Showing papers on "Monsoon published in 1990"

Synoptic climatology of transient tropical intraseasonal convection anomalies: 1975?1985

Abstract: Pentad mean anomaly maps were used to study the climatology of tropical intraseasonal convection anomaly (TICA) as a dynamic system. One hundred and twenty-two events were identified and classified into three categories: eastward (77), independent northward (27), and westward (18) propagation. The eastward propagation is more active in boreal winter than in summer, while the independent northward propagation, which is not associated with equatorial eastward propagation, occurs in boreal summer from May to October. The eastward moving TICA exhibits three major paths: 1) eastward along the equator from Africa to the mid-Pacific, 2) first eastward along the equator, then either turning north-east to the northwest Pacific or turning southeast to the southwest Pacific at the maritime continent, and 3) the main anomaly moves eastward along the equator with split center(s) moving northward over the Indian and/or western Pacific Oceans. The equatorial Indian Ocean and the western Pacific intertropical convergence zone are preferred geographic locations for their development, while the maritime continent and central Pacific are regions of dissipation. Independent northward propagation is confined to the Indian and western Pacific monsoon regions. Its existence suggests that the mechanism responsible for meridional propagation may differ from that for eastward propagation. The dynamic effect of the equator and the thermodynamic effect of the underlying warm ocean water are basic factors in trapping TICA in the deep tropics, while the annual march of maximum SST (thermal equator) and the monsoon circulation have profound influences on the annual variation and meridional movement of TICA.

Late Pleistocene variability of Arabian Sea summer monsoon winds and continental aridity: Eolian records from the lithogenic component of deep‐sea sediments

Abstract: The modern Indian Ocean summer monsoon is driven by differential heating between the Asian continent and the Indian Ocean to the south. This differential heating produces a strong pressure gradient which drives southwest monsoon winds during June, July, and August. Satellite and meteorological observations, aerosol measurements, sediment trap studies, and mineralogical studies indicate an atmospheric mode of transport for modern lithogenic sediments in the northwest Arabian Sea. Analyses of lithogenic grain size and mass accumulation rate (MAR) records from the Owen Ridge indicate that eolian transport has been the primary mode of transport for the past 370 kyr. Visual inspection shows that the MAR record is positively correlated with global ice volume as indicated by the marine δ18O record. In contrast, the grain-size record varies at a much higher frequency, showing little correlation to either the MAR or the δ18O records. Spectral analyses confirm these relationships, indicating that the lithogenic grain-size and MAR records are coherent only over the precession band whereby the grain size leads the MAR by 124° (∼8 kyr). We conclude that an eolian transport mechanism is the only mechanism that allows for this phase difference and at the same time is supported by comparison of the grain size and MAR with independent eolian records. We use lithogenic grain size as a paleoclimatic indicator of summer monsoon wind strength and lithogenic MAR as a paleoclimatic indicator of source-area aridity. These interpretations are supported by comparison of the lithogenic records to independent indicators of wind strength (Globigerina bulloides upwelling record) and aridity (a loess record from central China). Such comparisons indicate high coherence and zero phase relationships. Our work supports the findings of previous studies which have documented the link between monsoon strength and the Earth's axial precession cycles. Both the lithogenic MAR and the grain-size records have high coherency with precessional insolation. Maximum lithogenic MAR (source-area aridity) is in phase with δ18O (global ice volume) and leads maximum precessional insolation by 88° (∼6 kyr). We attribute this lead to the influence of glacial conditions on the aridity, and therefore the deflation potential, of the source areas. Maximum lithogenic grain size (summer monsoon wind strength) lags maximum precession by 148° (∼9 kyr). We attribute this lag both to the influence of global and/or local ice volume and to the availability of latent heat from the southern hemisphere Indian Ocean, the two of which combine to determine the strength of the Indian Ocean monsoon.

