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Sankar S.,Nansen Environmental Research Center India | Svendsen L.,Nansen Environmental and Remote Sensing Center | Svendsen L.,University of Bergen | Gokulapalan B.,Nansen Environmental Research Center India | And 2 more authors.
Tellus, Series A: Dynamic Meteorology and Oceanography | Year: 2016

Several studies have shown a statistically significant correlation between Atlantic multidecadal variability (AMV) and Indian summer monsoon rainfall (ISMR) since 1871 when instrumental data are available. In the instrumental records, both ISMR and North Atlantic sea surface temperatures (SSTs) have multidecadal variability with a period close to 60 yr, where periods of warm (cold) North Atlantic SSTs are accompanied by periods of wetter (dryer) ISMR and lower (higher) frequencies of dry years. We have studied both AMV and ISMR for the period from 1481 to present using several proxy reconstructions from both regions, as well as an extended instrumental data set for ISMR, to investigate multidecadal variability in the ISMR and the teleconnection to AMV. Previous studies investigating the relationship betweenAMVand ISMR in instrumental data have only used the period from 1871 onwards, whereas rain gauge data from the year 1844 are studied here, extending the instrumental record by 26 yr. We find that the observed link between AMV and ISMR is present in the extended instrumental data. We also find that multidecadal variability is present in the ISMR in all proxy records; however, all the proxy records for both ISMR and AMV diverge before the 1800s. In addition, the observed correlation betweenAMVand ISMR has weakened in the last decade. These results emphasise that it is not appropriate to use single proxy reconstructions to study past climates. © 2016 S. Sankar et al.


Abish B.,Nansen Environmental Research Center India | Joseph P.V.,Nansen Environmental Research Center India | Joseph P.V.,Cochin University of Science and Technology | Johannessen O.M.,Nansen Environmental and Remote Sensing Center
Advances in Atmospheric Sciences | Year: 2015

A study of six decades (1950–2009) of reanalysis data reveals that the subtropical jetstream (STJ) of the Southern (Northern) Hemisphere between longitudes 0°E and 180°E has weakened (strengthened) during both the boreal winter (January, February) and summer (July, August) seasons. The temperature of the upper troposphere of the midlatitudes has a warming trend in the Southern Hemisphere and a cooling trend in the Northern Hemisphere. Correspondingly, the north-south temperature gradient in the upper troposphere has a decreasing trend in the Southern Hemisphere and an increasing trend in the Northern Hemisphere, which affects the strength of the STJ through the thermal wind relation. We devised a method of isotach analysis in intervals of 0.1 m s−1in vertical sections of hemispheric mean winds to study the climate change in the STJ core wind speed, and also core height and latitude. We found that the upper tropospheric cooling of the Asian mid-latitudes has a role in the strengthening of the STJ over Asia, while throughout the rest of the globe the upper troposphere has a warming trend that weakens the STJ. Available studies show that the mid-latitude cooling of the upper troposphere over Asia is caused by anthropogenic aerosols (particularly sulphate aerosols) and the warming over the rest of the global mid-latitude upper troposphere is due to increased greenhouse gases in the atmosphere. © 2015, Chinese National Committee for International Association of Meteorology and Atmospheric Sciences, Institute of Atmospheric Physics, Science Press and Springer-Verlag Berlin Heidelberg.


Jayaram C.,East-West Center | Kochuparambil A.J.,Nansen Environmental Research Center India | Balchand A.N.,Cochin University of Science and Technology
International Journal of Remote Sensing | Year: 2013

The interannual variability of chlorophyll concentration along the southwest coast of India is studied using remote-sensing data from SeaWiFS. The data are analysed in conjunction with satellite-measured sea surface winds. The satellite-measured chlorophyll data for a period of 10 years from 1998 to 2007 were made use of for indexing the maximum offshore extent of chlorophyll along the coast for each month. From the empirical orthogonal functional analysis of chlorophyll data, it is observed that the dominant mode is annual. Interestingly, intraseasonal variability and the influence of climatic events like El Niño are observed in the secondary principle component of the time series. The variability of chlorophyll coincided well with variability of Ekman transport all along the coast with higher chlorophyll (>1 mg m-3) when the Ekman transport is greater than 1000 kg/m/s. During the years 2005-2007, reduction in the meridional (along shore) component of wind resulted in reduction of Ekman transport, the phenomenon which leads to a decrease in chlorophyll. This is due to the reduction in the amount of nutrients that entrained to surface layers during upwelling of the southwest monsoon. The chlorophyll-a is minimum when Ekman transport is less than 0.5 kg/m/s on the normalized scale. For higher values of chlorophyll, the Ekman transport is higher, indicating the contribution of wind in enhancing the already upwelled chlorophyll production. The smaller value of R2 infers that there exist other forces as well involved in augmenting the surface chlorophyll. The enhanced knowledge on the offshore extent and the intraseasonal and interannual variability of chlorophyll can provide valuable inputs on fisheries and primary productivity for this region. © 2013 Copyright Taylor and Francis Group, LLC.


