Ocean Monsoon Research Articles

Solar influence on the Indian Ocean Monsoon through dynamical processes

The result of paleoclimate studies on the relationship between the Indian monsoon and solar activity, inferred from the analysis of stalagmites in Oman, is confirmed by using a modern meteorological dataset from 1958–1999. The present result suggests that the solar influence on monsoon activity is not due to a change in radiative heating in the troposphere but, rather, originates from the stratosphere through modulation of the upwelling in the equatorial troposphere, which produces a north‐south seesaw of convective activity over the Indian Ocean sector during summer. Higher precipitation over Arabia and India, thus, occurs during high solar activity.

Outgoing long-wave radiation over the Tropical Pacific and Atlantic Ocean and Indian summer monsoon rainfall

In this paper, the interannual variability of satellite derived outgoing longwave radiation (OLR) is examined in relation to the Indian summer monsoon rainfall (June to September total rainfall; ISMR). Monthly grid point OLR field over the domain i.e. the tropical Pacific and Atlantic region (30° N to 30° S, 110° E to 10° W) and the ISMR for the period 1974–2001 are used for the study. A strong and significant north–south dipole structure in the correlation pattern is found between the ISMR and the OLR field over the domain during January. This dipole is located over the west Pacific region with highly significant negative (positive) correlations over the South China Sea and surrounding region (around north-east Australia). The dipole weakens and moves northwestward during February and disappears in March. During the month of May, the OLR over the central Atlantic Ocean shows a significant positive relationship with the ISMR. These relationships are found to be consistent and robust during the period of analysis and can be used in the prediction of the ISMR. A multiple regression equation is developed, using the above results, for prediction of the ISMR and the empirical relationships are verified using an independent data set. The results are encouraging for the prediction of the ISMR. The composite annual cycle of the OLR, over the west Pacific regions during extreme ISMR is found to be useful in the prediction of extreme summer monsoon rainfall conditions over the Indian subcontinent.

Variations in productivity and eolian fluxes in the northeastern Arabian Sea during the past 110 ka

High-resolution (one to two samples/ka) radionuclide proxy records from core 93KL in the northeastern Arabian Sea provide evidence for millennial climate variability over the past 110 ka. We interpret 230Th-normalized 232Th fluxes as a proxy for eolian input, and authigenic uranium concentrations as a proxy for past productivity. We attribute orbital and suborbital variations in both proxies to changes in the intensity of the southwest Indian Ocean monsoon. The highest 230Th-normalized 232Th fluxes occur at times that are consistent with the timing of the Younger Dryas, Heinrich events 1–7 and cold Dansgaard–Oeschger stadial events recorded in the GISP2 ice core. Such high dust fluxes may be due to a weakened southwest monsoon in conjunction with strengthened northwesterlies from the Arabian Peninsula and Mesopotamia. Authigenic uranium concentrations, on the other hand, are highest during warm Dansgaard–Oeschger interstadials when the southwest monsoon is intensified relative to the northwesterly winds. Our results also indicate that on orbital timescales maximum average eolian fluxes coincide with the timing of marine isotopic stage (MIS) 2 and 4, while minimum fluxes occur during MIS 1, 3 and 5. Although the forcing mechanism(s) controlling suborbital variabilities in monsoonal intensity is still debated, our findings suggest an atmospheric teleconnection between the low-latitude southwest monsoon and North Atlantic climate.

Palaeoclimatic interpretation of high-resolution oxygen isotope profiles derived from annually laminated speleothems from Southern Oman

High-resolution stable isotope profiles of three contemporaneously deposited stalagmites from a shallow cave in Southern Oman provide an annually resolved record of Indian Ocean monsoon rainfall variability for the past 780 years. Uranium-series age dating and counts of annual growth bands enable an excellent age calibration. Although modern speleothems do not grow in perfect isotopic equilibrium, oxygen isotope ratios (δ 18O) are a proxy for the amount of monsoon rainfall. This is supported by the statistically significant correlation between δ 18O and the thickness of annual bands, whereas δ 18O is inversely correlated with annual band thickness. Additionally, overlapping δ 18O profiles are very similar in pattern and range, indicating that sample specific noise did not blur the climatic signal. The longest oxygen isotope profile, derived from stalagmite S3, clearly shows the transition at ∼1320 AD from a generally wetter Medieval Warm Period to a drier Little Ice Age that lasted from approximately AD 1320–1660 in Southern Oman. The decrease in monsoon rainfall since the 1960s is also obvious in meteorological records from Northern Africa and India, indicating that our speleothem-based rainfall records do not only reflect local monsoon rainfall variability.

