year period – were
observed for lakes located in polar-oasis (thermal and biological)
environments (Fig. 4).
Summer ice minimum
The last date with a floating ice cover on the lake surface at the end of
summer was considered the ice minimum date. During years when lakes
maintained a multiyear ice cover, the summer ice minimum date was usually
observed in late August to mid-September when melt concludes and most lakes
start refreezing. The mean ice minimum date ranges from 12 July (DOY193,
Craig Lake) to 25 August (DOY237, Stanwell Fletcher Lake). From 1997 to 2011,
most lakes lost their summer ice cover during all years of investigation
(Table 4). Table 4 displays the range of observed summer ice minimum dates
(shown as DOYs) during this period, the mean summer ice minimum date for each
lake calculated based on 1997–2011 mean (excluding the years when lakes
maintained a partial summer ice cover) derived from spaceborne observations,
and the total days, representing the calculated earlier/later summer ice
minimum dates using the Mann–Kendall statistical test, relative to the
1997–2011 mean.
Dates (shown as day of the year – DOY) when minimum ice cover was
observed for the studied lakes from 1997 to 2011. Missing values (NA)
indicate the lack of available imagery. Total days for each individual lake
refer to the total number of days change during the 1997–2011 period and are
reported to the mean summer ice minimum DOY of the same period. The
statistical significance is indicated by the α values.
Year ofHazenCraigBuchananUnnamedHuntingUpperLowerEleanorUnnamedStanwellUnnamedobservations(Devon)CampMurrayMurray(Somerset)Fletcher(Baffin)199723220523324622423123222921521321519982091822142171862202202131992172031999249193208267205243243221215252215200025518220021420921221221721123721420012131881972452032202292242142532082002238194203230206NA2412272242572182003NA198197241200NANA212217258218200424621021824423821321324524425823320052091872032312062222232192232322232006234192202254211243257222213235215200722019120021419623423420820221920820082231911932271912132262082082382092009250205205226207262262218210232213201020618919621619921021220621223621520112231861842021952222491952022171981997–2011 mean2291932042322052272322182142372141997–2011 total days-9-2-23-30-15012-19-5-2-2α> 0.1> 0.10.050.1> 0.1> 0.1> 0.10.05> 0.1> 0.1> 0.1
Generally, the lake ice cover was observed to melt earlier in the season
during the years with positive annual air temperature anomalies and last
longer into the summer during the years with negative annual air temperature
anomalies. Similarly, the occasional multiyear ice on several lakes lasted
from one year to the other during the years with negative annual air
temperature anomalies. With the exception of the polar desert around Upper
and Lower Murray Lakes that maintained relatively consistent ice conditions
or a longer-lasting ice cover into the summer by 12 days (α=> 0.1, Lower Murray Lake) during the 15-year observation period, all other
lakes experienced earlier minimum ice during the summer months and thus
earlier ice-off dates. Lakes located in polar-oasis environments experienced
the earliest summer ice minimum dates, with lake L4 on Devon Island (30 days
earlier, α=0.1) and Buchanan Lake (23 days earlier, α=0.05) experiencing the greatest change. Eleanor Lake, despite being a
polar-desert type lake, experienced considerably earlier summer ice minimum
dates (19 days earlier, α=0.05). All typical polar-desert lakes
experienced minimum negative change, with a lower statistical significance
(2–5 days earlier, α=> 0.1) in the timing of the summer ice
minimum date (Fig. 5).
Multiyear ice was observed on occasional years for Lake Hazen (2000, 2004,
2009), Upper Murray Lake (1999, 2009), Lower Murray Lake (1999, 2002, 2006,
2009), lake L4 on Devon Island (1997, 1999, 2001, 2003–2004, 2006), and
Stanwell Fletcher Lake (2001–2004). Lakes formerly observed to maintain
multiyear ice covers, such as Lake Hazen in the 1950s (Hattersley-Smith,
1974) and Stanwell Fletcher Lake in the early 1960s (Coakley and Rust, 1968),
are shifting toward a more frequent seasonal ice cover. Similar to other High
Arctic lakes that are rapidly transitioning from a perennial (persistence
over decades or longer) or multiyear (persistence for > 1 year) to a
seasonal (annual melt out) ice cover (Paquette et al., 2015), lakes in the
central and eastern Canadian High Arctic seem to experience a similar shift.
