ZONAL INDEX INTRODUCTION

ZONAL INDEX INTRODUCTION
THE MIDDLE LATITUDES ARE CHARACTERIZED BY ZONAL W’LY WINDS & CHANGEABLE WX ALL THE YEAR AROUND. THE UPPER LEVEL FLOW PATTERN REVEALS A BROAD MEANDERING CIRCUMPOLAR W’LY CURRENT IN THE MIDDLE LATITUDES. THE STREAM EXHIBITS WAVELIKE PATTERN WITH 3-5 LARGE SCALE TROUGHS & RIDGES KNOWN AS LONG WAVES OF SMALLER AMPLITUDE & WAVELENGTH KNOWN AS SHORT WAVES WHICH CAN BE CONVENIENTLY LOCATED ON THE 500 hPa SURFACE.

ZONAL INDEX INTRODUCTION
THESE CIRCUMPOLAR ZONAL W’LIES REACH MAXIMUM STRENGTH NEAR THE TROPOPAUSE IN THE MIDDLE LATITUDES AND ITS CORE IS KNOWN AS JET STREAM. TWO SUCH DISTINCT JETS CAN BE SEEN IN THE MIDDLE LATITUDES W’LIES – ONE IN THE HIGHER MID LATITUDES NEAR 300 hPa LEVEL AND THE OTHER IN THE LOWER MID-LATITUDES (WITHIN SUB-TROPICS) AT 150 hPa LEVEL. THE FIRST ONE IS VARIABLY BOUND UP WITH THE DEPRESSIONS & FRONTS OF THESE LATITUDES. THE IDEA OF CIRCUMPOLAR W’LIES SERVES AS A BACKGROUND AGAINST WHICH CAN BE DESCRIBED THE VARIATIONS IN THE PATTERN.

ZONAL INDEX ZONAL PATTERN
THE CONCEPT OF ZI WAS INTRODUCED BY ROSSBY (1939). IT EXPRESSES NUMERICALLY (AT SEA LEVEL OR ALOFT) THE STRENGTH OF TEMPERATE LATITUDE W’LIES (35–55°N) OVER A HEMISPHERE. IT IS CALCULATED BY TAKING THE DIFFERENCE BETWEEN THE SEA LEVEL PRESSURE BETWEEN LATITUDES 35°N & 55°N, ALONG SEVERAL MERIDIANS IN THE HEMISPHERE AND AVERAGING THESE DIFFERENCES. HIGHER THE INDEX THE STRONGER IS THE WIND.

ZONAL INDEX ZONAL PATTERN
THE INDEX IS OBTAINED IN TERMS OF PRESSURE VALUES. HOWEVER, IT CAN BE EXPRESSED IN METRE PER SECOND AND CAN BE EASILY RECOGNIZED FROM THE CONTOUR PATTERNS OF 700 hPa CHART. 8 hPa OR MORE VALUES CORRESPONDS TO HIGH INDEX WHILE 3 hPa OR LESS CORRESPONDS TO LOW INDEX.

ZONAL INDEX RELATIONSHIP WITH GENERAL CIRCULATION
ZI IS FOUND TO BE RELATED TO GENERAL CIRCULATION. THE VARIATION IN ZI HAS BEEN FOUND TO BE ASSOCIATED WITH PRONOUNCED LATITUDINAL AS WELL AS LONGITUDINAL SHIFTS IN THE CHARACTERISTIC BRANCHES OF THE GENERAL CIRCULATION. FOR EXAMPLE, THE LOW INDEX PATTERN IS ASSOCIATED WITH LONGITUDINAL SHIFT IN CENTRES OF ACTION BUT ALSO LATITUDINAL (S’WARD) SHIFT TO MID-TROPOSPHERIC BAND OF MAXIMUM W’LY WINDS. THUS THE LOW INDEX OF TEMPERATE LATITUDES CAUSES STRONG W’LY FLOW IN THE MIDDLE & HIGH TROPOSPHERE OF SUB-TROPICS. IN THIS SENSE ‘HIGH’ & LOW INDEX ARE MISNOMERS.

