rainfall variability and the walker cell in the equatorial pacific ocean

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RAINFALL VARIABILITY AND THE WALKER CELL IN THE EQUATORIAL PACIFIC OCEAN By L. F. MUSK Department of Geography, University of Manchester S Barrett has written; ‘The tropics undoubtedly constitute one of the A largest single areas of sparse conventional data coverage remaining in the world today’ (Barrett 1967). The weather satellite, combining the roles of remote observing and remote sensing, is slowly proving an increas- ingly invaluable aid in the investigation of atmospheric circulations in remote areas of the globe, especially tropical oceanic areas, and has already suc- ceeded in helping to disprove the earlier ‘assumed simplicity’ of tropical weather. Previous studies have demonstrated the use of this photographic data- bank in investigating the Indian Monsoon (Hamilton 1974) and the cloud- forms associated with the Intertropical Convergence Zone (ITCZ) over the Indian Ocean (Hobbs 1974). The tropical Pacific Ocean constitutes another area where the use of satellite data has thrown new light on old climatologi- cal problems. In the mid-Equatorial Pacific Ocean, far away from any continental influences, there exists a zone of immense longitudinal extent which has not only low mean annual precipitation totals, but also very high rainfall variability. This zone exists where one would normally expect the moisture- laden north-east trades of the northern hemisphere to converge with the south-east trades of the southern hemisphere producing a single intertropical convergence zone, a zonal cloud band representing intense convective activity, very disturbed weather and consequential high rainfall. Describing the equatorial Pacific, Trewartha (1966) has stated ‘this long narrow zone represents one of the world’s most striking climatic anomalies’. This mid-Pacific dry belt has not been comprehensively documented to date because there is little conventional data (few stations lie directly beneath the mean climatological positions of either the northern limb of the ITCZ or the more ephemeral southern limb in this area), and because synoptic analysis in these parts is difficult due to the breakdown of the geostrophic wind relationship in equatorial latitudes. RAINFALL IN THE EQUATORIAL PACIFIC OCEAN Rainfall totals in the tropical Pacific (Fig. 1) have been mapped by a few workers (e.g. Schott 1933, Seelye 1950, Taylor 1970), who have demon- strated the main features of this region to be: 34

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RAINFALL VARIABILITY AND THE WALKER CELL IN THE EQUATORIAL PACIFIC OCEAN

By L. F. MUSK Department of Geography, University of Manchester

S Barrett has written; ‘The tropics undoubtedly constitute one of the A largest single areas of sparse conventional data coverage remaining in the world today’ (Barrett 1967). The weather satellite, combining the roles of remote observing and remote sensing, is slowly proving an increas- ingly invaluable aid in the investigation of atmospheric circulations in remote areas of the globe, especially tropical oceanic areas, and has already suc- ceeded in helping to disprove the earlier ‘assumed simplicity’ of tropical weather.

Previous studies have demonstrated the use of this photographic data- bank in investigating the Indian Monsoon (Hamilton 1974) and the cloud- forms associated with the Intertropical Convergence Zone (ITCZ) over the Indian Ocean (Hobbs 1974). The tropical Pacific Ocean constitutes another area where the use of satellite data has thrown new light on old climatologi- cal problems.

In the mid-Equatorial Pacific Ocean, far away from any continental influences, there exists a zone of immense longitudinal extent which has not only low mean annual precipitation totals, but also very high rainfall variability. This zone exists where one would normally expect the moisture- laden north-east trades of the northern hemisphere to converge with the south-east trades of the southern hemisphere producing a single intertropical convergence zone, a zonal cloud band representing intense convective activity, very disturbed weather and consequential high rainfall. Describing the equatorial Pacific, Trewartha (1966) has stated ‘this long narrow zone represents one of the world’s most striking climatic anomalies’.

This mid-Pacific dry belt has not been comprehensively documented to date because there is little conventional data (few stations lie directly beneath the mean climatological positions of either the northern limb of the ITCZ or the more ephemeral southern limb in this area), and because synoptic analysis in these parts is difficult due to the breakdown of the geostrophic wind relationship in equatorial latitudes.

