METD WEATHER
A. Deoras
Severe Weather Forecaster
Nagpur/New Delhi/Ludhiana
Concerning- Monsoon Track 2011
Writers note-
Dear Readers,as a part of the seasonal forecasts issued by METD WEATHER since 2009, I w'd like to introduce the 2011 Monsoon forecast or named as IndiMO
IndiMO is prepared seasonally since 2010 with the consideration of multi parameters relating to the Summer Monsoon.
Previous year, the focus was more on the Tropical Activity and the Interseasonal Variations.
This year, the IndiMO focuses on research aspects pertaining to the Madden Jullian Oscillation ( MJO) and its relation with the ENSO
The articles 1-4 are all educational articles particularly written to increase the knowledge of readers and clear their concepts.
The article 5 is the real monsoon forecast
The entire part of the article has Reserved Rights with METD WEATHER
(C) 2011 METD WEATHER
- A.Deoras
-----------------------------------------------------------------------------------------------------------------
Articles-
1) Introduction to the Summer Monsoon and its technicality
2) Gist of Summer Monsoon Activity
3) Introduction to Interseasonal Variations for the tropics
4) Analysis of ENSO ( El Nino Southern Oscillation),Relation with MJO
5) Expected parameters for 2011, REAL FORECAST
6) Synopsis
----------------------------------------------------------------------------------------------------------------
I- Introduction to the Summer Monsoon
Indian Summer monsoon is an annual phenomena happening briefly in the Indian Ocean,Arabian Sea and the Bay of Bengal. The summer monsoon is characterized by the development of monsoon current or a band of South West Winds carrying moisture lash the mainland bringing torrential rains.
The summer monsoon has two category on the basis of its splitting at Southernmost India
The Arabian Sea Branch
The Bay of Bengal Branch
This Monsoon current sets up because of differential heating of continental and sea air mass.
As the air over mainland gets heated more in summer period as the sun apparently migrates Northwards of Equator, rising warm air causes a decrease in air pressure over the Northern regions of India. As the Sea air mass has comparatively less cooling, a system of winds from South West develops which carries moisture with it.
The Somali Current plays a very important role in monsoon. The somali current generates the SW Monsoonal flow and the current typically resides in the 10-15N in Arabian Sea during the monsoonal period bending in and then entering bay of bengal.
The Ekman drift comes in effect in Arabian Sea when the ocean waters are directed at an angle of 45 degrees to the wind stress in the region. This arrangement directs the SW Monsoon winds towards mainland of India.
Right-
The IMD GFS Forecast map of 925mb ( Surface Winds) indicating well developed SW Winds as valid for 12UTC 22/05/11
The dark blue region indicate amplified winds pulling in moisture and regarded as SW Monsoon current.
This current originates from the Somali current
** The Inter-Tropical Convergence Zone or ITCZ which is an area of strong convective activity and rainfall can be seen as a region of converging winds as shown South of Sri Lanka.
ITCZ migrates North with torrential heating during summers and is over India during the Monsoon period.
(B)-
At Right, The GFS map of 850mb winds Valid 00z Fri,13th May shows Broad Anticyclonic circulation in Arabian Sea offshore India.
This circulation is regarded at "The Great Whirl"
and is present during summer in this region.
You can also see enhanced moisture SW of the Whirl which is the Somali Current developing.
THE GREAT WHIRL MOVEMENT CONTROLS THE DIRECTION OF MONSOON CURRENT AS IT ACTS LIKE A BLOCKING HIGH PRESSURE RESPONSIBLE FOR INCREASE IN HEATING OVER CENTRAL INDIA,WESTERN INDIA. ITS DISAPPEARANCE IMMEDIATELY REGULATES THE MONSOON CURRENT.
-----------------------------------------------------------------------------------------------------------------
II- GIST OF SUMMER MONSOON ACTIVITY
The summer monsoon activity formation requires following key factors-
1) Ample heating of Northern Plains of India and surrounding regions leading to formation of low pressure area due to land air mass lifting
2) No interference of Tropical Cyclones ( Which mostly forms within Monsoon trough) and limit the Monsoon wind amplification and disturbs the flow
3) An Enhanced Convection phase of Madden Jullian Oscillation ( Inter-Seasonal Variational Wind flowing Eastwards)
4) Reduction in intensity of North-East winds ( Trade Winds) permitting the flow
*The second parameter of Tropical Activity has been a major concern since last two years which had disrupted the Monsoonal Winds.
* A proven fact that the Madden Jullian Oscillation has widely influenced the quantum of Monsoon. We shall discuss the technicality,progression of this Inter-Seasonal Variation with the ENSO below.
----------------------------------------------------------------------------------------------------------------
III- Introduction to Inter-Seasonal Variation in the Atmosphere of Tropics-
Madden Julian Oscillation
MJO or Madden Julian Oscillation is a moving variation ( Travelling Variation) in the Tropical Atmosphere having a period of roughly 30-60 days.
This cycle moves Eastwards and is of the form of Enhanced or Suppressed Convection region depending on the Surface temperature of segment of Sea infront of them and behind them geometrically.
For higher latitudes, Jet Stream plays an important role in deciding the weather pattern. However for tropics, interseasonal variations are the deciding factor for the weather.
MJO can be determined studying the Outgoing Longwave Radiation OLR depicting Cloud tops temperature and thus forecasting the location of the phase.
The year 2009 turned to be a major rainfall deficiency year for India because of the absence of Enhanced Convection phase of the MJO. In addition to this years where strong deficiency or rainfall was seen in India, MJO wet phase was absent.
Right-
The 2009 plot showing the MJO. On Y-axis, period is plotted i.e months. On X axis, longitudes are mentioned.
