Summer Monsoon Activity Research Articles

The 30–60 day intraseasonal oscillation over the western North Pacific Ocean and its impacts on summer flooding in China during 1998

Eastern China experienced a series of severe floods during the summer of 1998. These floods are shown to be consistent with the propagation and activity of the 30–60 day intraseasonal oscillation (ISO) over the western North Pacific (WNP), where the monsoon trough and the subtropical anti‐cyclone appear as an anti‐clockwise propagation with the enhanced and suppressed convective anomalies in a 30–60 day period. The Meiyu circulation pattern in the lower atmosphere successively dominates the WNP, which results in southern China and the Yangtze River valley suffering much more rainfall and floods than normal. There are remarkable signals of 30–60‐day ISO in convective anomalies over the Bay of Bengal and east of Philippine Sea. During the cycle of the 30–60 day ISO, these convective anomalies move towards the South China Sea (SCS), where they affect the convective activity of the SCS summer monsoon, therefore maintaining the cycle of 30–60 day ISO over the WNP.

Causes and consequences of long‐term climatic variability on the Australian continent

Summary 1. Ice‐volume forced glacial–interglacial cyclicity is the major cause of global climate variation within the late Quaternary period. Within the Australian region, this variation is expressed predominantly as oscillations in moisture availability. Glacial periods were substantially drier than today with restricted distribution of mesic plant communities, shallow or ephemeral water bodies and extensive aeolian dune activity.2. Superimposed on this cyclicity in Australia is a trend towards drier and/or more variable climates within the last 350 000 years. This trend may have been initiated by changes in atmospheric and ocean circulation resulting from Australia's continued movement into the Southeast Asian region and involving the onset or intensification of the El Niño‐Southern Oscillation system and a reduction in summer monsoon activity.3. Increased biomass burning, stemming originally from increased climatic variability and later enhanced by activities of indigenous people, resulted in a more open and sclerophyllous vegetation, increased salinity and a further reduction in water availability.4. Past records combined with recent observations suggest that the degree of environmental variability will increase and the drying trend will be enhanced in the foreseeable future, regardless of the extent or nature of human intervention.

A Study on Relationship between Freezingthawing Processes of the Qinghai‐Xizang Plateau and the Atmospheric Circulation Over East Asia

AbstractThe seasonal freezing‐thawing process on the Qinghai‐Xizang Plateau is analyzed using the data of maximum depth of frozen soil from 64 stations, precipitation observations from 160 stations in China, and the NCAR/NCEP analysis. We have found that the seasonal freezing‐thawing process on the Qinghai‐Xizang Plateau not only affects the circulations above the plateau but also has impact on the circulation over East Asia. During the years when the frozen soil is shallower (deeper) compared to normal time, the South Asian high in July is stronger (weaker) and is located far to the west (east), the Indian cyclone is stronger (weaker), the subtropics high of the west Pacific is weaker (stronger) and is located far to the east (west), and the easterly jet stream over the southern plateau is stronger (weaker). We also found the correlation between freezing‐thawing processes and the summer monsoon activities. Three zones of significant correlation exist between the maximum depth of frozen soil and the summer (July) precipitation in China which has the same distribution as the summer mean rainfall patterns. Thus, the sensible and latent heating associated with the freezing‐thawing processes is an important factor that influences the climate over East Asia.

Sedimentary phosphorus record from the Oman margin: New evidence of high productivity during glacial periods

The northern region of the Arabian Sea is one of the biologically most fertile regions of the world oceans, with present productivity rates varying between 150 and 2500 mgC/m2 × day [Madhupratap et al., 1996]. This is related to the influence of the southwesterly summer monsoon which causes vigorous upwelling along the Oman margin. Upwelling ceases during northeasterly winter monsoon activity; productivity rates, however, remain relatively high (about 800 mgC/m2 × day), related to deep water mixing [Madhupratap et al., 1996]. The goal of this study is to verify if during the last glacial period, a period in which winter monsoon conditions prevailed, productivity rates were similarly high. With an analysis of phosphorus phases, stable nitrogen isotopes, organic matter content, and bulk mineralogy of the upper 10 m of the cores of ODP Hole 724C (corresponding to the last 140,000 years, sample resolution is ∼5 kyr), we provide new evidence of high productivity during this last glacial period (marine isotopic stages 2, 3, and 4). This was probably related to the combined effect of (1) increased eolian input of iron‐containing dust due to dryness on the adjacent continent and stronger winter monsoon, and (2) regeneration and diffusion of dissolved phosphorus from the sediments to the water column due to variations in the position and intensity of the Oxygen Minimum Zone. These findings suggest that there is no one‐to‐one relationship between summer monsoon activity and productivity, which emerges to be a quasi‐persistent phenomenon across glacial and interglacial stages.

