Allegra N. LeGrande

Configuration and assessment of the GISS ModelE2 contributions to the CMIP5 archive

We present a description of the ModelE2 version of the Goddard Institute for Space Studies (GISS) General Circulation Model (GCM) and the configurations used in the simulations performed for the Coupled Model Intercomparison Project Phase 5 (CMIP5). We use six variations related to the treatment of the atmospheric composition, the calculation of aerosol indirect effects, and ocean model component. Specifically, we test the difference between atmospheric models that have noninteractive composition, where radiatively important aerosols and ozone are prescribed from precomputed decadal averages, and interactive versions where atmospheric chemistry and aerosols are calculated given decadally varying emissions. The impact of the first aerosol indirect effect on clouds is either specified using a simple tuning, or parameterized using a cloud microphysics scheme. We also use two dynamic ocean components: the Russell and HYbrid Coordinate Ocean Model (HYCOM) which differ significantly in their basic formulations and grid. Results are presented for the climatological means over the satellite era (1980–2004) taken from transient simulations starting from the preindustrial (1850) driven by estimates of appropriate forcings over the 20th Century. Differences in base climate and variability related to the choice of ocean model are large, indicating an important structural uncertainty. The impact of interactive atmospheric composition on the climatology is relatively small except in regions such as the lower stratosphere, where ozone plays an important role, and the tropics, where aerosol changes affect the hydrological cycle and cloud cover. While key improvements over previous versions of the model are evident, these are not uniform across all metrics.

Mid-Holocene NAO: A PMIP2 model intercomparison

[1] The mid-Holocene (6000 years before present) North Atlantic Oscillation (NAO) from nine models in the Paleoclimate Modeling Intercomparison Project Phase 2 is studied, primarily through principal component analysis of winter time North Atlantic sea level pressure (SLP). Modeled mid-Holocene NAO and mean SLP show small changes compared to pre-industrial control runs, with a shift in mean state towards a more positive NAO regime for three of the models. Modeled NAO variability shows little change, with a small increase for some models in the fraction of time spent in the NAO-negative phase during the mid-Holocene. Proxy based reconstructions of the NAO indicate a more positive NAO regime compared to present day during the mid- Holocene. We hypothesise that there was a small NAO+ like shift in mean state during the mid-Holocene.

Subtropical Atlantic salinity variability and Atlantic meridional circulation during the last deglaciation

During the last deglaciation (ca. 21–10 ka), freshening of the North Atlantic surface likely caused reductions in Atlantic meridional overturning circulation (AMOC); the mechanisms related to AMOC recovery remain poorly understood. Here we present three new deglacial surface temperature and δ18Oseawater (δ18Osw) reconstructions from the western subtropical North and South Atlantic. Similarities to tropical Caribbean and western Atlantic δ18Osw records suggest that a salty surface water mass accumulated in the western Atlantic from 27°S to 33°N during periods of reduced AMOC. However, δ18Osw decreases led deep AMOC resumption by hundreds of years. We suggest that the northward export of salt previously trapped in the western Atlantic resulted in the early establishment of a shallow overturning circulation that eventually culminated in deep AMOC resumption, implying that AMOC may constitute a self-limiting system.

CMIP5 historical simulations (1850–2012) with GISS ModelE2

Observations of climate change during the CMIP5 extended historical period (1850-2012) are compared to trends simulated by six versions of the NASA Goddard Institute for Space Studies ModelE2 Earth System Model. The six models are constructed from three versions of the ModelE2 atmospheric general circulation model, distinguished by their treatment of atmospheric composition and the aerosol indirect effect, combined with two ocean general circulation models, HYCOM and Russell. Forcings that perturb the model climate during the historical period are described. Five-member ensemble averages from each of the six versions of ModelE2 simulate trends of surface air temperature, atmospheric temperature, sea ice and ocean heat content that are in general agreement with observed trends, although simulated warming is slightly excessive within the past decade. Only simulations that include increasing concentrations of long-lived greenhouse gases match the warming observed during the twentieth century. Differences in twentieth-century warming among the six model versions can be attributed to differences in climate sensitivity, aerosol and ozone forcing, and heat uptake by the deep ocean. Coupled models with HYCOM export less heat to the deep ocean, associated with reduced surface warming in regions of deepwater formation, but greater warming elsewhere at high latitudes along with reduced sea ice. All ensembles show twentieth-century annular trends toward reduced surface pressure at southern high latitudes and a poleward shift of the midlatitude westerlies, consistent with observations.

