Abstract
Abstract. Fresh water hosing simulations, in which a fresh water flux is imposed in the North Atlantic to force fluctuations of the Atlantic Meridional Overturning Circulation, have been routinely performed, first to study the climatic signature of different states of this circulation, then, under present or future conditions, to investigate the potential impact of a partial melting of the Greenland ice sheet. The most compelling examples of climatic changes potentially related to AMOC abrupt variations, however, are found in high resolution palaeo-records from around the globe for the last glacial period. To study those more specifically, more and more fresh water hosing experiments have been performed under glacial conditions in the recent years. Here we compare an ensemble constituted by 11 such simulations run with 6 different climate models. All simulations follow a slightly different design, but are sufficiently close in their design to be compared. They all study the impact of a fresh water hosing imposed in the extra-tropical North Atlantic. Common features in the model responses to hosing are the cooling over the North Atlantic, extending along the sub-tropical gyre in the tropical North Atlantic, the southward shift of the Atlantic ITCZ and the weakening of the African and Indian monsoons. On the other hand, the expression of the bipolar see-saw, i.e., warming in the Southern Hemisphere, differs from model to model, with some restricting it to the South Atlantic and specific regions of the southern ocean while others simulate a widespread southern ocean warming. The relationships between the features common to most models, i.e., climate changes over the north and tropical Atlantic, African and Asian monsoon regions, are further quantified. These suggest a tight correlation between the temperature and precipitation changes over the extra-tropical North Atlantic, but different pathways for the teleconnections between the AMOC/North Atlantic region and the African and Indian monsoon regions.
Highlights
Since their discovery in the North Atlantic marine sediment records and in the Greenland ice cores (Heinrich, 1988; Dansgaard et al, 1993), the abrupt events of the last glacial have been the topic of active research
The results shown here are transient responses to the corresponding fresh water perturbations. This is why we only briefly describe the Atlantic Meridional Overturning Circulation (AMOC) response and we focus most of our analysis on the surface climatic response, attempting to characterise the relationship between climate and AMOC states, between different aspects of surface climate in one region and on the relationships between climate responses in different regions of the globe for which fast teleconnection mechanisms have been suggested by previous work, as briefly summarised in the introduction
Over the rest of the Northern Hemisphere, and especially over the northeast Pacific and North America, no consistent cooling is simulated. This shows that the advection of the cold anomaly from the North Atlantic around the globe is not systematic for all models and that, as for Europe, potential impacts of atmospheric circulation changes can be important to set up the pattern of the thermal response to the AMOC decrease around the Northern Hemisphere
Summary
Since their discovery in the North Atlantic marine sediment records and in the Greenland ice cores (Heinrich, 1988; Dansgaard et al, 1993), the abrupt events of the last glacial have been the topic of active research. The objectives of each of these experiments were different, ranging from the sensitivity to hosing in different regions or to different hosing amplitudes (Roche et al, 2010; OttoBliesner and Brady, 2010) to the analysis of the impact of fresh water hosing on ENSO and its associated teleconnections (Merkel et al, 2010) or the impact of an AMOC collapse under a wide set of boundary conditions (Swingedouw et al, 2009; Singarayer and Valdes, 2010) It is, an ensemble which was not initially devised for a clean model intercomparison, but this ensemble gives a unique opportunity to compare model results for the glacial, as was done by Stouffer et al (2006) for present conditions, albeit with a non pre-determined fresh water hosing set up.
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