HR: 11:50h
AN: PP22A-07    [Abstracts]
TI: Does the Oceanic Meridional Overturning Cell Really Have More Than one Stable State ??
AU: * Nof, D
EM: nof@ocean.fsu.edu
AF: Florida State University, Departments of Oceanography, 4320, Tallahassee, FL 32306-4320
AU: van Gorder, S
EM: vangorder@ocean.fsu.edu
AF: Florida State University, Departments of Oceanography, 4320, Tallahassee, FL 32306-4320
AU: de Boer, A M
EM: adeboer@princeton.edu
AF: Princeton University/GFDL, PO Box 308, Princeton, NJ 08540
AB: Numerical climate and ocean models consistently show that the meridional overturning cell (MOC) has two stable states for the same fresh water flux (into the ocean). One of these two states usually corresponds to a high northward transport of surface water (and, hence, to a warm northern hemisphere climate) whereas the other corresponds to low northward transport (and, hence, to a cool northern hemisphere climate). This two-states scenario leads to the broadly quoted concern that our present day warm climate can perhaps spontaneously flip to a much cooler state. We present new analytical and numerical runs showing that these two states are an art-effect of the high eddy diffusivities most often used in the commonly employed numerical models. For the Atlantic, these diffusivities artificially introduce unrealistic upwelling into the thermocline within the limits of the Atlantic itself. These, in turn, introduce the familiar hysteresis corresponding to the two-states. In the real ocean, the convection in the north Atlantic is too strong and the vertical diffusivities in the thermocline are too small to allow for such upwelling within the limits of the Atlantic itself. As a result, the water which ultimately sinks in the north Atlantic is drawn into the Atlantic from regions far away--the Southern and Indian oceans. Both the analytical and the numerical results show that the two states converge into one in the limit of small diffusivity. This one-state scenario doesn't at all mean that the MOC cannot collapse due to a large fresh water flux--it certainly can do so if the fresh water flux is large enough. Rather, it means that there are no two-states for the same fresh water flux, implying that the transition from warm to a cold state cannot happen spontaneously.
DE: 4255 Numerical modeling
DE: 4263 Ocean prediction
DE: 4532 General circulation
DE: 1620 Climate dynamics (3309)
DE: 1635 Oceans (4203)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting