Summer rainfall India: A review of monsoonal and extramonsoonal aspects II

Atmosphere ISSN: 0004-6973 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/tato19 Summer rainfall India: A review of monsoonal and extramonsoonal aspects II A. K. Chakravarti To cite this article: A. K. Chakravarti (1968) Summer rainfall India: A review of monsoonal and extramonsoonal aspects II, Atmosphere, 6:3, 87-114 To link to this article: https://doi.org/10.1080/00046973.1968.9676553 Published online: 18 Apr 2011. Submit your article to this journal Article views: 330 View related articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalinformation?journalcode=tato19

ATMOSPHERE VOLUME 6, 1968 SUMMER RAIN FALL INDIA A REVIEW OF MONSOONAL AND EXTRAMONSOONAL ASPECTS - I1 A. K. CHAKRAVARTI IN THE EARLIER PART OF THlS STUDY (ATMOSPHERE VOL. 6. NO. 1,1968). IT WAS SHOWN THAT THE ATMOSPHERIC DISTURBANCES AND PERTURBATIONS RELATED TO SUM- MER RAINFALL IN INDIA CAN BE DIVIDED INTO THREE BROAD GROUPS, VIZ. - 1. AIR MASSES AND FRONTS IN THE MONSOON CIRCULATION, 2. UPPER ATMOSPHERIC PERTURBATIONS, AND 3. EXTRATROPICAL DISTURBANCES. MANY METEOROLOGISTS STILL BELIEVE IN AIR MASS ANALYSIS AND FRONTAL EX- PLANATIONS FOR THE SUMMER RAINFALL IN INDIA ALTHOUGH THERE IS MUCH CONTRO- VERSY REGARDING THE NATURE OF FRONTS AND THE ROLE OF THE INTERTROPICAL CONVERGENCE (ITC) ZONE. HOWEVER, WITH INCREASING KNOWLEDGE OF THE UPPER ATMOSPHERIC CONDITIONS, MORE AND MORE THEORIES ARE BEING EVOLVED REGARDING THE DIRECT CONNECTION BETWEEN SUMMER RAINFALL AND THE UPPER (500-TO 150-MB) LEVEL ATMOSPHERIC FLOW AND PERTURBATIONS. MOREOVER, EXTRATROPICAL DISTUR- BANCES ALSO CONTRIBUTE A FAIR AMOUNT OF RAINFALL DURING THE SUMMER MONSOON SEASON. IN THlS STUDY THE ROLE OF THESE REMAINING TWO GROUPS OF DISTURBANCES WILL BE EXAMINED. UPPER ATMOSPHERIC FLOW PATTERNS AND RAINFALL A. BURST OF MONSOON RAINFALL A VERY SIGNIFICANT STUDY IN THlS DIRECTION IS YlNlS (1949) HYPOTHESIS OF THE BURST (OR SUDDEN ONSET) OF THE MONSOON. HIS STUDY HAS SHOWN THAT THE ADVANCE OF THE MONSOON (EQUATORIAL CONVERGENCE ZONE) OVER INDIA IS CLOSELY LINKED WITH THE WESTWARD DISPLACEMENT OF A LOW LATITUDE TROUGH AT THE 500-MB. LEVEL FROM 85' E. LONGlTUDE TO 75' E. LONGITUDE. HE BELIEVES THAT THE ADVANCE OF THE MONSOON OVER INDIA IS RETARDED DUE TO A BRANCH OF THE WESTERLY JET STREAM SKIRTING THE SOUTHERN MARGIN OF THE HIMALAYAN MOUN- TAINS. AS SUMMER ADVANCES, THE BRANCH OF THE WESTERLY JET IS DISPLACED

NORTHWARD OF THE MOUNTAINS, THE TROUGH SHIFTS WESTWARD RAPIDLY. AND THE BURST OF MONSOON COINCIDES ABRUPTLY WlTH THE COLLAPSE OF THE SOUTHERN JET. ALTERNATE RETURN AND COLLAPSE OF THE JET SOUTH OF THE HIMALAYAS CAUSE BREAK AND BURST CONDITIONS IN THE MONSOON. ULTIMATELY, THE JET DISAPPEARS DURING THE FULL MONSOON SEASON. HOWEVER, ON THE BASIS OF INVESTIGATIONS MADE BY PISHAROTY AND DESAI (1956). YlNlS HYPOTHESIS MAY BE CRITICIZED ON TWO COUNTS: 1. THAT THE BURST OF THE MONSOON OVER SOUTH INDIA IS LARGELY UETER- MINED BY THE GENERAL CIRCULATION IN THE SOUTH INDIAN OCEAN AND OCCURS EVEN WITHOUT THE DISAPPEARANCE OF THE UPPER WESTERLY CUR- RENT SOOTH OF THE HIMALAYAS. 2. THAT THlS WESTERLY CURRENT MAY PREVENT THE BAY OF BENGAL BRANCH OF THE MONSOON FROM EXTENDING UP TO BIHAR OR UTTAR PRADESH, BUT DEFINITE INSTANCES HAVE BEEN FOUND WHEN THE ARABIAN SEA BRANCH OF THE MONSOON HAS EXTENDED UP TO PUNJAB DUE TO AN ACTIVE WESTERN DISTURBANCE SOUTH OF THE HIMALAYAS. ANOTHER STUDY, MADE BY KOTESWARAM (1958A) HAS ALSO INDICATED THAT THE WESTERLIES MAKE INCURSIONS INTO WEST PAKISTAN EVEN -AFTER THE BURST OF THE MONSOON. HOWEVER, THE WESTERLIES DO NOT DISAPPEAR COMPLETELY FROM THE SOUTH OF THE HIMALAYAS UNTIL THE SUB-TROPICAL ANTICYCLONE IS ESTABLISHED OVER THE HIMALAYAS. ON THE OTHER HAND, HE HAS GIVEN A DIFFERENT EXPLANATION OF THE BURST OF THE MONSOON OVER PENINSULAR INDIA. HE CONTENDS THAT THE BURST OF MONSOON ALONG THE MALABAR COAST IS ASSOCIATED WlTH THE ADVANCE OF THE EASTERLY JET STREAM (KOTESWARAM* 19586). FLOHNlS (1958) INVESTIGATION PRESENTS A MORE COMPLEX PATTERN OF UPPER ATMOSPHERIC CIRCULATIONS CONNECTED WlTH THE BURST OF THE MONSOON. HE THINKS THAT THE BURST OF THE MONSOON IS LINKED WlTH THE FORMATION OF A HlGH PRES- SURE AREA OVER THE TIBETAN PLATEAU DURING JULY AND AUGUST. THE FORMATION OF THlS CELL RESULTS IN THE REVERSAL OF THE NORMAL TEMPERATURE AND PRESSURE GRADIENTS BETWEEN THE 600- AND 300-MB. LEVELS. THlS REVERSAL ACTS LIKE A SWITCH COLLAPSING THE WESTERLY JET ALONG THE SOUTHERN HIMALAYAS, AND ALLOWING THE ADVANCE OF THE ITC ZONE THE EXTENSION OF THE EQUATORIAL WESTERLIES, AND THE BURST OF THE MONSOON. THE RECENT INVESTIGATIONS INTO THE EASTERLY JET STREAM AND THE TIBETAN HlGH PPESSURE CELL HAVE THROWN MUCH LIGHT ON THE MONSOON CIRCULATION AND CAUSES OF RAINFALL IN INDIA. IT IS REALIZED, HOWEVER, THAT A BETTER UNDER- STANDING OF THE NATURE OF THE BURST OF MONSOON AND SUMMER RAINFALL IN INDIA WILL BE ACHIEVED BY A THOROUGH KNOWLEDGE OF THE WESTERLY JET, THE EASTERLY JET, THE TIBETAN HlGH AND THEIR RELATIONSHIP WlTH THE MONSOON TROUGH. UNDER THE PRESENT CIRCUMSTANCES, SUCH A DETAILED INVESTIGATION WOULD APPEAR TO BE DIFFICULT DUE TO THE LACK OF UPPER AIR DATA PARTICULARLY FROM TIBET AND THE NEIGHBORING MOUNTAINOUS COUNTRIES.

B. EASTERLY JET STREAM AND RAINFALL SOME OF THE MOST REVOLUTIONARY CONTRIBUTIONS REGARDING THE SUMMER RAIN- FALL OF INDIA HAVE BEEN MADE WlTH THE DISCOVERY OF AN EASTERLY JET STREAM IN THE TROPICS. KOTESWARAM HAS POINTED OUT THAT THE CORE OF THE EASTERLIES IS NEAR 15' N. LATITUDE. THEY REACH VELOCITIES OF 80 TO 90 KNOTS OR MORE BETWEEN 30,000 TO 40,000 FEET ABOVE SEA LEVEL (KOTESWARAM, 1958A). IN THE LOW LATITUDES, THE WESTERLIES (SOUTHWEST MONSOON) ARE REPLACED EVERYWHERE BY THE EASTERLIES AT 500-MB. MUCH OF THE GENERAL DISTRIBUTION OF SUMMER RAIN- FALL IN INDIA HAS BEEN ACCOUNTED FOR BY THE EXISTENCE OF THlS EASTERLY JET STREAM. EXTENSIVE RAINFALL AND CLOUDINESS OVER NORTHEAST INDIA EAST OF 75' E. LONGITUDE, HAVE BEEN EXPLAINED AS A RESULT OF THE ENTRANCE AND ACCELERA- TION OF THE EASTERLY JET STREAM ASSOCIATED WlTH THE LARGE SCALE ASCENTOF THE AIR. ON THE OTHER HAND, DROUGHT PREVAILS WEST OF 75' E. DUE TO THE EXIT AND DECELERATION OF THE JET ASSOCIATED WlTH THE LARGE SCALE DESCENT OF THE AIR. SOUTH OF THE CORE, THE PATTERN IS REVERSED SO THAT THE WESTERN PART OF THE PENINSULAR INDIA IS RAINIER THAN THE EASTERN PART WlTH THE SLIGHT MODIFI- CATION IMPOSED BY THE WESTERN GHATS. THE RAINFALL PATTERN DURING JULY AS RELATED TO THE EASTERLY JET STREAM IS SHOWN IN FIGURE I. THERE IS, HOWEVER, SOME CONTROVERSY REGARDING THE NATURE AND THE SOURCE OF THE EASTERLY JET STREAM. KOTESWARAM (1958A) HAS OBSERVED THE FORMATION OF A HlGH PRESSURE AREA EXTENDING FROM THE 700- TO 500-MB. LEVELS DUE TO'THE HEATING OF THE TIBETAN PLATEAU. THE EASTERLY JET STREAM HAS BEEN DESCRIBED AS THE EQUATORWARD OUTFLOW FROM THlS HlGH PRESSURE CELL. IN A SUBSEQUENT STUDY, FLOHN (1958) HAS ALSO CONTENDED THAT THE FORMATION OF THE HlGH PRES- SURE CELL IS DUE TO THE HEATING OF THE MID-TROPOSPHERE OVER THE TIBETAN PLATEAU. ACCORDINGLY, THlS HlGH PRESSURE AREA LEADS TO THE FORMATION OF A DEEP AND PERSISTENT EASTERLY FLOW ABOVE THE 500-MB. LEVEL WHICH IS THEN CON- CENTRATED INTO THE EASTERLY JET STREAM ABOVE I50 MB. FLOHN STATES IN THE SAME PAPER THAT THE SEASONAL WARMING OF THE TIBETAN PLATEAU CAUSES A NORTHWARD MIGRATION OF THE TROPICAL EASTERLIES WHICH EXTEND TO THE STRATO- SPHERE TO FORM THE EASTERLY JET NEAR THE TROPOPAUSE. ON THE OTHERHAND, RAMANATHANIS (1958) VIEW IS THAT THE UPPER LEVEL EASTERLIES ORIGINATE FROM THE HlGH LEVEL ANTICYCLONIC CELL LYING OVER THE PACIFIC AND EASTERN ASIA. IT IS NOT CLEAR WHETHER THE EASTERLY JET IS PRODUCED BY - (I) AN OUTFLOW FROM THE SOUTHERN MARGIN OF THE TIBETAN HlGH PRESSURE CELL, 0 R (2) A VERTICAL EXTENSION AND CONCENTRATION OF THE TROPICAL EASTERLIES. FROM THE FOREGOING ANALYSIS, IT SEEMS MORE LIKELY THAT THE MERGER OF BOTH (I) AND (2) AND THEIR CONCENTRATION AT HIGHER LEVEL PRODUCES THE EASTERLY JET.