Hydrography and circulation off the west coast of India during the southwest monsoon 1987

Abstract: During June-August 1987 thirteen hydrographic sections, each approximately perpendicular to the coast, covered the coastal region off western India The southernmost section showed occurrence of upwelling, the nearshore surface temperature being about 25°C lower than farther offshore There was a shallow (approximately 75 to 100 m deep) equatorward surface current, below which there were signatures of downwelling indicative of a poleward undercurrent hugging the continental slope T-S characteristics showed that the undercurrent carried low salinity water found in the southwestern Bay of Bengal Conditions similar to those found at this section existed up to around 15N but the intensity of upwelling, and the signatures of the surface current and the undercurrent grew weaker progressively from the south to the north, and ceased to be noticeable at about 20N The width of the surface current was about 150 km, whereas the undercurrent was about 40 km wide The transport in the equatorward surface current increased from less than 05 × 106 m3/s to about 4 × 106 m3/s from the north to the southern end of the coast The winds during the observation varied between west-northwesterly near the southern end of the coast to west-southwesterly near the northern end The longshore component of the wind stress was generally equatorward Its magnitude was maximal (05 dyn/cm2) near the southern end On the basis of these observations we propose that the circulation off the west coast of India during the southwest monsoon, though weak, is dynamically similar to the wind-driven eastern boundary currents found elsewhere in the oceans

The Intraseasonal (30–50 day) Oscillation of the Australian Summer Monsoon

Abstract: The tropical intraseasonal (30–50 day) oscillation manifests itself in the Australian summer monsoon by a pronounced modulation of the monsoonal westerlies. These 30-50 day fluctuations of the monsoonal westerlies are coherent with rainfall and OLR across northern Australia. The OLR fluctuation originates in the Indian Ocean and systematically propagates eastward at 5 m s−1, consistent with previous studies of the intraseasonal oscillation. The detailed evolution of the intraseasonal oscillation of the monsoon is studied via composites of upper air data in and about the Australian tropics. During the summer periods 1957-87, 91 events were identified at Darwin, Australia. The composite oscillation at Darwin has a very deep baroclinic structure with westerlies extending up to 300 mb. The westerly phase lasts about ten days and lags a similar duration rainfall event by about four days. During the westerly phase, the upper troposphere is warm and the extreme lower troposphere is cool. This structure ...

Interaction between anomalous winter monsoon in East Asia and El Nino events

Abstract: Based on a series of data analyses, the intimate relations between anomalous winter monsoon in East Asia and El Nino are studied in this paper. Anomalistic circulation in the Northern Hemisphere caused by El Nino event can lead to enhancing the Ferrel cell and the westerlies in the mid—latitudes as the Hadley cell and result in the location of the front zone in East Asia to the north. These are unfavourable for the cold wave breaking out southward in East Asia. Therefore, there are warmer weather and weaker winter monsoon in East Asia in El Nino winter. There are stronger and frequent cold waves in East Asia during the wintertime prior to the occurrence of El Nino event. They will induce stronger winter monsoon in East Asia. Thus, the weakened trade wind and enhanced cumulus convection in the equatorial middle—western Pacific area caused by the stronger winter monsoon will play an important role in the occurrence of El Nino event. Therefore, the anomalously strong winter monsoon in East Asia during wintertime might be an important mechanism to cause El Nino event.

Impact of Indian Monsoon on the Coupled Atmosphere)Ocean System in the Tropical Pacific

Abstract: This study addresses the relationship between the Indian summer monsoon (ISM) and the coupled atmosphere/ocean system in the tropical Pacific on the interannual time scales. High positive correlations are found between ISM rainfall and both mixed layer sea water temperature (SWT) and sea surface temperature (SST) anomalies of the tropical western Pacific in the following winter. Negative correlations between ISM rainfall and SST in the central/eastern Pacific also appear to be most significant in the following winter. These parameters are correlated with each other mainly on a biennial time scale. Lag-correlations between the zonal wind and SST along the the equatorial Pacific show that the westerly (easterly) surface wind stress anomalies over the central/western Pacific are greatly responsible for the formation of negative (positive) SST/SWT anomalies in the western Pacific and positive (negative) SST/SWT anomalies in the central/eastern Pacific. Furthermore, it is evidenced that these lagcorrelations are physically based on the anomalies in the large-scale convection over the Asian monsoon region and the associated east-west circulation over the tropical Pacific, which first appear during the Indian summer monsoon season and evolve during the following autumn and winter. These results strongly suggest that the Asian summer monsoon may have an active, rather than a passive, role on the interannual variability, including the ENSO events, of the coupled atmosphere/ocean system over the tropical Pacific.

