Showing papers on "Monsoon published in 2003"

Holocene forcing of the Indian monsoon recorded in a stalagmite from southern Oman.

Abstract: A high-resolution oxygen-isotope record from a thorium-uranium-dated stalagmite from southern Oman reflects variations in the amount of monsoon precipitation for the periods from 10.3 to 2.7 and 1.4 to 0.4 thousand years before the present (ky B.P.). Between 10.3 and 8 ky B.P., decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in Greenland ice cores, indicating that early Holocene monsoon intensity is largely controlled by glacial boundary conditions. After approximately 8 ky B.P., monsoon precipitation decreases gradually in response to changing Northern Hemisphere summer solar insolation, with decadal to multidecadal variations in monsoon precipitation being linked to solar activity.

Abrupt changes in the Asian southwest monsoon during the Holocene and their links to the North Atlantic Ocean.

Abstract: During the last ice age, the Indian Ocean southwest monsoon exhibited abrupt changes that were closely correlated with millennial-scale climate events in the North Atlantic region1,2,3, suggesting a mechanistic link. In the Holocene epoch, which had a more stable climate, the amplitude of abrupt changes in North Atlantic climate was much smaller, and it has been unclear whether these changes are related to monsoon variability. Here we present a continuous record of centennial-scale monsoon variability throughout the Holocene from rapidly accumulating and minimally bioturbated sediments in the anoxic Arabian Sea. Our monsoon proxy record reveals several intervals of weak summer monsoon that coincide with cold periods documented in the North Atlantic region4—including the most recent climate changes from the Medieval Warm Period to the Little Ice Age and then to the present. We therefore suggest that the link between North Atlantic climate and the Asian monsoon is a persistent aspect of global climate.

Oceanic forcing of Sahel rainfall on interannual to interdecadal time scales.

Abstract: We present evidence, based on an ensemble of integrations with NSIPP1 (version 1 of the atmospheric general circulation model developed at NASA's Goddard Space Flight Center in the framework of the Seasonal-to-Interannual Prediction Project) forced only by the observed record of sea surface temperature from 1930 to 2000, to suggest that variability of rainfall in the Sahel results from the response of the African summer monsoon to oceanic forcing, amplified by land-atmosphere interaction. The recent drying trend in the semiarid Sahel is attributed to warmer-than-average low-latitude waters around Africa, which, by favoring the establishment of deep convection over the ocean, weaken the continental convergence associated with the monsoon and engender widespread drought from Senegal to Ethiopia.

The Indian monsoon and its variability

Abstract: For over 300 years, the monsoon has been viewed as a gigantic land-sea breeze. It is shown in this paper that satellite and conventional observations support an alternative hypothesis, which considers the monsoon as a manifestation of seasonal migration of the intertropical convergence zone (ITCZ). With the focus on the Indian monsoon, the mean seasonal pattern is described, and why it is difficult to simulate it is discussed. Some facets of the intraseasonal variation, such as active-weak cycles; break monsoon; and a special feature of intraseasonal variation over the region, namely, poleward propagations of the ITCZ at intervals of 2-6 weeks, are considered. Vertical moist stability is shown to be a key parameter in the variation of monthly convection over ocean and land as well as poleward propagations. Special features of the Bay of Bengal and the monsoon brought out by observations during a national observational experiment in 1999 are briefly described.

Possible impacts of Indian Ocean Dipole mode events on global climate

Abstract: Impacts of Indian Ocean Dipole mode (IOD) events on global climate are estimated by correlation/regression analysis. The analysis examined land rain and temperature and 3-dimensional atmospheric variables for a 42 yr period from January 1958 to December 1999. The correlation between IOD and the El Nino Southern Oscillation (ENSO) is accounted for using the multiple regression technique. We used partial correlation coefficients to describe the unique contribution of IOD to climate variability, independent of ENSO. In the Indian Ocean rim countries, IOD is associated with significant temperature and rain variability manifesting 2 large-scale patterns. In one, land tem- perature and rain are anomalously high over countries west of the Indian Ocean and anomalously low to its east. In the second pattern, enhanced rainfall is found over the Asian monsoon trough, extending from Pakistan up to southern China. Also noted are IOD impacts on several regions remote from the Indian Ocean. Strong correlation is found over Europe, northeast Asia, North and South America and South Africa concurrent with IOD events. Over these regions, positive IOD events are associated with warm land surface anomalies and reduced rainfall. The troposphere above the Indian Ocean exhibits strong variability during IOD events characterized by the following structures: (1) a Walker cell anomaly over the equator; (2) a deep modulation of monsoon westerlies; and (3) a Hadley cell anomaly over the Bay of Bengal. In the extratropics, IOD is associated with equivalent barotropic geopotential anomalies. These assume annular structure in the northern hemisphere, but Rossby wave train structure in the southern hemisphere.

