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As a major feature of tropical cyclones (TCs), controlling factors of TC outer size or size scaling remains a fundamental scientific question. The Rossby deformation radius and a natural extent associated with potential intensity have been proposed as two scalings of TC size. But neither of them satisfactorily captures the sensitivity of TC size to sea surface temperature (SST) in idealized f‐plane simulations. Inspired by the studies of the Hadley circulation, here we proposed a new TC scaling based on an energetic diagnostic scaling. TC size is primarily a ratio of the secondary circulation strength to subsidence velocity, further determined by the total atmospheric heating in the ascending area, the gross moist stability, the diabatic cooling, and the dry static stability. The former two is based on the moist energetic budget applied to the whole storm structure, while the latter two is based on the dry thermodynamic budget applied to the subsidence areas. The new scaling well captured the sensitivity of TC size to SST in idealized f‐plane simulations, partly resulted from expanded ascending area, increased surface moisture deficit, and weakened subsidence with increased SST. Plain Language Summary: Among the basic quantities of tropical cyclones (TCs), size is probably the most difficult to understand and predict. Due to lack of theories, our understanding of the controlling factors of TC size is rather limited. As a result, we have low confidence about the future change of TC size with warming. Idealized simulations have been used in recent years to study the environmental constraints of TC size, but the conventional scalings cannot capture the strong sensitivity of TC size to sea surface temperature (SST). In this study, we propose an energetic scaling that well predicts the simulated TC size variation with SST. We found that the secondary circulation strength and subsidence velocity essentially constrain the overall extent of TC. In addition, the framework also provides a new perspective to understand the limitation of conventional scalings. Key Points: A new energetic scaling successfully captures the outer size of tropical cyclone (TC) in f‐plane simulationsThe strong sensitivity of TC size to sea surface temperature (SST) depends on ascending area, surface moisture deficit, and subsidence velocityConventional TC size scalings underestimate or neglect the increases in inner‐core heating, which weaken their sensitivities to SST [ABSTRACT FROM AUTHOR]

Tropical cyclones (TCs) are an important source of turbulent mixing for the upper ocean at low latitudes, causing sea surface cooling and subsurface warming. A new estimate of annually accumulated sea surface cooling and upper ocean diapycnal diffusivity induced by TCs is obtained by using quantified cold wake sizes, which were largely ignored by previous studies. Both the annually accumulated tropical cyclone‐induced sea surface cooling and upper ocean diffusivity on a global scale show a significant decreasing trend over the period of 1982–2016, at a rate of −0.09 ± 0.03 °C/decade and −0.03 ± 0.01 cm2/s/decade respectively. The strengthening of ocean stratification with global warming contributes to the decrease of sea surface cooling and mixing, while the changes of tropical cyclone characteristics (such as translation speed, intensity, number, lifetime and size) contribute differently in various ocean basins. Plain Language Summary: The strong winds of tropical cyclones (TCs) greatly enhance upper ocean mixing. Cold water is mixed upwards, leaving cold wakes at the sea surface. Previous studies used cooling of the ocean surface induced by TCs to infer the upper ocean mixing, but some of their assumptions could lead to biases. In this study, based on quantified cold wakes, a new estimate of the sea surface cooling and upper ocean mixing induced by TCs is proposed. Based on observations, for the first time, we noted that the accumulated sea surface cooling and upper ocean mixing induced by global TCs have a significant decreasing trend over 1982–2016. This is primarily related to a more stable upper ocean and the decrease of tropical cyclone cold wake sizes with global warming. Key Points: With quantified cold wake size, a new estimate of sea surface cooling and upper ocean mixing induced by tropical cyclones is providedGlobal tropical cyclone‐induced annually accumulated ocean cooling and mixing have been decreasing in recent decadesThe increased ocean stratification and the decreased cold wake sizes contribute to the decrease of accumulated ocean cooling and mixing [ABSTRACT FROM AUTHOR]

