Formation process of intermediate water in baroclinic current under cooling


  • Yutaka Yoshikawa,

  • Kazunori Akitomo,

  • Toshiyuki Awaji


The formation process of intermediate water in baroclinic current under cooling is investigated using a nonhydrostatic numerical model. After baroclinic instability develops into finite amplitude in a short time, strong downdrafts with a horizontal scale of 1 km are generated near the density front and subduct surface water to depths (∼400 m) along isopycnals. As a result, patches of ventilated water of 10∼20 km horizontal scale with anticyclonic circulation are formed at intermediate depths. Combined effects of baroclinic instability and convection are key dynamics for these phenomena. Convection acts as an initiator for baroclinic instability at the onset and accelerates its subsequent growth by reducing stratification. Developed baroclinic wave forms an intense density front, and downdraft along isopycnals is generated through the frontogenetic process. Density change due to convection intensifies this frontal downdraft by strengthening the geostrophic forcing (tendency to destroy the geostrophic balance) and by reducing potential vorticity (static stability). Further, symmetric instability induced by the density change due to convection and intensified by the frontogenetic process drives slantwise convection, which in turn, enhances the frontal downdraft. Consequent downward velocity becomes >20 times as large as that of the frontal downdraft without convection (cooling) and twice larger than that of pure convection. Since the intensified frontal downdraft moves its position with time and induces divergent flow 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 formation process of intermediate water in this context is to enhance the frontal 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.