Rain‐formed barrier layer of the western equatorial Pacific warm pool: A case study

Abstract

During the intensive observation period of the Tropical Ocean ‐ Global Atmosphere / Coupled Ocean ‐ Atmosphere Response Experiment, a rain‐formed barrier layer was observed in a R/V 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 mixing was active during the survey. The isothermal layer deepened from 25 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 salinity change of 0.1–0.15 practical salinity units, but it had a temperature change (∼0.1°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 ‐ layer is defined as the vertical distance difference between a halocline and a thermocline, in which there is very little temperature change but a large salinity change. Thus the observations suggest that tropical rainfall has a greater impact on salinity than temperature. The descending low‐salinity water is slightly warmer during daytime and slightly colder during nighttime, reflecting a link with the diurnal cycle of solar radiation. I propose a mechanism for the formation of a rain‐induced barrier layer. When the temperature in the descending dilution water has been mixed to the same level as the environmental temperature, the salinity is mixed more slowly, so that a salinity difference exists between the dilution water and environmental water. Thus more time is required to reduce this rain‐induced salinity difference compared to the temperature, which is the cause of the barrier layer. A one‐dimensional, time‐dependent, rain‐fonned, barrier layer model is thus developed through integration of a set of one‐dimensional equations of temperature, salinity, and turbulent mechanical energy. The model shows that a rain‐formed barrier ‐ layer is sensitive to many atmospheric inputs, such as evaporation minus precipitation, surface heat flux and wind 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.