Response in atmospheric circulation and sources of Greenland precipitation to glacial boundary conditions
The response in northern hemisphere atmospheric circulation and the resulting changes in moisture sources for Greenland precipitation to glacial boundary conditions are studied in NCAR’s CCM3 atmospheric general circulation model fitted with a moisture tracking functionality. We employ both the CLIMAP SST reconstruction and a modification thereto with reconstructions of glacial ice sheets and land masks. The individual components of the boundary conditions are added first one at a time and, finally, together. These steps show the atmospheric circulation to respond approximately linearly to the boundary condition changes, and the full glacial change may thus be decomposed into contributions from SST and topography changes, respectively. We find that using the CLIMAP SST reconstruction leads to a shift from Atlantic toward Pacific source regions not found with the modified reconstruction having cooler tropics and less sea ice. The occurrence of such a shift depends chiefly on the SST reconstruction and not on the existence of the large northern hemisphere glacial ice sheets. The influence of these circulation changes on important factors for ice core interpretation such as precipitation seasonality, condensation temperatures and source temperatures are assessed.
Ice cores from the Greenland ice sheet are outstanding archives of past northern hemisphere atmospheric conditions. During the past decades, cores have thus been drilled in several Greenland locations in order to study past climatic conditions (Dansgaard and Johnsen 1969; Langway et al. 1985; Johnsen and Dansgaard 1992; Dansgaard et al. 1993; Mayewski et al. 1994; NGRIP members 2004). The abundance of one or both of the stable water isotopes (SWIs), H218O and HDO, are routinely measured throughout the depth of the core. Traditionally, the SWIs (typically written in δ notation relative to Standard Mean Ocean Water) are used as a proxy for local temperature at the time of deposition (e.g., Dansgaard and Johnsen 1969; Johnsen et al. 2001), while the so-called deuterium excess (δD−8δ18O) is interpreted as a proxy for conditions at the moisture source (e.g., Johnsen et al. 1989). However, factors such as wind speed, relative humidity and temperature at the time of evaporation and mixing and phase changes during transport complicate these relationships (e.g., Johnsen et al. 1989; Masson-Delmotte et al. 2005b).
In more recent ice coring efforts, SWIs are just a few out of a range of constituents that are measured (e.g., NGRIP members 2004), but they are nevertheless among the most important climatic proxies retrieved from ice cores (Johnsen and Vinther 2007). Despite the close connection observed between Greenland temperatures and δ18O during the past 100-200 years (Vinther et al. 2003, 2006), the nature of millennial scale δ18O variability is still poorly understood. Investigations of the Greenland ice cores that also included the available information from deuterium excess, have failed to produce δ18O based temperature reconstructions that are consistent with Greenland paleotemperatures derived from borehole thermometry (Masson-Delmotte et al. 2005b). Given our reliance on SWIs as a proxy for past temperatures, there is an urgent need to improve our understanding of the full range of mechanisms behind the observed SWI variability in the Greenland ice cores.
In this study we wish to dissect the relative roles of topography, sea surface temperatures (SSTs), sea ice, atmospheric transports, moisture sources and precipitation seasonality on Greenland precipitation-weighted temperatures. These are used here and by others (e.g., Krinner et al. 1997; Krinner and Werner 2003) as a proxy for SWIs. We employ a compartmentalization of the experiments reminiscent of those used by Rind (1987) and Yin and Battisti (2001). In the former, the LGM components are added in succession (SST, SST and flat ice sheets, SST and full-height ice sheets), while in the latter, different combinations of topographies and tropical and extratropical SSTs are used. We add topography and SST changes one at a time and finally together to yield the full set of LGM boundary conditions, and this permits us to assess the different contributions of the two components to the full change. To determine the sources of Greenland precipitation and monitor their changes with changing boundary conditions we follow the prescription by Joussaume et al. (1986) for moisture tracking (see description in the next section). This method has been used for over two decades (e.g., Koster et al. 1986, 1992; Charles et al. 1994; Armengaud et al. 1998; Delaygue et al. 2000; Werner et al. 2001; Noone and Simmonds 2002) in different models for both present day and glacial conditions, but we are unaware of any previous attempts to discern the relative roles of SSTs and topography in determining moisture sources and paths for Greenland precipitation. Comparisons of isotope records from multiple Greenland ice cores reveal that different locations are sensitive to different factors. For instance, Masson-Delmotte et al. (2005b) conclude that the NGRIP site may have a more stable moisture source during the Holocene than GRIP and NGRIP δ18O could therefore be the most reliable proxy for local temperature. GRIP deuterium excess may, on the other hand, to greater extent reflect North Atlantic conditions. Motivated by such inter-core differences, we have chosen to focus on six different Greenland ice core locations.
