Early Summer Monsoon Research Articles

Some aspects of an early summer monsoon onset over Andaman Sea and further progress towards the Indian mainland

This work addresses the mechanisms that leads to an early onset of monsoon over Andaman Sea but advances further rapidly (slowly) to the Indian mainland resulting in the early (delayed) onset over Kerala. The upper tropospheric temperature, production of kinetic energy (KE) and outgoing long wave radiation (OLR) from the month of May till onset over Kerala are analysed for two delayed onset years (1997, 1995) and two early onset years (2004, 1990). It is observed that the maximum temperature over Tibetan plateau (TP), an increase in the production of KE and strong equatorial convection in early May, is associated with early onset over Andaman Sea. However, when there is a lull in advance of monsoon after the early onset over Andaman Sea, shifting of the warm region south of TP, weak production of KE in the lower troposphere and convective region shifting to Western Pacific resulted in the delayed onset over Kerala in 1997 and 1995. During the early onset years viz. 2004 and 1990, the warm region moving westwards, high production of KE extending to mid troposphere and deep convection moving westwards in the north Indian Ocean (10–15°N) is noticed.

Aerosol and rainfall variability over the Indian monsoon region: distributions, trends and coupling

Abstract. Aerosol solar absorption over the Indian monsoon region has a potential role of modulating the monsoon circulation and rainfall distribution as suggested by recent studies based on model simulations. Prior to the onset of the monsoon, northern India is influenced by significant dust transport that constitutes the bulk of the regional aerosol loading over the Gangetic-Himalayan region. In this paper, a multi-sensor characterization of the increasing pre-monsoon aerosol loading over northern India, in terms of their spatial, temporal and vertical distribution is presented. Aerosol transport from the northwestern arid regions into the Indo-Gangetic Plains and over the foothills of the Himalayas is found to be vertically extended to elevated altitudes (up to 5 km) as observed from the space-borne lidar measurements (CALIPSO). In relation with the enhanced pre-monsoon aerosol loading and the associated solar absorption effects on tropospheric temperature anomalies, this paper investigates the monsoon rainfall variability over India in recent past decades from an observational viewpoint. It is found that the early summer monsoon rainfall over India is on the rise since 1950s, as indicated by historical rainfall data, with over 20% increase for the period 1950–2004. This large sustained increase in the early summer rainfall is led by the observed strengthening of the pre-monsoon tropospheric land-sea thermal gradient over the Indian monsoon region as indicated by microwave satellite measurements (MSU) of tropospheric temperatures from 1979–2007. Combined analysis of changes in tropospheric temperatures and summer monsoon rainfall in the past three decades, suggest a future possibility of an emerging rainfall pattern of a wetter monsoon over South Asia in early summer followed by a drier period.

Time-Space Characteristics of Seasonal and Interannual Variations of Atmospheric Water Balance over South Asia

The atmospheric water balance over dierent domains within the South Asian monsoon region has been studied using moisture convergence ðCÞ computed from JRA-25, ERA-40 and NCEP/NCAR reanalysis datasets, GPCP precipitation data ðPÞ and evaporation data ðEÞ as a residual of these two parameters. The seasonal clima- tology of P, C, and E for the selected regions shows generally large contribution of E to P. The inter-annual characteristics of P, C and E over selected key domains within the South Asian monsoon region have also been examined for both the early (June and July: JJ) and late summer (August and September: AS) monsoon periods from 1979 to 2000. The spatial and temporal characteristics of the hydrological cycle and the contribution of E and C to P are discussed in detail. One important aspect on the seasonal timescale is that from the dry regions in the northwest to the central and the wettest northeast regions, the monthly variations of E or C are large during the monsoon months specific to those regions. However, the interannual variability of P over each domain is not necessarily influenced by the same criteria like C or E, which influences the mean seasonal precipitation. It is also evident that the structure of variability for early (JJ) and late (AS) summer precipitation is dierent over the South Asian monsoon region. Over northwest India E is dominant on the seasonal timescale, but C contributes higher to interannual variability of P. On the other hand, over central India C is dominant during early summer (JJ) on the seasonal timescale, but E contributes higher to P variability on the interannual timescale, and during late summer (AS) E is dominant on the seasonal timescale, but C contributes higher to P variability on the inter- annual timescale. Over northeast India, C is dominant on the seasonal timescale, but E contributes higher to in- terannual variability of P. The importance of land-atmosphere interaction over each domain is discussed. The regionality in the mechanism of precipitation generation and its contribution to the India summer monsoon precipitation variability are also discussed in detail. The role of evaporation variability of precipitation is stronger over the Bay of Bengal sector and the role of convergence on the interannual variability of precipitation is stronger over the Arabian Sea sector.

