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THE TROPICAL EASTERLY JET AND OTHER REGIONALANOMALIES OF THE TROPICAL CIRCULATION

H. Flohn

University of Bonn, Germany

1. INTRODUCTION

Large-scale and regional anomalies of the atmos^-heric circ-ulation in tropical latitudes are the origin of the so-ca led "problemclimates" (Trewartha). Most of them can be understood if e properlytake into account several effects:

(a) Effects of the differential heat balance of ocea and/ \s in tropical latitudes (sections 2 and ).

(b) Effects of differential heating on a regional scale; forexample, the heating of high plateaux, cooling of lakes(section k).

(c) Divergence effects: directional divergence, speed diver-gence, frictional (stress) divergence (section 4).

(d) Effects of the asymmetry of the meteorological equator,produced by the different heat balances in the arctic *and antarctic regions (section 5)-

2. RADIATION AND HEAT BALANCE

On a rotating earth with a homogeneous surface weather andclimate would be functions only of latitude and season. On the realplanet earth, the differentiated radiation and heat balances of variousurfaces at various altitudes, together with the effects of frictionaldifferences, are responsible for the large-scale, regional and localgradients of temperature, humidity, pressure and wind. The primarycause of all effects mentioned above — including the physical originof most divergence effects - can be found in horizontal and seasonaldifferences in the heat balance of the surface of the earth.

The radiation and heat balance of the earth may be expressedby

Q = (s + H) (i - A) - (E - G)

effective incoming radiation effective outgoing radiation

Q - Us + Ua + Ue + Um

soll air evaporation meltingsea (sensible heat) (latent heat)

160

where

Q =S =H =A =

net radiationsolar radiationdiffuse sky radiationsurface albedo

EGU

161

outgoing terrestrial radiationatmospheric counter-radiationheat transfer

The tropical heat budget is shown in Table 1.

TABLE 1

Energy Budget (Percentage of Q)

Tropical Continents:

Equatorial humid zoneSemi-humid zone

Tropical Oceans:

Us

00

+5-10$

ua

22-30$50-65$ •

7-14$

ue

70-78$35-50$

80-98$

A special feature of the equatorial latitudes is that withthe near vanishing of the Coriolis parameter f = 2 {"!' sin 0 , severalterms in the equations of motion are no longer negligible, in contrastto the assumptions normally made in middle-latitude atmosphericdynamics. Another remarkable difference is connected with the occas-ional existence of "anomalous winds" (l), especially in cases of vort-;ity advection across the equator.

4

Since the tropical region - when defined between latitudes30°N and 30°S - covers one half of the earth1s surface with an averageland area of about 26 per cent, we should expect large-scale spatialvariations in its circulation patterns. This is especially true ifwe compare an oceanic section - äs in the central Pacific - with acontinental section, like Africa.

In an oceanic section the weak response of the air temperat-ure to the annual trend of the incoming radiation produces onlyinsignif icant seasonal differences in the f ields of tempera-tu/re ,pressure and wind. The physical cause of this weak response can befound (Table 1) in the fact that over the low-latitude oceans most ofthe net radiation is used for evaporation and only about 10$ is avail-able for heating the air. The annual average of Us is small, butdue to the heat storage capacity of the oceans rather large seasonalvariations of this term are observed. In oceanic sections thetropical easterlies between the subtropical anticyclonic cells decre-se with height and disappear between 6 and 10 kms, while at about12 kms (200 mbs) the westerlies from both hemispheres frequentlymerge above the intertropical convergence zone (ITC), which is partlyreplaced by weak but persisting cyclonic cells in equatoriallatitudes.

