Monsoon Variability and convection over the oceans

1. Monsoon Variability and convection over the oceans • IISc. • 24 July 2007

2. Links between the variability of the Indian monsoon on the synoptic and intraseasonal scales with convection over the surrounding seas and equatorial Indian ocean and on the interannual scale with convection over the (i) equatorial Pacific :ENSO • (ii) Equatorial Indian Ocean: EQUINOO and • (iii) Bay of Bengal via EQINOO, IOD

4. Figure 1a: IR for 30 June 2007, 7.30 am

5. Continental TCZ: Link to convection over the surrounding ocean • Continental TCZ is maintained partly by propagation of synoptic scale systems (lows /depressions ) or the planetary scale TCZ generated over the warm ocean (equatorial Indian Ocean, Bay of Bengal and the Arabian Sea) onto the subcontinent.

8. From Sikka & Gadgil 1980

9. From Srinivasan and Smith 1992

10. Variation of the region with deep clouds (identified as grids with OLR less than 200Wm−2 and 180Wm−2 at 90E during March-December 1979. The seasonal envelope is also indicated. (after Gadgil 2003)

11. Mean monthly all-India rainfall mo Most of the rainfall occurs during June-September-summer monsoon season; the focus of most studies is the summer monsoon Mo

12. Mean Summer monsoon (June-September) rainfall in cm

13. Interannual Variation of the anomaly of All-India summer monsoon rainfall (as % of the mean) std dev about 10% of mean

15. Relation with convection over the equatorial Indian ocean and over the west Pacific • Some of the disturbances over the Bay result from westward propagation of the disturbances from the Western Pacific • Thus the continental TCZ is maintained by propagations from the west Pacific/Bay of Bengal and the equatorial Indian ocean .

16. However, the relation to the convection over the equatorial Indian ocean is complex, since there is also competition between the continental and oceanic TCZ with active spells of one generally occurring during weak spells of the other (Sikka and Gadgil 1980 and many subsequent studies). • Typhoons over the west Pacific are also known to have an adverse impact on the Indian monsoon, particularly those with northward tracks

17. Gadgil and Joseph 2003

20. Experiencing Northward propagation of the TCZ ! 23jul07

21. Interannual Variation of the Monsoon • Link with ENSO: • high propensity of droughts during El Nino, excess rainfall during La Nina • (Sikka 1980, Rasmusson and Carpenter 1983)

22. monsoon seasons during the El Ninos of 1982,87 were droughts and during the La Nina of 1988 the rainfall was in excess.

23. Correlation between OLR at each grid point and area averaged OLR over the region marked OLR Correlation w.r.t. OLR over the central Pacific and East Pacific

24. IMPACT OF EL NINO/LA NINA ENSO index is the negative of the normalized SST anomaly of Nino 3.4 so that positive values are favourable for the monsoon

25. IMPACT OF EL NINO/LA NINA • Thus El Nino is associated with the suppression of convection over the entire north Indian ocean. • Given the role of convective systems over that ocean in maintaining the continental TCZ it is not surprising that the convection/rainfall over the Indian region is also suppressed.

28. EQUATORIAL INDIAN OCEAN OSCILLATION EQUINOO • when convection is enhanced over WEIO,it tends to be suppressed over EEIO &vice versa • sea level pressure anomalies are consistent-east west pressure gradient along equator • implies anomalies of zonal component of surface wind along the equator

29. Correlation of OLR. w.r.t. OLR over WEIO and EEIO for July-August

30. LINK TO THE EQUATORIAL INDIAN OCEAN • It appears that convection over the western equatorial Indian Ocean is favourable for the Indian monsoon while convection over the eastern equatorial Indian Ocean is unfavourable for the Indian monsoon.