Reconstruction of late Quaternary monsoon oscillations based on clay mineral proxies using sediment cores from the western margin of India

In order to reconstruct the variations in the intensity of summer monsoon precipitation during the late Quaternary, two sediment cores from the southwestern continental margin of India were studied for their clay mineral composition and grain size parameters. Kaolinite, illite and gibbsite were dominant in a core off Cochin. Illite with increasing kaolinite content towards the core top was found to dominate in a core from the continental slope off Goa. Major sources of clays at both the core sites were from the hinterland rocks and soils. Careful evaluations of several factors that could complicate the clay distribution in marine environment indicate that the clay mineral parameters can be used as proxies for the intensity of summer monsoon precipitation in the past. The relatively low values of humidity proxies (kaolinite content, K/C and K/I ratios) and higher illite crystallinity with significant variations indicate that the summer monsoons in general were weaker during the late glaciation, with distinct events of intensification at ∼28 000 and 22 000 yr BP. The last deglaciation was characterised by an increased terrigenous input with high values of humidity proxies during 15 700–14 800 cal yr BP, indicating an early strengthening of summer monsoon activity in the region. The most remarkable increase in kaolinite content, K/C and K/I ratios associated with much reduced values of chlorite and illite and illite crystallinity, however, occur between 8800 and 6400 cal yr BP. It appears that the Holocene precipitation maxima occurred after 9000 cal yr BP and lagged behind the precessional forcing. The late Holocene witnessed reduced rainfall activity and resultant decrease in hydrolysis starting at 5600 cal yr BP.

Storminess control over African dust input to the Moroccan Atlantic margin (NW Africa) at the time of maxima boreal summer insolation: a record of the last 220 kyr

Precessional forcing of dust flux from Northwest Africa to the Atlantic during the last 220 kyr is recognised through changes in physical and chemical sediment properties (sediment colour reflectance, magnetic susceptibility, X-ray fluorescence element intensity) in a core from the Moroccan margin. Sediment colour reflectance, magnetic susceptibility, and Ca and Fe element intensity changes are thought to be driven by dust input from North Africa. Spectral analysis of the sediment properties records displays a dominant periodicity in the 1/23 kyr−1 frequency band that is associated with the Earth’s orbital precession. Peaks in terrigenous supply match precessional minima. This suggests a close link between maxima of boreal summer solar radiation, monsoon activity, and dust generation. Enhanced precession-driven solar radiation would have caused increased seasonal temperature contrasts, which amplified atmospheric turbulence and stimulated storminess. Such a scenario is similar to today’s summer conditions, when frequent storms cause distinct dust plumes to form in the area and transport fine terrigenous material to the adjacent North Atlantic Ocean.

Analysis of freshwater flux climatology over the Indian Ocean using the HOAPS data

Estimation of freshwater flux over a certain region is very difficult, as it needs both accurate evaporation and precipitation rates over the study area. Over the Indian Ocean (IO), the precipitation estimates are totally lacking or unknown due to the lack of in-situ measurements. In the present study, we examine the climatology and the annual cycle of the freshwater flux over the IO using the newly available Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite data (HOAPS). This data set has been derived from passive microwave and infrared satellite data utilising the data from the Special Sensor Microwave/Imager (SSM/I) and the Advanced Very High Resolution Radiometer (AVHRR). We have compared the freshwater flux estimates from HOAPS data set with the earlier estimates to see how they differ from the older estimates. The evaporation rates from HOAPS are significantly different from the earlier estimates. The annual cycle of freshwater flux shows that the Bay of Bengal (BB) is a moisture convergence region most of the year as compared to the Arabian Sea (AS), which is a moisture flux divergence region. Further, the build-up of moisture in the southern Indian Ocean (SIO) is also clearly seen from March to June, indicating its major role in the summer monsoon activity over the Indian subcontinent. An analysis of monthly mean freshwater flux indicates that a small part of the southwest coast of India and southern parts of the BB region receives freshwater in April almost 2 months ahead of the onset of the summer monsoon.

