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Xingjian Jiang
and
Inez Fung

Abstract

An ocean general circulation model (OGCM) is used to study the response of ocean heat and mass transport to positive and negative heat flux anomalies at the ocean surface. As expected, tropical and low-latitude mixed layers respond rapidly (e-folding time about 50–70 years) to external forcing, while the response of the high latitude mixed layer, especially the Southern Ocean and northern North Atlantic, is very slow (e-folding time greater than 300 yr). The overall response is faster for negative than positive heat flux anomaly at the surface. The meridional heat transport changes by 15% in the first 50 yr in the southern high latitudes. Surprisingly, for the next 400–500 yr the change is very small. The analysis shows that the meridional mass transport intensifies in response to a negative surface heat flux anomaly but weakens in response to a positive heat flux anomaly. For example, at model year 100 the NADW is reduced from about 18 Sv to about 10 Sv for the positive heat flux experiment but increased to about 26 Sv for the negative heat flux experiment.

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Michael E. Schlesinger
and
Xingjian Jiang

Abstract

A simple atmosphere-Ocean model is developed to represent the 20-year 1 × C02 and 2 × C02 simulations obtained with a coupled atmosphere-ocean general circulation model for the purpose of obtaining a new estimate of the characteristic response time of the climate system that accounts for oceanic upwelling.

The simple atmosphere-generalized ocean model consists of a zonally averaged energy balance climate model and a zonally averaged multilayer ocean model. The high latitudes of both hemispheres are combined into a single polar region, and the low and middle latitudes into a single nonpolar region. The atmosphere is treated as a single layer and the ocean as an arbitrary number of layers. The simple model includes the meridional transport of heat between the nonpolar and polar regions for both the atmosphere and each ocean layer. The ocean model includes the vertical advective heat transfer by the vertical velocity, the latter of which is prescribed and can vary with depth in both the polar and nonpolar regions. The unknown parameters of the simple model are the meridional heat fluxes between the nonpolar and polar regions, the coefficients of heat transfer within the ocean, the heat transfer coefficient between the ocean and atmosphere, an additional ocean-atmosphere heat transfer parameter, and the climate sensitivity parameter.

The parameters of the simple model are determined from the 1 × C02 and 2 × CO2 simulations of the coupled atmosphere-ocean general circulation model. The simple atmosphere-ocean model is then used to project the response of the coupled atmosphere-ocean GCM from year 20 to year 100, and the resulting 2 × C02−1 × C02 differences are normalized by the estimated equilibrium temperature changes. From these projections it is estimated that the characteristic response time is between 40 and 60 years, in close agreement with the estimates of Schlesinger et al.

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