Unnikrishnan Alakkat

National Institute of Oceanography | nio · Division of Physical Oceanography (NIO)

A review of studies on past changes and future projections in extreme sea level in the tropical oceans with a focus on the north Indian Ocean is made. Studies based on historical tide-gauge data for different oceanic basins show that changes in extremes are caused primarily due to changes in mean sea level. Analysis of data from tide gauges along t...

A good understanding of the general circulation features of the oceans, particularly of the coastal waters, and ability to predict the key Oceanographic parameters with good accuracy and sufficient lead time are necessary for the safe conduct of maritime activities such as fishing, shipping, offshore industries etc. Considering these requirements a...

The present study focuses on the dynamics of M2 internal tides on the shelf and slope off the west coast of India (WCI) using a high-resolution (1/48∘) numerical ocean circulation model configured for the eastern Arabian Sea. The WCI is characterized by the presence of large internal tides where most of the internal tides generated on shelf-slope r...

The Indian Ocean Observing System (IndOOS), established in 2006, is a multinational network of sustained oceanic measurements that underpin understanding and forecasting of weather and climate for the Indian Ocean region and beyond. Almost one-third of humanity lives around the Indian Ocean, many in countries dependent on fisheries and rain-fed agr...

Energetics of semidiurnal barotropic and internal tides in the Bay of Bengal (BoB) and Andaman Sea (AS) are studied using the simulations from a high-resolution ocean general circulation model. Barotropic M2 tide, which is the largest tidal constituent in this region, loses about 65.5 GW of energy in the BoB and AS. Most of this barotropic energy (...

Capsule Design and performance of the High-resolution Operational Ocean Forecast and reanalysis System configured for generating analysis and forecasts in the Indian Ocean, especially in the coastal waters are discussed.

A good understanding of the general circulation features of the oceans, particularly of the coastal waters, and ability to predict the key oceanographic parameters with good accuracy and sufficient lead time are necessary for the safe conduct of maritime activities such as fishing, shipping, offshore industries etc. Considering these requirements a...

Tide–surge interaction serves a critical role in shallow coastal regions in modifying the characteristics of peak water levels during storm events. In the present study, we investigated the tide–surge interaction in the coastal waters at the head of the Bay of Bengal (BoB) during Cyclone Aila, a Category 1 cyclone. The space–time characteristics of...

Full Report of IndOOS-2. The Indian Ocean Observing System (IndOOS) is a network of all the sustained observations in the Indian Ocean. Here we present IndOOS-2, designed for the future, to address changing societal and scientific priorities.

Generation and propagation of internal tides in the western Bay of Bengal (BoB) are investigated using observations from Acoustic Doppler Current Profilers and simulations from a very high resolution numerical ocean model. Observations show that semidiurnal internal tides in the southern and northern parts of the western BoB are more energetic duri...

Indian Ocean decadal sea-level variability is an active research area, with many unresolved questions due to inadequate observational coverage. In this study, we analyse 26 Coupled Model Intercomparison Project (CMIP) pre-industrial simulations and isolate two consistent modes of Indian Ocean variability, which collectively explain about 50% of the...

The Indian Ocean Observing System (IndOOS) is an observing network of profiling floats (Argo), a moored tropical array (RAMA), repeat lines of temperature profiles (XBT network), surface drifters (GDP), and tide gauges. Augmenting these networks are satellite remotely-sensed observations, in particular of surface winds, sea level, sea surface tempe...

The Indian Ocean is warming faster than any of the global oceans and its climate is uniquely driven by the presence of a landmass at low latitudes, which causes monsoonal winds and reversing currents. The food, water, and energy security in the Indian Ocean rim countries and islands are intrinsically tied to its climate, with marine environmental g...

Emerging decadal climate predictions call for an assessment of decadal climate variability in the Coupled Model Intercomparison Project (CMIP) database. In this paper, we evaluate the influence of El Niño Southern Oscillation (ENSO) on Pacific Decadal Oscillation (PDO) in 10 control simulations from the CMIP3 and 22 from the CMIP5 database. All mod...

