AIR FORCE ADMINISTRATIVE COLLEGE COIMBATORE

1. NORTHEAST MONSOON AIR FORCE ADMINISTRATIVE COLLEGE COIMBATORE

2. INDIAN NORTHEAST MONSOONY.E.A. RajRegional Meteorological Centre, Chennaiyearaj@gmail.com

3. NORTHEAST MONSOON INTRODUCTION • THE INDIAN NORTHEAST MONSOON (NEM), ALSO KNOWN AS THE RETREATING SOUTHWEST MONSOON (SWM), IS A SMALL SCALE MONSOON CONFINED TO PARTS OF SOUTHERN INDIAN PENINSULA WITH DURATION OCTOBER-NOVEMBER-DECEMBER (OND). • IT IS ASSOCIATED WITH THE SEASONAL REVERSAL OF SURFACE AND LOWER TROPOSPHERIC WINDS FROM SOUTHWESTERLIES (DURING THE SWM SEASON OF JUNE–JULY-AUGUST-SEPTEMBER (JJAS) TO NORTHEASTERLIES, WHICH SET IN OVER THE INDIAN REGION IN OCTOBER

4. NORTHEAST MONSOON INTRODUCTION • NEM IS ALSO AN IMPORTANT RAINFALL SEASON FOR PARTS OF SOUTHERN PENINSULAR INDIA. THE STATE AND METEOROLOGICAL SUB DIVISION OF TAMIL NADU (TN) IS THE MAJOR BENEFICIARY OF THE NEM RAINFALL, WHICH IS PRODIGIOUS OVER THE BELT OF COASTAL TAMIL NADU (CTN) • OVER CTN, THE ONSET OF NEM, PRECEDED BY A WELL DEFINED REVERSAL OF LOW LEVEL WINDS FROM SOUTHWESTERLIES TO NORTHEASTERLIES, IS CLEARLY MARKED

5. REGIONS AFFECTED BY NEM GEOGRAPHICAL LOC • FIVE MET SUB DIVISIONS OF INDIA VIZ., COASTAL ANDHRA PRADESH (CAP), RAYALASEEMA (RYS), SOUTH INTERIOR KARNATAKA (SIK), KERALA (KER) AND TN (INCLUDING THE UT OF PONDICHERRY)

6. NORMAL RAINFALL CAP: RECEIVES 1024 MM OF ANNUAL NORMAL RAINFALL OUT OF WHICH 327 MM (32%) IS CONTRIBUTED BY NEM. RYS: RAINFALL OVER THIS SUB DIVISION DURING OND IS 219 MM (31% OF ANNUAL RAINFALL OF 706 MM) SIK: 1019 MM OF ANNUAL RAINFALL AND 210 MM DURING OND (21% OF ANNUAL)

7. NORMAL RAINFALL KER: 478 MM OF RAINFALL DURING OND. 16% OF THE ANNUAL RAINFALL OF 2928 MM TN: NEM RAINFALL OF TN (NRT) ACCOUNTS FOR 48% (OND, 438 MM) OF ITS ANNUAL RAINFALL OF 914 MM THE MAJOR BENEFICIARY OF NEM. COIMBATORE, RECEIVES 309 MM NEARLY 50% OF ANNUAL RAINFALL OF 614 MM.

8. DISTRIBUTION MONTHLY & SEASONAL RAINFALL • THE OND NEM RAINFALL IS HIGHEST ALONG SOUTH CAP (SCAP) AND NORTH CTN (NCTN) WITH SPATIAL RAINFALL VARIATION OF 70-100 CM • OVER CAP, RYS AND KER, OND RAINFALL DECREASES FROM NORTH TO SOUTH • DURING OND, THE MONTH OF OCT IS THE RAINIEST OVER NCAP, NORTH RYS, NORTH KERALA AND ITN. RAINFALL DURING DEC IS NOT VERY HIGH. IN MOST PARTS OF CTN, NOV IS RAINIEST MONTH. OVER SCAP, KER AND CTN, RAINFALL CONTINUES IN DEC.OVER PARTS OF CTN, THE RAINFALL DURING THE MONTH OF JAN IS ALSO HIGH

