Showing papers on "Monsoon published in 1994"

Surface circulation in the South China Sea

Abstract: A three-dimensional, primitive-equation model with a free surface is used to simulate the monthly circulation in the South China Sea. The model has a resolution of 0.4° in the horizontal and 21 layers in the vertical in a region from 2°N to 24°N and from 99°E to 124°E. Inflow and outflow in the Kuroshio, through the Taiwan Strait, and between the Sunda Shelf and the Java Sea are prescribed bimonthly. At the sea surface, the model is forced by monthly-averaged climatological winds and temperature and seasonally-averaged salinity. Several important features are reproduced in the model simulation. First, a strong coastal jet is present at the western boundary. The current is southward along the continental margin from China to southern Vietnam in winter. In summer, the current is northward and separates from the coast between 11°N and 14°N. The transition in September begins as a southward undercurrent, which is remotely forced by the northeast monsoon in the northern reaches of the South China Sea. The undercurrent extends to the surface in about a month. Second, inflow through the Luzon Strait from October to February transports the Kuroshio water in the top of 300 m of the water column westward along the continental slope south of China. In summer, eastward flow in the Luzon Strait transport surface water west of Luzon to the region east of Taiwan. Finally, a subsurface current, which is opposite to the surface current, exists over the Sunda Shelf and is driven by a pressure gradient set up by monsoon winds. These simulated currents are in qualitative agreement with the circulation inferred from the available observations.

Monsoons over China

Abstract: One: The Summer Monsoon in East Asia. Two: The Winter Monsoon in East Asia. Three: The Short-Range Fluctuations of Monsoons and Their Association with the Major Weather Events in China. Four: The Medium and Long-Range Fluctuations of Monsoons and Their Association with Floods and Droughts over China. Five: Effects of the Tibetan Plateau (Qinghai-Xizang) on the Monsoon. Six: Heat, Moisture and Energy Budgets over the Monsoon Regions of China and Some Aspects of Monsoon Dynamics.

Influence of the Land Surface in the Asian Summer Monsoon: External Conditions versus Internal Feedbacks

Abstract: The basic concept of land-sea temperature contrast and the strength of the Asian summer monsoon is investigated here by comparing the relative contributions of external conditions (involving surface albedo) and internal feedbacks (involving soil moisture) in a number of atmospheric general circulation model (GCM) mean climate simulations and in a GCM sensitivity experiment. All models are run with the same long-term mean sea surface temperatures so that only land-surface conditions affect the land-sea temperature contrast. There is a surprising consistency among the various models such that stronger summer monsoons (defined as high area-averaged precipitation over south Asia) are associated with greater land-sea temperature contrast (i.e., higher land temperatures), lower sea level pressure over land, less snow cover, and greater soil moisture. In a sensitivity study with land albedos uniformly raised from 0. 13 to 0.20 in one of the models, the winter-spring-summer sequence over southern Asia sh...

Interannual Variability of the Onset of the Indian Summer Monsoon and Its Association with Atmospheric Features, El Niño, and Sea Surface Temperature Anomalies

Abstract: The long-term mean date of the monsoon onset over Kerala (MOK) varies between 30 May and 2 June according to different estimates, with a standard deviation of 8–9 days. The earliest date of MOK, and the most delayed one, during the last 100 years differ by 46 days (7 May and 22 June, respectively). MOK switches on a spatially large and intense convective heat source over south Asia, lasting from June to September, whose moisture supply is made available through the cross-equatorial low-level jet stream. Superposed epoch analysis of 10 years of outgoing longwave radiation (OLR) data shows that MOK is a significant stage in the evolution of the OLR field in the tropics of the eastern hemisphere. At the time of MOK there is increased convection in a band about 5–10 degrees wide meridionally, extending from the south Arabian Sea to south China, and convection is suppressed all around, particularly in the western Pacific Ocean. In 1983 when MOK was delayed by about 3 pentads, OLR data showed that the ...

Coupled land-ocean-atmosphere processes and South asian monsoon variability.

