Showing papers on "Monsoon published in 1995"

Correlation between climate events in the North Atlantic and China during the last glaciation

Abstract: EPISODES of massive iceberg release (Heinrich events)1-3 into the North Atlantic Ocean during the last glaciation were associated with recurring episodes of unusually cold North Atlantic surface water (Bond cycles)4 and cold air temperatures over Greenland (Dansgaard-Oeschger events)5,6. Four of the youngest of these cold events have also been reported in climate records from sites outside the North Atlantic region7, but until now the entire suite has been identified only in North Atlantic marine sediments, Greenland ice-core records and, tentatively, in French lake sediments8. Here we examine grain-size data from Chinese loess and intercalated accretionary palaeosols of last-glacial age for evidence of similar climate signals remote from the North Atlantic region. We see grain-size maxima with ages that match those of the last six Heinrich events, which we interpret as an indication of the changing strength of the East Asian winter monsoon, which largely controls the transport and deposition of central Asian aeolian dust. Thus it seems that these Heinrich events have left their signature in the Chinese loess record. This is consistent with simulations of the glacial climate9, which imply that the climates of the North Atlantic and China were linked by the effect of westerly winds.

The Asian summer monsoon and ENSO

Abstract: The relationship between the evolution of the Asian summer monsoon and equatorial sea-surface-temperature anomalies has been studied using results from an integration with the UK Universities' Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (UGCM). the integration was performed as part of the Atmospheric Model Intercomparison Project and thus used the observed sea surface temperatures (SSTs) for the decade January 1979 to December 1988. the mean evolution of the Asian summer monsoon has been successfully simulated in terms of many aspects of the rapid transition of the large-scale circulation during the boreal spring and summer. However, the results for individual years showed considerable interannual variability, both in the strength of the monsoon and in the time of onset. A relationship has been identified between the evolution of the monsoon flow and the phase of the El Nino Southern Oscillation (ENSO). In agreement with observed results, years with warm SST anomalies in the equatorial central and east Pacific Ocean (El Nino) have a weaker monsoon circulation and a delayed onset. an opposite behaviour is noted for those years with cold Pacific SST anomalies (La Nina). Diagnostics from analyses from the National Meteorological Center and the European Centre for Medium-range Weather Forecasts, and from data on the outgoing long-wave radiation observed by the Advanced Very High Resolution Radiometer, have been used to verify the model results. A description of the mechanism by which the phase of ENSO remotely influences the dynamics of the Asian summer monsoon has been developed involving changes in the heating patterns over Indonesia and the west Pacific in the preceding spring.

Late Miocene environmental change in Nepal and the northern Indian subcontinent: stable isotopic evidence from paleosols

Abstract: Neogene sediments belonging to the Siwalik Group crop out in the Himalayan foothills along the length of southern Nepal. Carbon and oxygen isotopic analyses of Siwalik paleosols from four long Siwalik sections record major ecological changes over the past ∼11 m.y. The carbon isotopic composition of both soil carbonate and organic matter shifts dramatically starting ca. 7.0 Ma, marking the displacement of largely C3 vegetation, probably semi-deciduous forest, by C4 grasslands. By the beginning of the Pliocene, all the flood plains of major rivers in this region were dominated by monsoonal grasslands. The floral shift away from woody plants is also reflected by the decline and final disappearance of fossil leaves and the decrease in coal logs in the latest Miocene. A similar carbon isotopic shift has been documented in the paleosol and fossil tooth record of Pakistan, and in terrigenous organic matter from the Bengal Fan, showing that the floral shift was probably continentwide. The latest Miocene also witnessed an average change of ∼4‰ in the oxygen isotopic composition of soil carbonate, as observed previously in Pakistan. The reasons for this are unclear; if not diagenetic, a major environmental change is indicated, perhaps related to that driving the carbon isotopic shift. Recently described pollen and leaf fossils from the Surai Khola section show that evergreen forest was gradually displaced by semi-deciduous and dry deciduous forest between 11 and 6 Ma. The gradual nature of this floral shift, which culminated in the rapid expansion of C4 grasses starting ∼7.0 m.y. ago, is difficult to explain by a decrease in atmospheric pCO2 alone (Cerling et al., 1993) but fits well with a gradual onset of monsoonal conditions in the late Miocene in the northern Indian subcontinent. Himalayan uplift, driving both monsoonal intensification and consumption of CO2 through weathering, may be the common cause behind major late Miocene environmental change globally. However, the decline of effective moisture associated with monsoon development has probably slowed, not increased, the rate of consumption of CO2 by chemical weathering of Himalayan sediments.