9. INTER ANNUAL VARIATION OF NEM RAINFALL • STATISTICAL PARAMETERS OF OND NORTHEAST MONSOON RAINFALL OF FIVE SUB DIVISIONS AND THE SR, 1901-2010

10. INTER ANNUAL VARIATION OF NEM RAINFALL • DECADE WISE FREQUENCY ( NO. OF YEARS) OF EXCESS AND DEFICIENT YEARS OF NRS AND NRT, 1901-2010 NRS: OND northeast monsoon rainfall of Southern Region NRT: OND northeast monsoon rainfall of Tamil Nadu

11. MONTH WISE WIND FLOW PATTERN H H APR JAN L L H H H JUL OCT L L L L

12. Daily Rainfall Index • The DRI for a day has been defined as the percentage number of rainy days over a five day pentad, with the day in question as the central day, the number of rainy days counted over all the stations with available data. • Thus, for the Nth day, (N- 2), (N-1), N, (N+1) and (N+2)th days are considered. Out of 5M rainfall observations of M stations, if M1 observations correspond to a rainy day (day with at least 2.5 mm of rain), then DRI= (M1/5M)100 for the Nth day. Obviously, DRI varies between 0 and 100. • A threshold value of 40 has been chosen as the minimum DRI value a day must have for the northeast monsoon to be prevalent. This threshold value has been arrived at after a detailed examination of the intra seasonal variation of DRI for all the years. As it is well known that NEM rainfall occurs in spells with long dry spells in between (IMD,1973), the withdrawal of NEM should be identified with the end of the ultimate wet spell which marks the cessation of the season. • For identification of the date of withdrawal, the same methodology adopted by Raj (1998b) is followed.

13. ONSET, WITHDRAWAL AND DURATION ONSET • BY 15TH OCT SWM WITHDRAWS UP TO 15°N OVER PENINSULA. NORTH OF 20°N & IN MOST OF AREAS NORTH OF 17°N, THE WITHDRAWAL OF SWM IS ASSOCIATED WITH NEAR CESSATION OF RAINFALL AND RISE OF MAX TEMP IN OCT. • SOUTH OF 15°N, THERE IS A MARKED RISE IN RAINFALL FOLLOWED BY INCREASED CLOUDING AND CONTINUATION OF RAINFALL IN THE EASTERN PARTS OF THE PENINSULA. THIS MARKED INCREASE IN RAINFALL OCCURRING AFTER THE REVERSAL OF WINDS FROM SOUTHWESTERLIES TO NORTHEASTERLIES IS CONSIDERED AS THE NEM ONSET • NEM ONSET WAS FIRST MENTIONED IN THE YEAR 1923 BY IMD

14. ONSET, WITHDRAWAL AND DURATION ONSET • SINCE 1977, THE SETTING IN OF NEM HAS BEEN MENTIONED IN THE IDWRS EVERY YEAR • THERE WAS NO WELL DEFINED AND STANDARD CRITERIA TO DECLARE THE NEM ONSET AS MENTIONED IN IMD (1973) • THE NEM ONSET DATES OVER CTN WERE DERIVED FOR THE PERIOD 1901-1990 IN THIS STUDY BASED ON DAILY RAINFALL OF SIX STATIONS LOCATED IN CTN VIZ.,CHENNAI, CUDDALORE, NAGAPATTINAM, VEDARANYAM, PAMBAN AND TUTICORIN

15. CRITERIA FOR DETERMINATION OF DATES OF NEM ONSET THE FOLLOWING FIVE RULES (R1 TO R5) FORMULATED BY RAJ(1992) WERE USED: • R1: SOUTHWEST MONSOON SHOULD HAVE WITHDRAWN UP TO COASTAL ANDHRA PRADESH • R2: DEEP EASTERLIES SHOULD HAVE SET IN OVER TAMIL NADU OR SEASONAL LOW SHOULD HAVE ESTABLISHED IN SOUTH BOB ADJACENT TO TAMIL NADU COAST • R3: AFTER R1 AND R2 ARE SATISFIED, THE FIRST DAY OF FAIRLY WIDESPREAD (FW) RAINFALL OR A HIGHER CATEGORY OVER CTN WOULD BE THE DAY OF NORTHEAST MONSOON ONSET

