Intensifying tropical cyclones over the North Indian Ocean during summer
monsoon—Global warming
1. Introduction;
Tropical cyclone is one of the most destructive natural disasters that cause loss of lives and enormous property damages around the world. Anthropogenic climate change has the potential for slightly increasing the intensity of tropical cyclones through warming of sea surface temperature (Knutson and Tuleya, 2004). Recent publications linking an increase in cyclone intensity to increasing tropical sea surface temperatures (SSTs) (Webster et al., 2005; Trenberth, 2005; Emanuel, 2005a; Landsea, 2005; Landsea et al., 2006; Emanuel, 2005b) have fueled the debate on whether global warming is causing an increase in cyclone intensity (Pielke et al., 2005). Tropical cyclones are storm systems which gain energy from latent heat, i.e., from water vapor condensation. Tropical storms are tropical cyclones (TCs) which exhibit wind speeds in excess of 17 m/s. The development and the intensity of TCs depend on high underlying sea surface temperatures (at least 26.5 °C in the upper most ocean layer), a strong decrease of air temperature with altitude and low wind shear in the upper troposphere These factors permit strong convection and condensation. The initialization of a TC requires minimum of Coriolis force and a pre-existing disturbance in atmospheric circulati.
The assessment of climate change effects on tropical cyclones is necessary, both in terms of occurrences and tracks. Cyclone activity may be affected by the changes in sea surface temperature (SST). For instance, El Nino/Southern Oscillation is known to influence cyclone frequency in different ocean basins (Chan, 1985; Gray and Sheafer, 1991). Therefore the impacts of long-term SST trends on the cyclone frequency in each ocean basin need to be documented. Some investigators have studied the change in the tropical cyclone frequency in the North Indian Ocean (Ali, 1995; Joseph, 1995). A strong cross-equatorial Low Level Jet stream (LLJ) with its core close to the 850 hPa level exists over the Indian Ocean and south Asia during the summer monsoon season June to September —Joseph and Raman (1966) and Findlater (1969). The LLJ has two functions. It is a conduit carrying the moisture generated by the trade winds over the south Indian Ocean and evaporative flux from the Arabian Sea to the areas of monsoon rainfall production over south Asia. In addition the cyclonic vorticity north of the LLJ axis in the atmospheric boundary layer is a dynamic forcing for generation of vertical upward air motion and rainfall and the genesis of monsoon depressions and cyclones in north Bay of Bengal. Study of Joseph and Simon (2005) found that the tropical easterly jet strength during the summer monsoon period of June through September shows a strong decreasing trend in recent years. Attention of readers is drawn to Stephenson et al. (2001) who have shown that the monsoon wind shear indices of Webster and Yang (1992) and Goswami et al. (1999) have been decreasing at a rate of 0.1–0.3%/year based on NCEP/NCAR reanalysis data (1958–1998)
2. Data and methodology;
Monthly wind data at the 925-, 850-, 500-, 200- and 100 hPa levels from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis dataset (Kalnay et al., 1996). The NCEP-NCAR reanalysis project uses frozen assimilation technique to analyze past data and to eliminate noise due to different operational data assimilation schemes. The horizontal resolution of the reanalysis dataset is 2.5° latitude–longitude. In datasparse regions, the forecast model forms the first guess. The source of data of tropical cyclone frequency in the North Indian Ocean for the period 1877–2007 is an India Meteorological Department (1979, 1996). The data for 1986–2000 have been obtained from different volumes of the quarterly journal Mausam. Pentad running totals of frequencies in different months have been obtained and the linear trends have been computed using the least squared method. TC activity is tabulated using bet track data sets from 1986–2004 for the NIO. The best track data sets are the best estimates of the location and intensities of TCs at 6 h intervals produced by the national and international warning centers. The best track data from the Joint Typhoon Warning Center (JTWC) (Chu et al., 2002) .
