Sensitivity of the Size of a Tropical Cyclone to Sea Surface Temperatures in its Outer Region

We investigated the sensitivity of the size of a tropical cyclone to warming or cooling sea surface temperatures (SST) in its outer region by simulating the sea surface temperature beyond a radius of 200 km from the tropical cyclone center. Sensitivity experiments showed that an increased sea surface temperature outside the core region of the tropical cyclone had a negative effect on its size. Warming in the outer region contributed to the local enhancement of the latent heat flux from the sea surface, which promoted the development of small-scale convection and warmed the lower and mid-troposphere. This warming altered the local pressure gradient force in the upper and lower troposphere in such a way that it weakened the secondary circulation of the tropical cyclone and led to suppression of the spiral rainbands outside the eyewall. Further analysis showed that the outward-propagating rainband structure favored an increase in the size of the tropical cyclone. The diabatic heat released by the rainbands induced an inflow at lower levels, facilitating expansion of the tropical cyclone. The greater the distance of the rainbands from the center of the tropical cyclone, given the same amplitude of diabatic heating, the stronger the forced inflow, resulting in a faster increase in the size of the tropical cyclone.