The Effect of Eurasian Snow Cover on the Indian Monsoon

Abstract: The authors successfully model and simulate the observed evidence that anomalously high winter/spring Eurasian snow cover is linked to weak rainfall in the following summer Indian monsoon. It is shown that excessive snow cover in February reduces June to September precipitation over India. The excessive snow cover is associated with a weak monsoon characterized by higher sea level pressure over India, a weaker Somali jet, weaker lower tropospheric westerlies, and weaker upper tropospheric easterlies. The weak monsoon is also associated with weaker secondary circulations. The remote response to excessive Eurasian snow cover is to reduce the strength of trade winds in the eastern equatorial Pacific Ocean. Energy used in melting excessive snow reduces the surface temperature over a broad region centered around the Tibetan Plateau. Reduced surface sensible heat flux reduces the midtropospheric temperature over the Tibetan Plateau. The result is to reduce the midtropospheric meridional temperature gra...

Relation between century-scale Holocene arid intervals in tropical and temperate zones

Abstract: CLIMATE records from lake sediments in tropical Africa, Central America and west Asia show several century-scale arid intervals during the Holocene1–10. These may have been caused by temporary weakening of the monsoonal circulation associated with reduced northward heat transport by the oceans7 or by feedback processes stimulated by changes in tropical land-surface conditions10. Here we use a lake-sediment record from the montane Mediterranean zone of Morocco to address the question of whether these events were also felt in temperate continental regions. We find evidence of arid intervals of similar duration, periodicity and possibly timing to those in the tropics. But our pollen data show that the forest vegetation was not substantially affected by these events, indicating that precipitation remained adequate during the summer growing season. Thus, the depletion of the groundwater aquifer that imprinted the dry events in the lake record must have resulted from reduced winter precipitation. We suggest that the occurrence of arid events during the summer in the tropics but during the winter at temperate latitudes can be rationalized if they are both associated with cooler sea surface temperatures in the North Atlantic.

Scale interaction in the Western Pacific Monsoon

Abstract: The lower-tropospheric scale interactions occurring in the summer monsoon of the western North Pacific are reviewed and summarised in a conceptual model. Diabatic heating produces a circulation with similar characteristics to those that are observed. In the lower troposphere the advection of vorticity by the divergent wind produces a compact, and more intense response than in the upper levels. Subsequent phase dispersion westward, and group propagation eastwards, lead to a monsoon depression in convectively suppressed conditions, a westerly jet with cross-equatorial flow, and a strong confluence region to the east of the monsoon depression.

Biennial oscillation associated with the East Asian summer monsoon and tropical sea surface temperatures

Abstract: In this paper, the interannual variability of the East Asian summer monsoon (EASM) rainfall and the tropical sea surface temperature (SST) have been studied. It is found that the EASM rainfall prossesses a strong biennial signal, which is particularly pronounced over the southeast China. For the SST, the biennial oscillation is the second most significant quasi-periodic signal over the entire tropical Indian and Pacific Oceans. Results indicate that the biennial variations in the SST and EASM rainfall are closely linked. The SST pattern which is best correlated with EASM rainfall appears in the form of a double see-saw with quasi-stationary centers of action over the Indian Ocean, the Asian monsoon region and the eastern Pacific. The most pronounced SST signals are found in the equatorial eastern Pacific and Indian Ocean about two seasons preceding and following the EASM rainfall. Evidence is presented suggesting that the biennial variability of the EASM rainfall is phase-locked to a global scale biennial oscillation involving the interplay of the Asian monsoon, the Hadley and Walker circulations, and basin wide fluctuations in SST. In particular, the eastward propagation of zonal wind anomalies from the Indian Ocean to the western Pacific, which regulates the moisture fluxes from the western Pacific to the East Asian region, appears to be a key component of the biennial fluctuation associated with EASM rainfall. Results suggest that the relationship between the Asian monsoon and tropical SST is more robust in the biennial that the El Nino/Southern Oscillation (ENSO) time scale, hence raising the possibility that the biennial oscillation may be more fundamentally related to monsoon-ocean-atmosphere interaction than ENSO itself.

