Role of the North Atlantic in Indian Monsoon Droughts (original) (raw)
Related papers
The Indian subcontinent witnessed a severe monsoon drought in 2002, which largely resulted from a major rainfall deficiency in the month of July. While moderate El Nino conditions prevailed during this period, the atmospheric convective activity was anomalously enhanced over northwest and north-central Pacific in the 10–20N latitude belt; and heavy rainfall occurred over this region in association with a series of northward moving tropical cyclones. Similar out-of-phase rainfall variations over the Indian region and the northwest (NW) Pacific have been observed during other instances of El Nino/Southern Oscillation (ENSO). The dynamical linkage corresponding to this out-of-phase rainfall variability is explored in this study by conducting a set of numerical experiments using an atmospheric general circulation model. The results from the model simulations lend credence to the role of the tropical Pacific sea surface temperature anomalies in forcing the out-of-phase precipitation variability over the NW Pacific and the Indian monsoon region. It is seen that the ENSO induced circulation response reveals an anomalous pattern comprising of alternating highs and lows which extend meridionally from the equatorial region into the sub-tropic and midlatitude regions of west-central Pacific. This meridional pattern is associated with an anomalous cyclonic circulation over NW Pacific, which is found to favor enhanced tropical cyclonic activity and intensified convection over the region. In turn, the intensified convection over NW Pacific induces subsidence and rainfall deficiency over the Indian landmass through anomalous east-west circulation in the 10–20N latitude belt. Based on the present findings, it is suggested that the convective activity over NW Pacific is an important component in mediating the ENSO-monsoon teleconnection dynamics.
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.
Atlantic forced component of the Indian monsoon interannual variability
Geophysical Research Letters, 2008
1] The Indian monsoon interannual variability is modulated by the El Niño Southern Oscillation (ENSO), with a drier than normal monsoon season usually preceding peak El Niño conditions, and vice versa for La Niña phase. Pacific sea surface temperature (SST) anomalies, however, are not the only player. Building upon our recent discovery that atmospheric teleconnections between the tropical Atlantic and the Indian basin contributed to the weakening of the ENSO-monsoon anticorrelation during the '80s and '90s, we investigate the role of south equatorial Atlantic SSTs in forcing the Indian monsoon rainfall (IMR). Using two observational data sets and two ensembles of simulations we show that the residual in the IMR time series for observed and modeled data, obtained by subtracting the ENSO-forced component of the IMR that is linearly related to the NINO34 index, is significantly correlated with south equatorial Atlantic SSTs. Our results have important implications for seasonal forecast efforts.
Unusual circulation pattern during Indian summer monsoon failure in July 2002 and June 2009
Natural Hazards, 2012
The circulation patterns over the Indian Ocean and the surrounding continents have been studied during June 2009 and July 2002 to explain the failure of Indian summer monsoon (ISM) rainfall. This study presents evidences that the failure of the ISM during these 2 months was probably due to the development of cyclonic circulation anomaly over the Western Asia and anticyclonic circulation anomalies downstream of Eastern Asia. These circulation anomalies were associated with the equatorward advection of cold air up to 10°N. This may be due to the equatorward intrusion of midlatitude Rossby waves. We hypothesize that the intrusion of midlatitude Rossby wave is responsible for breaking the east-west circulation cell over the Indian region into two cells and weakening it. The weak east-west cell reduces the strength of the easterly wind field usually present over the monsoonal region, thus reducing the cross-equatorial moisture transport into the Indian subcontinent and decreasing monsoon rainfall.
