Extreme subseasonal tropical air-sea interactions and their relation to ocean thermal stratification

This thesis is concerned with extreme, rapid timescale tropical air-sea interactions and the influence of large-scale oceanic conditions on these interactions. The focus is on two types of extreme events: equatorial Indian Ocean cooling events and tropical cyclones. Cooling events occur on timescales of a few days to several weeks, in which atmospheric forcing causes Sea Surface Temperature (SST) cooling in the range of 1--5K, in both observational and coupled climate models. Cooling events are driven by changes in air-sea enthalpy fluxes and Ekman upwelling. Because the cooling due to Ekman upwelling depends on thermocline depth, large-scale oceanic conditions influence SST cooling. La Nina and negative Indian Ocean Dipole conditions are conducive to a shallower southwest equatorial thermocline, resulting in greater intraseasonal SST cooling during these interannual events; El Nino and positive Indian Ocean Dipole conditions lead to a deeper thermocline and reduced SST cooling. Results indicate that cooling events are related to the eastward propagation of convective patterns that resemble the Madden-Julian Oscillation.;For tropical cyclones, the response of intensity to cyclone-induced SST cooling was explored over 10-years of observational data. For slow moving (V/ f < 100km) tropical cyclones, it was found that the SST cooling response increases along with storm intensity from category 0--2 on the Saffir-Simpson scale. However, from category 2--5 the magnitude of SST cooling decreases. This result confirms model predictions indicating a prominent role for oceanic feedback controlling tropical cyclone intensity. Thus, only storms that develop in regions containing deep mixed layer and thermocline can achieve high intensity, and entrainment cooling is weaker for these storms.;The SST-intensity response in observations was compared to the GFDL Hurricane Forecast Model (GHM) for the periods 2005 and 2006--2009. The GHM was modified in 2006 to include a representation of warm core eddies and the loop current in the Gulf of Mexico, and a new parameterization of the drag coefficient. The 2006--2009 period contains more intense hurricanes (category 4 and 5) and the non-monotonic nature of the SST-intensity response is more similar to observations than in 2005. This result was attributed to weaker ocean thermal stratification in the Gulf of Mexico allowing for greater storm intensification. A very simple Conceptual Hurricane Intensity Model consisting of two coupled equations was formulated to account for the non-monotonic SST-intensity response.;Finally, dynamical oceanic changes in the tropical North Atlantic under climate change were examined across a range of climate models. Given the sensitivity of hurricane intensity to stratification, large-scale ocean changes must be understood in order to make robust intensity predictions. The models' mean state contained significant biases, and it is not clear whether these mean state biases are reduced in models with higher resolution. However, climate change projections indicate a robust subsurface warming response in the tropical North Atlantic that could impact hurricane intensity. The non-local air-sea processes that account for water mass biases were highlighted as an area for future research.