A Composite Study of Onset of the Australian Summer Monsoon

Abstract: The circulation changes that accompany an onset (defined as the first occurrence of wet 850-mb westerly winds at Darwin, Australia) of the Australian summer monsoon are documented by a composite study for the years 1957-1987. Composites of atmospheric fields at stations in and about the Australian tropics are constructed relative to the onset data at Darwin. It is shown that the composite onset is dominated by a slow eastward migration of a deep-baroclinic convective circulation displaced south of the equator. This propagating anomaly exhibited many features of the so-called 40-50 day oscillation, including an upper level anticyclone that accompanies the convective anomaly.

Decadal-scale changes of the circulation in the tropical atlantic sector associated with Sahel drought

Abstract: Trends in the general circulation setting during 1948–1983 accompanying the progressive aggravation of drought in sub-Saharan Africa are investigated from ship observations over the tropical Atlantic and surface station records in western Africa. Decadal-scale circulation changes in the Atlantic sector include a pressure rise over the tropical North Atlantic; southward displacement of the near-equatorial wind confluence and associated maximum cloud belt; acceleration of the north-east trades and possibly a strengthening of the South Atlantic trades, along with increasing cloudiness in the equatorial zone; increasing cloudiness over the tropical North Atlantic and Central American seas; and cooling of surface waters in a band across the tropical North Atlantic contrasting with warming in the South Atlantic. These changes occurred in all seasons, but were most pronounced at the height of boreal summer. Records at land stations in western Africa show that near the coasts the temperature and pressure trends are consistent with the adjacent ocean areas. In the interior of sub-Saharan Africa, warming trends prevail in boreal summer, presumably as a direct consequence of reduced inflow of cool maritime monsoon air; the concomitant downward pressure trends at continental stations in sub-Saharan Africa may reflect the hydrostatic effect of increasing temperature. In contrast to boreal summer, slight cooling trends and pressure rises prevail in the interior during winter. In the course of the past four decades, the near-equatorial wind confluence over the eastern Atlantic migrated southward by 200 km in July–August and 150 km for the rainy season as a whole. Given the steep meridional precipitation gradients across the Sahel, these circulation shifts are large enough to account for the observed downward trend in Sahel rainfall.

Oxygen isotope evidence for a stronger winter monsoon current during the last glaciation

Abstract: A VARIETY of palaeoclimate data from the northern Indian Ocean has shown that the Asian summer monsoon was weaker during the Last Glacial Maximum (∼18,000 years ago) than it is today1–5 It has also been inferred from these data that the winter monsoon was stronger at that time5 Here we present evidence that the transport of low-salinity water from the Bay of Bengal to the Arabian Sea, caused by the winter monsoon6 was higher during the Last Glacial Maximum This inference is based on a negative excursion, up to about 1‰ in magnitude, in the oxygen isotope ratios of three planktonic foraminiferal species, detected in a sediment core from the Arabian Sea by high-resolution (1–2 cm) sampling Our results place an upper limit of about 4,000 years on the length of time during which the winter monsoon was stronger than today

The Somali current at the equator: annual cycle of currents and transports in the upper 1000 m and connection to neighbouring latitudes

