Showing papers on "Monsoon published in 1986"

The Influence of Changing Orbital Parameters and Surface Boundary Conditions on Climate Simulations for the Past 18 000 Years

Abstract: General circulation model experiments at 3000-year intervals for the past 18 000 years were made to estimate the magnitude, timing, and pattern of the climatic response to prescribed changes of orbital parameters (date of perihelion, axial tilt, eccentricity) and glacial-age lower boundary conditions (ice sheets, land albedo, sea ice and sea surface temperature). The experiments used the Community Climate Model (CCM) of the National Center for Atmospheric Research (NCAR). The response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial-age boundary conditions. The continental interior of Eurasia was 2–4 K warmer in summer, and summer monsoon precipitation of North Africa-South Asia was increased by 10–20% between 12 000 and 6000 yr BP (before present) when perihelion occurred during northern summer (rather than in winter as now) and the earth's axial tilt was larger than now. Southern Hemisphe...

Coherent response of Arabian Sea upwelling and pollen transport to late Quaternary monsoonal winds

Abstract: The Indian summer monsoon causes large seasonal changes in the environments of both eastern Africa and the Arabian Sea. Here we compare time series (140 kyr long) of selected pollen types and foraminiferal upwelling faunas preserved in marine sediments, to identify the frequencies of their variability and the coherence and phase of the terrestrial and marine responses to monsoonal circulation. During interglacial intervals, the pollen and upwelling records of the western Arabian Sea indicate stronger monsoons and are both coherent and in phase at periodicities near the precession of the Earth's rotational axis (23 kyr). We conclude that the strength of the monsoonal southwesterlies accounts for this high coherence of marine and terrestrial records during interglacial intervals. Low correlation during glacial phases indicates weaker monsoons and some decoupling of these two records. These data indicate that two distinct modes of monsoon variability may exist: an interglacial mode with strong monsoons and effective coupling of terrestrial and marine records, and a glacial mode with both weaker monsoons and decreased linkage between Arabian Sea pollen and upwelling records.

Interannual Variability of the Australian Summer Monsoon at Darwin: 1952–82

Abstract: Fluctuations in the Australian summer monsoon over the period 1952–82 are described. The basis of the study is an objective definition of the major summer monsoon components based on the low-level zonal winds at Darwin; this is shown to be in good agreement with other large-scale indicators. Statistics are presented and discussed for the interannual variation in summer monsoon onset, extent, active and break conditions, circulation strength, and vertical structure. Some relationships with the Southern Oscillation are also described. These indicate that the Southern Oscillation Index (SOI) is highly correlated with the intensity and degree of convergence in the low-level monsoonal shear zone, and with the mean daily rainfall rate over northern Australia. There is also a significant correlation between the summer monsoon onset date and the SOI in the following spring, which has implications for El Nino teleconnections.

Synoptic-dynamic character of `bursts' and `breaks' in the South-West U.S. summer precipitation singularity

Abstract: Marked intraseasonal variations in the intensity of the south-west U.S. summer precipitation singularity (the ‘Monsoon’) are identified for the years 1980–1982 from a previously developed satellite cloud climatology. These ‘bursts’ and ‘breaks’ are examined synoptically by compositing sea level pressure and 500 mb height data. Both types of events are controlled by the upper-level flow. This typically involves, for bursts, the intrusion into subtropical latitudes of a trough in the westerlies with attendant cold air advection. In conjunction with intense surface heating, this produces widespread atmospheric destabilization over the South-west. Monsoon breaks are characterized by enhanced ridging of the Bermuda and North Pacific subtropical anticyclones with associated subsidence over southern Arizona/California. Between-year differences in these composite patterns are identified, and it is suggested that wetter summers may be characterized by larger within-season variations in 500 mb heights and isobaric curvature compared with drier summers. Several 500 mb synoptic types associated with burst events are determined subjectively, and composite patterns are derived. These indicate that bursts occasionally occur with anticyclonic conditions aloft, but the latitude of the ridge seems critical in determining the extent of moist air penetration from the south. Twice-daily aerological soundings at five stations in the South-west and northern Mexico are also used to determine the thermodynamic characteristics of bursts and breaks and the dominant source of monsoon moisture. Monsoon bursts in Arizona are evidently a combination of moisture from the Gulf of Mexico, transported on a light south-easterly flow, and more rapid lower-level ‘surges’ originating in the Gulf of California. Moisture from the latter source is transported rapidly into the Great Basin on a low-level wind-speed maximum of marked directional constancy. This study has implications for longer-term summer rainfall-synoptic circulation variations in the American South-west.