ZONAL INDEX PREDICTABILITY OF ZI
THE RELATION OF ZI WITH GENERAL CIRCULATION PATTERN FORCED RESEARCH EFFORT TOWARDS PREDICTING THE QUANTITY ZI. HOWEVER, NO FRUITFUL RESULT WAS OBTAINED. THE REASON STATED WAS THAT THE TOTAL HEMISPHERICAL ZI IS NOT A PRIMARY PARAMETER ACCOUNTED FOR ON THE BASIS OF ITS OWN PAST BEHAVIOUR; RATHER IT IS A DERIVED QUANTITY (DEPENDENT VARIABLE) WHOSE VARIATION CAN BE STATISTICALLY ACCOUNTED FOR ON THE BASIS OF ITS OWN PAST BEHAVIOUR; RATHER IT IS A DERIVED QUANTITY (DEPENDENT VARIABLE) WHICH REPRESENTS THE DEGREE OF LATITUDINAL ORGANIZATION OF CERTAIN LARGE SCALE ENERGY PRODUCING MECHANISMS IN THE MIDDLE & UPPER TROPOSPHERE.

ZONAL INDEX THE INDEX CYCLE
FROM WEEK TO WEEK EACH WINTER THERE ARE LARGE VARIATIONS OF THE ZI ABOUT ITS SEASONAL NORMAL, SHOWN IN THE FIG. 1. FROM THE FIGURE THE FOLLOWING FEATURES ARE REVEALED:- (A) DURING EACH YEAR THERE IS AT LEAST ONE PERIOD LASTING SEVERAL WEEKS IN WHICH THERE TAKES PLACE A GRADUAL DECLINE OF INDEX FROM HIGH TO LOW VALUES FOLLOWED BY A SIMILAR RISE. THE BEGINNINGS OF THESE PERIODS ARE MARKED BY ARROWS AND THE MEASURES OF THEIR INTENSITY DURATION IS INDICATED BY THE SHADED AREA. SUCH AN EXTENDED PERIOD OF FALLS & RISES HAS BEEN CALLED AN INDEX CYCLE. (B) THE INDEX CYCLE INTENSITY VARIES FROM YEAR TO YEAR. (C) MOST PRONOUNCED INDEX CYCLE OCCURS DURING ONE PARTICULAR TIME OF THE YEAR (LATE FEBRUARY).

ZONAL INDEX THE INDEX CYCLE
TO EXPLAIN THESE FEATURES THE FLOW PATTERNS ASSOCIATED WITH THE HIGH & LOW INDEX ARE TO BE UNDERSTOOD. FROM THE STUDY OF THE SEA LEVEL/MID TROPOSPHERIC HEMISPHERICAL MAPS, THE FOLLOWING DESCRIPTION OF THE INDEX CYCLE WAS GIVEN BY ROSSBY & WILLETT:- (A) INITIAL HIGH INDEX. (B) INITIAL LOWERING OF ZONAL INDEX. (C) LOWEST SEA LEVEL INDEX PATTERN. (D) INITIAL INCREASE OF INDEX PATTERN.

ZONAL INDEX ZONAL INDEX
(A) INITIAL HIGH INDEX (STRONG SEA LEVEL ZONAL WESTERLIES) CHARACTERISED BY:- (I) SEA LEVEL WESTERLIES STRONG AND NORTH OF THEIR NORMAL POSITION LONG WAVELENGTH PATTERN ALOFT. (II) PRESSURE SYSTEMS ORIENTED EAST - WEST, WITH STRONG CYCLONIC ACTIVITY ONLY IN HIGH LATITUDE. (III) MAXIMUM LATITUDINAL TEMPERATURE GRADIENT IN THE HIGHER MIDDLE LATITUDE, LITTLE AIR MASS EXCHANGE. (IV) THE CIRCUMPOLAR VORTEX AND JET STREAM EXPANDING AND INCREASING IN STRENGTH BUT STILL NORTH OF NORMAL SEASONAL LATITUDE.

