Showing papers on "Monsoon published in 1993"

Mantle dynamics, uplift of the Tibetan Plateau, and the Indian Monsoon

Abstract: Convective removal of lower lithosphere beneath the Tibetan Plateau can account for a rapid increase in the mean elevation of the Tibetan Plateau of 1000 m or more in a few million years. Such uplift seems to be required by abrupt tectonic and environmental changes in Asia and the Indian Ocean in late Cenozoic time. The composition of basaltic volcanism in northern Tibet, which apparently began at about 13 Ma, implies melting of lithosphere, not asthenosphere. The most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere. The initiation of normal faulting in Tibet at about 8 (± 3) Ma suggests that the plateau underwent an appreciable increase in elevation at that time. An increase due solely to the isostatic response to crustal thickening caused by India's penetration into Eurasia should have been slow and could not have triggered normal faulting. Another process, such as removal of relatively cold, dense lower lithosphere, must have caused a supplemental uplift of the surface. Folding and faulting of the Indo-Australian plate south of India, the most prominent oceanic intraplate deformation on Earth, began between about 7.5 and 8 Ma and indicates an increased north-south compressional stress within the Indo-Australian plate. A Tibetan uplift of only 1000 m, if the result of removal of lower lithosphere, should have increased the compressional stress that the plateau applies to India and that resists India's northward movement, from an amount too small to fold oceanic lithosphere, to one sufficient to do so. The climate of the equatorial Indian Ocean and southern Asia changed at about 6–9 Ma: monsoonal winds apparently strengthened, northern Pakistan became more arid, but weathering of rock in the eastern Himalaya apparently increased. Because of its high altitude and lateral extent, the Tibetan Plateau provides a heat source at midlatitudes that should oppose classical (symmetric) Hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons. For the simple case of axisymmetric heating (no dependence on longitude) of an atmosphere without dissipation, theoretical analyses by Hou, Lindzen, and Plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal (monsoonlike) circulation than such a source centered on the equator, but only if heating exceeds a threshold whose level increases with the latitude of the heat source. Because heating of the atmosphere over Tibet should increase monotonically with elevation of the plateau, a modest uplift (1000–2500 m) of Tibet, already of substantial extent and height, might have been sufficient to exceed a threshold necessary for a strong monsoon. The virtual simultaneity of these phenomena suggests that uplift was rapid: approximately 1000 m to 2500 m in a few million years. Moreover, nearly simultaneously with the late Miocene strengthening of the monsoon, the calcite compensation depth in the oceans dropped, plants using the relatively efficient C4 pathway for photosynthesis evolved rapidly, and atmospheric CO2 seems to have decreased, suggesting causal relationships and positive feedbacks among these phenomena. Both a supplemental uplift of the Himalaya, the southern edge of Tibet, and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the Himalaya that, in turn, enhanced extraction of CO2 from the atmosphere. Thus these correlations offer some support for links between plateau uplift, a downdrawing of CO2 from the atmosphere, and global climate change, as proposed by Raymo, Ruddiman, and Froehlich. Mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts, but might also play an important role in altering regional and global climates.

The Mexican Monsoon

Abstract: The pronounced maximum in rainfall during the warm season over southwestern North America has been noted by various investigators. In the United States this is most pronounced over New Mexico and southern Arizona; however, it is but an extension of a much larger-scale phenomenon that appears to be centered over northwestern Mexico. This phenomenon, herein termed the “Mexican monsoon,” is described from analyses of monthly mean rainfall, geostationary satellite imagery, and rawinsonde data. In particular, the authors note the geographical extent and magnitude of the summer rains, the rapidity of their onset, and the timing of the month of maximum rainfall. Finally, the difficulty in explaining the observed precipitation distribution and its timing from monthly mean upper-air wind and moisture patterns is discussed.

Century-scale events in monsoonal climate over the past 24,000 years

Abstract: BOTH the marine sediment record and numerical modelling of the atmospheric summer circulation over the northern Indian Ocean and southeast Asia have shown that the monsoonal climate exhibits a direct but nonlinear response to the intensity of solar insolation during summer, with a time lag of several thousand years1,2. Here we present evidence from a high-resolution record of oxygen isotopes and carbonate spanning the past 24,000 calendar years that the response of the southwest monsoon over the Arabian Sea to long-term, gradual insolation changes occurred in several distinct events of less than 300 years duration, at 14,300, 13,500, 13,060, 9,900, 8,800 and 7,30014C yr BP. Thus, during this transitional period from glacial to post-glacial conditions the slow solar forcing seems to have induced very rapid changes in local climate. We speculate that the rapid response may be related to albedo changes in Asia.

