Effects of convection instability due to incompatibility between ocean dynamics and surface forcings

W Cai,P C Chu

doi:10.1007/s00585-997-1067-1

Abstract

Abstract. The study demonstrates that an incompatibility between a surface temperature climatology and a given ocean model, into which the climatology is assimilated via Haney restoration, can cause model ocean climate drift and interdecadal oscillations when the ocean is switched to a weaker restoration. This is made using an idealized Atlantic Ocean model driven by thermal and wind forcing only. Initially, the temperature climatology is forcefully assimilated into the model, and an implied heat flux field is diagnosed. During this stage any incompatibility is suppressed. The restoring boundary condition is then switched to a new forcing consisting of a part of the diagnosed flux and a part of the restoring forcing in such a way that at the moment of the switching the heat flux is identical to that prior to the switching. Under this new forcing condition, the incompatibility becomes manifest, causing changes in convection patterns, and producing drift and interdecadal oscillations. The mechanisms are described.

Highlights

It is an open question as to whether or not an ocean general circulation model (OGCM) with prescribed physics is able to maintain a given set of prescribed surface climatology
Deser and Blackmon (1993) proposed that the general warming trend in the North Atlantic sea surface temperature (SST) during the 1920s±30s and the cooling in the 1960s is associated with changes in the Gulf Stream system. These new results emerge at a time when results from the Geophysical Fluid Dynamics Laboratory (GFDL) coupled oceanatmosphere model show the existence of similar oscillations in the thermohaline circulation in the North Atlantic with a comparable time scale of 40 to 50 years (Delworth et al, 1993)
We have argued that the incompatibility between the climatology and the internal model dynamics is responsible for the oscillation in C45

Summary

Introduction

It is an open question as to whether or not an ocean general circulation model (OGCM) with prescribed physics is able to maintain a given set of prescribed surface climatology. In a similar study, Deser and Blackmon (1993) proposed that the general warming trend in the North Atlantic sea surface temperature (SST) during the 1920s±30s and the cooling in the 1960s is associated with changes in the Gulf Stream system These new results emerge at a time when results from the Geophysical Fluid Dynamics Laboratory (GFDL) coupled oceanatmosphere model show the existence of similar oscillations in the thermohaline circulation in the North Atlantic with a comparable time scale of 40 to 50 years (Delworth et al, 1993). Chu: Eects of convection instability due to incompatibility between ocean dynamics and surface forcings variation of the surface heatux is modi®ed by forming a newux ®eld that is a linear combination of the diagnosedux and its zonal mean In this way, there is no addition or loss of heat associated with the redistribution, and the oceanic heat transport implied by the redistributedux ®eld is the same as that implied by the diagnosed ®eld.

The model and the thermal forcing condition

Drift and variability under the new forcing condition

Results under a weak restoration with no addedux term

Discussion and summary