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_at_reading.ac.uk
The Atlantic and Pacific switch

Summary
Europe freezes, the Sahara blooms

• 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 Earths 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