Wednesday, 12 February 2003: 4:45 PM
Precipitation and freshwater lens formation in the tropical western Pacific
The western equatorial Pacific contains the world’s highest open ocean sea surface temperatures (SST). Associated with these high SSTs is deep atmospheric convection, culminating in a maximum in global oceanic rainfall. As a result of the large amount of precipitation in the tropical western Pacific warm pool, substantial salinity stratification exists within the upper ocean. Analysis of upper ocean profile data obtained during the IOP of TOGA COARE has highlighted the formation of patches of fresher and colder surface water produced by localized rainfall. These areas of fresh surface water, termed “freshwater lenses”, are characterized by depressions in SST of 0.5 - 1.5 oC and in sea surface salinity of 0.2 - 2 psu. The freshwater can extend to depths well within the mixed layer; studies have found penetration depths of over 20 m. However it is difficult to determine accurately the relationship between precipitation and these freshwater lenses, due to the intermittency of rainfall and the difficulty of getting a true rainfall distribution on scales of tens of kilometers from ship-based observations. The spatial and temporal scale of the precipitation events may also play a role in freshwater lens formation. Precipitation influences the fluxes of heat, freshwater, and momentum at the ocean surface, resulting in a decrease in surface salinity, cooling of the surface, and changes in mixing associated with the buoyancy and momentum fluxes of rain. Different cloud types play different roles in the tropical western Pacific climate. The type of cloud associated with the precipitation and the timing of this precipitation with respect to the diurnal cycle may influence the impact of the precipitation on the tropical sea surface temperature and ocean mixed layer characteristics. Deep convective precipitation is typically associated with strong local surface winds, and the ensuing fresh water is easily mixed into the ocean below. By contrast, precipitation associated with shallow, isolated convection is generally accompanied by low wind speeds. This regime may result in the formation of a fresh water lens on the ocean surface that acts to stabilize the upper ocean and allows the sea surface temperature to warm radiatively. Thus the regime in which the precipitation occurs also influences the sea surface salinity and resulting temperature.
In order to determine the various effects of surface forcings on freshwater lens formation, a series of simulations have been performed using a tropical Pacific three-dimensional ocean model. The model has a realistic ocean mixed layer, and is forced with ECMWF model analyses and TRMM precipitation data. The period of 1998 – 1999 is simulated, and results in the western Pacific will be shown. The results of these simulations address the following issues: what initial conditions are most favorable for development and continuation of the freshwater lenses? How does the initial salinity structure affect the development and erosion of these lenses? Conditions favorable to continuation and enhancement of the localized freshwater lenses will be discussed, and an assessment of reduced temporal and spatial resolution of precipitation on these results is also described.