SLS - Raphael Rousseau-Rizzi (MIT)
Title: Potential Intensity and Atlantic Hurricane Activity
Potential intensity (PI), a thermodynamic bound on tropical cyclone (TC) maximum wind speeds, is an important predictor of both the intensity of a single event and of the collective activity of all TCs in a basin. Hence, understanding when PI is applicable is important for improving intensity forecasts, and understanding the causes of PI variability is important to help predict future variations in TC activity. This talk first validates PI as a bound for individual simulated TCs by addressing recent controversies arguing that the PI assumptions of steady-state and inviscid free troposphere make it inapplicable. It is shown that, at peak intensity, TCs are in steady-state for the purpose of comparisons with PI, and that the PI bound is valid in simulations with low parameterized mixing. Then, a linear model for interannual to multidecadal PI variations is introduced, which captures up to 95% of PI variance and allows one to identify global and local contributions to PI variations. The model shows that tropical North-Atlantic PI variations have been dominated by local influences over the last 40 years, and is useful for identifying the causes of past PI variations, and by extension of TC activity variations. The final part of the talk introduces an ongoing study of the causes of the hurricane drought of the 1970s and 1980s, a large and poorly explained decrease in Atlantic TC activity. The mechanism proposed here to explain this drought is that concurrent hemispherically asymmetric anthropogenic sulfate emissions caused a drying of the Sahel region and enhanced the emissions of dust from the Sahara and the Sahel. The radiative forcing by the dust lofted over the ocean then caused the observed decrease in TC activity.
About this Series
The Atmosphere, Ocean and Climate Sack Lunch Seminar Series is an informal seminar series within PAOC that focuses on more specialized topics than the PAOC Colloquium. Seminar topics include all research concerning the science of atmosphere, ocean and climate. The seminars usually take place on Wednesdays from 12-1pm in 54-915. The presentations are either given by an invited speaker or by a member of PAOC and can focus on new research or discussion of a paper of particular interest.