Showing papers on "Monsoon published in 2001"

A High-Resolution Absolute-Dated Late Pleistocene Monsoon Record from Hulu Cave, China

Abstract: Oxygen isotope records of five stalagmites from Hulu Cave near Nanjing bear a remarkable resemblance to oxygen isotope records from Greenland ice cores, suggesting that East Asian Monsoon intensity changed in concert with Greenland temperature between 11,000 and 75,000 years before the present (yr. B.P.). Between 11,000 and 30,000 yr. B.P., the timing of changes in the monsoon, as established with 230Th dates, generally agrees with the timing of temperature changes from the Greenland Ice Sheet Project Two (GISP2) core, which supports GISP2's chronology in this interval. Our record links North Atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer East Asian Monsoon originates.

Evolution of Asian monsoons and phased uplift of the Himalaya–Tibetan plateau since Late Miocene times

Abstract: The climates of Asia are affected significantly by the extent and height of the Himalayan mountains and the Tibetan plateau1,2,3,4 Uplift of this region began about 50 Myr ago, and further significant increases in altitude of the Tibetan plateau are thought to have occurred about 10–8 Myr ago4,5, or more recently However, the climatic consequences of this uplift remain unclear Here we use records of aeolian sediments from China6,7 and marine sediments from the Indian8,9,10 and North Pacific oceans11 to identify three stages of evolution of Asian climates: first, enhanced aridity in the Asian interior and onset of the Indian and east Asian monsoons, about 9–8 Myr ago; next, continued intensification of the east Asian summer and winter monsoons, together with increased dust transport to the North Pacific Ocean11, about 36–26 Myr ago; and last, increased variability and possible weakening of the Indian and east Asian summer monsoons and continued strengthening of the east Asian winter monsoon since about 26 Myr ago The results of a numerical climate-model experiment, using idealized stepwise increases of mountain–plateau elevation, support the argument that the stages in evolution of Asian monsoons are linked to phases of Himalaya–Tibetan plateau uplift and to Northern Hemisphere glaciation

Interannual Variability of the Asian Summer Monsoon: Contrasts between the Indian and the Western North Pacific–East Asian Monsoons*

Abstract: Analyses of 50-yr NCEP-NCAR reanalysis data reveal remarkably different interannual variability between the Indian summer monsoon (ISM) and western North Pacific summer monsoon (WNPSM) in their temporal- spatial structures, relationships to El Nino, and teleconnections with midlatitude circulations. Thus, two circulation indices are necessary, which measure the variability of the ISM and WNPSM, respectively. A weak WNPSM features suppressed convection along 108-208N and enhanced rainfall along the mei-yu/baiu front. So the WNPSM index also provides a measure for the east Asian summer monsoon. An anomalous WNPSM exhibits a prominent meridional coupling among the Australian high, cross-equatorial flows, WNP monsoon trough, WNP subtropical high, east Asian subtropical front, and Okhotsk high. The WNP monsoon has leading spectral peaks at 50 and 16 months, whereas the Indian monsoon displays a primary peak around 30 months. The WNPSM is weak during the decay of an El Nino, whereas the ISM tends to abate when an El Nino develops. Since the late 1970s, the WNPSM has become more variable, but its relationship with El Nino remained steady; in contrast, the ISM has become less variable and its linkage with El Nino has dramatically declined. These contrasting features are in part attributed to the differing processes of monsoon-ocean interaction. Also found is a teleconnection between a suppressed WNPSM and deficient summer rainfall over the Great Plains of the United States. This boreal summer teleconnection is forced by the heat source fluctuation associated with the WNPSM and appears to be established through excitation of Rossby wave trains and perturbation of the jet stream that further excites downstream optimum unstable modes.

Impact of the Indian Ocean dipole on the relationship between the Indian monsoon rainfall and ENSO

Abstract: The influence of the recently discovered Indian Ocean Dipole (IOD) on the interannual variability of the Indian summer monsoon rainfall (ISMR) has been investigated for the period 1958-1997. The IOD and the El Nino/Southern Oscillation (ENSO) have complementarily affected the ISMR during the last four decades. Whenever the ENSO-ISMR correlation is low (high), the IOD-ISMR correlation is high (low). The IOD plays an important role as a modulator of the Indian monsoon rainfall, and influences the correlation between the ISMR and ENSO. We have discovered that the ENSO-induced anomalous circulation over the Indian region is either countered or supported by the IOD-induced anomalous meridional circulation cell, depending upon the phase and amplitude of the two major tropical phenomena in the Indo-Pacific sector.

