A Literature Review: Effect of Climate Change on Tropical Cyclones
Effect of Climate Change on Tropical Cyclones
Since tropical cyclones(TCs) are one of the major geophysical cause of loss of life and property, it is important to understand if there is any change in the frequency and intensity of TCs due to anthropogenic climate change.
IPCC considers 0.25-0.5 C increase in warming over tropical oceans over the past few decades due to increase in greenhouse gas concentration over past 50years. During 6th International Workshop on Tropical Cyclones, a statement was released on the connection between the TCs and anthropogenic climate change. The statement was in response to the increase in number of recent high-impact TC events which includes, 10 land falling Tcs in Japan in 2004, 5 TCs affecting the Cook island in a five week period during 2005, cyclone Gafilo in Madagascar in 2004, cyclone Larry in Australia in 2006, typhoon Saomai in China in 2006, and violently active Atlantic TC season during the period of 2004 to 2005, including the catastrophic socioeconomic impact of Hurricane Katrina. A few recent articles  have noted a large increase in TC’s intensity, frequency and wind-speeds in some regions during past 5 decades, which could be attributed to the increase in the concentration of green house gases in past 50years. However, other studies explain this noticed increase as a result of better observations made and instruments used, making it easier to detect TCs. Consensus statement by the International workshop on TC-6 reported uncertain conclusions about the influence of climate change on TC after taking into account evidence both for and against.
It was concluded that no TC could be solely attributed to the anthropogenic climate change. Model and theory predicts 3-5% increase in wind speed per degree C increase in SST. But, there is inconsistency between the small change in wind speed projected by theory and modelling versus large variations reported by some observational studies. Significant limitation of measurements over some regions make detection of trends difficult. It was suggested that if increase in SST continues, susceptibility to TC storm surge flooding would strengthen.
Here we begin with a few papers published in support of the argument and then present unfavoured arguments by other authors. Intensity of a cyclone measure its destructive potential. Emanuel  in 1987 used a simple Carnot cycle model to measure the maximum intensity of a hurricane at higher temperature which he assumed as a result of C02 increase in the atmosphere. Using GCM with twice the present amount of C02, he predicted 40-50% increase in the destructive potential of the cyclones. In 1996,Nicholls et. al. proved downward trends in the frequency of intense Atlantic hurricanes during the past five decades. Theory(K.Emanuel-1987)  and Modelling(Knutson and Tuleya-2004)  anticipate increase in TC intensity with global warming. Emanuel(2005)  describes power dissipation index(PDI) which depend upon the storm intensity and lifetime. The author shows that the index has increased remarkably since the mid-1970s.
The index is shown to be very well correlated to the SST, reflecting climate signals including Multi- decadal oscillations in the north Atlantic and north pacific and global warming. His results indicates a rising trend in tropical cyclone destructive potential and significant increase in the hurricane related losses owing to the increase in coastal population.
According to IPCC 2001 report , the increase in droughts, TCs, and extreme high tides is probable at confidence level greater than 66% To understand the effect of model used in simulation, Knutson and Tulya in 2004 performed 1300 five day idealized simulations using high resolution versions of GFDL hurricane prediction models. After assessing 4 different moist convection parametrization and no convection parametrization in hurricane models, they concluded net increase in the storm intensity and near storm precipitation rates. The fractional change in precipitation was found to be more sensitive to the parametrization. Convective available potential energy(CAPE) was found to be 21% higher for more C02 amount in the atmosphere. On this basis, he concluded that even if the cyclone frequency remain constant with time, one should expect increase in the occurrence of highly intense storms, of category 5. According to Kevin Walsh , considering reliability of the model predictions, more intense cyclones can be detected in Atlantic only after 2050. A study made by Emanuel in 2007 indicated regional affect of climate change on TCs. Revised estimation of KE of the TCs in Atlantic and western North pacific was performed and it was found that Atlantic variability on time scales of few years or more is significantly correlated with tropical Atlantic SST while in the western north Pacific, this correlation was considerably weaker. Using basic theory and empirical statistical analysis it was shown that much of the fluctuation in the ocean basins can be explained by variations in potential intensity, low level vorticity and vertical wind shear.
In the Atlantic the potential intensity, low level vorticity, and vertical wind shear strongly covary and are highly correlated with SST unlike, Pacific where the three factors were weakly correlated. Publications against the view are as follows. A couple of studies performed on the TCs frequency detect absence of trends in their frequency(Landsea, Nichollas, & Gray-1996 and Chan & Shi -1996). Although some statistical methods predict more than 300% increase in TC intensity in Atlantic by the late 21st century, existing downscaling models or alternate statistical models does not support this dramatic variation(Vecchi et. al 2008). Atlantic, Solow and Moore (2002) showed absence of any periodicity in the N.Atlantic hurricane activity from 1900 to 1998. Nicholls et al. (1998) indicated that there has been downward trend in TC numbers in the Australian region since the late I960. In the northwest Pacific, Chan and Shi (1996) found a downward trend in the occurrence from 1959 to late 1980s, and there has been upward trend since then. No trend in the South Indian ocean (Henderson-Sellers et al. 1998) but a downward trend in the North Indian ocean (Raghavan and Rajesh 2003).The paper reviews present understanding of the influence of climate change on tropical cyclones. According to the author, there has been no change in the region of cyclone formation; however, there has been general agreement about the change in tracks and frequency. Changes in intensity should be detectable in Atlantic sometime after 2050.
Paleotempestology (Liu and Fearn 1993,2000, Donnelly et al. 2001a,b), which focus on the analysis of paleoclimate information of past hurricanes to extend our present understanding for a longer period than the observed one, has been involved in the study of climate change effects on tropical cyclones and it shows substantial change in incidence over the period of centuries and millennia.
Considering the results discussed so far, it is apparent that the intensity of tropical cyclones will increase in future with slight decrease in their frequency. The PDI index which is the measure of the destructive potential of a tropical cyclone seems strongly related to the SST and tropical cyclone intensity over the Atlantic ocean but not so in west pacific.
Effect of Climate change on tropical cyclones over Indian ocean:
TC Frequency over the North and South Indian ocean has been used as a measurement parameter to see any influence of climate change on TC but the intensity, the Power Dissipation Index, over the region has not been taken into account. Further work could be done by considering the intensity/PDI of TC over Indian ocean and checking its variability against climate change parameters for instance, SST and low level vorticity.
Arabian sea and the bay of Bengal constituting Indian ocean are of considerable importance, since the coastal regions in the vicinity of the Indian ocean is vulnerable to tropical cyclones, and the areas are densely populated. Most importantly the Indian ocean is characterised by statistically significant surface warming trends. This property is more prominent in Indian ocean then in North Pacific and North Atlantic ocean .