Abstract: Results from a global coupled ocean-atmosphere climate model and a model with specified tropical convective heating anomalies show that the South Asian monsoon was an active part of the tropical biennial oscillation (TBO). Convective heating anomalies over Africa and the western Pacific Ocean associated with the TBO altered the simulated pattern of atmospheric circulation for the Northern Hemisphere winter mid-latitude over Asia. This alteration in the mid-latitude circulation maintained temperature anomalies over South Asia through winter and helped set up the land-sea temperature contrast for subsequent monsoon development. South Asian snow cover contributed to monsoon strength but was symptomatic of the larger scale alteration in the mid-latitude atmospheric circulation pattern.

Evidence for eddy formation in the eastern Arabian Sea during the northeast monsoon

Abstract: The seasonal formation of a large (500–800 km diameter) anticyclonic eddy in the upper 300–400 m of the eastern Arabian Sea during the northeast monsoon period (December-April) is indicated from hydrographic and satellite altimetry sea level observations, as well as from numerical model experiments. The center of the eddy circulation is approximately 10°N, 70°E, just to the west of the north-south Laccadive Island chain. In this paper the eddy is called the Laccadive High (LH). In some ways it is a mirrorlike counterpart to the Great Whirl, which develops during the southwest monsoon off the Somali coast (western Arabian Sea). The LH occurs at the same latitude but on the opposite side of the basin during the reversed monsoon. It is different from the Great Whirl, however, in its formation process, its intensity, and its decay. The hydrographic data obtained from surveys all during a single season give sufficiently close station spacing to allow reasonable contouring of the geopotential surfaces and of the properties within and around the LH region with minimum time aliasing. The Geosat altimeter record extends over 4 years, during which the seasonal variability of the LH indicates a dynamic relief of approximately 15–20 cm, which is in good agreement with the hydrographic observations. The altimetry time series also suggests a westward translation of the LH by January with a subsequent dissipation in midbasin. The model used is a wind-forced three-layer primitive equation model which depicts a LH in agreement with the timing, position, and amplitude of both the hydrographic and altimetric measurements. The numerical simulation includes a passive tracer located in the western Bay of Bengal; the western advection of the tracer around the south coasts of Sri Lanka and India in December and January is consistent with the appearance of low-salinity water observed to extend into the Arabian Sea during this period. The modeling studies suggest that both local and remote forcing are important in formation of the LH.

Coarse fraction fluctuations in pelagic carbonate sediments from the tropical Indian Ocean: A 1500‐kyr record of carbonate dissolution

Abstract: We examined coarse fraction contents of pelagic carbonates deposited between 2000- and 3700-m water depth in the tropical Indian Ocean using ocean Drilling Program (ODP) sites 722 (Owen Ridge, Arabian Sea) and 758 (Ninetyeast Ridge, eastern equatorial Indian Ocean), and four giant piston cores collected by the French RN Marion Dufresne during the SEYMAMA expedition. Over the last 1500 kyr, coarse fraction records display high-amplitude oscillations with an irregular wavelength on the order of approximately 500 kyr. These oscillations can be correlated throughout the entire equatorial Indian Ocean, from the Seychelles area eastward to the Ninetyeast Ridge, and into the Arabian Sea. Changes in grain size mainly result from changes in carbonate dissolution as evidenced by the positive relationship between coarse fraction content and a foraminiferal preservation index based on test fragmentation. The well-known `'mid-Brunhes dissolution cycle'' represents the last part of this irregular long-term dissolution oscillation. The origin of this long-term oscillation is still poorly understood. Our observations suggest that it is not a true cycle (it has an irregular wavelength) and we propose that it may result from long-term changes in Ca++ flux to the ocean. Sites 722 and 758 deltaO-18 records provide a high-resolution stratigraphy that allows a detailed intersite comparison of the two coarse fraction records over the last 1500 kyr. Site 722 (2030 m) lies above the present and late Pleistocene lysocline. The lysocline shoaled to the position of site 758 (2925 m) only during the interglacial intervals that occurred between about 300 and 500 ka (Peterson and Prell, 1985a). Despite these supralysoclinal positions of the two sites, short-term changes in coarse fraction contents are correlatable from one site to another and probably result from regional (or global) dissolution pulses. By stacking the normalized coarse fraction records from sites 722 and 758, we constructed a Composite Coarse Fraction Index (CCFI) curve in which most of the local signals cancelled out. The last 800 kyr of this curve appear to compare extremely well with the Composite Dissolution Index curve from core V34-53 (Ninetyeast Ridge), which unambiguously records past variations of carbonate dissolution in the equatorial Indian Ocean (Peterson and Prell, 1985a). In the late Pleistocene the CCFI variations are mainly associated with glacial-interglacial changes. They show strong 100 and 41 kyr periodicities but no clear precession-related periodicities. As proposed earlier by Peterson and Prell (1985a), the lack of precession frequencies may suggest that the regional carbonate dissolution signal is driven by changes in deepwater circulation. We cannot totally reject the possibility, however, that low temporal resolution and/or bioturbation degrade somehow the precessional signal at ODP sites 722 and 758. In contrast, spectral density of dissolution cycles in the giant (53 m long) piston core MD900963 (Maldives area) displays clear maxima centered on the precession frequencies (23 and 19 kyr-1) as well as on the 29-kyr-1 frequency but shows little power at the 100-kyr-1 frequency. These high-frequency changes most probably result from changes in surface productivity associated with monsoon variability. Dissolution at this site may be ultimately controlled by the oxidation of organic matter which appears to be incorporated into the sediments in greater quantity during periods of weak SW monsoon and/or increased dry NE monsoon.