ZONAL INDEX ZONAL INDEX (B) INITIAL LOWERING OF ZI CHARACTERISED BY :-
(I) DIMINISHING SEA LEVEL W’LIES MOVING TO LOWER LATITUDES AND SHORTENING OF THE WAVE PATTERN ALOFT. (II) APPEARANCE OF COLD CONTINENTAL POLAR ANTICYCLONES IN HIGH LATITUDES, STRONG & FREQUENT CYCLONIC ACTIVITY IN MIDDLE LATITUDES. (III) MAXIMUM LATITUDINAL TEMPERATURE GRADIENT BECOMING CONCENTRATED IN LOWER MIDDLE LATITUDES, STRONG AIR MASS EXCHANGE IN THE LOWER TROPOSPHERE IN MIDDLE LATITUDES. (IV) MAXIMUM STRENGTH OF THE CIRCUMPOLAR VORTEX AND JET STREAM REACHED NEAR OR SOUTH OF THE NORMAL SEASONAL LATITUDES.

(C) LOWEST SEA LEVEL INDEX PATTERN CHARACTERISED BY :- (I) COMPLEX BREAK-UP OF THE SEA LEVEL ZONAL W’LIES IN LOW LATITUDES INTO CLOSED CELLULAR CENTRES, WITH CORRESPONDING BREAK DOWN OF THE WAVE PATTERN OF THE WAVE PATTERN ALOFT. (II) MAXIMUM DYNAMIC ANTICYCLOGENESIS OF POLAR ANTICYCLONES AND DEEP OCCLUSION OF STATIONARY CYCLONES IN MID-LATITUDES AND N-S ORIENTATION OF PRESSURE CELLS. (III) MAXIMUM EAST - WEST RATHER THAN NORTH - SOUTH AIR MASS & TEMPERATURE CONTRASTS. (IV) CUTTING-OFF OF WARM HIGHS IN HIGHER LATITUDES AND COLD CYCLONES IN THE LOWER LATITUDES.

(D) INITIAL INCREASE OF SEA LEVEL INDEX PATTERN CHARACTERISED BY:- (I) GRADUAL INCREASE OF SEA LEVEL ZONAL WESTERLIES WITH AN OPEN WAVE PATTERN ALOFT IN HIGH LATITUDES. (II) A GRADUAL DISSIPATION OF THE LOW LATITUDE CYCLONES AND A MERGING OF THE HIGHER LATITUDE ANTICYCLONES INTO THE SUBTROPICAL HIGH PRESSURE BELT. (III) A GRADUAL COOLING IN THE POLAR REGIONS AND HEATING OF THE COLD AIR MASSES AT LOW LATITUDES RE-ESTABLISHS A NORMAL POLEWARD TEMPERATURE GRADIENT IN THE HIGHER LATITUDES. (IV) DISSIPATION OF THE HIGH LEVEL CYCLONIC AND ANTICYCLONIC CELLS WITH A GRADUAL REESTABLISHMENT OF THE CIRCUMPOLAR JET STREAM IN THE HIGHER LATITUDES.

ZONAL INDEX CERTAIN OTHER FEATURES
FROM THE 700 hPa FIVE-DAY MEAN MAPS DURING THE FOLLOWING FEATURES ARE REVEALED:- (A) HIGH INDEX REQUIRES MID-LATITUDE BANDS OF CONFLUENCE WHERE COLD PC & WARM TROPICAL OR PM AIR MASSES ARE DRAWN TOGETHER TO FORM HIGH SPEED JET STREAMS (W’LIES). THESE CONFLUENCE ZONES ARE SEEN IN PREFERRED LOCATIONS – NW COAST OF US & EASTERN NORTH AMERICA. (B) LOW INDEX PATTERNS ARE FREE FROM MID- LATITUDE CONFLUENCE BUT THESE WHICH DO EXIST FORM IN LOW LATITUDES ONLY.

SINCE CONFLUENCE REQUIRES OUT-OF–PHASE WAVE SYSTEMS IN DIFFERENT LATITUDE BANDS, THE DIFFERENTIAL MOVEMENT OF TWO WAVE TRAINS AT DIFFERENT LATITUDES IN THE UPPER TROPOSPHERE (AS SHOWN IN FIG. 2 & 3) SHIFTS THE ZONE OF CONFLUENCE MERIDIONALLY (HENCE THE JET STREAM WHERE THE ENERGY OF W’LIES IS STORED IN NARROW ZONES) AND HENCE CAN BE SEEN THAT THE EQUATOR WARD FROM THE JET STREAM DURING HIGH INDEX PHASE.

