Showing papers on "Monsoon published in 2002"

Onset of Asian desertification by 22 Myr ago inferred from loess deposits in China

Abstract: The initial desertification in the Asian interior is thought to be one of the most prominent climate changes in the Northern Hemisphere during the Cenozoic era1,2,3,4. But the dating of this transition is uncertain, partly because desert sediments are usually scattered, discontinuous and difficult to date. Here we report nearly continuous aeolian deposits covering the interval from 22 to 6.2 million years ago, on the basis of palaeomagnetic measurements and fossil evidence. A total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils. This new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of Asia by the early Miocene epoch, at least 14 million years earlier than previously thought3,5. Regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in Asia.

Rainy Season of the Asian-Pacific Summer Monsoon(.

Abstract: To date, the monsoon-research community has not yet reached a consensus on a unified definition of monsoon rainy season or on the linkage between the onsets over the Asian continent and the adjacent oceans. A single rainfall parameter is proposed, and a suite of universal criteria for defining the domain, onset, peak, and withdrawal of the rainy season are developed. These results reveal a cohesive spatial–temporal structure of the Asian–Pacific monsoon rainy season characteristics, which will facilitate validation of monsoon hydrological cycles simulated by climate system models and improve our understanding of monsoon dynamics. The large-scale onset of the Asian monsoon rainy season consists of two phases. The first phase begins with the rainfall surges over the South China Sea (SCS) in mid-May, which establishes a planetary-scale monsoon rainband extending from the south Asian marginal seas (the Arabian Sea, the Bay of Bengal, and the SCS) to the subtropical western North Pacific (WNP). The ra...

Global air pollution crossroads over the Mediterranean

Abstract: The Mediterranean Intensive Oxidant Study, performed in the summer of 2001, uncovered air pollution layers from the surface to an altitude of 15 kilometers. In the boundary layer, air pollution standards are exceeded throughout the region, caused by West and East European pollution from the north. Aerosol particles also reduce solar radiation penetration to the surface, which can suppress precipitation. In the middle troposphere, Asian and to a lesser extent North American pollution is transported from the west. Additional Asian pollution from the east, transported from the monsoon in the upper troposphere, crosses the Mediterranean tropopause, which pollutes the lower stratosphere at middle latitudes.

Marine Reservoir Corrections for the Indian Ocean and Southeast Asia

Abstract: We have measured radiocarbon in prebomb known-age shells and coral from the Indian Ocean and southeast Asia to determine marine reservoir age corrections. Western Indian Ocean results show a strong 14C depletion due to upwelling in the Arabian Sea, and indicate that this signal is advected over a wide area to the east and south. In contrast, the surface waters of the South China Sea contain relatively high levels of 14C, due in part to the input of well-equilibrated water masses from the western Pacific. The easternmost regions of the Indian Ocean are also strongly influenced by the flowthrough of Pacific waters north of Australia.

