PNNL

Abstract

Summertime tropical upper tropospheric troughs (TUTTs) provide a unified framework to better understand how extratropical and tropical forcings jointly modulate regional tropical cyclone (TC) activity. In this study, we examine future changes in TUTTs and their implications for TC activity. Projections from 45 CMIP6 models suggest a contraction of the Pacific TUTT and a westward expansion of the Atlantic TUTT as the climate warms. Correspondingly, future changes in environment-based TC indices indicate that the large-scale conditions will become more favorable for TC genesis and intensification over the central North Pacific, but less favorable over the tropical North Atlantic and Gulf of Mexico. Utilizing a TC-permitting large-ensemble dataset (i.e., d4PDF) that reasonably captures the observed TUTT–TC relationships, we further confirm the impacts of a weakened Pacific TUTT and an intensified Atlantic TUTT on future TC activity. Nevertheless, CMIP6 projections of the TUTT show large inter-model spread, and d4PDF suggests that uncertainty in TUTT projections is linked to substantially different levels of TC risk over the central-to-eastern Pacific and the North Atlantic. Singular Value Decomposition further reveals that CMIP6 models projecting weaker Pacific and Atlantic TUTTs are characterized by stronger sea surface temperature warming in the Northern Hemisphere extratropics and tropical North Atlantic, but muted surface warming in the central-to-eastern equatorial Pacific and Southern Hemisphere. These models also tend to simulate weaker aerosol radiative forcing in the Northern Hemisphere, suggesting that changes in TUTTs and regional TC risk may be tied to uncertainties in modeled aerosol radiative effects.