Supplement of Arctic sea ice melt onset from passive microwave satellite data: 1979–2012

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Introduction
Changes in all aspects of the Arctic cryosphere observed by satellite since late 1978 have been dramatic over the last few decades.Record low annual sea ice extent minima were recorded numerous times in the last decade, most recently in September 2012 (Parkinson and Comiso, 2013).Sea ice is becoming increasingly young and thin (Maslanik et al., 2007(Maslanik et al., , 2011;;Kwok et al., 2009) and thus, is more susceptible to melting throughout the spring and summer months (Ngheim et al., 2007;Lindsay et al., 2009).The melt season is lengthening through changes in timing of the onset of melt in the spring and also by delaying the timing of freeze-up in the fall (Belchansky et al., 2004;Stroeve et al., 2006Stroeve et al., , 2014;;Markus et al., 2009).Lengthening melt seasons increase ice volume loss in the Arctic, in particular, through earlier melt onset which strengthens the sea ice albedo feedback loop (Stroeve et al., 2006(Stroeve et al., , 2014;;Markus et al., 2009).
The albedo changes on the sea ice surface that occur when melt begins allow for the absorption of solar radiation, which then increases the amount of melting that occurs Introduction

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Full within the ice-ocean system (Curry et al., 1995).An earlier date of melt onset on Arctic sea ice has a greater impact on the overall absorption of solar radiation in the ice-ocean system when compared to a lengthening of the melt season by a delay in the date of freeze-up in the fall (Perovich et al., 2007).Although no direct correlation between the melt onset date and September sea ice extent minima has been found (Wang et al., 2011), the date of melt onset in the Arctic signals the beginning of the melt season, and begins the ice-albedo feedbacks, which carry out through the remainder of the melt season (Stroeve et al., 2006;Markus et al., 2009).
Several algorithms exist to determine the date of melt onset on Arctic sea ice from passive microwave satellite observations (e.g.Smith, 1998;Drobot and Anderson, 2001;Belchansky et al., 2004;Markus et al., 2009) and also from active microwave satellite observations (e.g.Winebrenner et al., 1994;Forster et al., 2001;Kwok et al., 2003).However, melt onset dates from passive microwave observations are largely consistent for a longer time period (1979-present) than active microwave products.
We announce the release of the Snow Melt Onset Over Arctic Sea Ice from SMMR and SSM/I-SSMIS Brightness Temperatures, Version 3 (V3) data set that is now available for download from the National Snow and Ice Data Center (NSIDC) (Anderson et al., 2014), replacing the Version 2 (V2) data set.The melt onset (MO) dates in this updated data set are calculated using the Advanced Horizontal Range Algorithm (AHRA) developed by Drobot and Anderson (2001).The data set gives an annual view of the day of year (DOY) on which MO occurred at each pixel location.The data are available at a 25 km 2 resolution and are formatted using NSIDC's polar stereographic 304 × 448 pixel Northern Hemisphere grid.The data set has been reprocessed from passive microwave brightness temperatures (Tbs) to improve the consistency of data processing and extend the record of annual MO dates through the 2012 melt season.
In this work, we use this new data set to provide an updated statistical summary of MO dates for the 1979-2012 record and determine regional trends in the timing of MO for sea ice in the Arctic.Introduction

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AHRA melt onset date calculation
The AHRA described by Drobot and Anderson (2001) utilizes horizontally polarized, daily-averaged, Tbs from the 18/19 and 37 GHz channels.Tbs were obtained from the Scanning Multichannel Microwave Radiometer (SMMR) on board the NASA Nimbus-7 satellite platform and the series of Special Sensor Microwave Imagers (SSM/I) and the Special Sensor Microwave Imager and Sounder (SMMIS) from the Defense Meteorological Satellite Program's F8, F11, F13, and F17 platforms.SMMR Tbs were collected every second day, while SSM/I and SSMIS Tbs are available daily.Prior to the calculation of melt dates, the Tbs are corrected using linear regression coefficients determined from sensor overlap areas using DMSP F8 as the baseline sensor (Jezek et al., 1991;Abdalati et al., 1995;Stroeve et al., 1998;W. Meier, personal communication, October 2011).Tbs increase when liquid water is introduced to the snowpack atop the sea ice.The AHRA method tracks the difference between the 19 GHz (18 GHz for SMMR Tbs) and 37 GHz horizontally polarized Tbs at a given point.If the difference is > 4.0 K it is assumed that wintertime conditions exist at the point.If the difference is < −10.0 K then liquid water is likely present in the snow pack, causing a greater increase in the 37 GHz channel relative to the 18/19 GHz channel, and the date is recorded as the day of melt onset.If the difference falls between −10.0 and 4.0 K the 10 days prior and 9 days following the date in question are tested.If the difference between Tbs during the periods prior to and following the day in question is > 7.5 K a melt onset date is assigned.If this value is < 7.5 K no melt date is determined and the algorithm continues to the next day.During this stage of the algorithm, a large difference between the values prior to and following the date indicates a pattern shift in the time series of Tbs, thus melt onset has occurred.The use of the time series window surrounding the day makes the AHRA insensitive to spurious Tbs and weather interference.See Drobot and Anderson (2001) for full details on the algorithm.Introduction