Abstract: Current measurements were obtained with moored stations during October 1984 to October 1986 in two consecutive deployments across the Somali Current on the equator. For the transport calculations the deficiency of conventional subsurface moorings, i.e. no data from close to the surface, had to be overcome using the historical ship drift climatology. While the current structure during the summer monsoon is that typical of a western boundary current, the profile in winter is far from being a weaker southward reverse of the summer situation. Below a thin surface layer of southward flow, there is a northward undercurrent between about 120 and 400 m depth. Below that, the flow reverses again to southward. This results in drastic differences in cross-equatorial monsoon season transports. While the summer mean transport is 21 Sv for the upper 500 m, the winter monsoon mean for that depth range is close to zero. The annual mean transport in the upper 500 m is 10 Sv northward. Very little transport is measured in the 500–1000 m depth range in either season or the annual mean. The almost closed mass budget of the boundary current system during the winter circulation allows a calculation of cross-equatorial heat transport, which comes out to −3 × 1014 W (southward) for the northeast monsoon season mean. The heat flux associated with the annually varying part of the boundary current is small, only about −0.3 × 1014 W or about 5% of the total cross-equatorial heat flux as estimated by other methods. By combining the equatorial measurements with earlier off-equatorial current observations, particularly at 2°−4°S and 5°N, and with property distributions (salinity and oxygen) on isopycnal surfaces, analysed from the historical data file, a synopsis of the seasonal circulation changes of the entire Somali Current system between about 4°S and 12°N is then derived.

Circulation of the East China Sea, a numerical study

Abstract: A three-dimensional, primitive-equation model is developed to study how the Kuroshio, the monsoon, the Yangtze River outflow and the buoyancy forcing from the South China Sea affect the circulation of the East China Sea. It is found that the Kuroshio water usually intrudes into the East China Sea from both sides of Taiwan Island. Winter winds enhance the Kuroshio intrusion from northeast of Taiwan, but weaken it from the Taiwan Strait. Summer winds act in the opposite way. The increased presence of the Kuroshio water in the East China Sea in winter can be largely attributed to the shoreward surface Ekman drift associated with the northerly wind. In summer, theΩ-shaped plume emanating from the Taiwan Strait is, to a large extent, produced by the buoyancy forcing from the South China Sea.

Milankovitch and albedo forcing of the tropical monsoons: a comparison of geological evidence and numerical simulations for 9000 yBP

Abstract: Lake-level and palaeoecological evidence from Africa, Arabia and southern Asia for 9000 yBP suggests an intensification and increased poleward penetration of the northern monsoons. The vegetation belts shifted north by 4–6° latitude on the south side of the Sahara. In contrast, the monsoon over southern Africa was weaker than today. Calculations based on the new palaeogeographical map of Mali by Petit-Maire et al. (1988) indicated that the areaaveraged surface albedo decreased by 0·10–0·14 in the zone 16–24°N and that total annual precipitation increased by 150–320 mm north of the inland delta of the Niger (20–24° 15′N). Experiments with atmospheric general-circulation models suggest that this asymmetrical pattern of anomalies in the strength of the tropical monsoons can be explained in broad terms by the different orbital configuration of the Earth at 9000 yBP. Here, we describe a hitherto unpublished sensitivity experiment with the low-resolution (5° × 7·5°) version of the U.K. Meteorological Office 11-layer model, in which the albedo over Africa and Arabia between 15 and 30°N was reduced by between 0·04 and 0·06 to simulate the increase in vegetation cover at 9000 yBP. The results indicate that the surface-albedo change provides a significant positive feedback enhancing the direct climatic effects of Milankovitch forcing in the tropics.

Origin of low frequency (intraseasonal) oscillations in the tropical atmosphere. III - Monsoon dynamics

Abstract: The issue of the interaction of the monsoon large-scale circulation and intraseasonal oscillations is addressed, showing that, as a result of the interaction of the large scale monsoon flow with the near-equatorial intraseasonal oscillation, unstable baroclinic disturbances are generated over the monsoon region. From a linear stability analysis of quasi-geostrophic motion in a two-level model, it is shown that the westward propagating disturbances generated over the monsoon region are the manifestation of heat-induced unstable Rossby waves. The instability is favored in the region with large vertical wind shear and reduced effective static stability. The monsoon large scale circulation over India and southeast Asia and the plentiful supply of moisture in the region appear to be favorable for the development of these unstable waves.