Atmosphere-Warm Ocean Interaction and Its Impacts on Asian-Australian Monsoon Variation*

Abstract: Asian–Australian monsoon (A–AM) anomalies depend strongly on phases of El Nino (La Nina). Based on this distinctive feature, a method of extended singular value decomposition analysis was developed to analyze the changing characteristics of A–AM anomalies during El Nino (La Nina) from its development to decay. Two off-equatorial surface anticyclones dominate the A–AM anomalies during an El Nino—one over the south Indian Ocean (SIO) and the other over the western North Pacific (WNP). The SIO anticyclone, which affects climate conditions over the Indian Ocean, eastern Africa, and India, originates during the summer of a growing El Nino, rapidly reaches its peak intensity in fall, and decays when El Nino matures. The WNP anticyclone, on the other hand, forms in fall, attains maximum intensity after El Nino matures, and persists through the subsequent spring and summer, providing a prolonged impact on the WNP and east Asian climate. The monsoon anomalies associated with a La Nina resemble those durin...

Identification of three dominant rainfall regions within indonesia and their relationship to sea surface temperature

Abstract: The characteristics of climatic rainfall variability in Indonesia are investigated using a double correlation method. The results are compared with empirical orthogonal function (EOF) and rotated EOF methods. In addition, local and remote responses to sea-surface temperature (SST) are discussed. The results suggest three climatic regions in Indonesia with their distinct characteristics. Region A is located in southern Indonesia from south Sumatera to Timor island, southern Kalimantan, Sulawesi and part of Irian Jaya. Region B is located in northwest Indonesia from northern Sumatra to northwestern Kalimantan. Region C encompasses Maluku and northern Sulawesi. All three regions show both strong annual and, except Region A, semi-annual variability. Region C shows the strongest El Ni˜ no–southern oscillation (ENSO) influence, followed by Region A. In Region B, the ENSO-related signal is suppressed. Except for Region B, there are significant correlations between SST and the rainfall variabilities, indicating a strong possibility for seasonal climate predictions. March to May is the most difficult season to predict the rainfall variability. From June to November, there are significant responses of the rainfall pattern to ENSO in Regions A and C. A strong ENSO influence during this normally dry season (June to September) is hazardous in El Ni˜ no years, because the negative response means that higher SST in the NI ˜ NO3 of the Pacific region will lower the rainfall amount over the Indonesian region. Analyses of Indonesian rainfall variability reveal some sensitivities to SST variabilities in adjacent parts of the Indian and Pacific Oceans. Copyright  2003 Royal Meteorological Society.

Climate change at the 4.2 ka BP termination of the Indus valley civilization and Holocene south Asian monsoon variability

Abstract: [1] Planktonic oxygen isotope ratios off the Indus delta reveal climate changes with a multi-centennial pacing during the last 6 ka, with the most prominent change recorded at 4.2 ka BP. Opposing isotopic trends across the northern Arabian Sea surface at that time indicate a reduction in Indus river discharge and suggest that later cycles also reflect variations in total annual rainfall over south Asia. The 4.2 ka event is coherent with the termination of urban Harappan civilization in the Indus valley. Thus, drought may have initiated southeastward habitat tracking within the Harappan cultural domain. The late Holocene drought cycles following the 4.2 ka BP event vary between 200 and 800 years and are coherent with the evolution of cosmogenic 14C production rates. This suggests that solar variability is one fundamental cause behind Holocene rainfall changes over south Asia.

