Net accumulation of the Greenland ice sheet: High ... - CiteSeerX

GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 9, 1485, doi:10.1029/2002GL015742, 2003

Net accumulation of the Greenland ice sheet: High resolution modeling of climate changes Sissi Kiilsholm and Jens Hesselbjerg Christensen Danish Meteorological Institute, Lyngbyvej 100, Copenhagen Ø, Denmark

Klaus Dethloff and Annette Rinke Alfred Wegener Institute for Polar and Marine Research, Telegrafenberg A43, Potsdam, Germany Received 25 June 2002; revised 31 August 2002; accepted 22 November 2002; published 13 May 2003.

[1] High-resolution (50 km) climate change simulations for an area covering the entire Arctic have been conducted with the regional climate model (RCM) HIRHAM. The experiments were forced at the lateral boundary by largescale atmospheric conditions from transient climate change scenario simulations performed with the Max Planck Institute for Meteorology coupled ocean atmosphere general circulation model (OAGCM) ECHAM4/OPYC3 with a resolution of 300 km. The emission scenarios used were the IPCC SRES [Nakicenovic, 2000] marker scenarios A2 and B2. Three 30-year time slice experiments were conducted with HIRHAM for periods representing presentday (1961– 1990) and the future (2071– 2100) in the two scenarios. We find that due to a much better representation of the surface topography in the RCM, the geographical distribution of present-day accumulation rates simulated by the RCM represents a substantial improvement compared to the driving OAGCM. Estimates of the regional net balance are also better represented by the RCM. In the future climate the net balance for the Greenland Ice Sheet is reduced in all the simulation, but discrepancies between the amounts when based on ECHAM4/OPYC3 and HIRHAM are found. In both scenarios, the estimated melt rates are larger in INDEX TERMS: 1610 HIRHAM than in the driving model.

et al., 2000] on the basis of reanalysis data. Differences between the reanalysis data sets are enormous, and additional efforts are required in order to describe and understand the hydrological cycle over the entire Arctic. [Chen et al., 1997] and [Bromwich et al., 1998] used a statisticaldynamical model to downscale large scale dynamic and moisture information to a resolution comparable to the one adopted in the present HIRHAM simulation and obtained accumulation rates largely in agreement with the observations of [Ohmura and Reeh, 1991]. A more comprehensive approach to quantify accumulation over Greenland comes from high resolution RCMs [Kattsov et al., 2000; Christensen and Kuhry, 2000; Dethloff et al., 2002]. [3] The quality of such simulations are difficult to assess as the understanding of the mass balance of the Greenland Ice Sheet is still not well documented. Summing best estimates [Church et al., 2001] of the various mass balance components for Greenland gives a slightly negative balance 26 ± 32 mm/yr ignoring mass-discharge and bottom melting the net balance is 134 ± 24 mm/yr. Mass balance estimates for individual Greenland sectors are few and have only been made very recently [Thomas et al., 1998, 2000].

Global Change: Atmosphere (0315, 0325); 3349 Meteorology and Atmospheric Dynamics: Polar meteorology; 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854); 1833 Hydrology: Hydroclimatology. Citation: Kiilsholm, S., J. H. Christensen, K. Dethloff, and A. Rinke, Net accumulation of the Greenland ice sheet: High resolution modeling of climate changes, Geophys. Res. Lett., 30(9), 1485, doi:10.1029/ 2002GL015742, 2003.

2. Estimates of Accumulation and Mass Balance

1. Introduction [2] Climate variations are particularly large in the Arctic. Direct observations of Greenland precipitation are limited and the measurements of the mostly solid precipitation are strongly influenced by wind effects and mainly confined to complicated orographically influenced coastal stations. Reconstructions of net accumulation rates for the ice sheet region of central Greenland on the basis of annual layer studies yield values of 210 to 230 mm per year [DahlJensen et al., 1993]. Recently, the atmospheric moisture budget over the Arctic basin has been evaluated [Cullather Copyright 2003 by the American Geophysical Union. 0094-8276/03/2002GL015742$05.00