162 . • '•5 " '

In. a con.tln.en.tal section tlie response of'air temperaturesto tlie annual course of the incoming radiation is mucli stronger dueto tlie larger magnitude of Ua (Table 1). "We therefore observe a

. large seasonal shift of the zone with h.igh.est temperatures, lowestsurface pressure and converging winds from about 18°S in. January to18°N in July and under exceptional circumstances (äs in tlie Indo- ,Pakistan subcontinent) even t o 28 °N. In tlie equatorial regions weobtain - due to tlie dynamical effects described in section 5 - a-beltof äquatorial westerlies. In most areas tliis belt persists duringtlie whole year. In tlie neiglibouring tropical zones (approximately5-l8°Lat.) tliis large swing of the pressurexand wind belts leads totlie occurrence of a monsoonal sliifting of winds, with quasigeostrophicwes t erlies during summer and eas t erlies during winter. In tlie upperh.alf of tlie tropical troposphere, i.e. between 500 and 100 mbs( ~ .. 6-1 6 km) a steady easterly current prevails, separating the extra-tropical westerlies from both liemisplieres. Above tlie continent ofAfrica tliis pattern does not only persist during tlie extreme seasons, •but occurs also during tlie transition seasons. During tlie extremeseasons tliis wind pattern is correlated with. a strong troposplierictemperature gradient from tlie meteorological to tlie geographicalequator. Above North. Africa and ,India tlie äquatorial air at 200 or150 mbs is 7-11° cooler tlian above tlie central Saliara or tlieHimalayas.

. . 3. LARG-E-SCALB CIRGULATION PATTERNS .'• ,

Tlie belt of liigh.-troposplieric easterlies in tropical latit-udes is especially strong and persistent during tlie nortliern summer ; .above tlie Indo-African region. Here it is accentuated äs a TropicalEasterly Jet (TEj) (10), extending witn. an average core speed of50-70 knots in tlie layer 100-200 mb between long'itudes 100° and 30°E/and with. its cliaracteristic constancy from tlie Philippines to ¥est .'Africa, i.e. over a distance of 150° of longitude or nearly 17?000 kms.Unfortunately, a lack of internal consistency in tlie radioson.denetwork do-es not allow a reliable day-by-day analysis of tliis jet in ,its central region. However a statistical evaluation of all availableupper wind data at and above 200 mbs ( ~ 12 kms) during July andAugust 1956-62 above tlie Indo-Pakistan subcontinent and Africa leads '.to some general conclusions. They are based on tlie following maps '-'.aid diagrams :

(a) rresultant winds and isotachs at 'the, 200, 150 and 100 mblevelsf • • •

. (b) resultant winds and isotacns at tne level of jnaximum .speed

(c) meridional cross-sections of the zonal flow along .78 E ,and •10°E '" ' '

. : • (d) meridional cross —sections of temperature along 78 E(including Tibet). ..

In addition to tliis, an evaluation of the statistical param-eters of the wind distribution permits some synoptic conclusions.. In

163

its core near 15 N, which rises from 200 mbs at the southern' edge to100 mbs at the northern edge, the speed can reach 120-150 knots.Large-scale meandering Rossby waves are not observed, the prevailingmotion being more or less pulsatory, with only small deviations in ameridional direction. The pulses of the TEJ - with a length of3-6 days - seem to coincide with variations in the intensity of thewarm anticyclonic cell above the well-heated Tibetan plateau. Thisfeature was suggested in 1950 (3) and has recently been confirmed (6).Only a very small portion of the total kinetic energy (12-25 per cent)is due to travelling pulses or eddies and wind-constancy reaches97-<

If we define the long axes of the isotach ellipses äs jets, we observe a systematic deviation of the resultant winds: eastlongitude 80°E all resultant winds have a marked northerly compon-

ent, while west of this longitude a southerly component predominates.Only along the equatorial boundary are a few exceptions found. Thissuggests a large-scale non-geostrophic cross-circulation in which thedirection reverses from the entrance to the exit region of the jet, äsdescribed in the usual jet-stream models. Thus, in the entranceregion over SE-Asia one may observe a direct circulation cell in thetroposphere similar to that described by Koteswaram (10). In theexit region, however, extending from about 70°E to 10°¥, over 10,000kms, we observe an indirect tropospheric circulation with rising coolequatorial air and subsiding warm air at subtropical latitudes(20°-30°N).