Sensitivity of Eurasian Climate to Surface Uplift of the Tibetan Plateau

Abstract: Experiments with climate models are used to illustrate how earth's climate could be expected to change if surface uplift occurred. The experiments contrast two climate simulations: one with all continents at sea level and one with present-day mountains and plateaus. The simulations in the Eurasia sector show that surface uplift (increased elevation) produces marked increases in the intensity of summer and winter monsoons of Asia, with development of wetter climates to the south and east of the Tibetan plateau and drier climates to the north and west of the plateau; there is a general cooling in northern polar latitudes. Comparison of simulations from two different models shows that the magnitude of the simulated changes depends upon how the models calculate the land surface hydrology and the ocean temperature. The simulated changes produced by surface uplift agree in a general way with many geologic indicators of late Cenozoic climate change. The results suggest that surface uplift of mountains and platea...

Seasonality and interannual variability of particle fluxes to the deep Arabian sea

Abstract: Long-term sediment trap studies have been carried out since 1986 at three locations in the western, central and eastern Arabian Sea. Here we present total and bulk component fluxes measured for 3 years at the central station and for 4 years at the western and eastern stations. Particulate fluxes to the deep sea are controlled by the monsoons with generally higher fluxes during the SW and NE monsoons and lower fluxes during the intermonsoon periods. The increase of particle fluxes occurs simultaneously with a drop in surface water temperature, induced by wind-or convective-mixing and an associated entrainment of nutrients into the euphotic zone. More than 50% of the annual particle fluxes to the deep sea occurs during the SW monsoon at the western location due to the prolonged influence of the monsoonal upwelling as indicated by increased biogenic carbonate and opal fluxes. However, the opal fluxes peak a month later than the carbonate fluxes. The delayed onset of opal flux peak appears to be controlled by the observed premonsoon silica distribution in the Arabian Sea, where the subsurface waters are silica depleted down to the thermocline at 150 m. At the central location particle fluxes are of similar magnitude during the SW and NE monsoons. The interannual variability of particle fluxes at the eastern location is determined by the NE monsoon. At the western and central locations, on the other hand, maximum interannual variability of fluxes occurs during the SW monsoon and particle fluxes were higher during years of stronger SW monsoon. The results further suggest that, apart from monsoon strength, geographic shifts of the area of maximum wind-stress may produce significant variabilities in particle fluxes to the deep ocean at the western Arabian Sea site.

Reconstructing sea surface temperature and salinity using δ 18 O and alkenone records

Abstract: THE oxygen isotope (δ18O) composition of foraminiferal tests from deep-sea sediments is widely used as a palaeoclimate proxy, but it includes contributions from sea surface temperature, global ice volume and local salinity, which are difficult to separate. Recently a new technique for deriving palaeotemperatures has been developed which is based on the abundance ratios of unsaturated alkenones in phytoplankton algae1,2. Here we use a combination of oxygen isotope and alkenone records in a deep-sea core from the juncture of the Arabian Sea and the Bay of Bengal to extract the salinity signal from the former record. Variations in salinity are related to the balance between evaporation and precipitation3, and are thus a sensitive indicator of climate change. Our 170-kyr salinity record enables us to reconstruct changes in the Indian monsoon over this period, considerably extending earlier studies (which reached back to 18 kyr ago)4–8. Like these previous studies, we find that large variations in the monsoon occurred during the transition from the last glacial period to the present interglacial, but our results also provide a view of the monsoon throughout the last glacial and demonstrate the potential of this approach for reconstructing palaeosalinity.

