Unusual circulation pattern during Indian summer monsoon failure in July 2002 and June 2009 (original) (raw)
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Atmospheric Research
The seasonal summer monsoon rainfall over India has substantially depended on the synopticscale systems such as monsoon lows and depressions. India has received above-average rainfall during the 2020 summer monsoon season. Total 12 Low Pressure Areas (LPAs) formed in the north Indian Ocean during summer 2020 (in JJAS season). The significance of this monsoon season is that August 2020 received the highest all-India rainfall in the past 44 years since 1976. This is accompanied by around 50% of the total seasonal LPAs formed in August 2020, none of which intensified into a monsoon depression (MDs). This study attempts to understand the characteristic features of monsoon rainfall during August 2020 and explore the plausible mechanisms behind the LPAs not intensifying/concentrating as MDs. It is noted that the anomalous warming over the Northern Parts of the Arabian Sea (NPAS) resulted in increased convection over this region in August 2020, as a result, strong convergence of low-level wind is observed over NPAS region. In addition to this convergence, strong northwesterly winds emanating from central Asia merged with the enhanced cross-equatorial monsoon flow. However, this strong flow over the Arabian Sea sheared/dissociated into two branches: one extending up to northwest (NW) India along the monsoon trough, another one diverging into an anticyclone over the south BOB (SBOB), which reduced the horizontal shear there (Barotropic Instability). This anticyclone strength over the SBOB and its westward shift is determined by the western north pacific (WNP) anticyclone. Our analysis suggests that due to the poor barotropic instability over the head BOB, LPAs could not develop into MDs. Additionally, upper level (200 hPa) barotropic Rossby wave in August 2020 remains stationary over South Central Asia and retrogressed with a northeast to southwest orientation. It determined the path of movement of the low-level disturbance beneath and affected the all-India rainfall by virtue of enhanced rainfall over NW & Western Ghats (WG) regions. The interplay of the barotropic Rossby wave alongside an anticyclone over the WNP accompanied by local conditions caused the above normal rainfall over India in August 2020, even though there are adverse dynamical conditions. We have also verified these mechanisms in Community Earth System Model Large Ensemble (CESM-LE) model simulations, Analysis shows that model has a limited skill to simulate the changes in the monsoon rainfall and associated circulation and failed to capture the mid-latitude circulation impact on ISM rainfall.
Results from a 20-yr simulation of a high-resolution AGCM forced with climatological SST, along with simplified model experiments and supplementary data diagnostics, are used to investigate internal feedbacks arising from monsoon–midlatitude interactions during droughts in the Indian summer monsoon. The AGCM simulation not only shows a fairly realistic mean monsoon rainfall distribution and large-scale circulation features but also exhibits remarkable interannual variations of precipitation over the subcontinent, with the 20-yr run showing incidence of four ‘‘monsoon droughts.’’ The present findings indicate that the internally forced droughts in the AGCM emanate largely from prolonged ‘‘monsoon breaks’’ that occur on subseasonal time scales and involve dynamical feedbacks between monsoon convection and extratropical circulation anomalies. In this feedback, the suppressed monsoon convection is shown to induce Rossby wave dispersion in the summertime subtropical westerlies and to set up an anomalous quasi-stationary circulation pattern extending across continental Eurasia in the middle and upper troposphere. This pattern is composed of a cyclonic anomaly over west central Asia and the Indo- Pakistan region, a meridionally deep anticyclonic anomaly over East Asia (;1008E), and a cyclonic anomaly over the Far East. The results suggest that the anchoring of the west central Asia cyclonic anomaly by the stagnant ridge located downstream over East Asia induces anomalous cooling in the middle and upper troposphere through cold-air advection, which reduces the meridional thermal contrast over the subcontinent. Additionally, the intrusion of the dry extratropical winds into northwest India can decrease the convective instability, so that the suppressed convection can in turn weaken the monsoon flow. The sustenance of monsoon breaks through such monsoon–midlatitude feedbacks can generate droughtlike conditions over India.
Association of the Indian summer monsoon with the northern hemisphere mid-latitude circulation
International Journal of Climatology, 1997
The association between the mid±latitude circulation and rainfall over the Indian region on an intraseasonal time±scale is investigated by considering 11 years (1974±1984) of Northern Hemisphere 500 hPa geopotential heights and rainfall data for the Indian summer monsoon months June through to September. On the basis of extensive correlation analysis between the geopotential heights and rainfall, it is seen that three regions over the mid±latitudes, the Manchurian region, the Algerian region and the Caspian sea region show positive correlation with rainfall over India, with higher values north of 20 N latitude. Lead and lag correlations between the heights at the locations identi®ed above and rainfall over India reveals that some common element of low±frequency variability is in¯uencing the mid±latitude circulation and Indian rainfall. On the interannual scale the connections between the winter±time low±frequency patterns (the Paci®c/North Atlantic, the West Paci®c Oscillation, the North Atlantic Oscillation and the Eurasian) and Indian summer monsoon rainfall (ISMR) are investigated. Only the West Paci®c Oscillation pattern shows a signi®cant relationship with the ISMR. Further, the interannual and the decadal variability is examined by using the Northern Hemisphere zonal index data for the period 1900±1993. Results reveal that the decadal±scale variability of the ISMR and the circulation features of the Northern Hemisphere are connected.
