Icebergs have a potential impact on climate since they release freshwater
over a widespread area and cool the ocean due to the take-up of latent
heat. Yet, so far, icebergs have never been modelled using an ice-sheet
model coupled to a global climate model. Thus, in climate models their
impact on climate has been restricted to the ocean. In this study, we
investigate the effect of icebergs on the climate of the mid- to high
latitudes and the Greenland ice sheet itself within a fully coupled ice-sheet
(GRenoble model for Ice Shelves and Land Ice, or GRISLI)–earth-system (
During the last decade satellite observations have shown a reduction of the
Greenland ice sheet's height, by up to 1.5 m yr
There are numerous feedback mechanisms related to the growing and shrinking of ice sheets (Clark, 1999). First, changes in topography can lead to altered atmospheric circulation patterns (Ridley, 2005). Second, when an ice sheet is shrinking, there are fewer ice-covered areas and the resulting decrease in surface albedo enhances the uptake of heat by newly exposed land surfaces. Vizcaíno et al. (2008) showed that under future warming the decrease in both topography and albedo of the GrIS strongly enhances its decay. A further effect of the ice sheet's shrinking is enhanced runoff into the ocean and, as a consequence, a reduced sea surface salinity that increases the stability of the water column. This process, depending on the position and strength of the freshwater flux, might lead to a reduction or even collapse of the Atlantic Meridional Overturning Circulation (AMOC; e.g. Roche et al., 2010; Swingedouw et al., 2009). Besides runoff, iceberg calving is one of the main mechanisms of mass loss of ice sheets, and in a warming climate it is expected to increase. Recently, an increase in ice speed of the Greenland glaciers of up to 200 % and Arctic ice shelf break-ups led to enhanced ice discharge (e.g. Mueller et al., 2003; Rignot and Kanagaratnam, 2006; Nick et al., 2009). Since icebergs act as a mobile freshwater source and a sink of latent heat, they freshen and cool the ocean, thereby facilitating the stratification of the ocean as well as the formation of sea ice (Jongma et al., 2009).
Numerical ice-sheet models are valuable tools with which to study the evolution of the ice sheet during different climate states and its impact on climate. Therefore, they are used to better understand and investigate the aforementioned interactions between the GrIS and the other climate components. Most ice-sheet models currently used for performing longer time simulations are three-dimensional thermomechanical models, based on the shallow-ice approximation (Hutter, 1983; Morland, 1984). The ice sheet's thickness and extension are calculated at every time step. Some models also differentiate between fast- and slow-flowing ice, such as ice shelves and grounded ice, respectively, to allow for a dynamic computation of the grounding line (e.g. Huybrechts, 1990; Huybrechts and de Wolde, 1999; Greve, 1995, 1997; Ritz et al., 2001; Pollard and DeConto, 2007). These models are used, on the one hand, to predict the future development of the ice sheets and, on the other hand, to model their evolution during the past millennia and even millions of years.
The simplest approach to investigate the ice sheet's development over the past is by evaluating the impact of the forcing fields on it. This can be done either by using reconstructed air temperature and precipitation fields as input data (e.g. Ritz et al., 2001) or by using climate model output of specific time periods to drive the evolution of the ice sheet (e.g. Huybrechts et al., 2004; Charbit et al., 2007), or a combination of both (e.g. Gates, 1976; Pollard and Thompson, 1997; Broccoli, 2000). Using this set-up, the interactions are only one-sided as the climate is applied to the ice sheet but not altered by it.
A further and more complex approach is to couple ice-sheet models to earth system models of intermediate complexity (EMICs; Claussen et al., 2002) or to general circulation models (GCMs). In this case, the climate model (EMIC or GCM) and the ice-sheet model exchange input (temperature and precipitation) and output (albedo, topography, melting and calving of the ice sheet) fields (e.g. Wang and Mysak, 2002; Kageyama et al., 2004; Gregory et al., 2012). Therefore, the interactions are two-sided as the ice sheet's geometry and its freshwater fluxes are used as input for the climate model, where the runoff (surface and basal melt) as well as the ice discharge are considered as freshwater fluxes that are released into the ocean directly at the coastline (e.g. Bonelli et al., 2009; Vizcaíno et al., 2008; Goelzer et al., 2010) or over a pre-defined area (Ridley, 2005). Therefore, the meltwater released due to iceberg calving and the related take-up of latent heat by them is considered in the same way as the runoff and consequently spatially restricted to the coastline or homogenously distributed over a fixed region. A more complete description of coupled ice-sheet–climate modelling can be found in Pollard (2010).