A SIGNIFICANT POINT, HOWEVER, IS THAT A DISTINCTION HAS BEEN MADE BETWEEN THE UPPER EASTERLIES COMING FROM THE PACIFIC OCEAN OVER INDIA BETWEEN 500- TO 300-MB. LEVELS AND THE EASTERLY JET STREAM RUNNING OVER THE UPPER EASTER- LIES AT THE ZOO-ME. LEVEL (KOTESWARAM AND GEORGE. 1958). BELOW THE 500-MB. LEVEL OF UPPER EASTERLIES, THE SOUTHWEST MONSOON CURRENTS (EQUATORIAL WESTERLIES), EXTEND 10,000 TO 20,000 FEET ABOVE SEA LEVEL AND REACH AS FAR EAST- WARD AS THE SOUTH CHINA SEA AND WESTERN PACIFIC (KOTESWARAM, 1958A). THE EASTERLIES EXIST ONLY AT HIGHER LEVELS REPLACING WESTERLIES EVERYWHERE AT THE 500-MB. LEVEL EXCEPT IN A NARROW BELT BETWEEN THE AXIS OF THE MONSOON TROUGH AND THE SOUTHERN MARGIN OF THE HIMALAYAS, WHERE THEY DESCEND DURING THE SUMMER MONSOON SEASON, FIGURE 2. HOWEVER, RECENT lnvestlgatlons INTO THE CAUSES OF SUMMER RAINFALL IN INDIA HAVE EMPHASIZED THE DIRECT ROLE OF PERTURBATIONS AND VELOCITY INCREASES IN THESE UPPER EASTERLIES, AND HAVE ASSIGNED ONLY AN INDIRECT AND PASSIVE ROLE TO THE SOUTHWEST OR THE BAY OF BE NGAL MONSOONS. C. MONSOON DEPRESSIONS AND RAINFALL DUE TO UPPER ATMOSPHERIC PERTURBATIONS IT HAS BEEN DEMONSTRATED THAT - (1) THE SURGES IN THE EASTERLY JET STREAM, AND (11) THE EASTERLY WAVES IN THE UPPER EASTERLIES HAVE SEPARATE INFLUENCES IN CREATING DEPRESSIONS AND CAUSING RAl NFALL DURING THE SUMMER SEASON. (1) SURGES IN THE EASTERLY JET STREAM KOTESWARAM AND GEORGE (1958) HAVE OBSERVED THAT THE OCCASIONAL STRONG WINDS AS SURGES IN THE EASTERLY JET STREAM HAVE A DIRECT RELATIONSHIP WlTH THE FORMATION OF SURFACE DEPRESSIONS AND SUBSEQUENT RAINFALL. THEY HAVE NOTED THAT A PRE-EXISTING SURFACE TROUGH OVER THE NORTHERN PART OF THE BAY OF BENGAL INTENSIFIED INTO A DEPRESSION WlTH THE APPROACH OF A WlND MAXIMA OR A SURGE IN THE EASTERLY JET STREAM. FURTHER, THE DEPRE.SSION TENDED TO DISSIPATE AND THE CIRCULATION WEAKENED WlTH THE PASSAGE OF THE WlND MAXIMA. (11) EASTERLY WAVES AND MONSOON DEPRESSIONS THE SAME AUTHORS HAVE ALSO NOTED THE RELATIONSHIP BETWEEN THE EASTERLY WAVES AND THE FORMATION OF MONSOON DEPRESSIONS. THESE WAVE-LIKE Dl STUR- EANCES HAVE BEEN OBSERVED IN THE UPPER EASTERLIES BETWEEN 500- AND 300-MB. BELOW THE EASTERLY JET STREAM (KOTESWARAM AND GEORGE, 1958). THREE IMPORTANT FEATURES OF THE EASTERLY WAVES WERE POINTED OUT: 1. THE PASSAGE OF AN EASTERLY WAVE WAS ALWAYS PRECEDED BY UNSETTLED WEATHER OR THE EXTENSION OF THE SEASONAL (MONSOON) TROUGH FROM THE LAND TOWARDS SOUTHEAST INTO THE BAY OF BENGAL.

