ZONAL INDEX Mid latitudes are characterized by zonal westerlies This upper level flow pattern is circumpolar westerly current exhibit a wave pattern. The intensity of the zonal westerlies is one of the governing factors the behavior of weather pattern.

ZONAL INDEX The concept of Zonal index was introduced by rossby (1939) Numerical way of expressing the strength of the w’lies(at sea level or aloft) over a hemisphere in the temperate latitude (35–55°n). This relates to the form/state of general circulation, in terms of the longitudinal position and extent of centers of action. 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.

The index is obtained in terms of pressure values. Zonal index When the mean pressure difference between the latitudinal belts 350N and 550N is 8 hpa or more it corresponds to high index while it is 3 hpa or less it corresponds to low index. The index is obtained in terms of pressure values.

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 concept of zonal Index is helpful in assessing the general circulation pattern But its utility as a tool for predicting the circulation pattern has not found to be useful The reasons as suggested is that The total hemispherical Zonal index is not a primary parameter rather it is a derived quantity (dependent variable dependent on certain large scale energy producing mechanism)) While changes in the primary parameter can be accounted for based on its past behaviour, changes in the derived parameter can not be easily predicted in the same way Hence forecasting the ZI was not a very successful attempt .

ZONAL INDEX THE INDEX CYCLE From week to week there are large variations of the zi about its seasonal normal. The index cycle intensity varies from year to year. Most pronounced index cycle occurs during one particular time of the year (late february).

In each year atleast one period exists lasting for several weeks where in there take place a gradual decline of Index from high to low followed by a similar rise. B-is a(ATLANTIC) blocking wave defined as one in which 5-day mean sea-level pressure anomaly in the area500N to 700N and 200E to 500W has a positive value of 15 hPa or more and persists for atleast 10 days 5-day normal and monthly mean ZI bet 350N and 550N at 700 hPa over N- Hemisphere from 00 west ward to 1800 for winter months B B

ZONAL INDEX STAGES - - INDEX CYCLE There are four principal stages of Index Cycle (A) INITIAL HIGH INDEX. (B) INITIAL LOWERING OF ZONAL INDEX. (C) LOWEST SEA LEVEL INDEX PATTERN. (D) INITIAL INCREASE OF INDEX PATTERN.

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

(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) MAX LATITUDINAL TEMP GRADIENT BECOMING CONCENTRATEDIN 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.

ZONAL INDEX (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 aloft. (ii) 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.

ZONAL INDEX (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- establishes a normal pole ward 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 1944-49 THE FOLLOWING FEATURES ARE REVEALED:- (A) HIGH INDEX REQUIRES MID-LATITUDE BANDS OF CONFLUENCE WHERE COLD PC & WARM TROPICAL 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 INSTEAD ASSOCIATED WITH DIFLUENT PATTERN BUT THESE WHICH DO EXIST FORM IN LOW LATITUDES ONLY.

ZONAL INDEX Blocking is a necessary condition for inception of an Index cycle, but not a sufficient condition. So every blocking situation need not lead to initiation of an Index cycle. It is suggested that some external factors must be of importance.

Zonal Index CONDENSER THEORY The cold polar air is encompassed within the circumpolar westerly. The strength of the mid and upper tropspheric westerlies is a measure of the degree of containment of the cold air mass in the polar cap, Strong westerly forbid any extensive meridional transport. The radiational cooling enables this region to grow into a reservoir of cold air. The analogy is to the condenser. The containment is by the strong westerlies. Exchange of air between the poles and the tropics is a necessity for atmospheric heat balance. The blocking high waves are the ones which materialise in to Index cycle. This theory necessitate the building up of cold reservoir. ******FEB

The favourable period for the Index cycle is Feb Probable reason- The heat balance of the atmosphere demands an exchange of air between polar and equatorial regions. It is due to the slow moving blocking waves, with their strong and deep meridional component gradually enable depletion of the vast reservoir of cold air. The favourable period for the Index cycle is Feb Probable reason- Both the cold reservoir and the Atlantic blocking wave form at this period

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.

ZONAL INDEX REGIONAL INDEX AT TIMES in 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 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. FACTORS FAVOURABLE - EXTENSIVE COLD AIR RESERVOIR. - INITIAL FAVOURABLE ARLANTIC BLOCKING MECHANISM. - ACHIEVED BY LONG POLAR NIGHT OF LATE FEB.