The long-term upper-ocean (0-400 m) temperature trends between 1955 and 1994 are studied using the observations of expendable bathythermograph (XBT) and the simulation of an ocean general circulation model.

The observations show that the Pacific and Indian Oceans have a warming trend near the surface above 50 m and a cooling trend below 50 m. The warming trend near the surface extends relatively deeper in the Atlantic Ocean. These temperature trends are not zonally uniform in the Pacific Ocean, but relatively uniform in the Indian Ocean. In the tropical Pacific, the thermocline has a warming trend in the east but a cooling trend in the west. The warming in the east and cooling in the west are also manifested in the North Pacific mid-latitudes. These salient features of ocean temperature trends are well simulated in the Pacific and Indian Oceans, but not in the Atlantic Ocean.

The analysis of ocean heat budget shows that the temperature trends are dynamically controlled by ocean currents in the Pacific and Indian Oceans. The downward net surface heat flux, on the other hand, tends to damp the temperature trends near the surface. In the western tropical Pacific, the warming near the surface seems to be caused by the reduction of the South Equatorial Current, while the thermocline cooling is caused by the reduction of the equatorward convergent flow. In the central tropical Pacific, the warming near the surface is caused by the reduction of the off-equatorial Ekman flow, while the cooling in the thermocline is caused by the reduction of the equatorward convergent flow. In the eastern tropical Pacific, the warming appears to be caused by the reduction of the upwelling current. The change of these currents is caused by the decrease of the trade winds.

In the subtropical (30-40N) North Pacific, the warming in the east is caused by the reduction of the southward Ekman flow near the surface and the southward Sverdrup flow in the thermocline. The cooling in the west is caused by the reduction of the Kuroshio. The change of these currents is associated with the decrease of the westerlies.

In the tropical Indian Ocean, the surface warming is caused by the increase of the southward Ekman flow south of the equator, and the decrease of the southward Ekman flow north of the equator. The cooling in the thermocline is caused by the reduction of the off-equatorial divergent flow. The change of these currents is associated with the decrease of the winds in the Indian Ocean.

Sensitivity experiments of the Pacific Ocean demonstrate the robustness of the temperature trends and their mechanisms. The temperature trends are weaker near the surface and almost disappear in the thermocline without anomalous wind forcing, while the temperature trends do not change significantly in the absence of anomalous thermodynamic forcing (cloudiness, surface air temperature, and humidity) from the atmosphere. Furthermore, the temperature trends due to the anomalous forcing in the tropics and extratropics seem to be trapped locally.

Surface ocean temperature variability at the interannual (El Nino) time scale is also damped by the net downward surface heat flux in the tropical Pacific, but destabilized by ocean advective heat flux. The interannual temperature variability is enhanced by the off-equatorial Ekman divergent flow in the central tropical Pacific and the upwelling current in the eastern tropical Pacific, which are in turn associated with the westward trade winds. But the associations are not significant in the western tropical Pacific. This is consistent with Bjerknes positive feedback.

For more information: http://web.mit.edu/byh/www/thesis.html