Relationship between the potential and actual intensities of tropical cyclones on interannual time scales

Abstract

The thermodynamic theory for the physics of a mature tropical cyclone (TC) tells us that the cyclone's intensity cannot exceed an upper bound, the potential intensity (PI). This combined with an empirical result due to Emanuel leads to a prediction of average TC intensity change, given the change in PI. The slope of the predicted relationship between percentagewise variations in PI and those in intensity can vary between 0.5 and 1, depending on the mean PI and on what threshold is applied to the intensity data. For the Atlantic and Pacific, typical values are around 0.65 when tropical storms are excluded and 0.8 when they are included. The authors use best track data for the North Atlantic and western North Pacific, combined with PI computed from reanalysis data sets, to test these predictions. The results show that observed interannual variations of maximum TC intensity are consistent with the predictions of PI theory. Modest fractions of the variance in actual intensity are explained by PI variations. Much of the interannual variation in PI experienced by the storms comes from variation in TC tracks, so that the storms in different years are more or less likely to sample regions of high PI, rather than from variations in PI at a fixed location.

On the Seasonal Cycles of Tropical Cyclone Potential Intensity

“…The ratio C k /C D is an uncertain quantity and determining its value is an area of active research (e.g., Emanuel 2003, and references therein), but it is typically taken to be a constant. Here we set C k /C D 5 0:9 following the precedent of previous studies (Wang et al 2014;Wing et al 2015), but we have tested a range of reasonable ratios (0.4-1.0) and found our results to be qualitatively insensitive to the choice. The term (T s 2 T o )/T o is the thermodynamic efficiency of the potential tropical cyclone.…”

“…However, T s seasonal amplitude is generally larger than T o , and T s and T o may be related to one another through the outflow level, with key implications for the seasonal cycles of TC PI (see section 3). The final term (h 0 * 2 h*) is the thermodynamic disequilibrium between the sea surface and the free troposphere, which is principally controlled by sea surface temperatures (e.g., Emanuel 2007).…”

“…While it is recognized that warming sea surface temperatures (SSTs) have been and will continue to be a key driver for TC PI trends under anthropogenic climate forcing (e.g., Henderson-Sellers et al 1998;Emanuel 2005;Knutson et al 2010;Holland and Bruyère 2014;Strazzo et al 2015;Walsh et al 2016; and references therein), recent work has also focused on how lowerstratospheric temperatures influence TC PI (e.g., Emanuel et al 2013;Ramsay 2013;Wang et al 2014). Some studies have explored stratospheric linkages with TC PI trends over the past three decades (Emanuel et al 2013;Wing et al 2015), in the future under anthropogenic climate forcing (Vecchi et al 2014;Sobel et al 2016;Walsh et al 2016), and in a Montreal Protocol world-avoided scenario (Polvani et al 2016). Less attention has been given to interannual variability in TC PI and its connections with the middle atmosphere (Wing et al 2015), and only a few studies have considered the seasonality of potential intensity (Free et al 2004;Tonkin et al 2000).…”

“…Studies over the last few decades have established a theoretical framework for investigating the potential intensity (PI) that a TC may reach given local environmental conditions (e.g., Emanuel 1985;Holland 1997;Emanuel 1998, 2002, hereafter BE02). Evidence suggests that variations in actual tropical cyclone intensities scale with variability in potential intensity on multiple time scales (Emanuel 2000;Wing et al 2007). While it is recognized that warming sea surface temperatures (SSTs) have been and will continue to be a key driver for TC PI trends under anthropogenic climate forcing (e.g., Henderson-Sellers et al 1998;Emanuel 2005;Knutson et al 2010;Holland and Bruyère 2014;Strazzo et al 2015;Walsh et al 2016; and references therein), recent work has also focused on how lowerstratospheric temperatures influence TC PI (e.g., Emanuel et al 2013;Ramsay 2013;Wang et al 2014).…”

“…Some studies have explored stratospheric linkages with TC PI trends over the past three decades (Emanuel et al 2013;Wing et al 2015), in the future under anthropogenic climate forcing (Vecchi et al 2014;Sobel et al 2016;Walsh et al 2016), and in a Montreal Protocol world-avoided scenario (Polvani et al 2016). Less attention has been given to interannual variability in TC PI and its connections with the middle atmosphere (Wing et al 2015), and only a few studies have considered the seasonality of potential intensity (Free et al 2004;Tonkin et al 2000). In this study, we calculate the seasonal cycles of tropical cyclone potential intensity in the main TC development regions Supplemental information related to this paper is available at the Journals Online website: http://dx.doi.org/10.1175/ JCLI-D- 16-0827.s1. with 34 years of reanalysis data and use a decomposition method to determine the main factors that drive TC PI seasonality in these regions.…”