Indian monsoon

Was near coastal/marine, equatorial influence prevalent in North Indiasubcontinent 18.2 million years ago?
Ritesh Arya
168 Arath Bazaar, Kasauli, Himachal Pradesh
email: aryadrillers@gmail.com
Probably yes and this is what reinterpretation of the fossils and sedimentological data found from Kasauli Formation (dated to be Lower Miocene) and its equivalent in the Himalayas indicate. The sites from where fossils have been collected for the present study lie in Kasauli,Dharamsala, Udhampur, Jammu, Kalakot, Kargil, Alchi., Stok , Kangri range and Nyoma inthe northwestern Indian Himalayas. All these localities marked on the map and show an interesting structural pattern revealing a lot about the history of their genesis in geological time. Interestingly on ground all the fossil plant assemblage localities show great similarity in lithologies, fossils and type of preservations with each other. The most important being the taphonomic relationship between the fossil wood and the sandstone facies, which is quiet homogenous with other homotaxial localities or basins described above and show unmatched similarity in the environments of the deposition specially in the basins of Kasauli-Dagsahi, Dharamsala, Murree and Indus in Ladakh. Interesting observations made in the papers published earlier were reinterpreted based on the recent observations and fossil findings made by the author from differentlocalities along the Indus River in the high altitude, cold temperate climatic conditions of Ladakh, which indicate warmer and more humid climatic conditions at the time of deposition. Preliminary studies show that there was close similarity of the fossils of Garcinia, Gluta, Syzygium, Clinogene, Combretum and palms reported from Kasauli Formation with the extant taxa from coastal regions of Indo-Malayan including Borneo and Sumatra which indicate well hot humid equatorial climatic conditions of deposition withmajor representation of evergreen elements typical of near equatorial conditions and near shore coastal marine influence on otherwise terrestrial environment prevailed during thosetimes. Fossils of mega plants all along the Indo Malayan Indonesian islands establish land connections for their migration during those times. This fact on environmental regime (nearcoastal / marine transgression) at the time of deposition is further supported in Chitarwata Formation, west central Pakistan; Ladakh, North India Jammu, North India Dharamsala, Northwestern India; Kasauli, Northwest India Darjeeling,West Bengal, North Eastern Indiaand Bhutan. Based on the fossil findings of Mitrgyana from the Kasauli sediments elevation was also found to be less then 1300m in contrast to the present height of about 2000meters.This really meant that the Himalayas attained sufficient height to block the rain laden winds and thus the concept of monsoons was born. Presence of diverse assemblage of fossil flowers shows that seasonal changes had initiated during those times. It is very well documented fact that Subathu Formation enjoyed equatorial position during Lower Eocene times. From the above findings it can be interpreted that Kasauli Formation and its equivalent also enjoyed near coastal equatorial to subequatorial climatic conditions and latitudes may be around 5-10degrees north of equator at the time of deposition in contrast to 32 degrees. This implies that the major tectonic forces which were to give rise to the future Himalayas were not very active during those times . Northward dragging of the uplifted ztethyan basin (during Dagshai times) along with the Indian sub continent took place much later then 18.2 million years ago.

Sedimentary archives in the Himalayan foreland basin and Indus submarine fan provide the most detailed records of how changing monsoon strength may have affected erosion and the development of tectonic structures in the western Himalaya during the Neogene. Muscovite Ar-Ar ages show that fast exhumation of the Greater Himalaya was earlier in the west (20–35 Ma) than in the central Himalaya (10–25 Ma), probably driven by progressive lithospheric slab tearing that caused both rock uplift and an intensified summer monsoon to start earlier in the western Himalaya. Rates of exhumation reduced after ~ 17 Ma as uplift also slowed, apparently unrelated to climate change. However, further reduction at 6–8 Ma coincided with a time of weakening summer rains, as shown by carbon isotope data from the foreland and hematite-goethite records from ODP Site 730 on the Oman margin. Coming at a time of stronger winds, this drying was linked to global climatic cooling. Weakening of Late Miocene monsoon rains coincided with a southward migration of the ITCZ and faster erosion of the Lesser rather than Greater Himalaya. Unroofing of the Inner Lesser Himalaya, starting after 9 Ma, followed as a consequence of duplex formation enabled by focused erosion, although widespread exposure of that range was delayed until after 6 Ma. Inner Lesser Himalayan exposure, combined with unroofing of the Nanga Parbat Massif, caused a decrease in average ε Nd values of Indus River sediment after 5 Ma, rather than large-scale drainage capture.

