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Role of Ocean in Monsoon Variability in South Asia

This study explores the impact of the ocean on the variability of summer monsoon onset and rainfall in South Asia. The role of various oceanic factors such as sea surface temperature, ocean-atmosphere interaction, and the Tropical Biennial Oscillation is analyzed. The findings provide insights into the mechanisms behind the active-break cycle and interannual variability of the monsoon.

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Role of Ocean in Monsoon Variability in South Asia

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  1. Role of ocean in the variability of summer monsoon onset and rainfall of south Asia Dr. P.V.Joseph Professor Emeritus Department of Atmospheric Sciences Cochin University of Science and Technology e-mail: joporathur@gmail.com

  2. Monsoon Onset (Kerala) (2) Active – Break cycle (40 day) (3) Inter-annual variability (Biennial) (4) Decadal Variability (30 year epochs) (5) Longer term variability (Trends)

  3. MONSOON ONSET OVER SOUTHERN TIP OF INDIA (KERALA)

  4. Inter Tropical Convergence Zone (Jul-Aug, Jan-Feb)

  5. Rainfall (OLR) and wind at 1.5km (850hPa) at MOK [Average of 12 monsoon onsets] Joseph and Sijikumar (2004) J.Climate

  6. Schematic diagram showing the evolution of convection (rainfall) prior to Monsoon Onset over Kerala beginning 40 days (8 pentads) before monsoon onset over Kerala P-5,P-4 P0 P-8 P-3

  7. Hovmuller OLR 70E-85E Hovmuller 850hPa U-wind, 70-85E

  8. Pentad to Pentad changes in TMI SST prior to Monsoon Onset in 2003

  9. Composite TMI SST at P-7 (2-6May 2003)

  10. Composite TMI SST at P-5 (12-16May 2003)

  11. Composite TMI SST at P-2 (27-31May 2003)

  12. Composite TMI SST at P0 (6-10 June 2003)

  13. Bay of Bengal SST becomes high at P-8 and the Arabian sea SST becomes high before MOK. Mean TMI SST in (0C) for 2003 over Arabian Sea and Bay of Bengal boxes Days with respect to MOK

  14. Why the Bay of Bengal Warm pool forms before the Arabian Sea warm pool? • Observations show that the axis of maximum SST slopes from southwest to northeast over the Indian Ocean during March to May. • This slope is due to the large cooling of the Arabian Sea during the previous southwest monsoon season. • Thus the axis of warm pool is found to reach Bay of Bengal earlier than Arabian Sea as the season advances.

  15. Composite 1981-1990 average SST (a).1Apr-15Apr, (b). 16Apr-30Apr (c).1May-15May. Contours at intervals of 0.50C.  

  16. Global SST Anomalies are associated with the interannual variability of the dates of Monsoon Onset over Kerala. Anomalies are seen two seasons ahead (DJF & MAM) • Delays in onset are mostly associated with El Ninos particularly in their +1 years

  17. SST Anomaly April 1983 – Late Onset 13th June

  18. Correlation between HadISST and date MOK (1971- 2002) (MOK dates as revised by IMD) January February

  19. PRE MONSOON RAIN PEAK Joseph and Pillai (1988) – Current Science

  20. X is Pre Monsoon Rain Peak (1= 01 April) Y is Monsoon Onset Kerala (31= 01 May)

  21. ACTIVE – BREAK CYCLE OF THE MONSOON Joseph and Sabin (2008) in Journal ‘Climate Dynamics’

  22. Active Break cycle of Monsoon Joseph and Sijikumar (2004) J.Climate

  23. Cold Pool of the Bay of Bengal during Monsoon

  24. Zonal Wind through peninsular India at 850hPa during 1 June to 30 September 1998 (wind averaged over area 12.5N – 17.5N, 70E – 95E) Active Active Active Break Break

  25. Active Break cycle in Bay of Bengal SST in 1998 Joseph, Sooraj, Babu and Sabin (2005) Clivar Exchanges.

  26. N-Bay Cold Pool

  27. Convection in the Bay of Bengal is found to begin after the SST gradient in the Bay reaches a maximum. At this time SST in north bay also reaches a maximum. The 0-pentad of the Active – Break cycle is assumed to begin when the SST in the north Bay of Bengal box reaches a maximum • During July and August of 7 years with TMI SST (1998 to 2005 excluding 2002) there were 11 active-break cycles

  28. SST VARIATION OF NORTH BAY BOX IN COMPOSITE AB CYCLE OF JULY &AUGUST

  29. Composite Pentad SST anomaly in Active Break Cycle -2 +2 +3 -1 +4 0 +5 +1

  30. Composite Pentad GPI Rain in Active Break Cycle -2 2 -1 3 0 4 5 1

  31. Composite Pentad 850 hPa Wind in Active Break Cycle -2 2 -1 3 0 4 1 5

  32. Active-break cycle is hypothesised to be caused by ocean-atmosphere interaction in the time scale of about 40 days. There are several steps one following the other in this interaction process. We may do modelling experiments to verify the components (steps) of this interaction hypothesis.

  33. Interannual Variability of Monsoon Tropical Biennial Oscillation

  34. 306 Rain Guage Stations whose data used for Indian Summer Monsoon Rainfall. 1871-2003 Mean ISMR = 84.4 cm Std. Dev. = 8.3cm

  35. Indian Monsoon Rainfall (JJAS) during 1961 to 1990 Five Extreme WET (DRY) Monsoons in Green (Red)

  36. After a DRY monsoon tropical Indian ocean has a WARM SST anomaly and west Pacific ocean has a COLD SST anomalyand both persist till the following monsoon. Opposite sign anomalies occur after WET monsoons. If an El Nino and DRY monsoon occur together the SST anomalies are more intense (warm in the Indian and cold in the west Pacific oceans).

  37. These anomalies change the character of the monsoon of the following year (DRY to WET and WET to DRY). This phenomenon is called the Tropical Biennial Oscillation. It has also been seen that a DRY Indian monsoon is followed by a DRY Australian monsoon and a WET Indian monsoon is followed by a WET Australian monsoon. In the Tropical Biennial oscillation the tropical Indian and west Pacific oceans interact on a seasonal scale with the overlying atmosphere

  38. Composite JJAS 200hPa wind anomaly of 1965, 1972, 1979, 1982 & 1987 (5 DRY monsoons) C

  39. Composite JJAS 200hPa wind anomaly of 1961, 1970, 1975, 1983 & 1988 (5 WET monsoons) C

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