Atmospheric circulation changes stand out as a complicating factor for the simple temperature–SWI relationship. In a study by Schmidt et al. (2007) it was concluded that on modern-to-mid-Holocene timescales, local SWI signals at both low and high latitudes reflect non-local climate parameters rather than local ones. Even on present day time scales, interannual variability in the atmospheric circulation (such as the North Atlantic Oscillation, NAO) has influence on Greenland SWI composition unrelated to local temperatures (Sodemann et al. 2008b). Werner and Heimann (2002) found that precipitation-weighted inversion height temperatures, sea level pressure over Spain and relative humidity in a narrow mid-latitude Atlantic region constitute the best subset of climate variables to explain interannual δ18O variability. These findings are supported by the discovery of a significant NAO signal in δ18O from several Greenland ice cores (Vinther et al. 2003). This NAO imprint is in the companion papers by Sodemann et al. (2008a, b) shown to be a combined effect of variability in moisture sources and transport conditions. Cole et al. (1999) find that, on a world-wide scale, variations in moisture sources exert leading order influence on interannual δ18O variability. They speculate that this may be different for the larger temperature variations on ice age timescales, on which the traditional interpretation of δ18O reflecting local temperature and deuterium excess reflecting source regions probably is more valid. On orbital time scales, for instance, obliquity variations seem to influence the deuterium excess values in both Antarctic (Vimeux et al. 2001) and Greenland (Masson-Delmotte et al. 2005a, b) precipitation. One explanation for this obliquity imprint on deuterium excess is that obliquity variations dominate mean annual insolation at a given latitude (e.g., Loutre et al. 2004) and may thus induce changes in meridional SST gradients, meridional atmospheric moisture transports and, in turn, moisture sources.
Glacial atmospheric circulation changes arise as a combined response to topographic (massive ice sheets) and thermal (cooler SSTs and high-albedo continents) forcing (Rind 1987; Yin and Battisti 2001). Cook and Held (1988) compare stationary wave responses in a GCM and a linearized model, and conclude that the last glacial maximum (LGM) circulation was chiefly a linear response to the added ice sheets. Addition of the Laurentide ice sheet over northern North America induces a high pressure anomaly (relative to present day) centered slightly upstream of its peak (e.g., Rind 1987, his Fig. 22). This glacial anticyclonic circulation (as reported also by Cook and Held 1988; Krinner and Genthon 1998, and many others) advects cold air from the north onto the eastern flank of the ice sheet (near the east coast of northern North America) which influences the mass balance of the ice sheet itself (Roe and Lindzen 2001) but also tends to dampen the amplitude of the anomalous stationary wave (Cook and Held 1988). In this manner, the background meridional temperature gradient (and, by the thermal wind relation, the zonal wind) impacts the strength of the ice sheet-atmosphere interaction.
The influence of low-latitude SSTs on mid-to-high latitudes under LGM conditions has been investigated in several studies. Some conclude that mean tropical temperatures have a decisive impact on the radiative balance over the ice sheets (e.g., Rodgers et al. 2003). Yin and Battisti (2001), however, find that it is the tropical SST patterns rather than mean temperatures that dominate the high-latitude response. In fact, they conclude that low-latitude SST patterns play a role comparable to that of the ice sheet topography in determining the LGM stationary wave structure. Studying a subset of the Paleoclimate Modeling Intercomparison Project (PMIP, Joussaume and Taylor 2000) models, Kageyama et al. (1999) find that both the North Atlantic and North Pacific storm tracks shifted downstream in response to LGM conditions. This resulted in a reduction of the influence of North American continental conditions on the North Atlantic storm tracks relative to present day. Instead, sea ice conditions became the central influence on storm track position and extent (Toracinta et al. 2004; Byrkjedal et al. 2006).
Considerable uncertainty remains about the SST and sea ice distribution during the last glacial (see the discussion thereof in the next section). For a LGM scenario with a rather modest extent of sea ice in the Nordic Seas region, Byrkjedal et al. (2006) find that the Icelandic Low is much closer to its present-day state than in scenarios with more extensive sea ice. This central role of the sea ice thermal forcing on the regional atmospheric circulation is also called upon by Li et al. (2005) who conclude that North Atlantic/Nordic Seas sea ice reductions and concomitant shifts in precipitation seasonality are sufficient to explain the Dansgaard–Oeschger event temperature anomalies inferred from Greenland ice cores. They did, however, also speculate that these changes are not alone in determining the response; transport routes and moisture sources may also contribute.