In this case, it is po'ssible to compute the average " diverg-ence aloft in the exit region. Arranging all available upper windstatistics (averaged for the 8 months July-August 1957-60) above Africa(40-16°N) into a northern and a southern group, with a meridionaldifference ,of 6.7°Latitude = 7̂ -5 kms and a zonal distance of 5»5OO kms,den-Abidjan), we obtain the following humerical results for thelyer 500-100 mbs

9_v 3u. divh \3y 3x • ~ h

2-'65 - 0-83' = 1 - 8 2 (unit: 10 sec"1)

;<'The mass deficit produced by a meridional outflow in the

layer 5OO - 100 mbs could be balanced by an equivalent inflow in thelower troposphere below 5QO mbs and. in the stratosptiere above 10O mbs.In our model we disregard such possible stratospheric transport aridassume, under stationary conditions ( 3yO / at = 0) , a meridional cir-culation restricted to the troposphere. This necessitates an averagevertical uplift through the 500 mb level of ,1-13 cm sec~1 (about1 km per day) over an area of about 4-1 x 10° km^. .

A more detailed consideration of the data shows that theaxis of the TEJ core - when defined independe.ntly of height - deviatesby about 7° from a latitude circle, running from 83° to 263° in theusual notation (90° = E). In the southern group of wind data theaverage flow of the 500-1OO mb layer is also found to be 83°. This

12

164 ' .;'

J

*f

coincidence suggests a small rotation of the coordinate system in '."such, a sense that the x-axis runs parallel to 83° • In this system.the flow in the 500-100 mb layer.through tne southern boundary is '-.zero . ¥e may now assume a closed system with two solid vertical iwall s, one near 7°N, and the other near the siibtropical divergenceaxis at 25°N, •where the zonal flow in the layer 500-100 mb disappears.

, • v

In this1 model, the mass transport between 500 and 100 mbperpendicular to the jet -axis through the northern limit of the diver-gence area near 14°N is equivalent to an average wind of 135 cm se"c~^from 173°- This produces, in the larger northern convergence area,extending between approximately latitude 14°N and 25°N and covering.a"bout 6-7 x 10° km^, a downward transport through the 500 mb layer withan average forced subsid:ence of 0-69 cm sec or about 600 m per day.

t • "N

This strong cross-circulation in the exit region of the TEJsuggests a physical Interpretation of some climatological facts in thelower troposphere. Over the African continent the weather patternsat both sides of the ITO - when defined äs coinciding with the equato-rial pressure trough - are remarkably different (5) (?) : a"t thenorthern edge, we observe persistently dry, 'clear weather with. hazeand subsiding air, "but at the southern edge there occur several beltsof convective activity, increasing southward to a maximum at a merid-ional distance of about 600-800 kms. The usual explanation of thisasymmetry, äs a consequence of air-mass distribution, is not consist-ent with the length of the air-trajectories over the central Sudan.In addition to this, a systematic evaluation (5) of more than 200,000surface wind observations for the equatorial Atlantic, together with-all available pilot balloon data, has demonstrated that the sameasymmetric distrxbution of the horizontal divergence and vertical wind'component with respect to' the ITC is o"bserved over the ocean. This "result depends only on the observed wind field and suggests that in-this area vertical instability and moisture content are not conserva-tive air-mass properties, but are ' cons'equences of the dynamics of thewind-field. Earlier, no satisfactory physical e_xplanation of this;'asymmetry could "b'e given. ' •

i' • \g the season of the TEJ, that is from mid-June t o ''•" '

early September, the average position of the ITC in Africa is at about18°N, that is within the belt of forced subsidence at the northern .".edge of the TEJ. ' The region of greatest convective activity coin-..cldes with. the central part of the TEJ, with. divergence aloft and.large-scale lifting. Here and in this season, th.e ageostropliic . .. •circulation across the exit region of the TEJ allows a rational phy-•'sical Interpretation, including the extraordinary persistence of the•large summer-time upper anticyclone above Northern Africa and theNear East. In this area, the tropospheric circulation is oppositeto the usual sense of the Hadley circulation: here we observe," over:- .'.about one quarter of the earth's circumference, a strong anti-Hadley'circulation with rising cool air on the equatorial side and subsidingwarm air in the subtropical cell. For^the northern winter, the •numerical intensity of the planetary Hadley cell has been investigatedby Mintz and Lang (15) and Palmen, Riehl and Vuorela (l?)'- These -• •authors estimate the sinking motion in the subtropical cell to be

~ ~

1650.27 cm sec and 0.31 cm sec ', respectively. During summer, Mintzand Lang obtained only 0.05 cm sec~^: here the small latitudinal orplanetary average is nothing but the difference between two (or more)areas with comparatively large vertical motions of opposite sign.

