Vertical Wind Shear and Ocean Heat Content as Environmental Modulators of Western North Pacific Tropical Cyclone Intensification and Decay

Abstract

A general framework in which Ocean Heat Content (OHC) may modulate tropical cyclone intensification and decay in conjunction with Vertical Wind Shear (VWS) as the primary environmental control is formulated in terms of a two-dimensional phase diagram. Case studies of the life cycles of three tropical cyclones during the Impact of Typhoons on the Ocean in the Pacific (ITOP) field experiment during August-October 2010 are selected as examples owing to numerous Air-deployed eXpendable BathyThermographs (AXBTs) and special atmospheric observations. Vertical wind shears calculated from European Center for Medium-range Weather Forecasting analyses within a 3°-5° lat. annulus around the center may not properly represent the VWS in nonuniform, asymmetric, and time-varying conditions in the western North Pacific. As expected, VWS is the primary environmental control during the formation and early intensification stage over regions of large OHC, and during the decay phase over regions of small OHC. The challenging intensity forecasting problems are in the intermediate conditions of an intense tropical cyclone moving slowly over a region of low to moderate OHC when the negative feedback from the ocean may or may not lead to a decrease in intensity.