Stratigraphic exposures, fluvial archives and borehole data have been allowed to reconstruct the alluvial history of Late Pleistocene to Late Holocene and climate changes in the monsoonal wet – dry region of lower Damodar Basin (West Bengal, India). The facies architectures and climate proxies suggest that five to six climate changes occurred in between ~ 14 ka and 6 ka. Supporting evidence from the floodplain of Damodar River demonstrates that the successive phases of aggradation and incision were linked to the south-west monsoonal variability of Late Quaternary period. The onset of semi-arid climate was associated with caliches, pond and backswamp deposits of wanning low energy floods. The relatively warm-humid climate was associated with sandy bedforms, valley fills, slack water deposits and ferruginous nodules. This paper presents a synthesis of the available palaeoclimatic records from the lower Ganga Basin and the rivers of western and central India for the palaeoenvironmental significance of Late Quaternary deposits and discusses the influence of palaeoclimatic controls on the fluvial architectures and archives that developed below the floodplain of Damodar River. We have taken some representative studies from the region to reveal the spatial variability in fluvial successions in response to climate changes during this period.

Although there is some evidence for an Eocene monsoon the most important intensification of rainfall appears to start ~24 Ma, Early Miocene. Many paleoceanographic proxies for monsoon intensity are linked to wind and do not correlate well with humidity of the continental climate over tectonic timescales. Rainfall peaked in the middle Miocene (~15 Ma) with strong drying after 8 Ma. This timing does not correlate well with either initial uplift of the Tibetan Plateau or with the retreat of shallow seas from central Asia. The ~24 Ma onset of strengthening rainfall is associated with initiation of rapid erosion and cooling of Himalayan metamorphic rocks. The progressive detachment of the subducting Indian lithosphere from the eastern and western syntaxes at ~25 Ma to the east-central Himalaya at ~13–11 Ma would have produced corresponding propagation of rising Himalayan topography following release of the weight of the detached slab. Rapid uplift of the Himalayan barrier, blocking moisture-laden winds is considered the most likely trigger for a stronger summer monsoon in South Asia, which in turn allowed faster erosion and exhumation of the Greater Himalaya after 24 Ma.

Keynote address for the University of Virginia conference on "Crossings".  Now published in:"China and Beyond in the Medieval Period" edited by Dorothy Wong and Gustav Heldt, Cambria Press, 2014.  The beginning of my study of Maritime Buddhism.

The Indian Summer Monsoon (ISM), a significant part of the global monsoon system, is driven by several climate forcing parameters. With the growing pace of global climate change scenario, there is need to focus on generating more high-resolution records of past monsoon. The ISM reconstructions from Core Monsoon Zone (CMZ) of India, which represents all-India Summer Monsoon Rainfall, are useful for a better understanding of its past variability. Such reconstructions from the CMZ are rather sparse and require detailed study based on various climate proxies. Here, we focus on the reconstruction of ISM variability during the mid-Holocene, based on stalagmite oxygen isotope ratios from the Kotumsar cave, Central India. We show that with decreasing insolation, monsoon started declining at the beginning of the mid-Holocene from 8.5 to 6.5 ka BP, which is also observed in the previous ISM reconstructions with coarser resolutions. However, a gradual increase in the rainfall is observed from 6.5 to 5.6 ka BP, a feature which is also noted in the East Asian Monsoon reconstruction from the Dongge cave. Our record mainly emphasizes on the occurrence of several abrupt weak monsoon events throughout the mid-Holocene. The occurrence of 8.2 and 5.9 ka abrupt weak monsoon events suggest that ISM variability is tightly bound to North-Atlantic Oscillation (NAO). We also demonstrate that ISM during the mid-Holocene was partly sensitive to El-Nino Southern Oscillation (ENSO), and displayed an inverse relationship.