This article is organized as follows: Section 2 describes the experimental configuration, the moisture tracking technique and the boundary conditions. In Sect. 3, results are presented and we show the changes in the regional atmospheric circulation, the changes at six Greenland ice core sites and the changes at the source areas for Greenland precipitation. Section 4 discusses the findings in relation ice core data and previous modeling studies, before conclusions are offered in Sect. 5.
2 Experimental configuration
In this study, the National Center for Atmospheric Research’s CCM3 (Kiehl et al. 1996, 1998) is used with a series of different combinations of present day (PD) and LGM boundary conditions. The CCM3 is an atmospheric general circulation model and is run with a spectral resolution of T42 (corresponding to approximately 2.8° × 2.8°) and 18 levels in the vertical. The CCM3 is distributed with a land surface model (Bonan 1998) with non-interactive, user-specified geographies of surface and vegetation types. In our experiments, the model system is run in specified SST mode, in which the user specifies mid-monthly climatologies of sea surface temperatures and sea ice distributions which are then interpolated to each time step. It is important to note that, in specified SST experiments like these, only ocean grid point temperatures are specified; those of land points are prognosed by the model’s land module.
The experiments were run for 20 years after an initial spin up period. All results have been compared with calculations based on the first and last 10-year periods, and all features discussed here are robust across the three calculations.
Seven different experiments were run with different combinations of PD and LGM (21 kyr BP) settings. The imposed boundary conditions in terms of SSTs, topographies, land masks, orbital parameters and greenhouse gas concentrations are discussed in the following along with a description of the changes to the standard distribution of the model performed in order to track moisture from source region to precipitation site.
2.1 Sea surface temperatures
Present day SSTs are taken from the Shea et al. (1992) global climatology giving monthly 2° × 2° SSTs based primarily on the 1950–1975 period. For the LGM, obtaining a reliable SST data set is less straightforward. The Climate: Long-range Investigation, Mapping and Prediction project (CLIMAP 1981, 1994) provided the first global systematic reconstruction which was based on the transfer function method on marine sediment faunal fossils. This data set displays only modest LGM cooling in the tropics compared to today (∼1.5 K cooler) and very extensive high-latitude sea ice with perennial coverage in the North Atlantic reaching as far south as the British Isles. Both of these features have been contested.
As reviewed by, for instance, Crowley (2000) and Toracinta et al. (2004) studies of terrestrial tropical snow lines, corals and noble gas solubility in groundwater indicate more substantial, for example, on the order of 5 K, cooling at low-latitudes, while marine evidence points to a more intermediate cooling of 2–4 K. Specifically, newer reconstructions reduce the extent of the areas within the North and South Pacific gyres in which CLIMAP displayed warmer-than-modern SSTs (e.g, Hostetler et al. 2006). At high latitudes, the low diversity of planktonic foraminifera reduces the reliability of the transfer function method employed by CLIMAP, and large parts of the Nordic Seas are now believed to have been at least seasonally ice free (e.g., Meland et al. 2005).
All our experiments involving LGM SSTs have been doubled, using both the CLIMAP reconstruction and a data set with appropriate low- and high-latitude corrections. For the CLIMAP reconstruction which only provides February and August climatologies, we followed the recommendations of the Paleoclimate Modeling Intercomparison Project (PMIP, Joussaume and Taylor 2000) and used a sinusoidal interpolation between these two, taking them as the summer and winter extremes. Currently, no common reconstruction like CLIMAP has been made with the updates necessary to match the current state of knowledge. There is still some debate (Crowley 2000) and corrections tend to be created from study to study where LGM SST boundary conditions are needed (e.g, Krinner and Genthon 1998; Delaygue et al. 2000; Werner et al. 2000, 2001; Crowley 2000; Yin and Battisti 2001; Toracinta et al. 2004; Bromwich et al. 2004; Li et al. 2005; Hostetler et al. 2006; Byrkjedal et al. 2006). Rather than create yet a correction, we have chosen to follow the directions of Toracinta et al. (2004). The corrections performed therein are not necessarily better than the others, but they are simple and have also been re-used in another study (Bromwich et al. 2004).