Late Oligocene–Miocene paleoceanographic evolution of the southeastern Indian Ocean: evidence from deep-sea benthic foraminifera (ODP Site 757)

We quantitatively analyzed deep-sea benthic foraminifera from >125 μm size fraction from Ocean Drilling Program (ODP) Site 757 to understand deep-sea paleoceanographic changes in the southeastern Indian Ocean during the late Oligocene–Miocene (26.5–5.35 Ma). We used the knowledge of the ecology of Recent deep-sea benthic foraminifera for environmental interpretations. Factor and cluster analyses were run using percentages of 46 highest-ranked species that helped identify six biofacies defining six clusters of samples. The faunal data document a major shift in deep-sea ventilation, organic carbon flux and productivity at ∼8.3 Ma, coinciding with the major intensification of the Indian Ocean monsoon system. This marks a change from cold and well-oxygenated deep waters with low and pulsed organic carbon flux during 26.5 to ∼8.3 Ma to oxygen-poor deep waters with sustained flux of organic matter. From 15.0 to 7.6 Ma, the deep-sea currents were stronger in the southeastern Indian Ocean probably due to major expansion of the East Antarctic ice sheet and increased production of deep waters and in turn increased deep-sea circulation. The productivity increased in the Indo-Pacific region in the late Miocene and the modern monsoon regime probably established at 8.3 Ma.

Estimating Pearson's Correlation Coefficient with Bootstrap Confidence Interval from Serially Dependent Time Series

Pearson's correlation coefficient, rxy, is often used when measuring the influence of one time-dependent variable on another in bivariate climate time series. Thereby, positive serial dependence (persistence) and unknown data distributions impose a challenge for obtaining accurate confidence intervals for rxy. This is met by the presented approach, employing the nonparametric stationary bootstrap with an average block length proportional to the maximum estimated persistence time of the data. A Monte Carlo experiment reveals that this method can produce accurate (in terms of coverage) confidence intervals (of type bias-corrected and accelerated). However, since persistence reduces the number of independent observations, substantially more data points are required for achieving an accuracy comparable to a situation without persistence. The experiment further shows that neglecting serial dependence may lead to serious coverage errors. The presented method proves robust with respect to distributional shape (lognormal/normal) and time spacing (uneven/even). The method is used to confirm that a previous finding of a correlation between solar activity and Indian Ocean monsoon strength in early Holocene is valid. A further result is that the correlation between sunspot number and cosmogenic 10Be concentration vanishes after approximately 1870.

A 725 yr cycle in the climate of central Africa during the late Holocene

High-resolution analyses of the chemical and stable isotope composition of endogenic calcite from Lake Edward, an equatorial rift lake in central Africa, reveal large oscillations in African climate and moisture balance over the past 5400 yr. These oscillations indicate a series of droughts from 4900 to 400 yr B.P. that define a drought period of ∼725 yr. This period may be correlated to variations in the Indian Ocean monsoons, as documented by similar periods in sea-surface salinity records from the northern Indian Ocean. The similarities in periodicity of the Lake Edward and Indian Ocean records may indicate that low-frequency variability within the Indian and Pacific Oceans dominates millennial-scale African moisture balance and the Holocene climate of the tropical Indian Ocean.

