Control of Intense Tropical Cyclones by Upper-Ocean Stratification Variability

Strong winds associated with Tropical Cyclones (TCs) trigger intense mixing in the upper ocean, stirring warm surface water with cold water from the thermocline. This results in a surface cooling that feeds back negatively on TCs intensity.

It has been shown that upper-ocean stratification can modulate surface cooling induced by a TC by up to one order of magnitude (Vincent et al 2012). We here investigate the possibility that inter-annual variability of upper-ocean stratification (linked to e.g. El Niño) influences cyclonic activity by controlling the TC-induced cooling feedback.

Using the down-scaling methodology of Emanuel et al (2006), we simulate tens of thousands of axi-symmetric TC models coupled to varied ocean conditions representative of inter-annual variability in all oceanic basins. Only the ocean sub-surface properties are modified, sea surface temperature and atmospheric conditions are kept constant.

Sub-surface stratification properties are shown to modify the integrated Power Dissipation by 10-15% in the Pacific and South Indian Ocean basins. Ocean influence mostly concerns category 5 TCs with 20% to 40% difference in TC.days between years with favorable/unfavorable ocean conditions. Within category 5, TCs with 1-minute sustained winds greater than 170 knots (such as super-typhoon Haiyan) are most sensitive to upper-ocean stratification with an ~60% TC.days modulation in the West Pacific and Indian Ocean basins.

EM Vincent et al. (2012) Processes setting the Characteristics of Sea Surface Cooling induced by Tropical Cyclones. JGR, 117, C02020, DOI 10.1029/2011JC007396.

KA Emanuel et al. (2006) Climate and tropical cyclone activity: A new model downscaling approach. J. Clim., 19, 4797–4802.