2.2 Experiments and boundary conditions
For orbital parameters and greenhouse gases, guidelines from the Paleoclimate Modeling Intercomparison Project phase II (PMIP2, e.g., Crucifix et al. 2005) were used: Orbital parameters were taken for 0 kyr BP (eccentricity 0.016724, obliquity 23.446°, longitude of perihelion 102.04°) and 21 kyr BP (ecc 0.018994, obl 22.949°, lop 114.42°), and for greenhouse gas concentrations, pre-industrial (CO2 280 ppm, CH4 760 ppb, N2O 270 ppb) and LGM (CO2 200 ppm, CH4 350 ppb, N2O 190 ppb) values were specified. The solar constant was set to 1,365 Wm−2 in both cases.
The control experiment with PD SSTs, topography, orbital parameters, greenhouse gas (GHG) concentrations and land mask.
Orbital parameters, GHGs and land mask were changed to LGM values. This case was included to provide a control against which only SSTs and topographies were changed.
As in CTRLLND, but with topography (and surface type in glacier points) changed to LGM values.
As in CTRLLND, but with SSTs changed to CLIMAP LGM values.
Both topographies and SSTs changed to LGM values.
As SST but with LGM SSTs following the directions of Toracinta et al. (2004)
As LGM but with LGM SSTs following the directions of Toracinta et al. (2004).
Overview of the combinations of boundary conditions for the seven experiments. See text for details
2.3 Moisture tracking
For this study we have fitted the NCAR CCM3 with a moisture tracking functionality like those employed earlier in both the NASA GISS (by, e.g., Koster et al. 1992; Charles et al. 1994; Armengaud et al. 1998; Delaygue et al. 2000) and the ECHAM-4 (Werner et al. 2001) GCMs. The Earth’s surface is divided into a number of regions (in our case 17), and moisture evaporated from each of these is treated as a separate tracer until it leaves the atmosphere (due to, for instance, precipitation or condensation onto the land or ocean surface). During their lifetime in the atmosphere, the transports and phase changes of the tracers parallel those of the model’s moisture field. The tracers are thus subject to the same tendencies as the total moisture field and since the source regions cover the whole globe, the sum of the tracers equals, at each time step and in each grid box, the model’s moisture content. In this manner, precipitation at any point can be decomposed into its contributing source regions. Further details of the moisture tracking are given in the Appendix.
The names, abbreviations and latitude/longitude positions of the six ice core locations considered. See map in Fig. 3b
For the results presented here, the differences between the CTRL and the CTRLLND experiments are small compared to the changes associated with the other boundary condition variations. This is due to the fact that the SSTs are fixed to the present day values; if SSTs were allowed to adjust, the drop in greenhouse gas concentrations would cause the global climate to cool. In fact, the climate sensitivity (equilibrium response in global mean surface temperature to a doubling of CO2) of the CCM3 at the present resolution coupled to a slab ocean model is 2.3°C (Kothavala et al. 1999). However, since the CTRL and CTRLLND climates are so alike, we will in the following present all results relative to the CTRLLND experiment. This has the advantage that, when viewing results from, for instance, the TOPO or SST experiments, only topographies or SSTs have been changed; there are no land mask, orbital parameter or greenhouse gas changes.
3.1 Atmospheric circulation
The topographic forcing (Fig. 4a) contributes mainly with an increase in the 500 hPa height, especially in the Canadian Arctic and the Beaufort Sea region. The CLIMAP temperatures (Fig. 4b) contribute with a lowering of the 500 hPa height due to the colder temperatures, located mainly around high latitude ocean areas (where CLIMAP SSTs display the largest decreases). This lowering of the 500 hPa height is maximum near the present-day Aleutian Low. The Toracinta SSTs (Fig. 4c) yield a more uniform lowering of the height field, in line with the greater low-latitude cooling and smaller high-latitude cooling. The continental interiors are least affected by the SST changes.
In the mean sea-level pressure (not shown), the most pronounced difference between the two SST experiments is due to the differences in the location of the North Atlantic sea ice lines (Fig. 1). Typically, CLIMAP SSTs yield a weakening of the winter season Icelandic Low relative to present day (e.g., Rind 1987), but the much larger turbulent heat fluxes to the atmosphere in the SSTT and LGMT experiments fuel cyclogenesis in the region and yield a reduced weakening of the Icelandic Low compared to that seen with CLIMAP SSTs [as noted by Toracinta et al. (2004) and Byrkjedal et al. (2006)].
The spring and autumn patterns qualitatively resemble those shown for winter in Fig. 5 and the circulation differences described above thus carry over to the annual mean in spite of a more sluggish summer circulation (none of these seasons are shown) without the clear differences. While Fig. 5 shows the time mean situation, the height field weighted by Greenland precipitation (in line with the precipitation weighted quantities shown in the following) could alternatively have been calculated, thus focusing on the circulation patterns associated with Greenland precipitation. The simpler time mean circulation patterns displayed are, nevertheless, consistent with the moisture source differences described in the following.