Late Quaternary surface circulation in the east equatorial South Atlantic: Evidence from Alkenone sea surface temperatures

Abstract: Angola Basin and Walvis Ridge records of past sea surface temperatures (SST) derived from the alkenone Uk37 index are used to reconstruct the surface circulation in the east equatorial South Atlantic for the last 200,000 years Comparison of SST estimates from surface sediments between 5° and 20°S with modern SST data suggests that the alkenone temperatures represent annual mean values of the surface mixed layer Alkenone-derived temperatures for the warm climatic maxima of the Holocene and the penultimate interglacial are 1 to 4°C higher than latest Holocene values All records show glacial to interglacial differences of about 35°C in annual mean SST, which is about 15°C greater than the difference estimated by CLIMAP (1981) for the eastern Angola Basin At the Walvis Ridge, significant SST variance is observed at all of the Earth's orbital periodicities SST records from the Angola Basin vary predominantly at 23- and 100-kyr periodicities For the precessional cycle, SST changes at the Walvis Ridge correspond to variations of boreal summer insolation over Africa and lead ice volume changes, suggesting that the east equatorial South Atlantic is sensitive to African monsoon intensity via trade-wind zonality Angola Basin SST records lag those from the Walvis Ridge and the equatorial Atlantic by about 3 kyr The comparison of Angola Basin and Walvis Ridge SST records implies that the Angola-Benguela Front (ABF) (currently at about 14–16°S) has remained fairly stationary between 12° and 20°S (the limits of our cores) during the last two glacial-interglacial cycles The temperature contrast associated with the ABF exhibits a periodic 23-kyr variability which is coherent with changes in boreal summer insolation over Africa These observations suggest that surface waters north of the present ABF have not directly responded to monsoon-modulated changes in the trade-wind vector, that the central field of zonally directed trades in the southern hemisphere was not shifted or extended northward by several degrees of latitude during glacials, and that a cyclonic gyre circulation has existed in the east equatorial South Atlantic over the last 200,000 years This scenario contradicts former assumptions of glacial intensification of the Benguela Current into the eastern Angola Basin and increased coastal upwelling off Angola

A Model of the Asian Summer Monsoon.Part II: Cross-Equatorial Flow and PV Behavior

Abstract: The model developed and tested in Part I of this paper is used to investigate the mechanisms that sustain the low-level East African jet. The East African Highlands and a land/sea contrast in surface friction are shown to be essential for the existence and concentration of cross-equatorial flow. The question of inertial instability as air crosses the equator is addressed from a potential vorticity (PV) perspective. Surface friction and local diabatic heating provide mechanisms for material modification of PV and both are important for the maintenance of the jet. A compensation phenomenon that stabilizes the flow is suggested whereby reduced material tendencies of PV over the Indian Ocean are compensated for by increased modification over Africa. It is speculated that this stabilization may be a key ingredient in the monsoon circulation itself. However, in some cases there is intensification and increased variability of the wind speed maximum off the Somali coast, which may be an indication of red...

Multidecadal Variability in the Climate System over the Indian Ocean Region during the Austral Summer.

Abstract: Several independent historical studies of global atmospheric and oceanic parameters have identified low-frequency fluctuations in the global climate system. Much of this research has focused on Europe, the Atlantic Ocean, and North America. However, recent interest has begun to encompass decadal to multidecadal variability across the Indo-Pacific region. Such variability has been detected in sea surface temperature (SST), mean sea level pressure (MSLP), and surface wind fields over both the landmasses and the oceans. Around the Indian Ocean basin, the broad periods before and after the 1940s show important differences in features such as Indian southwest monsoonal rainfall and circulation patterns, relationships between austral summer rainfall in southern Africa and the El Ni˜o–Southern Oscillation phenomenon, and Australasian MSLP. Very little is known about this variability, particularly during the austral summer. In an effort to isolate such fluctuations and work toward understanding the physi...

