Wednesday, 12 February 2003: 4:30 PM
Effects of precipitation on the tropical western Pacific ocean using a coupled single–column model
The importance of salinity variability on the thermodynamics and dynamics of the upper ocean has gained an increased appreciation in the last few years. Ocean mixed layer models have been used to understand the extent to which precipitation in the tropical Pacific affects the upper ocean. In these cases, simulations with varying amounts of precipitation relative to the observed value have been performed, and the resultant SST difference examined. Over the four-month IOP, differences in SST even with significant changes in the precipitation amounts tend to be small (< ~ 0.2 °C). Thus the role of the surface salinity variability and resultant barrier layers on controlling the sea surface temperature in the western Pacific, at least over short time scales, is still in question. However, given that these investigations used an ocean mixed layer model solely, no information as to any amplifying effects that may occur as the atmosphere responds to small changes in the SST have been examined. In this study, a coupled atmosphere-ocean single-column model is used to investigate the sensitivity of the ocean and the ocean-atmosphere system to precipitation in the western tropical Pacific. The ocean component of the model was run in stand-alone mode with no precipitation at the surface. Comparisons with a simulation with observed precipitation shows very small deviations in sea surface temperature from the base simulation. This is consistent with other studies that have used a similar methodology with one-dimensional ocean mixed layer models. However, this approach ignores the possibility of feedbacks between changes in the ocean surface (specifically sea surface temperature) and atmospheric responses. A similar series of simulations were run using the coupled model. The coupled model shows stronger differences, indicating that the change in temperature induced by the incoming freshwater feeds back on to the atmosphere system, causing greater differences in sea surface temperature from the coupled model than the ocean stand-alone model. An initial analysis shows that the differences in the coupled model are not due to differences in shortwave radiation but differences in surface latent heat flux between the two simulations were observed. The single-column model can not reproduce feedbacks that may occur through non-local gradients in ocean temperature or salinity or atmospheric dynamics. Thus the variability occurring in the model when no precipitation is coming in through the ocean surface presents an incomplete picture. However, the results can be used to demonstrate that although sea surface temperature variability is small when considering the ocean in isolation, feedbacks to the atmosphere can amplify this ocean response as evidence by the more significant change in sea surface temperature from the coupled model with no surface precipitation than in the stand-alone ocean model with no precipitation. A more formal feedback analysis for quantifying the physics behind the amplified response will also be discussed.