Turning to the precipitation fields (not shown), linearity is not expected to hold as well as it did for the pressure fields due, for example, to the non-linearity of the Clausius–Clapeyron relation. Both when looking at absolute and relative changes, the glacial reduction in precipitation is exaggerated in the linear estimates in areas where both the topographic and thermal forcings yield decreases, such as southwest of Greenland, south of Iceland and west of Norway. These maritime areas experience, in the LGM and LGMT experiments, an annual mean drying of up to 80% locally. The fact that even the relative changes display non-linearity indicates that it is not only the exponential form of the Clausius–Clapeyron relation which is responsible for the non-linearity. This agrees with the finding of Krinner and Genthon (1998) that the Laurentide ice sheet induces circulation changes which modify precipitation rates beyond what is dictated by atmospheric temperatures alone. To the degree that the LGM and LGMT changes can be decomposed into the topographic and thermal contributions, however, the SST and SSTT changes tend to dominate. This is not surprising, considering the decrease in evaporation and holding capacity of the lower troposphere associated with the large temperature changes the surface undergoes in these experiments.
3.2 Site conditions
3.2.1 Precipitation rates
31 ± 7
22 ± 4
16 ± 2
17 ± 2
52 ± 9
85 ± 9
When going to LGM and LGMT conditions (Fig. 6a) all sites experience a drying, an effect which is most pronounced at the southeastern sites (RL and D3). There is, however, a qualitative difference between the changes at these sites and at the rest: Between the LGM and LGMT experiments RL and D3 encounter increases as opposed to the slight decreases seen at the other sites. The southeastern sites are more directly influenced by the cooler North Atlantic and reduced evaporation while the rest are governed by the changes in transport routes. The precipitation rates do not display linear responses to the boundary conditions as we saw for the 500 hPa height field. Especially for the RL and D3 sites, addition of topography alone has very limited effect (for ANN, DJF and JJA). Comparing SST and LGM, however, addition of topography has a marked drying effect. This can be seen from Fig. 9 (which will be discussed later) to be due to changes in the continental North American source: In both CTRL and TOPO, the sources are mainly Atlantic and addition of the Laurentide ice sheet has limited effect on the evaporative input relevant to Greenland. When Atlantic temperatures are lowered in the SST experiment; however, the North American source becomes more important and shutting this off by addition of the Laurentide in LGM has a large effect.
Figure 6d shows the seasonality of the accumulation rates by plotting DJF/JJA ratios. Validation of the present day ratio is not straightforward since there (to our knowledge) are no good, seasonally resolved observations of precipitation on the Greenland ice sheet. Steffen and Box (2001) report automatic weather station (AWS) estimates of monthly surface height changes from the period 1995–1999. These height changes are the combined result of precipitation, firn compaction, redistribution of snow by drifting and sublimation/condensation and can thus not be compared directly to our precipitation rates. For the Summit station, where the model (GR) gives a large summer dominance, the AWS shows no significant seasonality (although larger summer precipitation may be countered by firn compaction). At South Dome (∼200 km south of D3, where the model also shows larger summer rates), the AWS shows larger height changes during winter, but here the warm summer temperatures certainly give significant summer compaction contributions. Alternative information comes from Cappelen et al. (2001) who report monthly resolved climatologies from coastal stations around Greenland. All along the west coast, the stations have a summer maximum in precipitation. On the south-eastern coast, they have a winter maximum and on the mid-eastern coast (near RL), they show equal summer and winter rates. The modeled summer dominance at the CC, NE and NG sites thus resemble the west coast observations (although somewhat exaggerated). The RL site, showing almost equal summer and winter rates, is well in line with the observations. The summer dominance at the GR site is in line with the west coast observations but seems somewhat at odds with the flat AWS distribution of Steffen and Box (2001). The D3 site is close to both the west and east coasts showing summer and winter dominance, respectively, and it is thus unclear whether the modeled seasonality is realistic.
Going to the other experiments in Fig. 6d, a decrease in the DJF/JJA ratio relative to the CTRL value denotes a shift toward greater summer weighting and vice versa. Most sites encounter a shift toward greater summer weighting in most experiments. Exceptions are the northwestern sites, for example, CC, NE and NG, when CLIMAP SSTs are used (exps SST and LGM). These sites do not suffer under the vast winter North Atlantic sea ice cover, but are rather affected by the circulation changes and increased Pacific storminess which is most pronounced in winter.