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[4] To model climatic processes at the regional scale requires sufficient horizontal resolution, especially in regions with complex topography. Regional distribution of precipitation and temperature are generally realistically described in RCMs [Giorgi et al., 2001]. [5] In this study we have used the HIRHAM model [Christensen and Kuhry, 2000; Christensen et al., 1998; Dethloff et al., 1996] at 50 km horizontal resolution nested within two transient climate change simulations performed with the state-of-the-art OAGCM ECHAM4/OPYC3 [Roeckner et al., 1996; Stendel et al., 2000]. Although we are only concerned about the Greenland Ice Sheet here, the actual integration domain covers the entire Arctic north of 65°N with 110 by 100 grid points with an inter grid distance of 0.5° by 0.5°. The ability of HIRHAM to simulate present day Arctic conditions realistically has been thoroughly documented [Christensen and Kuhry, 2000; Dethloff et al., 1996; Dethloff et al., 2002]. Figure 1 compares the present day (model years 1961 – 1990) accumulation rates estimated as (solid) precipitation (i.e. [Ohmura et al., 1996]) minus evaporation (P – E) over Greenland as simulated by ECHAM4/OPYC3 with that of HIRHAM. While the orography used in ECHAM is based on the US Navy data at 10 - 1

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KIILSHOLM ET AL.: NET ACCUMULATION OF THE GREENLAND ICE SHEET

Figure 1. Estimate of accumulation rates over Greenland. Upper panel: P– E from ECHAM4/OPYC3 and Lower panel: P–E from HIRHAM [mm/y]. Contours for orography are shown for every 500 m.

arc min resolution [Joseph, 1980] HIRHAM utilized the more recent gtopo30 database, which has a resolution of 1 km [produced by the US Geological Survey’s EROS Data Center, http://edcdaac.usgs.gov/gtopo30/README.html]. In comparison to the US Navy data, the gtopo30 data provides a substantial improvement to the Greenland topography, although systematic errors adding up to app. 100 m at the 50 km resolution applied here, have recently been identified [Box and Rinke, 2003; Bamber et al, 2002]. It is seen that the HIRHAM simulation reveals regional details not present in ECHAM4/OPYC3. Dethloff et al. [2002] demonstrated, that HIRHAM depicts the regional accumulation patterns quite realistically when driven by analysis at the lateral boundaries by a comparison with an up-to-date accumulation map based on Ohmura and Reeh [1991] supplemented with new data material. Here we have chosen to compare with a map expressing individual measurements in order to highlight the complex situation in terms of the observational evidence. Figure 2 depicts observations from a dataset [Bales et al., 2001] and other sources of accumulation at Greenland. We have added 13 coastal data points from Ohmura et al. [1999] to get a better representation there. It is evident that in the coarse resolution ECHAM4/ OPYC3 many regional structures are not represented at all. Moreover, for Greenland at this resolution the land/sea and land/ice sheet contrasts does not match the orography due to

the steep slopes of the ice sheet in many areas. On the contrary, the HIRHAM model accounts well for the majority of these details. It is noted that the complex structure along the southern part with accumulation rates exceeding 1000 mm/yr seems well depicted in HIRHAM, while a much to wide region has high accumulation in the ECHAM4/OPYC3 simulation. Also elsewhere along the coast does HIRHAM pick up the regional details much more realistic than ECHAM4/OPYC3 does. Within the interior and in the northern parts, ECHAM4 shows very little structure, while HIRHAM simulates a wide Arctic desert area, with a tendency to overestimate the Arctic desert conditions in the northern most parts and the WestCentral plateau. Dethloff et al. [2002] addressed the observational evidence for the estimated accumulation rates in the northern most regions and found that previous maps (e.g. Ohmura and Reeh, 1991) are lacking observational support, as no stations are available. A good agreement in the regional precipitation patterns exists concerning the location and the values of very low accumulation and precipitation in the central northwest part of Greenland, but also many regional features in the coastal areas are in high agreement with the Figure 2 of Bales et al. [2001]. The simulated accumulation rates are 421 ± 183 mm/yr by the coarse resolution ECHAM4/OPYC3 model and 224 ± 35 mm/yr by HIRHAM. The uncertainties quoted are based on one standard deviation of the 30-year annual means. Both values are in good agreement with best estimates [Church et al., 2001] based on observations around 295 ± 15 mm/yr. [6] To estimate ablation we use an empirical relationship between the annual loss of mass and the mean air temperature above 2°C for the summer months June, July and August (JJA) as simulated by the models. A seasonal mean temperature of 2°C is the minimum temperature where mass loss due to ablation can be expected [Ohmura et al., 1996]. A linear relation for the trend has been calculated on basis of observations [Wild and Ohmura, 2000]. Refreeze on the surface is taken into account by this method. The physics behind this method can be questioned, as ablation probably to some extend is ruled by the occurrence of

Figure 2. Observed accumulation [mm/y]. Dot radius is related to the density of observations to ease readability.