Seasonal changes in the denitrification regime of the Arabian Sea

Abstract: Extensive observations made in the northeastern Arabian Sea during successive southwest (SW) and northeast (NE) monsoon seasons reveal significant changes in chemical composition of the oxygen-deficient waters. Distributions of nitrite and nitrate deficits, the latter computed from linear relationships established between the nitrate tracer ‘NO’ and potential temperature, and evaluated using a two end-member mixing model, are studied at four isopycnal surfaces to deduce some major features of the mid-depth circulation. The results suggest a complete reversal of the subsurface coastal circulation associated with the monsoons. During the SW monsoon, a northward undercurrent appears to carry low nitrite, low deficit and relatively well-oxygenated waters off the Indian continental margin, suppressing denitrification; this feature is absent during the NE monsoon. Vigorous horizontal mixing resulting from a seasonal reversal of currents at a depth of a few hundred metres, together with the lack of large east-west differences in the downward particulate flux, as revealed by some recent sediment trap experiments, could explain why the oxygen-deficient zone in the Arabian Sea is not confined to the upwelling centres along the boundaries. The results reinforce the assertion that mixed layer-induced nutrient pumping could account for a large fraction of new production in this region. Substantially higher deficits in inorganic combined nitrogen seem to occur during the NE monsoon suggesting that the renewal processes are most vigorous during the SW monsoon. It is suggested that the dominance of the NE monsoon circulation could lead to an intensification in denitrification during the glacial times.

The record of deglaciation in the Sulu Sea: Evidence for the Younger Dryas Event in the tropical western Pacific

Abstract: A high-resolution, accelerator mass spectroscopy 14C dated sediment record from the Sulu Sea clearly indicates that the Younger Dryas event affected the western equatorial Pacific. Planktonic foraminiferal δ18O and abundance data both record significant changes during Younger Dryas time. In particular, a 0.4‰ increase in the δ18O value of Globigerinoides ruber and the reappearance of the cool water planktonic foraminifera, Neogloboquadrina pachyderma, occur during the Younger Dryas at this location. These isotopic and faunal changes are a response to either surface water temperature or salinity changes, or some combination of the two. Changes in surface salinities could have been accomplished through either local or global processes. Intensification of the monsoon climate system and increased precipitation at approximately 11 ka is one mechanism that may have resulted in local changes in salinity. A meltwater pulse derived from the Tibetan Plateau is another mechanism which may have caused local changes in salinity. The presence of the Younger Dryas in the tropical western Pacific clearly indicates that this climatic event is not restricted to the North Atlantic or high latitudes, but rather is global in extent.

Late Cenozoic plateau uplift and climate change

Abstract: Sensitivity experiments with general circulation models show that uplift of plateau and mountain regions in Southern Asia and the American west during the late Cenozoic was an important factor in the evolution of Northern Hemisphere climate. The climatic trends simulated in the uplift experiments agree in direction with most trends observed in the geological record, including the tendencies toward greater regional differentiation of climate, and particularly the fragmentation into wetter and drier climatic patterns at middle latitudes. These climatic trends result from (1) increased orographic diversion of the mid-latitude westerlies, and (2) increased summer heating and winter cooling over the plateaus, which enhances seasonally reversing (monsoonal) changes in wind directions.Most previous hypotheses addressing the physical impact of orography on climate have focused on mountain ranges and have stressed relatively local responses such as upslope precipitation maxima, cooling of mountain crests due to lapse-rate effects on rising terrain, and lee-side rainshadow effects. In contrast, our results emphasise the importance of large-scale plateau orography. By redirecting the basic directions of wind flow both at surface and upper-tropospheric levels, these rising plateaux cause far-reaching climatic changes that extend across the continents as well as over the oceans.

Precipitation Fluctuation over Semiarid Region in Northern China and the Relationship with El Niño/Southern Oscillation

Abstract: In recent years the semiarid region of northern China, which has total annual precipitation between 200 and 500 mm, has shown signs of severe desertification. Intensive theoretical and observational studies are currently underway to examine the climate changes and other contributing factors. In this study, we used the 1951–86 monthly precipitation measurements in this region to study their fluctuations and relationship with the El Nino/Southern Oscillation. Three main features are identified: 1) a 2–3 year quasi-periodic fluctuation, 2) a tendency for rainfall deficiency for the whole region during ENSO years, and 3) a significant correlation between the precipitation fluctuation in the southern part of this region and Southern Oscillation index, with the former lagging the latter by 2–5 months. These features are also evident from analysis of the proxy data during the last hundred years. Discussions on the possible link between the precipitation fluctuation, the summer monsoon, the western Pacif...