Strong coherence between solar variability and the monsoon in Oman between 9 and 6 kyr ago

Abstract: Variations in the amount of solar radiation reaching the Earth are thought to influence climate, but the extent of this influence on timescales of millennia to decades is unclear. A number of climate records show correlations between solar cycles and climate1, but the absolute changes in solar intensity over the range of decades to millennia are small2 and the influence of solar flux on climate is not well established. The formation of stalagmites in northern Oman has recorded past northward shifts of the intertropical convergence zone3, whose northward migration stops near the southern shoreline of Arabia in the present climate4. Here we present a high-resolution record of oxygen isotope variations, for the period from 9.6 to 6.1 kyr before present, in a Th–U-dated stalagmite from Oman. The δ18O record from the stalagmite, which serves as a proxy for variations in the tropical circulation and monsoon rainfall, allows us to make a direct comparison of the δ18O record with the Δ14C record from tree rings5, which largely reflects changes in solar activity6,7. The excellent correlation between the two records suggests that one of the primary controls on centennial- to decadal-scale changes in tropical rainfall and monsoon intensity during this time are variations in solar radiation.

The Indian Ocean Experiment: Widespread Air Pollution from South and Southeast Asia

Abstract: The Indian Ocean Experiment (INDOEX) was an international, multiplatform field campaign to measure long-range transport of air pollution from South and Southeast Asia toward the Indian Ocean during the dry monsoon season in January to March 1999. Surprisingly high pollution levels were observed over the entire northern Indian Ocean toward the Intertropical Convergence Zone at about 6°S. We show that agricultural burning and especially biofuel use enhance carbon monoxide concentrations. Fossil fuel combustion and biomass burning cause a high aerosol loading. The growing pollution in this region gives rise to extensive air quality degradation with local, regional, and global implications, including a reduction of the oxidizing power of the atmosphere.

Subtropical Anticyclones and Summer Monsoons

Abstract: The summer subtropical circulation in the lower troposphere is characterized by continental monsoon rains and anticyclones over the oceans. In winter, the subtropical circulation is more strongly dominated by the zonally averaged flow and its interactions with orography. Here, the mechanics of the summer and winter lowertropospheric subtropical circulation are explored through the use of a primitive equation model and comparison with observations. By prescribing in the model the heatings associated with several of the world’s monsoons, it is confirmed that the equatorward portion of each subtropical anticyclone may be viewed as the Kelvin wave response to the monsoon heating over the continent to the west. A poleward-flowing low-level jet into a monsoon (such as the Great Plains jet) is required for Sverdrup vorticity balance. This jet effectively closes off the subtropical anticyclone to the east and also transports moisture into the monsoon region. The low-level jet into North America induced by its monsoon heating is augmented by a remote response to the Asian monsoon heating. The Rossby wave response to the west of subtropical monsoon heating, interacting with the midlatitude westerlies, produces a region of adiabatic descent. It is demonstrated here that a local ‘‘diabatic enhancement’’ can lead to a strengthening of the descent. Longitudinal mountain chains act to block the westerly flow and also tend to produce descent in this region. Below the descent, Sverdrup vorticity balance implies equatorward flow that closes off the subtropical anticyclone to the west and induces cool upwelling in the ocean through Ekman transport. Feedbacks, involving, for example, sea surface temperatures, may further enhance the descent in these regions. The conclusion is that the Mediterranean-type climates of regions such as California and Chile may be induced remotely by the monsoon to the east. Hence it can be argued that the subtropical circulation in summer comprises a set of weakly interacting monsoon systems, each involving monsoon rains, a low-level poleward jet, a subtropical anticyclone to the east, and descent and equatorward flow to the west. In winter, it is demonstrated how the nonlinear interaction between the strong zonal-mean circulation, associated with the winter ‘‘Hadley cell,’’ and the mountains can define many of the large-scale features of the subtropical circulation. The blocking effect of the longitudinal mountain chains is shown to be very important. Subsequent diabatic effects, such as a local diabatic enhancement, would appear to be essential for producing the observed amplitude of these features.

