RAIN-FORMED BARRIER LAYER OF THE WESTERN EQUATORIAL PACIFIC WARM POOL- A CASE-STUDY

During the intensive observation period of the Tropical Ocean-Global A tmosphere/Coupled Ocean-Atmosphere Response Experiment, a rain-formed barrier layer was observed in a RN Franklin survey. Currents measured with an acoustic Doppler current profiler were mainly southward. Winds were predominantly westerly with an average speed of 15 knots (7.5 m s(-1)), but they occasionally reached 25-30 knots (12.5 - 15 m s(-1)), during strong tropical storms that yielded 20-80 mm of rain. Wind mix ing was active during the survey. The isothermal layer deepened from 2 5 to 70 m in 5 days. Sea surface water diluted by rainfall penetrated deep under wind forcing through turbulent mixing and entrainment. The diluted water was strongly stratified in salinity with a vertical sali nity change of 0.1-0.15 practical salinity units, but it had a tempera ture change (similar to 0.1 degrees C) close to that of the isothermal layer. As a result, the halocline was shallower than the thermocline and a 10-m-thick barrier layer existed between the two. A barrier - la yer is defined as the vertical distance difference between a halocline and a thermocline, in which there is very little temperature change b ut a large salinity change. Thus the observations suggest that tropica l rainfall has a greater impact on salinity than temperature. The desc ending low-salinity water is slightly warmer during daytime and slight ly colder during nighttime, reflecting a link with the diurnal cycle o f solar radiation. I propose a mechanism for the formation of a rain-i nduced barrier layer. When the temperature in the descending dilution water has been mixed to the same level as the environmental temperatur e, the salinity is mixed more slowly, so that a salinity difference ex ists between the dilution water and environmental water. Thus more tim e is required to reduce this rain-induced salinity difference compared to the temperature, which is the cause of the barrier layer. A one-di mensional, time-dependent, rain-formed, barrier layer model is thus de veloped through integration of a set of one-dimensional equations of t emperature, salinity, and turbulent mechanical energy. The model shows that a rain-formed barrier-layer is sensitive to many atmospheric inp uts, such as evaporation minus precipitation, surface heat flux and wi nd forcing at the sea surface. The model proves that with both a small amount of warming but strong freshening and a small amount of cooling but strong freshening, a rain-formed barrier layer can be produced.