ABSTRACT The Indian Summer Monsoon (ISM) is crucial for agriculture and water resources in India. The large spatial and temporal variability of Indian Summer Monsoon Rainfall (ISMR) leads to flood and drought especially over northern plains of India, so quantitative and qualitative assessment of future projected rainfall will be important for policy framework. Evaluation of models performance in simulating rainfall and wind circulation of the Historical experiment (1961-2005) and its future projected change in RCPs (2006-2050) 4.5 and 8.5 in CMIP5 are carried out. In the Historical experiment, the model simulated rainfall is validated with observed rainfall of IMD (1961-2005) and GPCP (1979-2005) and only six (6) models BCC-CSM1.1(m), CCSM4, CESM1(BGC), CESM1(CAM5), CESM1(WACCM), and MPI-ESM-MR are found suitable in capturing ISMR and JJAS wind circulation at 850 & 200 hPa as in NCEP reanalysis, which shows anticyclonic circulation over Arabian Sea at 850hPa and cyclonic circulation at 200hPa along with excess and deficit rainfall over monsoon regions of NWI, NEI, WCI, CNI and PI at 99% & 95% confidence levels. Future projected change of JJAS wind shows anticyclonic circulation over Arabian Sea at 850hPa and cyclonic circulation around 400N,700E-900E at 200hPa which may be a possible cause of changes in JJAS rainfall over Indian regions.

Simulation of Indian Summer Monsoon (ISM) onset over South Asia Coordinated Regional Climate Downscaling Experiment (CORDEX) domain in pentad composite pattern is considered for this study. The latest version of the International Center for Theoretical Physics' (ICTP's) Regional Climate Model version 4.3 (RegCM-4.3) is used for the simulation of a pentad composite onset for three time period: Pre-onset, Onset and Post-onset periods of ISM. Each pentad composite is average of five consecutive days. 10 years (2001–2010) worth of pentad composites of rainfall, mean sea level pressure (MSLP), outgoing longwave radiation (OLR), wind at 850 hPa and 925 hPa levels till 7 days prior to the date of onset of monsoon and 7 days after the onset are analyzed to find out the predictive skill. Six Parameterization convection schemes (PCSs) viz. Kuo, Mix98, Mix99, Tiedtke, Emanuel and Grell are used in sensitivity experiment and estimation of their performance has been done. From the experiment, some modulation is found in the OLR field (≤ 200 Wm− 2) within the region 5°N–10°N and 70°N–75°E. Yearly analysis has shown the strength of wind at 925 hPa over the region 5°N–10°N and 70°E–80°E on pentad composite onset of ISM. Yearly analysis is conducted for finding the best fitted PCS which has provided the precursor for simulating the onset.

This article reviews the level of current scientific understanding regarding the impact of future change in the large-scale climate-earth system on ecosystem services. Impacts from sea level rise, ocean acidification, increases in ocean temperature, potential collapse of the thermohaline circulation; failure of the South Asia monsoon; the melting of sea ice, the Greenland Ice Sheet and the West Antarctic Ice Sheet; changes in water availability; and Amazonia forest dieback, are considered. The review highlights that while a number of uncertainties remain in understanding, there is evidence to suggest that climate change may have already affected some ecosystem services. Furthermore, there is considerable evidence to show that future climate change could have impacts on biodiversity, as well as secondary impacts on issues important to human society, including; habitability; land productivity and food security; water security; and potential economic impacts.

Somali upwelling history has been reconstructed for the last 18.5 ka BP based on biogenic silica fluxes estimated from a sediment core retrieved from the western Ara-bian Sea. Surface winds along the east African coast during the southwest monsoon (SWM) cause the Somali up-welling; thus, the intensity of this upwelling has been related to the variability of the SWM. Biogenic silica flux variation suggests periodic weakening and strengthening of the Somali upwelling. Weakened upwelling during the 18.5–15 ka BP period and strengthened upwelling during the Bølling–Allerød (15–12.9 ka BP) suggest the onset of the SWM. The Younger Dryas (12.9–11.7 ka BP) is marked by reduced upwelling strength, with an intensification of the So-mali upwelling observed at the beginning of the Holocene and a further decline at 8 ka BP. The increase in the upwelling strength recorded since 8 ka BP suggests SWM strengthening during the latter part of the Holocene. A comparison of upwelling variations with the SWM precipitation record demonstrates a reversal in the relationship between the strength of the Somali upwelling and SWM rainfall at the beginning of the Holocene. This observed shift has been attributed to the variation in the SWM strength due to the latitudinal shift of the intertropical convergence zone (ITCZ) associated with changes in moisture sources.

The late early to middle Miocene sediments at Varkala cliff section (SW India) offer a great potential to study paleoenvironmental and paleoclimatic conditions. The succession consists of coastal marine sands alternating with fine grained, organic-rich siliciclastics. Based on several palyno-samples, a mangrove flora could be reconstructed, composed of Rhizophoraceae, Avicennia, Xylocarpus, and Sonneratia. These grew in front of backswamps, followed by evergreen forests in hinterland with many rainforest elements.