The climate of the US Southwest

Abstract: This paper summarizes the current state of knowledge of the climate of southwest USA (the 'Southwest'). Low annual precipitation, clear skies, and year-round warm climate over much of the South- west are due in large part to a quasi-permanent subtropical high-pressure ridge over the region. However, the Southwest is located between the mid-latitude and subtropical atmospheric circulation regimes, and this positioning relative to shifts in these regimes is the fundamental reason for the region's climatic vari- ability. Furthermore, the Southwest's complex topography and its geographical proximity to the Pacific Ocean, the Gulf of California, and the Gulf of Mexico also contribute to this region's high climatic vari- ability. El Nino, which is an increase in sea-surface temperature of the eastern equatorial Pacific Ocean with an associated shift of the active center of atmospheric convection from the western to the central equa- torial Pacific, has a well-developed teleconnection with the Southwest, usually resulting in wet winters. La Nina, the opposite oceanic case of El Nino usually results in dry winters for the Southwest. Another im- portant oceanic influence on winter climate of the Southwest is a feature called the Pacific Decadal Oscillation (PDO), which has been defined as temporal variation in sea-surface temperatures for most of the Northern Pacific Ocean. The effects of ENSO and PDO can amplify each other, resulting in increased annual variability in precipitation over the Southwest. The major feature that sets the climate of the South- west apart from the rest of the United States is the North American monsoon, which in the US is most noticeable in Arizona and New Mexico. Up to 50% of the annual rainfall of Arizona and New Mexico oc- curs as monsoonal storms from July through September. Instrumental measurement of temperature and precipitation in the Southwest dates back to the middle to late 1800s. From that record, average annual rainfall of Arizona is 322 mm (12.7''), while that of New Mexico is 340 mm (13.4''), and mean annual tem- perature of New Mexico is cooler (12°C (53°F)) than Arizona (17°C (62°F)). As instrumental meteorolog- ical records extend back only about 100 to 120 yr throughout the Southwest, they are of limited utility for studying climate phenomena of long time frames. Hence, there is a need to extend the measured meteo- rological record further back in time using so-called 'natural archive' paleoclimate records. Tree-ring data, which provide annual resolution, range throughout the Southwest, extend back in time for up to 1000 yr or more in various forests of the Southwest, and integrate well the influences of both temperature and pre- cipitation, are useful for this assessment of climate of the Southwest. Tree growth of mid-elevation forests typically responds to moisture availability during the growing season, and a commonly used climate vari- able in paleo-precipitation studies is the Palmer Drought Severity Index (PDSI), which is a single variable derived from variation in precipitation and temperature. June-August PDSI strongly represents precip- itation and, to a lesser extent, temperature of the year prior to the growing season (prior September through current August). The maximum intra-ring density of higher elevation trees can yield a useful record of summer temperature variation. The combined paleo-modern climate record has at least 3 occurrences of multi-decadal variation (50 to 80 yr) of alternating dry (below average PDSI) to wet (above average PDSI). The amplitude of this variation has increased since the 1700s. The most obvious feature of the temperature record is its current increase to an extent unprecedented in the last 400 yr. Because this warming trend is outside the variation of the natural archives, it is possible that anthropogenic impacts, such as increased atmospheric concentrations of greenhouse trace gases, are playing a role in climate of the Southwest. Accordingly, this pattern merits further research in search of its cause or combination of causes.

Increase in the Asian southwest monsoon during the past four centuries

Abstract: Climate reconstructions reveal unprecedented warming in the past century; however, little is known about trends in aspects such as the monsoon. We reconstructed the monsoon winds for the past 1000 years using fossil Globigerina bulloides abundance in box cores from the Arabian Sea and found that monsoon wind strength increased during the past four centuries as the Northern Hemisphere warmed. We infer that the observed link between Eurasian snow cover and the southwest monsoon persists on a centennial scale. Alternatively, the forcing implicated in the warming trend (volcanic aerosols, solar output, and greenhouse gases) may directly affect the monsoon. Either interpretation is consistent with the hypothesis that the southwest monsoon strength will increase during the coming century as greenhouse gas concentrations continue to rise and northern latitudes continue to warm.

A unified monsoon index

Abstract: [1] There are several monsoon regions in the world. Some monsoon indices have been proposed to describe their variability, but a unified monsoon index suitable for all known monsoon regions has not yet been found. Here we present a unified dynamical index of monsoon, the dynamical normalized seasonality (DNS), and carry out an analysis of observation data over the past 40 years. The analysis shows that the DNS index can characterize the seasonal cycle and interannual variability of monsoons over different areas very well. The South Asia summer monsoon (SASM) sector (5°–22.5°N, 35°–97.5°E) is composed of two independent components, SASM1 (2.5°–20°N, 35°–70°E) and SASM2 (2.5°–20°N, 70°–110°E), with quite different relations with the monsoon rainfall over the South Asia. The African summer monsoon (ASM) is dominated by variability on the decadal time-scale, and its decadal abrupt decrease in 1967 may be an important cause of the persistent drought over the Sahel region. It is also found that there is a remarkable global correlation pattern between the South China Sea summer monsoon index (SCSSMI) and global precipitation during boreal summer.

