Ocean-atmosphere interactions in the North Indian Ocean play a vital role in the onset, progression and withdrawal of the Indian monsoon. This paper describes the Ocean Observation System (OOS), an operational observational programme of the Earth System Science Organization and the National Institute of Ocean Technology (ESSO-NIOT) under India's Ministry of Earth Sciences (MoES). Since 1997 it has provided oceanographic and surface meteorological data in real time for weather forecasting, climate research and several other applications. The programme focuses on understanding the phenomenon of the mean seasonal cycle of the Indian monsoon, the intra-seasonal to intra-decadal oscillations of air-sea interactions, trends that are related to tropical cyclones and the annual cycle balance in the exchange of waters between the two limbs of North Indian Ocean, i.e. the Arabian Sea and the Bay of Bengal. In situ observations are also used to develop, initialise and validate regional forecast models that provide high resolution data. There is also a growing need to understand the spatial phenomenon of oceans using satellite observations, wherein the quality of data needs to be validated and verified carefully. This paper also provides an overview of the scientific and societal impact of the Indian moored buoy network over two decades of operation. ARTICLE HISTORYHistorically, ocean surface measurements have been made via ships of opportunity, cargo vessels that sail the oceans to conduct maritime trade. These measurements were made for the safety of the ships' navigation,

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The Andaman coral reef region experienced mass bleaching events during 1998 and 2010. The purpose of this study is to investigate the role of the El Niño in the coral reef bleaching events of the Andaman region. Both Niño 3.4 and 3 indices were examined to find out the relationship between the mass bleaching events and El Niño, and correlated with sea surface temperature (SST) anomalies in the Andaman Sea. The result shows that abnormal warming and mass bleaching events in the Andaman Sea were seen only during strong El Niño years of 1997-1998 and 2009-2010. The Andaman Sea SST was more elevated and associated with El Niño Modoki (central Pacific El Niño) than conventional El Niño (eastern Pacific El Niño) occurrences. It is suggested that the development of hot spot patterns around the Andaman Islands during May 1998 and April-May 2010 may be attributed to zonal shifts in the Walker circulation driven by El Niño during the corresponding period.

The characteristic of cold core eddies and its influence on the hydrodynamics and biological production in Andaman waters were studied using in situ and satellite observations. The specific structure and patterns of the temperature-salinity (T-S) profiles, nutrients and chl a indicate the occurrence of the eddy, the spatial extent of which is well marked in sea surface height anomaly (SSHA). The Cyclonic Eddies are centered at 7N and 86E, 13N and 88E and 13N and 93E (CE1, CE2 and CE3 respectively). In situ measurements are done in the eastern flank CE1 along 8N and 92.5-93.5E. Vertical currents recorded using Acoustic Doppler Current Profiles (ADCP) shows northward flow along the track (0.3 m s -1 ) while along the western flank, the flow is weak and southward. This evidence the occurrence of cyclonic eddy and the altimetry derived SSHA depicts the spatial extent. Analysis to explore the possible forcing to induce the occurrence of eddy, indicate baroclinic instability (Ri <0.0001) in the water column due to vertical shear in the horizontal flow. Bay of Bengal (BoB) water evidenced from the T-S profiles and the semi-annual Rossby wave are the contributing factors of eddy formation. Whereas, the wind stress curl is not a major inductive of divergence in the region. The eddy influenced the nutrient pattern (NO2, NO3, PO4 and SiO4) and the biological production (chl a) in the region though the influence is less significant. CE1 and CE2 are similar in terms of forcing mechanisms while, CE3 is associated with convective mixing processes occurring along the northwest coast of Andaman due to the prevalent cold dry continental air from north east.