Triggering of the Positive Indian Ocean Dipole Events by Severe Cyclones over the Bay of Bengal

1. Triggering of the Positive Indian Ocean Dipole Events by Severe Cyclones over the Bay of Bengal Francis P. A., Sulochana Gadgil and P. N. Vinayachandran Center for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore-12, INDIA Abstract: In this paper, we suggest that positive Indian Ocean dipole (IOD) events could be triggered by the occurrence of severe cyclones over the Bay of Bengal during April/May. We show that all positive Indian Ocean dipole events during 1958-2003 are preceded by at least one such severe cyclone. We also propose the mechanism by which positive IOD events are triggered by severe cyclones. Figure 4. Composite maps of latitude-height sections of a) meridional velocity anomaly (m/s) and b) vertical velocity anomaly (dynes/sec), averaged over 85oE-110oE. Composites are made by considering 3 days during which cyclones were severe in 1982, 1994, 1997 and 2003. A Positive phase of Indian Ocean Dipole eventis characterized by negative (positive) SST anomaly in the eastern (western) equatorial Indian Ocean (E/WEIO) and positive (negative) OLR anomaly over the EEIO (WEIO) [Saji et al 1999, Webster et al 1999] Suppression of convection over the EEIO and triggering of the positive IOD events:Average OLR over WEIO is close to that over EEIO during mid-May to mid-August suggesting that the atmospheric dynamics is almost equally favourable over the two regions for supporting convection in this period. Suppression of convection over the EEIO leads to weakening of westerlies along the CEIO and enhanced convergence over WEIO. Suppression of convection over the EEIO leads to enhancement of convection over WEIO (Figure 5). Figure 1: Anomaly patterns of OLR (W/m2)and surface wind (m/s, left) and SST(oC) (right) during July-August 1994. The EEIO plays a crucial role in the development of positive IOD events [Annamalai et al 2003]. Among the factors that determine the evolution of the SST of EEIO, most important are: • the reduction of horizontal advection due to weakening of Wyrtki jet in the equatorial Indian Ocean [Vinayachandran 1999, 2002] and • the upwelling along the Sumatra coast forced by both anomalous easterlies along the equator and south-easterlies along the coast Figure 5. Daily variation of Convection Index (CI, which is defined as the sum of (200-OLR)W/m2 over all the grids in the specified region where OLR is below 200W/m2) over the EEIO (light grey), negative of the CI of WEIO (dark grey) and surface zonal wind (m/s, red curve) over the CEIO during 20 April-30 September for a) 1985, b) 1990 and c) 1997. • Daily evolution of OLR and wind during the 1997 IOD event shows a sharp transition in mid May with increase in OLR over the EEIO, strengthening of southeasterlies along the Sumatra coast and decrease in the westerlies along the equatorial Indian Ocean (Figure 2). • After this event, positive OLR anomalies persisted almost continuously for several months and the westerlies along the central equatorial Indian Ocean continued to decrease until it became easterly. This is followed by decrease in the SST in EEIO. In fact, all the positive IOD events during 1958-2003 are preceded by severe cyclones over the bay. Figure 2. Variation of a) OLR (W/m2), b) SST (oC) averaged over EEIO, c) zonal wind (m/s) along the central equatorial Indian Ocean (60oE-90oE, 2.5oS-2.5oN) and d) wind component (m/s) along the Sumatra coast during 1May- 31July 1997. Figure 6: Tracks of severe cyclones preceded IOD events during 1982-2003 This suggests that the positive IOD event was triggered around mid May in 1997. The evolution of IOD event in the spring/summer 2003 was also very similar to that of 1997 with a sharp transition in mid May. Figure 7: Schematic diagram representing the evolution of a positive IOD event. Severe cyclones over the Bay of Bengal and IOD events:It is interesting to note that both in 1997 and 2003 there were severe cyclones over the Bay of Bengal (the bay) around mid May. We next consider whether such cyclones could play a role in triggering the transition to positive IOD events. Severe cyclones over the Bay of Bengal is a necessary but not sufficient condition for the triggering of IOD events: Even though all the positive IOD events during 1958-2003 are preceded by severe cyclones over the Bay, not all the severe cyclones over the Bay are associated with positive IOD events (eg. 1979, 1990, 1991). Detailed analysis suggests that during these cyclones, the suppression of convection over the EEIO was not sufficient for triggering IOD events. In fact tracks of the severe cyclones in during May 1979 and 1990 were primarily to the western parts of the Bay. Thus suppression of convection over the EEIO should be sufficiently large for severe cyclones to trigger positive IOD events. Impact of severe cyclones over the bay:Important facets of the impact of severe cyclones over the bay in April-May are : a) an intensification of the meridional sea level pressure (SLP) gradient across the EEIO:Severe cyclones over the bay are associated with a sharp decrease in the surface pressure and hence strengthening of north-south pressure gradient across the EEIO. Since the surface wind parallel to the Sumatra coast closely follow the north-south surface pressure gradient across the EEIO, (Figure 3), this leads to enhanced upwelling along the Sumatra coast and thus decrease the SST in the EEIO. b) suppression of integrated water vapor content (IWVC) over the south-eastern parts of equatorial Indian Ocean: Severe cyclone over the bay also leads to a sharp reduction in the IWVC over the south-eastern parts of equatorial Indian Ocean (Figure 3). This could be due to the descent of dry air from the region of deep convection associated with the cyclone as seen from Figure 4. Concluding remarks: • The occurrence of severe cyclones over the Bay of Bengal during April-May is a necessary, but not sufficient, condition for the triggering of positive IOD events. • Major impacts of severe cyclones over the bay are the suppression of convection over the EEIO and intensification of southeasterlies off- Sumatra coast. • The suppression of convection over the EEIO leads to enhanced convection over the WEIO and anomalous surface easterlies over the CEIO. • Strong upwelling caused by the easterlies over CEIO and southeasterlies off Sumatra coast decrease SST in EEIO rapidly and hence positive IOD events could be triggered. Figure 3. Variation of the surface wind off Sumatra coast (m/s) and difference in SLP (hPa) averaged over the bay and region south of EEIO, during the severe cyclones of a) 1997 and c) 2003 (left). Variation of the IWVC (g/kg) over the EEIO during the severe cyclones of b) 1997 and d) 2003 (right). Symbols on SLP and IWVC curve represent the category of cyclone on each day: open circle-deep depression, close circle-cyclone and closed square-severe cyclone. References: Annamalai et al [2003], Deep Sea Research, 50, 2305–2330. Saji et al [1999], Nature, 401, 360–363. Vinayachandran, et al [1999], Geophys. Res. Lett., 26, 1613–1615. Vinayachandran, et al [2002], Deep Sea Research II, 49, 1573–1596. Webster et al [1999], Nature, 401, 356–360.) Acknowledgments:We thank Dr. Adam Sobel, Profs. Mark A. Cane, Ravi S. Nanjundiah and J. Srinivasan for useful discussions. Financial support from INDOMOD, Department of Ocean Development, Govt. of India, is gratefully acknowledged. Contact : francis@caos.iisc.ernet.in, sulo@caos.iisc.ernet.in, vinay@caos.iisc.ernet.in