Intraseasonal Oscillations and Interannual Variability of the Indian Summer Monsoon

Abstract: How and to what extent the intraseasonal oscillations (ISOs) influence the seasonal mean and its interannual variability of the Indian summer monsoon is investigated using 42-yr (1956–97) daily circulation data from National Centers for Environmental Prediction–National Center for Atmospheric Research 40-Year Reanalysis and satellite-derived outgoing longwave radiation data for the period of 1974–97 Based on zonal winds at 850 hPa over the Bay of Bengal, a criterion is devised to define “active” and “break” monsoon conditions The underlying spatial structure of a typical ISO cycle in circulation and convection that is invariant over the years is constructed using a composite technique A typical ISO has large-scale horizontal structure similar to the seasonal mean and intensifies (weakens) the mean flow during its active (break) phase A typical active (break) phase is also associated with enhanced (decreased) cyclonic low-level vorticity and convection and anomalous upward (downward) motion in

Active / break cycles: diagnosis of the intraseasonal variability of the Asian Summer Monsoon

Abstract: In this study, various diagnostics have been applied to daily observed outgoing longwave radiation (OLR) and ECMWF ReAnalysis (ERA) products to provide a comprehensive description of the active/break cycles associated with the Asian Summer Monsoon and to address the differing behaviour of the two dominant time scales of intraseasonal variability, 10–20 days and 30–60 days. Composite analysis of OLR based on filtered daily All-India rainfall (AIR) for the 40 day (30–60 days) intraseasonal mode indicates that during active phases, convection is significantly enhanced over the Indian continent, extending over the Bay of Bengal, Maritime continent and equatorial west Pacific, while convection is suppressed over the equatorial Indian Ocean and northwest tropical Pacific, resulting in a ‘quadrapole’ structure over the Asian monsoon domain. In response to this heating pattern, the large-scale Hadley (lateral) and the two east-west (transverse) tropical circulations are enhanced. There is also a significant impact on the extra-tropical circulation through excitation and propagation of Rossby waves. In contrast, the 15-day mode is more regional to the monsoon domain and has a prominent east-west orientation in convection. Only the local Hadley circulation over the monsoon region is modulated by this mode. The evolution of these two modes as revealed by POP (principal oscillation pattern) analysis, shows that the 40-day mode originates over the equatorial Indian Ocean. Once formed it has poleward propagation on either side of the equator, and eastward propagation into the equatorial west Pacific. From the equatorial west Pacific, northward propagation over the west Pacific and westward propagation into the Indian longitudes are prominent. The propagative features are complex and interactive and are responsible for the ‘quadrapole’ structure in convection seen from the composites. The interannual variability, assessed from the POP coefficient time series, indicates that the 40-day mode is strong during the onset phase of the monsoon in all the years but systematic propagation over the entire season depends crucially on the activity of the oceanic TCZ (tropical convergence zone). The POP analysis of the 15-day mode indicates that this event originates over the␣equatorial west Pacific, associated with westward propagating Rossby waves, amplifies over the northwest tropical Pacific and modulates both the continental and oceanic TCZs over Indian longitudes simultaneously. This mode is pronounced during the established phase of the monsoon. Due to the complexity in the propagational features of both the intraseasonal modes, it is concluded that understanding the subseasonal variability of one regional component of the Asian Summer Monsoon (ASM), requires understanding the entire ASM system.

Tibetan Plateau summer monsoon northward extent revealed by measurements of water stable isotopes

Abstract: A program of individual precipitation events and river water sampling and of water isotopic measurements (δD, δ18O) was carried out during summer 1996 along a northeast/southwest transect of the Tibetan Plateau. The spatial distribution of both δ18O and deuterium excess (d = δD-8*δ18O) of the precipitation reveals three distinct regions. Simulations with a simple isotopic model and seasonal isotopic variations measured at two extreme south and north locations support our interpretation in terms of different summer moisture origins: (1) South of the Himalayan mountains, the moisture provided by the Indian monsoon has been recycled over the Indian peninsula. (2) Between the Himalayas and the Tanggula mountains the oceanic moisture is directly transported from the Bay of Bengal along the Brahmaptra River valley. (3) North of the Tanggula mountains, the moisture is not provided by the monsoon anymore but by continental water recycling.

Seasonality and atmospheric dynamics of the teleconnection between African rainfall and tropical sea‐surface temperature: Atlantic vs. ENSO

