A Toy Model for the Global Annual Number of Tropical Cyclones

A Toy Model for the Global Annual Number of Tropical Cyclones

Yufeng Zhou¹, Yanluan Lin¹

¹Ministry of Education Key Laboratory for Earth System Modeling, Department of Earth System Science, Tsinghua University, Beijing 100084, China

Abstract

The annual number of global tropical cyclones (TCs) has remained relatively stable at around 90 over the past 50 years, but the underlying mechanisms are not well understood. TCs are more likely to be generated in favorable environmental conditions (e.g., warm sea surface temperature, moist atmosphere). At the same time, TCs can make surrounding environmental conditions hostile (e.g., cooling the sea surface temperature), and thus limit the subsequent TC genesis in a short term. In this study, a toy model is proposed to explore the potential factors that constrain global TC numbers by focusing on TC-environment interactions and the recovery process. The recovery time and main development region (MDR) are introduced in the model to represent the environmental fields and constrain TC genesis and activity. A series of sensitivity tests indicated that a longer recovery time, a larger influencing radius, and slower TC translation speed all lead to a significant reduction in TC numbers. The study suggests that the recovery time of TC-environment interactions may be a potential factor that limits the frequency of TCs. For instance, the recovery time of sea surface cooling effect is about 2-3 weeks, possibly constraining the annual TC number in the current climate. The toy model also provides new insights for future projections, and suggests that the reduction of TC numbers may be related to a slowdown in the recovery process of TC-environmental interactions. In addition, we acknowledge that TC-environment interactions not only manifest in the observed sea surface cooling effect (TC cold wake), but also in the large-scale environmental conditions, such as relative humidity. Therefore, the recovery time of TC-environment interactions, which includes both the ocean and atmosphere aspects, may be a potential factor limiting the frequency of global TCs. Overall, this study provides insights into the possible mechanisms and factors that influence global TC numbers, and contributes to our understanding of the potential impacts of climate change on TCs.

Key words: Tropical cyclones, TC-environment interactions, Cold wake, Toy model