Subtropical Westerly Jet Research Articles

The early summer seasonal change of Large-Scale circulation over east asia and its relation to change of the frontal features and frontal rainfall environment during 1991 summer

By using the rawinsonde data, upper cloud amount data and objective analysis data for global domain which all were produced by Numerical Prediction Division,JMA and by using daily and mean weather map issued byJMA and daily rainfall data over the Huaihe River Basin from China, an observational study to the early summer seasonal change of large-scale circulation over East Asia and its relation to change of the frontal features and environment for the frontal rainfall has been carried out. Following reaults have been obtained: (1) The early summer seasonal change of large-scale circulation was occurred during 20–23 May 1991, which was about 10 days earlier than the normal. During the period the subtropical westerly jet and tropical easterlies abruptly moved northward; (2) The northward movement of the tropical easterlies was not uniform, it was earlier at 100 hPa level and about 24 hrs late at 200 hPa level. The phenomenon was associated with earlier disappearence of the subtropical westerly jet at 100 hPa level; (3) During the seasonal change there were two westerly jets in the upper level and changed their intensity with the time. Before 18 May 1991, the southern one was more intense and then changed to more intense for the northern one and disappearence of the southern one, the phenomenon seemed to be appeared as northward movement of the southern jet; (4) A faster temperature(T) rising in the upper level over the Tibetan Plateau was associated with the seasonal change. From theT rising theT maximum moved onto the plateau, changing theT gradient from positive to nega-tive to the south flank of the plateau, the effect to reduce and disappear the southern westerlies. Also theT rising was associated with change of the frontal features over East Asia; (5) The seasonal change type during 1991 was same as that during 1992 and 1993, but different from that during 1990; (6) The environment for the frontal rainfall was change in the season, the differences were in the baroclinity in upper level and vertical wind speed and direction shear.

Tropical Impacts of SST Forcing. A Case Study for 1987 versus 1988

The response of the NASA/Goddard Institute for Space Studies general circulation model (GCM) to large tropical sea surface temperature (SST) anomalies is investigated by evaluating model simulations of the particularly contrasting summer monsoon seasons 1987 and 1988. These years are representative of the warm and cold phases, respectively, of a recent El Nino-Southern Oscillation (ENSO) event. An ensemble averaging the results of three simulations was considered for each season, using monthly mean observed SST anomalies for June-August 1987 and 1988 as lower boundary forcing. Consistent with the European Center for Medium Weather Forecast (ECMWF)-analyzed winds, the simulations based on 1988 as compared to 1987 SST exhibit stronger upper-tropospheric irrotational circulation between the monsoon regions and the Southern Hemispheric sub-tropical anticyclones, a stronger Pacific Walker cell and a weaker subtropical westerly jet over the South Pacific. In the same vein, the modeled precipitation, indicating a more northerly position of the Pacific Inter-Tropical Convergence Zone (ITCZ) in 1988 compared with 1987, is supported by satellite observations of outgoing longwave radiation and highly reflective clouds.

Case Studies of Extreme Climatic Events in the Amazon Basin

The mechanisms of climate anomalies in the Amazon basin were explored from surface climatological and hydrological series, upper-air, and satellite observations. The paper is focused on the March–April rainy season peak in the northern portion of Amazonia. Case studies for the moderately wet year 1986 (WET), showed a relatively far-southerly location of the Atlantic near-equatorial trough, and embedded intertropical convergence zone (ITCZ); strong ascending motion and vigorous convection over the Amazon basin, contrasting with pronounced subsidence off the west coast of South America,, and weak subtropical westerly jets (STWJ) in both hemispheres. In contrast, the extremely dry El Niño year 1983 (DRY), featured a more northward located ITCZ; subsidence over the Amazon basin; ascending motion and convective rainfall to the west of the Andes; and strong STWJ. In synthesis from these analyses, some major mechanisms of extreme rainfall events in northern Amazonia stand out, but only for the late austral summer, when the ITCZ in the tropical Atlantic-South American sector attains its southernmost position, as the intense summertime convection over Amazonia is an important component of the ITCZ. Thus, ascending motion over the northern part of the Amazon basin with an anomalously far-southward displaced ITCZ appears compatible with subsidence to the west of the Andes during the high phase of the Southern Oscillation (SO), which is defined by anomalously high/low pressure at Tahiti/Darwin. In contrast, ascending motion and convection over the easternmost equatorial Pacific, as is common during extreme events of the low-SO phase, require compensatory subsidence, and this may interfere with convection to the east of the Andes. However, hydrometeorological anomalies in Amazonia are not prevailingly related to the SO.