Despite these similarities between the Miocene and the modern vegetation, differences can be documented, e.g., the abundance of Nypa and the absence of gymnosperm in the Miocene vegetation. Based on the Coexistence Approach, a temperature is reconstructed, which is similar to today's (mean annual temperature 22.2–27.5 °C; coldest month temperature 20.6–23.6 °C; warmest month temperature 27.5–28.1 °C). However, in comparison to today, significantly lower values for the mean annual precipitation are reconstructed (1748–1958 mm), although a seasonality pattern was established (wettest month mean precipitation 225–358 mm; driest month mean precipitation 18–83 mm).

Comparison with other localities along the west coast of southern India hints at a similar vegetation and climate throughout the early and middle Miocene. Furthermore, the same results were obtained for SE India. This is in contrast to the strong west-east gradient in rainfall caused by the Indian Monsoon nowadays. Therefore, the paleovegetation of the early and middle Miocene of southern India appears to be more uniform than today. This outlines the complexity of describing the paleoclimatic evolution in the tropics based on plant fossils, but emphasizes the need for further study of this area to understand the early history of the Indian Monsoon.

Determining the interplay between tectonic deformation, climate, atmospheric CO2 concentrations and continental weathering and erosion is key to understanding the mechanisms that forced Cenozoic global cooling. In contrast with studies of paleo-climate and pCO2, the history of long-term silicate weathering in the Himalaya and Tibetan Plateau (HTP) during the late Cenozoic remains unclear. We reconstruct 5 m.y. of silicate sedimentary records at ODP Site 1143 in the South China Sea to explore the weathering history of the Mekong River basin that supplied the sediment. Coherent variation of weathering proxies from the South China Sea, Bay of Bengal, Loess Plateau, as well as the Yangtze and Yellow Rivers, indicates weakening chemical weathering intensity since the late Pliocene, as the climate cooled. This cooling, coupled with tectonic activity, shifted the dominant weathering regime from more transport-limited to more weathering-limited, causing less chemical depletion of silicate minerals. While silicate weathering rates became strongly correlated to erosion rates, they were decoupled from chemical weathering intensity. Physical denudation and associated silicate weathering rates in the HTP area increased in the Pliocene, driven by both rock uplift and stronger monsoon precipitation, decreasing atmospheric CO2 concentrations, and so contributing to Northern Hemisphere Glaciation (NHG).

Clay minerals from the Indus Canyon and eastern clinoform since ∼12 ka are uniformly rich in smectite and illite, similar to those from the Holocene Indus flood plains. A systematic enrichment of smectite in the proximal delta compared to the canyon and eastern clinoform argues for preferential capture of smectite close to the river mouth since ∼12 ka. There is a rapid shift to a more smectite-rich assemblage in the canyon and eastern clinoform after ∼5 ka. This change is probably caused by a change in sediment source, with less direct flux from the Himalaya and more erosion of older, weathered, smectite-rich sediment from the Indus River flood plains, driven by incision of the Indus and its tributaries into the floodplain as summer monsoon rains weakened. This influx of smectite is consistent with lower kaolinite/smectite values since ∼5 ka. The onset of large-scale agricultural activities since ∼5 ka, especially starting with the Harappan Civilization, may also have enhanced incision and erosion of floodplain sediments over the same time period. This study reports for the first time how monsoon strength variations since ∼12 ka affected the clay mineral assemblages and sediment provenance in a major submarine canyon.

Stable carbon and oxygen isotopic investigations are carried out on planktonic and benthic foraminifera from an AMS-dated sediment core of the northeast Indian Ocean (NEIO) to infer glacial to Holocene changes in surface and deep waters. The chronology of this gravity core (SK157-14; water-depth 3306 m; lat. 5°11'N; long. 90°05'E) was established using six AMS radiocarbon ages and oxygen isotope stratigraphy. Variations in δ18O and δ13C values of planktonic (Globigerinoides ruber) and benthic foraminifera (Cibicidoides spp.) are suggestive of large changes in the surface and deep water characteristics during the last ~ 60 ka. The δ18Opl values in core SK157-14 are significantly higher compared to the sediment cores in the northern Bay of Bengal and the Andaman Sea because of the diminished influence of riverine fresh water fluxes. Large variations in planktonic δ18Opl are noticed during the Marine Isotopic Stage (MIS) 3.1 and 3.3. Glacial to Holocene Δδ18Opl amplitude (1.8‰) is consistent with other published oxygen isotope records from the nearby locations. Maximum enrichment in δ18Opl occurs at 24-19 and the minimum during 7-6 ka BP. Spectral analysis of planktonic δ18Opl time series suggests a teleconnection between surface water δ18O and North Atlantic climate oscillations.

