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Robert G. Gallimore
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David D. Houghton

Abstract

The approximation of ocean heat storage by the net surface energy flux and the implications for zonal mean SST simulation using mixed layer ocean formulation are examined. The analysis considers both constant and variable depth mixed layers. Simulated zonal-mean net surface energy fluxes, taken from a low-resolution atmospheric GCM with prescribed SST, are compared to observed flux and ocean heat storage data. The impact of limitations of the mixed layer ocean formulation on SST simulation are assessed to impacts of simulation errors of the atmospheric model.

The results indicate the important in determining errors in the atmospheric model simulation for the net surface energy flux when assessing anticipated improvement in SST simulation as neglected physical processes (e.g., ocean heat transport) are incorporated in the ocean component of an interactive model. Noting the current limited availability of observations, the approximation of ocean heat storage by the simulated net surface energy flux is cautiously assessed for middle latitudes of the Northern Hemisphere. Due to the uncertainty in observational estimates for both seasonal net surface energy flux and seasonal ocean heat transport, the quality of the flux simulation and the question as to whether or not the model heat storage approximation would improve with the addition of seasonal ocean heat transport are assessed with less certainty.

The inferred annual cycle of zonal mean SST is calculated by applying both model and observed net surface energy fluxes (approximated heat storage) and observed heat storage data to the mixed-layer ocean formulations. The results show that the change from a constant 50 m depth to a variable depth mixed layer ocean formulation (after Meehl), yields significant improvement in zonal mean SST simulation with the sensitivity to the approximation for heat storage being a lesser factor. The large uncertainty in seasonal heat transport data, however, warrants sensitivity examination of their impact on climate in a coupled ocean–atmosphere model. The results demonstrate that examination of biases in atmospheric model simulation and calculation of inferred SST using the atmospheric model results can be useful in diagnosing SST simulation in coupled ocean–atmosphere models.

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Robert G. Gallimore
and
David D. Houghton

Abstract

The simulation of zonal mean ocean surface temperature and heat storage (time rate of change of heat content) obtained from a low resolution, atmospheric general circulation model (GCM) coupled to a variable depth upper-ocean formulation (A/VO model) is compared with observations and with a parallel simulation using a constant (50 m) depth upper-ocean component (A/CO model). Variable depths are used to specify the heat content in the seasonal mixed layer and thermocline; the depths are prescribed to vary latitudinally and seasonally but not longitudinally. The Northern Hemisphere depths are taken from Meehl; for the Southern Hemisphere, Meehl's depths are modified to account for differences in ocean thermal structure between the hemispheres.

The annual variation of upper ocean depth produces important effects on seasonal simulation of ocean surface temperature in the extratropics. In this study, the extremes of ocean temperature in both hemispheres occur earlier by about 30 days in the A/VO model, compared to the A/CO model. This leads to a warmer ocean in spring/summer and a colder ocean in fall/winter for the A/VO model. In contrast to the A/CO model, an important asymmetry in the structure of the monthly departures of ocean temperature from the annual mean is produced by the A/VO model. In the southern extratropics, the A/VO model simulates a reduced annual cycle of ocean temperature (by about 25–30%) from that produced by the A/CO) model.

Both models underestimate the observed seasonal amplitude of zonal mean heat storage; the underestimation is greatest for the A/VO model. The differences in simulations of heat storage are linked to differences in ocean surface temperature computation between the two models.

Errors in zonal mean ocean surface temperature for the A/VO model are less than for the A/CO model in the extratropics. particularly the Southern Hemisphere. However, the errors in the coupled A/VO model simulation are larger in the northern extratropics than in the previously published uncoupled calculations using prescribed heat storage estimates. It is argued that significant improvement in ocean temperature simulation by the A/VO model can be achieved with better approximation of heat storage in conjunction with a small adjustment to the prescribed variable depths.

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