3. Results and discussions;
The vertical wind shear, defined by the zonal wind difference between the upper (200 hPa) and lower (850 hPa) atmosphere, is averaged over the North Indian Ocean (00 –50 N and 400 –1000 E, Region for the summer monsoon season. The linear trend during this period of 50 years shows a decrease of 4 m/s in easterly shear, which is significant at 99.9% level by Students' t-test. The north–south temperature gradient for the layer, 500 hPa–100 hPa between 15°0 – 200 N a region for the same longitudinal belt are also obtained from the above said data source . The linear decreasing trend of temperature gradient for 150 latitude width is about 0.8 °C in 50 years which is also significant at 99.9% level. Now we show that over both the regions, R-I and R-II air temperature shows a strong environmental warming. Nevertheless, temperature over the lower latitudes increases at a higher rate than that of higher latitudes. The tropical NIO exhibited a strong warming trend during the last decades and shown in Fig. 3. SST trend observed during the period 1951–2000 was prescribed in the tropical NIO, yields reduced wind shear. In an earlier study it was found that the tropical easterly jet strength during the summer monsoon period of June through September shows a strong decreasing trend in recent years (Joseph and Simon, 2005). The 11 year running averages of temperature for the same vertical layer for the lower latitudes the increase in air temperature is 1.2 °C compared to 0.3 °C in the higher latitudes in 50 years This warming seems to be a part of the global warming trend known to be occurring since 1970s (Jones et al., 1999). Thus as expected, a decrease in the easterly jet shear seen earlier is due to decrease in the temperature gradient or relatively higher warming in the lower latitudes.
4. Conclusions;
The frequency of tropical cyclones in the North Indian Ocean has registered increasing trends during summer monsoon, which account for maximum number of intense cyclones. The increasing trend has been primarily due to decrease in the vertical wind shear. Thus, the future evolution of North Indian Ocean storm activity will critically depend on the warming of the sea surface waters and also the vertical wind shear. Likewise changes in ENSO statistics in the tropical pacific may become important, as they affect the SSTs in the all three tropical oceans. The stronger warming of tropical NIO during recent during the most years drove reduced vertical wind shear over the NIO and is thus responsible for the strong tropical cyclone activity observed. Given the strong correlation between the decreasing easterly wind shear and the increasing number of severe cyclonic storms, decreased TEJ may lead to additional severe tropical storms of hurricane intensity over North Indian Ocean. The catastrophic storms in June 2007 portend disastrous conditions for the large fraction of the global population in the Indian sub-continent and adjacent regions. Other parameters than SST, however, such as the vertical stability of the atmosphere or changes in oceanic mixed layer depth also need to be considered in future projections of cyclonic activity over the North Indian Ocean.
1. Introduction;
Tropical cyclone is one of the most destructive natural disasters that cause loss of lives and enormous property damages around the world. Anthropogenic climate change has the potential for slightly increasing the intensity of tropical cyclones through warming of sea surface temperature (Knutson and Tuleya, 2004). Recent publications linking an increase in cyclone intensity to increasing tropical sea surface temperatures (SSTs) (Webster et al., 2005; Trenberth, 2005; Emanuel, 2005a; Landsea, 2005; Landsea et al., 2006; Emanuel, 2005b) have fueled the debate on whether global warming is causing an increase in cyclone intensity (Pielke et al., 2005). Tropical cyclones are storm systems which gain energy from latent heat, i.e., from water vapor condensation. Tropical storms are tropical cyclones (TCs) which exhibit wind speeds in excess of 17 m/s. The development and the intensity of TCs depend on high underlying sea surface temperatures (at least 26.5 °C in the upper most ocean layer), a strong decrease of air temperature with altitude and low wind shear in the upper troposphere These factors permit strong convection and condensation. The initialization of a TC requires minimum of Coriolis force and a pre-existing disturbance in atmospheric circulati.