3.2.2 Precipitation weighted temperature
3.2.3 Moisture sources
One might suspect that the Pacific source and the cross-Arctic transport in SST and LGM could be an artifact of the topographic smoothing associated with the T42 model resolution, for example, that at higher resolution higher topography would more efficiently drain the moisture out of the atmosphere before reaching the Arctic Ocean and eventually Greenland. However, the height of the Brooks Range and the Mackenzie Mountains, which judged from the circulation patterns in Fig. 5c and e would constitute the prime blockers of the moisture transport, are not significantly lowered by the smoothing. Taking an approximate mean range height by averaging along latitude 68°N from 145–155°W, the Brooks Range is lowered from ∼1,400 m (at 1° × 1° resolution) to ∼1,200 m (at T42 resolution). For the Mackenzie Mountains we average, as an example, along longitude 129°W from 57 to 64°N, and for both resolutions the height is just under 2,600 m. Topographic smoothing thus seems incapable of playing the decisive role for the cross-Arctic transport in the present model configuration.
3.2.4 Contributions to changes in Tpw
This breakdown is shown for LGM and LGMT in Fig. 10b and c, respectively. The accuracy of our linear decomposition, indicated by the degree to which the thick and thin solid curves are equal, is seen to be quite convincing. The full LGM/LGMT curves (thick solid) are exactly the differences between the corresponding curves and the CTRL curves in Fig. 7a. The major contributor to these changes is in both cases the decrease in local condensation temperature. In LGMT (Fig. 10c), this cooling is offset everywhere by about 3 K due to the combination of seasonality and distribution changes. As seen in Fig. 6d, all cores experience a shift towards greater summer weighting in LGMT and this yields the warming effect shown by the dash-dotted curve. The small positive contribution from the distribution change owes mainly to the shift from the more distant North American source to the local Greenland source seen in all cores (Fig. 9, upper row).
In the LGM experiment (Fig. 10b), the results are less spatially uniform: The temperature contribution is smaller than in LGMT (due, as mentioned, to the much warmer low- to mid-latitudes) but has most weight closest to the heavily sea ice-influenced North Atlantic. This gradient is partially offset by an opposite gradient in the seasonality effect, which is positive in the southeast where there is a shift toward summer weighting and negative in the northwest where there is a shift toward winter weighting (Fig. 6d). In fact, this difference in the seasonality changes is exactly what allows the total Tpw cooling in LGM at the northwestern-most sites to be as large or larger than in LGMT despite the weaker direct cooling. The source distribution effect is small due to the competition between the cooling by the shift from Atlantic to Pacific sources and the warming by the shift from North American to Greenland sources. In both Fig. 10b and c, the changes at the NG and GR sites are almost equal, and in both cases is this due to a cancellation between temperature and seasonality effects.
3.3 Source conditions
The temperature/δ18O relationships reviewed by Johnsen et al. (2001) differ by approximately a factor of 3 depending on whether condensation temperatures or time mean surface temperatures (as inferred from borehole thermometry) are considered. This agrees well with our results in which both glacial runs produce condensation temperature cooling of 6–8 K (Fig. 10b, c) and time mean surface temperature cooling of up to 20 K (not shown). Krinner et al. (1997) and Werner et al. (2000) have previously found that much of the discrepancy between present day-tuned temperature/δ18O relationships and those inferred from boreholes stems from seasonality changes rather than atmospheric inversion height temperature changes. In our case, the “Temperature” curves in Figs. 10b and c show only a cooling of ∼10 K (rather than the 15–20 K at the surface) and the seasonality bias is only a few degrees. The “Temperature” curve carries the combined effects of condensation height temperature changes and the short-term (sub-monthly) precipitation bias; temperatures are recorded only during snow events and the very cold spells dragging down time-mean temperatures are not recorded. Our results are thus consistent with the suggestion of Johnsen et al. (2001) that “precipitating clouds retain much of their warmth, even during times of cold glacial climate”.
Based on GRIP deuterium excess and δ18O, Masson-Delmotte et al. (2005a) infer present-day to glacial source temperature cooling of about 6 K. This is much like the number we get for LGM (Fig. 12a), while the LGMT number is slightly larger 8.5 K (Fig. 12b). It is worth noting, however, that our source temperature calculations are limited by the source region resolution. We are thus reluctant to claim that this lends credence to one simulation over the other, but remark that both results are roughly in accordance with ice core inferences. This source region cooling is opposite to the GCM results of Delaygue et al. (2000) for Antarctica in which source temperatures increase when using CLIMAP SSTs due to sufficient equatorward shifts of source regions. This effect is absent when they use a cool-tropics version of CLIMAP.
