ENSO suppression due to weakening of the North Atlantic thermohaline circulation

Changes of the North Atlantic thermohaline circulation (THC) excite wave patterns that readjust the thermocline globally. This paper examines the impact of a freshwater-induced THC shutdown on the depth of the Pacific thermocline and its subsequent modification of the El Niño–Southern Oscillation (ENSO) variability using an intermediate-complexity global coupled atmosphere–ocean–sea ice model and an intermediate ENSO model, respectively. It is shown by performing a numerical eigenanalysis and transient simulations that a THC shutdown in the North Atlantic goes along with reduced ENSO variability because of a deepening of the zonal mean tropical Pacific thermocline. A transient simulation also exhibits abrupt changes of ENSO behavior, depending on the rate of THC change. The global oceanic wave adjustment mechanism is shown to play a key role also on multidecadal time scales. Simulated multidecadal global sea surface temperature (SST) patterns show a large degree of similarity with previous climate reconstructions, suggesting that the observed pan-oceanic variability on these time scales is brought about by oceanic waves and by atmospheric teleconnections

Un modèle simple pour comprendre pourquoi la couche de glace à la surface d'un plan d'eau tend à rester relativement mince = A simple model to understand why the layer of ice on the surface of water level tends to remain relatively thin

The ice covering salty or fresh water tends to remain rather thin, even if the air temperature is low for a long time. This is due to the insulating role of the ice cover itself, which slows down the transfer to the atmosphere of the heat produced by the solidification of the water. A simple thermodynamic model is developed to investigate the heat transfer processes associated with ice accretion. It is seen that the ice thickness tends to increase as the square root of the time elapsed and that the temperature profile in the ice layer is approximately linear. The stability of the solution obtained is examined. Finally, the simple model is applied to sea ice in the Arctic and Antarctic. The magnitude of the oceanic heat flux is shown to be partially responsible for the ice cover being generally thicker in the Arctic than in the Antarctic

Coupled climate model simulation of Holocene cooling events: oceanic feedback amplifies solar forcing

The coupled global atmosphere-ocean-vegetation model ECBilt-CLIO-VECODE is used to perform transient simulations of the last 9000 years, forced by variations in orbital parameters, atmospheric greenhouse gas concentrations and total solar irradiance (TSI). The objective is to study the impact of decadal-to-centennial scale TSI variations on Holocene climate variability. The simulations show that negative TSI anomalies increase the probability of temporary relocations of the site with deepwater formation in the Nordic Seas, causing an expansion of sea ice that produces additional cooling. The consequence is a characteristic climatic anomaly pattern with cooling over most of the North Atlantic region that is consistent with proxy evidence for Holocene cold phases. Our results thus suggest that the ocean is able to play an important role in amplifying centennial-scale climate variability

Holocene climate instability during the termination of the African Humid Period.

[1] The termination of the Holocene African Humid Period (similar to9 to similar to6 kyr BP) is simulated with a three-dimensional global coupled climate model that resolves synoptic variability associated with weather patterns. In the simulation, the potential for "green'' and "desert'' Sahara states becomes equal between 7.5 and 5.5 thousand years ago, causing the climate system to fluctuate between these states at decadal-to-centennial time-scales. This model result is supported by paleoevidence from the Western Sahara region, showing similar paleohydrological fluctuations around that time. For the present-day, only the desert Sahara state is stable in the model

Holocene climate instability during the termination of the African Humid Period

The termination of the Holocene African Humid Period (similar to9 to similar to6 kyr BP) is simulated with a three-dimensional global coupled climate model that resolves synoptic variability associated with weather patterns. In the simulation, the potential for "green'' and "desert'' Sahara states becomes equal between 7.5 and 5.5 thousand years ago, causing the climate system to fluctuate between these states at decadal-to-centennial time-scales. This model result is supported by paleoevidence from the Western Sahara region, showing similar paleohydrological fluctuations around that time. For the present-day, only the desert Sahara state is stable in the model

Impact of sea-ice formation on the properties of Antarctic bottom water

It is generally accepted that fresh-water fluxes due to ice accretion or melting profoundly influence the formation of Antarctic bottom water (AABW). This is investigated by means of a global, three-dimensional ice-ocean model. Two model runs were conducted. At the high southern latitudes, the control experiment exhibits positive (i.e. towards the ocean) fresh-water fluxes over the deep ocean, and large negative fluxes over the Antarctic continental shelf, because of the intense ice-production taking place in this region. The salinity of shelf water can increase in such a way that deep-water formation is facilitated. The simulated net fresh-water flux over the shelf has an annual mean value of -1 m a-1. This flux induces a transport of salt to bottom waters, which corresponds to an increase of their salinity estimated to be around 0.05 psu. In the second model run, the fresh-water fluxes due to ice melting or freezing are neglected, leading to a rearrangement of the water masses. In particular, the AABW-formation rate decreases, which allows the influence of North Atlantic deep water (NADW) to increase. As NADW is warmer and saltier than AABW, the bottom-water salinity and temperature become higher

Climate of the Last Glacial Maximum: sensitivity studies and model–data comparison with the LOVECLIM coupled model.

The Last Glacial Maximum climate is one of the classical benchmarks used both to test the ability of coupled models to simulate climates different from that of the present-day and to better understand the possible range of mechanisms that could be involved in future climate change. It also bears the advantage of being one of the most well documented periods with respect to palaeoclimatic records, allowing a thorough data-model comparison. We present here an ensemble of Last Glacial Maximum climate simulations obtained with the Earth System model LOVECLIM, including coupled dynamic atmosphere, ocean and vegetation components. The climate obtained using standard parameter values is then compared to available proxy data for the surface ocean, vegetation, oceanic circulation and atmospheric conditions. Interestingly, the oceanic circulation obtained resembles that of the present-day, but with increased overturning rates. As this result is in contradiction with the current palaeoceanographic view, we ran a range of sensitivity experiments to explore the response of the model and the possibilities for other oceanic circulation states. After a critical review of our LGM state with respect to available proxy data, we conclude that the oceanic circulation obtained is not inconsistent with ocean circulation proxy data, although the water characteristics (temperature, salinity) are not in full agreement with water mass proxy data. The consistency of the simulated state is further reinforced by the fact that the mean surface climate obtained is shown to be generally in agreement with the most recent reconstructions of vegetation and sea surface temperatures, even at regional scales