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.