Environmental variability within the Chinese desert-loess transition zone over the last 20000 years

The desert/loess transition zone in northern China is sensitive to climate variability, which is controlled mainly by the relative strengths of the East Asian summer and winter monsoons. Sandy loess layers found in the Loess Plateau and palaeosol sequences found in the sandy desert demonstrate latitudinal shifts of the southern desert margin over the last 20000 years. Stratigraphic investigations together with radiocarbon and some thermoluminescence dating, show that during the last glacial maximum the desert margin was at its most southerly position (38°N). During the early Holocene, it moved northward about three degrees in latitude (41°N). At present, the desert margin is again close to its most southerly position (38°21 N), yet the northern boundary position of modern summer monsoon activity is placed at 41°45 N, which is close to the estimated Holocene Optimum desert margin (41°N). This situation cannot be explained from natural climate models. Hence, an external driving mechanism needs to be considered, and the most obvious one is that caused by human activity on the natural environment over the last 3000 years. Historical evidence tends to reinforce this consideration.

Strong/ weak summer monsoon activity over the South China Sea and atmospheric intraseasonal oscillation

The circulation pattern corresponding to the strong / weak summer monsoon in the South China Sea (SCS) region and the associated characteristics of the abnormal rainfall in Eastern China have been studied by using the NECP reanalysis data and precipitation data in China. The results show that the climate variations in China caused by the strong / weak summer monsoon are completely different (even in opposite phase). The analyses of atmospheric intraseasonal oscillation (ISO) activity showed that the atmospheric ISO at 850 hPa near the SCS region is strong (weak) corresponding to the strong (weak) SCS summer monsoon. And the analyses of the circulation pattern of the atmospheric ISO showed that the strong / weak SCS summer monsoon circulation (200 hPa and 850 hPa) result mainly from abnormal atmospheric ISO.

Monsoonal variability in the northeastern Arabian Sea during the past 5000 years: geochemical evidence from laminated sediments

A high-resolution laminated sediment record from the oxygen-minimum zone in the northeastern Arabian Sea off Pakistan was studied to reconstruct climatic variations. To infer the monsoon-driven “moisture history” in the northeastern Arabian Sea during the past 5000 years, we have used three types of independant proxies: (1) varve thickness, (2) stable isotope ratios of planktic foraminifera; and (3) inorganic geochemical composition. On the basis of these proxies we have subdivided the sedimentary record into five intervals. The first time interval from 5000 to 3900 yr BP (Period I) is dominated by summer-monsoon activity. Deduced from Ti/Al, Zr/Al, K/Al and Sr/Ca ratios and oxygen-isotope data of planktonic foraminifera slightly enhanced winter-monsoon conditions can be inferred for the period from 3900 to about 3000 yr BP (Period II). Maximum precipitation during enhanced winter-monsoon activity at about 3100 yr BP was followed by an onset of aridification at 3000 yr BP which continued until about 2000–2200 yr BP (Period III). The textural, mineralogical and geochemical data suggest that this aridification was a gradual, century-scale process rather than an abrupt event. Comparison to late Holocene records in NW- and SW-India, the Near East and tropical Africa supports our interpretation that Period III from 3000 to 2000 yr BP is characterized by a gradual change in the atmospherical moisture budget over much of this area. Reduced monsoonal rainfall may be linked to variations of Eurasian snow cover. As deduced from varve thickness and element to Al-ratios, Period IV from 2000 to about 1500 yr BP is characterized by a summer-monsoon dominated sequence with higher precipitation than during Period III. The δ 18O record shows an abrupt shift at 2000 yr BP coinciding with an increase of varve thickness which suggests a cooling of surface waters in the northern Arabian Sea. Present-day conditions in the Makran area were established at 1500 yr BP (Period V). Based on inorganic geochemical data and meteorological records at several stations in the Makran we conclude that Period V is dominated by precipitation during the winter season. Here, declined rainfall associated with the summer-monsoon may be related to a less northward position of the intertropical convergence zone (ITCZ) during summer and a stronger influence of winter-monsoon linked phenomenona.

Magnetostratigraphy of Late Tertiary sediments from the Chinese Loess Plateau and its paleoclimatic significance

The Late Tertiary red clay/bed sediments underlying the Quaternary loess-paleosol in the Chinese Loess Plateau possesses high-resolution paleoclimatic changes related to the uplift of the Tibetan Plateau. Magnetostratigraphy and susceptibility measurements are discussed in this paper. The paleomagnetic results show that the red clay/bed began to accumulate at about 8.1 Ma, which represents the oldest Late Tertiary deposit continuously in the central Loess Plateau. The magnetic susceptibility curves show stepwise increases since the initiation of red clay, superimposed on several peaks. Moreover, the magnetic susceptibility increased abruptly since 3.8 Ma, which probably indicates the inception of the modern East Asia monsoon system. Between 3.8 and 2.6 Ma, the stepwise increases of susceptibility may imply progressively intensified East Asia summer monsoon activity related with the stepwise uplift process of the Tibetan Plateau.