The importance of icebergs has been shown in different studies where an iceberg module was coupled to climate models and forced with climatological data (e.g. Bigg et al., 1996, 1997; Gladstone et al., 2001; Death et al., 2006; Levine and Bigg, 2008; Green et al., 2011; Jongma et al., 2009, 2013). Jongma et al. (2009) highlighted the effect of icebergs under pre-industrial conditions using an EMIC that included an interactively coupled iceberg module based on Bigg et al. (1996). Focusing on the Southern Ocean, Jongma et al. (2009) revealed that icebergs significantly facilitate the formation of sea ice. Moreover, Levine and Bigg (2008), Green et al. (2011) and Jongma et al. (2013) highlighted the importance of including icebergs in model simulations of past ice shelf break-ups since the ocean, and consequently the AMOC, respond differently to them than to directly applied freshwater fluxes. A shortcoming of the studies done so far is that the locations and the amount of water used to generate icebergs have been prescribed according to observations and reconstructions. Recently, Martin and Adcroft (2010) coupled an iceberg module to a GCM. The climate model was used to generate icebergs at the coastal sites defined by the river routing system. This approach allows the background climate to define the number of icebergs generated under the assumption of an equilibrated ice sheet. Yet, none of these studies focusing on icebergs incorporated an ice-sheet model. Consequently, the interactions between the ice sheet and the icebergs were not taken into account.
Our aim is to include all the previously mentioned feedbacks (albedo,
topography, runoff and icebergs) in a fully coupled climate system.
Therefore, we use the
To achieve a fully coupled climate system, we further developed the model compared to previous studies (Jongma et al., 2009, 2013; Roche et al., 2014) by including the following two extensions. First, instead of prescribing the locations and the amount of icebergs being calved, they are now generated according to the ice lost by the dynamical ice-sheet model at the corresponding positions. Second, the water cycle is now closed between all the climate components. Therefore, the precipitation coming from the atmospheric model is used to build the ice sheet, its runoff is given to the river routing system and finally put into the ocean, and the calved mass is used to create icebergs that then release meltwater to the ocean. This fully coupled model set-up allows us to analyse the following questions. (1) How well are we able to reproduce the dynamics and main features of Greenland iceberg calving and ice-sheet development in a coupled climate model under pre-industrial conditions? (2) What is the influence of icebergs on the mid- to high-latitudinal climate and the modelled Greenland ice sheet itself? (3) How well can the effect of icebergs on climate be reproduced by freshwater fluxes that are applied at the same calving sites and with the same seasonal cycle, but lack the dynamic characteristics of icebergs? The difference between direct freshwater fluxes and icebergs has already been investigated by Jongma et al. (2009, 2013), but in their work the freshwater fluxes used to parameterise icebergs were distributed homogeneously around the Antarctic ice sheet (Jongma et al., 2009) or at a certain latitude belt in the North Atlantic (Jongma et al., 2013). In the present study, however, we introduce the freshwater fluxes into the ocean at the actual calving sites, a set-up that is closer to what has been done in other coupled climate models (e.g. Vizcaíno et al., 2008; Bonelli et al., 2009; Goelzer et al., 2010).
The questions stated here are addressed by performing and comparing four different model experiments that were all done under pre-industrial conditions and were performed until the ice sheet was equilibrated. The experiments differ in the way in which the freshwater fluxes (runoff and calving) of the ice sheet and the related uptake of latent heat are included in the climate model.