2. IF THE SEASONAL TROUGH HAD ALREADY EXTENDED OVER THE BAY OF BENGAL THEN THE PASSAGE OF THE WAVES RESULTED IN THE FORMATION OF A SURFACE DEPRESSION. 3. WHEN THE WAVE HAD OVERTAKEN AND PASSED THE TROUGH, THE DEPRESSION WEAKENED. A SIGNIFICANT CONCLUSION DRAWN FROM THIS STUDY IS THAT THE PASSAGE OF AN EASTERLY WAVE OR A SURGE IN THE EASTERLY JET STREAM WOULD USUALLY TRANS- FORM A TROUGH INTO A DEPRESSION. THE DEPRESSION BORN OVER THE BAY OF BENGAL TRAVELS INLAND AND PRODUCES WIDESPREAD RAINFALL ALONG ITS TRAJECTORY. THE PATH OF DEPRESSION MAY BE DETERMINED BY THE POSITION OF THE ITC ZONE. THE LATTER IN TURN MAY BE AFFECTED BY THE COLLAPSE OF THE WESTERLY JET STREAM SOUTH OF THE HIMALAYAS AND THE EXTENSION OF THE EASTERLYJET STREAM OVER INDIA. THUS THE FREQUENCY AND AERIAL EXTENT OF SUMMER RAINFALL IN INDIA ARE MORE DIRECTLY RELATED TO UPPER ATMOSPHERIC FLOW PATTERNS THAN TO THE SUR- FACE CIRCULATION. -.'HE ONLY OTHER DETAILED STUDY OF EASTERLY WAVES AND THEIR INFLUENCE ON RAINFALL IN INDIA IS THAT BY SRINIVASAN (1960). HIS STUDY IS SLIGHTLY DIFFERENT FROM THE PREVIOUS ONE. IN THE PREVIOUS WORK THE EMPHASIS WAS ON THE EASTER- LY WAVES OR THE SURGES IN THE JET STREAM AND THEIR ROLE IN THE FORMATION OF DEPRESSIONS. IN THE SUBSEQUENT WORK SRINIVASAN HAS ANALYZED THE NATURE OF EASTERLY WAVES OVER INDIA AND THEIR DIRECT ROLE IN DETERMINING THE RAINFALL PATTERN WITHOUT THE FORMATION OF SURFACE DEPRESSIONS. THE RAINFALL PATTERN ASSOCIATED WITH THE EASTERLY WAVES ACROSS INDIA IS DIFFERENT FROM THAT ASSOCIATED WlTH THE EASTERLY WAVES OBSERVED OVER THE CARIBBEAN SEA, ALTHOUGH BOTH TYPES OF WAVES MOVE FROM EAST TO WEST. IN THE CARIBBEAN EASTERLY WAVES, WHERE THE VELOCITY OF THE BASIC CURRENT DECREASES WITH HEIGHT, CONVERGENCE AND DIVERGENCE IN THE LOWER TROPOSPHERE PREDOMIN- ATE AND CREATE THE WEATHER IN THE REAR OF THE TROUGH LINE (RIEHL, ISS~I).ON THE OTHER HAND. IN THE EASTERLY WAVES OVER INDIA, WHERE THE VELOCITY OF THE UPPER EASTERLIES INCREASES WITH HEIGHT, THE UPPER DIVERGENCE AHEAD OF THE TROUGH AND THE UPPER CONVERGENCE IN THE REAR OF THE TROUGH ARE DOMINANT. THESE CAUSE WEATHER AND RAINFALL AHEAD OF THE TROUGH LINE (SRINIVASAN, 1960). HOWEVER, TWO IMPORTANT CONCLUSIONS MAY BE DRAWN REGARDING THE RAINFALL FROM THE EASTERLY WAVES DURING THE SUMMER MONSOONS~ 1 THAT OVER A MAJOR PORTION OF INDIA AND PARTICULARLY IN THE GANGES PLAINSTHE SUMMER RAINFALL IS PRIMARILY RELATED TO THE PASSAGE OF EASTERLY WAVES AND JET MAXIMA. 2. THAT THE PASSAGE OF THE EASTERLY WAVES CREATE ACTIVE CONDITIONS AND RAINFALL AND AN ABSENCE OF THESE WAVES MAY PRODUCE WEAK AND EVEN DRY MONSOON CONDITIONS.

THE INVESTIGATION BY SRINIVASAN HAS DEMONSTRATED THAT THE PASSAGE OF AN EASTERLY WAVE DOES NOT ALWAYS LEAD TO THE FORMATION OF A DEPRESSION AS CONTENDED BY KOTESWARAM AND GEORGE (1958). ANOTHER STUDY BY RAMANATHAN (1958) HAS ALSO CONFIRMED THAT THESE WAVES ARE NOT ESSENTIAL FOR DEPRESSION FORMA- TION. AN ANALYSIS OF THE SYNOPTIC CONDITIONS IN WHICH AN EASTERLY WAVE FAILS TO DEVELOP A MONSOON DEPRESSION MAY BE FOUND IN THE WORK OF CHAKRAVORTTY AND BASU (1957). THESE AUTHORS HAVE OBSERVED AN INVERSE RELATIONSHIP BETWEEN THE EASTERLY WAVES AND THE DISTURBANCES IN THE WESTERLIES OF THE NORTH. AC- CORDING TO THEM,A NUMBER OF CASES HAVE BEEN NOTICED WHEN THE EASTWARD PAS- SAGE OF WESTERN DISTURBANCES ACROSS OR NEAR THE EASTERN HIMALAYAS HAS PRE- VENTED THE EASTERLY WAVES FROM DEVELOPING INTO DEPRESSIONS. IT APPEARS, THEREFORE, THAT MONSOON DEPRESSIONS MAY FORM INDEPENDENTLY AND GIVE RAINFALL. SIMILARLY. EASTERLY WAVES MAY CAUSE RAINFALL WITHOUT ANY 1 SURFACE DEPRESSION. AND ON CERTAIN OCCASIONS AN EASTERLY WAVE MAY DEVELOP 1 A DEPRESSION