From 1997 to 2011, L4 on Devon Island and Stanwell Fletcher Lake were the
only lakes that preserved their ice cover for 2 or more consecutive years
(i.e., 2003–2004 for L4 and 2001–2004 for Stanwell Fletcher Lake).
Changes – shown as number of days – in the summer ice minimum date
of investigated lakes in the central and eastern Canadian High Arctic
(1997–2011). Number of days change is reported relative to the 1997–2011
mean summer ice minimum day derived from spaceborne observations during this
period. Lakes in polar-oasis environments are shown as blue bars and lakes in
polar-desert environments are shown as grey bars. The red line indicates the
1997–2011 mean number of days change for summer ice minimum.
Water-clear-of-ice dates shown as day of the year (DOY) for the
studied lakes from 1997 to 2011. Missing values (NA) indicate the lack of
available imagery. Dash indicates that complete melt did not occur. Asterisks
indicate the lakes that maintained an occasional perennial ice. Total days
for each individual lake refer to the total number of days change during the
1997–2011 period and are reported to the mean water-clear-of-ice DOY of the
same period. The statistical significance is indicated by the
α values.
Year ofHazen*CraigBuchananUnnamedHuntingUpperLowerEleanorUnnamedStanwellUnnamedobservations(Devon)*CampMurray*Murray*(Somerset)*Fletcher*(Baffin)1997233208234–225232NA23221623021619982131912152201872212212172042192071999250201209–207––2262162552202000–1952012172102192192182152392192001219196208–204220230225215–2102002241195204233209237–230227–2222003NA199198–202NANA214219–2202004–210219–244NANA248––23420052101882042332072232302202242372242006235194203–212249–225215236217200722120020121719723724121020322220920082241951942281952192282142092462102009–209206227208––219212234215201021019219721720021121720721323721920112251921852041972242511962032231991997–2011 mean2261982052222072272302202142342161997–2011 total days-9-4-24-8-15-213-20-5-4-5α> 0.1> 0.10.01> 0.1> 0.1> 0.1> 0.10.05> 0.1> 0.1> 0.1Water clear of ice
The end of break-up was indicated by the absence of an ice cover over lakes
(0 % ice), also known as WCI. For years with sparse satellite imagery at
the end of break-up, and thus with differences greater than 1 day between the
date of minimum ice cover and the WCI date, the day when the lake became ice
free was estimated by interpolating between the date of the last satellite
image (either SAR or Landsat) that indicated the presence of ice on lake
surface and the date of the next available satellite observation that showed
100 % open water, based on the observed rate of ice decay from previous
images.
The range of mean WCI dates for the observed lakes from 1997 to 2007 falls
between 17 July (DOY198, Craig Lake) and 22 August (DOY234, Stanwell Fletcher
Lake). The mean WCI date for Upper Murray and Lower Murray Lake is 15 August
(DOY227) and 18 August (DOY230), respectively. Lakes remained completely ice
free for several weeks prior to starting to refreeze, usually at the
beginning of September when below-freezing air temperatures returned. While
most lakes lost their ice cover every summer, observations indicate that a
few lakes did not completely melt during the summer months (Table 5). Table 5
displays the range of observed WCI dates (shown as DOYs) during this period,
the mean summer ice minimum date for each lake calculated based on 1997–2011
mean derived from spaceborne observations, and the total days, representing
the calculated earlier/later WCI dates using the Mann–Kendall statistical
test, relative to the 1997–2011 mean. Analysis indicates that the WCI date
was generally earliest for polar-oasis lakes: Lake Buchanan (by 24 days,
α=0.01), Hunting Camp Lake (biological oasis) on Bathurst Island (by
15 days, α=0.1), Lake Hazen (by 9 days, α=> 0.1), and
lake L4 on Devon Island (by 8 days, α=> 0.1). WCI for the
polar-desert Eleanor Lake occurred earlier by 20 days (α=0.05). The
polar-desert Lower Murray Lake experienced later WCI dates by 13 days
(α=> 0.1). Other than Lake Buchanan and Eleanor
Lake that showed a significant
statistical trend toward earlier open-water seasons, for all other lakes the
significance level is greater than 0.1. Comparative changes in timing of the
WCI date between lakes located in polar-desert environments and those in
polar-oasis environments are shown in Fig. 6.
