Temperature Trend of the Last 40 Yr in the Upper Pacific Ocean

The linear temperature trend of the last 40 yr (1955–94) in the upper Pacific Ocean above 400 m is studied using an objectively analyzed dataset and simulations of an ocean general circulation model. Both the data and simulations suggest a warming trend in the western tropical Pacific (108S–108N) near the surface and in the eastern tropical Pacific above 400 m but a cooling trend in the thermocline of the western tropical Pacific. In the midlatitude North Pacific (308–508N), the temperature trend is positive east of 1508W but negative to the west. Simulated heat budget indicates that the temperature trend in the tropical Pacific may result from oceanic advection. In the central and western Pacific, the surface warming is associated with the reduction of cold advection from the off-equatorial divergent flow and the South Equatorial Current, while the cooling in the thermocline is related to the reduction of equatorward warm advection. In the eastern Pacific, the warming is associated with the reduction of upwelling. The reduction of these ocean currents, in turn, may result largely from the weakening of the trade winds. In the midlatitude North Pacific, the ocean temperature trends similarly may result from the oceanic advection associated with the reduction of the westerlies. The effect of net surface heat flux into the ocean is a damping factor to the sea surface temperature. These studies highlight the importance of oceanic advection in producing long-term temperature trends.