WHICH SUBSEQUENTLY PRODUCES RAINFALL. THE GENERAL CAUSE OF RAINFALL IN OTHER PARTS OF INDIA HAS ALSO BEEN ATTRI- BUTED,'BY SRINIVASAN (1960),TO EASTERLY WAVES. HIS OBSERVATIONS ARE MADE OVER THE GANGETlC WEST BENGAL. THE EXTRAPOLATION WORK FOR THE REST OF THE COUN- TRY IS DOUBTFUL IN VIEW OF THE COMPLEX NATURE OF THE ITC ZONE, EASTERLY WAVES. WESTERN DISTURBANCES. AND THE EASTERLY JET STREAM. A MORE DETAILED INVESTIGATION IN OTHER PARTS OF INDIA IS NEEDED TO ASSESS THE INFLUENCE OF EASTERLY WAVES ON THE RAINFALL OVER THE REST OF THE COUNTRY. WESTERN DISTURBANCES AND SUMMER RAINFALL ANOTHER TYPE OF ATMOSPHERIC DISTURBANCE WHICH AFFECTS THE SUMMER RAIN- FALL OF INDIA IS KNOWN AS THE WESTERN DISTURBANCES. THESE DISTURBANCES, WHICH RESEMBLE EXTRATROPICAL CYCLONES, ARE THE DEVELOPMENT OF EASTWARD MOVING WAVES IN THE EXTRATROPICAL ZONAL WESTERLIES AND ARE SUPPOSED TO BE AN ASPECT OF THE INTERACTION BETWEEN THE TROPICS AND THE EXTRA TROPICS. HOW- EVER. ANY SHARP FRONTAL CHARACTERISTICS HAVE NOT BEEN RECORDED IN THE WES- TERN DISTURBANCES (RAMASWAMY. 1966). THESE DISTURBANCES ARE MOSTLY CONFINED TO NORTHERN INDIA GENERALLY NORTH OF 20' N. LATITUDE. UNTIL RECENTLY THE WESTERN DISTURBANCES WERE THOUGHT TO OCCUR ONLY DUR- ING THE WINTER. BUT NOW IT HAS BEEN FOUND THAT THEIR INFLUENCE DURING THE SUMMER MONSOON IS ALSO QUITE SIGNIFICANT. PARTHASARATHY (1958) HAS OBSERVED THAT THESE WESTERLY WAVES MOVING ACROSS THE LOWER HIMALAYAS ARE AS IMPOR- TANT AS THE EASTERLY WAVES IN DETERMINING THE INTENSITY AND DISTRIBUTION OF HEAVY RAINFALL DURING THE SOUTHWEST MONSOON PERIOD. A SIGNIFICANT FEATURE OF THE WESTERN DISTURBANCES, HOWEVER. IS THAT THEY PLAY A DUAL ROLE IN AFFECTING THE SUMMER RAINFALL OVER INDIA. UNDER PARTICU- LAR SYNOPTIC CONDITIONS. THE DISTURBANCES HAVE A POSITIVE EFFECT AND INCREASE THE RAINFALL. WHEREAS UNDER OTHERS, THEIR EFFECT IS NEGATIVE AND THEY DE- CREASE THE RAINFALL. - CONTINUED ON PAGE 109

- CONTINUED FROM PAGE 92 A. POSITIVE EFFECTS OF WESTERN DISTURBANCES ON SUMMER RAINFALL (1) BHULLAR (1952) CONTENDS THAT THE ONSET OF MONSOON OCCURS UNDER THE IN- FLUENCE OF A WESTERN DISTURBANCE. AN EXAMINAl ION OF RECORDS OF THE ONSET OF THE MONSOON FROM 1901 TO 1950 HAS INDICATED THAT FOR TWELVE YEARS THE WESTERN DISTURBANCES CAUSED THE ARRIVAL OF THE MONSOON EITHER FROM THE BAY OF BENGAL OR THE ARABIAN SEA. (11) PISHAROTY AND DESAl (1956) HAVE ALSO OBSERVED THAT DURING THE MONTHS OF APRIL AND MAY THE WESTERN DISTURBANCES MOVE ACROSS NORTH INDIA AS CLOSED CYCLONIC SYSTEMS ON THE SEA LEVEL CHARTS. ON THOSE OCCA - SIONS, WHEN THE WESTERLY TROUGH EXTENDS TO VERY LOW LATITUDE 20' TO 150 N., THE ADVANCE OF THE FRESH MONSOON OR EASTERLY WAVE FAVOURED THE FORMATION OR INTENSIFICATION OF DEEP DEPRESSIONS ON THE SOUTH- EASTERN COAST OF INDIA. (111) THE SAME AUTHORS HAVE ALSO NOTED DEFINITE INSTANCES WHEN AN ACTIVE WESTERN DISTURBANCE HAS SERVED TO EXTEND THE ARABIAN SEA BRANCH OF THE MONSOON (WITH HIGH HUMIDITY AND RAINFALL) INTO PUNJAB VIA EAST- ERN RAJASTHAN, BEFORE THE EXTENSION OF THE BAY OF BENGAL BRANCH EVEN UP TO THE EASTERN UTTAR PRADESH. (IV) IN ANOTHER INVESTIGATION. MOOLEY (1957) OBSERVES THAT THE PASSAGE OF A WESTERN DlSTURBANCE ACROSS NORTHWEST INDIA LEADS TO THE PRE-SEA- SONAL EXTENSION OF THE BAY OF BENGAL MONSOON CURRENT RESULTING IN WIDESPREAD THUNDER AND RAIN IN WESTERN UTTAR PRADESH AND PUNJAB. FURTHER, EVEN THE SEASONAL ACTIVITY OF THE MONSOON OVER PUNJAB AND WESTERN UTTAR PRADESH IS SOMETIMES INCREASED WITH THE PASSAGE OF WESTERN DISTURBANCES ACROSS THE NORTHERN FRINGE OF INDIA. (V) A VERY SIGNIFICANT CAUSE OF SUMMER RAINFALL OVER NORTHWESTERN INDIA WAS RECENTLY DISCOVERED BY VENKATARAMAN AND