Changes – shown as number of days – in the water-clear-of-ice date
of investigated lakes in the central and eastern Canadian High Arctic
(1997–2011). Number of days change is reported relative to the 1997–2011
mean water-clear-of-ice day derived from spaceborne observations during this
period. Lakes in polar-oasis environments are shown as blue bars and lakes in
polar-desert environments are shown as grey bars. The red line indicates the
1997–2011 mean number of days change for water that is clear of ice.
DiscussionChanges in lake ice regimes 1997–2011The break-up season
For the majority of lakes, the break-up season (1997–2011) covered the
months of June, July, and August, ice decay generally started in June and
transitioned toward an ice-free lake cover until the second/third week of
August. Depending on summer air temperatures and/or lake location and size,
some lakes become ice free before the end of July. The analysis focused on
the response of the lake ice cover to changes in air temperatures; however,
other factors that contribute to lake ice break-up exist, including but not
limited to on-ice snow depth and extent, wind action, and spring runoff.
While interannual variability in the MO dates existed from 1997 to 2011,
lakes generally experienced earlier MO during the years with positive air
temperature anomalies and later MO during the years with negative air
temperature anomalies. For instance, in 1997, a year with negative air
temperature anomaly at Alert, NU, MO for Lake Hazen was observed on 24 July
(DOY205), 31 days late compared to the 1997–2011 mean. MO for the same lake
occurred by 15 days earlier (8 June, DOY159) in 2010, when the air
temperature anomaly at Alert, NU, was positive. The large positive air
temperature anomalies (e.g., 2005, 2006, 2007, 2009, and 2011) is at Alert a
consequence of higher spring air temperatures during these years. Unusual
warm years have been associated with anomalous high-pressure atmospheric
events such as the one in the late winter of 1997/early spring of 1998,
resulting in above-average temperatures over the Canadian Arctic (Atkinson et
al., 2006). Furthermore, episodes of advection of moist, warmer air from the
anomalous higher sea surface waters of the northwestern Atlantic in spring/early
summer result in large positive anomalies in the near-surface temperature
(Sharp et al., 2011).
Given the high albedo of snow, MO (the first appearance of open water) could
be delayed if a layer of snow or snow ice formed during freeze-up is present
on lakes at the beginning of the break-up season. Previous field observations
revealed that the ice cover of Murray Lakes at the beginning of the 2005
break-up season entirely consisted of black ice, thus lacking the snow-ice
layer (Cook and Bradley, 2010). Additionally, most of the high-latitude
regions generally experience low amounts of snowfall
(< 158 mmyear-1). These facts could suggest that the presence
of snow ice and/or snow on High Arctic lakes is not a significant driver of
the break-up process. However, the sparse in situ snow accumulation,
thickness, and ice type measurements limit the evaluation of the importance of
snow in the timing of MO for the lakes in the CAA.
Another factor to be taken into consideration when discussing the timing of
MO for lakes is water inflow into the lakes. Eight out of the 11 lakes
included in this study have streams flowing into the lake. The origin of the
warmer streams flowing into the lakes could be from melting glaciers (i.e.,
Murray Lakes and Lake Hazen on Northern Ellesmere Island, Buchanan Lake on
Axel Heiberg Island) and/or runoff from snowmelt (e.g., lake L4 on Devon
Island). The ice break-up of Lake Hazen seems to be initiated by a runoff
stream from Craig Lake (Fig. 7).
Landsat images acquired at the start of ice break-up showing melt
and/or open water adjacent to water inflows: (a) Upper and Lower
Murray Lakes (17 June 2007); (b) Buchanan Lake (8 July 1999);
(c) Lake L6, (10 July 2003); (d) Lake Hazen and Craig Lake
(16 June 2001).
