Sea surface height evidence for long-term warming effects of tropical cyclones on the ocean

Abstract

Tropical cyclones have been hypothesized to influence climate by pumping heat into the ocean, but a direct measure of this warming effect is still lacking. We quantified cyclone-induced ocean warming by directly monitoring the thermal expansion of water in the wake of cyclones, using satellite-based sea surface height data that provide a unique way of tracking the changes in ocean heat content on seasonal and longer timescales. We find that the long-term effect of cyclones is to warm the ocean at a rate of 0.32 ± 0.15 PW between 1993 and 2009, i.e., ∼23 times more efficiently per unit area than the background equatorial warming, making cyclones potentially important modulators of the climate by affecting heat transport in the ocean-atmosphere system. Furthermore, our analysis reveals that the rate of warming increases with cyclone intensity. This, together with a predicted shift in the distribution of cyclones toward higher intensities as climate warms, suggests the ocean will get even warmer, possibly leading to a positive feedback.S trong winds associated with tropical cyclones (TCs) increase air-sea heat fluxes, favoring the intensification of storms, and generate vigorous vertical mixing in the upper ocean, stirring warm surface waters with colder waters below (1-6). The wake produced by the passage of TCs is thus characterized by a surface cold anomaly and a subsurface warm anomaly (1-3, 6, 7). After the TC passage, the sea surface cold anomaly dissipates quickly (8-10), due in part to anomalous air-sea heat fluxes (9, 11), whereas the subsurface warm anomaly is believed to persist over a much longer period (12). This has led to the suggestion that the net longterm effect of TCs is to pump heat into the ocean (13-16). Such a flux of heat into the low-latitude ocean has been proposed to be an important modulator of local and remote climate (12,(17)(18)(19)(20)(21)(22).During the past decade or so, several studies have been devoted to estimating the magnitude of this heating effect, using sea surface temperature (SST) data (13-16). However, owing to a lack of subsurface temperature observations, these studies relied upon many assumptions that led to large and poorly quantified uncertainties (SI Appendix, SI Results). Furthermore, it is currently highly debated how much (if any) of the estimated warming survives beyond winter season when the deepening of the mixed layer cools the upper ocean. To avoid the ideological and methodological challenges inherent in the previous work, we take a more straightforward approach that was first proposed by Emanuel (13, 23) and quantify the TC-induced warming effect on the ocean by estimating the thermal expansion of water in the wake of Northern Hemisphere TCs, using satellite-derived sea surface height (SSH) data (24) together with tropical cyclone best-track data (25,26). Combining these two datasets allows us to track the SSH anomalies (SSHAs) around the TC-generated wake beyond the winter season and thus provides a clear picture of the temporal evolution of the TC-induc...

Observational evidence supports the role of tropical cyclones in regulating climate

“…The findings of Mei et al (14) are not without limitations. For example, the upperocean response to tropical cyclones is largely controlled by the background conditions (e.g., stratification), which can vary by region and time of year (16).…”

“…1A) to estimate changes in vertically integrated ocean heat content. The results of Mei et al (14) significantly advance our understanding of the magnitude of tropical cyclone heat pumping by using satellite altimetry to estimate ocean heat content changes in the regions affected by the storms. The authors' techniques can detect small increases and decreases in sea surface height, which are closely linked to the thermal expansion and contraction of ocean water, thus providing a direct estimation of the changes in vertically integrated ocean heat content caused by tropical cyclone mixing and surface fluxes.…”

“…Results provide fresh evidence using comprehensive observational products to support the importance of tropical cyclones on global ocean dynamics by contributing to ocean mixing, heat budgets, and transports. Furthermore, Mei et al (14) find that the magnitude of this heat pumping increases with tropical cyclone intensity.…”

“…Regardless of the limitations, Mei et al (14) support important links between tropical cyclones and 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).…”

“…Quantifying the role of tropical cyclones in the Earth system may improve our understanding of the nature of climate change and reduce uncertainty in climate projections. In PNAS, Mei et al (14) help to fill this gap by providing valuable information about the effect of tropical cyclones on long-term warming of the global ocean. The authors employ innovative techniques using satellite altimetry to quantify changes in sea surface height in storm-affected regions during the months following tropical cyclones.…”