The Atlantic and Pacific switch

1. Response of the MOC in a reverse world experiment R.S. Smith+, J.M. Gregory +#, R. Thorpe# and J. Rodriguez# +NCAS-Climate, University of Reading, U.K. and # Hadley Centre, U.K. Met Office r.s.smith@reading.ac.uk Europe freezes, the Sahara blooms The Atlantic and Pacific switch Summary • The rotation of the Earth is reversed in a coupled AOGCM to investigate the dependence of the MOC on ocean basin asymmetries and freshwater transports • The current geometry of the oceans and freshwater catchments favours Atlantic overturning rather than Pacific The Atlantic MOC in a normal FAMOUS run (above) declines in the reverse experiment over a few hundred years towards the “off” state shown below. Deep convection in the north is suppressed by the increase in freshwater supplied to the North Atlantic catchment, whilst the spread of sea ice also impedes the formation of dense water by restricting heat loss. Changes in surface temperature (left) and precipitation (right) are caused by reversing the direction of the Earth’s rotation. Europe cools dramatically and winter sea ice extends well into the North Atlantic due to changes in ocean heat transport, whilst the North Pacific and West Asia heat up. Shifts in the prevailing winds greatly increase precipitation over North Africa, and the resultant increase in runoff makes the normally salty Mediterranean almost fresh. Not only does the direction of the tropical gyre and location of the boundary current reverse with the winds, but the usual North Atlantic Drift disappears completely and colder surface water flows down from the Arctic. Shown below are the ocean currents in the top 500m of the Atlantic for the normal (left) and reversed rotation (right) experiments. Spinning the world backwards It is still unclear why a meridional overturning circulation (MOC) capable of significant northward heat transport is found in the Atlantic and not the Pacific, and whether this preference is coincidental or a fundamental feature of the climate system. Text As the Atlantic MOC collapses, an overturning cell develops in the North Pacific (above, Pacific MOC for the normal case, below for the reversed rotation experiment). The change in planetary rotation causes the two ocean basins to swap characteristics, with the Pacific becoming more salty than the Atlantic, and developing a northward extension to its (eastern) boundary current. The climatic impact of the various geographical asymmetries and their role in forcing the Atlantic preference of the MOC is here investigated using a novel method in which the direction of rotation of the Earth is reversed. We use the FAMOUS AOGCM, a low resolution version of HadCM3 that has been shown to accurately reproduce its climate and sensitivity. Walker Institute for Climate System Research www.walker-institute.ac.uk