We examine the consistency of Indo-Pacific decadal sea level variability in 10 gridded, observation-based sea level products for the 1960–2010 period. Decadal sea level variations are robust in the Pacific, with more than 50% of variance explained by decadal modulation of two flavors of El Niño–Southern Oscillation (classical ENSO and Modoki). Ampl...

In the present study, we analysed 9-month long data from Acoustic Doppler Current Profilers (ADCP) deployed on the shelf off the east coast of India to study the characteristics of tidal currents in the region. The ADCPs were deployed at about 100–150 m depths off Cuddalore (CD, 12.0°N), Ramayapatnam (RM, 15.0°N), Kakinada (KN, 16.3°N) and South of...

Global tidal solutions are sometimes inaccurate in coastal regions. This is particularly true in some regions of the northern Indian Ocean, which are characterized by wide continental shelves and large tidal ranges. We show that these global solutions are inaccurate on the shelf region off Mumbai, off the west coast of India. In this region, the sh...

25-hour time series measurements of vertical salinity profiles and currents in Zuari estuary were done during both spring and neap tidal cycles of pre-monsoon (April–May), south-west monsoon (June–September) and post-monsoon (October–December) seasons during 2011 at four different stations along the estuary. Measurements were done at locations, nea...

The present article summarizes the research done at the CSIR–National Institute of Oceanography in 2014 in ocean science, resources and technology. Significant research has been conducted on air–sea interactions and coastal circulation, biogeochemistry, biology, marine geophysics, palaeoceanography, marine fishery, gas hydrates and wave energy. Tec...

Changes in extreme sea levels along the east coast of India and at the head of the Bay of Bengal are studied by analyzing hourly tide gauge data at Visakhapatnam, Chennai and Hiron Point. At Hiron Point, the increase in the 99th percentile of sea level shows an increase of 5.2 mm/yr, which is consistent with changes in mean sea level. This tide-gau...

The present communication discusses sea-level-rise trends in the north Indian Ocean, particularly off the Indian coasts, based on estimates derived from satellite altimeter and tide-gauge data. Altimeter data analysis over the 1993–2012 period reveals that the rate of sea-level rise is rather spatially homogeneous over most of the north Indian Ocea...

In his News and Analysis piece reporting on the newly released fifth assessment report (AR5) by Working Group I (WGI) of the Intergovernmental Panel on Climate Change (IPCC) (“A Stronger IPCC Report,” 4 October, p. [23][1]), R. A. Kerr highlights three fundamental conclusions about climate

ISI Document Delivery No.: 274ZI Times Cited: 0 Cited Reference Count: 0 Church, John A. Clark, Peter U. Cazenave, Anny Gregory, Jonathan M. Jevrejeva, Svetlana Levermann, Anders Merrifield, Mark A. Milne, Glenn A. Nerem, R. Steven Nunn, Patrick D. Payne, Antony J. Pfeffer, W. Tad Stammer, Detlef Unnikrishnan, Alakkat S. payne, antony/A-8916-2008;...

A vertically integrated 2D numerical model was developed for the simulation of major tidal constituents (M2, S2, N2, K1 and O1) in the Bay of Bengal. The bathymetry for the model domain was derived from an improved ETOPO5 dataset prepared in our earlier work. The simulated tidal elevations showed good agreement with the hourly tide gauge observatio...

In this study, we analysed decadal and long- term steric sea level variations over 1966–2007 period in the Indo-Pacific sector, using an ocean general circulation model forced by reanalysis winds. The simulated steric sea level compares favourably with sea level from satellite altimetry and tide gauges at interannual and decadal timescales. The amp...

The 27 November 1945 earthquake in the Makran Subduction Zone triggered a destructive tsunami that has left important problems unresolved. According to the available reports, high waves persisted along the Makran coast and at Karachi for several hours after the arrival of the first wave. Long-duration sea-level oscillations were also reported from...