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Updates to the data set
For Version 3 of the data set, some changes to the processing were made in addition to updating the record of annual MO dates through the 2012 melt season.The previous version of the data set (V2) was masked to the climatology of locations where a MO date had been calculated for every year in the 20 year period 1979-1998.This climatology mask was static and determined the pixels for which a melt date was calculated every year.The new data set (V3) no longer uses a static mask; instead, the MO dates are calculated for locations determined to be sea ice covered at the beginning of each melt season.The melt dates in a given year are calculated for pixel locations where sea ice concentration is ≥ 50 % on one or both of the first two days with data in March.The concentration data used here are Goddard merged sea ice concentrations available as part of the NOAA/NSIDC Arctic Sea Ice Climate Data Record (Meier et al., 2013).The beginning of March is used to represent full sea ice extent, since early March roughly corresponds to the annual maximum Arctic sea ice extent (e.g.Parkinson and Comiso, 2013).The first two days of data in March are used to account for days on which sea ice concentrations may be missing.Tbs were collected every second day during SMMR years (1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987); therefore, the sea ice concentrations used to create the ice mask for the MO dates data set may include two days during 1-5 March.
Since the sea ice mask is no longer static, the sea ice locations (especially along the ice edge) that experience MO throughout the melt season change from year to year.The annual MO date maps for 1979 and 2012 in Fig. 1 illustrate the changing sea ice mask and serve as sample data from the V3 data set.Due to the differences in swath width between the SMMR and SSM/I-SSMIS sensors, the data gap surrounding the North Pole (the pole hole) changes in diameter; examples of this can be seen in Fig. 1.The V2 climatology mask eliminated the difference between pole hole diameter that occurs; however, the reduction in diameter increases the amount of sea ice area for which MO is calculated, thus, increasing usefulness of the data for users who may subset the time series.Additionally, V2 of the data set included a 2 pixel buffer that Introduction

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Full eliminated coastal sea ice locations where possible uncertainties in the Tbs from landocean spillover can occur.Newer versions of the Tb data have now corrected for this spillover uncertainty (Cavalieri et al., 1999); therefore, the buffer is no longer used for V3.
As noted above, before MO is calculated, the Tbs are corrected to improve intersensor calibration using linear regression coefficients.Version 3 of the data set extends the record using Tbs from the DMSP F17 satellite for the years 2008-2012.To be consistent with the rest of the record, the F17 Tbs are also corrected back to F8 Tbs using regression coefficients provided by W. Meier (personal communication, October 2011).
Additionally, an erroneous application of the regression correction between SSM/I sensors on the DMSP F11 and F13 platforms was found and corrected for V3.

Calculation of statistics
All statistics reported here are calculated from pixel locations where a MO date exists in all 34 years of the data record.The sea ice locations (indicated by color) in Fig. 2 show the MO date climatology mask used in the calculation of statistics.Pixel locations in white do not have a melt date for one or more years and are excluded.Statistics are calculated for all of the Arctic sea ice cover (hereafter called the Arctic Region) and for smaller sub-regions of the Arctic that are identified by differing colors in Fig. 2. The area (in km 2 ) for each sub-region of the Arctic is not equal in this work because we restrict calculations of statistics to the MO date climatology mask and implicitly the sea ice extent.We then divide the Arctic into common geographic regions.The regional boundaries used here are the same as used by Meier et al. (2007) except we include sea ice locations within the Baltic Sea.These regional boundaries are also similar to those of other works including Markus et al. (2009) and Parkinson et al. (1999) except that the region mask used here divides regions within the Arctic Ocean into smaller seas.The sea ice area for each region (in km 2 ) is presented in Table 1.The area for the Arctic Region is the area sum of all 15 sub-regions.