RAINFALL IN THE EQUATORIAL PACIFIC OCEAN

Rainfall totals in the tropical Pacific (Fig. 1) have been mapped by a few workers (e.g. Schott 1933, Seelye 1950, Taylor 1970), who have demon- strated the main features of this region to be:

34

Fig. 1. Mean annual rainfall in the tropical Pacific Ocean in mm (after Sekiguchi 1952). Areas with over 2000 mm per annum are stippled

(i) a zonal equatorial dry belt with annual rainfall totals below 75 cm, extending from the South American coast to approximately 165OE;

(ii) wet zones flanking this linear dry belt (corresponding to the two zones of convergence of the double ITCZ described later);

(iii) decreasing rainfall with increasing latitude polewards of these wet belts, extending into the regions dominated by the trades and the subtropical high pressure systems;

(iv) a further region of high rainfall totals in the south-west in a band extending from Indonesia south-eastwards off the east coast of New Zealand towards higher southern latitudes.

These climatic divisions have also been highlighted by co-spectrum analysis of rainfall records in the area (Doberitz 1966), delineating areas with homogeneous precipitation and sea temperature regimes over the tropical Pacific Ocean. Doberitz’s main divisions correspond to the core of the Pacific dry belt, the surrounding areas of high rainfall and the areas under the dominating influence of the subsident trade-wind regime.

Apart from the low actual totals of precipitation in this region, there are two features of note. Firstly there is a marked variation in annual rainfall totals. Malden Island (4O03’N, 155O01 ’W) has probably the highest annual rainfall variability of any station in the world (Chang 1972). Annual totals here have varied from 3.55 cm to 142.0 cm; during 567 days in 1906-8 the island received only 14-48 cm of rain, while in the 12-month period from May 1914 to April 1915, 254.76 cm fell on the island. To take a further

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example, Ocean Island (0°52’S, 169O35’E), almost on the Equator, had a rainfall total of 24-7 cm in 1950, but one of 407.7 cm in 1940. Although the substance of the argument remains, this demonstrates the problems of using time divisions of months and years, when the irregular variations of the parameter being described do not conform to such divisions.

The second noteworthy feature of the rainfall regime is that rainfall totals tend to increase westwards. Taking an east-west transect along the dry zone, Galapagos Island, 700 miles west of South America, averages 10 cm of rain per year, Christmas Island at 157OW averages 91.2 cm, while Banaba at 170°E receives 190.6 cm.

The annual variability of rainfall in the dry belt was first mapped by Schott (1938), and his work was followed by that of Seelye (1950) over a more limited area. There are severe problems in analysing the variability of rainfall in the Equatorial Pacific due to breaks in the series of observations, the varying lengths and qualities of the records and the general paucity of data. However in Fig. 2 isopleths of the coefficient of variability have been drawn over the central western Pacific for the stations where data exist for the period 1951-60. This is a short period statistically but was chosen accord- ing to the availability of data from those stations with the required length of record. The coefficient of variability C.V. is defined by

G

X C.V.(%) = 1 x 100

where is the standard deviation and X is the mean value of the series. Despite the great variation in actual totals over the area, the most

noticeable feature of the map is its demarcation of a zone of maximum C.V. centred along an axis just south of the Equator and extending east-west over a considerable longitudinal extent. As with rainfall totals, the maximum variability is also at the western end of the dry belt (Ocean Island has a C.V. of 65.9 per cent).

SYNOPTIC INTERPRETATION

If one examines the synoptic climatology of the area, the origin of the climatic anomaly becomes more apparent. Because of the lack of conven- tional data and charts in an area where the synoptic network is sparse, weather satellite photographs have proved an invaluable tool for investiga- ting the climatic patterns. Examination and analysis of daily ESSA computer-rectified mosaics of the area have for the first time allowed an indirect analysis of the weather systems affecting the area.

Fig. 3 (see p. 39) shows the variation in monthly position and extent of regions of maximum brightness for the tropical Pacific Ocean from 30°N to 30°S taken from ESSA digital products for the year 1967. An area of maxi- mum brightness over the oceans on these time-composite diagrams represents an area with persistent, deep cloudiness. In contrast, cloud-free areas such as the Equatorial dry zone are conspicious on satellite photographs by the low

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Fig. 2. Coefficient of variability of rainfall over the Pacific Ocean, 1951-60

brightness (darker) areas. In particular the maps emphasise localities where the ITCZ persisted, where convective activity was persistent, and the areas frequented by frontal depressions.