Simulation of Indian summer monsoon rainfall and its intraseasonal variability in the NCAR climate system model

The broad climatological features associated with the Asian monsoon circulation, including its mean state and intraseasonal and interannual variability over the Indian subcontinent as simulated in the National Center for Atmospheric Research (NCAR) global coupled climate system model (CSM) in its control reference experiment, are presented in this paper. The CSM reproduces the seasonal cycle as well as basic observed patterns of key climatic parameters reasonably well in spite of some limitations in simulation of the monsoon rainfall. However, while the seasonality in rainfall over the region is simulated well, the simulated area-averaged monsoon rainfall is underestimated to only about 60% of the observed rainfall. The centers of maxima in simulated monsoon rainfall are slightly displaced southward as compared to the climatological patterns. The cross-equatorial flow in simulated surface wind patterns during summer is also stronger than observed with an easterly bias. The transient experiment with a 1% per year compound increase in CO2 with CSM suggests an annual mean area-averaged surface warming of about 1.73 °C over the region at the time of CO2 doubling. This warming is more pronounced in winter than during the monsoon season. A net increase in area-averaged monsoon rainfall of about 1.4 mm day–1, largely due to increased moisture convergence and associated convective activity over the land, is obtained. The enhanced intraseasonal variability in the monsoon rainfall in a warmer atmosphere is confined to the early part of the monsoon season which suggests the possibility of the date of onset of summer monsoon over India becoming more variable in future. The enhanced interannual and intraseasonal variability in the summer monsoon activity over India could also contribute to more intense rainfall spells over the land regions of the Indian subcontinent, thus increasing the probability of extreme rainfall events in a warmer atmosphere.

Low frequency modes and their link with summer monsoon activity described by wavelet analysis throughout India

The purpose of this paper is to propose a method and its application to attempt investigating temporal-frequency characteristics of low frequency modes throughout India. This diagnostic method combines wavelet analysis (WT) interlaced by a factor D and empirical orthogonal function (EOF). It is shown that the stability related to WT is ensured by the adequate choice of factor D and the sampling frequency. Thus, the reconstruction of a time series from its wavelet coefficients becomes possible (i.e., "wavelet synthesis," which can constitute a narrow bandpass). This method has been applied to daily pressure during summer monsoon activity for the year 1977. The results of analysis indicate a link between summer monsoon activity and both a 10-20-day and 25-50-day band. This link is expressed by the simultaneous occurrence of both modes throughout India with a different amplitude from one region to another expressed by the following factors. During summer monsoon onset, it is the 10-20-day mode that is predominant, while during the break, both modes have nearly the same amplitude. These modes have a maximum activity situated nearly at 23/spl deg/N, 83/spl deg/E.

Floods and droughts in association with cold and warm ENSO events and related circulation features

For the Indian subcontinent. the occurrence of floods and droughts is closely linked with the summer monsoon activity. The phenomenon of El Nino-Southern Oscillation (ENSO) has been established to be one of the major teleconnections of Indian Summer Monsoon. Also the relationship between the circulation features and summer monsoon activity is well documented in the literature. The interaction of F.NSO with monsoon system was known to the seasonal forecasters in India from the days of G. Walker. Northland (1953) summarising these results has remarked that ‘Monsoon has a prolonged influence on the global weather rather than global weather parameters influencing the monsoon’. 1990-94 was a prolonged period of warm ENSO producing weather anomalies in different regions of the globe. Yet during the same period all India rainfall was very close to normal and in fact. 1994 was a year of abundant rainfall for India. The aim of the study is to examine some of these features more critically.
 
 It is observed that ENSO has a modifying effect on the regional scale circulation pattern and possible interactions and/or phase-Locking with the planetary scale circulation pattern. which results into the occurrence or non-occurrence of an extreme event. Also, a qualitative analysis is carried for a period 1960-90 to assess how far the mid-season rainfall deficiency is made up at the end of the season. It is observed that even during drought years, the mid-season rainfall deficiency is made up at the end of the season for a considerable percentage of the total number of cases.