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Full All maps of summary statistics including the earliest MO date, latest MO date, range of MO dates, mean and standard deviation are calculated from the time series of MO dates at each individual pixel for 1979-2012.Regional statistics presented in Table 1 are calculated from the annual mean MO dates in each region (provided in Supplement Table S1).The mean earliest MO and mean latest MO values presented in Table 1 represent the earliest and latest of the annual mean MO dates, rather than the absolute earliest and latest MO dates from the 34 year record that appear in Fig. 3a and b.Regional trends are calculated from the slope of the least squares linear regression best-fit line on the time series of annual mean MO dates.

Melt onset statistics 1979-2012
Mean MO dates for the Arctic Region during the 34 year data record vary highly across the extent of sea ice cover (Fig. 3); however, the mean date of MO for the Arctic Region is 13 May (132.5 DOY) with a standard deviation of ±7.3 days (Table 1).In general, the mean MO dates occur earliest at sea ice locations along the periphery of the sea ice edge and in the southernmost locations such as the Sea of Okhotsk, Bering Sea, Hudson Bay, Gulf of St. Lawrence, Greenland Sea, Baltic Sea, and Barents Sea (Table 1, Fig. 3d).This indicates a general latitudinal dependence on the timing of MO; however, the standard deviation of MO dates can be large in portions of these early-melting regions.Regions with higher standard deviations in mean MO date have higher variability in MO timing from year to year.The regions with the highest standard deviations occur in parts of the Arctic Ocean, including: the Barents, Kara, Laptev, East Siberian, Chukchi, and Beaufort Seas with the greatest standard deviation (±14.5 days) occurring in the East Siberian Sea (Table 1).
The earliest MO dates during 1979-2012 occur at the beginning of the melt season, in early March, for most of the peripheral regions of the sea ice area (Fig. 3a).For portions of the Central Arctic, Canadian Archipelago, and the northern portion of the Beaufort Sea, the earliest MO dates do not occur until mid-late May.The earliest MO Introduction

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Full dates in other portions of the sea ice within the Arctic Ocean occur in late March and early April (warm colors in Fig. 3a).The latest MO dates in the record for much of the sea ice regions within the Arctic Ocean occur during August, while the coastal regions of the Arctic Ocean typically have the latest MO dates near the end of May through June (Fig. 3b).Two distinct areas of the sea ice cover appear to have a small range (warm colors in Fig. 3c), (1) in the peripheral sea ice regions (including the Sea of Okhotsk, the Bering Sea, the Labrador Sea (in the Baffin Bay region), and the southern Barents Sea) and ( 2) the North American side of the Arctic including parts of the Central Arctic, the northern Beaufort Sea, and the Canadian Archipelago regions.
The variability in MO dates described by both ranges and standard deviations for these locations is small; however, the timing of MO is distinctly different.In the southern, peripheral regions, where the sea ice is primarily composed of seasonal, first year ice, air temperatures warm to the melting point earlier in the year and early MO dates are observed.Conversely, sea ice in the Central Arctic is typically thicker, multiyear ice.Sea ice concentrations would tend to be higher in this area and air temperatures would warm later in the year than at southern locations, leading to the later mean MO dates observed.
The St. Lawrence Gulf and Baltic Sea regions have the earliest mean MO dates, occurring 15 March (73.2DOY) and 20 March (78.8DOY), respectively, although both areas are small (0.1×10 5 km 2 and 0.2×10 5 km 2 ) (Table 1).Other regions with relatively early mean MO dates (Table 1) are the Bering Sea, 21 March (79.9DOY); the Sea of Okhotsk, 22 March (80.8DOY); and the Barents Sea, 4 April (93.9 DOY).However, it is important to note that the early-melting sea ice in the Barents Sea is located in the southern, coastal portion of the region, while the sea ice in the northern half of the Barents, adjacent to the Central Arctic region, melts at a later date (Fig. 3d).The other (130.5 DOY) in the Kara Sea to 10 June (160.9DOY) in the Central Arctic region (Table 1).MO dates can vary widely from year to year in Arctic sub-regions depending on when the air temperatures in different regions reach the melting point.There is some latitudinal dependence on the timing as indicated by the general pattern of mean MO dates where earliest MO occurs along the ice edge and at southernmost regions (Fig. 3); however, higher standard deviations and larger mean ranges in some sub-regions such as the Barents, Kara, Laptev, East Siberian, and Chukchi Seas indicate that there is a good amount of year to year variability in the timing of MO at the regional scale.Although, on average, there is latitudinal dependence on timing of MO, springtime weather conditions and temperature anomalies are important for explaining the year to year variability in MO timing for much of the sea ice within the Arctic Ocean (Anderson and Drobot, 2001;Belchansky et al., 2004;Wang et al., 2011;Markus et al., 2009).Springtime weather conditions, including cyclonic activity, can have an influence on the air temperatures and the surface energy budget of the sea ice through the trapping of longwave heat when conditions are cloudy or through increased incoming shortwave radiation when conditions are cloud free and the sun rises in spring.