As an energy-consuming circulation, the indirect cell inthe TEJ exit region needs an external source of energy. This energyis most probably supplied by the remarkable heating of the middle andupper troposphere above the Tibetan plateau and adjacent highlands (6)(14), which increases the meridional pressure and temperature grad-ient towards the equator, and here leads to a marked baroclinic stateof the tropical atmosphere. In addition to this persistent transferof sensible heat from an elevated heated surface, the strong releaseof latent heat in the regions of the maximum orographic rainfall inwthe Himalaya and its extensions towards the SE in Assam and UpperH3urma, must also be considered.

For other continents thorough investigations have revealedthat no really comparable easterly jet exists below the tropical trop-opause. Vhile the cor'e of the TEJ is concentrated in the lowerstratosphere, in all other areas the easterlies are much weaker andincrease with height above 100 mbs. They represent obviously thelower boundary of the subtropical maximum of the stratospheric easter-lies in the summer hemisphere. The development of a strong andpersistent TEJ is restricted to Southern Asia and North Africa in thenorthern summer, due to the exceptional heat balance of the AsiaticPlateau during that season. Since no representative data for theheat balance of the Tibetan Highlands are available, we may comparethe data obtained by Zuev (20),Table 2, during August for the Pamir(Kara-Kul, 3990 m), which are practically the same äs for the adjacentlow-level desert Karakum. Obviously it is only the larger contrib-ution of advection and vertical mixing in elevated regions which pre-vents the temperatures from rising to the same values äs at sea—level.

TABLE 2

Energy Budget for the Pamir anc(units : g cal cm""

the Karakum Desertday"1

•

Effective incoming radiation (Effective outgoing radiationNet radiationHeat transfer surface-airHeat transfer surface-soilLatent heat of evaporation

S+H) (1-A)E - GQvausue

Pamir

5̂ 6-3052kl2kO

- 1718

Karakum

525~2kk :?,28127k.7O-'

From this reasoning we may also understand why the tropicalsummer rains do not extend (over Africa, Near East and parts of theArabian Sea) farther north than about 16-18°N, in contrast to the Sit-uation over North America or over the southern continents, where

12«

166*

tropical and continental summer rains (which can here hardly bedistinguished) extend up to 'Latitude 25° and more. Even in the GreatAustralian Desert summer precipitation is much more frequent than inthe extensive belt from the western Sahara to Baluchistan.

4. REGIONAL DIVERGENCE EFFECTS

¥ithin the equatorial rain belt in latitude 0-10 N, morethan 90% of the total area enjoys sufficient rainfall, especiallyduring the northern summer. Only one regional Interruption exlsts;this occurs at the Somali peninsula and extends from eastern Ethiopia .to the central Arabian Sea. In this. region we observe large aridareas with less than 500 mm of rain, in some areas less than 100 mm,'the bulk of which falls during spring and fall. During the summerperiod mid-June to early September, rainless weather with a strongsteady SV monsoon is predominant. Even on the mountain ridges theconvective showers during summer are much less effective than inspring and fall. As a remarkable exception tö the results in section3, arid conditions during the summer extend from northern Tanganyikathrough NE Kenya, Somalia and eastern Ethopia (Ogaden) to a triangulärsection .of the Arabian Sea reaching a point near 15°N, 65°E. Theusual explanation of this aridity is based on the parallelism betweenthe SW monsoon and the eastern coast of the Somali peninsula. Takinginto account the different surface friction (stress) over land andsea, we would expect a divergence zone along the coast. Furthennore

j at the ocean surface the wind stress produces an off-shore currentfrom W to E, together with cool upwelling water along the coast. Thisis well reflected in sea and air temperatures äs well äs in hazefrequency. But these explanations can be valid only for a smallcoastal zone.

Recent investigations (8) have shown that the spring and fallrains of Somalia are by no means exceptional, but coincide well (in »season and latitude) with the beginning and ending of the summer rainyseason in other sections of North Africa, for example along 32°E(Sudan-Ethiopian border) or 8°E (Nigeria). During the season of the.S¥ monsoon, however, two additional divergence effects occur togetherwith the coastal effects mentioned above.