Monsoon-forced chlorophyll distribution and primary production in the South China Sea: observations and a numerical study

Abstract: Although the South China Sea (SCS) exchanges water constantly with the western Philippine Sea, its nutricline is much shallower and its chlorophyll level in surface waters is twice as high. Analysis of CZCS-SeaWiFS data and shipboard data reveals a strong seasonality of chlorophyll in the SCS in three upwelling regions. A three-dimensional numerical model with coupled physics and biogeochemistry is developed to study the effect of monsoonal forcing on nutrient upwelling and phytoplankton growth in the SCS. The model has a horizontal resolution of 0.41 in the domain 2–24.81N and 99–124.61E and 21 layers in the vertical. The circulation is driven by monthly climatological winds. The nitrogen-based ecosystem model has four compartments: dissolved inorganic nitrogen (DIN), phytoplankton, zooplankton and detritus. The chlorophyll-to-phytoplankton ratio depends on light and DIN availability. The biological equations and parameters are taken from previous modeling studies of the Sargasso Sea. The model simulates the nitrate profile, the strong subsurface chlorophyll maximum, and the primary production in the central basin with reasonable success. It also generates intense chlorophyll patches in the monsoon-driven upwelling regions northwest of Luzon and north of the Sunda Shelf in winter and off the east coast of Vietnam in summer. The results are in reasonable agreement with shipboard observations and CZCS-SeaWiFS data. The primary production derived from SeaWiFS data shows a strong peak in winter and weak peak in summer with an annual mean of 354 mg C m � 2 d � 1 for the whole basin. The modeled primary production displays seasonal variation resembling the trend derived from SeaWiFS data, but the magnitude (280 mg C m � 2 d � 1 ) is smaller by 20%. The model also predicts an export fraction of 12% from the primary production in the euphotic zone. r 2002 Elsevier Science Ltd. All rights reserved.

Why is the Bay of Bengal less productive during summer monsoon compared to the Arabian Sea

Abstract: [1] The Bay of Bengal is traditionally considered to be a less productive basin compared to the Arabian Sea We explore the reasons for this in the central Bay during summer Copious rainfall and river water freshen the upper layers of the Bay by 3–7 psu during summer, and SST was warmer by 15–2°C than in the central Arabian Sea This leads to a strongly stratified surface layer The weaker winds over the Bay are unable to erode the strongly stratified surface layer, thereby restricting the turbulent wind-driven vertical mixing to a shallow depth of <20 m This inhibits introduction of nutrients from below, situated close to the mixed layer bottom, into the upper layers While advection of nutrients rich water into the euphotic zone makes the Arabian Sea highly productive, this process is unlikely in the Bay of Bengal

Intercomparison of the climatological variations of Asian summer monsoon precipitation simulated by 10 GCMs

Abstract: We assesses the overall performance of state-of-the-art atmospheric GCMs in simulating the climatological variations of summer monsoon rainfall over the Asian-Western Pacific region and the systematic errors that are common to a group of GCMs. The GCM data utilized are obtained from 10 GCM groups participated in the CLIVAR/Monsoon GCM Intercomparison Project. The model composite shows that the overall spatial pattern of summer monsoon rainfall is similar to the observed, although the western Pacific rainfall is relatively weak. For the simulated precipitation over the western Pacific, the models can be classified into two categories. The first category of models simulates the precipitation more confined to the equatorial region and weaker precipitation in the subtropical western Pacific compared to the observed. The second category of models simulates large precipitation in the subtropical western Pacific but the region is shifted to the north by 5–10°. None of the models realistically reproduce the observed Mei-yu rain band in the region from the East China Sea to the mid Pacific. Most of the models produce a rain band along the continental side of East Asia. The climatological variations of simulated summer rainfall are examined in terms of their amplitude and their principal EOF modes. All models simulate larger amplitudes of the climatological seasonal variation of Indian summer monsoon than the observed, though most models simulate smaller amplitudes in the western Pacific. The ten model composite produces four leading EOF modes over the Asian-western Pacific region, which are remarkably similar to the observed counterparts. The first and second eigenmodes, respectively, represent the smoothed seasonal march of broad-scale monsoon and the onsets of the Indian and East Asian summer monsoon. The third and fourth modes relate to the climatological intraseasonal oscillation (CISO). In contrast to the model composite, several models fail to reproduce the first principal mode, and most models do not reproduce the observed modes higher than the second. The CISO of precipitation is also examined over the Indian monsoon and the East Asia-western Pacific monsoon regions separately.