Water-clear-of-ice dates relative to the 0 ∘C spring
isotherm date between 1997 and 2011 for (a) Craig Lake (polar oasis)
and (b) Eleanor Lake (polar desert). Spearman's rank correlation
coefficient (R) is also shown.
The cumulative thawing degree days and ice fraction for Lake Hazen
during the 2010 break-up season.
Mean land surface temperature derived from MODIS Aqua/Terra over
the central/eastern CAA during the months of June, July, and August of 2004
(colder year) and 2011 (warmer year).
Similar to MO, timing of WCI is also dependant on air temperature. In order
to analyze the relation of WCI to air temperature, the 0 ∘C spring
isotherm dates were calculated based on the approach described in Bonsal and
Prowse (2003). The 0 ∘C spring isotherm date is considered as the
date when mean daily air temperature rises above 0 ∘C. Given the
large variability in daily air temperature, a 31-day running mean filter is
used for the mean daily air temperatures. Lake WCI dates relative to the
0 ∘C spring isotherm date, calculated based on the available weather
station temperature records and ERA-Interim data, from 1997 to 2011 are shown
in Fig. 8. The relation between the timing of WCI dates and 0 ∘C
spring isotherm date was determined with Spearman's rank correlation
coefficient (R). Analysis shows an overall correlation of R=0.60
between the WCI date timing and the 0 ∘C spring isotherm date for
lakes that were ice free every year between 1997 and 2011. As a result of the
presence of an occasional multiyear ice cover, the correlation between WCI
and the 0 ∘C spring isotherm date weakens.
The lower correlation for lakes that occasionally maintain an occasional
summer ice cover is likely related the limited ability of the gridded data to
accurately represent local climate conditions for lakes located further from
permanent weather stations. These lakes are influenced by local microclimates
due to the effect of the nearby glaciers and high mountains (Woo and Guan,
2006; Keatley et al., 2007). The presence of the Greenland Ice Cap (Alert,
Northern Ellesmere Island), glaciers (Northern Ellesmere Island, Devon
Island), and high topographic features (mountains > 2200 m around
Lake Hazen and near Eureka) could lead to discrepancies between the weather
station and reanalysis data. Hence, given the grid-cell size of the
reanalysis data (0.75∘), ERA-Interim records for the Northern
Ellesmere Island do not always capture the microclimates or warmer/colder
climatic episodes that develop in some of the smaller High Arctic areas
(L. C., Brown, personal communication, 2014).
AVHRR observations of WCI dates for Lake Hazen and Stanwell Fletcher Lake
from 1985 to 2004 reveal earlier break-up by < 10 days for the former and
by 4–6 days for the latter (Latifovic and Pouliot, 2007). The current
analysis shows that during the 1997–2011 period, break-up occurred earlier
by a total of 12 days for Lake Hazen and by 6 days for Stanwell Fletcher
Lake. Considering that break-up is highly correlated with air temperatures
(Duguay et al., 2006), the increase in the number of days revealing even
earlier WCI dates for Lake Hazen and Stanwell Fletcher Lake is reflective of
higher mean air temperatures during 1997–2011 shown by the gridded
ERA-Interim data.
Using all available RADARSAT-1/2, ASAR, and Landsat images from the beginning
to the end of the break-up period between 1997 and 2011, WCI timing was
determined with an accuracy of 1–3 days, 3 days being the longest period
with no available satellite imagery from any sensor at the end of break-up. A
time series of multiple-sensor acquisitions for Lake Hazen during the 2010
break-up season (Fig. 9) shows the ice cover changes from the beginning to
the end of break-up. Changes in the ice cover of Lake Hazen during the 2010
break-up season reflect the lake ice/temperature relation, the decrease in
the ice cover fraction being correlated (R=-0.94) with the number of
cumulative thawing degree days calculated based on the ERA-Interim
daily mean air temperatures (Fig. 10).