RAO (1965). THE HEAW RAINFALL OVER A LARGE AREA IN NORTHWEST INDIA, FAR FROM THE ORO- GRAPHIC INFLUENCE AND THE PATH OF A MONSOON DEPRESSION,COULD BE EX- PLAINED ONLY BY ANALYZING THE UPPER AIR CIRCULATION. IT WAS FOUND THAT UPPER LEVEL DIVERGENCE AHEAD OF A TROUGH IN THE WESTERLIES AT THE 500- TO 200-MB. LEVEL HAD CREATED LOW LEVEL CONVERGENCE AND WIDESPREAD RAINFALL. MOREOVER, UNDER THE INFLUENCE OF THE UPPER TROUGH, THE WESTERLY JET STREAM WAS PUSHED SOUTHWARD AND FURTHER CONTRIBUTED TO THE UPPER DIVERGENCE AND LOWER CONVERGENCE. THE DISCOVERY OF A DIRECT RELATIONSHIP BETWEEN THE WAVES IN THE WESTER- LIES AND THE RAINFALL DURING THE SUMMER MONSOON SEASON IN INDIA IS SIGNI- FICANT. IT INDICATES THAT THE SHORT WAVES IN THE HIGH LEVEL WESTERLIES, WHICH PROVIDE THE IDEAL SETTING FOR CYCLOGENESIS AND PRECIPITATION IN THE TEM- PERATE LATITUDES (RIEHL, 1965), ALSO CONTRIBUTE TO THE MONSOON RAINFALL OF

INDIA DURING THE SUMMER. THE GROWING EVIDENCE,THEREFORE, INDICATES THAT THE WEATHER DURING THE SUMMER IN INDIA UNDER THE TROPICAL MONSOON CIRCULA- TION IS NOT COMPLETELY INDEPENDENT AND SEPARABLE FROM THE WEATHER IN THE EXTRATROPICS (WESTERLIES). MANY HIGH LEVEL PERTURBATIONS OF THE WESTERLIES DO CROSS INTO INDIA. 8. NEGATIVE EFFECTS OF WESTERN DISTURBANCES ON SUMMER RAINFALL (1) IT IS INTERESTING TO NOTE THAT THE SAME WESTERN DISTURBANCES HAVE, ON OCCASIONS, CREATED A BREAK IN THE MONSOONS AND DROUGHT CONDITION. ACCORDING TO THE ANALYSIS BY PISHAROTY AND DESAl (1956). A QUICK SUC- CESSION OF WESTERLY WAVES DURING THE MONSOON SEASON LEADS TO THE IBREAK MONSOONl CONDITION. IN THIS CASE, WESTERLY WAVES DIVERT THE SOUTHEASTERN BAY OF BENGAL CURRENT INTO A SOUTHWESTERLY CURRENT WHICH THEN MOVES TOWARDS THE EASTERN HIMALAYAS PRODUCING RAIN IN THE LATTER AREAS. SIMILARLY, EASTERN DEPRESSIONS ARE ALSO DEFLEC- TED MORE AND MORE TO THE EAST BY THE WESTERN DISTURBANCES UNTIL THE MONSOON GRADUALLY WITHDRAWS FROM THE NORTHERN INDIA. (11) A DIFFERENT EXPLANATION HAS BEEN ADVANCED BY MOOLEY (1957) FOR THE IBREAK MONSOONl CONDITION. ACCORDING TO HIM, THE DROUGHT CONDITION IS DUE TO THE SIMULTANEOUS ABSENCE OF EASTERLY WAVES AND THE MORE FREQUENT PASSAGE OF WESTERN DISTURBANCES RESULTING IN THE NORTH- WARD SHl FT OF THE MONSOON TROUGH. THUS A BREAK IN THE MONSOON OCCURS OVER NORTH INDIA BUT THERE IS HEAVY RAINFALL IN THE FOOTHILLS OF THE HIMALAYAS. THE RAINFALL IN THE LATTER AREA IS, PROBABLY, DUE TO THE CONVERGENCE OF MONSOON CURRENTS AGAINST THE MOUNTAINS, AND THE WESTERN DISTURBANCES PLAY ONLY A PASSIVE ROLE. (111) IN ANOTHER INVESTIGATION. CHAKRAVORTTY AND BASU (1957) CONCLUDE THAT WHILE THE EASTERN DEPRESSIONS HELP THE ADVANCE OF THE MONSOON. THE WESTERN DISTURBANCES HAVE A IRETARDING EFFECTI. THE EASTERN DE- PRESSIONS DO FORM, BUT WEAKEN AND DISAPPEAR BEFORE PENETRATING FARTHER INTO THE INDIAN LANDMASS IF A WESTERN DISTURBANCE IS PRE- SENT THERE. THl S OBSERVATION CONTRADICTS THE CONCLUSIONS REACHED BY THE PREVIOUS AUTHORS NOTED UNDER THE POSITIVE EFFECTS. THE POSI- TIVE OR NEGATIVE INFLUENCE OF WESTERN DISTURBANCES ON THE SUMMER RAINFALL OF INDIA PRESENTS A VERY COMPLEX PROBLEM. DESPITE THE CON- TRADICTORY VIEWS, NO ATTEMPT HAS BEEN MADE TO EXPLAIN CLEARLY HOW THE WESTERN DISTURBANCES EITHER ATTRACT OR REPEL THE MONSOONS DUR- ING THE SAME SEASON. IT APPEARS, HOWEVER, THAT THERE ARE TWO MAIN FACTORS, VIZ., (I) THE FREQUENCY AND (2) THE PATH OF WESTERN DISTUR- BANCES WITH RESPECT TO THE AXIS OF THE MONSOON TROUGH, WHICH DETER- MINE WHETHER THERE WOULD BE RAINY OR DRY CONDITIONS. A DETAILED IN- VESTIGATION OF THESE ASPECTS SHOULD FURTHER OUR UNDERSTANDING OF THE NATURE OF SUMMER RAINS OVER NORTHWEST AND NORTH INDIA.