cyclones. Air temperature has a more profound influence on melt than previously has been acknowledged. The importance of the air temperature in estimating the melt is mainly due to the fact that the long wave atmospheric radiation is by far the most dominant heat source and that the majority of the atmospheric radiation received at the surface comes from the near-surface layer of the atmosphere [Ohmura, 2001]. On the Greenland Ice Sheet the 2°C isotherm for JJA is located at 1500 and 750 m above mean sea level in southern and northern Greenland, respectively, [Ohmura et al., 1996 and references therein]. A comparison of the simulated areal distribution of ablation zones with those provided by Abdalati and Steffen [1997] (not shown) indicate that ECHAM4/OPYC3 is too course meshed to allow for any detailed comparison, HIRHAM seems to capture the overall structure well and many regional features vary realistically. One exception is that no ablation is ever seen at altitudes above 1500 m, this may be an artifact of the seasonal temperature threshold adopted. [7] Figure 3 compares the total areal distribution of net balance as simulated by ECHAM4/OPYC3 and HIRHAM for present conditions. The net balance is 186 ± 186 mm/yr for ECHAM4/OPYC3 and 47 ± 62 mm/yr for HIRHAM. A recent assessment [Paterson and Reeh, 2001] of regional mass balance changes in a northern transect can also be compared to the present set of simulations, Table 1. The thickening of the ice sheet of northeast Greenland are captured well for ECHAM4/OPYC3 and HIRHAM,

Figure 3. Net balance: Combined accumulation and ablation rates over Greenland estimated from ECHAM4/ OPYC3 (top) and HIRHAM (bottom) [mm/yr]. 1961 –1990.

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Table 1. North Greenland Net Balance Change 1954 – 1995 76° – 79°N

27° – 30°W, (mm/y)

ECHAM4/OPYC3 HIRHAM OBSERVED

170 ± 40 75 ± 24 97 ± 84

60° – 65°W, (mm/y) 1390 ± 560 223 ± 189 310 ± 107

whereas the thinning of the ice sheet of northwest Greenland are less well simulated by ECHAM4/OPYC3 but reasonably simulated by HIRHAM.

3. Climate Change Simulations [8] In the climate change experiments the Global warming between the model years 1961 – 1990 and 2071 – 2100 results in a reduction of sea-ice cover in ECHAM4/OPYC3. One of the consequences of this is a change in the atmospheric general circulation over the Arctic. The model exhibits a realistic large-scale Arctic Oscillation (AO) pattern, which shows a clear trend towards a general positive phase towards the end of the 21st Century. In both scenarios the increased carbon dioxide leads to a warming of the high latitudes as a result of ice-albedo feedbacks and a reduction in the sea-ice cover. This triggers changes in the atmospheric circulation of the Arctic and a shift to a stronger positive AO phase [Fyfe et al., 1999]. At the same

Figure 4. Net balance: Combined accumulation and ablation rates over Greenland estimated from ECHAM4/ OPYC3 (top) and HIRHAM (bottom) [mm/yr]. 2071 – 2100 from scenario B2 simulations.

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Table 2. Net Accumulation for Scenario Runs SRES A2 and B2 SRES A2 HIRHAM ECHAM4/OPYC3

257 ± 81 200 ± 52

SRES B2 186 ± 57 90 ± 57

time the warmer atmosphere is able to hold an increased amount of moisture, which is released along the storm tracks. For Greenland this results in a considerable increase in P– E particularly over the Eastern parts. At the same time the warming results in an increase of the ablation zone, as the summer time zero degree isotherm is shifted to higher elevations. Figure 4 shows the net balance for the years 2071 – 2100 as simulated by ECHAM4/OPYC3 and HIRHAM, respectively. Many regional aspects are seen to differ between the two simulations. In the A2 scenario, the Greenland Ice Sheet will have a negative net balance considerably higher than for the B2 scenario. HIRHAM exhibits in general a higher negative net balance than ECHAM4/ OPYC3, see Table 2.

4. Discussion [9] We have demonstrated that at the coarse resolution typical for state-of-the-art coupled OAGCMs many of the regional features important for assessing the net balance of the Greenland Ice Sheet remains unresolved. This is particularly important if an attempt to analyze the stability of regional domains of the ice sheet is to be achieved. In the present study, we have documented that with the HIRHAM model utilized at a resolution of 50 km, it is possible to assess in a more consistent manner the areal distribution of present day accumulation zones as well as ablation areas. We propose that the high-resolution simulation gives increased credibility to the scenario estimate, because of what seems to be a more realistic simulation of the present day net balance.

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S. Kiilsholm and J. H. Christensen, Danish Meteorological Institute, Lyngbyvej 100, DK-2100 Copenhagen Ø, Denmark. ([email protected]; [email protected]) K. Dethloff and A. Rinke, Alfred Wegener Institute for Polar and Marine Research, Telegrafenberg A43, D-14473 Potsdam, Germany. (dethloff@ AWI-Potsdam.de; [email protected])