Seasonal estimates of global atmospheric sea‐salt distributions

Abstract: Seasonal estimates of sea-salt aerosol particle concentration distributions 15 m above the sea are presented on global contour maps. Measured data from a variety of sources relating atmospheric sea-salt concentration to wind speed have been combined, yielding relationships of form C = exp (as + b), where C is sea-salt concentration in micrograms per cubic meter and s is the horizontal wind speed in meters per second. These relationships, coupled with a Gaussian wind speed frequency distribution, allow us to calculate the atmospheric sea-salt concentration accounting for the variance about mean wind speeds. We use monthly wind mean speed and variance information in 5° × 5° latitude/longitude squares over the world ocean to estimate the global sea-salt aerosol particle mass distribution. The atmospheric sea-salt concentrations in the northern hemisphere marine troposphere display a substantial seasonal dependence. The 3-month seasonal average sea-salt concentrations in this region differ by a factor of 2–3 between the boreal winter and summer, and the highest values are between 40 and 49 μg m−3. The seasonal variability of atmospheric sea-salt concentrations in the high-latitude southern hemisphere is much less than that in the northern hemisphere, varying by less than a factor of 2 between the austral winter and summer, and again the highest values are about 45 μg m−3. The equatorial areas have uniformly lower atmospheric sea-salt concentrations than the high-latitude regions. The monsoonal winds over the Indian Ocean produce sea-salt concentrations in excess of 40 μg m−3 for the 3-month boreal summer average.

A study of circulation differences between East-Asian and Indian summer monsoons with their interaction

Abstract: Primarily based on the 1979 FGGE data an analysis is made of the circulation differences between the East-Asian and Indian summer monsoons together with their oscillation features and also the interplay be-tween various monsoon systems originating from the fact that the Asian monsoon area is divided into the East-Asian and Indian regions, of which the former is demarcated into the Nanhai (the South China Sea) and the Mainland subregions.

Mechanisms of Java Rainfall Anomalies

Abstract: The large-scale circulation departure patterns associated with the interannual variability of (July–June) rainfall in Java are studied on the basis of ship observations (1911–73) in the Indian Ocean and surface station records. Circulation mechanisms of interannual variability can, in part, be understood as modulations of the average annual cycle. Abundant rainfall years are characterized by an anomalously strong Northwest monsoon, and drought years by approximately inverse circulation characteristics. In December–January of the wet years, anomalously high pressure near Southeast Asia along with anomalously low pressure over Indonesia entail an enhanced meridional pressure gradient, stronger northeasterly flow over the South China Sea and Bay of Bengal, and enhanced northwesterlies over the Indonesian waters. The increased northerly wind component to the north and intensified westerlies over the equatorial Indian Ocean result in enhanced convergence and cloudiness over Indonesia, while surface wa...

Phytoplankton ecology and production in the Red Sea off Jiddah, Saudi Arabia

Abstract: Phytoplankton species diversity was generally high throughout the year at two stations in the central Red Sea (Lat. 21°30′N), and species of Mediterranean and Indian Ocean origin were represented, reflecting seasonal monsoonal influence. Low phytoplankton cell numbers accompanying high production rates suggest the significance of nanoplankton or picoplankton which were not enumerated. Production was high year-round, and averaged 390 gC m-2 yr-1, despite the virtual lack of nutrient additions from rainfall or land runoff or demonstrable upwelling. Highest nutrient levels followed the first seasonal peak of production. Biomass and production were seasonally bi- or tri-modal, with major peaks in December–February and June–August, 1977–1978. The first peak of production, populated by diatoms, occurred at the onset of seasonal stratification, but the second peak, populated by Trichodesmium spp., occurred at the height of seasonal stratification and lowest nutrient concentrations. There is no clear relationship between the timing of monsoon activity and the annual production cycle.