In January and February 1967 the northern limb of the ITCZ was well defined from South America westward at approximately 7-10°N; the southern ITCZ was non-existent except west of 150"W where it joined the very persistent cloudiness extending from well inside the tropics in the vicinity of Indonesia, south-eastwards off the coast of Australia towards Antarctica. In March the southern ITCZ became much more accentuated while the northern ITCZ lost a little definition. The extended tuning-fork configuration of the double ITCZ persisted until mid-May when the southern ITCZ disappeared in the mean situation; the northern ITCZ then intensified and dominated the atmospheric circulation of the area. In June and July the northern ITCZ continued to dominate while the southern ITCZ was absent or weak, and at the same time the 'frontal cloudiness' from Indonesia to Antarctica dissipated and broke up.

In August and September the northern ITCZ was dominant whereas the southern limb was weak, and then for the rest of the year the pattern was one of a strong northern cloudband, and a weak southern ITCZ, while the persistent cloudiness in the Indonesia-South Pacific sector moved equatorwards and eastwards to occupy a location similar to that of the

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southern ITCZ in earlier months. To summarise these monthly maps: 1. The northern ITCZ was well developed and persistent throughout the

year; i t underwent little seasonal migration, occupying a position between 7 and 12'N. There were longitudinal changes in intensity, and it was at its least persistent in March and April, the periods coinciding with the development of the southern ITCZ. From May 1967 it persisted with little variation with time until the end of 1968 (Gruber 1972).

2. The southern ITCZ, first described by Kornfield et al. (1967) was less well developed than its northern counterpart. In 1967 it reached its maximum development at 5's (90-135OW) from March to early May. (This happened again in the winter of 1968 and the spring of 1969 (Gruber 1972)). There was great longitudinal variation in the convective cloud intensity and rarely was it regularly developed. I t was most pronounced in its development from 90-140°W in April and May 1967 and from 90-115OW in April 1968. At the western end it frequently merged with the trailing end of the band of per- sistent cloudiness extending polewards from Indonesia, but in general was much more variable spatially and temporally than its northern counterpart.

3. There existed a belt of persistent cloudiness extending south-east- wards from Indonesia and New Guinea off the coast of New Zealand and Australia for most of the year, although it was not very well developed in July 1967. This belt appears to be anchored over Indonesia and strongly linked with the Indonesian convergence zone. The cloudiness curves cyclonically south-eastwards into higher lati- tudes marking the convergence of the southern trades with cooler air from high latitudes (Gruber 1972), especially west of 160OE. Gruber has shown this to be possible from analysis of cloud motions from ATS satellite photographs and mean gradient wind analysis. Several workers have noted its presence before (e.g. Streten 1970, Barrett 1970) and it is a major rainfall influence in the area.

This analysis of mean brightness for the one year 1967 has obvious deficiencies and limitations: the extrapolation from the particular year of 1967 as evidence for the general conditions prevailing and the use of satellite photographs taken once daily (i.e. an instantaneous snapshot). In order to test the validity of these generalisations a longer period of observations is needed. Sadler (1968) has produced satellite-derived maps of mean cloud cover over the tropics for the period February 1965 to January 1967. These emphasise the northern ITCZ and the persistent cloudiness off the coast of Indonesia, but the southern ITCZ was rather more ephemeral, extending from the west coast of South America with any persistence only in March 1965 and March 1966. Gruber (1972) has mapped the variations in the position of the ITCZ over the Atlantic and the Pacific Oceans for the period

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JhNUARY JULY

FEBRUARY

MARCH

APRIL

MAY

JUNE

AUGUSl

30' 20. to. 0' 10. 20. to'

30. 129' I IO'E 180' 150'W 120' 80'

SEPTEMBER

OCTOBER

NOVEMBER

DECEMBER

Fig. 3. Mean monthly positions of the maximum brightness axes in the tropical Pacific Ocean from ESSA 3 and 5 satellite evidence, January-December 1967

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January 1967 to February 1970, and in his analysis the northern limb again appears as the most persistent (Fig. 4).

It is apparent that the double cloudband of the ITCZ, when best developed (e.g. March 1967) has a characteristic shape of an extended tuning- fork lying along the equator with a wide complex single ITCZ over the Indonesian-New Guinea region which then splits at a bifurcation point around 160-170°E, the whole being of the order of 11 000 km long. There is little migration of the cloudbands with the seasonal migration of the sun; rather there is a more important pulsation in intensity of convective activity associated with westward moving disturbances embedded in the converg- ence lines and also linked with changes in intensity of convergence in the Hadley cell. On occasions the southern ITCZ may disappear completely for periods of a month.