Comparison of the Structure of the ITCZ in the West Pacific during the Boreal Summers of 1989–93 Using AMIP Simulations and ECMWF Reanalysis

The dynamic structure and interannual variability of the ITCZ in the western North Pacific and east Asia during the boreal summers of 1989-93 are investigated. European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses are used to characterize the dynamical structure of the ITCZ. The ITCZ structure in the National Center for Atmospheric Research (NCAR) Community Climate Model 3 (CCM3.6) and the National Aeronautics and Space Administration ( NASA) Goddard Institute of Space Studies (GISS) SI2000 GCM is also explored. The ITCZ axis can be partitioned into three sections in the western North Pacific and east Asia: the continental monsoon trough (CMT) in India, the Bay of Bengal, and Indo-China; the oceanic monsoon trough (OMT) in the Pacific warm pool; and the trade wind trough (TWT) in the central Pacific. The confluence point (CP) is defined as an area where the broad cyclonic flow in OMT changes to the easterly trades of the TWT. East of the CP, both GCMs simulate a weaker easterly trade wind regime with a broader ITCZ due to the dislocation of the Pacific high. To the west of the CP, both GCMs simulate a weaker cross-equatorial flow and OMT. Both models show skill in simulating the interannual variability of the location of the CP; however, for the peak ENSO warm phase years (1992-93), the spread among ensemble members of the CP longitude is comparable to the shift of ensemble mean CP location relative to 1989. That is, the internal variability of each GCM is on the same order as the simulated interannual variability in this region.

A 780‐year annually resolved record of Indian Ocean monsoon precipitation from a speleothem from south Oman

Meteorological records of monsoon rainfall in the Indian Ocean are generally less than 100 years long. The relative brevity of these records makes it difficult to investigate monsoon variation on decadal and centennial timescales, to determine what factors influence the intensity of rainfall on these timescales, or to place possible changes in the twentieth century into a broader historical context. Development of a geologic proxy for rainfall that records annual variation in the monsoon over much longer time periods than are covered by instrumental records would be a significant step forward. We have developed an annually resolved record of monsoon rainfall variation for the past 780 years based on annual layer thickness and stable isotope analyses of a laminated stalagmite from southern Arabia. Our results show that monsoon variation over the past century is not outside of the range of the past 800 years. Decreasing monsoon rainfall over the past century is related to increasing sea surface temperature in the Indian Ocean. Spectral analyses of the record are dominated by cycles that are similar to those observed in records of solar activity on centurial timescales. Decadal to interannual cycles in the record appear to originate in the tropical Pacific Ocean.

Holocene Variability in the Indian Ocean Monsoon: A Stalagmite- Based, High-Resolution Oxygen Isotope Record from Southern Oman

Holocene Variability in the Indian Ocean Monsoon: A Stalagmite- Based, High-Resolution Oxygen Isotope Record from Southern Oman

Bajada formation by monsoonal erosion of a subaerial forebulge, Sultanate of Oman

A vast, low gradient bajada is present in the interior of northern Oman. With an area of ∼40,000 km 2 and longitudinal gradients less than 0.3°, this Late Cenozoic bajada is 10 times larger and 10 times flatter than classical bajadas. The tectonic and climatic setting that favored aerially extensive bajada formation is investigated in this paper. A tectonic model is proposed that involves forebulge development associated with subduction of the Arabian plate beneath the Asian plate. The model identifies the southern Gulf of Oman as a foredeep, the Oman Mountains as a subaerial forebulge, the vast bajada as a deposit that blankets the distal forebulge, and the Umm as Samim sabkha as a backbulge basin deposit. The model is supported by an excellent match between modern topography and a theoretical flexural profile of the Omani lithosphere, by the lateral distribution of Pliocene–Pleistocene sediments, by the inferred Late Miocene to Pleistocene age span of the vast bajada, and by evidence of active surface uplift of the Oman Mountains. A second mechanism, post-orogenic exhumation of the pre-existing Oman Mountains, is likely to have accompanied and perhaps slightly preceded forebulge growth, facilitating erosion and transport of sediment across the forebulge. Evidence for a palaeoclimatic model is provided by previous sedimentological studies and stable isotope data of cements in channel gravels as well as proxy data from ODP Leg 117 (Oman Margin). Transport of coarse-grained sediment across the bajada is interpreted to have occurred mainly during the Late Miocene to Early Pleistocene when a humid-wet climate prevailed across Arabia. High water tables and accompanying cementation of bajada gravels may have aided long-distance transport. The evolution of surficial bajada sediments is correlated with changing climatic and hydrological conditions—from seasonal/perennial flows during the Pliocene (and before) to mainly ephemeral and finally flash flood flows (signifying increasing aridity) during the Pleistocene. These data imply that the Indian Ocean monsoon, represented by rainfall frequency and intensity, had a more northerly range in Arabia in the past compared to the present.

