Observational evidence supports the role of tropical cyclones in regulating climate.

Tropical cyclones are relatively rare weather events, yet they consistently rank among the world’s deadliest and costliest natural hazards. In the United States, tropical cyclones account for nearly $10 billion annually in economic damages (1), and damages from Hurricane Sandy (2012) are estimated to be over $50 billion. Understanding the role of these events within Earth’s climate system is of paramount importance to improving climate change projections and for constructing sound hazard assessments and adaptation strategies (2). The relationship between tropical cyclone activity and global warming is a controversial topic. Observational evidence suggests variability in sea surface temperature is linked to changes in tropical cyclone activity (3–5). However, questions about the effect of global warming on tropical cyclones are not fully answered, and projections of the response of tropical cyclones to future climate change have significant uncertainties (6–9). Mounting evidence indicates that tropical cyclones are not passive players in Earth’s climate system. Rather, the storms actively contribute to the dynamics of the global ocean (10–12) and atmosphere (13). 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. Changes in sea surface height are closely linked to changes in ocean heat content (e.g., thermal expansion), thus these results enable direct estimates of the vertically integrated changes in ocean temperatures caused by tropical cyclones. Results shed much-needed light on the interactions between tropical cyclones, ocean mixing, and ocean heat uptake. The ocean plays a central role in determining the Earth’s climate because it can accumulate enormous amounts of heat from the atmosphere and release it over long time periods. Mechanical ocean mixing provides an important source of energy that controls global circulation patterns and rates of ocean heat uptake (15). We currently lack a complete description of the physical processes responsible for this mixing. Identifying sources of mixing, as well as constraining mixing rates, is a major scientific challenge. Although tropical cyclones are highly localized and relatively short-lived weather events, they produce vigorous bursts of ocean mixing. This mixing can disrupt the usually well-stratified tropical ocean conditions, redistributing ocean heat vertically in regions affected by the storms. The signature response of tropical cyclone-induced mixing is surface cooling along the storm paths, accompanied by warming within the ocean interior (Fig. 1). The vertical and horizontal extent of the mixing depends on the intensity, size, and translational speed of the storm, as well as the background ocean conditions (16). Past studies have indicated that, when aggregated globally, tropical cyclones significantly contribute to global ocean mixing and energy budgets. The implications are that tropical cyclones can potentially contribute to ocean heat uptake and transport by pumping heat into the ocean interior (10–12). 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 (17–19). Previous efforts to quantify ocean heat pumping by tropical cyclones were limited by A B