Climate dynamics, 2007
The Indian subcontinent witnessed a severe monsoon drought in 2002, which largely resulted from a major rainfall deficiency in the month of July. While moderate El Nino conditions prevailed during this period, the atmospheric convective activity was anomalously enhanced over northwest and north-central Pacific in the 10-20°N latitude belt; and heavy rainfall occurred over this region in association with a series of northward moving tropical cyclones. Similar out-of-phase rainfall variations over the Indian region and the northwest (NW) Pacific have been observed during other instances of El Nino/Southern Oscillation (ENSO). The dynamical linkage corresponding to this out-of-phase rainfall variability is explored in this study by conducting a set of numerical experiments using an atmospheric general circulation model. The results from the model simulations lend credence to the role of the tropical Pacific sea surface temperature anomalies in forcing the out-of-phase precipitation variability over the NW Pacific and the Indian monsoon region. It is seen that the ENSO induced circulation response reveals an anomalous pattern comprising of alternating highs and lows which extend meridionally from the equatorial region into the sub-tropic and midlatitude regions of west-central Pacific. This meridional pattern is associated with an anomalous cyclonic circulation over NW Pacific, which is found to favor enhanced tropical cyclonic activity and intensified convection over the region. In turn, the intensified convection over NW Pacific induces subsidence and rainfall deficiency over the Indian landmass through anomalous east-west circulation in the 10-20°N latitude belt. Based on the present findings, it is suggested that the convective activity over NW Pacific is an important component in mediating the ENSO-monsoon teleconnection dynamics.
La Niña's Diminishing Fingerprint on the Central Indian Summer Monsoon
Geophysical Research Letters, 2020
La Niña years tend to provide increased Indian summer monsoon (ISM) rainfall. However, observations show 6-8% reduction in ISM rainfall during post-1980 La Niñas relative to pre-1980. Using a suite of atmospheric general circulation model experiments, we replicate this observed phenomenon and attribute it to a combination of weakening La Niña events themselves plus strongly warming tropical Indian Ocean. We demonstrate that half of the ISM rainfall reduction during post-1980 La Niñas can be attributed to changes in the spatial pattern and intensity of La Niña within the tropical Pacific Ocean. Warmer eastern-equatorial Pacific Ocean temperatures during post-1980 La Niñas weaken the Walker circulation, resulting in large-scale anomalous subsidence over the Indian subcontinent, thereby inhibiting the deep convection that drives ISM rainfall. Further, we demonstrate the declining central ISM rainfall during La Niña years with increasing tropical Indian Ocean warming, which has several serious concerns for regional water resources and stability. Plain Language Summary Historically, wetter conditions of Indian summer monsoon (ISM) during June to September of La Niña years are important for water resources in particular groundwater recharge. Observations from recent decades, however, show a reduction of 6-8% in ISM rainfall during post-1980 La Niñas relative to pre-1980 La Niñas, which is a serious concern for regional water resources and stability particularly if the trend continues. Using a suite of atmospheric model experiments, we replicate this observed phenomenon and attribute it to weakening La Niña events combined with a strongly warming tropical Indian Ocean. Model simulations indicate that 50% reduction in ISM rainfall during post-1980 La Niñas can be attributed to changes in the spatial pattern and intensity within the tropical Pacific Ocean. The tropical Pacific east-west atmospheric circulation pattern (Walker circulation) is crucial for deep convection over the South Asian region. We show that warmer eastern equatorial Pacific Ocean temperatures during post-1980 La Niñas weaken the Walker circulation, resulting in inhibition of deep convection over the Indian subcontinent, thereby reducing ISM rainfall. Furthermore, we demonstrate that ISM rainfall over central India during La Niña years is likely to decline with increasing tropical Indian Ocean warming, which has several important implications for agriculture and economy of the Indian subcontinent.
Drought in Central and Southwest Asia: La Nina, the warm pool, and Indian Ocean precipitation
Journal of Climate, 2002
Severe drought over the past three years (1998)(1999)(2000)(2001), in combination with the effects of protracted sociopolitical disruption, has led to widespread famine affecting over 60 million people in central and southwest (CSW) Asia. Here both a regional and a large-scale mode of climate variability are documented that, together, suggest a possible forcing mechanism for the drought. During the boreal cold season, an inverse relationship exists between precipitation anomalies in the eastern Indian Ocean and CSW Asia. Suppression of precipitation over CSW Asia is consistent with interaction between local synoptic storms and wave energy generated by enhanced tropical rainfall in the eastern Indian Ocean. This regional out-of-phase precipitation relationship is related to large-scale climate variability through a subset of El Niño-Southern Oscillation (ENSO) events characterized by an enhanced signal in the warm pool region of the western Pacific Ocean. Both the prolonged duration of the 1998-2001 cold phase ENSO (La Niña) event and unusually warm ocean waters in the western Pacific appear to contribute to the severity of the drought.
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.
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.