Abstract: A 47-year record (1951-1997) of gridded data covering Africa south of the Sahara was used to document the spatial and seasonal patterns of the correlation between precipitation and sea-surface temperatures (SST) in key tropical areas, as depicted by the NIN O3, South Atlantic and North Atlantic indices. El Nino -Southern Oscillation (ENSO) is confirmed as playing a dominant part in northeastern, eastern and southern Africa. However, its impact is also found over the Sahel during the northern summer, and other parts of the Gulf of Guinea region outside this season, a hitherto poorly documented feature. Over these two areas, ENSO and Atlantic SST (predominantly South Atlantic) contribute to different parts of the rainfall variance. The correlation with South Atlantic SST appears as a south-north dipole (positive/negative correlation) which shifts northward following the Inter-tropical Convergence Zone (ITCZ) translation between the northern low-sun and high-sun periods. A typing of the seasonal correlation patterns and a mapping of the multiple correlation coefficients are carried out in order to synthesize the space-time impacts of the three SST indices. Decadal-scale changes affect the strength of the teleconnections with both Atlantic and East Pacific SST, as reflected for instance by a small rise of the correlation with the NIN O3 index since 1970-1975 in the Sahel and southern Africa, and additional shifts for the Atlantic Ocean, but the main patterns remain generally apparent over the whole period. The circulation anomalies associated with the teleconnections were assessed using National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data. A study of the dataset accuracy in depicting long-term climatic variations revealed that a major shift, mainly artificial, is found in 1967-1968 in the time-series of most of the variables. The rest of the work thus concentrated on the 1968-1997 period. A number of changes in east-west circulation patterns have been found to be associated to ENSO variations. Over West Africa, El Nino events tend to result in enhanced northeasterlies/reduced monsoon flow, coupled to weakened upper easterlies, and hence dry conditions over West Africa close to the surface position of the ITCZ, in July-September, as well as January-March. Over the southwestern Indian Ocean, the positive equatorial temperature/geopotential height anomalies, which at 200 hPa accompany El Nino events, are conducive to an eastward shift of the mid-latitude upper troughs, thus being detrimental to summer rainfall over South Africa. Abnormally wet 'short rains' in East Africa can be accounted for by an ENSO-forced weakening of the equatorial Walker-type (east-west) cell which is found over the Indian Ocean during that season. By contrast, the impact of South Atlantic warmings is mostly shown in low-level dynamics, as exemplified by the weakened trades and monsoon flow which directly result in a southward shift of the ITCZ. The combination of ENSO and Atlantic SST anomalies are found to give rise to complex wind flow changes in the near-equatorial Atlantic. In addition to large-scale SST-forced atmospheric dynamics, a few regional atmospheric signals are found to explain residual parts of rainfall variance. For instance, a strengthening of the African Easterly Jet, or northerly wind anomalies across the Sahara, are shown to be related to drought conditions in the Sahel (July-September) and the Gulf of Guinea area (January-March), once the remote effect of SST anomalies is removed. Copyright © 2001 Royal Meteorological Society.

Seasonal stable isotope evidence for a strong Asian monsoon throughout the past 10.7 m.y

Abstract: O of wet-season rainfall was significantly morenegative (29.5‰ SMOW) prior to 7.5 Ma than after ( 26.5‰SMOW). If this change is attributable to a lessening of the amounteffect in rainfall, this agrees with floral and soil geochemical datathat indicate increasing aridity beginning at 7.5 Ma.Keywords: Tibetan Plateau, monsoon, stable isotopes, paleohydrology,seasonality.INTRODUCTIONThe Tibetan Plateau is the engine that drives the modern Asianmonsoon by generating a high-altitude region of low pressure in thesummer as the plateau heats, and a region of high pressure in the winteras the plateau cools (Hastenrath, 1991). During the summer, warm airrises from the plateau, pulling moist air off the ocean, across the Indiansubcontinent, and into the highlands; this results in heavy summer rain-fall on the subcontinent. The opposite occurs in the winter, resultingin cold dry air spilling off the plateau and effectively excluding rainfrom the subcontinent. Thus, the presence of a strong wet-season–dry-season alternation implies the presence of a plateau broad and highenough to drive the monsoon.The timing of the uplift of the plateau remains a matter of con-siderable debate because there are few direct indicators of paleotopog-raphy in the geologic record. Consequently, past workers in Tibet, re-lying on indirect indicators of uplift, have proposed dates ranging from40 to 3.4 Ma, on the basis of initiation of potassic volcanism (Chunget al., 1998; Turner et al., 1993) or extension on the plateau (Harrisonet al., 1995; Coleman and Hodges, 1995), changes in marine sedimen-tation rates (Burbank et al., 1993), sediment types (Rea et al., 1998),or biota (Nigrini and Caulet, 1992; Kroon et al., 1991), and changesin stable carbon isotope and palynological patterns on the Indian sub-continent (Quade et al., 1989; Chen, 1981). Although different areasof the plateau may have risen at different times, many workers haveinferred rapid simultaneous uplift of large areas of the plateau at 7–8Ma by a process such as lithospheric delamination (Molnar et al.,1993). This inference was based on the following approximately coevalphenomena: a major change in plant communities of the Indian sub-continent (Quade et al., 1989), and shifts in marine upwelling patternsthat are linked to an intense monsoon (Kroon et al., 1991). Althoughthere is strong evidence for significant climate change at 7–8 Ma, it isunclear whether this is the onset of the monsoon. The floral transitionseems to have been a global rather than local phenomenon (Cerling etal., 1997) and monsoonally driven upwelling may have already beenpresent by 10–12 Ma (Nigrini and Caulet, 1992; Kroon et al., 1991).Because there is an intimate association between the intense sea-sonality of the modern monsoon and a high Tibetan Plateau and be-cause evaporation can be unambiguously recognized in the d