The mean duration of the break-up season for Upper Murray Lake is 52 days and
for Lower Murray Lake is 55 days. Previous findings of ice regimes for Murray
Lakes between 1997 and 2007 indicate 16 August (DOY228) as a mean ice-off
date for Upper Murray Lake and 24 August (DOY236) for Lower Murray Lake, and
an average duration of the melt period of 74 and 81 days, respectively (Cook
and Bradley, 2010). The earlier timing of WCI dates and shorter break-up
seasons for Murray Lakes shown by the current study are indicative of
positive air temperature anomalies at Alert during all years from 2005 to
2011. Earlier positive air temperatures at the beginning of summer and
consecutive days with temperatures higher by 2–4 ∘C than
temperatures recorded during previous break-up seasons of 2007–2011 are
likely the main drivers of the earlier WCI dates and shorter break-up periods
for Murray Lakes shown in this study.
The greatest changes during the break-up season from 1997 through 2011 were
recorded for smaller lakes, in both polar-desert and polar-oasis environments
(i.e., lake L4 on Devon Island, Buchanan Lake and Hunting Camp Lake), and the
polar-desert type lake, Eleanor. These lakes experienced earlier MO by
20–30 days, earlier ice minimum dates by 15-30 days, and earlier WCI dates by
15–24 days. These findings suggest that following the considerably higher
temperature of polar deserts during recent years (Woo and Young, 2014),
polar-desert type lakes are starting to shift into polar-oasis type lakes,
the smaller ones showing the shift earlier.
The multiyear ice cover
The loss of the perennial/multiyear ice cover for most lakes is mainly a
consequence of the warmer air temperatures recorded in the High Arctic during
recent decades as recent studies show (Sharp et al., 2011; Zdanowicz et
al., 2012; Woo and Young, 2014). In general, during years when the
0 ∘C spring isotherm date occurred earlier in the spring, lakes
became ice free (e.g., in 1997 the lake L4 on Devon Island maintained a
multiyear ice cover in 1997 when the 0 ∘C spring isotherm date
occurred on DOY170 and, following a 0 ∘C spring isotherm date on
DOY157, it completely lost its ice cover early in 2011; DOY 204).
Conversely, during years with late 0 ∘C spring isotherm date lakes
maintained a multiyear ice cover. The persistence of ice throughout the
summer into early autumn when it starts refreezing (multiyear ice cover) on
occasional (cool) years in some lakes could be related to a multitude of
factors whose individual and/or combined actions allow the lake ice cover to
outlast from one season to another. As such, lower spring air temperatures
(e.g., 2004) delay ice break-up which combined with lower summer/fall air
temperatures promotes a multiyear ice cover. Additionally, the presence of
glaciers in the vicinity of some of the study lakes (i.e., Lake Hazen Upper
and Lower Murray Lake, lake L4 on Devon Island), through generally persistent
low air temperatures, stabilizes the lake ice cover (Doran et al., 1996).
Furthermore, the vicinity of a partially frozen Arctic Ocean during (extreme)
colder summer seasons cools the atmosphere around the lakes and thus supports
the presence of a summer ice cover.
Lake ice in a changing cryosphere
Since the mid-1990s, increasing summer and winter temperatures across the
entire Canadian Arctic, and highly noticeable on the eastern side, have been
recorded (Zdanowicz et al., 2012). These observations correspond with
positive summer-temperature anomalies of 1.5–2 ∘C between 2005 and
2009, 3 times higher than the mean of the 1960–2009 period (Fisher et
al., 2012). Air temperature changes at high latitudes impact the dynamics and
linkages between the different components of the cryosphere and thus also the
lake ice cover regimes. As a result of rising mean summer air temperatures in
the Canadian High Arctic during recent decades, the snow to total
precipitation ratio has been decreasing (Screen and Simmonds, 2011) and the
semi-permanent snow cover has been disappearing (Woo and Young, 2014).
Despite the enhanced runoff from the CAA glaciers since the 1990s (Gardner et
al., 2011; Lenaerts et al., 2013), without the semi-permanent snow, surface
and subsurface flows are not properly sustained. A declining semi-permanent
snow cover has implications for the local hydrology and could result in the
disappearance of wetlands and ponds in low-precipitation years when the
semi-permanent snow is the main source of runoff. Additionally, it also
affects the local ecology through changes in the habitat and food
availability and the vegetation cover. Moreover, following accelerated snow
ablation consequent to increased rainfall, early snowmelt occurs during warm
summers, exposing the underlying permafrost layer to more solar radiation and
thus deepening the active layer (Woo and Young, 2014).
