Discrete precipitation effects on seasonal mixed layer dynamics in the North Pacific Ocean.

Abstract

This study was conducted to examine the effects of discrete precipitation events on the short-term and seasonal evolution of ocean mixed layer temperature and salinity structure. This study was located at Ocean Station "P" (50°N, 145°W) in the Northeast Pacific Ocean. Two numerical modeling experiments were performed. The first was to simulate the response of the mixed layer to hypothetical discrete (isolated) precipitation events. This experiment showed that the effect of a single discrete rain event can van' with season, with the mixed layer depth (MLD) at onset of the rain event, and with the strength of wind stress forcing. A single rain event can have lasting effects on mixed layer depth and temperature for up to 55 days after the event, depending upon the season. The second experiment simulated quasi-realistic "complex" precipitation forcing, with a realistic distribution of synoptic events over a 13-month period. For this experiment, four different precipitation "intensities" were tested. The values of temperature, salinity, and MLD predicted by the model were compared with observed Conductivity Temperature Depth measurements and with the values predicted using constant precipitation forcing. In all experiments, the modelled MLD's approximated the observed MLD and temperature cycle. MLD's for all cases were too deep while temperature was estimated well in winter and was too cool in summer. Predicted salinity provided the greatest discrepancy between the modeled and the observed cycle. The 96 cm, 400-day quasi-realistic precipitation forced case best approximated the salinity observations though predicted salinity was fresher than observed in winter and saltier than observed in summer. Model results suggest that the amount of precipitation observed at Ocean Station "P" was too low to explain the observed and simulated ocean salinity and temperature structure for the year.