The West African Monsoon Dynamics. Part II: The “Preonset” and “Onset” of the Summer Monsoon

Abstract: The arrival of the summer monsoon over West Africa has been documented by using daily gridded rainfall data and NCEP‐NCAR reanalyses during the period 1968‐90, and OLR data over the period 1979‐90. Two steps have been characterized through a composite approach: the preonset and the onset of the summer monsoon. The preonset stage corresponds to the arrival in the intertropical front (ITF) at 15 8N, that is, the confluence line between moist southwesterly monsoon winds and dry northeasterly Harmattan, bringing sufficient moisture for isolated convective systems to develop in the Sudano‐Sahelian zone while the intertropical convergence zone (ITCZ) is centered at 58N. The mean date for the preonset occurrence is 14 May and its standard deviation is 9.5 days during the period 1968‐90. This leads to a first clear increase of the positive rainfall slope corresponding to the beginning of the rainy season over this Sudano‐Sahelian area. The onset stage of the summer monsoon over West Africa is linked to an abrupt latitudinal shift of the ITCZ from a quasi-stationary location at 58N in May‐June to another quasi-stationary location at 108N in July‐August. The mean date for the onset occurrence is 24 June and its standard deviation is 8 days during the period 1968‐ 90. This leads to a second increase of the positive rainfall slope over the Sudano‐Sahelian zone signing the northernmost location of the ITCZ and the beginning of the monsoon season. This abrupt shift occurs mostly between 108W and 58E, where a meridional land‐sea contrast exists, and it is characterized by a temporary rainfall and convection decrease over West Africa. Preonset dates, onset dates, and summer rainfall amount over the Sahel are uncorrelated during the period 1968‐90. The atmospheric dynamics associated with the abrupt ITCZ shift has been investigated. Between the preonset and the onset stages, the heat low dynamics associated with the ITF controls the circulation in the low and midlevels. Its meridional circulation intensity is the highest at the beginning of the monsoon onset. This can lead to 1) increased convective inhibition in the ITCZ through intrusion of dry and subsiding air from the north, and 2) increased potential instability through a greater inland moisture advection and a higher monsoon depth induced by a stronger cyclonic circulation in the low levels, through higher vertical wind shear due to westerly monsoon wind and midlevel African easterly jet (AEJ) increases, through enhancement of the instability character of the AEJ, and through increased shortwave radiation received at the surface. During the monsoon onset, once the rainfall minimum occurred due to the convective inhibition, the accumulated potential instability breaks the convective inhibition, the inertial instability of the monsoon circulation is released, and the associated regionalscale circulation increases, leading to the abrupt shift of the ITCZ. Then the ITCZ moves north up to 108N, where thermodynamical conditions are favorable. It is suggested by the authors that the abrupt shift of the ITCZ, initiated by the amplification of the heat low dynamics, could be due to an interaction with the northern orography of the Atlas‐Ahaggar Mountains. Subsidence over and north of this orography, due to both the northern branches of the heat low and of the northern Hadleytype cell, contributes to enhance the high geopotentials north of these mountains and the associated northeasterly winds. This leads to the development of a leeward trough that reinforces the heat low dynamics, maintaining an active convective ITCZ through enhanced moist air advection from the ocean, increasing the northern Hadley circulation, which reinforces the high geopotentials and the interaction with the orography through a positive feedback. The fact that an abrupt shift of the ITCZ is only observed on the western part of West Africa may result from the enhancement of moisture advection, which comes from the west and has a stronger impact west of the Greenwich meridian. The northwest‐southeast orientation of the Atlas‐Ahaggar crest can induce the interaction with the heat low, first in the east where the mountains are nearer to the ITF than in the west, and second in the west. Another consequence of the possible orography-induced interaction with the atmospheric circulation is that the induced leeward trough, increasing the cyclonic vorticity in the heat low, may stimulate moisture convergence in the oceanic ITCZ near the western coast of West Africa.

A synthesis of abrupt changes in the Asian summer monsoon since the last deglaciation

Abstract: We have compiled 36 previously published palaeoclimate records to determine the timing and spatial pattern of century-scale abrupt changes in Asian monsoon precipitation since the last deglaciation. We identify abrupt events from (1) the interpretations of the authors of these records and (2) the more objective moving t-test calculation. Our results indicate that abrupt climatic changes occurred at ~11.5 cal. ka, ~4.5- 5.0 cal. ka and ad 1300. At the start of the Holocene (~11.5 cal. ka), Asian monsoon precipitation increased dramatically. This climatic change is synchronous with an abrupt warming in the North Atlantic. During the middle Holocene, there was a time of preferred and widespread weakening in monsoon strength (~4.5- 5.0 cal. ka). This result contradicts previous notions of either a gradual trend towards drier conditions or a series of abrupt events that occurred in an unorganized fashion across space and time. The middle-Holocene abrupt event could have been synchronous with an abrupt cooling event in the North Atlantic, as well as a warming and intensie cation of internannual variability in the tropical Pacie c. In contrast to previous periods, precipitation changes at ad 1300 have a heterogeneous spatial pattern. We e nd no conclusive evidence for a change in the Asian monsoon at ~8.2 cal. ka, as suggested by several previous studies. More high-resolution data may be needed to observe this short-lived event. Overall, our results attest to the potential for rapid and major shifts in Asian monsoon precipitation that may be triggered by variations in other components of the climatic system.