16. CRITERIA FOR DETERMINATION OF DATES OF NEM ONSET • R4: IF THE DATE ARRIVED AT BY R3 HAPPENS TO BE EARLIER THAN 10 OCTOBER, THE WINDS /SURFACE CHARTS ARE TO BE SCRUTINISED TO DECIDE AS TO WHETHER THE ONSET OF EASTERLIES ARE TEMPORARY OR PERMANENT. IF IT IS PERMANENT, THEN THE DATE OF R3 SHOULD BE TAKEN AS THE ONSET DATE. IF THE EASTERLY ONSET IS TEMPORARY AND IF WESTERLIES APPEAR AGAIN IN THE LOWER TROPOSPHERE OVER CTN, THE DATE OF PERMANENT ONSET OF EASTERLIES IS TO BE DETERMINED AND R3 TO BE APPLIED AGAIN • R5: IN CASE, THE DATE FIXED IS COMPLETELY UNSATISFACTORY, A REVIEW IS TO BE MADE AND THE NEXT DATE OF FW RAINFALL IS TO BE CONSIDERED AS ONSET DATE.

17. DETERMINATION OF DATES OF WITHDRAWAL • REVERSAL OF LOWER TROPOSPHERIC WINDS FROM SOUTHWESTERLIES TO NORTHEASTERLIES OVER CTN IS TAKEN AS A PRE-REQUISITE FOR DECLARATION OF ONSET OF NORTHEAST MONSOON • BUT, NO SUCH CONSPICUOUS OR DISCERNIBLE CHANGE IN THE FLOW PATTERN EITHER AT LOWER OR UPPER LEVELS OF THE ATMOSPHERE OVER THE INDIAN REGION IS KNOWN TO BE ASSOCIATED WITH THE WITHDRAWAL OF NORTHEAST MONSOON OVER CTN • HENCE, THE RULES FOR DETERMINING THE DATE OF WITHDRAWAL OF NORTHEAST MONSOON HAVE BEEN FORMULATED BASED ON RAINFALL

18. DETERMINATION OF DATES OF WITHDRAWAL THE RULES FOR DETERMINATION OF DATE OF WITHDRAWAL OF NEM WHICH WERE FOLLOWED IN THE ABOVE STUDY ARE AS UNDER: • R1: THE DRI IS TO BE COMPUTED FOR EVERY DAY FOR THE PERIOD SEP TO FEB. IF DRI>40 FOR A DAY, THE DAY WOULD BE DEEMED TO BELONG TO A SIGNIFICANT RAIN SPELL • R2: IF NO SIGNIFICANT RAIN SPELL COMMENCED ON OR AFTER 1 JAN, THE FIRST DAY WITH DRI<40 WHICH IS NOT SUCCEEDED BY DATES OF DRI>40 IN THE CALENDAR EITHER UNTIL 31 DEC OR UP TO THE END OF THE SIGNIFICANT RAIN SPELL WHICH MAY HAVE COMMENCED ON OR BEFORE 31 DEC BUT CONTINUED THEREAFTER, WOULD BE DEEMED TO BE THE MIDDATE OF THE WITHDRAWAL PENTAD

19. DETERMINATION OF DATES OF WITHDRAWAL • R3: IF A SIGNIFICANT RAIN SPELL COMMENCED ON OR AFTER 1 JAN, THE WITHDRAWAL PENTAD IS TO BE DETERMINED BY CRITICALLY STUDYING THE JF RAINFALL. THE VARIOUS RAIN SPELLS, INTENSITY AND LENGTH OF EACH, DURATION OF DRY SPELLS IN BETWEEN – ALL ARE TO BE CONSIDERED BEFORE CONCLUDING WHETHER THE JF SPELL COULD BE CONSIDERED AS CONTINUATION OF NEM • R4: ONCE THE WITHDRAWAL PENTAD IS LOCATED, THE PRECISE DATE OF WITHDRAWAL COULD BE SELECTED FROM THE 5 DAYS OF THE PENTAD BY STUDYING THE SPATIAL DISTRIBUTION OF DAILY RAINFALL. PREFERABLY, DATE OF WITHDRAWAL SHOULD BE A DRY DAY OVER CTN

