Tropical Cyclone–Induced Upper-Ocean Mixing and Climate: Application to Equable Climates

Abstract

Tropical cyclones instigate an isolated blast of vigorous mixing in the upper tropical oceans, stirring warm surface water with cooler water in the thermocline. Previous work suggests that the frequency, intensity, and lifetime of these storms may be functions of the climate state, implying that transient tropical mixing could have been stronger during warmer equable climates with higher concentrations of carbon dioxide. Stronger mixing of the tropical oceans can force the oceans' meridional heat flux to increase, cooling tropical latitudes while warming higher ones. This response differs significantly from previous modeling studies of equable climates that used static mixing; coupling mixing to climate changes the dynamic response. A parameterization of mixing from tropical cyclones is developed, and including it leads to a cooling of tropical oceans and a warming of subtropical waters compared with control cases with fixed mixing. The mixing penetration depth regulates the magnitude of the response.

Observational evidence supports the role of tropical cyclones in regulating climate

“…Whether the extra ocean heat induced by tropical cyclones remains in the tropical oceans, gets carried poleward by ocean currents, or is released back to the atmosphere in the following winter (20), these results provide definitive and unique evidence that tropical cyclones are important contributors to the climate system with implications on the dynamics of the global ocean (10)(11)(12)(16)(17)(18)(19) and atmosphere (13,19). Considering these effects-as well as possible feedbacks-in our theoretical and numerical models of Earth's climate system may improve our understanding of past climate changes and reduce uncertainty in future climate projections.…”

“…Because ocean temperature is closely linked to the maximum potential intensity of a tropical cyclone, the fraction of intense events could increase further, causing additional ocean warming and thus closing the feedback loop. The existence of such a feedback is somewhat speculative, but it has been proposed as an explanation for sustaining past warm climate conditions (17,18) and warrants Mei et al significantly advance our understanding of the magnitude of tropical cyclone heat pumping.…”

“…Because tropical cyclone activity is sensitive to ocean temperature, feedbacks may exist in the climate system between tropical cyclones, ocean mixing, and ocean heat content that are not captured in the current generation of climate models. This feedback may be important for understanding past and potential future changes in the Earth's climate system (17)(18)(19).…”

Enhancement of subduction/obduction due to hurricane-induced mixed layer deepening

“…In addition, several recent studies have focused on the important role of TC stirring in the global heat budgets of past climates (Emanuel, 2002;Korty et al, 2008;Jansen and Ferrari, 2009;Fedorov et al, 2010;Jansen et al, 2010), the ocean-atmosphere CO2 exchange (Bates et al, 1998) and its biological effect (Babin et al, 2004). However, the impact of subduction/obduction enhancement induced by tropical cyclones to the oceanic general circulation remains unclear at the present time.…”

Climate modelling and deep-time climate change

“…Tropical cyclone frequency and intensity are predicted to increase in a warmer world; by increasing vertical mixing in the ocean, this could lead to stronger poleward heat transport (Emanuel, 2001(Emanuel, , 2002. However, direct implementation of a temperature-dependent ocean mixing rate in a climate model, designed to mimic the eff ect of tropical cyclones, failed to show much increase in heat transport to the high latitudes (Korty et al, 2008 ).…”