Seasonal variability of sea surface salinity and salt budget of the mixed layer of the north Indian Ocean

Abstract: [1] A subset of the recently published salinity database of the global oceans is utilized to characterize and explain the observed seasonal variability of sea surface salinity of the north Indian Ocean, in greater detail than has been possible previously. The influence of salinity on the seasonal evolution of near-surface mixed layer depth is highlighted. The relative importance of freshwater flux (evaporation minus precipitation) and horizontal advection in accounting the observed seasonal variability of sea surface salinity is evaluated. The influence of massive river outflow in producing the observed sea surface salinity minima in the coastal northwestern Bay of Bengal during August–September is highlighted. The observed interannual variability of sea surface salinity along two major shipping lanes in the tropical Indian Ocean in relation to El Nino is examined. The annual average of sea surface salinity shows contrasting distributions in the Arabian Sea and the Bay of Bengal due to differences in hydrological forcing. The seasonal variability of sea surface salinity is most pronounced in the coastal region of the northern Bay of Bengal, northwestern Arabian Sea, and the southeastern Arabian Sea. The incorporation of salinity effect reduces the thickness of the near-surface mixed layer, and this reduction is most pronounced in the Bay of Bengal, where it builds up from June to July and becomes most prominent by February in the following year, when the freshening effects of hydrological forcing through local rainfall and river discharges are felt the most on the near-surface layers. The salt budget analysis of the mixed layer shows a broad agreement between the patterns of observed and diagnosed seasonal changes caused by freshwater flux and horizontal advection, despite limitations in the accuracy of these estimates. The freshwater input through rainfall and river discharges in the Bay of Bengal far exceeds evaporation, causing surplus freshwater for export. Horizontal advection of salinity is found to be important in the southeastern Arabian Sea during winter and in the western and eastern Arabian Sea during the summer monsoon season and in the Bay of Bengal throughout the year with the exception of premonsoon season. The pronounced dilution observed during the height of the summer monsoon season in the coastal northwestern Bay of Bengal is attributed to peak discharges from major rivers. Historic data along two major shipping lanes in the tropical Indian Ocean have clearly revealed the signature of El Nino in the interannual variability of sea surface salinity.

The El Nino Impact on the Summer Monsoon in Brazil: Regional Processes versus Remote Influences

Abstract: The El Nino impact on Brazil's summer monsoon has not been adequately assessed through seasonal analysis because it shows significant subseasonal variations. In this study, the El Nino influence on the summer monsoon circulation, rainfall, and temperature is analyzed with monthly resolution, using data from a dense network of stations. The expected precipitation percentiles during the monsoon season of El Nino (EN) events are calculated, as well as anomalies of surface temperature and thermodynamic parameters. This information is analyzed jointly with anomaly composites of several circulation parameters. The analysis shows that some precipitation and circulation anomalies, which are consistent and important during part of the season, are smoothed out in a seasonal analysis. There are abrupt changes of anomalies within the summer monsoon season, suggesting the prevalence of regional processes over remote influences during part of the season. The probable role of remote influences and regional processes is assessed. The anomalous heat sources associated with El Nino perturb the Walker and Hadley circulations over South America and generate Rossby wave trains that produce important effects in the subtropics and extratropics. In the early summer monsoon season, remotely produced atmospheric perturbations prevail over Brazil. Anticyclonic low-level anomalies predominate over central-east Brazil, in the Tropics and subtropics, due to the subsidence over the Amazon and to Rossby waves in the subtropics. Easterly moisture inflow from the Atlantic is favored, but diverted toward northern South America (SA) and south Brazil. There are negative precipitation anomalies in north and central-east Brazil and positive ones in south Brazil. These precipitation anomalies are favored by the perturbation in the Walker and Hadley circulation over the east Pacific and South America, and by a Rossby wave train over southern SA that originates in the eastern Pacific. In January, with the enhancement of the continental subtropical heat low by anomalous surface heating during the spring, there is anomalous low-level convergence and cyclonic circulation over southeast Brazil, while at the upper levels anomalies of divergence and anticyclonic circulation prevail. This anomalous circulation directs moisture flux toward central-east Brazil, causing moisture convergence in this region. A favorable thermodynamic structure enhances precipitation over central-east Brazil, the dry anomalies in north Brazil are displaced northward, and the anomalies in south Brazil almost disappear. In February, after the above-normal precipitation of January, the surface temperature anomalies turn negative and the precipitation diminishes in central-east Brazil. There are negative rainfall anomalies in north Brazil and in the South Atlantic convergence zone (SACZ) and positive ones in south Brazil. Influence function analysis shows that while the anomalies of circulation over southeast Brazil in the spring of El Nino years are mostly due to remote influences from the tropical east Pacific, those in January are probably due to local influence. During this month the monsoonlike circulation is enhanced. Simultaneous and lagged correlation analysis of SST and rainfall in central-east Brazil shows that SST anomalies in the Atlantic Ocean off the southeastern coast of Brazil fluctuate on the same timescale as the circulation and precipitation anomalies.