THE CONCEPT OF INDEX VARIATION CAN BE UNDERSTOOD FROM THE VARIATIONS IN WIND PROFILES ALSO. IT IS SEEN THAT THE AREA UNDER THE WIND PROFILE (WIND SPEED Vs LATITUDE) WHICH REPRESENTS THE TOTAL MOMENTUM OF THE W’LIES ALOFT NEARLY REMAINS CONSTANT FROM WEEK TO WEEK. COMPUTATION OF MOMENTUM & ITS COMPARISON WITH ZONAL INDEX SHOWS GREATER VARIABILITY OF ZONAL INDEX THAN OF TOTAL MOMENTUM FROM WEEK TO WEEK OR MONTH TO MONTH.

THIS CONCEPT IS ILLUSTRATED IN FIG. 4 WHERE THE ZONAL WIND PROFILES DURING HIGH & LOW INDEX STAGES OF THE INDEX CYCLES OF TWO DIFFERENT YEAR ARE PLOTTED. IT SUGGESTS THAT THERE IS A NET LOSS OF MOMENTUM AT INTERMEDIATE LATITUDES WHICH IS MADE-UP BY AN INCREASE IN LOWER LATITUDES. THE COMPENSATION WOULD BE MORE EXACT IF THE ENTIRE HEMISPHERE IS CONSIDERED FOR COMPUTATION INSTEAD OF THE REGION 22½°N – 72½°N BETWEEN 0 & 180°W AS IS THE CASE IN THE ABOVE EXAMPLE.

IT IS SEEN FROM THE ABOVE THAT THE TOTAL MOMENTUM OF THE MID-TROPOSPHERIC W’LIES AROUND THE HEMISPHERE REACHES A CERTAIN VALUE AT ANY GIVEN PERIOD EACH YEAR, AND IT IS ONLY THE DISTRIBUTION VARIATIONS IN ZI (OR ANY ZONAL FLOW BETWEEN FIXED LATITUDES) AND DUE TO THE DEGREE OF ORGANIZATION OF THE LARGE SCALE MID-TROPOSPHERIC WAVE SYSTEMS. IF THEY ARE MORE ORGANIZED THEY CAUSE CONFLUENCE PATTERN WHILE IF THEY ARE MORE DISORGANIZED THEY CAUSE DIFFERENT PATTERN. CONFLUENCE PATTERN STORES ENERGY IN JET STREAM WHILE DIFFLUENCE PATTERN REPRESENTS AREAS OF STRONG BLOCKING.

THEREFORE INDEX VARIATION IS DUE TO FACTORS LEADING TO PECULIAR LONG WAVE STRUCTURES FAVOURABLE TO SUSTAINED CONFLUENCE (FOR HIGH INDEX ) AND DUE TO FACTORS LEADING TO BREAKDOWN OF THESE WAVE STRUCTURES INTO DIFFLUENCE (BLOCKING ) PATTERNS WITH THEIR RESULTING LOW INDEX.

ZONAL INDEX INITIAL STAGE OF THE DECLINE
FROM THE EXAMINATION OF 700 hPa FIVE-DAY MEAN MAPS DURING THE INDEX CYCLE IT IS SEEN THAT EACH CASE OF INDEX DECLINE WAS ATTENDED BY A WAVE OF BLOCKING IN THE ATLANTIC IS BY A STRONG LOCALIZED REDUCTION IN ZI OVER ATLANTIC. OUT OF THREE REGIONS NAMELY ATLANTIC, NORTH AMERICA (65°W – 125°W) & EASTERN PACIFIC (135°W – 175°E ) ATLANTIC CONTRIBUTES MOST HEAVILY TO THE INITIAL INDEX DECLINE. THUS ‘BLOCKING IS NECESSARY BUT NOT A SUFFICIENT CONDITION FOR THE INCEPTION OF INDEX CYCLE’.

