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

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)

MONSOON ONSET OVER SOUTHERN TIP OF INDIA (KERALA)

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

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

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

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

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

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

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

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

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

Mean TMI SST in (0C) for 2003 over Arabian Sea and Bay of Bengal boxes 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

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.

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

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

SST Anomaly April 1983 – Late Onset 13th June

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

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

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

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

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

Cold Pool of the Bay of Bengal during Monsoon

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 Break

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

N-Bay Cold Pool

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

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

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

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

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

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.

Interannual Variability of Monsoon Tropical Biennial Oscillation

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

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

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).

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

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

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

ASIA PACIFIC WAVE

DECADAL VARIABILITY OF MONSOON

MONSOON RAINFALL OF INDIA 1875 TO 2002 From IITM web site: www.tropmet.res.in 1900 WET DRY WET DRY 1870 1930 1960 1990

June-Sept 200 hPa U wind– mean of 1950 to 1959 (Magnitude 9m/s and more at 3m/s intervals) 400N

U-Wind 200 hPa (mag in m/s) decade 1960s minus 1950s (2m/s and more at intervals of 1m/s) 40N

U-Wind 200 hPa (mag in m/s) decade 1970s minus 1950s (2m/s and more at intervals of 1m/s) 400N

U-Wind 200 hPa (mag in m/s) decade 1980s minus 1950s (2m/s and more at intervals of 1m/s) 400N

It is hypothesised that in the DRY epochs when the mid latitude westerlies move southwards over Asia, the divergence area around the monsoon convective heat source acts on the westerlies and produce the Asia Pacific Wave which has one spatial phase in deficient monsoon years (trough over India) and another spatial phase in excess monsoon years (ridge over India)

The correlation between Indian monsoon rainfall and its predictors has varied considerably over the last 100 years. The DRY epochs have high correlation. WET epochs have poor correlation. It is hypothesised that only when westerlies intrude southwards over Asia that the monsoon heat sources influence the westerlies and create large interannual variability of the monsoon leading to the observed high correlation

Extracted from Meehl (1997)

In Meehl’s model for Tropical Biennial Oscillation the mid-latitude westerlies intrude into south Asia as a wave number 3, but really it is a wave number 6 (the Asia Pacific Wave) The westerly intrusion in a DRY 30-year epoch is like the one given by Meehl (Wave number 3)

Longer term variability (Trends)

In the Global Warming scenario, the Sea Surface Temperature (SST) of equatorial Indian Ocean has warmed much more than the other tropical oceans during the period 1950 to the present. Indian ocean has warmed through about 1.5 degree Celsius. The impact (teleconnection) of this rapid warming on the monsoon and Jetstreams are discussed

RAPID WARMING OF INDIAN OCEAN 1950 to 2003 (June-Sept SST 1999-2003 minus 1950-1954) 0C From Joseph and Sabin - Paper presented at WMO conference on Reanalysis at Tokyo, January 2008

Jun-Sept. SST averaged over 60-90E, 7S-3N (Box) and linear trend line 1950 to 2003

OLR showing intensity of vertically growing clouds ( rainfall) (Lower the OLR higher the amount of clouds and rainfall)

OLR (1996-2002) minus (1979-1985) in W/m2 for July & August

) ( 20N 0 30S TEJ STJ Monsoon Hadley Cell with its Ascending limb at 20N and descending limb at 30S shown. With increasing SST near equator when equatorial convection strengthens, the Monsoon Hadley Cell gets weakened which weakens both TEJ and STJ. LLJ

MONSOON HADLEY CELL During the months June to September, the upward limb of the monsoon Hadley Cell is at lat. 20N and the downward limb at 30S. When equatorial convection increases, the tropo-spheric temperature gradient between equator and 20N decreases, weakening the monsoon Hadley Circulation and Jetstreams. The Low Level Jetstream (1.5Km), the Tropical Easterly Jetstream (14Km) and the southern Hemisphere Sub-Tropical Jetstream (12Km) are found to weaken since 1950 .

Monsoon Season Wind Flow through Peninsular India Box 1950 to 2003 at 850hPa (1.5 Km ASL) Joseph and Simon (2005), Clivar Exchanges / Current Science

A TYPICAL MONSOON DEPRESSION VORTEX m/s

HadISST difference in C (1994-03) minus (1951-60) at top and (1941-50) minus (1891-1900) at bottom

Mean 200 hPa U-wind in m/s of July and August (1950 -1959) at top and (1990-1999) at bottom (NCEP data) Weakening of TEJ and southern hemisphere STJ seen.

* 3

hPa m/s Vertical section Jul & Aug U-wind (NCEP) averaged 00-180E longitudes showing TEJ (10N) and STJ (30S) for period 1950-1959 (left) and 1990-1999 (right)

There is strong interaction between Asian Summer Monsoon and Tropical Indian and west Pacific Oceans on Intra-seasonal, Interannual, decadal and longer time scales. For understanding and predicting variability of monsoon on these time scales, study of ocean-atmosphere interaction is very important.

GREAT HOPES FOR THE FUTURE With our capability to monitor in real time the Atmosphere, Ocean and Land and the availability of high computer power to run sophisticated global models, it is hoped that we will be able to understand and predict monsoon and its variability on various time and space scales in the near future

THANK YOU