The assessment of climate change effects on tropical cyclones is necessary, both in terms of occurrences and tracks. Cyclone activity may be affected by the changes in sea surface temperature (SST). For instance, El Nino/Southern Oscillation is known to influence cyclone frequency in different ocean basins (Chan, 1985; Gray and Sheafer, 1991). Therefore the impacts of long-term SST trends on the cyclone frequency in each ocean basin need to be documented. Some investigators have studied the change in the tropical cyclone frequency in the North Indian Ocean (Ali, 1995; Joseph, 1995). A strong cross-equatorial Low Level Jet stream (LLJ) with its core close to the 850 hPa level exists over the Indian Ocean and south Asia during the summer monsoon season June to September —Joseph and Raman (1966) and Findlater (1969). The LLJ has two functions. It is a conduit carrying the moisture generated by the trade winds over the south Indian Ocean and evaporative flux from the Arabian Sea to the areas of monsoon rainfall production over south Asia. In addition the cyclonic vorticity north of the LLJ axis in the atmospheric boundary layer is a dynamic forcing for generation of vertical upward air motion and rainfall and the genesis of monsoon depressions and cyclones in north Bay of Bengal. Study of Joseph and Simon (2005) found that the tropical easterly jet strength during the summer monsoon period of June through September shows a strong decreasing trend in recent years. Attention of readers is drawn to Stephenson et al. (2001) who have shown that the monsoon wind shear indices of Webster and Yang (1992) and Goswami et al. (1999) have been decreasing at a rate of 0.1–0.3%/year based on NCEP/NCAR reanalysis data (1958–1998)
2. Data and methodology;
Monthly wind data at the 925-, 850-, 500-, 200- and 100 hPa levels from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis dataset (Kalnay et al., 1996). The NCEP-NCAR reanalysis project uses frozen assimilation technique to analyze past data and to eliminate noise due to different operational data assimilation schemes. The horizontal resolution of the reanalysis dataset is 2.5° latitude–longitude. In datasparse regions, the forecast model forms the first guess. The source of data of tropical cyclone frequency in the North Indian Ocean for the period 1877–2007 is an India Meteorological Department (1979, 1996). The data for 1986–2000 have been obtained from different volumes of the quarterly journal Mausam. Pentad running totals of frequencies in different months have been obtained and the linear trends have been computed using the least squared method. TC activity is tabulated using bet track data sets from 1986–2004 for the NIO. The best track data sets are the best estimates of the location and intensities of TCs at 6 h intervals produced by the national and international warning centers. The best track data from the Joint Typhoon Warning Center (JTWC) (Chu et al., 2002) .
3. Results and discussions;
The vertical wind shear, defined by the zonal wind difference between the upper (200 hPa) and lower (850 hPa) atmosphere, is averaged over the North Indian Ocean (00 –50 N and 400 –1000 E, Region for the summer monsoon season. The linear trend during this period of 50 years shows a decrease of 4 m/s in easterly shear, which is significant at 99.9% level by Students' t-test. The north–south temperature gradient for the layer, 500 hPa–100 hPa between 15°0 – 200 N a region for the same longitudinal belt are also obtained from the above said data source . The linear decreasing trend of temperature gradient for 150 latitude width is about 0.8 °C in 50 years which is also significant at 99.9% level. Now we show that over both the regions, R-I and R-II air temperature shows a strong environmental warming. Nevertheless, temperature over the lower latitudes increases at a higher rate than that of higher latitudes. The tropical NIO exhibited a strong warming trend during the last decades and shown in Fig. 3. SST trend observed during the period 1951–2000 was prescribed in the tropical NIO, yields reduced wind shear. In an earlier study it was found that the tropical easterly jet strength during the summer monsoon period of June through September shows a strong decreasing trend in recent years (Joseph and Simon, 2005). The 11 year running averages of temperature for the same vertical layer for the lower latitudes the increase in air temperature is 1.2 °C compared to 0.3 °C in the higher latitudes in 50 years This warming seems to be a part of the global warming trend known to be occurring since 1970s (Jones et al., 1999). Thus as expected, a decrease in the easterly jet shear seen earlier is due to decrease in the temperature gradient or relatively higher warming in the lower latitudes.
4. Conclusions;
The frequency of tropical cyclones in the North Indian Ocean has registered increasing trends during summer monsoon, which account for maximum number of intense cyclones. The increasing trend has been primarily due to decrease in the vertical wind shear. Thus, the future evolution of North Indian Ocean storm activity will critically depend on the warming of the sea surface waters and also the vertical wind shear. Likewise changes in ENSO statistics in the tropical pacific may become important, as they affect the SSTs in the all three tropical oceans. The stronger warming of tropical NIO during recent during the most years drove reduced vertical wind shear over the NIO and is thus responsible for the strong tropical cyclone activity observed. Given the strong correlation between the decreasing easterly wind shear and the increasing number of severe cyclonic storms, decreased TEJ may lead to additional severe tropical storms of hurricane intensity over North Indian Ocean. The catastrophic storms in June 2007 portend disastrous conditions for the large fraction of the global population in the Indian sub-continent and adjacent regions. Other parameters than SST, however, such as the vertical stability of the atmosphere or changes in oceanic mixed layer depth also need to be considered in future projections of cyclonic activity over the North Indian Ocean.
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