Medium Range Prediction of Summer Monsoon Activities over India vis∓a∓ vis Their Correspondence with the Observational Features

A detailed examination on various aspects of the southwest monsoon over the Indian subcontinent using analyses and forecasts of the NCMRWF global data assimilation and forecast system for the years 1994 and 1996 is carried out in this study. Objective procedures developed by Ramesh et al. (1996) based on the monsoon 1995 data are employed in the present work. It is found that all the deterministic features of the summer monsoon viz., the onset, its advancement, stagnation / revival and the withdrawal, can reasonably by prognosticated after employing the objective methodologies and monitoring daily variations of certain derived quantities from the large scale analyses and forecasts. Further, a good correspondence of the observed large scale / synoptic scale circulation features and the observed rainfall etc. with the deterministic characteristics of the summer monsoon show a good prospect for real time prognosis of the important summer monsoon activities over the Indian subcontinent.

Remotely and Regionally Forced Pre‐Monsoon Signals Over Northern India and Neighbourhood

AbstractA detailed diagnostic examination of the National Centre for Environmental PredictiodNational Centre for Atmospheric Research Reanalysis data supplemented by a theoretical study is undertaken with the goal of investigating the dynamics of pre‐monsoon stationary wave anomalies occurring over northern India and adjoining region prior to weak Indian summer monsoons. It is seen from the analysis that the rotational wind anomalies preceding weak monsoon years are primarily forced by tropical divergent wind anomalies located over the tropical central‐eastern Pacific ocean, south Indian ocean and locally over north‐west India. the divergent wind anomalies over the central‐eastern Pacific ocean can be linked to anomalous changes in the tropical Pacific sea surface temperature arising due to El Nioñ/Southern Oscillation (ENSO). the upper level divergent wind forcing overlying the south Indian ocean seems to be locally connected to the warm sea surface temperature anomalies over that region.Results of simple experiments using a forced divergent barotropic vorticity model establish that the maintenance of rotational wind anomalies over north India prior to weak monsoons is attributable to the existence of anomalous divergent wind forcing over the aforementioned regions in the tropics. A normal mode experiment shows that the climatological basic flow alone is not sufficient to support these pre‐monsoon vorticity perturbations. Furthermore, calculation of vorticity generation terms suggests that the divergent wind advection and Rossby source terms make significant contributions to the production of rotational anomalies over north India during the pre‐monsoon months. Based on the above results, it is conjectured that energy injection into the subtropics via Rossby wave dispersion could be a foremost dynamical mechanism which efficiently sustains rotational wind perturbations over north India and neighbourhood. an assessment of the relative importance of the ENSO and non‐ENS0 divergent forcings, shows that the latter tend to exert a larger influence on the pre‐monsoon signals over north‐west India.The implications of using the pre‐monsoon signals for long range forecasting of Indian summer monsoon activity are determined by two factors viz., (i) the robustness and intensity of the pre‐monsoon signals and (ii) the persistence of the signals during the following months. the first factor in turn depends on the manner in which the different forcing anomalies co‐ordinate. the amplitude of the pre‐monsoon signals can be large (small) depending on whether the ENSO and non‐ENSO boundary forcings act in a mutually co‐operative (competitive) manner. the second factor, which is related to the maintenance of anomalous quasi‐stationary patterns during the following months, depends crucially on how the different boundary forcings evolve during the summer monsoon season.

Dynamics of upper tropospheric stationary wave anomalies induced by ENSO during the northern summer: A GCM study

Ensemble seasonal integrations are carried out with the COLA GCM, with a view to understand the dynamical connection between warm SST anomalies in the equatorial central-eastern Pacific Ocean and the upper level stationary wave anomalies seen during drought years over the Indian summer monsoon region. In addition, experiments with and without orography are performed in order to examine the role of the Himalayas in modulating the El Niño induced stationary wave anomalies over the summer monsoon region.The GCM simulations show a statistically significant weakening of the summer monsoon activity over India in response to the SST forcing in the equatorial Pacific Ocean. This weakening of the summer monsoon appears to be largely related to modifications of the local Hadley and Walker cells over the summer monsoon region. In addition, it is seen that the anomalous ENSO divergent forcing over the tropical Pacific Ocean can act as a potential source for Rossby wave dispersion. Here one finds the possibility of meridionally propagating Rossby waves, which emanate from the ENSO forcing region, to interact with the subtropical westerlies and generate anomalous highs and lows in the subtropics and extratropics. The quasi-stationary perturbations seen over west Asia, Pakistan and northwest India during drought years, seem to be generated by the above mechanism. An alternate mechanism that could be important for the persistence of the quasi-stationary perturbations seems to be based on the dynamic excitation of middle latitude normal modes which can extract energy from the zonally varying unstable basic flow.It is seen from the GCM simulations, that the Himalayan orography plays a crucial role in anchoring the El Niño induced extratropical westerly troughs far to the west in the high latitude belt. In the absence of orography it is seen that the ENSO induced extra-tropical cyclonic anomalies tend to intrude southward into the monsoon region thereby destroying the regional scale circulations completely. Another effect due to the Himalayas is to generate lee waves on the eastern side of the topographic barrier which encircle the globe in the subtropics and midlatitudes.

