Analysis of the dynamic mechanisms of upwelling in deep ocean water
caused by typhoons
Abstract
A typhoon (hurricane) is a very strong local disturbance that can affect
ocean water as deep as 1000 m. According to observations and numerical
simulations, the decrease in ocean temperature is believed to be caused
by inertial pumping, entrainment, upwelling, and a pressure-gradient
force; however, these assumptions are mostly based on modeling and do
not have a clear dynamic mechanism. Therefore, in this study, the
dynamic mechanism of ocean water in a highly-idealized ocean and wind
field was calculated. Within the maximum wind range, upwelling in the
ocean surface is caused as per the classical Ekman layer theory. Then, a
pressure-gradient force drives a cyclo-geostrophic current. Because it
provides centripetal force, the pressure-gradient force decreases with
increasing depth. These different currents generate friction that is
similar to bottom Ekman pumping, leading to upwelling. In the actual
ocean, the pressure-gradient force is not only balanced by centripetal
force but also baroclinic force. When these forces are balanced, the
pressure-gradient force disappears, and the dynamic motion of the ocean
water stops. Thus, upwelling no longer occurs. The depth at which this
balance occurs is considered to be the maximum depth that a typhoon can
impact. Therefore, the pressure-gradient force, caused by typhoons and
then offset by centripetal and baroclinic forces, is the original
dynamic of upwelling in deep ocean water.