13 research outputs found

    ASIRI: An Ocean–Atmosphere Initiative for Bay of Bengal

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    Air–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange along the periphery of Sri Lanka, which includes the 100-km-wide East India Coastal Current (EICC) carrying low-salinity water out of the BoB and an adjacent, broad northward flow (∼300 km wide) that carries high-salinity water into BoB during the northeast monsoon. Atmospheric boundary layer (ABL) observations during the decaying phase of the Madden–Julian oscillation (MJO) permit the study of multiscale atmospheric processes associated with non-MJO phenomena and their impacts on the marine boundary layer. Underway analyses that integrate observations and numerical simulations shed light on how air–sea interactions control the ABL and upper-ocean processes

    Evaluation of bio-optical inversion of spectral irradiance measured from an autonomous underwater vehicle

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    Autonomous underwater vehicles (AUVs) can map water conditions at high spatial (horizontal and vertical) and temporal resolution, including under cloudy conditions when satellite and airborne remote sensing are not feasible. As part of the RADYO program, we deployed a passive radiometer on an AUV in the Santa Barbara Channel and off the coast of Hawaii to apply existing bio-optical algorithms for characterizing the optical constituents of coastal seawater (i.e., dissolved organic material, algal biomass, and other particles). The spectral differences between attenuation coefficients were computed from ratios of downwelling irradiance measured at depth and used to provide estimates of the in-water optical constituents. There was generally good agreement between derived values of absorption and concurrent measurements of this inherent optical property in Santa Barbara Channel. Wave focusing, cloud cover, and low attenuation coefficients influenced results off the coast of Hawaii and are used to evaluate the larger-scale application of these methods in the near surface coastal oceans

    ASIRI : an ocean–atmosphere initiative for Bay of Bengal

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    Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 97 (2016): 1859–1884, doi:10.1175/BAMS-D-14-00197.1.Air–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange along the periphery of Sri Lanka, which includes the 100-km-wide East India Coastal Current (EICC) carrying low-salinity water out of the BoB and an adjacent, broad northward flow (∼300 km wide) that carries high-salinity water into BoB during the northeast monsoon. Atmospheric boundary layer (ABL) observations during the decaying phase of the Madden–Julian oscillation (MJO) permit the study of multiscale atmospheric processes associated with non-MJO phenomena and their impacts on the marine boundary layer. Underway analyses that integrate observations and numerical simulations shed light on how air–sea interactions control the ABL and upper-ocean processes.This work was sponsored by the U.S. Office of Naval Research (ONR) in an ONR Departmental Research Initiative (DRI), Air–Sea Interactions in Northern Indian Ocean (ASIRI), and in a Naval Research Laboratory project, Effects of Bay of Bengal Freshwater Flux on Indian Ocean Monsoon (EBOB). ASIRI–RAWI was funded under the NASCar DRI of the ONR. The Indian component of the program, Ocean Mixing and Monsoons (OMM), was supported by the Ministry of Earth Sciences of India.2017-04-2

    Numerical Modelling of Tidally Generated Internal Wave Radiation From the Andaman Sea Into the Bay of Bengal

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    Semi-diurnal internal waves generated by tides in a high resolution numerical model that includes the Andaman Sea archipelago are found to propagate into the central Bay of Bengal and reach the coasts of India and Sri Lanka. The waves are also present in subsurface temperature records from RAMA moorings, and their propagation speed across the Bay of Bengal agrees well with satellite remote sensing from MODIS imagery in spite of the hydrostatic nature of the model. The internal waves are simulated by a fully coupled ocean-atmosphere prediction system, exchanging surface fluxes between the air and sea at high frequency and at high resolution. For the ocean, a hydrostatic model including diurnal and semi-diurnal tides provides a 2 km resolution representation of the entire Bay of Bengal. In the ocean model simulations, the semi-diurnal internal waves interact with the mesoscale circulation and surface waves and modify the flow and the stratification. By comparing coupled ocean-wave model runs with tides and without tides, but forced by identical surface fluxes from the atmosphere, it is demonstrated that the inclusion of diurnal and semi-diurnal tides act to cool the core of the thermocline while increasing the temperature above and below it along the pathway of the internal waves, a result that likely is due to vertical mixing by the waves

    Eddies, topography, and the abyssal flow by the Kyushu-Palau Ridge near Velasco Reef

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    © The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Andres, M., Siegelman, M., Hormann, V., Musgrave, R. C., Merrifield, S. T., Rudnick, D. L., Merrifield, M. A., Alford, M. H., Voet, G., Wijesekera, H. W., MacKinnon, J. A., Centurioni, L., Nash, J. D., & Terrill, E. J. Eddies, topography, and the abyssal flow by the Kyushu-Palau Ridge near Velasco Reef. Oceanography, 32(4), (2019): 46-55, doi: 10.5670/oceanog.2019.410.Palau, an island group in the tropical western North Pacific at the southern end of Kyushu-Palau Ridge, sits near the boundary between the westward-​flowing North Equatorial Current (NEC) and the eastward-flowing North Equatorial Countercurrent. Combining remote-sensing observations of the sea surface with an unprecedented in situ set of subsurface measurements, we examine the flow near Palau with a particular focus on the abyssal circulation and on the deep expression of mesoscale eddies in the region. We find that the deep currents time-averaged over 10 months are generally very weak north of Palau and not aligned with the NEC in the upper ocean. This weak abyssal flow is punctuated by the passing of mesoscale eddies, evident as sea surface height anomalies, that disrupt the mean flow from the surface to the seafloor. Eddy influence is observed to depths exceeding 4,200 m. These deep-​reaching mesoscale eddies typically propagate westward past Palau, and as they do, any associated deep flows must contend with the topography of the Kyushu-Palau Ridge. This interaction leads to vertical structure far below the main thermocline. Observations examined here for one particularly strong and well-sampled eddy suggest that the flow was equivalent barotropic in the far field east and west of the ridge, with a more complicated vertical structure in the immediate vicinity of the ridge by the tip of Velasco Reef.We gratefully acknowledge the help of Captain David Murline and the crew of R/V Roger Revelle and the shore-based assistance of Lori Colin and Pat Colin of the Coral Reef Research Foundation. We sincerely thank Terri Paluszkiewicz for her steadfast support of basic research programs, including FLEAT, during her many years of service to the community as Office of Naval Research (ONR) Physical Oceanography Program Manager. MA was supported by ONR grant N000141612668, MS and MAM by N00014-16-1-2671, MHA and JAM by N00014-15-1-2264 and N00014-16-1-3070, GV by N00014-15-1-2592, DLR by N00014- 15-1-2488, and STM and EJT by N00014-15-1-2304. VH and LC were supported by ONR grant N00014-15-1-2286 and NOAA GDP grant NA15OAR4320071. RCM was supported by the Postdoctoral Scholar Program at the Wood Hole Oceanographic Institution, with funding provided by the Weston Howland Jr. Postdoctoral Scholarship. We thank the Palau National Government for permission to carry out the research in Palau
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