Changes in near-surface air and surface temperature
The 1 km resolution combined MODIS Aqua/Terra LST maps represent a valuable
tool in obtaining consistent observations of “surface skin” temperature
over land at high latitudes. MODIS observations acquired during the break-up
season of 2 extreme years, 2004 (negative air temperature anomalies), and
2011 (positive air temperature anomalies) show up to 5 ∘C
differences in the mean summer air temperatures of the 2 years, over all
study sites (Fig. 11).
Differences in the surface “skin” temperature from MODIS discriminate the
warmer land areas from the colder ones. During years with negative annual air
temperature anomalies, these differences range from 5 to 15 ∘C for
the polar-oasis areas around Lake Hazen on Northern Ellesmere Island and
Buchanan Lake on Axel Heiberg Island and from 1 to 4 ∘C for the
polar-oases areas around lake L4 on Devon Island and Hunting Camp Lake on
Bathurst Island. The air temperature differences between the investigated
polar oases and the surrounding areas during years with positive annual air
temperatures range from 10 to 19 ∘C around lake Hazen and Buchanan
Lake and from 9 to 16 ∘C for areas around lakes L6 and L7.
In June 2004, the areas around Lake Hazen and Buchanan Lake experienced
higher temperatures than the surrounding areas that are controlled by a
typical polar-desert climate by 5–12 ∘C. No air temperature
difference was observed in the case of areas around lakes L4 and Hunting Camp
Lake during the same month. In 2011, the June air temperature differences
between the warmer polar oases and surrounding areas ranged from
3 to 4 ∘C (lake L4 and Hunting Camp Lake) and 10 to 16 ∘C (Lake
Hazen and Buchanan Lake).
During the month of July, all areas around the investigated polar oases
experienced higher air temperature than the surrounding areas. In 2004, these
differences ranged from 3–4 ∘C (lake L and Hunting Camp Lake) to
3–11 ∘C (Lake Hazen and Buchanan Lake). These differences were
considerably higher in 2011 and ranged from 12–15 ∘C (lakes L6
and L7) to 9–19 ∘C (Lake Hazen and Buchanan Lake).
During the month of August 2004 and 2011, no noticeable “skin” temperature
differences were observed for areas around lakes L4 and Hunting Camp Lake in
comparison to the usually colder surrounding areas. Air temperatures around
Lake Hazen and Buchanan Lake during the same month were higher than the
neighboring areas by 9–12 ∘C in 2004 and by 13–16 ∘C in
2011.
Analysis of MODIS data shows that the greatest differences in surface
temperature of polar oases and those polar-desert locations investigated in
the current study occur in June and July, with significant surface
temperature differences during years with higher air temperature anomalies of
the same month. Moreover, differences in surface temperature between polar
oases and the surrounding polar-desert environments is considerably higher
around Lake Hazen on Northern Ellesmere Island and Buchanan Lake on Axel
Heiberg Island and less around lake L4 on Devon Island and Hunting Camp Lake
on Bathurst Island. In years with high positive air temperature anomalies,
these polar “hot spots” extend over larger areas and could be impacting the
otherwise typical polar-desert climate of the neighboring areas.
Analysis of air temperature as recorded at the weather stations with
available data reveals that temperature anomalies during the same month of
the 2 years are greatest during the month of June at all stations and
smallest in August when solar radiation drops off rapidly as a result of a
declining solar angle (Woo and Young, 1996). The smallest positive air
temperature anomaly between 2004 and 2011 was recorded in July at Alert
(0.67 ∘C), and the greatest positive anomaly was observed in July at
Resolute (5.09 ∘C). In 2004, delayed WCI dates were observed for
most lakes, with Craig Lake, lake L11 on Baffin Island, Hunting Camp Lake on
Bathurst Island, and Eleanor Lake experiencing the latest WCI dates during the
15-year record. Lake Hazen and lake L4 on Devon Island also maintained a
multiyear ice cover throughout the 2004 summer. Conversely, in 2011 earlier
MO and WCI was observed for most lakes, with extreme earlier WCI dates for
Lake Buchanan, lake L4, Hunting Camp Lake, Eleanor Lake, lake L9, and lake L11.