Upwelling along the coasts of Java and Sumatra and its relation to ENSO

Abstract: Upwelling along the Java-Sumatra Indian Ocean coasts is a response to regional winds associated with the monsoon climate The upwelling center with low sea surface temperature migrates westward and toward the equator during the southeast monsoon (June to October) The migration path depends on the seasonal evolution of alongshore winds and latitudinal changes in the Coriolis parameter Upwelling is eventually terminated due to the reversal of winds associated with the onset of the northwest monsoon and impingement of Indian Ocean equatorial Kelvin waves Significant interannual variability of the Java-Sumatra upwelling is linked to ENSO through the Indonesian throughflow (ITF) and by anomalous easterly wind During El Nino episodes, the Java-Sumatra upwelling extends in both time (into November) and space (closer to the equator) During El Nino (La Nina), the ITF carries colder (warmer) water shallowing (deepening) thermocline depth and enhancing (reducing) upwelling strength

ENSIP: The El Niño simulation intercomparison project

Abstract: An ensemble of twenty four coupled ocean-atmosphere models has been compared with respect to their performance in the tropical Pacific. The coupled models span a large portion of the parameter space and differ in many respects. The intercomparison includes TOGA (Tropical Ocean Global Atmosphere)-type models consisting of high-resolution tropical ocean models and coarse-resolution global atmosphere models, coarse-resolution global coupled models, and a few global coupled models with high resolution in the equatorial region in their ocean components. The performance of the annual mean state, the seasonal cycle and the interannual variability are investigated. The primary quantity analysed is sea surface temperature (SST). Additionally, the evolution of interannual heat content variations in the tropical Pacific and the relationship between the interannual SST variations in the equatorial Pacific to fluctuations in the strength of the Indian summer monsoon are investigated. The results can be summarised as follows: almost all models (even those employing flux corrections) still have problems in simulating the SST climatology, although some improvements are found relative to earlier intercomparison studies. Only a few of the coupled models simulate the El Nino/Southern Oscillation (ENSO) in terms of gross equatorial SST anomalies realistically. In particular, many models overestimate the variability in the western equatorial Pacific and underestimate the SST variability in the east. The evolution of interannual heat content variations is similar to that observed in almost all models. Finally, the majority of the models show a strong connection between ENSO and the strength of the Indian summer monsoon.

Modulation and amplification of climatic changes in the Northern Hemisphere by the Indian summer monsoon during the past 80 k.y

Abstract: High-frequency suborbital variations (Dansgaard-Oeschger cycles) characterize the climatic history of the Northern Hemisphere as observed in Greenland ice cores, deep-sea sediments of the North Atlantic, the Californian borderland, the Arabian Sea, the South China Sea, and the Chinese loess area. Paleoceanographic data from core KL126 from the Bay of Bengal in combination with data from the other Asian monsoonal areas indicate that the feedback processes involving snow and dust of the Tibetan Plateau vary the summer monsoon capacity to transport moisture into central South Asia and into the atmosphere. We postulate that the summer monsoon initiates, amplifies, and terminates these cycles in the Northern Hemisphere.

Spatial and Temporal Variability in the Stable Isotope Systematics of Modern Precipitation in China: Implications for Paleoclimate Reconstructions