Long-period variations of sea-level in Australasia

Abstract: Summary. The ability of the Australian sea-level monitoring network is assessed in the investigation of long-period sea-level signals. Through the character of coastal long waves, seasonal variations in level and inter-annual level anomalies, the importance of the south coast of the Continent is identified as a coherent indicator of large-scale marine and atmospheric teleconnections. The source of the sea-level signal is investigated by the tracing of progressive features, by the numerical modelling of wind stress over the Southern Ocean, by the modelling of the effect of monsoonal rains over the lndian Ocean and the mass transport through the lndonesian Strait. These features are related to the ENSO cycle which for the first time is linked, inter &a, with Southern Ocean mechanisms.

Mesoscale analyses of the Sichuan flood catastrophe 11-15 July 1981

Abstract: During the period 11–15 July 1981, heavy rainfall occurred over the Sichuan basin in China, resulting in severe floods that took a large toll in human life and property damage. Synoptic analyses indicate that early in this period the southerly monsoon flow was particularly strong near the basin because of a favorable positioning of the Pacific subtropical high and the Indian monsoon depression. The passage of a deep midlatitude trough from the Lake Baikal region brought colder, drier air from Siberia into southwest China. The Siberian air stream met the monsoon current over the eastern plateau and the Sichuan basin, creating a region of large-scale, low-level convergence. Mesoscale analyses show that the flood was directly related to the extreme development of a long-lived mesoscale vortex [called south (SW) vortex by Chinese meteorologists] over the basin as it merged with another mesoscale vortex generated over the Tibetan Plateau. Mesoscale heat and moisture budgets suggest that the latent hea...

Late Quaternary paleoceanography of the tropical Atlantic, 2: The seasonal cycle of sea surface temperatures, 0–20,000 years B.P.

Abstract: Foraminiferal species abundances are used to estimate seasonal variability of late Quaternary sea surface temperatures in the tropical Atlantic Ocean. Empirical orthogonal functions analysis of 28 time series isolates two patterns (modes) of variation. The dominant mode 1 reflects seasonal temperature contrast of the South Equatorial Current region 5°–6°C higher at the glacial maximum than at present, probably indicating larger seasonal variations of the southern trade winds. Forcing for this pattern may have come from equatorward compression of glacial climate zones due to high-latitude cooling in both hemispheres and/or suppression of meridional monsoonal circulation via feedback from northern hemisphere ice cover. When compared with CLIMAP estimates of lower seasonal variations of glacial age sea ice fronts in Antarctica, mode 1 suggests decoupling of low- and high- latitude seasonal cycles in the southern hemisphere on a glacial-interglacial scale. Mode 2 variations reflect seasonal contrast in sea surface temperature of 2°–3°C less 9000 to 14,000 years B.P. than at present along the equator and in the eastern subtropical Atlantic, perhaps related to an equatorial position of the Intertropical Convergence Zone, weakened trade winds, and/or strengthened monsoonal circulation during deglaciation. Our findings emphasize the need to isolate spatially coherent (and thus dynamically linked) patterns of climate change from a complex record of multiple climatic effects. Only then can effective tests of hypotheses be made with a coupled strategy of data acquisition and climate modeling.

A model study of the seasonal cycle of the Arabian Sea surface temperature

Abstract: The Annual variation of the SST along a zonal strip from the coast of Somalia to the southwest coast of India was simulated using available data (monthly-mean heat and momentum fluxes across the air-sea interface, surface advective field, etc.) as input to a Kraus-Turner mixed-layer model. Three cases were examined. In the first, influence of surface fluxes alone was considered. The second included the effects of surface fluxes and vertical advection. Then, effect of horizontal advection was added. The model forced with the surface heat and momentum fluxes alone simulated reasonably well the SST variability throughout the year except during the May-August (southwest monsoon) cooling phase. The model was found to be inadequate to handle the coastal areas during this phase. Over the open-sea regime the performance of the model was better; and, it improved when the influence of advection was included. The important contribution of the horizontal advection during June-August was to remove most of the heat gained at the surface during the course of a year. Though downwelling in the open-sea had little influence on the SST, it had noticeable impact on the vertical heat transport. The numerical experiments suggest that the Kraus-Turner thermodynamics alone dominate the Arabian SST variability throughout the year except during the southwest monsoon, when dynamics too playa significant role.