A review of Central Asian glaciochemical data

Abstract: The glaciers of central Asia provide suitable locations from which to recover continuous, high-resolution glaciochemical records on a continental scale. Although the glaciochemical investigations undertaken to date in central Asia are few in number and limited in terms of spatial coverage and length of record, some preliminary observations can be made concerning regional and seasonal trends in snow chemistry in this region. The sodium chloride ratio for most snow samples collected in central Asia approaches the ratio found in sea water (0.86 in /Leq kg-I), reflecting a marine source for these constituents. Sodium and chloride concentrations are, on average, 3-10 times higher in the Himalayas than in the Karakoram, demonstrating the greater influence of monsoonal sources of moisture in the Himalayas. Very high sodium concentrations from Khel Khod Glacier probably reflect a local crustal source from surrounding ice-free areas. Low nitrate concentrations were found in snow collected from the southern margin of the Himalayas and high concentrations in snow deposited on the north margin of the Himalayas. This strong regional trend in the spatial distribution of nitrate suggests the influx of continental aerosols, rich in nitrate, originating from the arid regions of central Asia. High calcium concentrations measured in snow from Mount Everest and the north-west corner of China are also indicative of dust derived from the arid regions of central Asia. Very high sulfate concentrations found in snow from the Tien Shan and the Bogda Shan most likely reflect local anthropogenic sources. The altitude effect on isotopic composition is not apparent from snow samples collected in central Asia. Understanding the processes which control the chemical content of snow, the local-to-regiona l scale complexities,

Abundance of Oil Sardine (Sardinella longiceps) and Upwelling on the Southwest Coast of India

Abstract: The abundance of oil sardine (Sardinella longiceps) on the Malabar coast is highly variable on the decadal scale. During this century there have been several periods of relatively high abundance, and several major population crashes. O-group recruitment to the fishery begins towards the end of the summer monsoon, and its success is statistically related to sea level at Cochin just prior to onset of the monsoon. At this time, sea level indicates remote forcing of upwelling, rather than the wind-driven upwelling that occurs during the monsoon. Unusually early remote-forcing appears to inhibit subsequent recruitment, perhaps through exclusion of spawning fish from the neritic zone by oxygen-deficient upwelled water.

A 350 ka history of the Indian Southwest Monsoon—evidence from deep-sea cores, northwest Arabian Sea

Abstract: The Indian summer Southwest Monsoon plays an important part in influencing, and regulating, the productivity and sedimentation in the northwest Arabian Sea at the present day, by driving coastal upwelling. This leaves permanent sedimentological and geochemical records in the accumulating deep-sea sediments. Cores 722B and 724C were raised from the Owen Ridge and Oman Margin, respectively, during Leg 117 of the Ocean Drilling Program and have been subjected to geochemical analyses and α-spectrometry. A comparative core, CD17–30, situated on the adjacent Indus Fan abyssal plain, has also been studied. The chronostratigraphy of the cores has been established with δ 18O stratigraphy, giving a 350 ka climate record. Changes in the total sediment mass accumulation rates occur on glacial/interglacial time scales, with maximum fluxes occurring during glacial episodes. The high fluxes are predominantly due to wind-transported dust at the ridge and margin sites. Compositional parameters (e.g. the Ti/Al ratio) indicating the proportion of heavy minerals present within the dust, suggests that strong winds associated with the Southwest Monsoon, occur with Milankovitch periodicities, and are dominated by the precession (23 ka) frequency. The wind strength controls the proportion of heavy minerals transported to the Arabian Sea, whilst continental aridity influences the timing of deflation from the Arabian and Somalian peninsulas. Tracers of palaeoproductivity (Ba/Al) indicate strong coherence and phase with the proxy ice volume (foraminiferal δ 18O) signal, suggesting global climate parameters (ice volume, continental aridity) determine coastal productivity by influencing nutrient supply. In relation to productivity, the roles of oceanic circulation/stratification and nutrient supply through continental runoff are discussed. This study shows that the Southwest Monsoon appears to only affect the shorter period (precession cycle, 23 ka band) productivity signal. Evidence from excess 230Th suggests deep oceanic circulation (at about 2000 m depth) was more intense 110 ka BP decreasing toward 40 ka BP. By the use of these various geochemical tracers a new, and comprehensive, view of the interaction of the Monsoon and global climate with marine productivity through the late Pleistocene has been obtained.