Contrasting the results for the LGM and LGMT runs, it is intriguing that the differences in source areas for the precipitation arriving in Greenland are so large. In the LGM run most ice core sites on the Greenland ice sheet receive significantly increased relative amounts of precipitation from Pacific sources as compared to the CTRL run (Fig. 9). This change toward a Pacific source region is not seen in the LGMT run that shows unchanged or even decreasing amounts of precipitation from Pacific sources when compared to the CTRL run. This difference between source areas in the LGM and LGMT runs is important because a marked depletion of stadial stable isotope data from the Camp Century and NGRIP ice cores points to increased precipitation in central and northern Greenland stemming from a northerly (i.e., Pacific) source during the LGM (Johnsen and Vinther 2007).
Realizing that precipitation δ18O is also influenced by changes in temperatures in the source area (as the fractionation is a process that is dependent on the temperature change from the source area to the condensation area) we have also calculated the changes in temperature difference between source area temperatures and condensation temperatures. The resulting patterns of change can be seen in Fig. 13b. Again, the LGM run is most similar to the observed δ18O changes in the northwestern region. With or without taking source temperatures into account the main result of the comparison between modeled proxy and observed changes at the drill sites is, however, the same. A large increase in Pacific moisture at the CC, NE and NG sites is seen even in the annual average in the LGM experiment (Fig. 9) and yields larger decreases in the proxy to the northwest. This supports the suggestion made by Johnsen and Vinther (2007) that the observed pattern of Holocene to LGM δ18O changes in the ice cores are best explained by Pacific moisture transported to the northern and central parts of the Greenland ice sheet by more northerly winds.
Such shifts in Greenland moisture sources in response to CLIMAP sea surface temperatures and glacial topography have been found in some, but not all, similar simulations. Charles et al. (1994), for example, find that addition of the Laurentide ice sheet gives a large drop in North American contribution and the extensive sea ice gives a large drop in the Nordic Seas contribution. These changes give a zonal change throughout Greenland: North Atlantic sources dominate southern Greenland while North Pacific sources dominate northern Greenland much like in the present study. They find that North Pacific moisture arrives in Greenland much more depleted (order of 15‰) in δ18O and the shift would thus (even without changes in temperature) yield significant drops in δ18O. In contrast, Werner et al. (2001), also using CLIMAP SSTs, find no shift in source regions toward Pacific influence. Moreover, they find no significant differences when modified CLIMAP SSTs (with cooler tropics and warmer North Atlantic) are used. Their modeled LGM—present change in δ18O is on the low side, but although one possible reason could be the lack of a shift toward Pacific moisture, this would, they argue, not resolve their problem of a positive deuterium excess change (rather than the observed decrease). They do find an increase in the northerly component of the mean flow over central Greenland, but this leads only to a cooling and drying but no increase in Pacific moisture. They speculate that different model sensitivities to SST forcing or different ice cap specifications are possible explanations. Based on our compartmentalized experiments, however, where the increased Pacific influence on the northwestern cores was seen with and without changed topography (Fig. 9) we are reluctant to ascribe such inter-model differences to different specifications of ice sheets. In their high-resolution regional model, Bromwich et al. (2004) find that the existence of a pronounced split jet stream hinges also on model physics, resolution and glacial SST reconstruction.
Resolution may, in fact, be a part of the reason for our apparently contradictory result that the CLIMAP reconstruction yields results better in agreement with ice core inferences than does the modified and supposedly improved reconstruction. Sodemann et al. (2008a) find with their Lagrangian backtrajectory technique no Pacific moisture contribution to present-day Greenland winter precipitation, regardless of site, in a simulation with considerably higher-resolution ERA-40 data (T159). Thus, if the present-day Pacific influence in our study and those of Charles et al. (1994) and Werner et al. (2001) is merely an artifact of resolution, the shift in this quantity may also be so. This does not, however, explain the ice core indications of a changed source. Alternatively, the tropical cooling of 4 K in the modified reconstruction is too strong (e.g., Crowley 2000) or the tropical SST pattern is more important than the absolute tropical temperature (Yin and Battisti 2001; Hoerling et al. 2004). Perhaps a different tropical pattern (with larger-than-CLIMAP cooling) could give the right changes in moisture sources. Hostetler et al. (2006) suggest a different, oceanographically based LGM reconstruction which in the GENESIS model yields temperatures and precipitation changes which (to the degree paleodata exist) match better than CLIMAP. Finally, it may be that the models’ responses in atmospheric circulation simply are too weak as found by Hurrell et al. (2004) for 20th century North Atlantic Oscillation changes.
