The Spatial and Temporal Impact of an Idealized SST Gradient on Simulations of an Idealized Tropical Cyclone

Abstract:

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 TCs 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. The authors seek to understand whether the conclusions made in previous works can be applied in the case of a TC.

To address this, the effects of an idealized sea surface temperature (SST) gradient on a simulated TC are investigated using the Weather Research and Forecasting (WRF) model at 2km grid spacing. An idealized analytic vortex with no accompanying background wind is used to simulate the TC in one run with a horizontal north-south SST gradient and in a second run with the SST gradient flipped south-north. A third model run with constant 28^oC SST everywhere is also conducted. Model runs with the SST gradient show asymmetries in convection and structure develop quickly before and after the cyclone develops. In these runs the cyclone drifts one direction depending on the location of colder SST. This is consistent with a thermal wind which forms due to the SST gradient. The SST gradient runs develop the storm faster and with higher peak intensity than in the constant SST run.