Joseph P.V.,Nansen Environmental Research Center India | Joseph P.V.,Cochin University of Science and Technology
Surveys in Geophysics | Year: 2014

Asian summer monsoon sets in over India after the Intertropical Convergence Zone moves across the equator to the northern hemisphere over the Indian Ocean. Sea surface temperature (SST) anomalies on either side of the equator in Indian and Pacific oceans are found related to the date of monsoon onset over Kerala (India). Droughts in the June to September monsoon rainfall of India are followed by warm SST anomalies over tropical Indian Ocean and cold SST anomalies over west Pacific Ocean. These anomalies persist till the following monsoon which gives normal or excess rainfall (tropospheric biennial oscillation). Thus, we do not get in India many successive drought years as in sub-Saharan Africa, thanks to the ocean. Monsoon rainfall of India has a decadal variability in the form of 30-year epochs of frequent (infrequent) drought monsoons occurring alternately. Decadal oscillations of monsoon rainfall and the well-known decadal oscillation in SST of the Atlantic Ocean (also of the Pacific Ocean) are found to run parallel with about the same period close to 60 years and the same phase. In the active-break cycle of the Asian summer monsoon, the ocean and the atmosphere are found to interact on the time scale of 30-60 days. Net heat flux at the ocean surface, monsoon low-level jetstream (LLJ) and the seasonally persisting shallow mixed layer of the ocean north of the LLJ axis play important roles in this interaction. In an El Niño year, the LLJ extends eastwards up to the date line creating an area of shallow ocean mixed layer there, which is hypothesised to lengthen the active-break (AB) cycle typically from 1 month in a La Niña to 2 months in an El Niño year. Indian monsoon droughts are known to be associated with El Niños, and long break monsoon spells are found to be a major cause of monsoon droughts. In the global warming scenario, the observed rapid warming of the equatorial Indian ocean SST has caused the weakening of both the monsoon Hadley circulation and the monsoon LLJ which has been related to the observed rapid decreasing trend in the seasonal number of monsoon depressions. © 2014 Springer Science+Business Media Dordrecht.


Abish B.,Nansen Environmental Research Center India | Joseph P.V.,Nansen Environmental Research Center India | Joseph P.V.,Cochin University of Science and Technology | Johannessen O.M.,Nansen Environmental and Remote Sensing Center
Journal of Climate | Year: 2013

Recent research has reported that the tropical easterly jet stream (TEJ) of the boreal summer monsoon season is weakening. The analysis herein using 60 yr (1950-2009) of data reveals that this weakening of the TEJ is due to the decreasing trend in the upper tropospheric meridional temperature gradient over the area covered by the TEJ. During this period, the upper troposphere over the equatorial Indian Ocean has warmed due to enhanced deep moist convection associated with the rapid warming of the equatorial Indian Ocean. At the same time, a cooling of the upper troposphere has taken place over the Northern Hemisphere subtropics including the Tibetan anticyclone. The simultaneous cooling of the subtropics and the equatorial heating has caused a decrease in the upper tropospheric meridional thermal gradient. The consequent reduction in the strength of the easterly thermal wind has resulted in the weakening of the TEJ. © 2013 American Meteorological Society.


Jayakumar A.,Indian Institute of Tropical Meteorology | Jayakumar A.,Nansen Environmental Research Center India | Gnanaseelan C.,Indian Institute of Tropical Meteorology | Sabin T.P.,Indian Institute of Tropical Meteorology
International Journal of Climatology | Year: 2013

Sea surface temperature (SST) in the region off southern tip of India (STI, 75-83°E, 5-8°N) exhibited a prominent variability in the intraseasonal time scale (both 30-90 d and 10-30 d band) during boreal summer. Mechanisms associated with this intraseasonal variability are studied using three-dimensional ocean general circulation model (OGCM) sensitivity experiments, satellite observed outgoing longwave radiation, SST and winds for the period 1998-2007. The background oceanic structure of the STI characterized by a shallow thermocline and moderate mixed layer provided ideal conditions for strong oceanic sub-surface processes. The model mixed layer heat budget reveals that the oceanic processes such as horizontal advection and vertical processes are the dominant mechanisms in the STI region as compared with air-sea flux. Sensitivity experiments with the OGCM reveals that the ocean dynamical processes contribute to most of the intraseasonal SST variability and the wind stress contributes to 85% of the variability whereas surface flux contributes to only 15% for the 30-90 d SST variability. Higher amplitude of surface flux perturbation and its contribution to SST in the 10-30 d as compared with the 30-90 d band are evident in the model experiment and are consistent with the observational analysis. There is year-to-year variability in the relative role of horizontal and vertical processes for different intraseasonal SST events over STI. © 2012 Royal Meteorological Society.