The western boundary current of the seasonal subtropical gyre in the Bay of Bengal

Abstract: Hydrographic data collected during March–April 1991 show the presence of a poleward current along the western boundary of the Bay of Bengal north of about 10° N carrying warmer waters of southern origin. The inshore side of the current was marked by cooler, more saline waters brought to the surface due to the presence of the current which transported approximately 10 × 106 m3/s. The hydrography is suggestive of many of the features that have been associated with the western boundary currents of the subtropical gyres of the world oceans: a recirculation zone, waves, eddies, etc. These features, however, were not satisfactorily resolved in the data. Using available climatologies of monthly mean ship drifts, seasonal hydrography, and monthly mean wind stress, we propose that the poleward current is the western boundary current of a seasonal anticyclonic subtropical gyre which forms in the Bay during January, is best developed during March-April, and decays by June. The gyre and the western boundary current are unique because of their seasonal character. The pattern of circulation leading to formation and decay of the gyre is reproduced reasonably well in the computation of the monthly mean barotropic transport induced by the curl of wind stress, which has a well-defined annual cycle due to the monsoons and which is conducive to the formation of an anticyclonic gyre only during the months of January-May. The pattern of circulation due to baroclinic transport induced by the wind stress curl, however, is not known at present, and this makes it difficult to conclude unequivocally that the wind stress curl over the bay is the sole mechanism to force the gyre.

A 300 KYR Record of Upwelling Off Oman during the Late Quaternary: Evidence of the Asian Southwest Monsoon

Abstract: In the northwest Arabian Sea upwelling occurs each summer, driven by the strong SW monsoon winds. Upwelling results in high biological productivity and a distinctive assemblage of plankton species in the surface waters off Oman that are preserved in the sediments along the Oman continental margin, creating a geologic record of monsoon-driven upwelling. Sediments recovered from the Oman continental margin during Ocean Drilling Program leg 117 provide an opportunity to examine how upwelling has varied during the late Quaternary, spanning a longer interval than piston cores recovered prior to the ODP cruise. Variations in foraminifer shell accumulation and in the relative abundance of Globigerina bulloides indicate dominant cycles of variation at 1/100 kyr, the dominant frequency of glacial-interglacial variations, and at 1/23 kyr, the frequency of precessionally driven cycles in seasonal insolation. The strongest monsoon winds (indicated by increased upwelling) occurred during interglacial times when perihelion was aligned with the summer solstice, an orbital change that increased the insolation received during summer in the northern hemisphere. During glacial times upwelling was reduced, and although the precessional cycles were still present their amplitude was smaller. At both frequencies the upwelling cycles are in phase with minimum ice volume, evidence that glacial-interglacial climate changes also include changes to the climate system that influence the low-latitude monsoon. We attribute the decrease in the monsoon winds observed during glacial times to changes in bare land albedo over Asia and/or to changes in the areal extent and seasonal cycle in Asian snow cover that decrease the summer land-sea temperature contrast. Other mechanisms may also be involved. These new upwelling time series differ substantially from previous results, however the previous work relied on cores located farther offshore where upwelling is less intense and other physical mechanisms become important. Our results support the observations derived from atmospheric general circulation models of the atmosphere that indicate that both glacial boundary conditions, and the strength of summer insolation are important variables contributing to cycles in the monsoon winds during the late Quaternary.

South asian summer monsoon variability in a model with doubled atmospheric carbon dioxide concentration.

Abstract: Doubled atmospheric carbon dioxide concentration in a global coupled ocean-atmosphere climate model produced increased surface temperatures and evaporation and greater mean precipitation in the south Asian summer monsoon region. As a partial consequence, interannual variability of area-averaged monsoon rainfall was enhanced. Consistent with the climate sensitivity results from the model, observations showed a trend of increased interannual variability of Indian monsoon precipitation associated with warmer land and ocean temperatures in the monsoon region.

Influences of High- and Low-Latitude Processes on African Terrestrial Climate: Pleistocene Eolian Records from Equatorial Atlantic Ocean Drilling Program Site 663

Abstract: High- and low-latitude forcing of terrestrial African paleoclimate variability is demonstrated using 900 ka eolian and biogenic component records from Ocean Drilling Program site 663 in the eastern equatorial Atlantic. Terrigenous (eolian dust) and phytolith (savannah grass cuticle) accumulation rate records vary predominantly at 100 and 41 kyr periodicities and spectral phase estimates implicate high-latitude forcing. The abundance of freshwater diatoms (Melosira) transported from dry African lake beds varies coherently at 23–19 kyr orbital periodicities and at a phasing which implicates low-latitude precessional monsoon forcing. Modeling studies demonstrate that African climate is sensitive to both high- and low-latitude boundary conditions. African monsoon intensity is modulated by direct insolation variations due to orbital precession, whereas remote high-latitude forcing can be related to cool North Atlantic sea surface temperatures (SSTs) which promote African aridity and enhance dust-transporting wind speeds. The site 663 terrigenous and phytolith records covary with North Atlantic SST variability at 41 °N (site 607). We suggest that Pleistocene African climate has responded to both high-latitude North Atlantic SST variability as well as low-latitude precessional monsoon forcing; the high-latitude influence dominates the sedimentary record. Prior to circa 2.4 Ma, terrigenous variations occurred primarily at precessional periodicities (23–19 kyr), indicating that African climate was largely controlled by low-latitude insolation variations prior to the onset of high-amplitude glacial-interglacial climate change.