A recent study noted reduced rainfall and cloud fraction over cold wakes induced by tropical cyclones, but a quantification of top‐of‐atmosphere (TOA) radiation change due to these cold wakes has not been attempted. Based on global TOA radiative flux observations, we show that TOA shortwave and longwave radiations increase by 0.76 W m−2 (0.2%) and 0.74 W m−2 (0.3%) over the cold wake area relative to local climatology, respectively. Due to the cancelation between the shortwave and longwave components, daily average TOA net radiation is only marginally modulated by cold wakes, but stands out in the day and night time average. In addition, the seasonal basin‐wide regulation of TOA net radiation by cold wakes can be up to 1.0 W m−2, locally comparable to the magnitude of radiative forcing due to man‐made aerosols. The regional impact of cold wakes on TOA radiations is therefore highly relevant and potentially important. Plain Language Summary: The upper ocean stirred by tropical cyclones leaves a cold wake at the surface. Though the impact of cold wakes on ocean heat pumping, heat transport, and storm intensity were extensively explored for years, only recently have their modulation effects on rainfall and clouds been demonstrated. A further question is whether cold wakes significantly modify the local and seasonal basin‐wide energy budget. Here, we show that in observations, the top‐of‐atmosphere (TOA) shortwave and longwave radiations can be modified over cold wakes by ∼0.7 W m−2, while the daily‐averaged net radiation is unremarkable due to the cancelation between the shortwave and longwave components. However, the modulation effect of cold wakes on the local net radiation in the daytime and nighttime is much larger than the daily‐averaged radiation. Moreover, the basin‐wide radiative effect is one order larger than the local effect. The regulation of TOA radiation by cold wakes, particularly for the regional impacts and diurnal cycles, is therefore highly suggested for future assessment of Earth's energy budget. Key Points: Local average top‐of‐atmosphere (TOA) longwave and shortwave radiations can be modulated by tropical cyclone cold wakes by ∼0.7 W m−2TOA net radiation change over cold wakes is small due to the cancelation between longwave and shortwave radiationsImpact of cold wakes on TOA net radiation differs over different basins due to different climatological mean radiations [ABSTRACT FROM AUTHOR]

Mixing of the upper ocean by the wind field associated with tropical cyclones (TCs) creates observable cold wakes in sea surface temperature and may potentially influence ocean heat uptake. The relationship between cold wake size and storm size, however, has yet to be explored. Here we apply two objective methods to observed daily sea surface temperature data to quantify the size of TC‐induced cold wakes. The obtained cold wake sizes agree well with the TC sizes estimated from the QuikSCAT‐R wind field database with a correlation coefficient of 0.51 and 0.59, respectively. Furthermore, our new estimate of the total cooling that incorporates the variations in the cold wake size provides improved estimates of TC power dissipation and TC‐induced ocean heat uptake. This study thus highlights the importance of cold wake size in evaluating the climatological effects of TCs. Plain Language Summary: The wind field of a tropical cyclone can mix warm water downward, leaving a cold wake at the surface. This effect may influence ocean heat uptake within the climate system. We derive a novel oceanic metric of tropical cyclone size based on its induced cold wake using sea surface temperature data for a period from 2002 to 2011. The cold wake size agrees well with the wind field size determined from QuikSCAT surface winds. Furthermore, with the cold wake size incorporated, total cooling provides better quantifications of tropical cyclone power dissipation and cyclone‐induced ocean heat uptake. More reliable estimates of cyclone power dissipation and sea surface cooling may be useful for both understanding and forecasting impacts of tropical cyclones on the climate system. Key Points: The size of the tropical cyclone wind field correlates reasonably well with the size of its mixing‐induced cold wakeWith the cold wake size incorporated, TC‐induced sea surface total cooling has a better correspondence with TC power dissipationCompared to sea surface cooling with a fixed cold wake area, the total cooling provides an improved estimate of TC‐induced ocean heat uptake [ABSTRACT FROM AUTHOR]