Benthic foraminiferal δ18Oben values indicate deep water cooling of ~ 1.5 °C during the last glacial maximum. The δ13Cben values are generally higher for the Holocene foraminifera suggesting greater contribution from the North Atlantic Deep Water (NADW). However glacial benthic foraminifera are characterized by lower δ13Cben values. Highly depleted δ13Cben values during the ~ 60-50, 21-17 and 13-11 ka BP intervals suggest decrease contribution from the North Atlantic Deep Water (NADW) and increase influx from the Southern Ocean Deep Water (SODW). In addition, oxidation of organic matter and ageing of the deep water may have contributed in the pronounced decrease in δ13Cben during the glacial intervals.

Three pairs of small, fourth-to sixth-order catchments (approximately 5-80 km 2), draining the margin of the Darjeeling Himalayas into the piedmont, were selected for a comparison of the land-use impact on the morphology and sedimentology of their stream channels. Each pair experienced similar annual rainfall and comprised similar metamorphic bedrock, steep topography, and brown soils; the members of each pair contrast with respect to their land-use structure as they comprise large forest cover (forest > 90%) and a significant contribution of agricultural land with tea plantations (forest < 63%), respectively. A set of rainfall, cross-sectional, and sediment data were collected for the characterization of the stream channels. The obtained results revealed that, under extreme rainfall and high denudation rates as well as frequent flash floods, the geomorphic response to land-use changes involving agricultural expansion at the expense of forest cover was less pronounced in the mountain channels than in the piedmont. Significant differences between the studied pairs of mountain streams draining forested and agricultural catchments were observed only at bankfull width and baseflow depth in fourth-order streams and baseflow depth in fifth-order streams. Local environmental factors such as bedrock channel boundaries, steep topography, high material supply to river networks, and selective sediment mobilization during extreme rainfall and floods override the effects of land use and exert dominant control over stream channels in mountain catchments. However, the effects of agricultural activity in mountains are propagated downstream. In the agricultural catchment in the piedmont, a local rapid decrease in river slope and an increase in water infiltration into alluvia facilitates the deposition of eroded material. In addition, the stream in the agricultural catchment exhibited a rise in the riverbed with the expansion of a braided channel over a dozen kilometers below the Himalayan front in the piedmont, while the streams in the forested catchment revealed a distinct tendency towards incision. The present-day channel morphologies and sediment patterns observed in forested catchments are most likely similar to those that existed in the studied rivers prior to the development of tea plantations and settlements in the mid-19th century.

The aim of the present study is to find out the possible linkage between the North Atlantic Oscillation (NAO), a major ocean–atmosphere coupled phenomena believed to be responsible for the variability of Indian Summer Monsoon Rainfall (ISMR) and its relation with Quasi-Biennial Oscillation (QBO) using 60 years (1953–2012) period. Spring North Atlantic Oscillation Index (NAOI) has been found to be inversely correlated with the ISMR, which has been highly enhanced during the east years when the NAOI data have been stratified according to the phases of QBO. The effect of NAOI on the ISMR is shown to be strengthening significant in the later years of the study as compared to that of the initial years of the study. This interesting result provides an input to use it as a predictor of the ISMR.