Multiyear ice was not observed on any of the 11 lakes at the end of the
2010/11 ice season.
Changes in atmospheric/oceanic circulation patterns
Air temperatures at high latitudes are associated with changes in the coupled
atmosphere–ocean system (Trenberth and Hurrell, 1994). Warmer air
temperatures and a higher Arctic troposphere will continue to reduce the
pressure gradient between northern and southern latitudes and will lead to
substantial changes in atmospheric circulation patterns (Watanabe et
al., 2006). Extreme phases of atmospheric circulation patterns, also known as
teleconnections, have been shown to influence ice phenology of lakes in the
Northern Hemisphere (Bonsal et al., 2006), such as the major shift in the
Pacific Decadal Oscillation (PDO) in the mid-1970s toward a positive phase when
North American lakes experienced earlier break-up and shorter ice seasons
(Benson et al., 2000) or the strong positive phase of El Niño–Southern
Oscillation (ENSO) in 1998 that resulted in extreme ice events (i.e., later
ice freeze-up and earlier break-up, anomalously thin ice) for many of the
lakes in the High Arctic (Atkinson et al., 2006) and when none of the study
lakes maintained a multiyear ice cover. The North Atlantic Oscillation (NAO)
and the Arctic Oscillation (AO), highly related to each other, also play a
significant role in the winter/early spring (November–April) Arctic
atmosphere, NAO in particular influencing the air temperatures on the eastern
side of North America (Bonsal et al., 2006). The relation between Arctic lake
ice phenology and NAO/AO patterns during recent years has not yet been
thoroughly investigated. However, it has been hypothesized that the shorter
ice seasons in northeastern Canada and Baffin Bay are likely associated with
the persistent positive air temperature anomalies in the area from around
2000 (Prowse et al., 2011), coinciding with a trend toward a more negative
NAO/AO values (Overland and Wang, 2005).
The lack of a strong correlation between ice break-up of High Arctic lakes
and teleconnections from 1997 through 2011 as indicated by a preliminary
analysis (not shown) could be explained by the fact that neither PDO nor the
NAO/AO has been in a phase to contribute to the Arctic warming during the
past several years (AMAP, 2011). On a background of increased warming at
higher latitudes, the previous strong correlation between AO and ice regimes
of lakes north of 65∘ (Bonsal and Prowse, 2003) could weaken in
the forthcoming decades. However, in order to determine the impact that
atmospheric circulation patterns have had on ice phenology during recent
years, a more comprehensive analysis is needed.
Summary and conclusions
This study provides an assessment of
lake ice conditions in the central and eastern Canadian High Arctic and
reveals changes in the break-up dates and the summer ice cover that these
lakes experienced between 1997 and 2011. Analysis of the available SAR and
Landsat data from 1997 to 2011 indicates that the start of ice break-up (melt
onset) is occurring by 14–39 days earlier for polar-oasis lakes (out of five
investigated polar-oasis lakes, one showed a statistically significant trend
at the 0.01 level and two at the 0.05 level) and by 3–23 days earlier for
polar-desert lakes (out of six investigated polar-desert lakes, two showed a
statistically significant trend at the 0.05 level). Changes were also
observed in the summer ice minimum, with ice generally disappearing earlier
on all lakes, by 9–30 days earlier in polar-oasis environments (out of five
investigated polar-oasis lakes, one showed a statistically significant trend
at the 0.05 level and one at the 0.1 level) and by 2–19 earlier in polar-desert environments (out of six investigated polar-desert lakes, one showed a
statistically significant trend at the 0.05 level). Timing of the WCI dates
ranges from 9 to 24 days earlier in polar-oasis environments (out of five
investigated polar-oasis lakes, one showed a statistically significant trend
at the 0.01 level) and from 2 to 20 days earlier in polar-desert environments
(out of six investigated polar-desert lakes, one showed a statistically
significant trend at the 0.05 level). The only lake with later WCI dates is
Lower Murray Lake (13 days, statistically significant at the > 0.1 level).