On the Maintenance of Short-Term Subtropical Wind Maxima in the Southern Hemisphere during SOP-1, FGGF

The extent to which divergent circulations, induced by tropical heating, help to maintain westerly maxima in the Southern Hemisphere subtropics during the SOP-1 of FGGE is explored using Level III-b analyses from the Goddard Laboratory for Atmospheres. The contribution of the divergent wind component to the total ageostrophic flow in the subtropics is examined, as are the roles of other forcing terms in the localized Eliassen-Palm (1961) flux zonal momentum equation. Throughout the summertime subtropics, it is generally found that the dominant term in the zonal momentum budget is the Coriolis force applied to the diabatically driven meridional circulation. The largest positive tendencies due to this term are found in the entrance regions of the subtropical westerly maxima, and divergent circulations account for nearly all of the total ageostrophic flow. In the SPCZ region, however, it is found that transient eddies play an important role by partially offsetting the strong Coriolis acceleration in the entrance region of the local jet, and they help accelerate the westerly flow in the exit region through both barotropic and baroclinic processes. Energetically, the dominant term in the rotational kinetic energy budget throughout the subtropical belt is the conversion of divergent to rotational kinetic energy. The evidence supports the view that tropical heating in transient events drives or enhances local meridional overturning in the atmosphere, which, in turn, strengthens the summer subtropical westerly jet stream.

FGGE冬期における熱帯循環のエネルギー変換

FGGE level-IIIb data produced by the European Centre for Medium Range Weather Forecasts (ECMWF) for the DJF 1978-79 is used to analyse the kinetic energy budget over the global tropical belt. A detailed energetics of the mean and transient fluctuations, in particular, the generation, the exchange between the mean flow and the transients and the dissipation of kinetic energy are estimated for the tropics and compared with the earlier studies. It is found that about 60% of the kinetic energy generated in the tropics is dissipated by the frictional mechanisms and remaining 40% is transported to the middle latitudes of both the hemispheres. The subtropical westerly jet stream of the winter hemisphere plays a dominant role in the process of kinetic energy flux transport. It is observed that the inter-hemispheric exchange of kinetic energy is negligible. Further, the study confirms the fact that the tropical circulation is mainly maintained by the mean flow while in the extra tropics (beyond 30° latitude) the transients are quite important.

Air-sea interaction on a seasonal scale over north Indian Ocean -Part II. : Monthly mean atmospheric and oceanic parameters during 1972 and 1973

Monsoon-Indian Ocean interaction is examined in detail using 5-dcgree square monthly mean data of ocean and atmosphere of two consecutive years of contrasting monsoon rainfall, 1972 and 1973. There was a major monsoon failure (drought) in 1972; in 1973 India had excess rainfall during the monsoon season.
 It is found that north Indian Ocean is colder than normal (negative SST anomaly) during the months prior to monsoon of 1972" During this –period upper tropospheric sub-tropical westerlies (monthly mean) intruded equatorwards over soJ1tlfAsia, farthest south over the Arabian Sea longitudes. The weak monsoon of June to Septemb~72 produced a warm SST anomaly over tropical Indian Ocean on account of decreased cooling of the ocean surface layer during the monsoon season due to decreased upwelling, particularly off the coasts of Somalia and Arabia, decreased wind mixing, decreased evaporation, and decreased clo1.lding. The positive SST anomaly of large spatial extent thus created persisted from October 1972 to May 19-73. The upper tropospheric circulation over south Asia during this period had equatorial easterlies (instead of westerlies) and increased strength of the sub-tropical westerly jet stream over north India. These results agree with observations and GCM simulations over Pacific Ocean with warm SST anomalies. Monsoon of 1973 was good and cold SST anomalies again appeared over north Indian Ocean from October 1973.

The sensitivity of the northern hemisphere summer mean meridional circulation to changes in the large scale eddy forcing

A zonally averaged version of the Goddard Laboratory for Atmospheric Sciences (GLAS) climate model is used to study the sensitivity of the northern hemisphere (NH) summer mean meridional circulation to changes in the large scale eddy forcing. A standard solution is obtained by prescribing the latent heating field and climatological horizontal transports of heat and momentum by the eddies. The radiative heating and surface fluxes are calculated by model parameterizations. This standard solution is compared with the results of several sensitivity studies. When the eddy forcing is reduced to 0.5 times or increased to 1.5 times the climatological values, the strength of the Ferrel cells decrease or increase proportionally. It is also seen that such changes in the eddy forcing can influence the strength of theNH Hadley cell significantly. Possible impact of such changes in the large scale eddy forcing on the monsoon circulation via changes in the Hadley circulation is discussed. Sensitivity experiments including only one component of eddy forcing at a time show that the eddy momentum fluxes seem to be more important in maintaining the Ferrel cells than the eddy heat fluxes. In the absence of the eddy heat fluxes, the observed eddy momentum fluxes alone produce subtropical westerly jets which are weaker than those in the standard solution. On the other hand, the observed eddy heat fluxes alone produce subtropical westerly jets which are stronger than those in the standard solution.

