Changes in Tropical Cyclone Frequency not Strongly Correlated with Sea Sur-face Temperature

Despite the devastating weather and climate events that have occurred globally throughout the past half-century, tropical cyclone<!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> activity has decreased. This is true even while sea surface temperatures (SST<!–[if supportFields]> XE “sea surface temperature (SST)” <![endif]–><!–[if supportFields]><![endif]–>), which are positively correlated with potential energy in tropical storms, have increased.  A scientific dilemma is why, with the increase in SSTs and global warming that are necessary for typhoon genesis, would there be a decrease in numbers of tropical storms. Zhou et al. (2010) imply that the explanation for this phenomenon is based in the second law of thermodynamics. They arrived at this by examining the tropical storm genesis in concordance with the inter-tropical convergence zone (ITCZ<!–[if supportFields]> XE “ITCZ” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “intertropical convergence zone, ITCZ” <![endif]–><!–[if supportFields]><![endif]–>).  Their results suggest that sea surface temperatures are only one of the necessary conditions for tropical cyclone genesis, and that the low-level vorticity, or tendency for elements of the fluid to “spin”, associated with ICTZ variations should be a fundamental factor for tropical cyclone genesis. They conclude that the causality between SSTs and tropical storm frequency is suggested is not yet fully understood and should be examined further. —Brian Nadler
 Zhou, X., Liu, C., Liu, Y., Xu, H., and Wang, X., 2011. Changes in tropical cyclone<!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> number in the Western North Pacific in a warming environment as implied by classical thermodynamics. International Journal of Geosciences 2, 29–35.

Zhou et al. examined the Western North Pacific (WNP) as a model that might explain the relationship between unusual SSTs and tropical storm number. They used NCEP/NCAR 2.5°x2.5° resolution reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric research to examine the correlation between SST<!–[if supportFields]> XE “sea surface temperature (SST)” <![endif]–><!–[if supportFields]><![endif]–> and surface wind divergence/convection<!–[if supportFields]> XE “convection” <![endif]–><!–[if supportFields]><![endif]–>. Zhou and company then used the second law of thermodynamics to try and explain the implications of SST variation and why sea surface warming around the western Pacific is not as dramatic as it is over the central and eastern Pacific.  Then, they developed equations, showing a predictable pattern in the changes in SST in various regions, which also helped to explain further temperature variation.
They discovered that there is a gradual increase in the 20-year mean of SSTs over the North Pacific, related to surface wind convection<!–[if supportFields]>XE “convection” <![endif]–><!–[if supportFields]><![endif]–> over the ITCZ<!–[if supportFields]> XE “ITCZ” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “intertropical convergence zone, ITCZ” <![endif]–><!–[if supportFields]><![endif]–>, but that originally warmer areas will experience a much weaker warming, because of heat lost through diffusion. This leads to the weakening of the ICTZ trough as seen in recent studies, thereby leading to the decreased number of tropical storms. Additionally, a wind component was discovered to have a much more important role over thermodynamic factors than previously considered.
The authors found that warmer SSTs in the western North Pacific can cause fewer tropical cyclones<!–[if supportFields]> XE “cyclones” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–>, indicating that sea surface temperatures are only one of the necessary conditions and do not definitively lead to an increase in tropical storm numbers. Examining the effects of other variables on tropical cyclone frequency will help better understand the results proposed by this study. 

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