During the 15-year period covered by this study, the MO and WCI dates
occurred earlier for all 11 lakes, with the exception of Lower Murray Lake on
Northern Ellesmere Island that, despite experiencing earlier MO, had an ice
cover lasting longer into the summer or even occasionally persisting from one
year to another. The lakes with the greatest changes in the timing of MO date
were Buchanan Lake on Axel Heiberg Island (24 days early), Eleanor Lake on
Cornwallis Island (20 days early), lake L7 on Bathurst Island (15 days
early), Lake Hazen (9 days early), and lake L4 on Devon Island (8 days
early). Earlier summer ice minimum was also observed on these lakes. Given
that with the exception of Eleanor Lake, the lakes with the shortest ice
seasons are located in polar-oasis areas, environments dominated by milder
temperatures, comes to reinforce the strong relation between air temperature
and lake ice break-up. The increasing positive air temperature anomalies are
likely the cause of multiyear ice loss for lakes. Lakes that preserved their
ice cover from one season to another on a consistent basis (e.g., Lake Hazen,
Stanwell Fletcher Lake) in previous decades are transitioning toward a
seasonal ice cover, with sparse or no multiyear ice seasons. Some of the
lakes on Northern Ellesmere Island (i.e., Lake Hazen and Murray Lakes), along
with lake L4 on Devon Island and Stanwell Fletcher Lake are the only lakes
with observed occasional multiyear ice. From 2007 to 2011, Lake Hazen and
Murray Lakes were the only ones with observed multiyear ice cover in 2009.
However, given the short record of this study, these results are indicative
of a possible change in lake ice regimes during the 15-year period and likely
are also reflective of yearly variability and variability over the study
area.
In an Arctic that has warmed during recent decades and that will likely
continue to be driven by above-normal air temperatures, shorter ice seasons,
with later freeze-up and earlier break-up dates, complete loss the perennial
ice cover (Brown and Duguay, 2011), and major biological changes within the
High Arctic lakes are predicted to persist in forthcoming decades.
Studies suggest that consequent to ice loss and longer open-water seasons,
Arctic lakes and ponds have the potential to experience a large leap in
productivity and more rapid nutrient cycling (Perren et al., 2003; Smol and
Douglas, 2007; Paul et al., 2010). A possible scenario consequent to a
reduced ice cover involves abundance of periphytic diatoms in shallow lakes
(Smol et al., 2005), diversification of the planktonic flora (Keatley et
al., 2008), and an overall increase of the primary production rate (Smol et
al., 2005). Lakes within the polar deserts may virtually start to respond and
act like those within polar oases consequent to the changing climate
conditions (Young and Abnizova, 2011). Studying the ice phenology of lakes
that are presently located in polar-oasis environments could provide an
insight as to how the ice conditions of polar-desert lakes may be in the near
future.
The Arctic cryosphere is a complex system driven by strong interactions among
the atmosphere, land, and ocean. Under projected amplified warming of polar
regions, ice break-up of inland lakes will be prone to a greater change as
ice decay is more responsive to changes in air temperature (Brown and Duguay,
2011). Considering the dynamic nature of the ocean–atmosphere–land
linkages, changes within the lake ice cover are likely to be more prominent,
result in more extreme ice conditions associated with warmer events (e.g.,
extremely late freeze-up, extremely early break-up; Benson et al., 2011), and
shift from a perennial/multiyear to a seasonal ice cover.
The results presented in this paper document changes in the ice cover of
lakes in the Canadian High Arctic in recent years, as observed by a
combination of SAR and optical sensors, and present a preview of changes that
Arctic lakes are likely to undergo in future decades. The combination of
radar satellite missions, the new Sentinel-1A/B and the forthcoming RADARSAT
constellation, and the recently launched optical Sentinel 2-A Multi-Spectral
instrument, with frequent revisit times, will be invaluable tools that will
enable consistent monitoring of High Arctic lakes in a dynamic and rapidly
changing climate. The 15-year ice records for the observed 11 lakes in the
CAA set the baseline for a long-term monitoring database for High Arctic
lakes that can be consolidated through observations from future satellite
missions.
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