A climate model for cyclones over the Arabian Sea simulated by researchers from Princeton University, USA in collaboration with National Oceanic and Atmospheric Administration (NOAA), USA represented among one of the first times that a projected climate model of cyclones synchronized with actual observations of cyclonic storm activity.
A successful prediction
The paper on the climate model by Hiroyuki Murakami et al. (2017) which predicted that global warming could lead to an increase in extremely severe cyclonic storms (ESCS) formation in the Arabian Sea by 2015, was published in the journal Nature Climate Change in November 2017. The HiFLOR climate model of cyclones in the Arabian Sea utilized in the study successfully predicted the extremely severe cyclonic storms that were observed over the Arabian Sea in 2014. The climate model of cyclones over the Arabian Sea found that the occurrence of post-monsoonal extremely severe cyclonic storms showed significant increase in activity between 1990 and 2015, out of which, the predictions for the latter period matched the recent extremely severe cyclonic storms that occurred over the Arabian Sea.
There is a limited amount of accurate long-term data on extremely severe cyclonic activity over the Arabian Sea because in the period before 1998, full coverage of the area by weather satellites was not present. However, in applying climate science to more recent developments, NOAA and Princeton University’s climate model of cyclones over the Arabian Sea accurately predicted in particular the gamut of Cyclone Nilofar that formed in October 2014 over the Arabian Sea, Cyclone Chapala in 2015 which was the strongest cyclone ever recorded over the Arabian Sea, and the recent Cyclone Ockhi of 2017 that devastated Southern India.
NOAA and Princeton University’s climate model of cyclones over the Arabian Sea made its most notable prediction when it projected that post-monsoonal extremely severe cyclones would increase in occurrence and intensity by around 2015, as reflected in the 3 devastating extremely severe cyclonic storms between 2014 and 2017 (Science Daily, 2017).
The number and strength of storms is highly variable from year to year, which makes it challenging to detect trends in the frequency or intensity of hurricanes over time. The advent of satellite technology in the 1970s made it possible to more consistently track cyclones. Storm counts and strength measurements from before to the 1970s are less consistent, further complicating the study of long-term trends. To help address these challenges, scientists run climate models calibrated with observations over the historical period to project future trends and understand the major factors driving these trends.
Recent research in this area in other basins also suggests that there has been an increase in intense hurricane activity over the past 40 years (IPCC, 2012). It must be noted however, that in a study by O P Singh et al. (2000) the frequency of cyclonic storms in the northern Indian Ocean over 122 years between 1877 and 1998 was studied, where enhanced cyclogenesis was observed over the years in the period during November and May. The maximum number of extremely severe cyclonic storms however, occurred over the Bay of Bengal, which reported a doubling of cyclonic storm activity during this time period. The ratio of such storms forming over the Bay of Bengal as against the Arabian Sea is 4:1.
A case of climate change
Although the scientific community has much to be optimistic about, given the successful prediction of this climate model of cyclones over the Arabian Sea, the grey area remains as to whether the phenomena is a result of climate change. The climate model of cyclones in the Arabian Sea used by NOAA and Princeton University posited that its principal variables behind formation of the extremely severe cyclonic storms were above-normal temperatures along with largely anthropogenic atmospheric pollutants—however, this requires considerable scrutiny.
The variable used by Murakami et al. (2017) in their HiFLOR simulation model implied two scenarios. The first scenario factored in variability in temperatures between time periods that involved previously observed variations in extremely severe cyclonic storms in the northern Indian Ocean that were reproduced and then projected to create simulations of natural variability in storm formation over the Arabian Sea. The second scenario used simulations that included the results of increases in the concentrations of atmospheric pollutants such as organic carbon, black carbon, sulphates, etc., based on their projected concentrations for time periods till 2015 (Murakami et al., 2017).