CONCLUSIONS THE POPULAR EXPLANATIONS THAT THE SUMMER RAINFALL IN INDIA IS RELATED TO ELEMENTS WITHIN THE WARM MOIST MONSOON CURRENTS SUCH AS DIFFERENT AIR MASSES, FRONTS, DEPRESSIONS OR EVEN THERMAL CONVECTIONS AND OROGRAPHIC CON- VERGENCE ARE FAR FROM ADEQUATE. IN FACT, IT IS OBSERVED THAT THE DEPTH OF MONSOON WESTERLIES DURING SUMMER MAY INCREASE UP TO 6 KM. OR MORE EVEN OVER NORTH INDIA YET, THERE MAY BE NO RAINFALL (SRINIVASAN, 1960). THERE IS GROWING EVIDENCE THAT MOST DISTURBANCES LEADING TO PRECIPITATION ARE INI- TIALLY GENERATED BY THE UPPER ATMOSPHERIC FLOW PATTERNS. GREAT SIGNIFICANCE HAS BEEN ATTACHED TO THE ITC ZONE IN DETERMINING THE PATH AND FREQUENCY OF MONSOON DEPRESSIONS, THE CONVERGENCE OF DIFFERENT AIR MASSES, AND THE INTENSITY AND PATTERN OF THE DISTRIBUTION OF RAINFALL OVER INDIA. HOWEVER, THE POSITION. MOVEMENT AND THE STRENGTH OF THE ITC ZONE IN TURN, ARE DETERMINED BY HlGH LEVEL FLOW PATTERNS SUCH AS THE WESTERLY JET, THE EASTERLY JET AND THE TIBETAN HlGH PRESSURE CELL. ALSO THE TBURSTT OR THE IBREAKT OF THE MONSOONS ARE PRINCIPALLY ASSOCIATED WlTH THESE UPPER ATMOS- PHERIC CIRCULATION PATTERNS. HlGH LEVEL ATMOSPHERIC PERTURBATIONS MAKE A SIGNIFICANT CONTRIBUTION TO THE SUMMER RAINFALL. THE EASTERLY WAVES AND THE SURGES IN THE EASTERLY JET STREAM ON MANY OCCASIONS HAVE LED TO THE FORMATION OF SURFACE DEPRESSIONS OR HAVE INDUCED RAINFALL WITHOUT SUCH DEPRESSIONS. SIMILARLY, THE HEAVY PRE - ClPlTATlON ALONG THE WESTERN GHATS HAS BEEN FOUND TO BE ASSOCIATED WlTH THE LOW LEVEL WESTERLY JET STREAM OR WlTH SURGES IN THE EQUATORIAL WESTERLIES AND IS NOT DUE MAINLY TO OROGRAPHIC OR CYCLONIC CONVERGENCE. FURTHERMORE, IT HAS BEEN DISCOVERED THAT THE SUMMER RAINFALL IN INDIA IS ALSO AFFECTED BY THE WESTERLIES OF THE MIDDLE LATITUDES. THE WESTERN DIS- TURBANCES HAVE BEEN FOUND TO HAVE BOTH POSITIVE AND NEGATIVE EFFECTS ON THE RAINFALL OF THE COUNTRY. THE PROBLEMS OF POSITIVE OR NEGATIVE EFFECTS ARE NOT CLEARLY UNDERSTOOD AND NEED FURTHER INVESTIGATION. BESIDES THE INFLUENCE OF WESTERN DISTURBANCES, THE RAINFALL OVER NORTHWESTERN INDIA IS ALSO RE- LATED TO WAVES IN THE UPPER WESTERLIES AND THE WESTERLY JET STREAM. THE GENERAL SUMMER RAINFALL PATTERN IN INDIA IS PARTLY DETERMINED BY THE EASTERLY JET STREAM THROUGH CONVERGENCE AND DIVERGENCE ASSOCIATED WlTH ITS ENTRANCE AND EXIT OVER THE COUNTRY. HOWEVER, THERE ALSO SEEMS TO BE SOME CONCENTRATION OF SPECIFIC ATMOSPHERIC ACTIVITIES PRODUCING PRECIPITATION IN PARTICULAR PARTS OF THE COUNTRY. FOR EXAMPLE, GREATER ACTIVITY OF - (A) THE ITC ZONE, EASTERLY WAVES AND SURGES IN THE EASTERLY JET OVER THE GANGES PLAIN AND THE SURROUNDING AREAS. (6) THE WAVES IN THE WESTERLIES AND THE WESTERLY JET STREAM OVER THE NORTHWESTERN INDIA.