The paper is structured as follows: first the global climate model
The earth system model of intermediate complexity used in this study is the
so-called
The atmospheric model ECBilt (Opsteegh et al., 1998) is a quasi-geostrophic,
spectral model calculated on a horizontal T21 truncation (5.6
The GRenoble model for Ice Shelves and Land Ice (GRISLI) is a
three-dimensional thermomechanical model which was first developed for the
Antarctic (Ritz et al., 1997, 2001) and then further expanded for the
Northern Hemisphere (Peyaud et al., 2007). In the present study only the
Northern Hemisphere grid is used with a horizontal resolution of 40
As explained in detail in Sect. 2.4 and in Roche et al. (2014), the yearly runoff is added to the atmospheric model ECBilt and recomputed to fit its time step of 4 h.
We use the optional dynamic-thermodynamic iceberg module (Jongma et al., 2009, 2013; Wiersma and Jongma, 2010) with the same parameter set as in Jongma et al. (2009). It is based on the iceberg-drift model published by Smith and coworkers (Smith and Banke, 1983; Smith, 1993; Loset, 1993) and was further developed by Bigg et al. (1996, 1997) and Gladstone et al. (2001). It was implemented in CLIO by Jongma et al. (2009) and Wiersma and Jongma (2010). The icebergs are calculated on the CLIO grid and moved according to the Coriolis force; the air, water and sea-ice drag; the horizontal pressure gradient force; and the wave radiation force. These forces depend on the wind and the ocean currents calculated in ECBilt and CLIO, which are then interpolated linearly from the surrounding grid corners to fit the icebergs location. Melting of the bergs occurs due to basal melt, lateral melt and wave erosion. As the icebergs melt, their length-to-height ratio changes and they are allowed to roll over. Yet, break-up of icebergs is not considered. The meltwater fluxes are added to the ocean's surface layer of the current grid cell, and the latent heat fluxes needed to melt the icebergs are taken from the ocean layer according to the depth of the iceberg.
In contrast to Jongma et al. (2009, 2013), who prescribed the release
position and amount of icebergs, we have coupled the iceberg module to
GRISLI. Thus, we generate icebergs according to the mass loss that is
calculated by GRISLI over 1 year and then given to the iceberg module.
Therefore, we divide the yearly amount of mass at the calving sites into
monthly values considering the seasonality of calving. We follow the results
of Martin and Adcroft (2010), with the maximum occurring in spring and the
minimum in late summer (Fig. 2a). The monthly mass is then transformed into
a daily available mass as follows in Eqs. (1) and (2):
Furthermore, 10 size classes of bergs have been computed as defined by Bigg
et al. (1997) and used and stated by e.g. Gladstone et al. (2001),
Death et al. (2006) and Jongma et al. (2013). These size classes are based on
present-day observations in the Arctic done by Dowdeswell et al. (1992).
Each class corresponds to a defined percentage of the daily available
amount. Thus, every day we produce icebergs of the 10 different size classes
as
Summary of treatment of freshwater fluxes coming from the ice sheet and of latent heat fluxes related to iceberg melting. Runoff: basal and surface melting of the ice sheet; iceberg FWF: melt flux related to iceberg calving; direct FWF: input of calving mass as freshwater flux into the first ocean cell next to the ice-sheet margin instead of forming icebergs; local LHF: take-up of latent heat at the position where the freshwater related to iceberg melting is put into the ocean; homogeneous LHF: parameterisation of freshwater fluxes related to iceberg calving as take-up of latent heat homogenously around Greenland.