The influence of Indian Ocean atmospheric circulation on Warm Pool hydroclimate during the Holocene epoch

Existing paleoclimate data suggest a complex evolution of hydroclimate within theIndo-Pacific Warm Pool (IPWP) during the Holocene epoch. Here we introduce a new leafwax isotope record from Sulawesi, Indonesia and compare proxy water isotope data withocean-atmosphere general circulation model (OAGCM) simulations to identifymechanisms influencing Holocene IPWP hydroclimate. Modeling simulations suggestthat orbital forcing causes heterogenous changes in precipitation across the IPWPon a seasonal basis that may account for the differences in time-evolution of the proxydata at respective sites. Both the proxies and simulations suggest that precipitationvariability during the September–November (SON) season is important for hydroclimatein Borneo. The preeminence of the SON season suggests that a seasonally laggedrelationship between the Indian monsoon and Indian Ocean Walker circulation influencesIPWP hydroclimatic variability during the Holocene.

Future climate change under RCP emission scenarios with GISS ModelE2

We examine the anthropogenically forced climate response for the 21st century representative concentration pathway (RCP) emission scenarios and their extensions for the period 2101–2500. The experiments were performed with ModelE2, a new version of the NASA Goddard Institute for Space Sciences (GISS) coupled general circulation model that includes three different versions for the atmospheric composition components: a noninteractive version (NINT) with prescribed composition and a tuned aerosol indirect effect (AIE), the TCAD version with fully interactive aerosols, whole-atmosphere chemistry, and the tuned AIE, and the TCADI version which further includes a parameterized first indirect aerosol effect on clouds. Each atmospheric version is coupled to two different ocean general circulation models: the Russell ocean model (GISS-E2-R) and HYCOM (GISS-E2-H). By 2100, global mean warming in the RCP scenarios ranges from 1.0 to 4.5°C relative to 1850–1860 mean temperature in the historical simulations. In the RCP2.6 scenario, the surface warming in all simulations stays below a 2°C threshold at the end of the 21st century. For RCP8.5, the range is 3.5–4.5°C at 2100. Decadally averaged sea ice area changes are highly correlated to global mean surface air temperature anomalies and show steep declines in both hemispheres, with a larger sensitivity during winter months. By the year 2500, there are complete recoveries of the globally averaged surface air temperature for all versions of the GISS climate model in the low-forcing scenario RCP2.6. TCADI simulations show enhanced warming due to greater sensitivity to CO2, aerosol effects, and greater methane feedbacks, and recovery is much slower in RCP2.6 than with the NINT and TCAD versions. All coupled models have decreases in the Atlantic overturning stream function by 2100. In RCP2.6, there is a complete recovery of the Atlantic overturning stream function by the year 2500 while with scenario RCP8.5, the E2-R climate model produces a complete shutdown of deep water formation in the North Atlantic.