The 10–20-Day Mode of the 1979 Indian Monsoon: Its Relation with the Time Variation of Monsoon Rainfall

Abstract: The synoptic structure of the 10–20-day monsoon mode and this intraseasonal monsoon mode's relationship with the Indian monsoon rainfall are examined with the 1979 summer First GARP Global Experiment IIIb data of the European Centre for Medium-Range Weather Forecasts and the daily 1° × 1° rainfall estimates retrieved from the satellite data by the Goddard Laboratory for Atmospheres. The major findings of this study are as follows. 1) The 10–20-day monsoon mode exhibits a double-cell (either double-high or double-low) structure; one cell is centered at about 15°–20°N and the other at the equator. 2) Both cells of the 10–20-day monsoon mode propagate coherently westward along the Indian monsoon trough and along the equator, respectively. 3) Based upon the zonal wind and local Hadley circulation, the vertical structure of the 10–20-day monsoon mode does not exhibit a phase change. 4) A significant rainfall occurs around low centers of the 10–20-day monsoon mode through the modulation of this monsoon...

Homogeneous Indian Monsoon rainfall: Variability and prediction

Abstract: The Indian summer monsoon rainfall is known to have considerable spatial variability, which imposes some limitations on the all-India mean widely used at present. To prepare a spatially coherent monsoon rainfall series for the largest possible area, fourteen subdivisions covering the northwestern and central parts of India (about 55% of the total area of the country), having similar rainfall characteristics and associations with regional/global circulation parameters are merged and their area-weighted means computed, to form monthly and seasonal Homogeneous Indian Monsoon (HIM) rainfall series for the period 1871–1990. This paper includes a listing of monthly and seasonal rainfall of HIM region. HIM rainfall series has been statistically analysed to understand its characteristics, variability and teleconnections for long-range prediction.

Observations and dynamics of semiannual and annual sea levels near the eastern equatorial Indian Ocean boundary

Abstract: Monthly sea level records from the western coasts of Thailand and Indonesia are examined together with atmospheric pressure, wind data, and a linear model to document and understand eastern equatorial Indian Ocean semiannual and annual sea level fluctuations. The semiannual sea level fluctuations are approximately constant in amplitude and phase for six sea level stations on a 4000-km stretch of coastline extending from 8.12°S to 7.83°N. Semiannual zonal equatorial winds in the Indian Ocean basin remotely generate these sea level fluctuations. In sharp contrast to the semiannual sea level, the phase and amplitude of the annual sea level fluctuations vary strongly along the coast. The annual fluctuations are forced predominantly by the coastal alongshore monsoon wind. For the six months from April to September this monsoon wind has a southeasterly component toward India, raising coastal sea level in the Northern Hemisphere and lowering it in the Southern Hemisphere; during the other six months win...

Sensitivity of Asian and African climate to variations in seasonal insolation, glacial ice cover, sea surface temperature, and Asian orography

Abstract: A general circulation model was used to investigate the sensitivity of Asian and African climate to prescribed changes in boundary conditions with the objective of identifying the relative importance of individual high-latitude glacial boundary conditions on seasonal climate and providing a physical basis for interpreting the paleoclimate record. The circulation model is described and results are presented. Insolation forcing increased summer Asian monsoon winds, while increased high-latitude ice cover strengthened winter Asian trade winds causing decreased precipitation. These factors had little effect on African climate. Cooler North Atlantic sea surface temperatures enhanced winter trade winds over North Africa, southern Asian climate was relatively unaffected. Reducing Asian orography enhanced Asian winter circulation while decreasing the summer monsoon. These model results suggest that African and southern Asian climate respond differently to separate elements of high-latitude climate variability.

