The Effects of SST Gradients on Tropical Convective Systems and Implications for Tropical Cyclogenesis

It has long been known that generally the warmer the sea surface temperature (SST), the more possible tropical cyclone (TC) genesis is, assuming the atmosphere is supportive. The conventional wisdom has been that – apart from what the TC cools through upwelling -- one value of SST represents the state of the ocean surface in the region of the storm's inner circulation. With the advent of the satellite era and fine resolution SST datasets now becoming available, we know that in reality there are gradients of SST across which developing TCs move. The influence of those gradients on tropical convection and TC genesis is largely unknown at this time.

Previous studies have shown that SST gradients can significantly impact the overlying ocean surface winds leading to areas of enhanced convergence/divergence and Vorticity (Chelton et al. 2004; O'Neill et al. 2005, 2010). The magnitude of this effect approximately increases as the surface wind increases. Work by Minobe et al. (2008) concluded that a sharp SST Gradient, over the Gulf Stream for instance, could produce enough surface wind convergence to maintain a band of precipitation along the ocean front. An analysis of satellite derived SST data over the Atlantic show that it is not uncommon for SST gradients of 2 Co/200km or more to exist in the immediate environment of a Tropical System. The authors seek to understand whether the conclusions made in previous works can be applied in the case of a developing Tropical System and whether SST Gradients exist in the Tropical Atlantic to a degree that would influence the cyclogenesis process.

To address this, the effects of SST gradients on tropical cyclogenesis processes are investigated using model simulations of the Weather Research and Forecasting Model (WRF). WRF is run at cloud permitting scales (2km) for real cases of co-location between a tropical system and an SST gradient exceeding 2 Co/200km in the environment of the system. In subsequent runs to this control run, the SSTs are modified to give a smaller or larger SST Gradient with the same atmospheric conditions. All cases are chosen from Atlantic Hurricane Seasons between 2002-2011. The results are then analyzed in the framework of previous studies that have sought to model and understand tropical cyclogenesis using WRF (Nolan 2007; Fang and Zhang 2010).