Abrupt Seasonal Change of Large-Scale Convective Activity over the Western Pacific in the Northern Summer

Abstract: Seasonal variations of large-scale convective activity and wind over the western Pacific are examined using Geostationary Meteorological Satellite infrared equivalent blackbody temperature (T BB ) and European Center for Medium range Weather Forecast (ECMWF) global analyses over a 10-year period from 1980 to 1989. In particular, this study describes an abrupt northward shift of large-scale convective activity over the western Pacific around 20°N, 150°E in late July. The enhanced convective activity is coincident with strong cyclonic circulation there which induces westerlies to the south of the cyclone and easterlies to the north of it. It is emphasized that this strong cyclonic circulation appears suddenly over the subtropical western Pacific region. Monsoon westerlies to the west of 110°E are not similary accelerated at the same time, indicating that this abrupt change is independent of the Asian monsoon system. To the north, an anticyclonic circulation is generated, which corresponds to the withdrawal of the Baiu season over Japan. Furthermore, this abrupt northward shift of large-scale convective activity is shown to be associated with tropical cyclone activity. In the mid latitudes, geopotential height pattern between pre- and post-northward shifts of the large-scale convective activity in late July exhibit equivalent barotropic vertical structure, suggesting the Rossbywave propagation emanating northeastward from the enhanced convective region around 20°N, 140°E (western Pacific) to as far north as 60°N, 180° (Bering Sea). Another feature is that the seasonal increase of sea surface temperature (SST) over the key area (20°N, 150°E) precedes abrupt convective enhancement by about 20 days, exceeding 29°C in early July. It is inferred that the northeastward extension of the warm SST tongue is intimately associated with the enhanced convection in late July. This result suggests that SST warming is not a sufficient condition but certainly one important ingredient for the abrupt northward shift of convections.

An Observational Study of the South China Sea Monsoon during the 1979 Summer: Onset and Life Cycle

Abstract: The onset and life cycle of the 1979 South China Sea summer monsoon were examined in the context of the 30–60- and 12–24-day monsoon modes. The former intraseasonal mode formed the northward-migrating monsoon trough/ridge, while the latter intraseasonal mode propagated westward in the South China Sea. The monsoon in this region exhibited three cycles over the summer (May–August), with the onset taking place about one cycle ahead of the onset of the Indian and Japanese monsoons. Climatologically, a summer trough line radiated out from the Indian monsoon trough region, across Indochina, to the northern South China Sea. The monsoon onset occurred when the 30–60-day monsoon trough and the 12–24-day low center arrived simultaneously at the northern South China Sea, close to the climatological summer trough line, in the middle of May. The breaks occurred when the 30–60-day monsoon ridge lines and the 12–24-day high center met near 15°–20°N in the northern South China Sea. The South China Sea monsoon wa...

Model Climatology of the Mexican Monsoon

Abstract: The Mexican monsoon is a significant feature in the climate of the southwestern United States and Mexico during the summer months. Rainfall in northwestern Mexico during the months of July through September accounts for 60% to 80% of the total annual rainfall, while rainfall in Arizona for these same months accounts for over 40% of the total annual rainfall. Deep convection during the monsoon season produces frequent damaging surface winds, flash flooding, and hail and is a difficult forecast problem. Past numerical simulations frequently have been unable to reproduce the widespread, heavy rains over Mexico and the southwestern United States associated with the monsoon. The Pennsylvania State University/National Center for Atmospheric Research mesoscale model is used to simulate 32 successive 24-h periods during the monsoon season. Mean fields produced by the model simulations are compared against observations to validate the ability of the model to reproduce many of the observed features, includ...