The simulations from the regional climate model, PRECIS (Providing REgional Climates for Impacts Studies), were analysed for the occurrence of tropical cyclones in the Bay of Bengal in a baseline scenario (1961-1990) and a future climate scenario (2071- 2100), A2. The analysis showed an increase in the frequency of cyclones in the Bay of Bengal dur...

Estimates of return periods of extreme sea level events along the coast are useful for impact assessment. In this study, a vertically integrated 2D model was developed for the simulation of storm surges in the Bay of Bengal. The bathymetry for the model was derived from an improved ETOPO-5 data set, which was prepared in our earlier work. The meteo...

Various numerical models used to study the dynamics and horizontal distribution of salinity in Mandovi-Zuari estuaries, Goa, India is discussed in this chapter. Earlier, a one-dimensional network model was developed for representing the complex estuarine system of Mandovi and Zuari. The network model could simulate and explain the observed decay of...

The maritime trade of Orissa with transoceanic countries is evident from the early historical period onwards. It has been observed that new ports were set up after the decline of old ports in the nearby vicinity; however, information on the decline of ancient ports is very sparse. The archival and historical records of the 16th and 19th centuries a...

Present study reviews the characteristics of tidal propagation on the continental shelf off the central west coast of India. Available observations from tide gauges are used to describe the variations of tidal ranges along the west coast. Amplitudes and phases from a global tidal model in the open ocean and those at the coast are used to construct...

IntroductionEvidence for Changes in Extreme Sea Levels and Waves in the Recent PastMid-Latitude and Tropical Storms: Changes in the Atmospheric Drivers of Extreme Sea LevelFuture Extreme Water LevelsFuture Research NeedsConclusions AcknowledgmentsReferences

The Mandovi and Zuari are two tropical estuaries along the west coast of India, characterized by mixed tides with semidiurnal dominance and heavy fresh water influx during the southwest monsoon (June to September). Previous observations of tides in these estuaries show that the tidal effects in the upstream regions are subject to decay due to fresh...

Harmonic analysis of the observed and simulated tides in the Mandovi and Zuari estuaries along the west coast of India was carried out. Tidal constituents derived from this process were analyzed to study the tidal asymmetry in these estuaries. Sea level observations during March-April 2003 at 13 stations in the Mandovi and Zuari estuaries were used...

Analysis of daily rainfall data over India during the period 1951-2007 reveal an increased propensity in the occurrence of ldquomonsoon-breaksrdquo over the subcontinent. The increasing trend is seen both in the duration and frequency of monsoon-breaks over the subcontinent, the causes for which are investigated using in situ , satellite, and reana...

The observed large tidal range (up to 3 m during spring tide) at the Mumbai High offshore region located near the continental shelf break, off the central west coast of India, is described based on simultaneous tidal measurements (30 s average) at 15 min sampling interval using four tide gauges deployed from an oil drilling platform of the Oil and...

The coastal ocean accounts for only 7% of the total oceanic area, but it plays a very important role in biogeochemical cycles. It not only exchanges energy and matter with the open ocean, but terrestrial inputs of materials such as freshwater, sediments, dissolved/particulate nutrients and organic matter by surface runoff and groundwater flow have...

Mean-sea-level data from coastal tide gauges in the north Indian Ocean were used to show that low-frequency variability is consistent among the stations in the basin. Statistically significant trends obtained from records longer than 40 years yielded sea-level-rise estimates between 1.06–1.75 mm yr− 1, with a regional average of 1.29 mm yr− 1, when...

Ocean modellers use bathymetric datasets like ETOPO5 and ETOPO2 to represent the ocean bottom topography. The former dataset is based on digitization of depth contours greater than 200 m, and the latter is based on satellite altimetry. Hence, they are not always reliable in shallow regions. An improved shelf bathymetry for the Indian Ocean region (...

The diurnal cycle of rainfall over the eastern equatorial Indian Ocean was studied for the period 23rd October 2001 to 31st October 2003 using hourly data from the Triton buoy positioned at 1.5° S and 90° E. An analysis of the active and weak spells of rainfall for different seasons revealed peaks in the late evening hours in Winter, Summer and Fal...