Seasonal Migration of Monsoons between the Northern and Southern Hemisphere as Revealed from Equatorially Symmetric and Asymmetric OLR Data.

The climatological pentad mean OLR data are partitioned into symmetric component OLR´, and asymmetric component OLR´´, with reference to the equator. Objective criteria are then introduced to define the intensity, center and areal extent of strong convections with OLR´ (OLR´´) of less than 220 (-20) Wm-2 over the equatorial (subtropical) domains, and seasonal migration of monsoons between the two hemispheres is investigated. Over the equatorial continents, such as Africa and South America, OLR´ exhibits a common feature of distinct semi-annual standing oscillations, while OLR´´ is of annual standing oscillation character that is peculiar to the monsoon system. Coexistent of two standing oscillations of different periodicity implies occurrence of a systematic cross-equatorial propagation of strong convections twice a year, northward during the spring transition, and southward in fall. The manner in which the monsoon transits from one hemisphere to another is distinctly different between the Indian Ocean and Western Pacific. This is associated with difference in the systematic equatorial basic flow which is, to a large extent, regulated by equatorial convections occurring over the maritime continent. Eastward (westward) of the maritime continent around 125°E, is a Kelvin (Rossby)-type basic flow regime; that is subject to significant annual variations. During late fall to early winter over the equatorial Western Pacific is a very pronounced Kelvin-type basic flow enhanced by northerly surges emanating out of the Siberian and North Pacific high pressure systems. The symmetric Kelvin flow acts as a bridge through which convections migrate southward from the WNPM (Western North Pacific Monsoon) to WSPM (Western South Pacific Monsoon) domain by crossing the equator in November and December. Gradual weakening of the Kelvin-type flow regime after winter inhibits northward return journey of convections from WSPM to WNPM in April and May. During spring and early summer over the eastern equatorial Indian Ocean is a prominent symmetric Rossby-type basic flow, which serves as a medium for convections to propagate northward, although not well organized, from SIOM (South Indian Ocean Monsoon) to SEAM (Southeast Asian Monsoon). The fall transition is of different character. The asymmetric convection center established in SEAM migrates southward, until reaching the equator by October. Instead of penetrating into the South Indian Ocean, the convective center suddenly changes its direction and propagates eastward along the equator, reaching near Borneo by the end of the calender year.

Ocean-atmosphere Structure in the Tropical Indian Ocean during a Ship Cruise 1995/96

The structure of the upper ocean and surface atmospheric conditions are described during a ship cruise across the central Indian Ocean from December 1995 to January 1996. In situ data on currents, temperature, salinity and surface heat fluxes are described and compared with expected climatological values. Thermocline uplift in the 6-7°S latitude band is linked to the cyclonic shear of near-surface currents. A comparison of quiescent and windy periods demonstrates that evaporative fluxes become amplified near cyclonic vortices fed by southerly meridional winds. The ocean density structure is influenced by salinity gradients, driven by the precipitation-evaporation balance. Near the inter-tropical convergence zone (ITCZ), freshwater fluxes create a stable surface layer and helps to maintain the eastward equatorial counter-current. An analysis of the atmospheric boundary layer from NCEP re-analysis data seeks to place the in situ results into the context of weather conditions at the time of the cruise. Further studies of this kind will improve our understanding of relationships between the Indian Ocean monsoon and surrounding climates.

Getting a grip on Indian Ocean monsoons

An improved understanding of the Indian Ocean monsoon season could help researchers to better forecast floods and the associated spread of cholera in low-lying Bangladesh. In a joint effort by the University of Colorado at Boulder, the Asian Disaster Preparedness Center, and the Bangladesh government, researchers are using a variety of monitoring and forecast modeling tools to better understand and characterize the monsoon season's active and calm periods. By studying Indian Ocean climatic conditions and probabilities that lead to regular flooding of the Bangladesh delta, researchers also can provide probabilities concerning outbreaks of cholera, an intestinal disease that infects large segments of that country's population.

