Formation process of intermediate water in baroclinic current under cooling

The formation process of intermediate water in baroclinic current under coo ling is investigated using a nonhydrostatic numerical model. After baroclin ic instability develops into finite amplitude in a short time, strong downd rafts with a horizontal scale of I km are generated near the density front and subduct surface water to depths (similar to 400 m) along isopycnals. As a result, patches of ventilated water of 10 similar to 20 km horizontal sc ale with anticyclonic circulation are formed at intermediate depths. Combin ed effects of baroclinic instability and convection are key dynamics for th ese phenomena. Convection acts as an initiator for baroclinic instability a t the onset and accelerates its subsequent growth by reducing stratificatio n. Developed baroclinic wave forms an intense density front, and downdraft along isopycnals is generated through the frontogenetic process. Density ch ange due to convection intensifies this frontal downdraft by strengthening the geostrophic forcing (tendency to destroy the geostrophic balance) and b y reducing potential vorticity (static stability). Further, symmetric insta bility induced by the density change due to convection and intensified by t he frontogenetic process drives slantwise convection, which in turn, enhanc es the frontal downdraft. Consequent downward velocity becomes > 20 times a s large as that of the frontal downdraft without convection (cooling) and t wice larger than that of pure convection. Since the intensified frontal dow ndraft moves its position with time and induces divergent how at depths, a patch of ventilated water with a horizontal scale much larger than that of the frontal downdraft is formed. The role of convection (cooling) in the fo rmation process of intermediate water in this context is to enhance the fro ntal downdraft rather than to deepen the mixed layer This scenario is quite different from the one realized when baroclinic instability and convection do not coexist.