Description of a Monsoon Gyre and Its Effects on the Tropical Cyclones in the Western North Pacific during August 1991

Abstract: This paper describes the character and evolution of the low-level wind, sea level pressure, and satellite-observed cloudiness over the western North Pacific (WNP) during August 1991 when the low-level monsoon circulation there became organized as a monsoon gyre The specific configuration of the monsoon circulation, which herein is called a monsoon gyre, is an episodic event—occurring roughly once per year, for two or three weeks during July, August, or September As a monsoon gyre, the low-level circulation of the WNP becomes organized as a large cyclonic vortex associated with a nearly circular 2500-km-wide depression in the contours of the sea level pressure A cyclonically curved band of deep convective clouds rims the southern through eastern periphery of this large vortex Once this pattern is established, it becomes a prolific generator of mesoscale vortices that emerge from the downstream end of the major peripheral cloud band These mesoscale vortices form the seed disturbances for midge

Currents and transports of the Monsoon Current south of Sri Lanka

Abstract: The zonal monsoon circulation south of India/Sri Lanka is a crucial link for the exchange between the northeastern and the northwestern Indian Ocean. The first direct measurements from moored stations and shipboard profiling on the seasonal and shorter-period variability of this flow are presented here. Of the three moorings deployed from January 1991 to February 1992 along 80°30′E between 4°11′N and 5°39′N, the outer two were equipped with upward looking acoustic Doppler current profilers (ADCPs) at 260-m depth. The moored and shipboard ADCP measurements revealed a very shallow structure of the near-surface flow, which was mostly confined to the top 100 m and required extrapolation of moored current shears toward the surface for transport calculations. During the winter monsoon, the westward flowing Northeast Monsoon Current (NMC) carried a mean transport of about 12 Sv in early 1991 and 10 Sv in early 1992. During the summer monsoon, transports in the eastward Southwest Monsoon Current (SMC) were about 8 Sv for the region north of 3°45′N, but the current might have extended further south, to 2°N, which would increase the total SMC transport to about 15 Sv. The circulation during the summer was sometimes found to be more complicated, with the SMC occasionally being separated from the Sri Lankan coast by a band of westward flowing low-salinity water originating in the Bay of Bengal. The annual-mean flow past Sri Lanka was weakly westward with a transport of only 2–3 Sv. Using seasonal-mean ship drift currents for surface values in the transport calculations yielded rather similar results to upward extrapolation of the moored profiles. The observations are compared with output of recent numerical models of the Indian Ocean circulation, which generally show the origin of the zonal flow past India/Sri Lanka to be at low latitudes and driven by the large-scale tropical wind field. Superimposed on this zonal circulation is local communication along the coast between the Bay of Bengal and the Arabian Sea.

Circulation and water masses of the Arabian Sea

Abstract: The dynamics and thermodynamics of the surface layer of the Arabian Sea, north of about 10N, are dominated by the monsoon-related annual cycle of air-sea fluxes of momentum and heat. The currents in open-sea regime of this layer can be largely accounted for by Ekman drift and the thermal field is dominated by local heat fluxes. The geostrophic currents in open-sea subsurface regime also show a seasonal cycle and there is some evidence that signatures of this cycle appear as deep as 1000 m. The forcing due to Ekman suction is an important mechanism for the geostrophic currents in the central and western parts of the Sea. Recent studies suggest that the eastern part is strongly influenced by the Rossby waves radiated by the Kelvin waves propagating along the west coast of India.