Evidence of meridional motion in the summer lower stratosphere adjacent to monsoon regions

Abstract: Twenty-one years of rawinsonde data were used together with 8 years of uninitialized European Centre for Medium-Range Weather Forecasts (ECMWF) analyses to describe the climatological structure of large-scale circulations adjacent to monsoon regions in northern and southern hemisphere summers. In the upper troposphere and lower stratosphere, equatorward and poleward motions occur on the eastern and western sides of monsoon regions, respectively. It is shown that significant meridional velocities (>1 ms−1) penetrate the lower stratosphere up to a maximum height of 50–30 mbar. Largest meridional velocities are observed in connection with the Asian monsoon in northern summer. Although evanescent in height, these motions are relatively important for horizontal transport of constituents in the summer lower stratosphere, when planetary waves are otherwise small. Asian and Mexican monsoons in this season are displaced sufficiently far from the equator, in close proximity to the tropopause break, to have a significant role in stratosphere-troposphere (S/T) exchange. The companion paper by Chen (1995) provides evidence of irreversible S/T exchange in the “upper middle world” during northern summer.

Monsoonal Interactions Leading to Sudden Tropical Cyclone Track Changes

Abstract: Sudden poleward track changes of tropical cyclones embedded in monsoon gyres in the western North Pacific are documented. During these track changes, which are generally not well forecast, the cyclones are often accompanied by a separate comma-shaped area of gale-force winds and deep convection along the eastern periphery. This monsoon surge is distinct from the tropical cyclone. Synoptic analyses often reveal a building anticyclone to the east or southeast of the monsoon gyre. The hypothesis that the sudden track change is initiated by a binary interaction of the tropical cyclone and monsoon gyre is tested with a nondivergent barotropic model. Tropical cyclone-scale vortices with initial positions within the eastern semicircle of a larger monsoon gyre-scale vortex initially coalesce with the monsoon gyre and then exhibit sudden poleward track changes that are similar to the observations. During the coalescence phase, the large and relatively weak monsoon gyre undergoes a β-induced dispersion in ...

Tropical Circulations Associated with Southwest Monsoon Onset and Westerly Surges over the South China Sea

Abstract: The earliest onset of the Asian summer monsoon occurs in early to middle May over the South China Sea. This onset is signified by the development of low-level westerlies and leads to heavy convective rainfall over southern China (pre-Mei-Yu). In June, low-level westerly surges over the northern South China Sea are associated with the Mei-Yu rainfall system in the Yangtze region and southern Japan. In this work, the ECMWF data for 1981–86 are used to study the tropical circulations associated with the development of low-level westerlies during both events. Composites of horizontal wind, geopotential height, moisture, and vertical velocity during six May onsets and nine June surges, respectively, indicate that both events occur with the approach of a midlatitude trough–front system. The possible triggering of the South China Sea summer monsoon onset by the midlatitude system may explain why the South China Sea onset occurs prior to other regions of the Asian monsoon. During boreal spring, this is t...

June-september rainfall in north-eastern Africa and atmospheric signals over the tropics: A zonal perspective

Abstract: The connection between rainfall in northeast Africa (Djibouti, Eritrea, Ethiopia, Kenya, Somalia, Sudan, and Uganda) and various atmospheric indicators from the tropics have been investigated for the northern summer season. Variables used include zonal wind at 700 hPa and 200 hPa, sea-level pressure, and rainfall over other tropical areas, for the period 1951–1988. Strong significant correlations are shown with the Southern Oscillation and the components of the Walker cell in the Pacific Ocean. Droughts in the Ethiopia-Uganda area are associated with El Nino events and, moreover, with above normal pressure and droughts over India. Other significant relationships exist with the African monsoon. Westerly (easterly) anomalies in the lower (upper) levels are associated with abundant summer rainfall years in north-east Africa. These results are all consistent with the previously defined Walker-type circulations over the tropics, particularly as far as the Indo-Pacific and African cells are concerned. Although the Southern Oscillation has a dominant impact in this respect, the specific coherence of the African cell is further evidenced. Preliminary findings suggest that the summer rainfall areas of East Africa could qualify as a location for the rising branch of this cell.

