On the Seasonal Cycles of Tropical Cyclone Potential Intensity

Abstract

Recent studies have investigated trends and interannual variability in the potential intensity (PI) of tropical cyclones (TCs), but relatively few have examined TC PI seasonality or its controlling factors. Potential intensity is a function of environmental conditions that influence thermodynamic atmosphere-ocean disequilibrium and the TC thermodynamic efficiency-primarily sea surface temperatures and the TC outflow temperatures-and therefore varies spatially across ocean basins with different ambient conditions. This study analyzes the seasonal cycles of TC PI in each main development region using reanalysis data from 1980 to 2013. TC outflow in the western North Pacific (WNP) region is found above the tropopause throughout the seasonal cycle. Consequently, WNP TC PI is strongly influenced by the seasonal cycle of lower-stratospheric temperatures, which act to damp its seasonal variability and thereby permit powerful TCs any time during the year. In contrast, the other main development regions (such as the North Atlantic) exhibit outflow levels in the troposphere through much of the year, except during their peak seasons. Mathematical decomposition of the TC PI metric shows that outflow temperatures damp WNP TC PI seasonality through thermodynamic efficiency by a quarter to a third, whereas disequilibrium between SSTs and the troposphere drives 72%-85% of the seasonal amplitude in the other ocean basins. Strong linkages between disequilibrium and TC PI seasonality in these basins result in thermodynamic support for powerful TCs only during their peak seasons. Decomposition also shows that the stratospheric influence on outflow temperatures in the WNP delays the peak month of TC PI by a month.

Interbasin Differences in the Median and Variability of Tropical Cyclone MPI in the Northern Hemisphere

“…This means that Tout is the warmest and OTL is the lowest in the NA among four basins (Figures c and d). These are consistent with the results in Gilford et al (), who found that OTL in the NA is lower than that in the WNP but is closer to that in the ENP, and correspondingly the outflow layer temperature in the NA is warmer than that in the WNP and ENP (see their Figure 2b). Gilford et al () also found that in the summer months when SSTs are the warmest, outflow temperatures are anomalously warm in the seasonal cycle in addition to being deeper into the stratosphere.…”

“…These are consistent with the results in Gilford et al (), who found that OTL in the NA is lower than that in the WNP but is closer to that in the ENP, and correspondingly the outflow layer temperature in the NA is warmer than that in the WNP and ENP (see their Figure 2b). Gilford et al () also found that in the summer months when SSTs are the warmest, outflow temperatures are anomalously warm in the seasonal cycle in addition to being deeper into the stratosphere. Compared with both QPBL and SST‐T1000 (Figures a and b), overall Tout and OTL show larger variabilities at low SST than at high SST (Figures c and d) across all four basins, and are the largest in the WNP for SST below 29 °C.…”

“…Similarly, Wing et al () showed that the TC MPI in the Pacific is about 5 m s −1 higher than that in the NA (see their Figure 1). More recently, Gilford et al () studied the seasonal cycle of TC MPI and also found that the MPI is higher in the WNP than in the NA and ENP in their corresponding TC seasons (see their Figure 1d).…”

Seasonal and Diurnal Variations of Tropical Tropopause Layer (TTL) Over the Indian Peninsula

“…This difference in seasonal anomalies between the two stations could be closely associated with the difference in topography (Trivandrum being a coastal station and Gadanki an inland station). Other investigators also observed very weak (~1 K) and unclear seasonal variation in temperature in the altitude region below TTL base except in the ABL at tropical stations (e.g., Gilford, Solomon, & Emanuel, ; Hashiguchi et al, ). While the T ′ in the bottom layer of TTL is highest during premonsoon season (~1 K) and lowest during postmonsoon season (<1 K), at Trivandrum, this region is cooler (by 0.5 to −1 K) during the October to February period (postmonsoon and winter seasons) and warmer (by~1.5 K) during March to September period (premonsoon and summer monsoon) at Gadanki.…”

“…These anomalies, quite significant in the upper layer of the TTL, decrease at higher altitudes and become minimal at ~25 km height. The seasonal anomalies reported at other tropical regions also showed large values above CPT with significant peak at ~18 .5 km, but large seasonal contrast is observed between winter and summer with a peak‐to‐peak difference of ~5–8 K (Gilford et al, ; Hashiguchi et al, ; Kim & Son, ; Yulaeva, Holton, & Wallace, ). These features above CPT could be explained by the relatively long radiative relaxation time scale in the region (Randel, Garcia, & Wu, ), tropical upwelling near the CPT (Boehm & Lee, ), and the annual cycle of large‐scale circulations in the troposphere and stratosphere and their associated processes (Yulaeva et al, ).…”