TROPICAL CYCLONE GENESIS FACTORS DURING THE HOLOCENE (Invited Presentation)

Proxies of past storm activity have revealed a fascinating picture of variability over recent millennia. The climate of the preindustrial-era Holocene was forced by natural variations in solar activity, the periods and dominant phases of ENSO, volcanic activity, and, on longer time intervals, variations in Earth’s orbit. To provide context for the proxies of storm activity, it is important to understand whether (and if so, how) changes in forcing can affect tropical cyclone genesis. We analyzed how the large-scale climate changes in paleoclimate simulations affect factors known to influence genesis of tropical cyclones. While there are several impressive (and counterintuitive) changes in genesis factors in simulations of very different climates such as the Last Glacial Maximum and hothouse periods in the early Cenozoic, there is a broad stability for many of these factors across the Holocene. Nevertheless, some factors undergo subtle changes that have the potential to affect interpretation of proxies; we present our analysis of these findings here.

We find that the distribution of top-of-the-atmosphere radiation during the Mid-Holocene 6000 years ago (6ka) altered the Northern Hemisphere seasonal cycle of the thermodynamic limit on tropical cyclone intensity. The shift in radiation also increases the difference between middle tropospheric and boundary layer entropy, a parameter that has been related to the incubation period required for genesis. The Southern Hemisphere, which receives more solar radiation during its storm season today than it did 6ka, displays slightly more favorable thermodynamic properties during the Mid-Holocene than in the pre-industrial era control. Surface temperatures over the ocean in both hemispheres respond more slowly than do those in upper levels to radiation anomalies, altering the thermal stability.

Volcanism produces a sharp but transient temperature response in a simulation of the last millennium that strongly reduces potential intensity during the seasons immediately following a major eruption. The differential vertical temperature response is key: temperatures in the lower and middle troposphere cool while those near the tropopause rise. Aside from these deviations, there is no substantial variation in thermodynamic properties over the 1000-year simulation.