Fig. 4. Mean annual position of maxi- mum brightness axes in the tropical Pacific Ocean from satellite evidence (after Gruber 1972)

THE WALKER CELL

Combining the information from the satellite photographs with the con- ventional climatic data it has been suggested that in the mid-Pacific there is a unique east-west zonal circulation dominated by subsidence between the two ascending limbs of the double ITCZ. This circulation pattern has come to be known as the Walker Cell (named after Sir Gilbert Walker who first implied the existence of such a cell in his work on the ‘southern oscillation’ (Walker 1924)).

This zone of active subsidence represents a region of local divergence and high pressure between the two convergence lines of the ITCZ. In practical terms the Walker Cell is defined by the two parallel bands of convective cloud clusters, the two limbs of the ITCZ which are situated approximately 10-15O of latitude on either side of the equator. The cloudbands represent ‘the loci of cloud clusters associated with westward propagating wave dis- turbances’ (Lockwood 1974), where deep hot-tower convection is occurring and instability is being released locally. Time-composite photographs of the area (Chang 1972) have demonstrated that these clusters migrate westward along the band with an average wavelength of 2000-10 000 km and a lifetime of 4-7 days; they can be followed right across the Pacific, maintained in a quasi-steady state. These cloud clusters, first described as ‘equatorial waves’ (Palmer 1952), represent concentrations of cyclonic vorticity at low levels and anticyclonic vorticity at 200 mb, with strong ascent at middle levels. The clear areas between the clusters are marked by weak subsidence together with vertical distributions of vorticity and divergence opposite in sign to those within the clusters (Williams 1970). Much of the rainfall of the area

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is associated with thz convective activity within these clusters. In climatic terms they provide the ascending branch of the meridionally operating Hadley cell of low latitudes, separating the zonally organised subsident region of light winds and suppressed convection, the Walker cell.

Thus the satellite with its visual evidence of this cloud-free region of divergence has produced important corroborative evidence of a new type of

Fig. 5. Models of the Hadley and Equatorial cells: (a) simple classical model, with one Hadley cell in each hemisphere; (b) Fletcher's (1945) model, emphasising the Equatorial cells between the two Hadley cells; (c) Asnani's (1968) model with a pre- dominating southern ITC cell; and (d) Bunker's simplified model of the actual circulalion over the Pacific in April I967 (modified, after Bunker 1971)

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atmospheric circulation, a tropical east-west zonal circulation with an upper westerly return flow.

Asnani (1968) has examined the viability of the double ITCZ structure working from dynamic considerations, and suggests that ‘there is an equatorial cell separating the classic two Hadley cells from each other. In this cell the air approaching the equator in the lower layers and receding from it aloft suffers subsidence’ (Fig. 5). Hastenrath (1968) has also demon- strated the viability of such a circulation regime; he considers there to be twin equatorial cells with convection (at the ITCZ) on their poleward limits and subsidence in the centre, enclosed between the Hadley cells. He suggests that because of the important influence of the cold oceanic currents in forcing the circulation, the double structure could not occur over continents.

Many workers have observed and analysed the atmospheric circulation cell structures over the Pacific which are different from the mean global pattern. Bunker (1971) has recently studied the energy transfer and tropical cell structure of the central Pacific. The circulation he found consisted of a shallow northerly cell, and an extensive equatorial cell centred south of the Equator, with maximum subsidence over the Equator and a strong southern Hadley cell (Fig. 5). Part of the high level outflow of the southern Hadley cell subsides across the Equator and decreases the activity of the northern convergence zone. Equatorial easterlies were observed and re- lated to the cold upwellings of equatorial waters. Krueger and Winston (1974) have recently analysed the variation of the Pacific Ocean Walker circulation, comparing the strong, well-developed circulation of February 1971 with the much weaker circulation of February 1969. They suggest that another factor controlling the potency and form of the circulation may well be dynamic instability in the westerlies of the winter hemisphere’s subtropical jet stream.