Abrupt intensification of the SW Indian Ocean monsoon during the last deglaciation: constraints from Th, Pa, and He isotopes

Sediments from western Arabian Sea core 74KL representing the last 23 ka were analyzed for helium, thorium, and protactinium isotopes. Assuming global average fluxes of extraterrestrial 3He and 230Th, the average 3He-derived sediment mass accumulation rate (MAR) is a factor of 1.8 higher than the average 230Th-derived MAR. 3He- and 230Th-derived MARs converge, however, during the Younger Dryas (YD) and during the peak of the early Holocene humid interval. These features, not seen anywhere else in the world, probably reflect a combination of climate-driven changes in the flux of 230Th and 3He. Ratios of xs231Pa/xs230Th, proxies of paleoproductivity, are lowest during the last glacial maximum (LGM), and increase abruptly during the Bolling–Allerod. Later, following a sudden decrease to near-LGM values during the YD, they rise abruptly to maximum values for the entire record in the early Holocene. We hypothesize that low xs231Pa/xs230Th ratios reflect low productivity due to the decreased intensity of the SW monsoon, whereas the opposite is true for high ratios. The correlation between Arabian Sea productivity and monsoonal upwelling, on the one hand, and North Atlantic climate variability, on the other, suggests a linkage between high- and low-latitude climates caused by changing patterns of atmospheric circulation.

Exploratory Long-Range Models to Estimate Summer Climate Variability over Southern Africa

Abstract Teleconnection predictors are explored using multivariate regression models in an effort to estimate southern African summer rainfall and climate impacts one season in advance. The preliminary statistical formulations include many variables influenced by the El Nino–Southern Oscillation (ENSO) such as tropical sea surface temperatures (SST) in the Indian and Atlantic Oceans. Atmospheric circulation responses to ENSO include the alternation of tropical zonal winds over Africa and changes in convective activity within oceanic monsoon troughs. Numerous hemispheric-scale datasets are employed to extract predictors and include global indexes (Southern Oscillation index and quasi-biennial oscillation), SST principal component scores for the global oceans, indexes of tropical convection (outgoing longwave radiation), air pressure, and surface and upper winds over the Indian and Atlantic Oceans. Climatic targets include subseasonal, area-averaged rainfall over South Africa and the Zambezi river basin, an...

Colonial Science and Ecological Change: Tanzania's Mlalo Basin, 1888–1946

East Africa's coastal mountains, island massifs jutting up out of the surrounding savannas, have a thirty-million-year natural history as biological refuges. Their slopes have been watered for millennia by the Indian Ocean monsoons and have fostered the evolution of a broad diversity of flora and fauna.1 African farmers have long sought out these climatic and botanical havens and developed around their water sources a sophisticated agricultural production system designed to mitigate the severe droughts which periodically plague eastern Africa. Although long-term settlement in the mountains caused environmental change, its character and scope has yet to be adequately explained. Nonetheless, over the past seventy years, colonial and postcolonial governments have moved to ameliorate what they have perceived as human-induced ecological stress. This article examines the early phase of the colonial debates over environmental degradation and its remedies in the Mlalo basin, an upland region in the West Usambara mountains of Tanzania. The policy discussions regarding Mlalo occurred in the context of a general crisis in colonial society between 1929 and 1940, during which colonial perceptions of agroecological decline and the economic pressures of depression challenged European colonial governments in Africa and Asia. In terms of ecological issues, worldwide concern over erosion in the American Dust Bowl resonated across East Africa, resulting in an official preoccupation with soil conservation. Tanganyika Territory's Mlalo basin came to be cited in the 1930S as one of several particularly striking manifestations of soil degradation and agroecological decline.2 When a food shortage known as njaa ya Chankola (Chankola's famine) struck between 1941 and 1946, the specter of human and environmental catastrophe justified colonial degradation scenarios, which in turn guided the colonial government's attempt between 1946 and 1949 to modernize husbandry via the Mlalo Basin Rehabilitation Scheme.3 Recent scholarship in the social sciences has raised a number of questions regarding the course and interpretation of environmental degradation in Africa and Latin America. Many of these studies cite the historical tendency of Western (or