Effects of irrigation and vegetation activity on early Indian summer monsoon variability

AbstractWe examined the effects of land cover change over the Indian subcontinent during pre‐monsoon season (March, April, and May—MAM) on early Indian summer monsoon (ISM) rainfall using observed Normalized Difference Vegetation Index (NDVI) and July precipitation for the period of 1982–2003. MAM NDVI anomalies have increased in the Indian subcontinent and the increases are significantly correlated with increases in the irrigated area, not preceding rainfall. July rainfall significantly decreased in central and southern India, and the decrease is statistically related to the increase in the preceding MAM NDVI anomalies. Decreased July surface temperature in the Indian subcontinent (an expected result of increased evapotranspiration due to irrigation and increased vegetation) leads to a reduced land–sea thermal contrast, which is one of the factors driving the monsoon, and therefore weakens the monsoon circulation. A weak early ISM appears to be at least partially a result of irrigation and the resultant increased vegetation and crop activity prior to the monsoon. Copyright © 2008 Royal Meteorological Society

Origin and transport of tropical cirrus clouds observed over Paramaribo, Suriname (5.8°N, 55.2°W)

The Intertropical Convergence Zone (ITCZ) passes twice a year over tropical Suriname, bringing two wet and two dry seasons. During a pilot study campaign in Suriname, cirrus clouds were observed with a mobile aerosol Raman lidar (MARL) and with balloon sondes containing a frost point hygrometer called Snow White, over the period October–November 2004. These observations are used to study the origin of cirrus clouds and the dynamical processes that determine their transport, using European Centre for Medium‐Range Weather Forecasts (ECMWF) operational analyses. The height of cirrus occurrence is in phase with the height of the cold point tropopause, with maximum heights occurring during Northern Hemisphere winter that are about 2 km above the minimum values in summertime. The occurrence of cirrus often corresponds with a northerly meridional wind flow (in a layer underneath the tropopause), also when the ITCZ lies to the south in the period January–May. ECMWF analyses point out that inertial instability flow, in the form of vertically stacked meridional circulation cells in the upper troposphere (UT), can explain the transport of these cirrus events. Also evident is that radiative cooling of a moist layer transported in the UT leads to a thermal wind in the form of an easterly/westerly jet associated with the southward/northward transport of moist air. An interactive play between the inertial instability and thermal wind mechanisms explains many of the observed features of cirrus occurrence over Suriname. The observed cirrus mostly originates from the ITCZ or from deep convective centers to the south that form during the early summer monsoon.

PDO, ENSO and the early summer monsoon rainfall over south China

Interdecadal variations in the early (May–June) summer monsoon rainfall over South China (SCMR) are found to be related to the ENSO (El Niño/Southern Oscillation) and the PDO (Pacific Decadal Oscillation). An interdecadal variation in SCMR can be identified, with more dry (wet) monsoon years during the periods of high (low) PDO index. Such variations are also related to ENSO in association with PDO. When ENSO and PDO are in phase, i.e. high PDO phase/El Niño events, or low PDO phase/La Niña events, the SCMR tends to be below or above normal respectively more often. But when the ENSO and PDO are out‐of‐phase, the SCMR has no wet or dry preference. Such relationships appear to be related to the intensity of the subtropical high determined by the superposition of the effects of ENSO and PDO.