Three million years of monsoon variability over the northern Sahara

Abstract: We present a 3 million year record of aeolian dust supply into the eastern Mediterranean Sea, based on hematite contents derived from magnetic properties of sediments from Ocean Drilling Program Site 967. Our record has an average temporal resolution of 3400 years. Geochemical data validate this record of hematite content as a proxy for the supply of aeolian dust from the Sahara. We deduce that the aeolian hematite in eastern Mediterranean sediments derives from the east- ern Algerian, Libyan, and western Egyptian lowlands located north of the central Saharan watershed (321� N). In corroboration of earlier work, we relate dust flux minima to penetration of the African summer monsoon front to the north of the central Saharan wa- tershed. This would have enhanced soil humidity and vegetation cover in the source regions, in agreement with results from ''green Sahara'' climate models. Our results indicate that this northward monsoon penetration re- curred during insolation maxima throughout the last 3 million years. As would be expected, this orbital pre- cession-scale mechanism is modulated on both short (3100-kyr) and long (3400-kyr) eccentricity time scales. We also observe a strong expression of the 341- kyr (obliquity) cycle, which we discuss in terms of high- and low-latitude mechanisms that involve Southern Hemisphere meridional temperature contrasts and shifts in the latitudes of the tropics, respectively. We also ob- serve a marked increase in sub-Milankovitch variability around the mid-Pleistocene transition (30.95 Ma), which suggests a link between millennial-scale climate variability, including monsoon dynamics, and the size of northern hemisphere ice sheets.

Indian Ocean Climate and an Absolute Chronology over Dansgaard/Oeschger Events 9 to 13

Abstract: Oxygen-isotope ratios of a stalagmite from Socotra Island in the Indian Ocean provide a record of changes in monsoon precipitation and climate for the time period from 42 to 55 thousand years before the present. The pattern of precipitation bears a striking resemblance to the oxygen-isotope record from Greenland ice cores, with increased tropical precipitation associated with warm periods in the high northern latitudes. The largest change, at the onset of interstadial 12, occurred very rapidly, in about 25 years. The chronology of the events found in our record requires a reevaluation of previously published time scales for climate events during this period.

Mediterranean Moisture Source for an Early-Holocene Humid Period in the Northern Red Sea

Abstract: Paleosalinity and terrigenous sediment input changes reconstructed on two sediment cores from the northernmost Red Sea were used to infer hydrological changes at the southern margin of the Mediterranean climate zone during the Holocene. Between approximately 9.25 and 7.25 thousand years ago, about 3 per thousand reduced surface water salinities and enhanced fluvial sediment input suggest substantially higher rainfall and freshwater runoff, which thereafter decreased to modern values. The northern Red Sea humid interval is best explained by enhancement and southward extension of rainfall from Mediterranean sources, possibly involving strengthened early-Holocene Arctic Oscillation patterns and a regional monsoon-type circulation induced by increased land-sea temperature contrasts. We conclude that Afro-Asian monsoonal rains did not cross the subtropical desert zone during the early to mid-Holocene.