Interannual Variations of Summer Precipitation in the Arid/semi-arid Regions in China and Mongolia : Their Regionality and Relation to the Asian Summer Monsoon

Abstract: In this study, interannual variations of summertime precipitation over arid and semi-arid regions in China and Mongolia are investigated. To clarify the regionality of the interannual variability in summer precipitation, an analysis technique of rotated empirical orthogonal functions is applied for a recent 40-year (1951-1990) period of summer precipitation. As a result of the REOF, five regions have been determined : I) Taklimakan Desert, II) Loess Plateau, III) North China to central and the southeastern part of Mongolia, IV) the north of Tianshan Mountains, and V) the northern part of Mongolia. Summertime precipitation over Region III) shows a significant decreasing trend after 1955. Next, to examine how the variations in precipitation in these regions are influenced by the Asian (Indian) summer monsoon activity in the mid-latitudes, correlations with all-India monthly and seasonal rainfall (IMR) are investigated. Further, the change of atmospheric circulation patterns with the interannual variation of summer precipitation of Regions I and II are also examined. The results are summarized as follows : 1) The interannual variation of summer precipitation of Region I (Taklimakan Desert) is mainly related to the windward mid-latitude circulation and eastward (westward) shift of the Tibetan High in a wet (dry) year. This region shows a clear negative correlation with IMR in June and July, and the relationships are caused by a rather local circulation change with IMR variation over Central Asia. 2) In Region II (Loess Plateau, the middle reaches of Yellow River), interannual variation of summer precipitation shows a positive correlation with IMR through the summer monsoon season. It shows a clear 2-3 year periodic oscillation, and seems to be closely related to the atmosphere/ocean interaction in the equatorial Pacific.

Water circulation in the Gulf of Papua

Abstract: The Gulf of Papua has the shape of a half-moon of radius of about 200 km and mean depth 1 m s−1. The dominant M2 tide propagates from the Coral Sea through the Gulf to enter both Torres Strait and the large estuaries of Papua New Guinea. The low-frequency currents have, in costal waters, little vertical shear associated with the salinity stratification, but, at the shelf break, a strong vertical shear in the well-mixed layer typically 100 m thick. A dominant forcing of the circulation in the Gulf is the eastward-flowing Coral Sea Coastal Current in the Northwest Coral Sea. This current appears to generate a counter-clockwise rotating eddy in the Gulf. The wind fluctuations result in the brackish water leaving the Gulf alternatively at its western and eastern sides. The residence time of river runoff in the Gulf, estimated using a three-dimensional hydrodynamic model, is about 2 months and this estimate agrees with that from freshwater budget estimates. Brackish water intrudes in the Torres Strait where tidal mixing maintains vertical homogeneity. The tidal mixing front is located near the northern tip of the Warrior Reefs and the intrusion is strongest in the monsoon season.

Climatic response of the Indian subcontinent to doubled CO2 concentrations

Abstract: Results from the United Kingdom Meteorological Office (UKMO) coupled climate model have been analysed over the Indian subcontinent in order to validate the model's performance and to assess the changes in climate and its variability in a simulation with a 1 per cent increase per year in CO2 (compound). The model produces a reasonable simulation of present-day climate over the Indian subcontinent. At the time of CO2 doubling, the model simulates temperature increases of the order of 1 K to 4 K over the Indian subcontinent during winter and monsoon seasons. The model-simulated monsoon circulation shifts by 10° latitude towards the north and intensifies by approximately 10 per cent in the warmer atmosphere. The interannual variability of monsoon onset dates and intraseasonal variability of monsoon rainfall are not significantly different when the CO2 concentration doubles. However, the model simulates increased interannual variability of the monsoon rainfall and a greater number of heavy rainfall days during the monsoon.

Multiyear present‐day and 2 × CO2 simulations of monsoon climate over eastern Asia and Japan with a regional climate model nested in a general circulation model