In what can be seen as an ominous sign, one can see that NOAA and Princeton University’s climate model of cyclones over the Arabian Sea includes the two chief concomitants of climate change as the principal variables in their simulations—temperature rise and atmospheric pollutants. There are however, other studies that argue that climate models of cyclones and cyclonic system formation must take in more factors than to be inferred at only by looking at increases in temperature. Other studies argue that there can be in fact many other factors that can contribute to cyclonic storm formation rather than simply looking at a rise in temperatures or changes in the levels of atmospheric pollutants as factors.
For example, M R R Kumar and S Sankar (2010) carried out a study of storms over the northern Indian Ocean for the period between 1951 to 2007 using multiple datasets utilising analytical methods that included extended reconstruction sea surface temperature (ERSST). The study found that in spite of a substantial rise in sea surface temperatures over the Bay of Bengal in the period between 1951 and 2007 as compared to 1901-1951, a substantial rise in the frequency of storms over the region was
The study inferred that a factor other than warming, namely mid-tropospheric humidity over the Bay of Bengal decreased during 1951-2007 and as such warmer sea surface temperatures cannot alone account for how convective systems are initiated over the Bay of Bengal which influence cyclonic storm formation, especially wind speeds. The climate model of cyclones in the Arabian Sea used by NOAA and Princeton University thus cannot be seen as a universal method of climate simulation for all cases. Kumar and Sankar for example, point out how low-level vorticity, vertical wind shear and mid-tropospheric humidity which are atmospheric characteristics can play significant roles in determining cyclone formation and intensification. One thus cannot only rely on temperature increases alone as a variable when it comes to factoring climate models, although there can be some cases where it might be a significant factor.
In a study carried out by T R Knutson et al. (2010) on the effects that climate change might have on tropical cyclones, a great difficulty is expressed in charting long-term trends in tropical cyclone frequency and intensity due to a warming climate. Long-term projections are in particular complicated by large amplitude fluctuations in the intensity and frequency of tropical cyclones. Another drawback is also a lack of a comprehensive global historical record of cyclonic storms and a lack of data. Scientists largely rely on theoretical models that are dynamic high-resolution computer simulations which largely point towards the fact that the globally averaged intensity will shift towards stronger storms in the case of tropical storms. Knutson’s study found that tropical cyclone intensity could increase by 2 to 11 per cent by 2100.
While one cannot look at all times at just temperature rise as the only factor in determining whether an extremely severe cyclonic storm might occur in a simulated climate model of cyclones and their possible formation and frequency, we must remember that with constantly rising temperatures, we may have a greater possibility of extremely severe cyclonic storm formation. Even in the most basic sense, it would take only the chance changes in convective patterns considering that cyclonic storms use warm, moist air as fuel increasing the likelihood of cyclonic storms.
IPCC. 2012. Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation. Special Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
Knutson, T.R., McBride, J. L., Chan, J., Emanuel, K., Holland, G., Landsea, C., Held, I., P. Kossin, J.P., A. K. Srivastava, A.K., and Sugi, M., 2010. Tropical cyclones and climate change. Nature Geoscience (3): 157–163.
Kumar, M.R.R., and Sankar, S., 2010. Impact of global warming on cyclonic storms over north Indian Ocean. Indian Journal of Marine Sciences, 39(4): 516-520.
Murakami, H. Vecchi, G. A., and Underwood, S., 2017. Increasing frequency of extremely severe cyclonic storms over the Arabian Sea. Nature Climate Change, 7(12): 885-889.
Science Daily. December 6, 2017. Birth of a storm in the Arabian Sea validates climate model. Available at: https://www.sciencedaily.com/releases/2017/12/171206141643.htm
Singh, O.P. Khan, T.M.A and Rahman, M. S., 2000. Has the frequency of intense tropical cyclones increased in the north Indian Ocean? Current Science, 80(4):575-580.
Inputs from : Dr Ajit Tyagi, Air Vice Marshal (Retd.), Former DG, IMD. firstname.lastname@example.org