Check the vertical column above 70-100E region.
In the map, Orange-Yellow shading indicates suppressed region of rains while the blue shows wet phase of MJO
For the entire year of 2009, there was a remarkable absence of Wet phase of MJO leading to depreciation in the quantum of rainfall
Above-
Two phase of Madden Julian Oscillation in progress. The top image is of 14th May 2011 of the Outgoing Longwave Radiation showing a suppressed phase over India and South East Asia ( Depicted by the brown region in the right side of image)
Bottom image shows the Enhanced phase of MJO depicted by the precipitation region and green blue area over SE Asia. At the same time, the opposite phase can be seen near Atlantic Ocean.
II- MJO Formation
The air flows out from the suppressed convection region due to the absence of primary lifting required for convection. This air progresses towards the Enhanced convection region.
The circulating air carries moisture with it. When it reaches the area of Enhanced Convection,the development of cyclonic circulation at lower troposphere layers ( Around 850mb) leads to convergence of moist air. The moist air rises inside a convection region and hence a Wet phase of MJO develops.
The dry phase of MJO is a region having Cyclonic spin at the 200mb ( In contrast to wet phase mentioned above having an anticyclonic spin at 200mb) while dry phase has anticyclonic or clockwise spin in lower profiles.
This is a textbook description why a broad region ( The Great Whirl mentioned above) is seen during dry MJO in Arabian Sea.
The MJO are driven by the Easterlies which intensify during La Nina
-----------------------------------------------------------------------------------------------------------------
IV- Analysis of ENSO ( El Nino Southern Oscillation),Its Relation with MJO and quantum of Monsoon rain
As discussed above, inter-seasonal variation plays a great role in deciding the quantum of monsoon rainfall in India.
However, it has been noted that there is a relation between ENSO and MJO
ENSO refers to the oscillational transition taking place in the Equatorial Central Pacific due to changes in the Sea Surface Temperatures which is attributed to variations in the intensity of trade winds.
ENSO has two phases -
El Nino and La Nina
Above- During a La Nina period in June-August, regions of SE Asia experience Wet and Cool weather.
The Reason is not known why there is such anomaly during La Nina in SE Asia
The Warm Phase of ENSO i.e El Nino
During El Nino,Trade winds weaken leading to accumulation of Sea waters near Indonesia due to which temperature rises ( SST).
During El Nino, there is deficient rainfall in India as shown above.
Relation of the MJO with ENSO-
Considering the ENSO,MJO events since 1978-2010
El Nino Years- ( After 1978) ( S for Strong,M for Moderate,W for Weak)
1982 ( S)
1983 first half (S),
1987(M),
1991(S),
1992(M)
2002(M)
2004,2006 (W)
2009(S)
The MJO was mostly positive (+1.5 to 2) all these years especially during the Summer Monsoon period i.e June-Sept
clearly depicting its direct relation.
All related data can be availed here
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_mjo_index/pentad.shtml
La Nina years
Brief year 1988(S)
1998,1999 (M)
2010(M)
It can be seen that during this period, the MJO phase was mostly Wet or Negative ( Anomaly of -1 to -2)
For images click above
III -
Neutral Phase-
1978-1982 ( Neut)
and some more years
It has been observed that the MJO wave has a nature depending on the intensity of Neutral.
During the period 1978-1982 when the SST was showing weak negative anomaly,
those years witnessed more number of wet phase of mjo than the dry
-----------------------------------------------------------------------------------------------------------------
2011 CONDITIONS AND SUMMER MONSOON FORECAST
1) Somali Current-
Somali Current has well set and is already bringing heavy rains.
The Great Whirl located in Central Arabian Sea is acting like a blocking zone.
Right-
Satellite image of India ( Water Vapor section) shows the dark black region. Its the Blocking High pressure region and you can see a nice bend in the Moisture winds due to it
The HP is expected to move away shortly and shall permit movement of winds
2) MJO-
An Unexpected Suppressed Phase of MJO seems to be at place over India depicted by the OLR and other maps. This is suppressing the convection as mentioned above.
The phase is now into a transition to active phase as seen from the CHI 200mb forecast
Right-
The Green shading indicates passing of Wet phase of MJO from 25th May to 14th June.
However,
There seems to be a lack of activity in June month ( Mid June and possibly till Mid July)
This means Monsoon shall progress initially till Mid June and marking its time but the current shall weaken in June end
ENSO-
The persisting weak la nina is going to get converted to a Neutral ENSO phase.
In Neutral ENSO, the MJO is highly active and so in 2011 we might get to see variation in Rainfall from months to months unlike previous years mentioned when only one type of MJO phase was in place
2011 Monsoon timings-
The SW Monsoon shall be reaching SW India i.e Kerala by 31st May
Also, Tamil Nadu,Karnataka,Andhra Pradesh,Lakshadweep regions will receive monsoon shower in the first few weeks of June ( by 10th June 2011)
Then after 15th June, I expect a weakening in the monsoon and hence its progress as well
-----------------------------------------------------------------------------------------------------------------
Primary forecast ( Synopsis)
* Forecast for 2011 Monsoon calls for a Normal or slightly less than normal
rainfall accumulation
*Monsoon shall mark time or reach at the usual time for Southern India
* Slow Monsoon phase in end of June
* Monsoon shall be overall deficient till mid July but shall recover in August and September
* Tropical Activity is expected to be low overall this year ( May-June period)
Possible Storms in Wet phase ( After 22nd May to 17th June)
Minor chance now
EXPERIMENTAL PERCENTAGE-
90% NORMAL
10% DEFICIENT
No comments:
Post a Comment