Tropical Cyclone Activity in an Aquaplanet General Circulation Model

Abstract

Understanding the influence of large-scale environmental factors on tropical cyclone (TC) formation is a task of great scientific and societal importance. While much has been learned about the genesis of tropical cyclones over recent decades, a dynamical theory linking the annual global frequency of TCs to the large-scale environment remains elusive. This investigation is motivated by previous studies that have shown that future projections of TC activity will depend critically on the reliability of projections of the change in the pattern of sea surface temperature (SST).

Atmospheric general circulation models are now routinely run at sufficient resolution to enable the internal generation of TC-like disturbances within the simulations and can thus provide an important tool for investigating the behavior of TCs in different climates. This study employs an aquaplanet configuration of the High-Resolution Atmospheric Model (HiRAM) developed at NOAA's Geophysical Fluid Dynamics Laboratory. Through systematically modifying HiRAM’s lower boundary condition, this research has explored the impact of changes in the meridional and zonal pattern of SST on the simulated TC activity.

Within these simulations we find that TCs prefer to form near 15N on the poleward flank of the Intertropical Convergence Zone (ITCZ). The underlying meridional SST pattern influences the position of the ITCZ, which in turn influences the frequency of TC genesis: the number of TCs increase as the mean ITCZ latitude shifts poleward. The changes in frequency can be related to both the upward mass flux and ambient vorticity of the large-scale environment.

The evolution of TC along-track intensity was also explored. The intensity of TCs decrease as the separation in the mean latitude of the maximum SST and ITCZ decreases. The average lifetime maximum intensity of TCs is positively related to the period of intensification and the latitude at which the maximum intensity is acquired.

With the introduction of a zonally asymmetric SST perturbation the TCs preferentially form over the regions of relatively warm SST. As the mean zonal gradient of SST increases the TC occurrence density increases over the warmest regions, however the total global frequency remains relatively insensitive to the zonal SST gradient.