31. EQUINOO Index : • EQWIN: negative of anomaly of zonal surface wind over 60o-90oE, 2.5oS-2.5oN (normalized by its standard deviation), so that positive values of EQWIN are favourable for the monsoon • EQWIN is highly correlated with the difference in the OLR over EEIO and WEIO. :

32. July 1994: OLR & surface wind anomalies

33. INTERANNUAL VARIATION OF ISMR • Note that positive values of EQWIN and ENSO index are favourable for the monsoon . • Consider the June-September averages of ISMR (normalized by its std deviation), EQWIN, ENSO index

34. Extremes of the Indian Summer Monsoon Rainfall and 1997 (period:1979-2003) • 1997: ENSO index and EQWIN comparable but opposite; excess seasons: 88 La Nina& EQWIN favourable, 83,94 EQWIN favourable & ENSO unfavourable; droughts 82,87 El Nino; 79,85 EQWIN unfavourable, 1986, 2002 both unfavourable

35. ISMR for all the June- September seasons between 1958-2004 ISMR Anomaly <-1.5 Maroon -1.5 to -1.Red -1. to-.5Orange -0.5 to .5 Grey 0.5 to 1.0 Green !.0 to 1.5 Blue >1.5 Dark blue a) ISMR in the phase plane of June-September average values of the ENSO Index and EQWIN for all the June- September seasons between 1958-2004. Red (dark red) represents seasons with ISMR deficit greater than 1 and 1.5 standard deviation respectively; whereas blue (dark blue) represents seasons with ISMR excess of magnitude greater than 1 and 1.5 standard deviation respectively. . Green (orange) represents moderate positive (negative) ISMR anomaly of magnitude between 0.5 and 1 standard deviation. b) Same as Fig 12 a, but for July-August all India rainfall.

36. Strong relationship between large anomalies of ISMR and a composite index which is a linear combination of the indices for ENSO and EQUINOO with all seasons with large deficits (excess) characterized by small (large) values of the index • Gadgil Sulochana, P N Vinayachandran, P A Francis and Siddhartha Gadgil 2004 GRL

37. Assn. also for Jul-Aug

38. Prediction?

39. We have focussed so far, on the links between the monsoon and EQUINOO i.e. atmospheric convection/circulation over the equatorial Indian ocean rather than SST.

40. In the coupled system SST, OLR, wind are all interrelated. However, often the SST responds to changes in OLR, wind and there are lags. • EQUINOO has been considered to be the atmospheric component of the Indian Ocean Dipole (IOD) just as the southern oscillation is the atmospheric component of the coupled ENSO mode over the Pacific. • The index used for the ocean component is DMI defined by Saji et al which is the difference between the SST anomalies of EEIO and WEIO.

42. Thus on the seasonal scale, the variation of ISMR does not appear to be related to that of DMI. • This is consistent with Saji et al., (1999) who showed that while DMI is highly correlated with rainfall over eastern Africa and western equatorial Indian Ocean, the correlation with the rainfall over the Indian region is poor.

43. Results about the link of the Indian monsoon with EQUINOO and lack of a relationship with DMI are supported by a recent study by Mark Cane’s group.

44. Indian summer monsoon rainfall and its link with ENSO and Indian Ocean climate indices • Chie Ihara,a, Yochanan Kushnir,a Mark A. Canea and Victor H. De La Pe˜nab • Int. J. Climatol. 27: 179–187 (2007)

45. Investigation of the relationship of the variation of the Indian monsoon with ENSO, EQUINOO and IOD, using data for a much longer period (from 1881 to 1998) • Suggests that the variation of ISMR is better described by use of indices of ENSO as well as EQWIN than ENSO alone, and they conclude that the relation between Indian Monsoon, equatorial wind and ENSO holds good for the entire period. • In contrast to EQWIN, no skill is added to the specification of ISMR by the DMI index

46. Relation between the ocean component of IOD and EQUINOO • In fact, whereas there is a tight linkage between the southern oscillation in the atmosphere and the fluctuations between El Nino and La Nina in the ocean, with the southern oscillation index being highly correlated to the different El Nino indices (correlation coefficient of 0.86 for the Nino3.4 index), the correlation between EQWIN and DMI is only 0.52 for the period 1958-1997 .

47. Positive dipole events are associated with large positive values of DMI which are generated by strong positive phase of EQUINOO July 1994 OLR & surface wind anomalies SST anomalies

48. Important features of the circulation in 1994 • (i) strong winds parallel to the Sumatra coast and • (ii) easterlies over the central equatorial Indian ocean In fact these features play an important role in the development phase of positive dipole events, in producing upwelling over EEIO and hence generating negative SST anomalies.

49. However, strong positive phase of EQUINOO does not always lead to a positive IOD event since the large upwelling in EEIO may not lead to sufficient cooling if the upper layer of the EEIO is deep e.g. 2003-aborted IOD. • Hence the relatively small correlation