Variability of Air-Sea Interactions over the Indian Ocean Derived from Satellite Observations

Novel ways of monitoring the large-scale variability of the southwest monsoon in the Indian Ocean are presented using multispectral satellite datasets. The fields of sea surface temperature (SST), surface latent heat flux (LHF), net surface solar radiation (SW), precipitation (P), and SW − LHF over the Indian Ocean are analyzed to characterize the seasonal and interannual variability with special emphasis on the period 1988–90. It is shown that satellite data are able to make a significant contribution to the multiplatform strategy necessary to describe the large-scale spatial and temporal variability of air–sea interactions associated with the Indian Ocean Monsoon. The satellite data analyzed here has shown for the first time characteristics of the interannual variability of air–sea interactions over the entire Indian Ocean. Using monthly means of SST, LHF, SW, P, and the difference SW − LHF, the main features of the seasonal and interannual variability of air–sea interactions over the Indian Ocean are characterized. It is shown that the southwest monsoon strongly affects these interactions, inducing dramatic exchanges of heat between air and sea and large temporal variations of these exchanges over relatively small timescale (with regards to typical oceanic timescales). The analyses indicate an overall good agreement between satellite and in situ (ship) estimates, except in the southern Indian Ocean, where ship sampling is minimal, the disagreement can be large. In the latitudinal band of 10°N–15°S, differences in climatological in situ estimates of surface sensible heat flux and net longwave radiation has a larger influence on the net surface heat flux than the difference between satellite and in situ estimates of SW and LHF.

Variability of Air–Sea Interactions over the Indian Ocean Derived from Satellite Observations

Abstract Novel ways of monitoring the large-scale variability of the southwest monsoon in the Indian Ocean are presented using multispectral satellite datasets. The fields of sea surface temperature (SST), surface latent heat flux (LHF), net surface solar radiation (SW), precipitation (P), and SW − LHF over the Indian Ocean are analyzed to characterize the seasonal and interannual variability with special emphasis on the period 1988–90. It is shown that satellite data are able to make a significant contribution to the multiplatform strategy necessary to describe the large-scale spatial and temporal variability of air–sea interactions associated with the Indian Ocean Monsoon. The satellite data analyzed here has shown for the first time characteristics of the interannual variability of air–sea interactions over the entire Indian Ocean. Using monthly means of SST, LHF, SW, P, and the difference SW − LHF, the main features of the seasonal and interannual variability of air–sea interactions over the Indian Ocean are characterized. It is shown that the southwest monsoon strongly affects these interactions, inducing dramatic exchanges of heat between air and sea and large temporal variations of these exchanges over relatively small timescale (with regards to typical oceanic timescales). The analyses indicate an overall good agreement between satellite and in situ (ship) estimates, except in the southern Indian Ocean, where ship sampling is minimal, the disagreement can be large. In the latitudinal band of 10°N–15°S, differences in climatological in situ estimates of surface sensible heat flux and net longwave radiation has a larger influence on the net surface heat flux than the difference between satellite and in situ estimates of SW and LHF.

Dynamics of the seasonal variations in the Indian Ocean from TOPEX/POSEIDON sea surface height and an ocean model

Observations from the TOPEX/POSEIDON (T/P) altimeter reveal that the sea level in the Indian Ocean varies strongly with season. Surface undulations propagate eastward along the equator and westward outside the equatorial waveguide. The forcing mechanisms responsible for such variations are investigated in this paper by analyzing T/P data and surface wind‐stress forcing, and by using a nonlinear, 2.5‐layer, reduced gravity model. Both the data and the model show that Kelvin and Rossby waves forced by the Indian Ocean monsoon are primarily responsible for the seasonal sea‐level change in the tropical and the subtropical Indian Ocean.