ZONAL INDEX INITIAL STAGE OF THE DECLINE
IN FIG.1 THE BEGINNING OF BLOCKING WAVES IN ATLANTIC ARE SHOWN BY ‘B’s WITH THE PERIOD OF BLOCKING AS THEIR SUBSCRIPT. HERE THE PERIOD OF ATLANTIC BLOCKING WAS DEFINED AS ONE IN WHICH 5- DAY MEAN SEA LEVEL PRESSURE ANOMALY IN THE AREA 50°N – 70°N & 20°E- 50°W HAS A POSITIVE VALUE OF 15 hPa OR GREATER AND PERSISTS FOR AT LEAST 10 DAYS. FROM THE FIGURE IT IS SEEN THAT DURING THE INDEX CYCLE A BLOCKING IS SEEN BUT AFTER EVERY BLOCKING WAVES AN INDEX CYCLE IS NOT SEEN. FROM THE MAPS, IT IS FOUND THAT ALL THE BLOCKING CASES INDICATED NO FUNDAMENTAL DIFFERENCES. THEREFORE, SOME EXTERNAL FACTORS (OUT SIDE ATLANTIC) LIKE THE DIFFERING QUALITY & QUANTITY OF RADIATION FROM THE SUN SHOULD BE OF IMPORTANCE IN THE INDEX DECLINE PHASE.

ZONAL INDEX THEORY OF INDEX CYCLE: CONDENSER THEORY
THIS THEORY WAS PROPOSED BY NAMIAS. IT IS AS FOLLOWS. ‘THE HEAT BALANCE OF ATMOSPHERE DEMANDS AN EXCHANGE OF AIR BETWEEN POLAR & EQUATORIAL LATITUDES. THIS EXCHANGE COULD OPERATE IN SHORT WEEK TO WEEK OSCILLATIONS OF THE INDEX BUT FOR THE SLOW MOVING BLOCKING WAVES WITH THEIR STRONG MERIDIONAL COMPONENTS OF FLOW GRADUALLY OPERATING TO DEPLETE VAST RESERVOIRS OF COLD AIR WHICH ARE IMPRISONED AT HIGH LATITUDES W’LIES. THUS THESE BLOCKING WAVES WHICH OCCUR SIMULTANEOUSLY WITH GREAT RESERVOIR OF COLD POLAR AIR (AFTER AN EXTENDED HIGH INDEX PHASE) ARE THE ONES WHICH MATERIALISE INTO AN INDEX CYCLE.

TO SUBSTANTIATE THIS THEORY THE 700 hPa ZI VARIATION IN WINTER MONTHS FOR TWO SIX YEAR PERIODS (1944 – 49: 1932 – 39) IS SHOWN IN FIG. 6. THE SIX YEAR PERIOD 1944 – 49 IS SAME AS THAT OF FIG 1. EXCEPT THAT THE AVERAGE ZI FOR 10 DAYS PERIOD IS PLOTTED HERE. FROM FIG. 6, IT IS SEEN THAT THOUGH THE SAME MAJOR INDEX CYCLE IS PRESENT IN THE 1932 – 39 DATA AS IN THE 1944 – 49 DATA, THE AMPLITUDE OF EARLIER (VARIATION) IS LESS. THIS IS DUE TO ASSUMPTION MADE IN CONSTRUCTING THE 1932 – 39 CURVE DUE TO LACK OF DATA. THE ASSUMPTION WAS THAT THE LAPSE RATE OVER ATLANTIC & PACIFIC WERE MOIST ADIABATIC. THE SECOND REASON IS IN CASE OF 1932 – 39) CURVE THE AREA CONSIDERED WAS 1/3rd MORE IN LONGITUDINAL EXPANSE THAN THAT OF AREA. THESE TWO FACTORS INCREASE THE VARIATION IN ZI IN PERIOD, TO THE ORDER SEEN IN PERIOD. THUS THERE IS A DISTINCT SUGGESTION OF A GLOBAL SINGULARITY IN THE GENERAL CIRCULATION.