Trends in melt onset dates
Trends in the time series of annual mean MO dates indicate that MO is occurring earlier in the year for the majority of Arctic sea ice over the 1979-2012 data record (Fig. 4).For the Arctic Region, a statistically significant trend (99 % confidence level) of −6.6 days decade −1 exists, indicating that MO is occurring earlier in the year in re- East Siberian Sea.R 2 values vary, but are strongest for the Arctic Region and the Central Arctic where the R 2 value is at least 0.76 (Fig. 4).Statistically significant trends also exist in the Bering Sea and Baffin Bay although at a 95 % confidence level with weak R 2 values (Fig. 4).Southerly, peripheral regions of the sea ice where the mean MO dates occur earliest, as described in Sect.3, tend to have very weak R 2 relationships and insignificant trends, although the trend is negative for nearly all regions.An interesting finding to note is the statistically significant (95 % confidence level) positive trend occurring in the Bering Sea.The Bering Sea is the only region of sea ice that shows a trend towards later MO dates through the data record.The relationship is weak (R 2 of 0.18) and the area of sea ice in the region is small (2.7 × 10 5 km 2 ), however, this region is showing an anomalous change in MO that is different from all other regions.Calculations for these trends and statistics are normalized to locations where MO dates exist in all years of the data record; however, the ice edge in this data set changes from year to year with the extent of sea ice at the beginning of March.Therefore it is interesting to note that the sea ice cover is actually more extensive in the Bering Sea in recent years than in the earliest years of the data record.For an example of this, see Fig. 1, where Bering Sea ice extent (using the 50 % concentration threshold) is greater in 2012 than in 1979.
The mean MO dates show a significant trend towards increasingly early MO for the majority of Arctic sea ice, in agreement with the works of others (e.g.Stroeve et al., 2006Stroeve et al., , 2014;;Markus et al., 2009).Earlier MO on sea ice increases the amount of solar radiation that can be absorbed by the ice-ocean system by reducing surface albedo during the time of the year when solar radiation is greatest (Perovich et al., 2007).Increased absorption of solar radiation during the spring and can lead to increased heating in the Arctic, extensive loss of sea ice volume, and a delay in freeze-up following the melt season (Stroeve et al., 2014).

Summary
We have described an updated record of MO dates over Arctic sea ice that is now available for download from NSIDC (Anderson et al., 2014).This new data set utilizes the AHRA method for calculating the date of MO from passive microwave satellite data, which has improved consistency and been updated to include recent data from the SSMIS satellite sensor through 2012.
Based on this 34 year record of MO dates on Arctic sea ice we have shown that the timing of MO, on average, has some dependence on latitude.Typically, the sea ice periphery and southerly-located seas experience MO early in the year during the months of March and April, while northerly locations, in the central and western Arctic Ocean, experience MO in mid-late May.However, increased variability in regions within the Arctic Ocean shows that there is considerable year to year variability in MO timing which is attributed to variability in springtime weather conditions.
The 34 year record of MO dates shows significant, negative trends for the majority of the Arctic that indicate earlier MO.These trends in MO are on par with the warming trends observed in the Arctic over recent decades and the overall reduction of sea ice volume.However, the positive trend in the Bering Sea indicate the regional nature of MO timing and the need for more investigation into the variability of regional-scale atmospheric conditions surrounding the timing of MO.Full

The
Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | peripheral and southern regions including: Hudson Bay, Baffin Bay, and the Greenland Sea have a mean MO date which occurs in the latter half of April.The remaining regions are located within the Arctic Ocean and have mean MO dates that range from 11 May Discussion Paper | Discussion Paper | Discussion Paper | cent years when compared to the earliest years of the data record.Statistically significant negative trends also exist for sub-regions of the Arctic Ocean including: the Barents, Kara, Laptev, East Siberian, Chukchi, and Beaufort Seas, and the Canadian Archipelago and the Central Arctic region (99 % confidence level).These trends range from −4.6 days decade −1 in the Canadian Archipelago to −11.8 days decade −1 in the Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Figure 4 .
Figure 4. Time series of annual mean MO date and least squares linear regression trend for the Arctic Region and sub-regions.The R 2 value and decadal trend (days decade −1 ) are shown for each region.Bold trends are statistically significant at a 99 % confidence level.An * indicates statistically significant trends at a 95 % confidence level.