The simulated Indian monsoon: A GCM sensitivity study

Abstract: A series of sensitivity experiments are conducted in an attempt to understand and correct deficiencies in the simulation of the seasonal mean Indian monsoon with a global atmospheric general circulation model. The seasonal mean precipitation is less than half that observed. This poor simulation in seasonal integrations is independent of the choice of initial conditions and global sea surface temperature data used. Experiments are performed to test the sensitivity of the Indian monsoon simulation to changes in orography, vegetation, soil, wetness, and cloudiness. The authors find that the deficiency of the model precipitation simulation may be attributed to the use of an enhanced orography in the integrations. Replacement of this orography with a mean orography results in a much more realistic simulation of Indian monsoon circulation and rainfall. Experiments with a linear primitive equation model on the sphere suggest that this striking improvement is due to modulations of the orographically forced waves in the lower troposphere. This improvement in the monsoon simulation is due to the kinematic and dynamical effects of changing the topography, rather than the thermal effects, which were minimal. The magnitude of the impact on the Indian monsoon of the other sensitivity experiments varied considerably, but was consistently less than the impact of using the mean orography. However, results from the soil moisture sensitivity experiments suggest a possibly important role for soil moisture in simulating tropical precipitation, including that associated with the Indian monsoon.

Simulation of the Northern Summer Monsoon in the ECMWF Model: Sensitivity to Horizontal Resolution

Abstract: The ability of the ECMWF model (cycle 33) to simulate the Indian and East Asian summer monsoons is evaluated at four different horizontal resolutions: T21, T42, T63, and T1O6. Generally, with respect to the large-scale features of the circulation, the largest differences among the simulations occur at T42 relative to T21. However, on regional scales, important differences among the high-frequency temporal variability serve as a further critical rest of the model's ability to simulate the monsoon. T106 best captures both the spatial and temporal characteristics of the Indian and East Asian monsoons, whereas T42 fails to correctly simulate the sequence and development of synoptic-scale milestones that characterize the monsoon flow. In particular, T106 is superior at simulating the development and migration of the monsoon trough over the Bay of Bengal. In the T42 simulation, the development of the monsoon occurs one month earlier than typically observed. At this time the trough is incorrectly locate...

Last glacial maximum in China: comparison between land and sea

Abstract: As a result of the sea-level drop at the last glacial maximum (about 100–120 m in the South China Sea, and 130–150 m in the East China Sea), the size and configurations of the China Seas changed greatly. The East China Sea with the Yellow Sea and Bohai Gulf was reduced into an elongated trough, while the South China Sea became a semi-enclosed gulf. Due to the southern shift of the polar front in the North Pacific and the reorganization of the surface current system, the winter temperature in the South China Sea was 6–10°C colder, and the seasonality was much stronger than it is now. The reduced sea area, and thus the increased distance to the coast, together with the declined sea surface temperature, led to intensified aridity on the land. The enhanced eolian processes related to the intensification of the winter monsoon caused an expansion of the loess deposition area reaching the southeastern part of China to the south of the Yangtze River and off the modern sea coast. Vegetation zones greatly shifted to the southeast. In north China, deserts and steppes were much more extensive than they are now and the modern areas of warm-temperate, broad-leaved forests were occupied by steppes or woodland. In south China, the deciduous, broadleaved forests were developed in areas now covered by evergreen forests, and the monsoon rain forest completely disappeared. The changes in the climate on land and the configurations of seas altered the sediment supply and sedimentation rate in the China Seas, giving rise to the Atlantic-type late Quaternary CaCO3 curve there.