Annual river discharge in southeastern Australia related to El Nino‐Southern Oscillation forecasts of sea surface temperatures

Abstract: Annual natural discharge (Q) of the River Murray and its most extensive tributary, the Darling River system, is often inversely related to sea surface temperature (SST) anomalies in the eastern tropical Pacific Ocean. These SST variations are components of a planetary-scale phenomenon referred to as El Nino-Southern Oscillation (ENSO). Darling and Murray river historical values of Q indicate that annual surface runoff from regions dominated by subtropical summer monsoon precipitation and annual surface runoff primarily responding to temperate winter storms are both strongly influenced by ENSO cycles. Forecasting, approximately 1 year in advance, of ENSO-related SST from geophysical model calculations thus provides a mechanism for estimating probabilities of annual river discharge amount. Contingency tables relating annual Q to SST, based on combining observed data for 95 years and forecast SST over a period of 15 years, provide probabilities of expected annual Q as a function of forecast SST. The SST of the eastern tropical Pacific was successfully forecast to be appreciably warmer than long-term mean conditions for much of the year beginning in mid 1991. Precipitation data through 1991 indicated that annual natural Q for the Darling River was probably substantially below the mean. However, winter precipitation in higher-runoff portions of the Murray Basin was above average during this El Nino episode, contrary to the trend for most such events over the past Century.

Interannual variability of rainfall in the eastern horn of Africa and indicators of atmospheric circulation

Abstract: Relationships between rainfall variations in the Eastern Horn of Africa, the Southern Oscillation, and the Indian Ocean temperature and pressure surface fields are studied for the period 1932–83. Rainfall data consist of stations and regional indices calculated for three selected areas experiencing quite different rainfall patterns. The results indicate significant negative correlations between northern autumn rains in Somalia and the Southern Oscillation during the same season. These rains are also negatively correlated with pressure in the Western Indian Ocean, and positively correlated in the Eastern Indian Ocean. The reverse pattern is shown with sea-surface temperature. This agrees very well with the observations made in Kenya as far as the short rains of October-November are concerned. Central Ethiopia summer rains, in contrast, indicate significant positive correlations with the Southern Oscillation, at zero and 3 months lag. El Nino years often correspond to drought years in this region. Evidence of an inverse relationship between the amount of rainfall in Ethiopia during summer (especially during September) and both the pressure and sea temperature over the Indian Ocean is also given. Correlations with the Arabian Sea are particularly strong. However, teleconnections between summer rains in Northern Ethiopia (Eritrea) and the ENSO or the Indian Ocean fields are much weaker. These results suggest that, although summer rains over Ethiopia are said to be related to the monsoon air flow from the Congo Basin and the Atlantic Ocean, there also exist quite strong connections with the surface conditions prevailing in the Indian and Pacific Oceans.

Space-Time Evolution of Meteorological Features Associated with the Onset of Indian Summer Monsoon

Abstract: To study the climatological structure of the atmospheric fields during the onset phase of the Indian summer monsoon, a composite analysis of different meteorological parameters over Indian stations is carried out. The composites are constructed relative to a uniform set of onset dates over south Kerala. Over the peninsular Indian stations, the rainfall composites show sudden and sharp increases with onset except in the case of east coast stations, where rainfall does not substantially change with the onset of the summer monsoon. The composite wind analysis demonstrates how the upper-tropospheric subtropical westerlies weaken and shift poleward and the tropical easterlies strengthen and spread north with the onset of the monsoon. The onset vortex that takes the monsoon northward along the west coast in many years is clearly discernible between 600 and 400 hPa in the composite streamline charts. The relative humidity builds up suddenly in the vertical a few days before the onset at the respective s...

Simulation of Late Permian Climate and Biomes with an Atmosphere-Ocean Model: Comparisons with Observations

Abstract: A climate model is used to simulate the climate of the Late Permian. The climate model employs more detailed prescriptions of land-ocean boundaries, topography, and inland lakes and seas than were used in previous climate simulations of supercontinents with idealized land-ocean boundaries and no topography. The presence of mountains and plateaus and of inland seas and lakes produce large differences in the simulated climate compared to simulations that omit these features. Mountains and plateaus become focal points for enhanced precipitation and also help to intensify the monsoon circulations. Extensive inland seas and lakes exert a strong local damping of the seasonal range of temperature and also cause changes beyond the lakes region due to dynamical and hydrological effects. Using the climate-biome classification scheme of Walter, the simulated distribution of climates-biomes is compared to the observed distribution of Late Permian vegetation-biomes. Agreement is good in all but two areas.