Study of the Dynamics of the Intertropical Convergence Zone with a Symmetric Version of the GLAS Climate Model

Abstract The results of some calculations with a zonally symmetric version of the Goddard Laboratory of Atmospheric Sciences (GLAS) climate model are described. The model was first used to study the nature of symmetric circulation in response to various zonally-averaged latent heating fields based on observations. Three experiments with distribution of Intent beating corresponding to the equinox condition, Northern Hemisphere summer condition and south Asian monsoon condition showed reasonable similarity to the observed distribution of surface easterlies and westerlies and the subtropical westerly jets. In the south Asian monsoon experiment, surface westerlies as well as the upper-level easterly jet in the subtropics of the Northern Hemisphere were found. The strength of the subtropical westerly jet increased with decrease in the vertical eddy viscosity. Additional experiments were carried out in which the model was allowed to determine its own latent beat sources and the results were analyzed to examine ...

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Based on the ERA-40 reanalysis data, dryness and wetness over the Tibetan Plateau are categorized according to the monthly standardized precipitation index. The atmospheric features associated with severe and extreme dryness and wetness reveal two cross-Eurasia wave trains: the Scandinavia-East Asia wave train and the Mediterranean-East Asia wave train. Severe and extreme dryness is associated with an anomalous cyclone over south and southeast Asia, which directs the moisture supply from the Arabian Sea, the Bay of Bengal, and the South China Sea directly eastward towards the western Pacific, thus bypassing the Tibetan Plateau. This cyclone anomaly constitutes part of the Scandinavia-East Asia wave train, which is sustained by the divergence and convergence of the anomalous transient eddy heat transport associated with a more southwest-northeast oriented North Atlantic storm track and a northward shift of the polar front jet in the North Atlantic.In contrast, severe and extreme wetness over the Tibetan Plateau is associated with a more zonally elongated North Atlantic storm track; wave trains excited from there have a high probability to reach the Mediterranean region and to propagate eastward following the subtropical westerly jet. This Mediterranean-East Asia wave train generates anticyclonic anomalies around the Tibetan Plateau and East China, which bring more moisture supply from the Arabian Sea, the South China Sea, and the western Pacific towards the Tibetan Plateau and enhance the moisture convergence there. This paper demonstrates how atmospheric bridging processes affect regional climate variability under present day climatic conditions, which are also relevant for understanding past climates.

Jet streams at 150 mb during a large scale Drought in Indian summer monsoon

During 1972 a year of large scale drought in the Indian summer monsoon rainfall, the mean u-field at 150 mb shows a weaker than normal tropical easterly jet stream over south Asia and a stronger than normal sub-tropical westerly jet stream over Australia; the mean v-field shows that the cross equatorial northerlies are weaker and displaced eastwards by about 20° longitude.
 
 The two years 1970 and 1975 of highest monsoon rainfall for India of the decade 1970-1979 were compared with the two years of lowest monsoon rainfall 1972 and 1979, In the strength of the sub-tropical Jet stream (STJ) over Australia as seen from an analysis of monthly mean u-fields of July and August. It is seen that during the poor rainfall years, the STJ is about 20% stronger than in the good monsoon years.

Relations between theZCriterion for the Subtropical High, Hadley Cell Parameters and the Rainfall in Northern Ghana

We use radiosonde data from Tamale in northern Ghana to evaluate the Z criterion latitude ϕz. We find it correlates well with other Hadley cell parameters since its variations are apparently controlled by the meridional movement of the subtropical westerly jet. Since the Hadley cell parameters correlate well with Tamale rainfall, we conjecture that it should be possible to predict northern Ghana rainfall on the basis of the southward extension of the subtropical jet once more data have been collected by the Tamale radiosonde. This conjecture is supported by data from other stations in West Africa.

Late quaternary climatic changes in Australia and North Africa: A preliminary interpretation

