Upper Ocean Structure: Responses to Strong Atmospheric Forcing Events

Abstract

Over the past decade, progress in measurement and modeling of the oceanic response to strong forcing events such as tropical cyclones (TCs) is given with a focus on western parts of the North Atlantic and North Pacific Ocean basins. These regimes are known for their energetic upper ocean currents that drive the basin circulation. In terms of the upper response, considerable attention has focused on the cold wake structure beginning just in back of the eye-wall of TCs. In these regimes, sea surface cooling levels of more than 3°C often result in the negative feedback primarily due to vigorous shear-induced mixing at the base of the ocean mixed layer through wind-driven currents. This effect has been modeled extensively often assuming that the basic state (or pre-TC condition) is at rest. However, the background oceanic state, and in particular in the western parts of the oceanic basins in the Northern Hemisphere the flows are energetic. The ensuing upper ocean thermal response is considerably weaker (less surface cooling) to strong TC forcing where the warm layers extend to greater depth. That is, strong currents transport warm water poleward (Loop Current, Florida Current, Gulf Stream, and the Kuroshio). In these regimes, wind-driven shears are arrested by these background currents which does not allow strong sea surface cooling through mixing processes. In addition, oceanic heat content levels remain relatively high during TC passage in these regimes. Thus, a key part of this interaction is the ocean's momentum balance (e.g., currents and shears). This less-negative feedback implies that there is a more sustained ocean-to-atmosphere heat and moisture (enthalpy) flux to the TC and represents an important mechanism for observed deepening of severe TCs. Recently developed assimilation schemes for models and new measurement tools for oceanic observations are briefly discussed herein where the focus is on future avenues for basic and applied research in improving our understanding of these coupled responses during TC passage.