Abstract: Two continuous 5-year-long simulations over eastern Asia and the Japan islands, one for present-day climate (control) and one for climate under doubled carbon dioxide concentration (2×CO2) are completed with a regional climate model (RegCM) nested, in a one-way mode, within a general circulation model (GCM). The GCM is run at R15 resolution (4.5×7.5° latitude × longitude), and the RegCM is run at 50-km grid point spacing. In the control run, both the GCM and the RegCM reproduce the seasonal migration of the westerly jet but produce too strong a monsoonal circulation, which results in a significant overestimate of summer precipitation over the eastern Asian continent. The temporal evolution of the eastern Asia summer monsoon, with steady phases separated by more rapid transitions, is reproduced, but the monsoon rain belt reaches too far north, and the occurrence of tropical storms is underestimated. Regionally averaged surface air temperatures are mostly within 1–3°C of observations. Seasonal precipitation amounts over Japan are within 10–35% of observed ones. The narrow Korea and Japan land masses, which are not captured by the GCM grid, substantially affect the simulated surface precipitation climatology in the RegCM. Under 2×CO2 forcing, warming in the range of 4–11°C is simulated, greater in winter than in summer and increasing toward high latitudes. The strength of the monsoonal circulation increases in 2×CO2 conditions, leading to a general increase in precipitation over all regions by 10–30%. The simulated precipitation change shows significant regional and, within Japan, subregional structure. The Japan and Korea land masses substantially affect the summer precipitation changes, indicating the need to capture them in numerical simulations of climate change. Because of the uncertainties in the control simulations, our 2×CO2 results are intended not to provide climate change scenarios for impact assessments, but only to illustrate the model sensitivity to different forcings. Plans are under way to further test and improve the RegCM performance over the region and to use more recent GCM simulations to drive the RegCM.

Study of seasonal transport variations in the Indonesian seas

Abstract: Seasonal transport variations between the Pacific and Indian Oceans via the Indonesian seas were studied by the Euler-Lagrangian method. The velocity field was calculated with a fairly high resolution robust diagnostic model. The model well reproduces the features of seasonal variations in the Indonesian seas. The total volume transport of the Indonesian throughflow is 20±3 Sv (1 Sv = 106 m3 s−1), the maximum being from boreal spring to boreal summer and the minimum in boreal winter. The values are similar to those of previous general circulation models with a wide Indonesian passage despite resolution of the presence of the many small islands in the Indonesian seas. Although a large portion of the net transport is contained in the upper layer, deep transport below 1000-m depth is about 5 Sv. This value corresponds to approximately 25% of the total transport, which means that disregard of the deep transport leads to underestimation of the volume transport of the throughflow. Tracking of numerous labeled particles in the calculated velocity field clarified the sources and pathways of the Indonesian throughflow. The major route is a western one through both the Makassar and Lombok Straits. Most of the North Pacific water supplied from the Mindanao Current passes along this route, entering the Indian Ocean within several months with almost no loss of its properties (intense vertical mixing around the Lombok sill reported by observations could not be reproduced in our model). In contrast, South Pacific water takes the eastern route into the eastern Indonesian seas and subsequently mixes with waters from the North Pacific and Indian Oceans in the Banda Sea, which means that it has a long travel time (at least a few years). Water taking the eastern route therefore loses its original properties before arriving in the Indian Ocean. The transport processes also are significantly affected by seasonal variations in equatorial circulation in the western Pacific. In the surface layer, North Pacific water is vigorously supplied to the western route only from boreal spring to summer in association with the linkage between the current flowing through the Makassar Strait and the Mindanao Current. In other seasons, because the Mindanao Current is strongly linked with the North Equatorial Countercurrent and the New Guinea Coastal Current primarily by northeasterly monsoonal winds, its upper water flows back to the Pacific Ocean. In the subsurface layer, a pronounced inflow of Mindanao Current water into the western route occurs from boreal winter to spring, when the subsurface link between that current and the Equatorial Undercurrent tends to weaken. In the deep, the quasi-steady transport of Pacific water into the Indian Ocean via the eastern route is fed by the westward deep current in the equatorial Pacific.