Abstract: The stable isotopic composition of materials such as glacial ice, tree rings, lake sediments, and speleothems from low-to-mid latitudes contains information about past changes in temperature (T) and precipitation amount (P). However, the transfer functions which link NOp to changes in T or P, dNOp/dT and dNOp/dP, can exhibit significant temporal and spatial variability in these regions. In areas affected by the Southeast Asian monsoon, past variations in N18O and ND of precipitation have been attributed to variations in monsoon intensity, storm tracks, and/ or variations in temperature. Proper interpretation of past NOp variations here requires an understanding of these complicated stable isotope systematics. Since temperature and precipitation are positively correlated in China and have opposite effects on NOp, it is necessary to determine which of these effects is dominant for a specific region in order to perform even qualitative paleoclimate reconstructions. Here, we evaluate the value of the transfer functions in modern precipitation to more accurately interpret the paleorecord. The strength of these transfer functions in China is investigated using multiple regression analysis of data from 10 sites within the Global Network for Isotopes in Precipitation (GNIP). NOp is modeled as a function of both temperature and precipitation. The magnitude and signs of the transfer functions at any given site are closely related to the degree of summer monsoon influence. NOp values at sites with intense summer monsoon precipitation are more dependent on the amount of precipitation than on temperature, and therefore exhibit more negative values in the summer. In contrast, NOp values at sites that are unaffected by summer monsoon precipitation exhibit strong relationships between NOp and temperature. The sites that are near the northern limit of the summer monsoon exhibit dependence on both temperature and amount of precipitation. Comparison with simple linear models (NOp as a function of T or P) and a geographic model (NOp as a function of latitude and altitude) shows that the multiple regression model is more successful at reproducing NOp values at sites that are strongly influenced by the summer monsoon. The fact that the transfer function values are highly spatially variable and closely related to the degree of summer monsoon influence suggests that these values may also vary temporally. Since the Southeast Asian monsoon intensity is known to exhibit large variations on a number of timescales (annual to glacial^interglacial), and the magnitude and sign of the transfer functions is related to monsoon intensity, we suggest that as monsoon intensity changes, the magnitude and possibly even the sign of the transfer functions may vary. Therefore, quantitative paleoclimate reconstructions based on NOp variations may not 0012-821X / 04 / $ ^ see front matter < 2004 Elsevier B.V. All rights reserved. doi:10.1016/S0012-821X(04)00036-6 * Corresponding author. Tel. : +1-510-642-9539; Fax: +1-510-643-9980. E-mail address: kathleen@eps.berkeley.edu (K.R. Johnson). EPSL 6996 17-3-04 Earth and Planetary Science Letters 220 (2004) 365^377 R Available online at www.sciencedirect.com www.elsevier.com/locate/epsl be valid. < 2004 Elsevier B.V. All rights reserved.

Climate Change in China from 1880 to 1998 and its Impact on the Environmental Condition

Abstract: The global mean surface air temperature (SAT) or the Northern Hemisphere mean SAT has increased since the late nineteenth century, but the mean precipitation around the world has not formed a definite tendency to increase. A lot of studies showed that different climate and environmental changes during the past 100 years over various regions in the world were experienced. The climate change in China over the past 100 years and its impact on China's environmental conditions needs to be investigated in more detail.Data sets of surface air temperature and atmospheric precipitation over China since 1880 up to the present are now available. In this paper, a drought index has been formulated corresponding to both the temperature and precipitation. Based on three series of temperature, precipitation, and drought index, interdecadal changes in all 7 regions of China and temperature differences among individual regions are analyzed. Some interesting facts are revealed using the wavelet transform method. In Northeast China, the aridification trend has become more serious since 1970s. Drought index in North China has also reached a high value during 1990s, which seems similar to that period 1920s–1940s. In NorthwestChina, the highest temperature appeared over the period 1930s–1940s. Along the Yangtze River valley in central eastern China and Southwest China, interdecadal high temperature occurred from 1920s to 1940s and in 1990s, but the drought climate mainly appeared from 1920s to early 1940s. In South China, temperature remained at a high value over the period 1910s–1940s,but the smaller-scale variation of drought index was remarkable from 1880 to 1998. Consequently, the quasi-20-year oscillation (smaller-scale variation) and the quasi-70-year oscillation (secular variation) obviously exist in temperature and precipitation series in different regions over China.Climate change and intensified human activity in China have induced certain environmental evolutions, such as the frequency change of dust-storm event in northern China, no-flow in the lower reaches of the Yellow River and the runoff variation in Northwest China. On the other hand, frequent floods along the Yangtze River and high frequency of drought disaster have resulted in tremendous economic losses in the last decade in China. The primary reason for these happenings may be attributed to the evolution of the monsoon system in East Asian.

Possible Roles of Atlantic Circulations on the Weakening Indian Monsoon Rainfall–ENSO Relationship

Abstract: Since the 1970s, the inverse relationship between the Indian monsoon rainfall and the El Nino–Southern Oscillation (ENSO) has weakened considerably. The cause for this breakdown is shown to be most likely the strengthening and poleward shift of the jet stream over the North Atlantic. These changes have led to the recent development of a significant correlation between wintertime western European surface air temperatures and the ensuing monsoon rainfall. This western Europe winter signal extended eastward over most of northern Eurasia and remained evident in spring, such that the effect of the resulting meridional temperature contrast was able to disrupt the influence of ENSO on the monsoon.