Climate model faces considerable difficulties in simulating the rainfall characteristics of southwest summer monsoon. In this study, the dynamical downscaling of European Centre for Medium-Range Weather Forecast's (ECMWF's) ERA-Interim (EIN15) has been utilized for the simulation of Indian summer monsoon (ISM) through the Regional Climate Model version 4.3 (RegCM-4.3) over the South Asia CoOrdinated Regional Climate Downscaling EXperiment (CORDEX) domain. The complexities of model simulation over a particular terrain are generally influenced by factors such as complex topography, coastal boundary, and lack of unbiased initial and lateral boundary conditions. In order to overcome some of these limitations, the RegCM-4.3 is employed for simulating the rainfall characteristics over the complex topographical conditions. For reliable rainfall simulation, implementations of numerous lower boundary conditions are forced in the RegCM-4.3 with specific horizontal grid resolution of 50 km over South Asia CORDEX domain. The analysis is considered for 30 years of climatological simulation of rainfall, outgoing longwave radiation (OLR), mean sea level pressure (MSLP), and wind with different vertical levels over the specified region. The dependency of model simulation with the forcing of EIN15 initial and lateral boundary conditions is used to understand the impact of simulated rainfall characteristics during different phases of summer monsoon. The results obtained from this study are used to evaluate the activity of initial conditions of zonal wind circulation speed, which causes an increase in the uncertainty of regional model output over the region under investigation. Further, the results showed that the EIN15 zonal wind circulation lacks sufficient speed over the specified region in a particular time, which was carried forward by the RegCM output and leads to a disrupted regional simulation in the climate model.

Agronomic terms for interactions between pedosphere, atmosphere, biosphere, hydrosphere and lithosphere are explained; likewise the distinction between meteorological, agricultural and hydrological droughts. Two droughts in Maharashtra – 1970s, 2012 – are compared and it is shown on the basis of data from a Maharashtra State Commissionariate of Agriculture internal Memorandum on Drought Situation in Rabi 2012 and Mitigation Plan (revised, 2013) that the 2012 one was to some extent agricultural but mainly hydrological: its economic consequences were far worse than a 90%-good monsoon would likely have caused in the first half of the 20th century. Arguments drawing upon work of the Indian Institute of Tropical Meteorology (IITM) are advanced to the effect that such tragedies will recur until environmentally sound policies have been devised and well implemented: Maharashtran farmers and government policies should expect “unpredictable anomalies” getting worse. An appendix by Pranav Bhatnagar provides additional, district-by-district data to this effect.
      Long-term damage due to the Green Revolution is analysed. Remedial proposals are set out under five headings. 1: Rejuvenate aquifers and prevent erosion by means of micro-watershed management. 2: Incentivise farmers to grow well chosen combinations of cultivars adapted to local conditions of pedosphere, hydrosphere and atmosphere. 3: Reallocate uses of some of the state’s land according to its long-term biosustainable capabilities. 4: Persuade and enable people to use water less wastefully and do rain-water harvesting. 5: Get politicians to reverse the environmentally catastrophic policy of building more big dams.

The 14C ages of planktonic foraminifers Globigerinoides sacculifer bracketing the Younger Dryas in a δ18O record of Globigerinoides ruber from a laminated sediment core on the Pakistani continental margin suggest that surface reservoir ages in the Arabian Sea were in excess of 1000 years during the deglaciation. A least squares error fit of a detailed 14C chronology to the (atmospheric) tree ring record gave variable early Holocene reservoir ages between 780 and 1120 years, well above the prebomb value of 640 years. Mid-Holocene reservoir ages are less well constrained but were probably closer to the prebomb value. The method used to fit individual core sections to the tree ring record was designed to require only a rough a priori estimate of the time spans, which in the core presented here were taken from each section's range of 14C ages. A significant 220-year quasi-oscillation was present in the δ18O record during the early Holocene but not thereafter. This frequency and amplitude pattern resembles an early Holocene 207- to 227-year oscillation previously observed in the atmospheric 14C record, which is generally interpreted as reflecting solar irradiance variability. An early Holocene climate event at 8150-8400 calendar years B.P. observed elsewhere within the Asian-East African monsoon system was again found in our record, suggesting a reduction in precipitation over NW India and Pakistan.

Recent studies using coupled atmosphere–ocean models have shown that the tropical Atlantic has a significant impact on the Indian monsoon. In this article, the observational basis for this teleconnection is examined and the physical mechanism responsible for bridging sea-surface temperatures (SSTs) in the Atlantic and precipitation over India is investigated with idealized atmospheric general circulation model (AGCM) experiments in which constant SST anomalies are prescribed and ‘switched on’ in the tropical Atlantic region. A simple Gill–Matsuno-type quadrupole response is proposed to explain the teleconnection between the tropical Atlantic and the Indian basin, with an enforcement of the eastward response likely due to nonlinear interactions with the mean monsoon circulation. The simplicity of this mechanism suggests the reproducibility of this result with a broad range of AGCMs. Copyright © 2009 Royal Meteorological Society