^

Due to the increase of the zonal pressure gradient polewardfrom the equator, the average speed of the S¥ monsoon at sea increaseswith distance from the equator: Latitude 0-5°N Beaufort 3-9, 5°-10°NBeavifort 5-6 and Latitu.de 1O°—15°N Beau-fort 6. O, with. a constancy >of

-, 8O— 90/&. The same speed divergence can be expected over land, forexample over interior Somalia and Ogaden, even if conclusive evidencewill hardly be obtainable. . :

- i . '

Furthermore, an inspection of the obse'rvations of surfacewinds taken at noon — that is around the time of greatest vertical mom—entum exchange - shows a marked directional divergence between a flowfrom S or SSE in the area south of the Ethopian highland (Lodwar,Marsabit, Moyale, Neghelli) and one from the SS¥ to S¥ along the Kenya-Somalia border (Mandera, ¥ajir) and in southern Somalia (Mogadishu,Belet Uen). In the region between Lake Rudolf and the Somalia borderthe belt of low-level equatorial westerlies (in the northern summer)

16?

is obviously interrupted, due to the large-scale influence of th.e,Ethopian highlands. The physical cause of this divergence can beattributed to a thermally driven regional circulation around theEthopian highlands, with convergence in the lower layers and anti-cyclonic flow patterns probably at 600 mbs and above. During thesummer rainy season, the main heating effect of the highlands may bedue to the release of latent heat "in the cloud layer, that is mostlybetween 700 and 550 or 500 mbs. **".-'

During the season of the fully developed S¥ monsoon (mid-June to early September) all possible divergence effects act togetherto produce large-scale subsidence with a maximum along the eastern.coast of the Somalia peninsula. These effects are

(a) directional divergence due to the heating of the Ethopianhighland

(b) speed divergence due to a northward increase of the pressuregradientf '

(c) frictional stress divergence between land and sea in thecoastal zone

(d) anticyclonic (off-shore) deviation of the wind-driven oceancurrents with upwelling cool water along the coast.

Taken together, these effects .are obviously much stronger than thelarge-scale divergence aloft äs described in section 3«

5. HEMISPHERIC ASYMMETRY EFFECTS

One of the strongest anomalies of the tropical circulation isthe contrast of an equatorial rain belt north of the equator with ary equatorial zone just south of the equator. This pattern is öbser-ed not only in the eastern and central parts of the Pacific, but'alsoin the eastern and central parts of the equatorial Atlantic. Theexplanation of the equatorial rain belt by the "clash of the trades"along the ITC is well-known: we have only to explain why the ITC rem-ains during the whole year in the northern hemisphere. Usually theaverage position of the meteorological equator north of the geograph-ical equator häs been explained äs a consequence of the warming ofthe northern continents during summer. But this hypothesis cannotbe valid during winter and over the largest of the oceans. Recentinvestigations liave sliown that tlae extraordinarry strength of -tlie south—ern westerlies - which are stronger, even in the southern summer, thanthe northern wes'terlies during winter (1l) - is correlated with ashift of the southern subtropical anticyclonic cells towards 'theequator. This foroes the powerful trades of the southern hemisphereto cross the equator and penetrate into the northern hemisphere.

In low latltudes a very strong correlation exists between thezonal wind component u and the vertical stability of the lower trojpos-phere. Generally speaking westerly/easterly winds are correlated withunstable/stable conditions, and therefore with high/low values forcloudiness and rain frequency. One can try to Interpret this statis- .

tical result by considering the equation of" vertical motion (w) to-gether with the frequently neglected vertical•component of.the •Goriolis force. • • .

dw 1 9P . qdt /<> 3z

l. u ( = 2 w cos

Tliis equation however, yields greatly exaggerated numerical resultsfor w. • A more rational physical interpretation has been given by. .Lettau (13)} who introduces eddy viscosity terms into the equation ofmotion and derives, under reasonable assumptions, an expression forthe height-variation of the turbulent deviations ("w1 ) of the ve,rticalwind component (w) from its' temporal mean T7 ("w1 = w - "w) . This./expression is . *•

9z I. U

Under these assumptions, an-increase/decrease of cofrvecti-veactivity with height should be correlated with westerly/easterly •winds, äs observed in climatological statistics. In individual cases,such a statistical tendency can certainly be overbalanced by synopticor large^-scal'e convergence effects, ,as for example in the equatorialrain belt north of the equator where easterly surface winds still •prevail. . . : . .