Variations of eastern Asian monsoon over the last 140,000 years

Abstract: The Weinan loess-soil sequence in central China provides lithogenic, geochemical and palaeopedological evidence of past changes in the eastern Asian monsoon climate. The chemical weathering index as defined by Nesbitt and Young (1982) and the SiOJTiOz ratio are found to be sensitive indicators of the strengths of the summer and winter monsoons, respectively. Over the last climatic cycle, strong summer monsoon occurred during six time intervals with an apparent - 20,000 years cycle, the major component of the Earth 's orbital precession. Strengthening of the summer monsoon and weakening of the winter monsoon is approximately in phase. Micromorphological study of the palaeosols reveals a landscape variability from north temperate steppe (Haplic Chernozems) to subtropical forest (Chromic Luvisol) environments for the interstadial and interglacial periods. Changes in both orbitally produced north summer insolation and glacial age boundary condition are necessary to explain the major shifts of the monsoon climate. Within the constraints of the used time scale, the variations of the monsoon climate are basically coeval with the variations of the global ice volume as indicated by the marine 6180 record. Moreover, the amplitudes of monsoon variability and the landscape evolution display _ however striking discrepancies with the global ice volume, which may be better explained by the variations of the summer insolation in the northern I hemisphere with an apparent time lag of several thousands years for the loess sequence.

All-India Summer Monsoon Rainfall and Sea Surface Temperatures around Northern Australia and Indonesia

Abstract: The relationship between Indian summer (June–September) monsoon rainfall and sea surface temperatures around northern Australia–Indonesia has been explored using data from 1949 to 1991. Warm sea surface temperatures are generally associated with a good monsoon; a poor monsoon is usually accompanied and preceded by low sea surface temperatures. This finding confirms, on independent data, a suggestion made a decade ago. This study also confirms a relationship between changes in Darwin pressure and Indian monsoon rainfall. Thew two relationships appear to provide a method for predicting Indian summer monsoon rainfall a month or two before the onset of the monsoon season. Two predictors (April sea surface temperatures and the change in Darwin pressure from January to April) together account for about 50% of the variance in Indian monsoon rainfall if the data are adjusted to remove possible artificial trends in the ocean temperatures. The northern Australia–Indonesia region is clearly an important com...

Wind relaxation as a possible cause of the South China Sea Warm Current

Abstract: Winter appearance of a northeastward warm current off the southern coast of China against gale force winds is well documented but lacks a plausible explanation. Relaxation of northeasterly winds is envisaged here as a possible cause of the South China Sea Warm Current in winter. A three-dimensional circulation model for the South China Sea is first driven to equilibrium by climatological forcings. Thereafter, wind forcing is relaxed from the 15th day of each month for 9 days. In winterlike months from December to April, the wind relaxation invariably triggers a northeastward current of which the location and alongshore span are comparable to that of the observed warm current. This current is driven by the pressure gradient along the northwestern boundary of the South China Sea, sea level being high to the southwest and low to the northeast. The sea level gradient is built up by the monsoon-driven southwestward coastal current along the northwestern boundary and, after wind relaxes, triggers a return current and a sea level drop that expand southwestward from the southern coast of China to the east coast of Vietnam. The current is initially barotropic, becoming increasingly baroclinic in time as warm waters from the south are advected northeastward. The model also suggests that the sea level gradient is present in most of the months of the year, but is not as dramatic as in winter to trigger fundamental changes in the circulation of the South China Sea.

Relationships between regional indian summer monsoon rainfall and eurasian snow cover

Abstract: In this work, correlation analysis is applied to study the interannual relationships between Indian summer mon-soon rainfall of different homogeneous regions and Eurasian Snow Cover (ESC) during winter and spring seasons for the time period from 1973 to 1992. The monsoon rainfall of the western and central regions of India, as well as the all-India monsoon rainfall, is significantly negatively correlated with the ESC averaged for the months December-March and with, especially, the ESC of February. This study may provide some useful information for the long-range prediction of the regional Indian monsoon rainfall.

A 2,200 years periodicity in the Asian Monsoon System

Abstract: We have carried out a high-resolution study of fluctuations in upwelling intensity along the Oman Margin, western Arabian Sea, through the last 19,130 years based on several marine microfossil indices of upwelling. We document a periodicity of 2,200 years in the upwelling indices, which would be governed by the south-west (SW) monsoon with such a periodicity. Our data further demonstrate greater amplitude in the variability of the SW monsoon during the Holocene than during the last glacial period. Our reported 2,200 years periodicity has two implications (1) oceanic circulation changes partly influence monsoon strength at sub-Milankovitch cycles (2) previously documented 2,300 years periodicity in atmospheric 14C might be induced by oceanic circulation changes.