Geological and biological data for the period 0–40,000 B.P. suggest that variations in precipitation and evaporation along the temperate and tropical margins of Australia and North Africa are closely related to variations in the position and strength of the subtropical anticyclones. Such changes in the subtropical anticyclones seem related to changes in the position and wave amplitude of the subtropical westerly jet stream. A dynamic interpretation of the inferred palaeoclimates suggests that important additional factors were the distribution of surface temperature anomalies over Siberia and the Sahara at 30,000 B.P.; the location of North Pacific and North American surface temperature anomalies at 24,000 B.P.; compression of the westerlies over Tasmania during 20,000–15,000 B.P.; breakdown of the summer monsoon during 17,000–12,000 B.P.; and a possible change in the slope and nature of the Intertropical Convergence during 11,000–7,000 B.P. Major environmental changes associated with the above influences were heavy rainfall, high lake levels, and increased fluvial activity in southern Australia and along both margins of the Sahara between 40,000 and 20,000 B.P.; low lake levels in the Afar at 30,000 B.P.; intertropical aridity and dune-building along the tropical margins of the Sahara and Australia, and desiccation in semi-arid New South Wales during the interval 17,000–12,000 B.P.; and very high lake levels and increased precipitation in the Sahel and in northern and southern Australia from 11,000 B.P. to 5,000 B.P. The past and present role of the desert anticyclones in controlling the start and the close of the arid phases is demonstrated, as is the correlation between Southern Hemisphere pressure systems and Saharan rainfall, and between Northern Hemisphere circulation changes and Australian rainfall. The subtropical anticyclones may act as buffers when the thermal balance between the two hemispheres is upset by an excess of surface ice in any one hemisphere, and this influences the climate well beyond the deserts.

Tropical East–West Circulations During the Northern Winter

Abstract In this Paper we present the geometry and intensity of the mean east–west circulation during the northern winter. We show that near the equatorial latitudes two pronounced regions of divergent mass outflow in the upper troposphere are found near the convective regions over the northwestern part of South America and Indonesia. The intensity of the east–west circulation is shown to be of the order of 1 m sec−1 which is comparable to the intensity of the Hadley circulation. The divergent streamlines are shown to be important for the maintenance of the three waves of the subtropical westerly jet in the Northern Hemisphere, and are shown to exhibit asymptotes of convergence in the regions of mid-oceanic upper tropospheric troughs over the tropical southern oceans. Kinetic energy exchanges for a tropical belt 15S to 15N at 200 mb are expressed as a function of zonal wavenumber. Results for northern summer and winter seasons are compared. We find that wave interactions with the mean zonal flow differ in...

A study of vertical wind profile of the tropical easterly jet stream over Madras

Rawin observations for the months June to September during five consecutive year (1961-65) have analysed to study the vertical wind profile of the tropical easterly jet stream over Madras, Visakhapatnam and Trivandrum. The main finding is that the stronger the tropical easterly jet stream, the higher is the altitude of maximum wind. This is just the opposite of the author's earlier finding in the case of sub-tropical westerly jet stream where stronger the jet maximum the lower is its level of occurrence. It is also seen that the tropical easterly jet strengthens as one towards south from Visakhapatnam to Trivandrum while the level of its occurrence lowers.

A study of vertical wind profile of the Westerly Jet stream over Delhi, using radar wind data

Radar wind observations for the months December to February (winter season) during three consecutive years (1963-1966) have been utilised to study the vertical wind profile of the sub-tropical westerly jet stream over Delhi and the observed features have been discussed. The main finding is that the stronger the sub tropical westerly jet stream, the lower is the altitude of maximum wind.

Breaks in the Indian summer monsoon as a phenomenon of interaction between the easterly and the sub-tropical westerly jet streams

Breaks in the Indian summer monsoon as a phenomenon of interaction between the easterly and the sub-tropical westerly jet streams

Breaks in the Indian summer monsoon as a phenomenon of interaction between the easterly and the sub-tropical westerly jet streams

The present paper contains the results of the study of a synoptic situation over India during a period of 8 days in which normal monsoon conditions were followed by a break and were further followed by a return to normal monsoon. This study has shown that during breaks in the monsoon, a trough in the middle latitude westerlies with a jet embedded in it increases considerably in its amplitude and gets retarded as it moves into the Tibetan Plateau on account of its entering a region of weaker basic current. The large-amplitude trough weakens or destroys completely the Tibetan high at the 500 mb level and appreciably weakens the easterlies at the higher levels to the south of the Himalayas. Protruding into India and Pakistan, the large amplitude trough contributes to the development of heavy rainfall along and near the foot of the Himalayas which is characteristic of the break conditions. The southward protrusion of the trough and its subsequent movement eastward, is also indirectly responsible for a corresponding shift of the high over Iran and Arabia, which, in its turn, seems to lead to the formation of a secondary jet core in the easterlies west of 80° E between 20° N and 25° N. Consequent on the above developments, the westerly jet which had retreated to the north of the Himalayas at the time of the onset of the monsoon reenters the Indo-Pakistan sub-continent during the breaks. Thus we see during break conditions, the remarkable spectacle of two jets of entirely different types-the easterly and the sub-tropical westerly—within a short latitudinal distance of each other and dynamically interacting with each other.
 The above detailed study, supplemented by a general examination of the monthly mean 700 mb charts for the northern hemisphere for a period of 10 years, suggests that active monsoon over India is closely associated with high index circulation in middle latitudes over Asia and neighbourhood, while weak monsoon is closely associated with low index circulation over the same region.