Pacific–East Asian Teleconnection. Part II: How the Philippine Sea Anomalous Anticyclone is Established during El Niño Development*

Abstract: The anomalous Philippine Sea anticyclone (PSAC) conveys impacts of El Nino to east Asian climate during the mature and decay of an El Nino (from the winter to ensuing summer). It is shown that the anomalous PSAC forms in fall about one season prior to the peak El Nino; its strength increases with the El Nino intensity and its sign reverses during a La Nina. The PSAC formation concurs with abnormal deepening of the east Asian trough and with increasing number of northward recurvature of tropical storms in the western Pacific. The PSAC establishment is abrupt, coupling with a swing from a wet to dry phase of an intraseasonal oscillation (ISO) and often concurrent with early retreat of the east Asian summer monsoon. The ISO becomes inactive after PSAC establishment. The development of the PSAC is attributed to combined effects of the remote El Nino forcing, tropical- extratropical interaction, and monsoon-ocean interaction. The developing El Nino induces off-equatorial as- cending Rossby wave responses and land surface cooling in northeast Asia; both deepen the east Asian trough in fall and induces vigorous tropical-extratropical exchange of air mass and heat, which enhances the cold air outbreak and initiation of the PSAC. Through exciting descending Rossby waves, the El Nino-induced Indonesian subsidence generates low-level anticyclonic vorticity over south Asia, which is advected by mean monsoon westerly, instigating the anomalous PSAC. The ISO interacting with the underlying ocean plays a critical role in the abrupt establishment of PSAC. The wind-evaporation/entrainment feedback tends to amplify (suppress) ISO before (after) winter northeasterly monsoon commences, suggesting the roles of atmosphere-ocean inter- action and the seasonal march of background winds in changing the Philippine Sea ISO intensity and maintaining PSAC.

The Boreal Summer Intraseasonal Oscillation: Relationship between Northward and Eastward Movement of Convection

Abstract: The summertime intraseasonal oscillation (ISO) is an important component of the south Asian monsoon. Lagged regressions of intraseasonally filtered (25‐80 days) outgoing longwave radiation (OLR) reveal that centers of convection move both northward and eastward from the central equatorial Indian Ocean subsequent to the initiation of an ISO. Eastward movement of convection is also seen at Indian subcontinent latitudes (10 8‐208N). Based on the regression results, the summertime ISO convection signal appears as a band tilting northwestward with latitude and stretching from the equator to about 208N. Viewed along any meridian, convection appears to propagate northward while equatorial convection propagates to the east. To examine the robustness of the connection between eastward and northward movement, individual ISOs are categorized and composited relative to the strength of the large-scale eastward component of convection in the central equatorial Indian Ocean. It is found that the majority of ISOs that exhibit northward movement onto the Indian subcontinent (42 out of 54 ISOs, or 78%) also exhibit eastward movement into the western Pacific Ocean. It is also found that when convection in the central Indian Ocean is not followed within 10‐20 days by convection in the western Pacific Ocean (12 out of 54 ISOs, or 22%), the independent northward movement of convection in the Indian Ocean region is somewhat stunted. The link between the eastward and northward movement of convection is consistent with an interpretation of the summertime ISO in terms of propagating equatorial modes. The northward moving portion of convection is forced by surface frictional convergence into the low pressure center of the Rossby cell that is excited by equatorial ISO convection. A similar convergence pattern is seen for the northern winter ISO, but it does not generate poleward movement due to relatively cool SSTs underlying the surface convergence.