Summer Sahel-ENSO teleconnection and decadal time scale SST variations

Abstract: The correlation between Sahel rainfall and El Nino–Southern Oscillation (ENSO) in the northern summer has been varying for the last fifty years. We propose that the existence of periods of weak or strong relationship could result from an interaction with the global decadal scale sea surface temperature (SST) background. The main modes of SST variability have been extracted through a principal component analysis with Varimax rotation. The correlations between a July-September Sahel rainfall index and these SST modes have been computed on a 20-year running window between 1945 and 1993. The correlations with the interannual ENSO-SST mode are negative, not significant in the 1960s during the transition period from the wet climate phasis to the long-running drought in the Sahel, but then were significant since 1976. During the former period, the correlations between the Sahel rainfall index and the other SST modes (expressing mostly on quasi and multi-decadal scales) are the highest, in particular correlations with the tropical Atlantic “dipole”. Correlations between Sahel and Guinea Coast rainfall are also significantly negative. After 1970, the Sahel-Guinea Coast rainfall correlations are no longer significant, and the ENSO-SST mode becomes the only one significantly correlated with Sahel rainfall, especially due to the impact of warm events. The partial correlations between the ENSO-SST mode and the Sahel rainfall index, when the influence of the other SST modes are eliminated, are significant over all the 20-year running periods between 1945 and 1993, suggesting that this summer teleconnection could be modulated by the decadal scale SST background. The NCEP/NCAR reanalyses reproduce accurately the interannual variability of the atmospheric circulation after 1968. In particular a regional West African Monsoon Index (WAMI), combining wind speed anomalies at 925 and 200 hPa, is highly correlated with the July-September Sahel rainfall index. A warm ENSO event is associated both with an eastward mean sea level pressure gradient between the eastern tropical Pacific and the tropical Atlantic and with a northward pressure gradient along the western coast of West Africa. This pattern leads to enhanced trade winds over the tropical Atlantic and to weaker moisture advection over West Africa, consistent with a weaker monsoon system strength and a weaker Southern Hemisphere Hadley circulation. The NCEP/NCAR reanalyses do not reproduce accurately the decadal variability of the atmospheric circulation over West Africa because of artifical biases. Therefore the impact of the decadal scale pattern of the atmospheric circulation has been investigated with atmospheric general circulation model (AGCM) sensitivity experiments, by forcing the ARPEGE-Climat model with different combinations of an El Nino-like SST pattern with the pattern of the main mode of decadal scale SST variability where the hightest weights are located in the Pacific and Indian basins. AGCM outputs show that the decadal scale SST variations weakly affect Sahel rainfall variability but that they do induce an indirect effect on Sahel rainfall by enhancing the impact of the warm ENSO phases after 1980, through an increase in the fill-in of the monsoon trough and a moisture advection deficit over West Africa.

Primary productivity and its regulation in the Arabian Sea during 1995

Abstract: The annual cycle of monsoon-driven variability in primary productivity was studied in 1995 during the Arabian Sea Expedition as part of the United States Joint Global Ocean Flux Studies (US JGOFS). This paper describes the seasonal progression of productivity and its regulation on a section which ran from the coast of Oman to about 1000 km offshore in the central Arabian Sea at 65°E. During the SW Monsoon (June–mid-September), the coolest water and highest nutrient concentrations were close to the coast, although they extended offshore to about 800 km; during the January NE Monsoon, deep convective mixing provided nutrients to the mixed layer in the region 400 – 1000 km offshore. As expected, the SW Monsoon was the most productive season (123±9 mmol C m−2 d−1) along the southern US JGOFS section from the coast to 1000 km offshore, but productivity in the NE Monsoon was surprisingly high (112±7 mmol C m−2 d−1). There was no onshore/offshore gradient in primary productivity from 150 to 1000 km off the Omani coast in 1995, and there was no evidence of light limitation of either primary productivity or photosynthetic performance (PoptB) from deep convective mixing during the NE Monsoon, deep wind mixing during the SW Monsoon or offshore Ekman downwelling during the SW Monsoon. Productivity during the Spring Intermonsoon (86±6 mmol C m−2 d−1) was much higher than in oligotrophic regions such as the tropical Pacific Ocean (29±2 mmol C m−2 d−1) or the North Pacific gyre region (32±8 mmol C m−2 d−1). The 1995 annual mean productivity (111±11 mmol C m−2 d−1) along this section from the Omani coast to the central Arabian Sea was about equal to the spring bloom maximum (107±23 mmol C m−2 d−1) during the 1989 North Atlantic Bloom Experiment (NABE) and the equatorial, 1°N–1°S wave guide maximum (95±6 mmol C m−2 d−1) in the Pacific Ocean during the 1992 EqPac study. The 1995 SW Monsoon primary productivity was similar to the mean value observed in the same region in 1994 by the Arabesque Expedition (127±14 mmol C m−2 d−1) and in 1964 by the ANTON BRUUN Expedition (115±27 mmol C m−2 d−1). During the 1995 SW Monsoon, strong, narrow and meandering current filaments extended from the region of coastal upwelling to about 700 km offshore; these filaments had levels of biomass, primary productivity, chlorophyll-specific productivity and diatom abundance that were elevated relative to other locations during the SW Monsoon. The SW Monsoon was the most productive period, but SW Monsoon primary productivity values were lower than predicted because efficient grazing by mesozooplankton kept diatoms from accumulating the biomass necessary for achieving the high levels of primary productivity characteristic of other coastal upwelling regions. The high rates of chlorophyll-specific productivity (PoptB>10 mmol C mg Chl−1 d−1) observed in the 1995 SW Monsoon, together with the observed dust flux and iron concentrations, indicate that the Arabian Sea was more iron replete than the equatorial Pacific Ocean or the Southern Ocean.