Climatic regimes of tropical convection and rainfall

Abstract: Annual distribution and phase propagation of tropical convection are delineated using harmonic and amplitude-phase characteristics analysis of climatological pentad mean outgoing longwave radiation and monthly frequencies of highly reflective cloud. An annual eastward propagation of peak rainy season along the equator from the central Indian Ocean (60°E) to Arafura Sea (130°E) is revealed. This indicates a transition from the withdrawal of the Indian summer monsoon to the onset of the Australian summer monsoon. Significant bimodal variations are found around major summer monsoon regions. These variations originate from the interference of two adjacent regimes. The convergence zones over the eastern North Pacific, the South Pacific, and the southwest Indian Ocean are identified as a marine monsoon regime that is characterized by a unimodal variation with a concentrated summer rainfall associated with the development of surface westerlies equatorward of a monsoon trough. Conversely, the central Nor...

A Warm and Wet Little Climatic Optimum and a Cold and Dry Little Ice Age in the Southern Rocky Mountains, U.S.A.

Abstract: The zenith of Anasazi Pueblo Indian occupation in the northern Colorado Plateau region of the southwestern U.S.A. coincides with the Little Climatic Optimum or Medieval Warm Period (A.D. 900-1300), and its demise coincides with the commencement of the Little Ice Age. Indexes of winter (jet-stream derived) and summer (monsoon derived) precipitation and growing season length were developed for the La Plata Mountains region of southwestern Colorado. The results show that during the height of the Little Climatic Optimum (A.D. 1000–1100) the region was characterized by a relatively long growing season and by a potential dry farming zone or elevational belt (currently located between 2,000 m and 2,300 m elevation) that was twice as wide as present and could support Anasazi upland dry farming down to at least 1,600 m, an elevation that is quite impossible to dry farm today because of insufficient soil moisture. This expanded dry-farm belt is attributable to a more vigorous circulation regime characterized by both greater winter and summer precipitation than that of today. Between A.D. 1100 and 1300 the potential dry-farm belt narrowed and finally disappeared with the onset of a period of markedly colder and drier conditions than currently exist. Finally, when the Little Ice Age terminated in the mid A.D. 1800s and warmer, wetter conditions returned to the region, another group of farmers (modern Anglos) were able to dry farm the area.

On the dynamical basis for the Asian summer monsoon rainfall-El Niño relationship

Abstract: The dynamical basis for the Asian summer monsoon rainfall-El Nino linkage is explored through diagnostic calculations with a linear steady-state multilayer primitive equation model. The contrasting monsoon circulation during recent El Nino (1987) and La Nina (1988) years is first simulated using orography and the residually diagnosed heating (from the thermodynamic equation and the uninitialized, but mass-balanced, ECMWF analysts) as forcings, and then analyzed to provide insight into the importance of various regional forcings, such as the El Nino–related heating anomalies over the tropical Indian and Pacific Oceans. The striking simulation of the June–August (1987–1988) near-surface and upper-air tropical circulation anomalies indicates that tropical anomaly dynamics during northern summer is essentially linear even at the 150-mb level. The vertical structure of the residually diagnosed heating anomaly that contributes to this striking simulation differs significantly from the specified canonic...

Glacial‐Holocene carbonate dissolution and sea surface temperatures in the south China and Sulu seas

Abstract: Sediment cores from the South China and Sulu seas have been used to study sea surface temperature changes in these two western equatorial Pacific basins during the last 25,000 years. Sea surface temperature (SST) estimates were derived using the planktonic foraminiferal transfer function FP-12E previously developed by Thompson (1981). The water depths for the cores range from 500 m to more than 4,000 m and thus provide a good opportunity to evaluate the effect of carbonate dissolution on quantitative paleotemperature estimates. The sea surface temperature time series from shallow, well-preserved cores indicate that average winter and summer temperatures during the Holocene were approximately 27°C and 29.5°C, respectively, for both the South China and Sulu seas. These estimates agree well with modern observations. During the last glacial maximum, summer sea surface temperatures were approximately 28.5°C in the South China Sea and 29°C in the Sulu Sea and thus were very similar to the Holocene. In contrast, glacial winter sea surface temperatures are estimated at approximately 21°C for the South China Sea and 24°C for the Sulu Sea. This decrease in glacial winter sea surface temperatures results in a much larger seasonality during the last glacial (5°-8°C) compared to the Holocene (2°C). These seasonal contrasts are much greater than those estimated by Climate: Long-Range Investigation, Mapping, and Prediction Members (1981) for this region of the western equatorial Pacific. Variation in intensity of the monsoon system and surface water exchange rates between these basins and the open ocean are the major factors controlling glacial-interglacial SST fluctuations in the South China and Sulu seas. One factor influencing the accuracy of the SST estimates is the quality of preservation of the planktonic foraminiferal assemblages. Our results demonstrate that depth-dependent increases in dissolution result in systematically cooler SST estimates. This is due to the fact that warm water planktonic foraminifera tend to be more solution susceptible, and as dissolution progresses, the assemblage becomes enriched in the more resistant, cooler water taxa. Since dissolution is more intense during interglacials than glacials in the Pacific, dissolution tends to reduce the amplitude of the glacial-interglacial temperature difference.