- u

a

- 26-

2 25- a

24-

z \

w

\ \ FEBRUARY - 23-

22-

\ I ‘\\

--- AUGUST

~ 211

- 2 7 Fig. 6. Equatorial distribution of mean sea surface temperatures in the Pacific Ocean for February and - 26

August (after Saha 1970) - 2s

- 24

- 23

:-22

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coast of South America and flowing out westwards to the central Pacific along the Equator (Fig. 6). This cold current is mainly due to cold upwelling but it is strengthened by advection from the Peru current and the presence of the cold Cromwell current (an equatorial undercurrent at a depth of approximately 100 my discovered in 1952). The cprrent is driven by the low level easterlies of the Walker Cell; its importance is that it produces local heat sources and heat sinks in an east-west sense, masking the larger scale global heating patterns determining the meridional sense of the general atmospheric circulation which are here dominated by the local air-sea interaction in the atmospheric boundary layer.

Thus the eastern half of the Equatorial Pacific (the eastern half of the Walker cell) with negative sea temperature anomalies acts as a large-scale mass source, a region of the atmosphere which is continually being cooled, giving rise to localised high pressure. The western half acts as a large-scale mass sink, a region of the atmosphere being warmed in the boundary layer with a consequent fall of pressure. This regime reaches a maximum in August when the equatorial easterlies are at their strongest (helping to extend the longitudinal effect of the cold current further westwards). The current also partially explains the tuning fork configuration of the Walker cell. Bjerknes (1966) has suggested that the ITCZ is located where ocean temperatures are highest in a meridional sense, apd this has recently been substantiated by theoretical modelling (Pike 1971). If this is so, then in the Pacific these zones of maximum sea temperature are shifted away from the Equator by the upwelling. In the west the single form of the ITCZ corres- ponds to the traditionally observed single ITCZ formed at the convergence of the two hemispheric trade wind systems over warm land surface and warm equatorial waters. In the east the ITCZ forms where the maximum oceanic temperature occurs (either side of the Equator), while over the cold current itself subsident divergent f3ow occur$ producing the cloudfree conditions. At the bifurcation point over Indonesia, Bjerknes has suggested that the low level convergence from north, south and east stimulates a major hub of convective activity. The ITCZ is therefore situated at a compromise position between: (1) boundary layer convergence and (2) con- ditional instability released in hot-tower convection in association with the high sea surface temperature.

TABLE 1. Surface wind summaries for years with extreme rainfall at Malden Island (4"3'N, 155"Ol'W) (after Seelye 1960)

Yearcommencing Rainfall(cm) N N E E SE S SW W NW Calm May 1914 254.76 11 17 38 1 1 1 1 3 6 12 April 1925 17.37 0 7 8 8 4 0 0 0 0 1

According to Chang the driest years at Malden Island correspond with periods of undisturbed easterly winds when the Walker cell is well developed (a fact recognised as early as 1926), whereas the yet years are marked by the appearance of westerly winds (Table 1). Bjerknes stated that when the cold

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ocean water along the equator is well developed the air above it is too cold to undergo buoyant convection in the rising limb of the Hadley circulation. Instead, the equatorial air flows westwards between the two Hadley cells of the two hemispheres to the warmer west Pacific where, having been heated and supplied with moisture from the warmer waters, the air can take part in larger-scale wet convection (Bjerknes 1969). He suggested that the axis of this Walker circulation was at 160°E with the sinking extending eastwards to 8OoW, with rising air to about 130OE. The westerly flow of the upper Walker cell has been further confirmed by Sadler (1 959) who stated: ‘The upper tropospheric flow reverses near 150°F where the west components increase towards the east and the east components towards the west. The reversal region is a semipermanent feature with a seasonal and annual longitudinal variation’. Rainfall variability itself reflects the fact that the western end of the Walker cell varies in position from year to year.

AIR-SEA INTERACTION

Sea-surface temperatures affect both the rates of local evaporation and local atmospheric stability. Bjerknes (1969) has studied the link between rainfall and air-sea surface temperature differences for Canton Island (2O48’S, 171 O43’W). Upwelling cold water affects the atmosphere around the island for most of the year, but occasionally the upwelling ceases and the sea-surface temperature becomes warmer than the atmosphere (e.g. late 1957, early 1958, late 1963 and late 1965). Bjerknes showed that during the period 1950-67, large monthly totals of rain at Canton Island occurred only during periods when the ocean was warmer than the atmosphere.