In a belt of width 400 kms on both sides of the ITC, 60-<of the total kinetic energy of the surface winds is contributed byeddies, in contrast to a figure of only 35-̂ 0$> in the area of the un-disturbed trades'(5)- • . •

If we neglect,. at'first, certain anomalous periods withlengths of soine months, the extreme aridity of the equatorial dry zone,especially in .the vicinity. of the equator, .can be interpreted interms of a second statistical correlation (5)- This is weaker thanthe first one, but ' still significant. and states that in tropical . lati-tudes, winds directed towards the .poles/equator are correlated with atendency for lif ting/subsid'ence, and therefore with high/low valuesfor cloudiness and rain frequency. A physical interpretation ofthis result can be given only in a climatological sense. This refersto a time-averaged wind system H, ~ with 9U/ax = o and theCoriolis parameter f close to zero.

¥e. find that

9v / 8y = - /s f"1 v

/S = -df = J_ d f/ dy R

_LR

R0f!•,00

= eax-tli1 s= latitude= coriolis parameter= 2 «o cos0=' earth' s angular velocity

••..,:/;V',..'.; V • In ..this , case . we ofotain for, w. at'. height z (w )

9_v dz = J_cot0 / v dz-' .9Y R L

^

- ' 1 6 9

Under these assumptions we expect, in a. steady current witha meridional wind component towards the equator/poles a tendency forsubsidence/lif ting. This rapidly increases (with ctg $> —>• 4- °° )•wh.en approaching the equator (and is invalid at 0 = 0) . Additionalstatistlcal evidence for this correlation has been found at Eniwetolc(12) and in other latitudes by Molla and Loisel (16).

The above e'xplanation refers to those areas where the ITCremains, even in northern winter (southern summer), at a sufficientdistance north of the • e.quator, and where steady ESE Trades cross theequator. Anothe'r-physical cause of this correlation is furnishedby the well-lcnown behaviour_of easterly waves, where convergencenormally occurs at .the rear together with winds with a component fromthe equator..

In the dry equatorial zones at both the Pacific and the"Atlantic, äs well äs on the western coasts of Africa and South Americabetween Latitude 2° -and 12°S, large-scale anomalies of precipitationoccur together with remarkable anomalies of the se^i temperature."While the coastal and oceanic areas in this belt are normally dry withupwelling cool water alpng the coast, in some periods the ocean temp-eratures rise to the"normal values of 26°-27°C together with consider-able rainfall. . Over land, serious floods occur. in the usually dryriver beds . This phenomenon has been described, along the coasts ofnorthern Peru and Ecuador, äs "El Ninö". Similar phenomena havebeen described from the Loango coast 'of West Africa (3°-5°), but occurin the saine longitudes from Gabun to central Angola (2°-12°s).

In this connection recent investigations (2) have shown th'atthe rainfall anomalies on the Angola coast (Luanda 8.8°S) are signif-icantly correlated not only with those of the Island of FernandoNoronha (3.8?S), situated at a distance of about 5000 kms near the NE,coast of Brazil, but also with those of Continental stations in the-northern part of the semi-arid, drought-affected triangle of NE Brazil

able 3 A). . Since the highest correlation is found with a time-lagof 3-4 months, the Interpretation of this teleconnection can be basedon oceanic advection of areas of cool (or warm) water with the South-Equatorial Current.- Some evitdence for this hypothesis can be foundby correlating ocean temperatures near Fernando Noronha with the windfield in equatorial. oceanic areas and with rainfall anomalies in theinterior of NE Brazil.. The problem of the physical cause of thesesea-temperature -anomalies remains unsolved; most probably they ar-e, dueto large-scale anomalies of the atmosphe.ric circulation in subtroptLcallatitudes. The stränge delay of the rainy season in NE Brazil alsoseems to be caused by- the same mechanism.