Relationship between Indian Summer Monsoon Rainfall and Location of the Ridge at the 500-mb Level along 75°E

Abstract: Banerjee et al. showed for the first time that the number of Indian subdivisions with normal or above-normal monsoon rainfall is related to the location of the April 500-mb ridge along 75°E. Thapliyal brought out the relationship between monsoon rainfall of peninsular India and this ridge. A detailed investigation of the relationship between all-India (India taken as one unit) monsoon rainfall, as well as monsoon rainfall of the subdivisions of India, and the location of the April 500-mb ridge along 75°E, and of the stability and consistency of the relationship has been made in this study, which is based on data for 1939–84. The relationship between all-India monsoon rainfall and the ridge location is positive (correlation coefficient = 0.71) and is significant at the 0.1% level and that between the subdivisional monsoon rainfall and the ridge is significant at 5% or above for all the subdivisions lying west of 84°E and north of 12°N. During the years when all-India monsoon rainfall is deficient,...

Deep-sea sediments in the eastern equatorial Atlantic off the African coast and meteorological flow patterns over the Sahel

Abstract: Deep-sea mineral dust deposits are a powerful tool allowing to deduct atmospheric flow patterns. Here deep-sea sediments from the eastern tropical Atlantic were used to derive climatic conditions over the African continent where in summer the dust is lifted in the transition zone of the Sahara and the Sahel. The meteorological process causing this aeolian dust injection are the squall lines, the rain bearing disturbances of the Sahel. They produce a high surface wind velocity area in their northern vicinity. The deep-sea deposits of sediments and their spatial patterns allowed to conclude that the dust raising mechanism connected with the squall lines were similar for present day conditions and for the time of the last glacial maximum. The Saharan trades and the African monsoon had about the same extension in either time slice coupled with an identical travel belt of the squall lines. However, the intensity and the frequency of the squall lines were lower in glacial times.

Application of Extended Empirical Orthogonal Function Analysis to Interrelationships and Sequential Evolution of Monsoon Fields

Abstract: Extended empirical orthogonal function (EEOF) analysis has been employed to study linear relationships among the mean sea level pressure, 700 mb height and rainfall over India, and their low-frequency sequential evolution during the peak summer monsoon months. The interrelationships between these fields are strongest over central India and, while the rainfall activity is colocated with the corresponding changes in the 700 mb heights, it is displaced southward with respect to the pressure changes. The first two EEOF's of all the three fields (averaged over 5 or 7 days) show that the dominant low-frequency sequential evolution is associated with north and northeastward movement of the anomaly centers with a recurrence period of about 40 days. In addition, the presence of a westward moving wave in sea level pressure anomalies located roughly near 15°N latitude is revealed by the third EEOF.

Forecasting of monsoon performance over India

Abstract: The technique of screening-regression analysis is applied to forecasting the monsoon performance (percentage rainfall departure) over India from the immediately preceding May's meteorological variables. The resulting prediction equation with five variables performed well, with a few exceptions, on independent data. The forecasting scheme shows some promise of providing a useful forecast of monsoon rainfall departure over India before the beginning of the season.

Fluctuations of precipitable water over the Indian Ocean during the 1979 summer monsoon

Abstract: Fluctuations of precipitable water (PW) over the Indian Ocean during the 1979 summer season are studied. Time series of space average PW over the Arabian Sea show the existence of a well-marked oscillation of humidity related to the active/break cycle of the monsoon. This cycle around 30–50 days was the major feature of the 1979 monsoon. It also appeared on the pressure and zonal wind fields. This oscillation resulted from perturbations originating over the equatorial eastern Indian Ocean and propagating northward toward India. The arrival over India of a trough-ridge characterized by humid/dry conditions gave rise to an active/break monsoon. It is also suggested that this type of oscillation might be related to the date of the onset of the monsoon. Another periodicity around 10–12 days is also found in the PW and meridional wind fields. This fluctuation appears to be associated with a westward propagating mode north of the equator.