Middle atmosphere dynamics and monsoon variability

Abstract: Some dynamical parameters of the middle atmosphere such as quasi-biennial oscillation in the low-latitude stratospheric zonal winds, interannual variability of the high-latitude middle atmosphere and vertical motions in the tropical middle atmosphere are studied to examine their roles in determining the variability of rainfall over India during the summer monsoon season. It is believed that the amount of sulphurous gases released in the stratosphere from the great volcanic eruptions modifies in the climate (rainfall). An attempt has also been made in this paper to present the work done so far in this direction.

The summer monsoon of 1988

Abstract: Time averaged, monthly mean fields of a number of climate parameters such as sea surface temperature anomalies, outgoing longwave radiation anomalies, 200 mb velocity potential field, streamfunction anomaly at the lower and the upper troposphere and percentage rainfall amounts are presented, for the summer of 1988 over the regions of Asian summer monsoon. Above normal rainfall occurred over most of the Indian subcontinent, southeast Asia and eastern China during 1988. In comparison, 1987 was a drought year. This paper presents a comparison of some of the salient aforementioned parameters. The evolution of the planetary scale divergent motions and the streamfunction anomaly exhibit prominent differences during the life cycle of the monsoon in these two years. The velocity potential field exhibits a pronounced planetary scale geometry with the divergent outflows emanating from the monsoon region during 1988. The descending branches of these time averaged circulations are found over the Atlantic ocean to the west and over the eastern Pacific and North America to the east. The immense size of this circulation is indicative of an above normal monsoon activity. During 1987, the outflow center was located much further to the southeast over the western Pacific ocean. The longitudinal extent of the monsoonal divergent circulations were much smaller in 1987. The positive sea surface temperature anomaly of the El Nino year 1987 is seen to move westwards to the western Pacific in 1988, a warm anomaly also appears over the eastern equatorial Indian ocean and the Bay of Bengal at this time. The latter contributes to the supply of moisutre during the active monsoon season of 1988. The outgoing long wave radiation anomalies evolve with a westward propagation of strong positive anomalies from the central Pacific ocean consistent with the evolution of divergent circulation. The streamfunction anomalies basically show westerly zonal wind anomalies being replaced by easterly anomalies during 1988 over the upper troposphere of the monsoon region. The lower tropospheric streamfunction anomaly during the drought year 1987 showed a pronounced counter monsoon circulation. That feature was absent in 1988.

