Sensitivity of the tropical Pacific Ocean to precipitation induced freshwater flux

We have performed experiments using an ocean model to study the sensitivity of tropical Pacific Ocean to variations in precipitation induced freshwate r fluxes. Variations in these fluxes arise from natural causes on all time scales. In addition, estimates of these fluxes are uncertain because of dif ferences among measurement techniques. The model used is a quasi-isopycnal model, covering the Pacific from 40 degrees S to 40 degrees N. The surface forcing is constructed from observed wind stress, evaporation, precipitatio n, and sea surface temperature (SST) fields. The heat flux is produced with an iterative technique so as to maintain the model close to the observed c limatology, but with only a weak damping to that climatology. Climatologica l estimates of evaporation are combined with various estimates of precipita tion to determine the net surface freshwater flux. Results indicate that in creased freshwater input decreases salinity as expected, but increases temp eratures in the upper ocean. Using the freshwater flux estimated from the M icrowave Sounding Unit leads to a warming of up to 0.6 degrees C in the wes tern Pacific over a case with zero net freshwater flux. SST is sensitive to the discrepancies among different precipitation observations, with root-me an-square differences in SST on the order of 0.2-0.3 degrees C. The change in SST is more pronounced in the eastern Pacific, with difference of over 1 degrees C found among the various precipitation products. Interannual vari ation in precipitation during El Nino events leads to increased warming. Du ring the winter of 1982-83, freshwater flux accounts for about 0.4 degrees C (approximately 10-15% of the maximum warming) of the surface warming in t he central-eastern Pacific. Thus, the error of SST caused by the discrepanc ies in precipitation products is more than half of the SST anomaly produced by the interannual variability of observed precipitation. Further experime nts, in which freshwater flux anomalies are imposed in the western, central , and eastern Pacific, show that the influence of net freshwater flux is al so spatially dependent. The imposition of freshwater flux in the far wester n Pacific leads to a trapping of salinity anomalies to the surface layers n ear the equator. An identical flux imposed in the central Pacific produces deeper and off-equatorial salinity anomalies. The contrast between these tw o simulations is consistent with other simulations of the western Pacific b arrier layer formation.