Evidence for a counter-wind current in winter off the southeast coast of China

Abstract: Regarding the current pattern in winter in the near-sea region of Zhejiang, Fujian and Guangdong (including the western and central parts of the Taiwan Strait), oceanographers both at home and abroad had considered unanimously that under the intense influence of the northerly monsoon, the current (not only at the surface but also in the deep and near-bottom layers) flows southwestward with the wind. It was not until the end of the fifties that scientists began to question such a traditional concept.

Near-Surface Meanders in the Equatorial Indian Ocean

Abstract: Drifting buoys were released in the western Indian Ocean from 1979 to 1982. They reveal meanders of the eastward flowing monsoon drift in August–September 1979, 1980 and 1981. Oscillating meridional buoy drifts reach 80 cm s−1 and meridional displacements can exceed 3° for motions at a period close to 25 days. In 1979, this is related to oscillation below the thermocline, sensed by an array of current meters in the western equatorial Indian Ocean. It is likely that the origin of the oscillations is in the surface currents north of the equator. The currents change in October with the formation of an intense eastward equatorial current in which most of the buoys are entrained. As the buoys drift rapidly towards the eastern Indian Ocean, meridional motions are still present but at shorter periods (12 days) than is observed below the thermocline (25 days). It is possible that is still a manifestation of the same oscillations, but with Doppler shifting and a strong influence of nonlinearities. Waves a...

The equatorial undercurrent: 100 years of discovery

Abstract: The Equatorial Undercurrent is a narrow ribbon of eastward flow centered on the equator in the upper thermocline. It is a permanent feature of the general circulation in the Atlantic and Pacific oceans and is present in the Indian Ocean in northern winter and spring during the northeast monsoon. It reaches speeds of 50–100 cm s −1 below the westward flow of the South Equatorial Current, and in the Pacific transports as much mass on average (40×106 m3 s −1 ) as the Florida Current, which feeds the Gulf Stream.

Potential Predictability of Lower-Tropospheric Monsoon Circulation and Rainfall over India

Abstract: The potential predictability of the lower tropospheric circulation and the rainfall over India during the peak summer monsoon season (July–August) is studied by analyzing the signal-to-noise ratio. Daily 700-mb heights, mean sea level pressure anomaly and rainfall at 220 stations for 21, 30 and 19 years, respectively, are used to represent the circulation and rainfall fields. The predictability of the circulation fields in general increases with decreasing latitude but is low over the area normally occupied by the monsoon trough. The potential predictability of rainfall is about 50% over the major parts of the country.

Evolution of the Net Surface Shortwave Radiation over the Indian Ocean during Summer MONEX (1979): A Satellite Description

Abstract: The evolution of the net shortwave (NSW) radiation fields during the monsoon of 1979 was analyzed, using geostationary satellite data, collected before, during, and after the monsoon onset. It is seen, from the time sequence of NSW fields, that during the preonset phase the characteristics of the NSW field are dominated by a strong maximum in the entire Arabian Sea and by a strong minimum in the central and eastern equatorial Indian Ocean, the minimum being associated with the intense convective activity occurring in that region. As the season evolves, the minima of NSW associated with the large scale convective activity propagate westward in the equatorial ocean. During the monsoon onset, there occurs an explosive onset of the convection activity in the Arabian Sea: the maximum has retreated towards the Somalia coast, and most of the sea then experiences a strong minimum of NSW associated with the intense precipitation occurring along the southwestern coast of the Indian subcontinent.