We have performed four different experiments (Table 1) that vary in the
implementation of the freshwater fluxes (runoff and calving) calculated in
GRISLI and the uptake of latent heat needed to melt the calving flux. All
experiments have in common that GRISLI is coupled to
In the control (CTRL) experiment, the freshwater cycle is closed between the atmospheric model ECBilt and the oceanic model component CLIO. In ECBilt, the precipitation (solid and liquid) is computed every 4 h and the solid precipitation is added to the snow layer. To prevent the model from piling up too much snow in areas with a positive snow mass balance, the height of the snow layer is not allowed to exceed a pre-defined threshold (10 m). If the snow layer exceeds this threshold, the amount of snow above it (the so-called excess snow) is melted, and it is added to the soil moisture (in a bucket model) and routed into the ocean when the maximum, pre-set soil water holding capacity is exceeded. In the CTRL experiment where icebergs are not explicitly modelled, their cooling effect is parameterised using the excess snow. Therefore, the heat needed to melt the excess snow is taken up homogenously around Greenland from the ocean surface layer in the ocean model CLIO (Fig. 1a). The solid precipitation that is falling on the ice sheet is given to GRISLI, where it is used to calculate the surface mass balance. However, it is not removed from ECBilt because in CTRL the water cycle between ECBilt-CLIO and GRISLI is uncoupled, implying that GRISLI is not incorporated into the freshwater cycle of ECBilt–CLIO.
In the calving (CALV), the “fresh” freshwater (FWFf) and the “cold” freshwater (FWFc) experiments, the freshwater cycle is closed between ECBilt, CLIO and GRISLI. Therefore, the precipitation given from ECBilt to GRISLI is removed from ECBilt. GRISLI uses the precipitation to calculate the surface mass balance. At the end of one model year it provides ECBilt with the amount of the computed runoff (surface and basal melt) and CLIO with the ice discharge. In ECBilt the runoff is incorporated into the land routing system and distributed to the ocean. The ice discharge in CLIO is either used to generate icebergs (CALV experiment) or melted instantaneously at the ice-sheet border (FWFf and FWFc experiments). The ice discharge has to be melted before being supplied to the ocean as a freshwater flux, and the treatment of the heat needed to do this differs between the CALV, FWFc and FWFf experiments. In CALV and FWFc, this heat is taken up from the ocean cell corresponding to the position where the ice discharge is added to the ocean either in the form of an iceberg melt flux (CALV) or in the form of a freshwater flux at the ice-sheet margin (FWFc). In FWFf the ice discharge is melted at the ice-sheet border without taking up heat; instead the latent heat related to the excess snow is taken up homogenously around Greenland, identical to the CTRL experiment. This allows us to separate the freshening and the cooling effect of icebergs.
Schematic representation of the water cycle between the atmospheric component ECBilt, the ice-sheet module GRISLI, the iceberg module and the oceanic component CLIO; numbers correspond to experiments (1: CTRL; 2: CALV; 3: FWFf; 4: FWFc).
A schematic representation of the water cycle between the atmosphere (ECBilt), ocean (CLIO), ice sheet (GRISLI) and iceberg model is displayed in Fig. 3. Volume changes of the ice sheet are reflected in the resulting calving flux and runoff. The latter is given to the land routing system of ECBilt and transported into the ocean. Runoff is included in all the experiments except the CTRL run.
Summary of anomalies analysed.
When we compare these four experiments (Table 2), we can analyse the impact of the icebergs on the climate of the mid- to high latitudes caused by the distribution of their meltwater and the related cooling and freshening of the ocean (CALV–CTRL). Moreover, we can separately analyse the impact of freshening (FWFf–CTRL) and of cooling the ocean (FWFc–FWFf) as the freshwater experiments only differ in the treatment of latent heat. Further, the differences between simulated icebergs and directly applied freshwater fluxes (CALV–FWF), which ignore the spatial distribution of the meltwater, are investigated.
All runs were done under pre-industrial conditions (orbital parameters and greenhouse gas concentrations corresponding to the year 1850), and the ice sheet was initialised from present-day observations (Bamber et al., 2001). The experiments were continued until the ice sheet was equilibrated, which took about 11 000 model years. In total the experiments were performed for 12 000 model years, and the results of the last 1000 model years are presented in the following section. The climate is equilibrated, with no detectable drift in the deep-ocean temperature.
First row: ice-sheet thickness (m):
1000-year averages. First row: mass balance (m):
Before analysing our results, we briefly summarise the main properties of
the CTRL ice sheet. In CTRL, the resulting modelled ice-sheet volume
(3.8
Summary of computed ice discharge (Calvflux) as calculated in GRISLI, surface mass balance (SMB) of the Greenland ice sheet, runoff as calculated in GRISLI, and sea-ice volume and area as computed in CLIO.