Role of the North Atlantic in Indian Monsoon Droughts
arXiv: Atmospheric and Oceanic Physics, 2019
The forecast of Indian monsoon droughts has been predicated on the notion of a season-long rainfall deficit linked to warm anomalies in the equatorial Pacific. Here, we show that in fact nearly half of all droughts over the past century were sub-seasonal, and characterized by an abrupt decline in late-season rainfall. Furthermore, the potential driver of this class of droughts is a coherent cold anomaly in the North Atlantic Ocean. The vorticity forcing associated with this oceanic marker extends through the depth of the troposphere, and results in a wavetrain which curves towards the equator and extends to East-Asia. This upper-level response triggers an anomalous low-level anticyclonic circulation late in the season over India. This teleconnection from the midlatitudes offers an avenue for improved predictability of monsoon droughts.
Impact of convection over the equatorial trough on summer monsoon activity over India
International Journal of Remote Sensing, 2005
The cause for the disruption of rainfall (break in monsoon conditions) over the Indian subcontinent during the monsoon months, for the period, 1979 -1998, are investigated using the pentad rainfall data from the Global Precipitation Climatology Project (GPCP). Most (about 73%) of the break in monsoon (BM) events were associated with the convective activity (rainfall more than 30 mm/pentad) over the equatorial trough (ET) region. The association between these events and the convective activity over the western (WET) and eastern equatorial trough (EET) regions of the tropical Indian Ocean were further explored. These relationships were tested for different (deficit, normal and excess) monsoon conditions over the Indian subcontinent and the El Nino conditions in the Pacific Ocean.
Remotely and Regionally Forced Pre-Monsoon Signals Over Northern India and Neighbourhood
Quarterly Journal of the Royal Meteorological Society, 1999
A 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 centraleastern Pacific ocean, south Indian ocean and locally over northwest 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 NiiiolSouthern 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 northwest 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 coordinate. The amplitude of the pre-monsoon signals can be large (small) depending on whether the ENSO and non-ENS0 boundary forcings act in a mutually cooperative (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.
The authors investigate the life cycle of a strong subtropical stratospheric intrusion event and propose a hypothesis through which it might reduce the intensity of the Indian summer monsoon (ISM) rainfall (ISMR) after the monsoon onset during June 2014. The diagnostic analysis of ERA-Interim data revealed that stratospheric intrusion occurs in the region of the subtropical westerly jet (SWJ) as a result of Rossby wave breaking (RWB). The RWB event is associated with eddy shedding. These eddies transport extratropical stratospheric mass and energy fluxes downward and southward to north India (NI). As a result, the intrusion spreads dry, cold, and ozone-rich air deep into the troposphere (;500 hPa) over the NI. It enhanced the static stability and weakens the north-south upper-tropospheric temperature gradient. The intrusion of cold and dry air persisted for the entire June, which might have inhibited northward propagation of ISM convection and could be responsible for prolonged hiatus in northward phase propagation of the ISM after onset. The relation between stratospheric intrusion events and ISMR from long-term data (1979-2007) is also investigated. The analysis shows that the stronger negative anomalies of rainfall are associated with stratospheric intrusions during break spells. Thus, the study reveals that stratospheric intrusion is an important factor that may influence ISMR deficit.
The movement of cold fronts with associated westerly waves from west coast of South Africa (10°E: even from 40°W) to west coast of Australia (120°E) during south west monsoon season influences Indian summer monsoon rainfall significantly.Moderate/ deep cold fronts have been observed in southern hemisphere east of 30°E and north of 30° south/ 25° south, during normal/ excess Indian summermonsoon months. Feeble cold fronts, which are observed during deficient monsoonmonths, do not penetrate north of 30°south. Westerly waveshave been observed from 850 hPa to 200 hPa or even up to 150 hPa pressure heights. It is well known thatcold fronts are closely associated with low pressure systems, normally lying at the leading edge of high pressure systems. They tend to move towards the equator and eastward. It is also well known that low and medium clouds such as Cumulus (CU), Cumulonimbus (CB), Altocumulus (AC) and Altostratus (AS) are observed at the cold front. In the rear of a cold front AC and AS clouds are observed. Because of presence of high pressure area, in the rear of a cold front, strong pressure gradient is observed from South Indian Ocean to north of the equator. Moisture generatedby the low level westerly waves/ troughs, in South Indian Ocean, can be observed by presence of thick AS clouds in the rear of cold fronts. Moisture generated (cold air mass), below 8000 feet (base of AS clouds) is transported to Indian Seas by low level subtropicalanti cyclones (from 850 hPa onwards) located between 40°W to 120°E, through southeasterly trades.This has been confirmed by 850hPa relative humidity (RH) and winds anomalies, for 21 normal/ excess and 20 deficient monsoon months for 31years period. So, the region from 40°W-120°E and north of 30°S in southern hemisphere is most vital for Indian Summer Monsoon Rainfall.
Meridionally Extending Anomalous Wave Train over Asia During Breaks in the Indian Summer Monsoon
Earth Systems and Environment, 2019
Anomalous interactions between the Indian summer monsoon (ISM) circulation and subtropical westerlies are known to trigger breaks in the ISM on subseasonal time-scales, characterised by a pattern of suppressed rainfall over central-north India, and enhanced rainfall over the foothills of the central–eastern Himalayas (CEH). An intriguing feature during ISM breaks is the formation of a mid-tropospheric cyclonic circulation anomaly extending over the subtropical and mid-latitude areas of the Asian continent. This study investigates the mechanism of the aforesaid Asian continental mid-tropospheric cyclonic circulation (ACMCC) anomaly using observations and simplified model experiments. The results of our study indicate that the ACMCC during ISM breaks is part of a larger meridional wave train comprising of alternating anticyclonic and cyclonic anomalies that extend poleward from the monsoon region to the Arctic. A lead–lag analysis of mid-tropospheric circulation anomalies suggests tha...