Surface and upper air temperatures over India in relation to monsoon rainfall

Abstract: The relationship between the all-India summer monsoon rainfall and surface/upper air (850, 700, 500 and 200 mb levels) temperatures over the Indian region and its spatial and temporal characteristics have been examined to obtain a useful predictor for the monsoon rainfall. The data series of all-India and subdivisional summer monsoon rainfall and various seasonal air temperatures at 73 surface observatories and 9 radiosonde stations (1951–1980) have been used in the analysis. The Correlation Coefficients (CCs) between all-India monsoon rainfall and seasonal surface air temperatures with different lags relative to the monsoon season indicate a systematic relationship. The CCs between the monsoon rainfall and surface-air temperature of the preceding MAM (pre-monsoon spring) season are positive over many parts of India and highly significant over central and northwestern regions. The average surface air temperature of six stations i.e., Jodhpur, Ahmedabad, Bombay, Indore, Sagar and Akola in this region (Western Central India, WCI) showed a highly significant CC of 0.60 during the period 1951–1980. This relationship is also found to be consistently significant for the period from 1950 to present, though decreasing in magnitude after 1975. WCI MAM surface air temperature has shown significant CCs with the monsoon rainfall over eleven sub-divisions mainly in northwestern India, i.e., north of 15 °N and west of 80 °E. Upper air temperatures of the MAM season at almost all the stations and all levels considered show positive CCs with the subsequent monsoon rainfall. These correlations are significant at some central and north Indian stations for the lower and middle tropospheric temperatures. The simple regression equation developed for the period 1951–1980 isy = − 183.20 + 8.83x, wherey is the all-India monsoon rainfall in cm andx is the WCI average surface air temperature of MAM season in °C. This equation is significant at 0.1% level. The suitability of this parameter for inclusion in a predictive regression model along with five other global and regional parameters has been discussed. Multiple regression analysis for the long-range prediction of monsoon rainfall, using several combinations of these parameters indicates that the improvement of predictive skill considerably depends upon the selection of the predictors.

Low frequency variation of tropical convergence zones

Abstract: Intraseasonal variation of tropical convergence zones (TCZ) is studied focussing on the three major features of the TCZ over the Indian longitudes during the summer monsoon viz. (i) the oscillation between active and weak spells, (ii) the occurrence of two favourable zones-one over the equatorial oceans and another over the heated continent and (iii) poleward propagations of the oceanic TCZ onto the heated continent. An observational study of the intraseasonal variation over different parts of the tropics has shown that the first feature may be an ubiquitous feature of the TCZ variations, the second occurs only over the Asian summer and winter monsoon zones, and the third only over the Asian summer monsoon. Analysis of a simple monsoon model has revealed that poleward propagation occurs in the presence of a meridional surface temperature gradient because the convective heating is asymmetric, with more heating on the poleward side. Preliminary analysis of the T-21 version of the ECMWF model has shown that it is capable of simulating the three major features of the intraseasonal variation of the TCZ over the Indian longitudes during the summer monsoon.

Variability in upwelling fields in the northwestern Indian Ocean 1. Model experiments for the past 18,000 years

Abstract: A nonlinear reduced gravity ocean model is used to assess the effects of changes in the monsoon winds during glacial and interglacial conditions on the seasonal circulation in the northwestern Indian Ocean. Winds from the National Center for Atmospheric Research Community Climate Model simulations for 18 kyr B.P. (the most recent glacial maximum and a period of weaker monsoon winds), 9 kyr B.P. (near the beginning of the present interglacial and a period of stronger monsoon winds), and present-day (0 kyr B.P.) conditions are used to drive the model to a steady seasonal cycle. Strength of upwelling fields are inferred in each case by integrating upward displacements of the model pycnocline over the primary upwelling season of mid-April to mid-August. In both the 0 kyr B.P. and the 9 kyr B.P. cases, a broad band of upwelling and decreased model upper layer thickness extends along the coast of the Arabian Peninsula out 350–500 km offshore during the southwest monsoon. This upwelling is driven by the strongly positive wind stress curl beneath the cyclonic side of the atmospheric Findlater Jet, located to the north and west of the jet axis. Farther offshore, in the anticyclonic region to the south and east of the jet axis, the negative wind stress curl drives downwelling and increased upper layer thickness. In the 9 kyr B.P. case, the atmospheric jet is much stronger and narrower than in the 0 kyr B.P. case, with stronger values of both positive and negative curl to the northwest and southeast, respectively, of the jet axis. This drives much stronger upwelling to the northwest of the jet axis and downwelling to the southeast. In the 18 kyr B.P. case, the Findlater Jet is very weak, and upwelling patterns are uniformly weak across the basin. The spatial distribution and temporal variation of the upwelling fauna in the sediment record is consistent with the model upwelling fields in all three cases. The ocean model thus provides the link between the climate model and the sedimentation data, verifying the hypothesis that variations in upwelling driven by variations in the strength of the monsoon jet are responsible for the observed variations in the sediment record.