THE CONDENSER THEORY CAN BE FURTHER SUBSTANTIATED FROM PRE- & POST-INDEX CYCLE TEMPERATURE CONSIDERATIONS OVER THE POLAR REGION. IT IS SEEN WHILE COLDEST TEMPERATURES ARE SEEN DURING JANUARY (PRE-INDEX CYCLE PERIOD), COMPARATIVELY (WARMER) HIGHER TEMPERATURES ARE SEEN DURING POST-INDEX CYCLE PERIOD. A BETTER MEASUREMENT OF INTENSITY CYCLE IS DETERMINED FROM THE WIND PROFILES OF THE LOW INDEX PORTION OF THE CYCLE. THE MOMENTUM IS COMPUTED FROM THESE PROFILES DURING THE LOW INDEX PERIOD (MID FEB – MID MAR) AND PLOTTED AGAINST THE JANUARY THICKNESS OF 1000 – 700 hPa LAYER (TEMPERATURES PRECEDING THE CYCLE OVERT THE POLAR REGION) OVER 55°N – 70°N & 0° - 180°N.

FROM FIG. 5 IT IS SEEN THAT LEAST MOMENTUM (LOSS OF MOMENTUM DURING INDEX DECLINE PHASE FEB-MAR) IS PRODUCED BY COLDEST JAN TEMPERATURES. THUS THE INTENSITY OF LONG INDEX CYCLES IS LARGELY DETERMINED BY THE RESERVOIR OF COLD AIR PRECEDING THEIR ONSET. THUS IN THE BLOCKING WAVES MARKED IN FIG. 1 MANY OF THESE FAILED TO PRODUCE A MAJOR CYCLE WERE TO CLOSE TO A RECENT PERIOD IN WHICH THE ATMOSPHERIC CONDENSER HAD DISCHARGED IN THE FORM OF AN INDEX MINIMUM WELL BELOW NORMAL. CERTAIN RECOVERY TIME IS NEEDED TO PRODUCE ANOTHER DISCHARGE.

THE FLOW PATTERN DURING THE INITIAL DECLINE OF THE INDEX CYCLE OVER ATLANTIC FAVOUR CELLULAR STRUCTURES WITH WARM HIGHs AT HIGH LATITUDES AND COLD LOWs AT LOW LATITUDES. DURING LATE WINTER BECAUSE OF SURFACE STABILITY IN NORTHERN LATITUDES IN WARN HIGHs BECOME MORE STABLE AND PERMANENT. IN LOWER LATITUDES BECAUSE OF STRONG VERTICAL TEMPERATURE GRADIENT & UNDERLYING WATER SURFACE THE INSTABILITY CAUSES CONVECTIVE SHOWERs THEREBY REDUCING LATENT HEAT. THIS ARRESTS THE FILLING OF THE COLD LOWs AND MAINTAIN THEM. THUS THE FLOW PATTERN AND THE COLD RESERVOIR ARE FAVOURABLE IN LATE FEB FOR INDEX DECLINE.

BLOCKING HIGH CONTOURS ARE OUT OF PHASE WITH THE THERMAL PATTERN. APPRECIABLE AMOUNTS OF WARM AIR ADVECTION ARE FOUND TO THE WINDWARD OF THE UPPER LEVEL RIDGE, WHICH USUALLY MOVES VERY SLOWLY, GRADUALLY EXPANDS NORTHWARDS AND AFTER SOME CONSIDERABLE TIME THE STAGE SHOWN IN FIG 2 IS REACHED. DURING THIS STAGE THE FLOW AROUND THE RIDGE HAS CHANGED FROM A WAVE-SHAPE TO AN OMEGA () SHAPED PATTERN, AS SHOWN IN (FIG. 2) IN FAVOURABLE CIRCUMSTANCES, THE BLOCKING HIGH ACQUIRES APPRECIABLE DIMENSIONS AND INTENSITY; AND THE ZONAL CURRENT DEVELOPS INTO TWO DISTINCT BRANCHES. SHARP TROUGHS OR CUT-OFF LOWS DEVELOP TO THE SW & SE AS SHOWN IN FIG 3

ZONAL INDEX INITIAL INCREASE OF ZI FOLLOWING THE MINIMUM
THE RECOVERY OF ZI FROM ITS MINIMUM OF THE CYCLE TAKES PLACE BY EARLY MARCH. THE CONTRIBUTION MAINLY IS FROM WESTERN ATLANTIC ZI. THE RECOVERY COULD BE POSSIBLY DUE TO INCREASE IN INSOLATION WHICH DESTROYS THE ELEMENTS RESPONSIBLE FOR LOW INDEX THE COLD POOLS OF LOW LATITUDES & WARM POOLS OF HIGH LATITUDES.