Using the NCAR CCM3 atmospheric GCM fitted with a moisture tracking functionality we have studied the changes in atmospheric circulation and sources for Greenland precipitation resulting from different components of glacial boundary conditions. For the glacial SSTs we have used both the CLIMAP reconstruction and the modified version thereof by Toracinta et al. (2004) displaying 4 K cooler tropics and less extensive sea ice cover, especially in the North Atlantic. The ICE-5G reconstruction of Peltier (2004) was used to determine ice sheets, topography and land mask. Before changing both SSTs and topography to glacial conditions, a series of intermediate steps was taken, in which the two components were changed alone. This allows us to monitor the contributions of each of them to the full glacial changes.
The changes in the 500 hPa height field was found quite accurately to respond linearly to the changing boundary conditions: For both of the SST reconstructions, the change due to adding only topography (TOPO–CTRL) plus the change due to SST changes (SST–CTRL and SSTT–CTRL, respectively) equals approximately the full glacial changes (LGM–CTRL and LGMT–CTRL, respectively). This linearity permits us to ascribe certain changes in the full glacial experiments to contributions from the individual factors. We find, for instance, a shift toward a cross-Arctic flow at the 700 hPa level and an associated shift in sources of Greenland precipitation from the Atlantic to the Pacific as a result of the CLIMAP SSTs. This shift occurs with or without changing the topography and does not occur in any case with the modified temperature reconstruction. Our experiments thus indicate that the temperature reconstruction is more important than the ice sheet reconstruction for inducing such shifts.
Although the glacial cooling tends to lead to a drying of the climate, the northwestern-most of the Greenland cores, which in SST and LGM acquire a new Pacific source of moisture, do not encounter as massive a drying during winter as do the more Atlantic-influenced cores. This leads to two different directions of change in seasonal weighting for the two groups, which in turn leads to two different signs of the seasonal bias in precipitation weighted temperatures. Interestingly, stable isotope data from the northwestern-most Greenland ice core (Camp Century) lend support to the proposition that a Pacific source played an important role for northwestern Greenland precipitation during the glacial (Fig. 13).
During winter when continental sources are insignificant, addition of the ice sheets (yielding no decisive circulation changes) does not make a large difference and the SST reconstruction is most important for determining the change in condensation temperature (Fig. 7b). In summer, where the North American contribution is important and the shift toward Pacific moisture is less pronounced, addition of the Laurentide Ice Sheet becomes the dominant factor in determining the change in condensation temperature (Fig. 7c).
In the annual mean, we find that it is possible to decompose the LGM and LGMT changes into linear contributions from temperature changes, seasonality changes and source distribution changes (Fig. 10b, c). In both cases, the direct source distribution effect is rather small and the warm bias from the seasonality change tends to counter some of the direct cooling. The exception to this is for the northwestern cores in the LGM case, where the opposite seasonality effect tends to lower the condensation temperatures even further. The low-latitude cooling in LGMT yields a cooling of the high-latitude atmosphere which is re-found as a larger drop in the condensation temperatures relative to the LGM case—again, except for the northwestern cores where the seasonality biases are opposite.
The source temperatures generally decrease and they do so more for LGMT than for LGM (Fig. 11). Shifts toward more equatorward source regions are insufficient to give any visible warming effect on the resulting source temperatures. When the circulation changes and the Pacific becomes more influential during winter in the LGM case, a non-linear effect of a shift from a region of large cooling (the Atlantic) to a region of much smaller cooling (the Pacific) yields a warming.
Our key finding is that SSTs seem to be more important than topography for qualitatively changing moisture sources for Greenland precipitation. In fact, the apparent dominance of the SST effects—be it global mean temperature, tropical SST patterns or meridional gradients—suggests that also the Eemian may have been characterized by qualitatively different source patterns. Since at least the glacial changes are more pronounced the more northwesterly the location of the core, this may become particularly important when it comes to interpretation of the (not yet drilled) NEEM core which is expected to give an undisturbed Eemian record.
Note that precipitation rates and accumulation rates are not exactly the same since the latter also includes effects of sublimation.
Both authors are supported by the Carlsberg Foundation. This work was supported in part by a grant of HPC resources from the Arctic Region Supercomputing Center (ARSC) at the University of Alaska Fairbanks as part of the Department of Defense High Performance Computing Modernization Program. We thank Harald Sodemann and an anonymous reviewer for comments and suggestions that have significantly improved the manuscript.
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