Differences in heat budgets of the near-surface Arabian Sea and Bay of Bengal: Implications for the summer monsoon

Abstract: An analysis of the heat budgets of the near-surface Arabian Sea and Bay of Bengal shows significant differences between them during the summer monsoon (June-September). In the Arabian Sea the winds associated with the summer monsoon are stronger and favor the transfer of heat to deeper layers owing to overturning and turbulent mixing. In contrast, the weaker winds over the bay force a relatively sluggish oceanic circulation that is unable to overturn, forcing a heat budget balance between the surface fluxes and diffusion and the rate of change of heat in the near-surface layer. The weak winds are also unable to overcome the strong near-surface stratification because of a low-salinity surface layer. This leads to a shallow surface mixed layer that is stable and responds quickly to changes in the atmosphere. An implication is that sea surface temperature (SST) in the bay remains higher than 28°C, thereby supporting large-scale deep convection in the atmosphere during the summer monsoon. The atmospheric heating associated with the convection plays a critical role in sustaining the monsoon winds, and the rainfall associated with it, not only over the bay but also over the Indian subcontinent, maintains a low-salinity surface layer. In the Arabian Sea the strong overturning and mixing lead to lower SST and weak convective activity, which in turn, lead to low rainfall and runoff, resulting in weak stratification that can be overcome easily by the strong monsoon winds. Thus, in both basins, there is a cycle with positive feedback, but the cycles work in opposite directions. This locks monsoon convective activity primarily to the bay.

Climatology of Cyclogenesis Mechanisms in the Mediterranean

Abstract: A general climatology of the main mechanisms involved in Mediterranean cyclogenesis is presented. A diagnostic study of both composite means and case studies is performed to analyze processes occurring in different seasons, and in different cyclogenetic regions within the same season. It is shown that cyclones that developed over the three most active areas in winter—the Gulf of Genoa, the Aegean Sea, and the Black Sea—are essentially subsynoptic lows, triggered by the major North Atlantic synoptic systems being affected by local orography and/or low-level baroclinicity over the northern Mediterranean coast. It is also suggested that cyclones in two, or all three, of these regions often occur consecutively, linked to the same synoptic system. In spring and summer, thermally induced lows become progressively more important, despite the existence of other factors, such as the Atlas Mountains contributing to lee cyclogenesis in northern Africa, or the extension of the Asian monsoon into the eastern ...

The East African March–May Rainy Season: Associated Atmospheric Dynamics and Predictability over the 1968–97 Period

Abstract: This paper focuses on the East African March–May “long rains.” Particularly, it investigates the atmospheric patterns associated to the March–May rainfall anomalies, then proposes a seasonal prediction model. In a preliminary step, in order to define a regional rainfall index, a new form of extended principal component analysis is performed, aimed at capturing both the spatial and intraseasonal rainfall coherence. What emerges is that although the long rains exhibit a low temporal coherence, calling for a separation between the months of March–April and May in teleconnection studies, they show a relatively strong spatial consistency over the Kenya–Uganda inland region. From composite analyses performed using NCEP–NCAR reanalyzed atmospheric data, three major signals appear for that region. Two are during March–April involving ENSO and the latitudinal location of the ITCZ, and ENSO interactions with the northern extratropical dynamics (by way of cool surges toward the Tropics and upper-ridge–troug...

A teleconnection pattern in upper-level meridional wind over the North African and Eurasian continent in summer

Abstract: One-point correlation analysis on upper-level meridional wind identified the existence of a teleconnection pattern in July, which emerges from North Africa to East Asia along the westerly jet in the middle latitudes. We examined the spatial and temporal structures of this teleconnection pattern, and found the unique characteristics rather different from the patterns in other elements such as geopotential height, streamfunction and vorticity. We also investigated the relationship between this teleconnection and precipitation, and suggested that the teleconnection is a possible linkage of the EASM to the Indian monsoon, and even to subtropical heating anomalies over Atlantic.

Monsoon Breaks and Subseasonal Sea Surface Temperature Variability in the Bay of Bengal

Abstract: The Indian southwest monsoon directly affects the lives of over one billion people, providing almost 90% of the annual precipitation to the Indian subcontinent. An important characteristic of the southwest monsoon is variability on subseasonal timescales, with “active” periods of heavy rain interrupted by drier “break” periods. Both the number of monsoon breaks in a season and the timing of these breaks profoundly impact agricultural output from the Indian subcontinent. Most research on monsoon breaks has emphasized possible atmospheric mechanisms. However, new satellite data reveal large-amplitude basin-scale subseasonal sea surface temperature (SST) variability in the Bay of Bengal (BoB), in which northern BoB cooling precedes monsoon breaks by about 1 week. The relationship is statistically significant at the 95% level over the 3 yr examined, and so offers a potential statistical predictor for short-term monsoon variability. The basinwide averaged amplitude of SST changes is 1°–2°C and local c...