Simulation of Summer Monsoon Climate over East Asia with an NCAR Regional Climate Model

Abstract: A summertime season climate over east Asia is simulated with a regional climate model (RegCM) developed at the National Center for Atmospheric Research (NCAR) to validate the model's capability to produce the basic characteristics of monsoon climate over the region. The RegCM used here is a modified version of the NCAR-Pennsylvania State University Mesoscale Model (MM4), in which the Biosphere-Atmosphere Transfer Scheme and a detailed radiative transfer package have been implemented for climate application. The model horizontal resolution is 50 km, and the domain covers a 5200 km × 4700 km area encompassing eastern Asia and adjacent ocean regions. The simulation period is June–August 1990, and the model-driving initial and lateral boundary conditions are from European Centre for Medium-Range Weather Forecasts analyses of observations. The simulated patterns of the monsoon circulation, precipitation, and land-surface temperature are in general agreement with observations, although the model is som...

Seasonal Variability of Intrusion of the Kuroshio Water across the Continental Shelf Northeast of Taiwan

Abstract: The currents over the East China Sea continental slope northeast of Taiwan were monitored from mid-July to early December, 1992 and again from late March to early August, 1993, covering the two transition periods of monsoon seasons. In 1992, we found massive Kuroshio intrusion (onshore velocity reaching 25 cm/s or more) occurred at mid-October, a lag of one month after the wind pattern changed from southerly to northeasterly. By comparing the surface meteorological with observed cross-slope velocity, we suggest that cooling rather than direct wind forcing, i.e., mid-October cold air outbreaks associated with the Siberia highs moving across the East China Sea, may be the major cause for triggering the observed winter intrusion event. The intrusion continued until the end of the 1992 data in early December and again in late March and early April of 1993 when the data became available. In mid-April, 1993, the intrusion suddenly halted when the lows replaced the highs and the wind reversed with warm air from south. In addition, we found that short-period intrusions appeared in August and early September, 1992 and in late July, 1993. All of these summer intrusions coincide with the occurrence of typhoon passages over or further offshore of the area northeast of Taiwan.

Fluxes of material in the Arabian Sea and Bay of Bengal : sediment trap studies

Abstract: In order to investigate how monsoons influence biogeochemical fluxes in the ocean, twelve time-series sediment traps were deployed at six locations in the northern Indian Ocean In this paper we present particle flux data collected during May 1986 to November 1991 and November 1987 to November 1992 in the Arabian Sea and Bay of Bengal respectively Particle fluxes were high during both the SW and NE monsoons in the Arabian Sea as well as in the Bay of Bengal The mechanisms of particle production and transport, however, differ in both the regions In the Arabian Sea, average annual fluxes are over 50gm-2y-1 in the western Arabian Sea and less than 27gm-2 y-1 in the central part Biogenic matter is dominant at sites located near upwelling centers, and is less degraded during peak flux periods High particle fluxes in the offshore areas of the Arabian Sea are caused by injection of nutrients into the euphotic zone due to wind-induced mixed layer deepening In the Bay of Bengal, average annual fluxes are highest in the central Bay of Bengal (over 50gm-2y-1) and are least in the southern part of the Bay (37gm-2y-1) Particle flux patterns coincide with freshwater discharge patterns of the Ganges-Brahmaputra river system Opal/carbonate and organic carbon/carbonate carbon ratios increase during the SW monsoon due to variations in salinity and productivity patterns in the surface waters as a result of increased freshwater and nutrient input from rivers Comparison of S years data show that fluxes of biogenic and lithogenic particulate matter are higher in the Bay of Bengal even though the Arabian Sea is considered to be more productive Our results indicate that in the northern Indian Ocean interannual variability in organic carbon flux is directly related to the strength and intensity of the SW monsoon while its transfer from the upper layers to the deep sea is partly controlled by input of lithogenic matter from adjacent continents