Atmospheric circulation characteristics during some El-Nino years in relation to Indian summer monsoon rainfall

Based on observed rainfall data of India Meteorological Department (IMD), correlation coefficients (CCs) have been computed between Indian summer monsoon rainfall (ISMR) and sea surface temperature (SST) anomalies over different Nino regions and standardised pressure difference between Tahiti and Darwin. Significant positive CCs are found between the Southern Oscillation Index (SOI) in winter and subsequent June rainfall over India. Concurrent with and subsequent to Indian summer monsoon, SOI shows significant positive CC with the mean rainfall of July to September (JAS). Significant negative CCs are found between JAS mean rain and the concurrent and following SST anomalies over Nino-3.4 region. On the basis of these correlations, it is proposed that the entire period of summer monsoon from June to September could be divided into two sub-periods such as: early summer (June) and mid-late summer (July to September) monsoon for prediction of ISMR in the extended range. In order to examine the characteristics of atmospheric circulation during some El-Nino years, divergent flow at 200 hPa and omega field at 500 hPa based on NCEP/NCAR reanalysis have been studied in detail. Major significant southward shift of upper level divergent field from India are related to El-Nino and this shift may be responsible for causing droughts during several El-Nino years over India. Also vertical wind fields at 500 hPa show sinking motion over large parts of India and west Pacific and ascending motion over southern Indian Ocean, central and eastern Pacific during major drought years.

Lagged relationships between ENSO and the Asian Summer Monsoon in the CSIRO coupled model

The lagged relationship between the El Nino‐Southern Oscillation (ENSO) and Asian Summer Monsoon (ASM) variability is investigated using the Commonwealth Scientific and Industrial Research Organization (CSIRO) Mark3 coupled model. Composite analyses of 17 warm and 16 cold events from an extended unforced run reveal that the model captures the asymmetric structures of rainfall and SST anomalies over the tropical Indian Ocean during northern hemisphere spring which is one of the major precursory signals of anomalous ASM variability. As a remote forcing mechanism prior to the ASM onset, simulated anomalous convection over the northern Indian Ocean strongly influences the land surface hydrologic conditions over central and southwest Asia. Simulated land‐surface processes result in anomalous temperatures over the land, which in turn produce a change in the land‐ocean thermal contrast over the domain. The results are consistent with observed features, which show a delayed and indirect impact of ENSO on the ASM variability during the early summer monsoon period, but not during the rest of the monsoon season.

A modeling study of the effects of anomalous snow cover over the Tibetan Plateau upon the South Asian summer monsoon

The effect of anomalous snow cover over the Tibetan Plateau upon the South Asian summer monsoon is investigated by numerical simulations using the NCAR regional climate model (RegCM2) into which gravity wave drag has been introduced. The simulations adopt relatively realistic snow mass forcings based on Scanning Multi-channel Microwave Radiometer (SMMR) pentad snow depth data. The physical mechanism and spatial structure of the sensitivity of the South Asian early summer monsoon to snow cover anomaly over the Tibetan Plateau are revealed. The main results are summarized as follows. The heavier than normal snow cover over the Plateau can obviously reduce the shortwave radiation absorbed by surface through the albedo effect, which is compensated by weaker upward sensible heat flux associated with colder surface temperature, whereas the effects of snow melting and evaporation are relatively smaller. The anomalies of surface heat fluxes can last until June and become unobvious in July. The decrease of the Plateau surface temperature caused by heavier snow cover reaches its maximum value from late April to early May. The atmospheric cooling in the mid-upper troposphere over the Plateau and its surrounding areas is most obvious in May and can keep a fairly strong intensity in June. In contrast, there is warming to the south of the Plateau in the mid-lower troposphere from April to June with a maximum value in May. The heavier snow cover over the Plateau can reduce the intensity of the South Asian summer monsoon and rainfall to some extent, but this influence is only obvious in early summer and almost disappears in later stages.