Stratospheric aerosols and the Indian monsoon

Abstract: This paper reports on a research study of the association between stratospheric aerosols and Indian monsoon precipitation. It was found that low-latitude aerosols precede below average precipitation and high-latitude aerosols precede above average precipitation. A chi-squared analysis shows that the association is above the 99% level of significance. The second part of the research concerns the transformation of low-latitude aerosols into high-latitude aerosols as they move poleward in the second year of their existence. The model predicts that below average monsoon precipitation of the first year after a low-latitude eruption would be followed by above average monsoon precipitation in the second year. The findings support the prediction. Table 7 shows that below average monsoon years are twice as likely to be followed by above average monsoon years as by below average monsoon years. Because aerosol research suffers from inadequate reporting of the events (see appendices A, B, and C), the third section of this research study uses El Nino events as a proxy for the presence of low-latitude stratospheric aerosols. Previous research of Handler (1984, 1986) and Parker (1986) has shown that there is a high level of statistical association (>98%) between the presence of low-latitude stratospheric aerosols and El Nino events. The 22 moderate and strong El Nino events (Quinn et al., 1978) are used as a proxy for the presence of low-latitude aerosols. Six of the 22 events are not useful for the analysis because of the presence of interfering aerosols. Of the 16 possible El Nino events with no interfering aerosols present, 13 show the proper sequence of below average and then above average monsoon precipitation. Since recent aerosols data are generally more complete, a separate analysis of the 1942–1984 period is presented. In that 43 year period, the Indian monsoon fits the stratospheric forcing model very closely. Low-latitude aerosols are associated with below average monsoon precipitation and high-latitude aerosols are associated with above average monsoon precipitation. Six years are indeterminate because of the interfering effects of high- and low-latitude aerosols. The data are consistent with the global climate models of Kutzbach and Guetter (1986). The forcing of a low-latitude aerosol is assumed equivalent to a decrease in solar radiation in that both reduce the equator-to-pole temperature gradient. The Kutzbach and Guetter study has shown that a small change of solar radiation can produce a large change in monsoon precipitation. Finally, the research suggests that the long term, secular behavior of Indian monsoon precipitation can be related to the frequency of low-latitude volcanic eruptions. During the early part of the 20th century, low-latitude eruptions were frequent and the average monsoon precipitation was low. During the period 1942–1962 the frequency of low-latitude eruptions was low and the average monsoon precipitation was high.

Equatorial Indian Ocean evaporation estimates from operational meteorological satellites and some inferences in the context of monsoon onset and activity

Abstract: The estimation of evaporation from the sea surface is not yet achieved adequately by remote sensing techniques, in general. However, for approximate averaged estimates over moderate space and time scales over a specific tropical region, e.g., weekly values over the Indian Ocean as needed in monsoon moisture diagnosis, it may be possible to extrapolate satellite wind and humidity data to the ocean surface and then use bulk aerodynamic parameterization for estimating evaporation. In the present investigation, GOES low-level cloud winds and TIROS-N moisture profiles over the Indian Ocean are extrapolated to the ocean surface. The planetary boundary layer (PBL) wind shear is obtained over different sub-regions and periods during the monsoon season, by reference to objectively analysed fields. These shear values are applied to GOES satellite winds to obtain sea-surface winds. The humidity extrapolation was based on (i) an exponential fit for water vapour density and (ii) a vertical distribution of relative humidity approximately proportional to atmospheric pressure. The exchange coefficient is varied slightly depending on wind speed and boundary-layer stability inferred approximately from TIROS-N sea surface temperatures and temperature profiles. The evaporation estimate as based on these satellite parameters is assessed by comparison with ships' surface observations. Sensible heat exchange is also estimated and assessed. Some inferences based on these estimates are also presented, in relation to monsoon onset and activity.

On the interannual variability of the Indian monsoon and the Southern Oscillation

Abstract: The mechanisms of the interannual variability of the Indian monsoon are investigated from observations in the Indian Ocean sector through correlation and stratification techniques Years with abundant Southwest monsoon rainfall in India are characterized by anomalously low pressure over South Asia and the adjacent waters, enhanced cross-equatorial flow in the western, and increased cloudiness over the northern portion of the Indian Ocean, continuing from the pre-monsoon through the post-monsoon season; positive temperature anomalies over land and in the Arabian Sea in the pre-monsoon season, changing to negative departures after the monsoon onset The following causality chain is suggested: The anomalously warm surfaces of South Asia and the adjacent ocean in the pre-monsoon season induce a thermal low, thus enhancing the northward directed pressure gradient, and favoring a vigorous cross-equatorial flow over the Indian Ocean After the monsoon onset the land surfaces are cooled by evaporation, and the Arabian Sea ausrface waters by various wind stress effects However, latent heat release over South Asia can now maintain the meridional topography gradients essential to the monsoon circulation