Bacterioplankton distributions and production in the northwestern Indian Ocean and Gulf of Oman, September 1986

Abstract: Bacterial abundance and 3H-thymidine incorporation were measured throughout the water column during September–October 1986, along transects in the north western Arabian Sea and Gulf of Oman. Bacterial abundances and production estimated from incorporation rates were high (> 1 × 109 cells 1−1 and 30–92% of primary production, respectively) along the oceanic portions of the transects. These elevated levels may indicate a response to the decline of summer phytoplankton blooms stimulated by monsoonal deepening of the mixed layer. Bacterial production and abundance profiles had complex vertical structure with multiple subsurface maxima related to chlorophyll and oxygen distributions. Production and abundance both declined exponentially with depth below 100–200 m. Rates of dissolved organic carbon (DOC) release from the sinking particle flux may have been adequate to support bacterial production in the Gulf of Oman, but in the open Arabian Sea this source appeared to be insufficient to meet the bacterial demand. The bacterial production estimates reported herein are very conservative because very low conversion factors were used. In general these results suggest that the carbon sources usually assumed to support bacterial production (e.g., phytoplankton exudation, particle breakdown) supply only a fraction of the bacterial demand in the northwest Indian Ocean.

Regional distribution of monsoon and desert dust signals recorded in Asian glaciers

Abstract: Short-term (6 months to 17 years) glaciochemical records have been collected from glacier basins throughout the mountains of central Asia. The spatial distribution of snow chemistry in central Asia is controlled predominantly by the influx of dust from the arid and semi-arid regions in central Asia. The glaciochemical data suggests that glaciers which are removed from large source areas of mineral aerosol, such as those in the Himalaya, the Karakoram, and the southeastern Tibetan Plateau, are the ones most likely to contain longer-term glaciochemical records which detail annual to decadal variation in the strength of the Asian monsoon and long-range transport of Asian dust.

The 30-60-Day Convection Seesaw between the Tropical Indian and Western Pacific Oceans.

Abstract: The tropical Indian and western Pacific oceans are two prominent action centers for tropical 30–60-day convective variability. When convection is enhanced over the equatorial Indian Ocean, the tropical western Pacific often experiences an abnormal dry condition (phase I), whereas the development of the convection over the tropical western Pacific tends to be accompanied by suppressed convection in the equatorial Indian Ocean (phase II). This convection seesaw is a fundamental characteristic of the tropical 30–60-day oscillation. The seesaw is intimately associated with the activity of propagating low-frequency convective systems (LFCSs). Its formation process is season dependent. Typical boreal summer seesaw results from a time-lagged development of two systems: a western system that originates in the equatorial Indian Ocean and moves eastward and/or northward and an eastern system that develops in the western Pacific monsoon region and moves westward and/or northward. The boreal winter seesaw, o...

Large scale aspects of India-China summer monsoon rainfall

Abstract: This study investigates the dominant modes of variability in monthly and seasonal rainfall over the India-China region mainly through Empirical Orthogonal Function (EOF) analysis. The EOFs have shown that whereas the rainfall over India varies as one coherent zone, that over China varies in east—west oriented bands. The influence of this banded structure extends well into India. Relationship of rainfall with large scale parameters such as the subtropical ridge over the Indian and the western Pacific regions, Southern Oscillation, the Northern Hemispheric surface air temperature and stratospheric winds have also been investigated. These results show that the rainfall over the area around 40°N, 110°E over China is highly related with rainfall over India. The subtropical ridge over the Indian region is an important predictor over India as well as over the northern China region. '

The tropopause cold trap in the Australian monsoon during STEP/AMEX 1987

Abstract: The relationship between deep convection and tropopause cold trap conditions is examined for the tropical northern Australia region during the 1986-87 summer monsoon season, emphasizing the Australia Monsoon Experiment (AMEX) period when the NASA Stratosphere-Troposphere Exchange Project (STEP) was being conducted. The factors related to the spatial and temporal variability of the cold point potential temperature (CPPT) are investigated. A framework is developed for describing the relationships among surface average equivalent potential temperature in the surface layer (AEPTSL) the height of deep convection, and stratosphere-troposphere exchange. The time-mean pattern of convection, large-scale circulation, and surface AEPTSL in the Australian monsoon and the evolution of the convective environment during the monsoon period and the extended transition season which preceded it are described. The time-mean fields of cold point level variables are examined and the statistical relationships between mean CPPT, surface AEPTSL, and deep convection are described. Day-to-day variations of CPPT are examined in terms of these time mean relationships.