The results of the CALV experiment reveal that the modelled calving sites
and iceberg tracks fit the observations reasonably well. As is shown in the
Arctic Monitoring and Assessment Programme (AMAP) plot (Fig. 2b), calving
occurs along almost the entire coast of Greenland, with major calving sites
in Baffin Bay and along the southeast coast of Greenland. Despite the
coarse resolution of GRISLI and the simplified calving scheme used, these
calving sites are generally well captured (Fig. 6a). The sites in the northeast of
Greenland are overestimated, which is probably caused by overestimated
ice-sheet thickness there compared to observations (Fig. 4b
compared to a). The modelled calving flux (2.0
The mean yearly distribution of icebergs (Fig. 6b) illustrates that the
majority of bergs travels along the east and west coast of Greenland
reaching as far south as about 50
CALV–CTRL differences of 1000-year averages:
We find that including icebergs in the model set-up (CALV experiment) causes
a cooler ocean state than the CTRL run around Greenland (Fig. 7a). This is
due to the transportation of the icebergs by winds and ocean currents,
leading to a more extensive distribution of the meltwater, reaching up to
Svalbard and Iceland (Fig. 1d). In accordance with the icebergs' melt flux,
the sea surface temperatures (SSTs, Fig. 7a) decrease around the GrIS as a
result of the take-up of latent heat needed to melt the icebergs as well as
the incoming cold meltwater (Fig. 1b, d). Strong differences are found in
Baffin Bay due to the large number of icebergs in this region. The increased
melt flux enhances the sea-ice thickness (SIT) up to 0.7 m (Fig. 7b). In
Baffin Bay the response in sea surface salinity (SSS, Fig. 7d) is two-sided.
North of 65
A thicker sea-ice cover in combination with a higher albedo and generally
lower SSTs cause a cooler atmospheric state (Fig. 7e, f) in CALV compared to
CTRL, since less heat is exchanged between the ocean and the atmosphere.
Thus, the air temperatures over the whole region decrease by up to
From the comparison of CALV with CTRL we conclude that icebergs cause an overall colder climate in the mid- to high latitudes. This pattern is not captured by the homogeneous uptake of latent heat in the CTRL run because the parameterisation of icebergs used underestimates the freshening and cooling (Fig. 1a, b). Overall, the CALV ice sheet is too extensive and thick compared to observations, as is the CTRL one. But explicitly modelling icebergs increases (decreases) the ice-sheet thickness along the western (eastern) Greenland margin (Fig. 4c), which fits better to observations (not shown). This is caused by the local effect of the icebergs on the sea-ice thickness and the atmospheric temperatures.
FWFf–CTRL differences of 1000-year averages:
In the FWFf experiment the calving flux (1.9
The altered ocean conditions in FWFf compared to CTRL are evident in the
increased surface albedo due to the thickened sea ice. Its shielding effect
causes lower air temperatures of up to
Applying the calving fluxes in the form of instantaneous freshwater fluxes
(2.0
CALV–FWFf differences of 1000-year averages:
Using the calving mass calculated by GRISLI to generate icebergs (as in
CALV) that freshen and cool the ocean unevenly instead of applying this mass
in the form of local freshwater fluxes and homogenous take-up of latent heat
(as in FWFf) results in different ice-sheet topographies at the end of the
experiments. Explicitly modelling icebergs has a stronger cooling effect
close to the Greenland ice sheet than FWFf, since most of the iceberg melt
flux (IMF) is released close to the ice sheet. The melting of icebergs
extracts heat from the upper layers of the ocean, depending on the size and
the number of bergs (Fig. 1b, d). In FWFf, however, this spatial pattern of
take-up of latent heat is ignored. Therefore, CALV results in lower SST
values (
In the Labrador and GIN seas the explicit modelling of icebergs causes a weakened convection compared to FWFf because the icebergs withdraw the latent heat they need to melt from the respective ocean layer, thereby stabilising the water column. Due to the distribution of icebergs, less melt flux is released in the Greenland Sea than in FWFf because the calved bergs are moved southward (Fig. 1f). This is displayed in the higher SSS (0.2 psu) compared to FWFf (Fig. 9d).
