There is a strong negative correlation (significant at 99.9% confidence level) between the absolute easterly shear and the number of severe storms suggesting that a decrease in easterly shear is favorable for the formation of more severe storms. in Modeling Earth Systems (JAMES), Journal of Geophysical Research Its strongest winds are in September while it begins shifting back towards the equator. Stable isotopic (δ18O and δ2H) and geospatial approach for evaluating extreme rainfall events. The correlation coefficient (cc) between the two variables is 0.94 significant at the 99.9% confidence level by students' two sided t‐test. Presently these intense systems are known to form only in the pre and post monsoon seasons, when the vertical wind shear is small. During January, the African easterly jet lies at 3,000 metres (9,800 ft) above sea level at five degrees north latitude. However, several recent papers [Nyberg et al., 2007; De Maria, 1996] suggest that the VWS plays the deciding role in the subsequent intensification to the hurricane stage. [5], Tropical cyclones and climate change § Hurricanes. Thus the tendency to increase severe storms together with the increase of extreme rain events increases substantially the disaster potential in this region. Similarly, the increase of SST is slightly favorable (because the cc between SST and the number of STS is not significant) but significantly reduces monsoon depressions. [13] The TEJ strength during the summer monsoon period of June through September shows a strong decreasing trend in recent years. These variations are particularly high above 500 hPa. [1], The mid-level African easterly jet is considered to play a crucial role in the southwest monsoon of Africa,[3] and helps form the tropical waves that move across the tropical Atlantic and eastern Pacific oceans during the warm season. This shows that the higher values of SST inhibit the formation of monsoon depressions. [5] Figure 2 displays the average vertical wind shear (VWS) (zonal wind, U at 200 hPa minus U at 850 hPa) for the region 0°–5°N and 40°–100°E (R1) for the period 1950–2007 and computed from the NCEP/NCAR reanalysis data [Kalnay et al., 1996]. Bulletin of the American Meteorological Society. Moreover, the TEJ over West Africa is significantly correlated with summer Sahel rainfall on interannual and decadal time scales. That is, the decrease of VWS associated with weakening of TEJ is favorable for the higher number of STS and lower number of depressions. In another study [De Maria, 1996], using observational and theoretical analysis it was shown that weaker vertical shear is favorable for the tropical cyclone development and intensification. [6] Figure 3 presents VWS anomalies for the period 1950–2007. [2007] have included depressions and STS together. Weakening Trend of the Tropical Easterly Jet Stream of the Boreal Summer Monsoon Season 1950–2009. The result is a strong area of high altitude winds commonly called the African Easterly Jet. The TEJ is a typical characteristic of the north Indian ocean summer climate. An observational evidence of decrease in Indian summer monsoon rainfall in the recent three decades of global warming era. Thin lines show NCEP/NCAR data and thick lines show DOE data. Intensification of premonsoon tropical cyclones in the Bay of Bengal and its impacts on Myanmar. In particular from about the year 2001 all the anomalies are positive. [2004] found that the TEJ strength during the Asian summer monsoon season shows a strong decreasing trend in recent years. In September, winds maximize near 50 km/h (31 mph) between the 12th and 13th parallels. [12] To see how SST affects monsoon depressions, we computed the cc between the number of monsoon depressions during the period 1950–2007 and SST. The TEJ was first discovered by Koteswaram [1958]. Response of tropical cyclone potential intensity over the north Indian Ocean to global warming. Thanks are also due to the reviewers for useful suggestions. We showed that the physical mechanisms responsible for the development are different for depressions and STS, thus justifying the separation. Depressions and severe storms are defined according to the wind intensity up to 33 knots and greater than 48 knots respectively [Indian Meteorological Department, 1979]. Cyclone trends constrain monsoon variability during Late Oligocene sea level highstands (Kachchh Basin, NW India). The number of STS in the recent decade 1998–2007 was 6, while in the earlier decade 1989–1998 it was only 3! It is seen in Table 1 that the correlation between the number of depressions and the absolute VWS is positive and highly significant, i.e. Figure 4 presents the monthly variation of VWS for R1, R2 and R3 regions and the total number of STS for the period 1950–2007. Note that Dash et al. This decrease of monsoon depressions and increase of severe storms seem to be associated with the decrease of moderate rain events and increase of extreme rain events during 1981–2000 noted by Goswami et al. In addition, we used the recent data of Kanamitsu et al. Geophysics, Geomagnetism Environmental conditions regulating the