ZONAL INDEX QUESTIONS & ANSWERS
QUESTION 1: WHY DURING EACH YEAR IS THERE A TENDENCY FOR THE OCCURRENCE OF AT LEAST ONE INDEX CYCLE LASTING SEVERAL WEEKS? ANSWER: BECAUSE THE HEAT BALANCE OF THE ATMOSPHERE DEMANDS AN EXCHANGE OF AIR BETWEEN POLAR AND EQUATORIAL LATITUDES. THIS EXCHANGE COULD OPERATE IN SHORT WEEK-TO-WEEK OSCILLATIONS OF THE INDEX, WERE IT NOT FOR SLOW-MOVING BLOCKING WAVES WITH THEIR STRONG, DEEP, MERIDIONAL COMPONENTS OF FLOW GRADUALLY OPERATING TO DEPLETE VAST RESERVOIRS OF COLD AIR WHICH ARE IMPRISONED AT HIGH LATITUDES DURING EXTENDED PERIODS BY STRONG MID-LATITUDE W’LIES.

QUESTION 2: WHY DO THESE CYCLES VARY IN INTENSITY FROM ONE YEAR TO THE NEXT? ANSWER: BECAUSE, FOR REASONS UNKNOWN, THE QUASI-PERMANENT ANCHOR TROUGHs & RIDGEs OF THE MID-TROPOSPHERE ARE FIXED IN DIFFERENT REGIONS IN DIFFERENT YEARS AND THUS IN SOME YEARS THE FLOW PATTERNS BECOME MORE HIGHLY ORGANISED INTO CONFLUENCE (HIGH INDEX) BANDS THAN OTHERS. IN THIS MANNER, THE COLD RESERVOIR BECOMES STORED OR DEPLETED AT DIFFERENT TIMES OF THE YEAR. SINCE THE INTENSITY OF THE CYCLE APPEARS TO BE DEPENDENT UPON THE AMOUNT OF STORED COLD AIR, WHICH IN TURN DEPENDS BOTH ON THE NET OUTGOING RADIATION & UPON THE STORAGE TIME, IT IS CLEAR THAT THERE MUST BE CONSIDERABLE VARIATION IN THE CHARACTER OF INDEX CYCLES THROUGH THE YEARS. WHAT WE DO NOT YET KNOW IS WHY THE QUASI-PERMANENT ANCHOR TROUGHs & RIDGEs OF MID-TROPOSPHERE ARE FIXED IN DIFFERENT REGIONS IN DIFFERENT YEARS; IT IS QUITE POSSIBLE THAT LAG EFFECTS OF OCEAN CURRENTS, SNOW COVER, ETC. ARE DOMINANT FACTORS.

QUESTION 3: WHY DOES THERE APPEAR TO BE ONE PARTICULAR PERIOD (LATE FEBRUARY) WHEN THE MOST PRONOUNCED CYCLE OCCURS? ANSWER: BECAUSE THE TWO PRIMARY FACTORS RESPONSIBLE FOR THE CYCLE, AN EXTENSIVE COLD AIR RESERVOIR AND THE INITIAL, STABLE, CELLULAR-TYPE. ATLANTIC BLOCKING-MECHANISM ARE BOTH FAVOURED AT THIS TIME OF YEAR. THE COLD RESERVOIR IS FAVOURED BY THE LONG POLAR NIGHT OVER A PRE-ESTABLISHED, EXTENSIVE, SNOW COVER. THE ATLANTIC WARM ANTICYCLONE TO THE NORTH AND THE COLD LOW TO ITS SOUTH, CONCOMITANT FEATURES OF BLOCKING, ARE BOTH FAVOURED BY CONSIDERATIONS OF VERTICAL STABILITY IN THE FRICTION LAYER AS DEEP POOLS OF POLAR AIR MOVE INTO S’LY LATITUDES. THE STABILITY UNDER THE WARM HIGHS RESULTS IN LESS “LEAKAGE” FROM BELOW, WHILE THE INSTABILITY AT LOW LATITUDES, EXTENDING THROUGH A DEEP LAYER OF AIR OVER WATER SURFACES, MORE THAN COMPENSATES THE FRICTIONAL FILLING BY PRODUCING COPIOUS SHOWERS RELEASING MUCH LATENT HEAT.