(C) THE SURGES IN THE EQUATORIAL WESTERLIES ALONG THE WESTERN COAST n HAVE BEEN OBSERVED DURING THE MONSOON SEASON. HOWEVER, IN ANY ANALYSIS OF THE GENESIS OF WEATHER OR RAINFALL IN INDIA, THREE IMPORTANT FACTORS MUST BE TAKEN INTO CONSIDERATION: 1. THE HIGH MOUNTAIN BARRIERS IN THE NORTH RESTRICTING OR MODIFYING THE DIRECT NORTHERN INFLUENCES OVER THE COUNTRY. 2. THE LOCATION OF THE NORTHERN PART OF THE COUNTRY IN A TRANSITIONAL ZONE WlTH RESPECT TO THE INFLUENCES OF BOTH THE WESTERLY AND THE EASTERLY JET STREAMS DURING THE SUMMER. 3. INDIAIS APPENDAGE TO THE SOUTH OF THE BIGGEST LANDMASS OF THE WORLD AND AT THE SAME TIME ITS PENINSULAR PROJECTION INTO THE SUB-TROPICAL AND TRO- PICAL SEAS AND OCEANS. ALL THESE FACTORS MAKE INDIA A PECULIAR REGION SUSCEPTIBLE TO MULTIPLE WEATHER INFLUENCES. THE PRESENT PROBLEM IS NOT ONLY TO EXAMINE THE INDI- VIDUAL ROLE OF SURFACE DISTURBANCES AND UPPER AIR PERTURBATIONS WHlCH DETER- MINE THE RAINFALL, BUT TO ESTABLISH THE INTER-RELATIONSHIP BETWEEN THEM AND TO EVOLVE AN INTEGRATED PICTURE. THE FOCUS OF RECENT INVESTIGATIONS HAS AL- READY SHIFTED FROM THE SURFACE TO THE UPPER ATMOSPHERIC,CONDITIONS. IT HAS BEEN DEMONSTRATED THAT WHILE THE MONSOONS PROVIDE THE SOURCE OF MOISTURE, THE UPPER ATMOSPHERIC PERTURBATIONS ACT AS ITRIGGERS' FOR RELEASING RAIN- FALL. IT WILL, THEREFORE, BE PREPOSTEROUS TO LIMIT THE CONCEPT PURELY TO MONSOONS, SINCE THE CAUSES OF SUMMER RAINFALL IN INDIA APPEAR TO BE SlGNlFl- CANTLY LINKED TO THE EXTRAMONSOONAL CIRCULATION AND PERTURBATIONS. REFERENCES BHULLAR, G. S. (1952) ONSET OF MONSOON OVER DELHI, INDIAN JOURNAL OF METEOR- OLOGY AND GEOPHYSICS. VOL. 3, PP. 25-30. CHAKRAVORTTY, K. C. THE INFLUENCE OF WESTERN DISTURBANCES ON THE WEATHER AND OVER NORTHEAST INDIA IN MONSOON MONTHS, INDIAN JOURNAL BASU, S.C. (1957) OF METEOROLOGY AND GEOPHYSICS, VOL. 8, PP. 261-272. FLOHN, H. (1958) RECENT INVESTIGATIONS ON THE MECHANISM OF THE SUMMER MONSOON OF SOUTHERN AND EASTERN ASIA. MONSOONS OF THE WORLD, SYMPOSIUM, NEW DELHI. PP. 75-88. K IT IS ADMITTED, HOWEVER,THAT SUCH REGIONAL CONCENTRATION OF ACTIVITIES MAY HAVE BEEN OBSERVED DUE TO THE PARTICULAR NATURE OF INVESTIGATION INAN AREA WlTH SPECIFIC OBJECTIVES INVOLVED, AND THUS MAY BE BIASED.

KOTE SWARAM. P. (1958A) THE EASTERLY JET STREAM IN THE TROPICS. TELLUS. VOL. 10, PP. 43-57. KOTESWARAM. P. (19588) THE ASIAN SUMMER MONSOON AND THE GENERAL CIRCULATION OVER THE TROPICS. MONSOONS OF THE WORLD, SYMPOSIUM, NEW DELHI. PP. 105-110. KOTESWARAM. P. AND GEORGE. C.A. (1958) ON THE FORMATION OF MONSOON DEPRESSIONS IN THE BAY OF BENGAL. INDIAN JOURNAL OF METEOROLOGY AND GEOPHYSICS. VOL. 9. PP. 9-24. THE ROLE OF WESTERN DISTURBANCES IN THE PRODUCTION OF WEATHER OVER INDIA DURING DIFFERENT SEASONS. INDIAN JOURNAL OF METEOROLOGY AND GEOPHYSICS. VOL. 8. 1957. PP. 259-260. SOME ASPECTS OF THE RAINFALL IN INDIA DURING THE SOUTH- WEST MONSOON SEASON, MONSOONS OF THE WORLD. SYMPO - SIUM. NEW DELHI. PP. 185-194. PISHAROTY, P. R. AND DESAI. 6. N. (1956) RAMANATHAN. K. R. (1958) WESTERN DISTURBANCES AND INDIAN WEATHER. INDIAN JOURNAL OF METEOROLOGY AND GEOPHYSICS. VOL. 7, PP. 333-338. MONSOON AND THE GENERAL CIRCULATION OF THE ATMOSPHERE. MONSOONS OF THE WORLD. SYMPOSIUM, PP. 53-64. RAMASWAMY. C. (1 966) THE PROBLEM OF FRONTS IN THE INDIAN ATMOSPHERE, INDIAN JOURNAL OF METEOROLOGY AND GEOPHYSICS. VOL. 17. NO. 2, PP. 151-170. INTRODUCTION TO THE ATMOSPHERE, MCGRAW-HILL BOOK COMPANY, PP. 151-155. SOUTHWEST MONSOON RAINFALL IN GANGETIC WEST BENGAL AND ITS ASSOCIATION WlTH UPPER AIR FLOW PATTERNS. INDIAN JOURNAL OF METEOROLOGY AND GEOPHYSICS. VOL. 11. PP. 5-18. VENKATARAMAN. K. S. AND RAO. BHASKARA* N. S. (1965) EFFECT OF HIGH LEVEL DIVERGENCE ON THE RAINFALL DISTRI- BUTION OVER NORTHWEST INDIA ASSOCIATED WITH A SOUTH- WEST MONSOON DEPRESSION. INDIAN JOURNAL OF METEOROLOGY AND GEOPHYSICS, VOL. 16, PP. 411-420. YIN, M.T. (1949) A SYNOPTIC AEROLOGICAL STUDY OF THE ONSET OF THE SUM- MER MONSOON OVER INDIA AND BURMA. JOURNAL OF METEOROL- OGY, VOL. 6, PP. 393-000.

H) TIBETAN HIGH PRESSURE... :'... i. ISOHYET FIGURE 1 km. SCHEMATIC PROFILE OF THE WIND SYSTEMS ALONG 78" E. DURING JULY - AUGUST Q) JET STREAMS, :::::.LOWLEVEL JET (AFTER: FLOHN, KOTESWARAM, JOSEPH 8 RAMAN) FIGURE 2