Active and break phases in the South American monsoon system

Abstract: The South American monsoon system (SAMS) refers to the austral summer season features of deep convective activity and large-scale circulation. This study examines intraseasonal variations in the low-level wind circulation in the Amazon and their modulating effects on active and ''break'' phases in SAMS. Daily averages of outgoing longwave radiation (OLR), NCEP-NCAR reanalysis, and gridded rainfall station data in Brazil are used from 1 November to 28 February 1980-99. The direction of wind anomalies (10-70 days) in the Rondonia State, Brazil, is used to classify periods of westerly (W) and easterly (E) low-level wind regimes. Composites of W regime show low-level wind anomalies crossing the equator southward and closing in a cyclonic anomalous circulation off the coast of Argentina and Uruguay. Broad areas of enhanced convection and rainfall are observed in central and southeast Brazil. Suppressed convection is observed over the Bolivian Altiplano and in northern South America. In contrast, in the E regime, opposite patterns are observed in the low-level circulation, con- vection, and rainfall anomalies. The duration of active (W regimes) and break (E regimes) periods are quite similar, with median values of 4 and 5 days, respectively. Further investigation showed that the region of convection and rainfall anomalies over Venezuela and northwest Brazil is observed only in the 10-30-day band. Comparison of the results shown here with previous studies indicates the importance of intraseasonal variations in the activity of SAMS.

A 780‐year annually resolved record of Indian Ocean monsoon precipitation from a speleothem from south Oman

Abstract: [1] Meteorological records of monsoon rainfall in the Indian Ocean are generally less than 100 years long. The relative brevity of these records makes it difficult to investigate monsoon variation on decadal and centennial timescales, to determine what factors influence the intensity of rainfall on these timescales, or to place possible changes in the twentieth century into a broader historical context. Development of a geologic proxy for rainfall that records annual variation in the monsoon over much longer time periods than are covered by instrumental records would be a significant step forward. We have developed an annually resolved record of monsoon rainfall variation for the past 780 years based on annual layer thickness and stable isotope analyses of a laminated stalagmite from southern Arabia. Our results show that monsoon variation over the past century is not outside of the range of the past 800 years. Decreasing monsoon rainfall over the past century is related to increasing sea surface temperature in the Indian Ocean. Spectral analyses of the record are dominated by cycles that are similar to those observed in records of solar activity on centurial timescales. Decadal to interannual cycles in the record appear to originate in the tropical Pacific Ocean. INDEX TERMS: 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854); 1620 Global Change: Climate dynamics (3309); 1699 Global Change: General or miscellaneous; KEYWORDS: Monsoon, paleoclimate, rainfall, Indian Ocean, speleothem, oxygen isotopes

The Tropospheric Biennial Oscillation and Asian–Australian Monsoon Rainfall

Abstract: In the context of the Asian‐Australian monsoon, the tropospheric biennial oscillation (TBO) is defined as the tendency for a relatively strong monsoon to be followed by a relatively weak one, and vice versa. Therefore the TBO is not so much an oscillation, but a tendency for the system to flip-flop back and forth from year to year. The more of these interannual flip-flops or transitions, the more biennial the system. The transitions occur in northern spring for the south Asian or Indian monsoon and in northern fall for the Australian monsoon involving coupled land‐atmosphere‐ocean processes over a large area of the Indo-Pacific region. There is considerable seasonal persistence from the south Asian to Australian monsoon as noted in previous studies, with a strong south Asian or Indian monsoon tending to precede a strong Australian monsoon and vice versa for weak monsoons. Therefore, transitions from March‐May (MAM) to June‐September (JJAS) tend to set the system for the next year, with a transition to the opposite sign the following year. Quantifying the role of the conditions that contribute to these transitions in the TBO and their relationship to ENSO is crucial for verifying their accurate representation in models, which should lead to improved seasonal forecast skill. An analysis of observed data shows that the TBO (with roughly a 2‐3-yr period) encompasses most ENSO years (with their well-known biennial tendency) as well as additional years that contribute to biennial transitions. Thus the TBO ←