ZONAL INDEX REGIONAL INDEX
AT TIMES ONE SECTOR OF THE HEMISPHERE, THE CIRCULATION MAY HAVE A HIGH INDEX CHARACTER, WHILE IN ANOTHER, PERHAPS ADJACENT, THE INDEX MAY BE LOW (NAMIAS (1952)). THUS THE STUDY OF REGIONAL INDEX (ZONAL) FOR CUT-OF LOW WAS ATTEMPTED BY WG CDR AJIT TYAGI. IT WAS FOUND OUT THAT REGIONAL INDEX WAS BELOW NORMAL ON THE DAYS OF CUT-OFF LOW. IT WAS OBSERVED THAT FEBRUARY & MARCH MONTHS ARE FAVOURABLE FOR THE DEVELOPMENT OF CUT-OFF LOWs OVER NW INDIA.

ZONAL INDEX SUB-TROPICAL WESTERLIES IN RELATION TO LARGE SCALE FAILURE OF MONSOON – PV JOSEPH PERSISTENT & DEEP S’WARD INTRUSION OF SUB-TROPICAL W’LIES OF THE UPPER TROPOSPHERE INTO AREAS IMMEDIATELY WEST OF INDIA & ADJOINING INDIAN AREAS DURING THE MONSOON SEASON IS ONE OF THE PRINCIPAL CAUSES FOR THE LARGE SCALE FAILURE OF INDIAN MONSOON. THE S’WARD INTRUSION OF W’LIES AS SEEN FROM MONTHLY MEAN WIND CHARTS AND INFERRED FROM CYCLONIC TRACKS, HAS A PERSISTENCE OF A FEW MONTHS PRIOR TO THE MONSOON SEASON AND ON MANY OCCASIONS RIGHT FROM THE PREVIOUS WINTER/POST-MONSOON SEASON.

ZONAL INDEX MONSOON INDEX
SIMILAR TO ZI – DEFINED IN TERMS OF METRE PER SECOND AS DIFFERENCE BETWEEN VELOCITY 10ºN & 25ºN AT 700 hPa. IN TERMS OF PRESSURE AT SURFACE, MONSOON INDEX OF 11 hPa IS CONSIDERED AS NORMAL. HIGHER THE MONSOON INDEX BETTER IS THE MONSOON.

ZONAL INDEX WHY INDEX CYCLE ? TO MAINTAIN HEAT BALANCE.
WHY THEY DIFFER IN INTENSITY ? FAVOURABLE LOCATIONS OD SEMI-PERMANENT SYSTEMS (TROUGHS & RIDGES) REQUIRED FOR MAXIMIMUM TRANSFER OF POLAR COLD AIR MASS TO TROPICS. FACTORS FAVOURABLE - EXTENSIVE COLD AIR RESERVOIR. - INITIAL FAVOURABLE STABLE CELLULAR ARLANTIC BLOCKING MECHANISM. - ACHIEVED BY LONG POLAR NIGHT OF LATE FEB.

ZONAL INDEX ANTICYCLONICITY INDEX (BRYSON & LABEY (1958)) 55ºN ZI1
ANTICYCLONICITY INDEX (AI) = ZI1 - ZI2

ZONAL INDEX ANTICYCLONICITY INDEX (BRYSON & LABEY (1958)) H - 20
For Example, ºN ZI1 ºN H - 05 ZI ºN - 20 ZI1 = 15 – 5 = 10 & ZI2 = (-5) – (-20) = 15; Therefore, AI = - 5 metre per second

ZONAL INDEX REFERENCE:
‘THE INDEX CYCLE AND ITS ROLE IN GENERAL CIRCULATION’ BY JEROME NAMIAS [CLIMATE IN REVIEW – MC BOYLE PAGE 48 (LIBRARY REF NO ]

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