This paper describes a sectorially averaged seasonal model developed for simulating the longterm response of the climate system to the astronomical forcing. The model domain covers the northern hemisphere. The atmospheric dynamics is represented by an improved zonally averaged quasi-geostrophic model. It includes a new parameterization of the meridional transport of quasigeostrophic potential vorticity and a parameterization of the Hadley sensible heat transport. The atmosphere interacts with the other components of the climate system (ocean, sea ice, and land surface covered or not by snow and ice) titrough vertical fluxes of momentum, heat and water vapor. The model explicitly incorporates detailed radiative transfer, surface energy balances, and snow and sea ice budgets. The vertical profile of the upper ocean temperature is computed by an integral mixed-layer model which takes into account merldional convergence of heat. Sea ice is represented by a thermodynamic model including leads and a new parameterization for lateral accretion. This paper presents the model climate for present conditions and results of sensitivity experiments obtained by modifying some internal parameters or by deactivating certain parameterizations in the model. Simulation of the present climate shows that the model is able to reproduce the main characteristics of the general circulation and, in particular, the surface wind field. The seasonal cycles of oceaxfic mixed layer, sea ice, and snow cover are also well reproduced. Sensitivity experiments show the importance of the meridional sensible heat transport by the Hadley circulation in the tropics, the seasonal cycle of the oceanic mixed-layer depth and sea ice formation in latitude bands where the average water temperature is above the freezing point. In a forthcoming paper, this model will be coupled to an ice sheet model and applied to the simulation of the last glacial cycle in the northern hemisphere. 1. for a glacial age to occur, northern high-latitude summers must be cold enough to prevent the winter snow from melting. This allows a positive annum budget of snow and initiates a positive feedback cooling over the Earth through a further extent of the snow cover and a subsequent increase of the surface albedo. At the present time, the astronomical task since interactions between the different parts of the cli-theory has passed severe statistical tests (e.g., Berger [1989] mate system are very complex and occur on a wide range of for a comprehensive review), but we do not yet have a suftemporal and spatial scales. ficient understanding of the physical mechanisms by which Among the different temporal scales which characterize the climatic evolution, the last glacial cycle seems to be particularly interesting to study, because it is well marked in Paper number 91JD00874. 0148-02.27/91/91JD-00874 $05.00 the climate system responds to these astronomical changes of the incoming solar radiation. At the seasonal time scale, for example, these changes affect snow precipitation and surface...
<p>The future surface mass balance (SMB) will influence the ice dynamics and the contribution of the Antarctic ice sheet (AIS) to the sea-level rise. Most of recent Antarctic SMB projections were based on the 5th phase of the Coupled Model Intercomparison Project (CMIP5). However, new CMIP6 results have revealed a +1.3&#176;C higher mean Antarctic near-surface temperature than in CMIP5 at the end of the 21st century enabling estimations of future SMB in warmer climates. Here, we investigate the AIS sensitivity to different warmings with an ensemble of four simulations performed with the polar regional climate model MAR forced by two CMIP5 and two CMIP6 models over 1981--2100. Statistical extrapolation allows us to expand our results to the whole CMIP5 and CMIP6 ensembles. Our results highlight a contrasting effect on the future grounded ice sheet and the ice shelves. The SMB over grounded ice is projected to increase as a response to stronger snowfall, only partly offset by enhanced meltwater runoff. This leads to a cumulated sea-level rise mitigation (i.e. an increase in surface mass) of the grounded Antarctic surface by 5.1 &#177; 1.9 cm sea-level equivalent (SLE) in CMIP5-RCP8.5 and 6.3 &#177; 2.0 cm SLE in CMIP6-ssp585. Additionally, the CMIP6 low-emission ssp126 and intermediate-emission ssp245 scenarios project a stabilised surface mass gain resulting in a lower mitigation to sea-level rise than in ssp585. Over the ice shelves, the strong runoff increase associated with higher temperature is projected to lower the SMB with a stronger decrease in CMIP6-ssp585 compared to CMIP5-RCP8.5. Ice shelves are however predict to have a close-to-present-equilibrium stable SMB under CMIP6 ssp126 and ssp245 scenarios. Future uncertainties are mainly due to the sensitivity to anthropogenic forcing and the timing of the projected warming. Furthermore,&#160; we compare the MAR projected SMB to the ISMIP6-derived SMB, revealing large local and integrated differences between MAR and the respective forcing ESM highlighting the need of additional projections relying on more models including both RCMs and ESMs. While ice shelves should remain at a close-to-equilibrium stable SMB under the Paris Agreements, MAR projects strong SMB decrease for an Antarctic near-surface warming above +2.5&#176;C limiting the warming range before potential irreversible damages on the ice-shelves. Finally, our results reveal the existence of a potential threshold (+7.5&#176;C) that leads to a lower grounded SMB increase. This however has to be confirmed in following studies using more extreme or longer future scenarios.</p>
Abstract. A set of coupled ocean-atmosphere simulations using state of the art climate models is now available for the Last Glacial Maximum and the mid-Holocene through the second phase of the Paleoclimate Modeling Intercomparison Project (PMIP2). This study presents the large scale features of the simulated climates and compares the new model results to those of the atmospheric models from the first phase of the PMIP, for which sea surface temperature was prescribed or computed using simple slab ocean formulations. We consider first the large scale features of the climate change, pointing out some of the major differences between the different sets of experiments. Then we quantify the latitudinal shift of the location of the ITCZ in the tropical regions during boreal summer. It is shown that this shift is limited for LGM, whereas a northward shift and an increase of precipitation are well depicted for mid-Holocene in continental regions affected by monsoon precipitation. In the last part we quantify for both periods the feedback from snow and sea-ice in mid and high latitudes. We show that it contributes for half of the cooling in the northern hemisphere for LGM, the second half being achieved by the reduced CO2 and water vapour in the atmosphere. For mid-Holocene the snow and albedo feedbacks strengthen spring cooling and enhance boreal summer warming, whereas water vapour reinforces the late summer warming. These feedbacks are modest in the southern hemisphere. For LGM most of the surface cooling is due to CO2 and water vapour.
ABSTRACT. [ t is ge nerally accepted th a t fresh-wa ter flu xes du e to ice accre ti o n or melting p rofo undl y influence the formati o n o f An tarctic bottom water (AA B\V ). Thi s is investigated by means of a globa l, three-dim ensiona l ice ocea n model. Two model runs were co nducted. At th e hi gh so uth ern latitudes, the co ntrol ex periment ex hibits positive (i. e. toward s the ocean ) fr esh-water flu xes over the deep ocea n, a nd la rge negative fluxes over th e Anta rctic contin ental shelf, because of the intense ice-production ta king place in this region. Th e salinity of shelf water can increase in such a way th at deep-water form ati on is facilitated. Th e simulated net fresh-water flu x ove r the shelf has a n a nnual mean va lue of -1 m a I. T hi s flu x induces a transport of salt to bottom wa ters, which corres po nd s to a n increase o[their salinity es tim a ted to be a round 0.05 psu. In th e seco nd m odel run, the fresh-water Duxes due to ice melting or freez ing a re neglected, leading to a rearrangement of the water m asses. In particul a r, the AABW-form ation rate decreases, which all ows th e influence of North Atl antic deep water (NADW ) to increase. As NADW is warm er a nd saltier th an AABW, th e bottom-water sa lini ty a nd temperature become higher.
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