A Hydrological Definition of Indian Monsoon Onset and Withdrawal

Abstract: A diagnostic criterion that retrospectively assesses the onset and withdrawal dates of the Indian monsoon is derived from variability in the large-scale hydrologic cycle. The method is proposed as an improved means with which to understand interannual variability in the monsoon transitions as compared to criteria that rely heavily on rainfall variability over limited spatial domains (e.g., individual Indian districts). The hydrologic cycle is chosen as a key physical basis for monitoring the monsoon due to the essential roles played by zonal and meridional gradients in water vapor, clouds, and rainfall in driving the large-scale monsoon circulation. Moreover, as rainfall is greater than evaporation in wet monsoonal areas, lateral transports of water vapor are required for the existence of monsoonal rains. To diagnose onset and withdrawal, vertically integrated moisture transport (VIMT) is therefore used instead of rainfall, which over the large scale is often poorly measured and modeled. In contrast to rainfall, VIMT is generally well modeled and observed, and its variability, particularly over the Arabian Sea, is substantial during both monsoon onset and withdrawal. An index, named the hydrologic onset and withdrawal index (HOWI), is thus formed from those regions where VIMT variability is pronounced at the beginning and end of the monsoon season. The HOWI offers several advantages as the index is based on fields that are better modeled and measured than rainfall, and the index is indicative of the transition in the largescale monsoon circulation rather than being highly sensitive to synoptic variability and the spatial complexity of the monsoon transitions. The HOWI is shown to be both robust to bogus monsoon onsets and reflective of the timing, rather than the spatial character, of the transitions. Analysis of interannual variability in monsoon onset and withdrawal dates based on the HOWI reveals robust associations that are weak and insignificant when assessed using other onset criteria. For example, the associations between total June‐July‐August‐September (JJAS) rainfall and both monsoon onset and withdrawal are weak (correlations are weaker than 20.11) when onset dates from the Indian Meteorological Department (IMD) or other objective methods are considered. However, the HOWI criterion shows strong correlations between total JJAS rainfall and both onset (0.30) and withdrawal (20.49). Thus, the length of the monsoon season is shown to be strongly related to its overall strength. In addition, while the correlation between IMD onset date and

Coupled dynamics over the Indian Ocean: spring initiation of the Zonal Mode

Abstract: Atmosphere and ocean model assimilated products, in conjunction with observed precipitation and ocean model estimates of Indonesian Throughflow (ITF) transport and barrier layer thickness, are analyzed to elucidate the role of external (ENSO and ITF) and internal (monsoon) factors in the initiation of the Indian Ocean Zonal Mode (IOZM). The diagnostics show that there exists a natural mode of coupled variability in the eastern equatorial Indian Ocean (EEIO) that is weak on its own but intensifies in boreal spring/early summer, usually when ENSO-like conditions exist in the western Pacific, as implied by the Southern Oscillation Index (SOI). In the EEIO, there exists a ‘time window’ in the annual cycle—boreal spring—during which the ocean–atmosphere system is particularly sensitive to external forcing. At interannual timescales, spring atmospheric conditions in the EEIO are remotely controlled by SST in the equatorial western-central Pacific. Warm SST anomalies there cause changes in the Pacific Walker circulation and induce subsidence over the EEIO that results in negative precipitation anomalies:(i) Forced by this heat sink, an anticyclone develops in the lower atmosphere over the southeastern Indian Ocean as a Rossby-wave response, and the alongshore upwelling-favorable winds off Java–Sumatra are enhanced. (ii) The reduced surface fresh-water flux and enhanced upwelling reduce the barrier layer in the upper ocean. These processes along-with the reduction of ITF help trigger the IOZM. Once triggered, IOZM grows in summer by the Bjerknes feedback. Its interactions with the monsoon heat source result in enhanced precipitation along the monsoon trough in July–August. This north–south heating gradient favors a local meridional circulation with increased alongshore winds off Sumatra, implying the potential role of the monsoon background cycle. The hypothesis that the equatorial western-central Pacific SST anomalies control the spring precipitation variations in the EEIO/maritime continent is demonstrated by sensitivity experiments with an atmospheric general circulation model. During the spring initiation stages of the IOZM, an analysis of the mixed layer heat budget in an ocean general circulation model indicates that cooling off Java is primarily due to entrainment and also due to latent cooling, both caused by enhanced upwelling-favorable winds. r 2003 Elsevier Science Ltd. All rights reserved.