Ratna S.B.,Centro Euro Mediterraneo sui Cambiamenti Climatici | Cherchi A.,Centro Euro Mediterraneo sui Cambiamenti Climatici | Cherchi A.,Italian National Institute of Geophysics and Volcanology | Joseph P.V.,Nansen Environmental Research Center India | And 4 more authors.
Climate Dynamics | Year: 2016

The Indo-Pacific Ocean (i.e. region between 30°E and 150°E) has been experiencing a warming since the 1950s. At the same time, the large-scale summer monsoon rainfall over India and the moisture over the East Africa/Arabian Sea are both decreasing. In this study, we intend to investigate how the decrease of moisture over the East Africa/Arabian Sea is related to the Indo-Pacific Ocean warming and how this could affect the variability of the Indian summer monsoon rainfall. We performed the analysis for the period 1951–2012 based on observed precipitation, sea surface temperature and atmospheric reanalysis products and we verified the robustness of the result by comparing different datasets. The decreasing trend of moisture over the East Africa/Arabian Sea coincides with an increasing trend of moisture over the western Pacific region. This is accompanied by the strengthening (weakening) of the upward motion over the western Pacific (East Africa/Arabian Sea) that, consequently, contributes to modulate the western Pacific-Indian Ocean Walker circulation. At the same time, the low-level westerlies are weakening over the peninsular India, thus contributing to the reduction of moisture transport towards India. Therefore, rainfall has decreased over the Western Ghats and central-east India. Contrary to previous decades, since 2003 moisture over the East Africa/Arabian Sea started to increase and this is accompanied by the strengthening of convection due to increased warming of sea surface temperature over the western Arabian Sea. Despite this moisture increase over the Arabian Sea, we found that moisture transport is still weakening over the Indian landmass in the very recent decade and this has been contributing to the decreased precipitation over the northeast India and southern part of the Western Ghats. © 2015, Springer-Verlag Berlin Heidelberg.


Nandini Menon N.,Nansen Environmental Research Center India | Singh T.,Wageningen University | Pettersson L.H.,Nansen Environmental and Remote Sensing Center
Eos | Year: 2014

Coastal zones around the world are extremely vulnerable today because of the unprecedented pressures of industrial and urban development as well as climate change related devastations, such as the growing intensities of cyclonic storms, the rise in sea surface temperature, sea surges, and sea level rise. In India, where about 35% of the population lives within 100 kilometers of the coastline, fisheries are a major driver and safety net for economic development and coastal livelihoods. Coastal ecosystems are closely linked with socio-economic systems, which require carefully planned coastal zone management (CZM) actions. ©2014. American Geophysical Union. All Rights Reserved.


Nair S.,Nansen Environmental Research Center India
IAHS-AISH Publication | Year: 2010

The impact of environmental changes on the hydrology of small basins is a serious challenge to the safety of water, food and energy supplies in the state of Kerala, India. Human interference and climate changes have altered the hydrology of most of the basins here, with significant impacts on all facets of life. Conservation and management practices are inadequate and implementation of projects and policies often fail because of various social, economic and political problems. An assessment of the impact of climate change and human interference on the small basins of Kerala has been carried out. Changes in water availability under a predicted change in climate have been estimated using the modified hydrological model. Sediment transport in selected basins has been analysed. A review has been made of both the existing programmes and projects for the protection and management of basins, as well as of current policies and adaptation strategies. Copyright © 2010 IAHS Press.


Nair A.,Nansen Environmental Research Center India | Ajith Joseph K.,Nansen Environmental Research Center India | Nair K.S.,Nansen Environmental Research Center India | Nair K.S.,Center for Earth Research and Environment Management
Atmospheric Environment | Year: 2014

Kerala, a maritime state of India is bestowed with abundant rainfall which is about three times the national average. This study is conducted to have a better understanding of rainfall variability and trend at regional level for this state during the last 100 years. It is found that the rainfall variation in northern and southern regions of Kerala is large and the deviation is on different timescales. There is a shifting of rainfall mean and variability during the seasons. The trend analysis on rainfall data over the last 100 years reveals that there is a significant (99%) decreasing trend in most of the regions of Kerala especially in the month of January, July and November. The annual and seasonal trends of rainfall in most regions of Kerala are also found to be decreasing significantly. This decreasing trend may be related to global anomalies as a result of anthropogenic green house gas (GHG) emissions due to increased fossil fuel use, land-use change due to urbanisation and deforestation, proliferation in transportation associated atmospheric pollutants. We have also conducted a study of the seasonality index (SI) and found that only one district in the northern region (Kasaragod) has seasonality index of more than 1 and that the distribution of monthly rainfall in this district is mostly attributed to 1 or 2 months. In rest of the districts, the rainfall is markedly seasonal. The trend in SI reveals that the rainfall distribution in these districts has become asymmetric with changes in rainfall distribution. © 2014 Elsevier Ltd.

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