アンサンブル気候実験で再現される東アジアの冷夏・暑夏

An ensemble of three 40-year parallel simulations was performed using a T42 AGCM version of the Japan Meteorological Agency global model to answer the question why extraordinary cool anti hot summers in East Asia, especially Japan and Korea, tend to occur very frequently in recent years from the late 1970s to the early 1990s. Three independent long-term integrations from January 1955 to December 1994 were forced by the same SST boundary condition observed on the global scale. Our AGCM simulations employing prescribed observed SSTs were successful in reproducing extratropical circulation anomalies that bring about the decadal-scale amplitude modulation of interannual variations of summer mean temperatures in the vicinity of Japan. During the period from the beginning of 1980s to the early 1990s, the interannual variability of the east-west gradient of summertime SST anomalies between the South China Sea and the tropical western Pacific east of the Philippines became appreciably large, was accompanied by anomalous cumulus convection around the Philippines, and its phases coincided quite well with those of model-simulated lower-tropospheric geopotential height variations near Japan. The anomalous convective heating substantially affected summertime lower tropospheric circulation anomalies in East Asia through the dynamic process of the excitation of PJ teleconnection pattern (Nitta, 1987). The anomalous SST forcing from the tropics is crucially responsible for the frequent occurrence of extreme cool and hot summers in Japan and Korea from the late 1970s to the early 1990s. The presence of strong east-west gradient of SST anomalies across the Philippines is primarily attributed to the significant coupling of weak (strong) South Asian summer monsoon and the warm (cold) episode of ENSO. The warm episodes that occurred during the period from the late 1970s to the early 1990s are appreciably different from a typical model of El Nino event exemplified by Rasmusson and Carpenter (1982) in terms of seasonal evolution. It is anticipated that both unusually persistent ENSO signals from the preceding winter until summer and the associated South Asian summer monsoon activity strongly regulate the formation of the east-west SST gradient near the Philippines in boreal summer.

Evolution of the summer monsoon regime over the Loess Plateau of the last 150 ka

On the magnetic susceptibility data of 30 sections covering the Loess Plateau, contour maps of susceptibility are drawn out for 5 time intervals including the last second glaciation (150 ka B.P.), the last interglaciation (130 - 73 ka B.P.), the interstadial of the last glaciation (24 - 59 ka B.P.), the last glacial maximum (18 ka B.P.) and the Holocene optimum (9ka B.P.). The susceptibility value of the last glaciation maximum loess is regarded as the base value of pedogenic loess or paleosol. The susceptibility increment to the base indicates the summer monsoon activity and its strength. On this assumption and the susceptibility contour maps, the variation sequence of the summer regime are reconstructed for the last 150ka. The extent and the northern boundaries of the summer monsoon front activity are also estimated for every stage. It is suggested that the evolution history of the summer monsoon regimes revealed by the susceptibility contour maps not only provides information of position, strength and their configuration of the climate members influencing the summer monsoon, but also provides some climatic signals from the polar region and southern hemisphere.

North-Eastward propagating 25–50-day mode and its link with Indian summer monsoon activity

Spatio-temporal characteristics of the 25–50-day oscillations are investigated using the empirical orthogonal function (EOF) decomposition and spectral analysis with the Maximum Entropy Method (MEM). Daily pressure values over India during 1978 are used in this study. Power spectra of the temporal coefficients of the two leading EOFs show the existence of a low frequency oscillation with a period range 25–50-day over all India. An analysis using extended empirical orthogonal function (EEOF) indicates North-Eastward propagating 25–50-day mode. The analysis EOF has allowed to establish a relationship between the 25–50-day oscillation and the activity of the summer monsoon. The North-Eastward propagation of this mode is also confirmed by the simple EOF analysis.