The sensitivity of tropical cyclone activity to off-equatorial thermal forcing in aquaplanet simulations

GFDL's High Resolution Atmospheric Model (HiRAM) is able to explicitly simulate tropical cyclones (TCs) and has previously been shown to well reproduce the geographical, seasonal, and interannual variation in observed TC activity when forced with observed monthly-mean sea surface temperature (SST) fields (Zhao et al., 2009). This motivates a sensitivity study that considers the response of simulated tropical cyclones to variations in the underlying SST pattern.

We present results from a suite of experiments in which HiRAM was employed in two different aquaplanet configurations: one set with a fixed, zonally-symmetric SST boundary condition and the other set with interactive SSTs determined by a slab ocean boundary condition. By manipulating the meridional SST profile directly, or through prescribing a cross-equatorial ocean heat flux in the slab ocean, we vary the latitude and strength of the intertropical convergence zone (ITCZ).

We compute the total accumulated cyclone energy (ACE) per year within each set of simulations as a measure of the tropical cyclone activity. This metric incorporates both the frequency of tropical cyclogenesis, as well as the duration and evolution of cyclone intensity along the tracks. We present the TC statistics with respect to the various SST profiles and the associated precipitation (a proxy for the ITCZ).

We further explore the genesis frequency with respect to several environmental parameters. In particular we consider the mean strength of vertical velocity |ω500hPa| and the absolute vorticity of the zonal mean flow (η = ζ + f) in the regions where storms are generated. The results show that the ITCZ plays an important role in TC genesis in this aquaplanet framework.