Tropical Cyclones (TC)

Tropical Cyclones (TC)Tropical Cyclone

Tropical Cyclones

Analytical models that relate TC genesis to moist convective instabilities and/or numerical models with a detailed description of turbulence and microphysical processes in clouds are today available. Although these models are still far from providing solutions to forecasting problems, consensus for the main physical mechanisms relates to the heat release from water vapor condensation in intense moist convection over a thermally inhomogeneous surface of the ocean. In a rotating stratified atmosphere, this heat source induces a radial mass flux directed toward the center of a developing low level disturbance. From the law of conservation of absolute angular momentum, such a flow can then acquire an intense cyclonic rotation. At the initial stage of TC intensification, a nearly steady mass inflows to the center at low levels, with vertical ascent in the axial area, and outflow at the top (tropopause), see Figure 1 above.

In central areas of TCs, strong and positive wind stress curl dominates. For slow-moving storms, quasi-circular wind patterns trigger strong Ekman pumping, driving surface water away from the storm center, with associated isopycnal uplifts that can typically reach 50–100 m. In that context, thanks to newly available satellite estimates, the role of upper salinity distribution can also be investigated. Peculiar vertical salinity stratification can act to reduce the SST cooling (Balaguru. et al., 2012; Reul et al., 2014, Balaguru et al., 2020).

Balaguru et al. (2020) further evidenced ​a strong inverse relationship between salinity and TC RI in the eastern Caribbean and western tropical Atlantic due to near-surface freshening from the Amazon-Orinoco River system. In this region, rapidly intensifying TCs induce a much stronger surface enthalpy flux compared to more weakly intensifying storms, in part due to a reduction in SST cooling caused by salinity stratification. As the atmosphere warms under climate change, its capacity to hold water vapor increases. This is quantified by the Clausius-Clapeyron relationship, which explains that the atmosphere will hold about 7% more moisture for every degree Celsius of warming. That generally implies more evaporation in areas that are already dry and increased precipitation in regions that already receive high rainfall. Thus we can expect ​increasing droughts in dry areas and more floods in regions now prone to flooding (Durack et al., 2012). This expected increase in the water cycle dynamic will affect the distribution of freshwater at the ocean surface and therefore the vertical density stratification, which, in addition to upper ocean warming might have severe impact on the RI of the most intense storms in river-water affected regions, such as the Bay of Bengal or the western tropical Atlantic.