The small differences in the resulting ocean state are also reflected in the
atmosphere, which displays lower temperatures (
From our studies we conclude that the experiments with the freshwater fluxes from the ice sheet (runoff and calving) implemented (CALV, FWFf, FWFc) result in a colder climate than CTRL and an up to 300 m altered ice-sheet thickness. Even though the resulting ocean and atmospheric state is comparable in all the freshwater experiments, the ice-sheet topographies vary up to 150 m. The differences in ice-sheet thickness arise due to the different spatial pattern of the added calving flux, either directly at the calving site (FWFf, FWFc) or distributed by the icebergs (CALV), and especially due to the spatial pattern of the take-up of latent heat needed to melt it. Using local freshwater and latent heat fluxes to parameterise icebergs (FWFc) results in a similar ice sheet and climate as the explicit modelling of icebergs (CALV), whereas the differences are stronger if the take-up of latent heat is computed homogenously (FWFf).
In the presented study the coupling between the ice-sheet model GRISLI and
the earth system model of intermediate complexity
So far, icebergs have mostly been parameterised using freshwater fluxes to
save computation time. To study the impact of such parameterisations, we
compared dynamically included icebergs to freshwater fluxes released at the
same locations and according to the same seasonal cycle as the icebergs and
found noticeable differences. Icebergs cause thicker sea ice all around
Greenland and especially in Baffin Bay and the Arctic Ocean compared to the
freshwater fluxes being applied at the calving locations together with
homogeneous take-up of latent heat around Greenland. This is comparable to
the findings of Jongma et al. (2009), who performed sensitivity studies under
pre-industrial conditions, where they investigated the different impact of
icebergs compared to homogeneously distributed freshwater fluxes south of
55
The presented coupled model set-up offers a great approach to conduct long-term experiments to better understand the role of icebergs and the interactions between the different climate components during abrupt climate changes. This is feasible with the presented model since the computation time for 1000 model years is about 2 days in the fully coupled set-up. A useful next step could be to use this model set-up to study Heinrich events in detail, as the crucial question as to how the icebergs' feedback was on climate under colder and more instable times has not yet been fully addressed.
We have coupled the ice-sheet model GRISLI to the earth system model
According to our study, implementing the freshwater fluxes (calving and
runoff) from the Greenland ice sheet causes a colder climate at mid- to high
latitudes. Explicitly including icebergs results in an increased sea-ice
thickness all around the Greenland ice sheet, especially north of
65
From the presented analysis we conclude that the strongest impact of calving on the climate is due to the spatial distribution of the take-up of latent heat needed to melt the ice mass and that the freshening due to the released meltwater has a smaller impact. Applying direct freshwater fluxes that absorb the latent heat locally at the calving sites results in a similar climate and ice-sheet geometry as in the CALV experiment. However, directly applied freshwater fluxes together with homogenous take-up of latent heat lead to an underestimated cooling at the ice-sheet border and an overestimated one temperatures further away. The warmer surface temperatures over southern Greenland cause higher ablation rates and result in a 150 m reduction in ice-sheet thickness compared to the iceberg experiment.
In the present study the resulting climate conditions and ice-sheet geometries differ between the experiments even though they were done under pre-industrial conditions where the calving rates are relatively constant and small. The impact of icebergs on the ice sheet's development is thought to be stronger during colder climate conditions with higher calving rates.
M. Bügelmayer is supported by NWO through the VIDI/AC2ME project no.
864.09.013. D. M. Roche is supported by NWO through the VIDI/AC2ME project
no. 864.09.013 and by CNRS-INSU. The authors wish to thank Christophe Dumas
for his advice and help in the coupling of the ice-sheet model to the
iceberg module and Catherine Ritz for the use of the GRISLI ice-sheet model.
Further, the authors wish to thank the reviewers for providing constructive
comments on how to improve the quality of this paper. The authors also gratefully
acknowledge the Institute Pierre Simon Laplace for hosting the