Earliest Holocene south Greenland ice sheet retreat within its late Holocene extent

Early Holocene summer warmth drove dramatic Greenland ice sheet (GIS) retreat. Subsequent insolation-driven cooling caused GIS margin readvance to late Holocene maxima, from which ice margins are now retreating. We use 10Be surface exposure ages from four locations between 69.4°N and 61.2°N to date when in the early Holocene south to west GIS margins retreated to within these late Holocene maximum extents. We find that this occurred at 11.1 ± 0.2 ka to 10.6 ± 0.5 ka in south Greenland, significantly earlier than previous estimates, and 6.8 ± 0.1 ka to 7.9 ± 0.1 ka in southwest to west Greenland, consistent with existing 10Be ages. At least in south Greenland, these 10Be ages likely provide a minimum constraint for when on a multicentury timescale summer temperatures after the last deglaciation warmed above late Holocene temperatures in the early Holocene. Current south Greenland ice margin retreat suggests that south Greenland may have now warmed to or above earliest Holocene summer temperatures.

Surface‐melt driven Laurentide Ice Sheet retreat during the early Holocene

Received 21 September 2009; revised 3 November 2009; accepted 30 November 2009; published 30 December 2009. [1] To better understand mechanisms of ice-sheet decay, we investigate the surface mass balance of the Laurentide Ice Sheet (LIS) during the early Holocene, a period of known rapid LIS retreat. We use a surface energy-mass balance model (EMBM) driven with conditions derived from an equilibrium atmosphere-ocean general circulation model 9 kilo-years ago simulation. Our EMBM indicates a net LIS surface mass balance of 0.67 ± 0.13 m yr 1 , with losses primarily due to enhanced boreal summer insolation and warmer summers. This rate of loss compared to LIS volume reconstructions suggests that surface ablation accounted for 74 ± 22% of the LIS mass loss with the remaining loss likely driven by dynamics resulting in basal sliding and calving. Thus surface melting likely played a governing role in the retreat and disappearance of this ice sheet. Citation: Carlson, A. E., F. S. Anslow, E. A. Obbink, A. N. LeGrande, D. J. Ullman, and J. M. Licciardi (2009), Surface-melt driven Laurentide Ice Sheet retreat during the early Holocene, Geophys. Res. Lett., 36, L24502, doi:10.1029/ 2009GL040948.

Southern Laurentide ice-sheet retreat synchronous with rising boreal summer insolation

Establishing the precise timing for the onset of ice-sheet retreat at the end of the Last Glacial Maximum (LGM) is critical for delineating mechanisms that drive deglaciations. Uncertainties in the timing of ice-margin retreat and global ice-volume change allow a variety of plausible deglaciation triggers. Using boulder 10 Be surface exposure ages, we date initial southern Laurentide ice-sheet (LIS) retreat from LGM moraines in Wisconsin (USA) to 23.0 ± 0.6 ka, coincident with retreat elsewhere along the southern LIS and synchronous with the initial rise in boreal summer insolation 24–23 ka. We show with climate-surface mass balance simulations that this small increase in boreal summer insolation alone is potentially sufficient to drive enhanced southern LIS surface ablation. We also date increased southern LIS retreat after ca. 20.5 ka likely driven by an acceleration in rising isolation. This near-instantaneous southern LIS response to boreal summer insolation before any rise in atmospheric CO 2 supports the Milankovic hypothesis of orbital forcing of deglaciations.

Evaluating climate model performance in the tropics with retrievals of water isotopic composition from Aura TES

We evaluate the NASA Goddard Institute for Space Studies ModelE2 general circulation model over the tropics against water isotope (HDO/H2O) retrievals from the Aura Tropospheric Emission Spectrometer. Observed isotopic distributions are distinct from other observable quantities and can therefore act as an independent constraint. We perform a small ensemble of simulations with physics perturbations to the cumulus and planetary boundary layer schemes. We examine the degree to which model-data agreement could be used to constrain a select group of internal processes in the model, namely, condensate evaporation, entrainment strength, and updraft mass flux. All are difficult to parameterize but exert strong influence over model performance. We find that the water isotope composition is more sensitive to physics changes than precipitation, temperature, or relative humidity in the lower and upper tropical tropospheres. Among the processes considered, this is most closely, and fairly exclusively, related to midtropospheric entrainment strength. Our study indicates that water isotope observations could provide useful constraints on model parameterizations.