Influence of Soil Moisture on the Asian and African Monsoons. Part I: Mean Monsoon and Daily Precipitation

Abstract: Soil moisture responds to precipitation variability but also affects precipitation through evaporation. This two-way interaction has often been referred to as a positive feedback, since the water added to the land surface during a precipitation event leads to increased evaporation, and this in turn can lead to further rainfall. Various numerical experiments have suggested that this feedback has a major influence on tropical climate variability from the synoptic to the interannual timescale. In the present study, ensembles of seasonal simulations (March–September) have been performed in order to investigate the sensitivity of the Asian and African monsoon rainfall to regional soil moisture anomalies. After a control experiment with free-running soil moisture, other ensembles have been performed in which the soil water content is strongly constrained over a limited area, either south Asia or Sudan–Sahel. Besides idealized simulations in which soil moisture is limited by the value at the wilting poi...

BOBMEX: The Bay of Bengal Monsoon Experiment

Abstract: The first observational experiment under the Indian Climate Research Programme, called the Bay of Bengal Monsoon Experiment (BOBMEX), was carried out during July-August 1999. BOBMEX was aimed at measurements of important variables of the atmosphere, ocean, and their interface to gain deeper insight into some of the processes that govern the variability of organized convection over the bay. Simultaneous time series observations were carried out in the northern and southern Bay of Bengal from ships and moored buoys. About 80 scientists from 15 different institutions in India collaborated during BOBMEX to make observations in most-hostile conditions of the raging monsoon. In this paper, the objectives and the design of BOBMEX are described and some initial results presented. During the BOBMEX field phase there were several active spells of convection over the bay, separated by weak spells. Observation with high-resolution radiosondes, launched for the first time over the northern bay, showed that the magnitudes of the convective available potential energy (CA-PE) and the convective inhibition energy were comparable to those for the atmosphere over the west Pacific warm pool. CAPE decreased by 2-3 kJ kg(-1) following convection, and recovered in a time period of 1-2 days. The surface wind speed was generally higher than 8 m. s(-1). The thermohaline structure as well as its time evolution during the BOBMEX field phase were found to be different in the northern bay than in the southern bay. Over both the regions, the SST decreased during rain events and increased in cloud-free conditions. Over the season as a whole, the upper-layer salinity decreased for the north bay and increased for the south bay. The variation in SST during 1999 was found to be of smaller amplitude than in 1998. Further analysis of the surface fluxes and currents is expected to give insight into the nature of coupling.

Coherent Intraseasonal Oscillations of Ocean and Atmosphere during the Asian Summer Monsoon

Abstract: The space-time evolution of the ocean and atmosphere associated with 1998-2000 monsoon intraseasonal oscillations (ISO) in the Indian Ocean and west Pacific is studied using validated sea surface temperature (SST) and surface wind speed from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager, and satellite outgoing longwave radiation. Monsoon ISO consist of alternating episodes of active and suppressed atmospheric convection moving northward in the eastern Indian Ocean and the South China Sea. Negative/positive SST anomalies generated by fluctuations of net heat flux at the ocean surface move northward following regions of active/suppressed convection. Such coherent evolution of SST, surface heat flux and convection suggests that air-sea interaction might be important in monsoon ISO.