Atmospheric Controls on Soil Moisture-Boundary Layer Interactions. Part II: Feedbacks within the Continental United States

Abstract: The CTP-HIlow framework for describing atmospheric controls on soil moisture‐boundary layer interactions is described in a companion paper, Part I. In this paper, the framework is applied to the continental United States to investigate how differing atmospheric regimes influence local feedbacks between the land surface and the atmosphere. The framework was developed with a one-dimensional boundary layer model and is based on two measures of atmospheric thermodynamic properties: the convective triggering potential (CTP), a measure of the temperature lapse rate between approximately 1 and 3 km above the ground surface, and a low-level humidity index, HIlow. These two measures are used to distinguish between three types of early-morning atmospheric conditions: those favoring moist convection over dry soils, those favoring moist convection over wet soils, and those that will allow or prevent deep convective activity, independent of the surface flux partitioning. Analyses of multiyear CTP-HIlow scatterplots from radiosonde stations across the contiguous 48 United States reveal that during the summer months (June, July, and August) positive feedbacks between soil moisture and moist convection are likely in much of the eastern half of the country. Over the western half of the country, atmospheric conditions and the likelihood of moist convection are largely determined by oceanic influences, and land surface conditions in the summer are unlikely to impact convective triggering. The only area showing a potential negative feedback is in the dryline and monsoon region of the arid Southwest. This potential arises because of the topography of this and surrounding regions. A relatively narrow band of stations lies in between the eastern and western portions of the country, in some years behaving like the stations to the west and in other years behaving like the stations to the east.

Coupling between Northward-Propagating, Intraseasonal Oscillations and Sea Surface Temperature in the Indian Ocean*

Abstract: Using a hybrid atmosphere‐ocean coupled model, it is shown that during the boreal summer northwardpropagating, intraseasonal oscillations (NPISOs) are strongly coupled to the underlying sea surface temperature (SST) in the Indian Ocean sector. On the one hand, the intraseasonal atmospheric convection changes the SST through solar radiation, latent heat flux, and mixed-layer entrainment; on the other, the induced SST fluctuations feed back to affect the intraseasonal convection. The preferential northward, rather than southward, propagation of boreal summer ISOs in the Indian Ocean is partially explained by an interaction among the summer-mean climate state, the atmospheric disturbances, and the ocean surface temperature. A solution to an atmosphere-only model forced with daily SST produces much stronger NPISOs than a similar solution forced with monthly mean SST (AMIP-type run). The atmosphere-only model, however, even when it is forced by daily SST from the coupled model (with a small amount of noise in the initial and/or boundary conditions), is unable to reproduce the NPISOs in the coupled case. In the coupled system, intraseasonal SST anomalies are forced by intraseasonal atmospheric convection, and hence are in quadrature with the convection. In the stand-alone atmospheric model, however, SST acts only as a boundary forcing, and the resultant atmospheric convection has almost the same phase with the underlying SST. One consequence is that the intensity of the SST-forced intraseasonal convection in the stand-alone atmospheric model is considerably weaker than in the coupled model. Finally, solutions indicate that the northward movement of the off-equatorial convection in the northern Indian Ocean is more closely related to local intraseasonal SST anomalies than to the equatorial eastward-moving Madden‐Julian oscillation: Positive (negative) SST anomalies in the northern Indian Ocean lead the active (break) phases of the intraseasonal convection by about 2 pentads (10 days). Therefore, intraseasonal SST anomalies in the northern Indian Ocean are potentially a useful index to forecast active (break) spells of the south Asian summer monsoon.

Biogeochemistry of the Bay of Bengal: physical, chemical and primary productivity characteristics of the central and western Bay of Bengal during summer monsoon 2001

Abstract: Reliable data on biological characteristics from the Bay of Bengal are elusive. In this paper, we present results on physics, chemistry and biology simultaneously measured during the summer monsoon, 2001 from open ocean and coastal areas of the region. It was characterised by cold core eddies and thermocline oscillations. However, these were capped by prevalent low saline upper regime and prevented surfacing of nutrients. The river plume effects were evident from the low salinity values observed in the surface layers of the upper bay, but this did not bring-in significant amount of nutrients. The chlorophyll a concentrations (10-20 mg m -2 ) and primary productivity values (40-502 mg C m -2 d -1 ) were low and not up to Arabian Sea values for the same season. Diatoms dominated the phytoplankton community and consisted of more number of genera compared to the Arabian Sea. Large colonies of the tunicate, Pyrosoma, which occurred at the surface as well as mid-depths could have consumed a part of the phytoplankton population. These results, albeit limited, have implications on the biogeochemistry of the region.