Doberitz (1966) has also examined the problem of teleconnections and phase relationships between rainfall and sea temperatures in the Pacific. He used 35 rainfall and five sea temperature series of varying lengths over the period 1890-1 965 and concluded that cross-spectra relationships between rainfall and sea temperature generally exhibit either a simultaneous march of anomalies or a small time lag of sea temperature after rainfall.

This close relationship between annual sea temperature and rainfall totals is shown in the scatter diagram of Fig. 7, showing the relationship between annual rainfall and mean annual sea temperature for Canton Island over the period 1950-65. Years with low mean sea-surface temperature tend to be the dry years (when the cold current is driven across the Pacific by strong easterlies in the Walker cell) while the wet years correspond to conditions of warm average sea surface temperature. The statistical unit of one year is open to criticism, yet the relationship still yields a correlation coefficient of 0.785 between the two variables, a value which is statistically significant at the 99 per cent confidence level.

Thus the dry zone of the central Pacific largely corresponds with the zonally organised region of subsidence of the Walker cell, along the axis of the Peruvian current. The regions with the highest coefficients of variability,

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I .

2.

Ocean to the

The annual variability of the Peruvian current and the concomitant change in potency of the atmospheric mass sources and sinks associated with the cold current in any one year; the small migration of the ITCZ associated with trade wind variations, and changes in the convective intensity of the ITCZ. Island, the station with the highest coefficient of variability, lies close main position of the boundary separating the very moist, unstable

weather of the ITCZ and the very stable, dry and subsident airflow of the dry zone; thus a slight change in position of either of these two influences may be critical for local rainfall totals.

REPERCUSSIONS

The traditional approach to modelling the general circulation of the atmosphere has been based on meridional circulation and north-south cells of the Hadley type. The frame of reference has been determined by the traditional pole-equator cross-section with little inherent scope for any longitudinal temperature contrast acting as the atmospheric forcing mechanism. Krishnamurti (1971) has recently proposed the existence of other tropical zonal circulations as well as the supposed Walker cell. He suggested that the equatorial Walker circulation is mainly the southern extension of a much more vigorous east-west circulation extending up to

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40°N during summer, but the question concerning its importance in the functioning of either the atmospheric circulation of the tropics or the general circulation as a whole remains unanswered. The other significant question which arises from this work is the importance elsewhere of ocean currents and air-sea interaction as determinants of large-scale climatic patterns. Perhaps GATE, which took place in the summer of 1974 in the eastern Atlantic, will throw some light on this question.

ACKNOWLEDGMENTS

I wish to thank Messrs M. Tristram and C. Thomas for preparing the diagrams.

REFERENCES ASNANI, G. C. 1968

BARRETT, E. C. 1967

1970

BJERKNES, J. 1966

1969

BUNKER, A. F. 1971

CHANG, C. P. 1970

CHANG, J-H. 1972

DOBERITZ, R. 1968

GRUBER, A. 1972

HAMILTON, M. G. 1974

HASTENRATH, S. L. 1968

HOBBS, J. E. 1974

KORNFIELD. J.. HASLER. 1967 A. F., HANSON, K. J., and SUOMI, V. E.

KRISHNAMURTI, T. N. 1971

KRUEGER, A. F. and 1974

LOCKWOOD, J. G. 1974

WINSTON, J. S.

PALMER, C. E. 1952

PIKE, A. C. 1971

The equatorial cell in the general circulation. J . Atrn.

Viewing weather from space. (Longmans, London),

The estimation of monthly rainfall from satellite data. Mon. Wea. Rev., 98, pp. 322-327

A possible response of the atmospheric Hadley circulation to equatorial anomalies of ocean temperature. Tellus, 18, pp. 82G829

Atmospheric teleconnections from the equatorial Pacific. Mon. Wea. Rev., 97, pp. 163-172

Energy transfer and tropical cell structure over the central Pacific. 1. Atrn. Sci., 28, pp. 1101-1116

Westward propagating cloud patterns in the tropical Pacific as seen from time composite satellite photographs. Ibid., 27, pp. 133-138

Atmospheric circulation systems and climates. (Oriental Pub. Co., Honolulu, Hawaii), 328 pp.