Similar correlations can be found — according to prelimina'ryinvestigations (Table 3 B) -'between the anomalies' on the Peru-Ecuador'coast and those in the'-vast area of the dry equatorial zone of the :

Pacific. This extends over about 12,000 Icms far int o the Pacific.At the remote islands in; this area we likewise observe a retardationof the rainy season into.-the first half of the year. In this . . .connection Schott (19) has given some indications of a coincidence ö.fanomalous years over large dis'tances but with. apparently no time .lagin the teleconnections. Over the equatorial Indian Ocean quite

170

different patterns are observed: here we find an equatorial belt ofweak, unsteady westerlies, which prevails during the greater part ofthe year, between 2°S and 2°N (3, 6, 7). In contrast to earliertextbooks, the trades from both hemispheres are here deflectedbefore reaching the equator, to a more westerly direction (NE into NW,SE into S¥). A physical explanation may be based on the occurrenceof a weak zonal pressure gradient in the equatorial latitudes of theIndian Ocean and the consequent relatively strong meridional compon-ents of both trades. Assuming quasigeostrophic conditions togetherwith v > ü and f ~ 0, Hollrnan (9) has derived, for a wind systemapproaching the equator, an additional westerly component V u givenby

V u = IFT1f~2Vg2

(v = geostrophic part of v, R = earth1 s radius , l = 2<o cos0 )g /

This term is based on the increasing imbalance betweenpressure gradient and horizontal Coriolis force due to the inertia ofa predominantly meridional current approaching the equator from bothhemispheres; it produces a deflection into westerlies before crossingthe equator. The decreasing speed and deflection into westerliesproduce (in a statistical sense) a tendency for convergence with con-sequent instability and lifting, thus preventing the occurrence of adry equatorial belt such äs is observed in the eastern Pacific and ;Atlantic. Other causes may be found in the frequent extension ofAntarctic troughs in the upper westerlies to and even across theequator. This is true not only in the whole Indian Ocean - coincidingwith the widespread penetration of Antarctic water and air into temp-erate latitudes - but also in the Pacific between 160°E and 170°¥. Inthese areas the trades are much more disturbed than in other regionsand owing to the occurrence of low-level disturbances (with consequentdecreased vertical stability) convective activity and rainfall aredistinctly greater in these regions. - J

TABLE 3 x

Teleconnections in the Equatorial Dry Belt

A. Angola - NE Brazil r n .

R Luanda X-V - R Ascension II-XI O•38 k$3 -•

R Luanda I-III - R Fernando Noronha IV-VIII 0.77 35

R Luanda XII-III - R Porangaba I-V 0-50 3̂

R Luanda XII-III - R Quixeramobim I-V ' 0.4? 59

R Luanda XII-III - R Sobral III-VI

Fernando de Noronha II-VII : R - t

TABLE. 3 (contd. )

B. Ecuador-Peru - Central. Pac'ific r n

R Guayaquil XI -IV - R Funafuti' II-VII ' 0.6? 2?

R Guayaquil XI-IV - R Punafuti IX-II 0.50 27

t P. Chicama I-IV - R Panning Isl. IV -V 0.58 24. "W r :r ,--•• —

t P. CtLicama II-III - R Funafuti VI -VII 0.47 18•wt Puerto Chicama I-IV - R Nauru IV-VII 0.49 26w ' .

"R Maiden III-VI - R Nauru II -V ' '0.44 18

R = rainfall, t = ocean temperature , I — XII = months ,\f

r = correlation coefficient, n = number of pairs ; significancelevel for r shown by underlining: ̂ level (_ _ )? Q>1^ level f _

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Alaka M. A. , 196l : Nat . Hurricane Res. Proj. Report 45-

Eickermann, ¥. , H. Flonn, 1962 : Bonner Meteor. Abhandl .

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4. Flonn H.., .1953

5. Flonn H., 1957

6.- Flonn H. , I960

7. Flonn H., 1902

Q. Flonn H. ,'

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Z. f. Meteor. 6, 97-107.

Beitr. Phys. Atmos. 30, 18-46.

in : Monsoons of the World, New Delhi, 75-88

in : Tropical Meteorology in Africa, Nairobi

Würzburger Geogr. Abnandl. (in print)

i. Hollmann G., 1955 = Meteor. Rundsch. 8, 79-82.

10. Kotes-waram P. , 1958 : Tellus 10, 43-57-

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