Sea level changes can be of two types: (i) changes in the mean sea level and (ii) changes in the extreme sea level. The former is a global phenomenon while the latter is a regional phenomenon. Estimates of mean sea level rise made from past tide gauge data at selected stations along the coast of India indicate a rise of slightly less than 1 mm/year...

This paper describes the hydrographic observations in the southeastern Arabian Sea (SEAS) during two cruises carried out in March–June 2003 as part of the Arabian Sea Monsoon Experiment. The surface hydrography during March–April was dominated by the intrusion of low-salinity waters from the south; during May–June, the low-salinity waters were begi...

Hourly tide gauge data were analyzed for extreme sea level for three stations, Paradip, Vishakhapatnam, and Chennai, along the east coast of India. These stations are tidally dominant, with the magnitudes of tides exceeding those of surges, in general. During the period of analysis (1974-1988), two to four storm surge events were identified in a ye...

A finite element (fe) model using the software package ADCTRC was developed to simulate the tides and currents in the Gulf of Kutch, located on the northwest coast of India. The surface elevations from the model were analysed for the amplitudes and phases of four major tidal constituents (Md2, Sd2, Kd1 and Od1) and the principal overtide (Md4) and...

The monsoon currents are the seasonally reversing, open-ocean currents that flow between the Arabian Sea and the Bay of Bengal, the two wings of the north Indian Ocean. The Summer Monsoon Current (SMC) flows eastward during the summer monsoon (May–September) and the Winter Monsoon Current (WMC) flows westward during the winter monsoon (November–Feb...

A 2D barotropic numerical model is developed for the Gulf of Kutch with a view to synthesize available information on tides and currents in the Gulf. A comparison of model results with moored current meter observations shows that the model reproduces the amplitudes and phases of surface elevations and currents satisfactorily relative to the observa...

Analysis of hydrographic data collected along 80 degrees, 84 degrees and 88 degrees E between 5 degrees N and 3 degrees S during peak south-west monsoon (July-August) of the eastern equatorial Indian Ocean (EEIO) reveals thermohaline variability and anomalous current structure in the upper ocean. A warm (28.4-28.8 degrees C) and less-saline (33 to...

Geostrophic circulation derived from hydrographic data collected during July-August, 1994 along 80, 84 and 88E between 5N and 3S latitudinal belt in the eastern equatorial Indian Ocean is presented. A broad westward flow north of the equator and an eastward flow, constituting the southwest monsoon current (SWMC), in the vicinity of the equator char...

A 2D barotropic numerical model is developed for the Gulf of Kutch with a view to synthesize available information on tides and currents in the Gulf. A comparison of model results with moored current meter observations shows that the model reproduces the amplitudes and phases of surface elevations and currents satisfactorily relative to the observa...

The continental shelf on the west coast of India is widest off Bombay and leads into a strongly converging channel, the Gulf of Khambhat. Tides in the Gulf are among the largest on the coast. We use data on amplitude and phase of major semi-diurnal and diurnal constituents at forty-two ports in the Gulf and surrounding areas to define characteristi...

The Mandovi-Zuari estuarine network on the west coast of India consists of shallow strongly converging channels, that receive large seasonal influx of fresh water due to the monsoons. The main channels, the Mandovi and Zuari estuaries, connect the network to the Arabian Sea. Observations show that tidal amplitude in the channels remains unchanged o...

Determination of amplitudes and phases for the annual and semi-annual cycle of the temperature in the Indian Ocean north of 20S from Levitus temperature climatology (1982) gives maximum amplitudes of the seasonal cycle at 100 m with the dominance of semi-annual cycle in the equatorial region and annual cycle elsewhere in the domain. The Bay of Beng...

The seasonal cycle of temperature in the monthly mean Levitus (1982) data is analyzed by a least squares method fitting the observed temperature with a cosine function having a period of 1 year. It is found that the signal is most pronounced in the subsurface levels (100-200 m), with high amplitudes in the western and eastern parts of the Bay of Be...