The unusual summer of 1994 in East Asia: IOD teleconnections

Abstract: [1] An extremely hot and dry summer of 1994 was reported in East Asian countries Using observational data, we have demonstrated that the Indian Ocean Dipole (IOD) is at least one possible cause of the abnormal East Asian summer climate An anomalous cyclonic circulation over the western Pacific and the southern China weakened the monsoonal northward flow in the lower troposphere An anomalous anticyclonic circulation with the equivalent barotropic structure around Japan, Korea and the northeastern part of China caused the hot and dry summer of 1994 This accumulation of the lower potential vorticity in the Far East is related to the wave activity from the Mediterranean/Sahara region The monsoon-desert mechanism connects a Rossby wave source with the IOD-induced diabatic heating around the Bay of Bengal Another Rossby wave-train pattern was generated in the upper troposphere and propagates northeastward from the southern China Both the Rossby wave patterns influenced the circulation changes over East Asia

Arctic Oscillation signals in the East Asian summer monsoon

Abstract: [1] The present study examines the relationship between the Arctic Oscillation (AO) and the East Asian summer monsoon. Two rainfall data sets are used. One is obtained from 10 stations along the Yangtze River to the southern Japan and the other from gridded global land rainfall data for the period 1900–1998. All data are high-pass filtered before analyzing to highlight the interannual variability. Results show that the AO significantly influences on year-to-year variations in the East Asian summer monsoon rainfall. When AO leads by one month, the correlation between May–July AO and summer total rainfall is −0.44. When AO leads by two months, correlation becomes −0.32. Of all monthly, May AO shows the strongest connection to the summer monsoon rainfall. Correlation coefficient between them is −0.45. The large-scale atmospheric circulation patterns in East Asia in association with the AO are also evident. A positive phase of the AO in late spring is found to lead to a northward shift in the summertime upper tropospheric jet stream over East Asia. This northward shift of the jet stream is closely related to anomalous sinking motion in 20°–40°N and rising motion in surrounding regions. These changes give rise to a drier condition over the region extending from the Yangtze River valley to the southern Japan and a wetter condition in the southern China. Possible mechanisms connecting the late spring AO and summer monsoon rainfall are suggested.

Clustering of synoptic activity by Indian summer monsoon intraseasonal oscillations

Abstract: [1] Active and break phases of the Indian summer monsoon are characterized by enhancement and decrease of precipitation over the monsoon trough region. Using genesis data of monsoon low pressure systems (LPS) and circulation data for the period 1954 to 1993, it is shown that the frequency of occurrence of LPS is nearly 3.5 times higher in the active phase of monsoon as compared to the break phase. In addition, the tracks of these synoptic systems are also strongly spatially clustered along the monsoon trough during the active phase of the monsoon. The enhanced (decreased) frequency of occurrence of LPS during active (break) phases is due to modulation of meridional shear of zonal winds and cyclonic vorticity along the monsoon trough by the intraseasonal oscillations (ISO).

The Relation between the Tibetan Winter Snow and the Asian Summer Monsoon and Rainfall: An Observational Investigation

Abstract: Utilizing winter (November–March) accumulated snow depth data at 60 stations over the Tibetan Plateau (TP) for the period 1960–98, three typical patterns of the TP snow anomaly's spatial distribution were objectively classified by means of empirical orthogonal function (EOF) analysis. They are characterized by light snow over the entire Tibet region (LS pattern), by an eastern Tibet heavy snow (ETHS pattern), and by a southwestern Tibet heavy snow (SWTHS pattern), respectively. The possible relations between various patterns of the Tibet winter snow anomaly and subsequent summer monsoon and rainfall over south, southeast, and east Asia are investigated using composite analysis. In ETHS and SWTHS years, the south and southeast Asian summer monsoon becomes weak and there is less summer rainfall over south and southeast Asia than in normal years. In LS years, the anomalies of the subsequent summer monsoon and rainfall are opposite to those in ETHS and SWTHS years. The physical mechanism is, in part,...

A new monsoon index and the geographical distribution of the global monsoons

Abstract: A new monsoon index, the dynamical normalized seasonality (DNS), is introduced to study the issue of monsoons. This DNS index can describe both seasonal variation and interannual variability of different monsoon regions. It can also be used to delimit the geographical distribution of the global monsoon systems. Furthermore, it is pointed out that the index is very useful for understanding deeply the monsoons to study the difference, relationship, and interactions among the classical monsoon, ordinary monsoon and monsoon-like system.