Koharenzanalyse von Niederschlag und Wassertem- peratur im tropischen Pazifischean Ozean. Ber. dtsch. Wetterd. (Offenbach), 15, (112), 22 pp.

Fluctuation in the position of the ITCZ in the Atlantic and Pacific Ocean. 1. Atm. Sci., 29,

A satellite view of the South Asian summer monsoon. Weather, 29, pp. 82-95

On mean meridional circulation in the tropics, 1. A tm. Sci., 25, pp. 979-983

A complex inter-tropical convergence zone-some examples from the Indian Ocean. Weather, 29,

Photographic cloud climatology from ESSA 111 and V computer-produced mosaics. Bull. Amer. Met.

Tropical east-west circulations during the northern summer. J . Arm. Sci., 28, pp. 1342-1347

A comparison of flow over the tropics during two contrasting circulation regimes. Zbid., 31, pp.

World climatology, an environment approach. Arnold, London, 330 pp.

Tropical meteorology. Quart. J . R . Met. Soc., 78,

Intertropical convergence zone studied with an inter- acting atmosphere and ocean model. Mon. Wea. Rev . , 99, pp. 469-471

Sci., 25, pp. 133-134

140. PP.

pp. 193-197

pp. 122-143

SOC.. 48, pp. 878-883

35 8-3 70

pp. 126-164

46

SADLER, J. C.

SAHA, K.

SCHOTT, G.

SEELYE, C. J.

SEKIGUCHI, T.

STRETEN, N. A.

TAYLOR, R. C.

TREWARTHA, G. T.

WALKER, G.

WILLIAMS, K.

1959

1968

1970

1933

1950

1952

1970

1970

1966

1924

1970

Wind regimes of the troposphere and stratosphere over the equatorial and sub-equatorial central Pacific. In Vol. 13, Secretariat, North Pacific Congress, Dept. of Science, Bangkok

Average cloudiness in the tropics from satellite observations, International Indian Ocean Expedi- tion. Met. Mono. No. 2 , Dept. of Geophysics, Univ. of Hawaii

On the nature and origin of the double intertropical convergence zone. Proc. Symposium Tropical Met. Honolulu (Am. Met. SOC.), E 11, 8 pp.

Die jahrlichen Niedersclagsmengen auf dem Indischen und Stillen Ozean. Ann. Hydrog. Mart. Meteor., 61, pp. 1-12

Rainfall and its variability over the central and southwestern Pacific. N Z J. Sci and Tech., (B),

The rainfall distribution in the Pacific region, Proc. Pacific Sci. Congr. 7th Congr. 111 (Wellington,

A note on the satellite observed zone of high cloudiness in the central South Pacific. Austral. Met. Mag., 18, pp. 31-38

The distribution of rainfall over the tropical Pacific Ocean, deduced from island, atoll, and coastal stations. Proc. Symposium Tropical Met. , Honolulu (Am. Met. Soc.), J 111, 8 pp.

The earth’s problem climates. McGraw Hill, New York, 334 pp.

Correlations of seasonal variations of weather, IX, A further study of world weather. Mem. of Indian Met. Dept., 25, pp. 275-332

Characteristics of the wind, thermal and moisture fields surrounding the satellite observed meso- scale trade wind cloud clusters of the western north Pacific. In Proc. Symposium Tropical Met . , Honolulu (Am. Met. Soc.), D IV, 8 pp.

32, 2, pp. 11-24

NZ), pp. 101-102

RAINFALL IN THE SEYCHELLES 1941 TO 1970

By D. ASPIN International A erudio Limited

AH& the main Island of the Seychelles Group, lies in the Indian M Ocean at 4O40’S 55O30’E, some 1100 km north-east of Diego Suarez in northern Malagasy and 1800 km east of Mombasa on the coast of Kenya (Fig. I ) . The island is 26 km long and 8 km wide, and composed mostly of granite. It has very little coastal plain and rises quickly to an average height of between 500 m and 600 m. The highest peak, Morne Seychellois (905 m above mean sea level), is approximately 3 km south-west of the old meteor- ological station on the Long Pier at Victoria (Fig. 1). This site was closed at the end of June 1971 when the meteorological office transferred to the new site at the international airport.

Rainfall averages are available monthly and annually for the period 1902 to 1955 (Poona, Indian Met. Dept. 1965; Wright and Ebdon 1968), but there is little information about the site of the rain-gauges prior to

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