20. DETERMINATION OF DATES OF WITHDRAWAL IMD CRITERIA (ONSET) THE CRITERIA FOR DECLARING ONSET OF NEM WAS SET BY IMD IN AUG 88 BY MEANS OF AN OFFICIAL CIRCULAR, WHICH WAS AMENDED FURTHER IN AUG 06 (IMD, 2008). THE CRITERIA FOR COMMENCEMENT OF NEM RAINS AS PER THE LATEST CIRCULAR ARE: • WITHDRAWAL OF SWM UP TO LATITUDE 15°N • ONSET OF PERSISTENT SURFACE EASTERLIES OVER TAMIL NADU COAST • DEPTH OF EASTERLIES UP TO 850 HPA OVER TAMIL NADU COAST • FAIRLY WIDESPREAD RAINFALL OVER COASTAL TAMIL NADU, SOUTH COASTAL ANDHRA PRADESH AND ADJOINING AREAS.

21. DETERMINATION OF DATES OF WITHDRAWAL IMD CRITERIA (WITHDRAWAL) • AS FOR NEM WITHDRAWAL, IMD STARTED ANNOUNCING WITHDRAWAL DATES OF NEM ONLY FROM THE YEAR 1993 AND PRIOR TO THAT NO SUCH DECLARATION APPEARS TO HAVE BEEN MADE • IMD HAS DECLARED CESSATION OF NEM RAINFALL BY CONSIDERING RAINFALL AND MAY BE A FEW OTHER PARAMETERS SUCH AS DEPTH OF MOISTURE, TEMPERATURE ETC. IN A SUBJECTIVE WAY BUT NO OBJECTIVE CRITERIA OR GUIDELINES TO DETERMINE NEM HAVE SO FAR BEEN PUBLISHED

22. DETERMINATION OF DATES OF WITHDRAWAL IMD CRITERIA (WITHDRAWAL) • AS FOR NEM WITHDRAWAL, IMD STARTED ANNOUNCING WITHDRAWAL DATES OF NEM ONLY FROM THE YEAR 1993 AND PRIOR TO THAT NO SUCH DECLARATION APPEARS TO HAVE BEEN MADE • IMD HAS DECLARED CESSATION OF NEM RAINFALL BY CONSIDERING RAINFALL AND MAY BE A FEW OTHER PARAMETERS SUCH AS DEPTH OF MOISTURE, TEMPERATURE ETC. IN A SUBJECTIVE WAY BUT NO OBJECTIVE CRITERIA OR GUIDELINES TO DETERMINE NEM HAVE SO FAR BEEN PUBLISHED

23. CONCLUSION • THOUGH A MONSOON OF SMALLER SCALE OVER INDIAN REGION AND A BIT DISORGANISED, THE INDIAN NORTH EAST MONSOON IS CHARACTERISED BY SEVERAL FEATURES NORMALLY ASSOCIATED WITH MONSOONS OF LARGER SCALE • IT CAN ALSO BE CONSIDERED AS A COMPONENT OF THE SOUTH EAST ASIAN WINTER MONSOON ALBEIT IN A RESTRICTED WAY • WITH THE AVAILABILITY OF SUBSTANTIAL QUANTUM OF SATELLITE DATA AND DATA GENERATED FROM MODERN OBSERVING SYSTEMS SUCH AS DOPPLER WEATHER RADARS ESPECIALLY OVER THE BAY OF BENGAL SEA AREA, THERE IS PLENTY OF SCOPE FOR FURTHER RESEARCH IN INDIAN NEM TO UNRAVEL ITS COMPLEXITIES

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