formation of super tropical cyclone during pre-monsoon transition period over Bay of Bengal. With the above noted decreasing trend of wind shear, the season of tropical cyclones of hurricane intensity might extend in future to the months of June and September because of presently existing lesser wind shear in these months compared to that of July and August. Weaker tropospheric vertical wind shear is shown to be an important factor for the development of intense or major hurricanes in the Atlantic basin [Klotzbach and Gray, 2006]. The ccs between the monthly variation of absolute VWS for the 3 regions and the number of STS are respectively 0.01, −0.42 and −0.69**. Thus if the present decreasing trend of TEJ intensity continues there is a strong likelihood of the formation of tropical cyclones of hurricane intensity even during the summer monsoon. Also shown is the VWS for the same region for the recent period 1979–2007 computed from the NCEP/DOE data [Kanamitsu et al., 2002]. But the decrease of vertical shear associated with the decrease of TEJ strength [Rao et al., 2004] might generate conditions favorable for the development of cyclones of hurricane intensity over the north Indian ocean basin even during the summer monsoon season. The thickness values increase to the north indicating the increase of temperature to the north. [2002] for the period 1979–2007. All the values are means for the 4‐month season: June, July, August and September, and for the period 1950 through 2007. Arrows indicate the wind vectors at 150 hPa level. [3] The data used in this study were: wind and geopotential height data for the period 1950–2007 obtained from the NCEP‐NCAR (National Centers for Environmental Predictions/National Center for Atmospheric Research) reanalysis data set [Kalnay et al., 1996] and data for the cyclonic systems over the north Indian ocean for the same period obtained from the Indian Meteorological Department [1979, 1996, 2007]. Tropical Easterly Jet Stream occurs near the tropopause over Southeast Asia, India, and Africa during summer. This paper compares the characteristics of the Tropical Easterly Jet (TEJ) and upper‐level winds in six reanalysis products, compares them with soundings at seven West African locations, examines the relationship between Sahel rainfall and the TEJ, and examines factors influencing the TEJ. Properties of Rocks, Computational [2004] found a strong negative correlation between the TEJ strength (negative values of zonal wind) and the number of Bay of Bengal cyclonic systems of which monsoon depressions form a major part. Enter your email address below and we will send you your username, If the address matches an existing account you will receive an email with instructions to retrieve your username, Dynamics of The Tropical Atmosphere and Oceans, Meteorological Measurements and Instrumentation, Fluid Dynamics of the Mid-Latitude Atmosphere, Time Series Analysis in Meteorology and Climatology: An Introduction, The Atmosphere and Ocean: A Physical Introduction, 3rd Edition, Quarterly Journal of Royal Meteorological Society Special Collections, Meteorological Applications Special Collections, I have read and accept the Wiley Online Library Terms and Conditions of Use. The standard deviation of wind over the two‐month intervals is also indicated. Anomalies of vertical wind shear. In addition to region R1, the regions R2 (5°N–15°N) and R3 (15°N–20°N) for the same longitude belt as R1 are also used to calculated cc values. Composition and Structure, Atmospheric Generation of the African Easterly Jet and Its Role in Determining West African Precipitation. In an earlier paper, Haltiner [1967] showed that sensible heat reduces the baroclinic instability of short and medium scale waves, such as the monsoon depressions. [4] Figure 1 shows the region under study. This suggests that if the present decreasing trend of TEJ intensity continues there is a strong likelihood of the formation of tropical cyclones of hurricane intensity even during the summer monsoon. The values of cc for the recent period 1979–2007 for R1, R2 and R3 are, respectively, −0.34, −0.53** and −0.39*. Variability of upper‐ocean characteristics and tropical cyclones in the South West Indian Ocean. In any case, further detailed analysis is needed to confirm these results. Broken lines show the thickness between 200 and 850 hPa levels. [4] The jet exhibits both barotropic and baroclinic instability, which produces synoptic scale, westward-propagating disturbances in the jet known as African easterly waves or tropical waves. The region under study. Use the link below to share a full-text version of this article with your friends and colleagues. Journal of Geophysical Research: Atmospheres. Number of times cited according to CrossRef: Warming Trends in the Central Equatorial Indian Ocean and the Associated Coupled Feedback Processes. Please check your email for instructions on resetting your password. In a recent paper [Dash et al., 2007] noted that the number of weak low pressure systems (wind speed less than 8.5 m s−1) have increased while depressions and cyclonic storms decreased.