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

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.

Palaeoclimates in central and south America since 18,000 BP based on pollen and lake-level records

The last 18 ka of palaeoclimates are discussed at 3 ka intervals based on palaeoenvironmental changes shown in pollen and lake level records from Central and South America. A total of 61 pollen records fulfill the requirements of continuity and chronology to be used in this synthesis: 8 records from tropical and subtropical lowlands in Central and South America; 7 records from the Central American Highlands; 26 records from the High Andes, north and south of the equator; 2 records from the southern subtropics; and 18 from the southern Andes and subantarctic islands. Palaeoclimatic interpretation of the pollen records is semiquantitative, based on bioclimatic parameters such as e.g. elevational temperature gradients or latitudinal precipitation gradients that can be related to vegetation zonation. Additional palaeoclimatic information is obtained from lake-level records, derived either from dated shorelines, or from the limnological history of lake sediment sections. Large-scale palaeoclimatic comparison of the proxy data records indicates that full- and late-glacial climates throughout Central and South America were cooler than today, by about 4 to 5°C, but that moisture patterns showed regional differentiation. Thus, in both the southern subtropics and high latitudes in southern Chile, conditions were substantially moister than today during full- and late-glacial times, whereas everywhere else conditions were far drier than today. In terms of palaeocirculation, this scenario can be interpreted as a poleward shift of the subtropical as well as the westerly circulation, a scenario that has been suggested by palaeoclimatic simulations from general circulation models (GCM). Another major palaeoclimatic aspect predicted by the GCM simulations is the suppressed early Holocene summer monsoon in the southern subtropics. Although proxy records are few from this zone, their palaeoenvironmental history during that time shows a period of aridity that would support the model. One of the most controversial aspects of South American palaeoclimates concerns the existence of a Younger Dryas equivalent, a cold period between 11 and 10 ka BP. Based on the existing data from South America, that show a great diversity of palaeoclimatic trends, the concept of an interval that is characterized by the same climatic signal as documented for the North Atlantic seems too simplistic. Moreover, at this point, only very few records have been analysed in sufficient detail to yield unambiguous climatic information for the interval in question. The principal outcome of this palaeoenvironmental and palaeoclimatic synthesis is the better understanding of the type and quality of data available at present, and the realization of regions and periods where much more and concerted effort is needed to address problems of intra- and interhemispheric palaeoclimatic correlations. Only then can questions concerning the mechanism of climatic change be tackled successfully.

The Structure and Vorticity Budget of an Early Summer Monsoon Trough (Mei-Yu) over Southeastern China and Japan

One of the most persistent rain-making events over East Asia is the development of an early summer monsoon trough (Mei-Yu) which extends from southeastern China to southern Japan. This work studies the structure and vorticity budget of a Mei-Yu system for the period 10–15 June 1975. Subjectively analyzed grid-point data are time composited with respect to the trough axis along three cross sections over southeastern China (western section), southern East China Sea (central section) and southern Japan (eastern section), respectively, during the mature and decaying stages of the trough. The results indicate that the structure of the eastern and central sections resembles a typical midlatitude baroclinic front with strong vertical tilt toward an upper level cold core and a strong horizontal temperature gradient. On the other band, the western section resembles a semitropical disturbance with an equivalent barotropic, warm core structure, a weak horizontal temperature gradient, and a rather strong horizontal wind shear in the lower troposphere. Cumulus convection activity south of the 850 mb trough is significant in all three sections and contributes substantially to the thermally direct secondary circulation, but the large-scale organizing mechanism differs from one section to another. In the eastern and central sections it is mainly due to differential vorticity advection while in the western section it is due to Ekman pumping (CISK). The generation of cyclonic vorticity is counteracted by cumulus damping in the eastern section and by boundary layer friction in the mountainous western section.