Plunge pools and paleoprecipitation

Abstract: Sedimentary sequences in the form of ridges or terraces surrounding plunge pools at the base of waterfalls can provide records of past discharge and as a consequence an indirect measure of rainfall variations over many thousands of years. Waves generated by the waterfall deposit sands and pebbles as a beach at the perimeter of the plunge pool. As climatic conditions change, plunge pools expand and contract, leaving these beaches as relic sedimentary deposits. The first sedimentary sequence of this kind to be analyzed for records of Quaternary climate changes is in Kakadu National Park, Northern Territory, Australia. Temporally, this record correlates closely with other paleoclimate proxies on both the Australian and African continents as well as the Indian subcontinent. The stratigraphic data indicate that this part of northern Australia was much wetter during the early to mid-Holocene (∼10-5 ka) and also during the last glacial maximum (∼22-18 ka). These pluvial periods are attributed to a strengthening of the northwest monsoon despite a considerably lower sea level during the last glacial maximum.

Interannual Variation of the Indian Monsoon Simulated by the NCAR Community Climate Model: Effect of the Tropical Pacific SST

Abstract: Previous studies have shown diagnostically and statistically that the interannual variation of the Indian monsoon is closely correlated with the tropical Pacific sea surface temperature (SST). It seems likely that the interannual variation of the Indian monsoon results from the response of this monsoon system to the interannual variations of the Pacific SST. This hypothesis has not been substantiated in the past. In order to test it, Version 1 of the National Center for Atmospheric Research Community Climate Model (CCM) was used to perform two parallel climate simulations: a control run using the 12 calendar month climatological SST and a run using real-time Pacific SST. The SST data used in this study are derived from the Comprehensive 0cean-Atrnosphere Data set. Significant interannual variations of the Indian monsoon circulation are generated by the real-time Pacific SST experiment, but not the clmatological SST control experiment in real-time Pacific SST simulation weakened during the 1982 an...

Progress in Dendroclimatic Studies in Indonesia

Abstract: A 416-year (1514-1929) tree-ring width chronology of teak (Tectona grandis L.F.) from Cepu, Central Java, Indonesia (111°35'E,7°06'S), first published by Berlage over 60 years ago, remains one of the few high-resolution paleoclimatic archives available for the western equatorial Pacific. Here we present an update of this chronology from 1880-1991, which extends the original series by 63 years. As was found for the original Berlage record, the updated chronology is correlated positively with rainfall and inversely with sea level pressure during the dry monsoon season (around May to October) just prior to the period of growth in Java. This teak record is also related to several indices of the El Nino-Southern Oscillation (ENSO), including sea level pressure at Darwin, Australia, Wright's rainfall index for the equatorial Pacific, and the Quinn historical record of ENSO. The years 1737 and 1925,rated as strong ENSO events by Quinn, are ranked among the three lowest index years in the Berlage teak record.1967,which is not considered to bean ENSO year, has the lowest ring width index in the updated series from1880-1991,and the lowest dry season rainfall on record for Jakarta, Indonesia (since 1869). The dry monsoon drought during the extreme 1982-83 ENSO event (although devastating for much of Indonesia) was not unusually severe based on either the tree-ring or Jakarta rainfall records.

Diurnal Cloud-to-Ground Lightning Patterns in Arizona during the Southwest Monsoon

Abstract: Cloud-to-ground (CG) lightning shows great variability across Arizona from one year to the next as well as from one day to the next. Availability of moisture, location of the subtropical ridge axis, transitory troughs in both the westerlies and easterlies, and low-level moisture surges from the Gulf of California can affect thunderstorm occurrence, which, in turn, will affect lightning production. Diurnal CG lightning patterns in Arizona are also determined by daily heating cycles and topography. Six years of Bureau of Land Management CG flash data are used in this investigation. In Arizona, lightning usually starts first, on a daily basis, in the plateau region and extends in an arc from the White Mountains of eastern Arizona westward across the Mogollon Rim and then northward onto the Kaibab Plateau of northern Arizona. Flash activity moves in a more or less continuous fashion off the plateau, south and westward down the topography gradient, and enters the lower desert by early evening. At the ...