The upper-ocean response to monsoonal forcing in the Arabian Sea: seasonal and spatial variability
01 Jan 2000-Deep-sea Research Part Ii-topical Studies in Oceanography (Pergamon)-Vol. 47, Iss: 7, pp 1177-1226
TL;DR: In this article, the seasonal and spatial variability of the upper ocean response to the Monsoon cycle in the Arabian Sea was examined using four towed profiler surveys conducted between December 1994 and October 1995.
Abstract: Observations from four towed profiler surveys undertaken between December 1994 and October 1995 examine the seasonal and spatial variability of the upper ocean response to the Monsoon cycle in the Arabian Sea. Although observed atmospheric forcing agrees well with modern climatologies, cross-basin patterns of mixed-layer depth and water properties observed in 1994–1995 are not entirely consistent with an upper-ocean response dominated by Ekman pumping. During the winter monsoon, the mixed-layer deepens dramatically with distance offshore. Surface cooling intensifies with offshore distance, and a one-dimensional response dominated by convective overturning could explain observed wintertime mixed-layer depths. Except for waters associated with a filament extending offshore from the Omani coast, mixed-layer depths and water properties show only modest cross-basin contrasts during the Southwest Monsoon. Filament waters differ from surrounding mid-basin waters, having shallow mixed-layers and water properties similar to those of waters upwelled near the Omani coast. In September, following the Southwest Monsoon, waters within 1000 km of the Omani coast have cooled and freshened, with marked changes in stratification extending well into the pycnocline. Estimates of Ekman pumping and wind-driven entrainment made using the Southampton Oceanographic Center 1980–1995 surface flux and the Levitus mixed-layer climatologies indicate that during the Southwest Monsoon wind-driven entrainment is considerably stronger than Ekman pumping. Inshore of the windstress maximum, Ekman pumping partially counters wind-driven entrainment, while offshore the two processes act together to deepen the mixed-layer. As Ekman pumping is too weak to counter wind-driven mixed-layer deepening inshore of the windstress maximum, another mechanism must act to maintain the shallow mixed-layers seen in our observations and in climatologies. Offshore advection of coastally upwelled water offers a mechanism for maintaining upper ocean stratification that is consistent with observed changes in upper ocean water properties. Ekman upwelling will modulate wind-driven entrainment, but these results indicate that the primary mechanisms acting inshore of the windstress maximum are wind-driven mixing and horizontal advection.
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TL;DR: In this article, the authors review observations, theory and model results on the monsoon circulation of the Indian Ocean and discuss possible physical mechanisms behind seasonal variability of the meridional overturning streamfunction and heat flux.
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TL;DR: PISCES-v2 as mentioned in this paper is a biogeochemical model which simulates the lower trophic levels of marine ecosystems and the bio-ochemical cycles of carbon and of the main nutrients (P, N, Fe, and Si) and is intended to be used for both regional and global configurations at high or low spatial resolutions as well as for short-term (seasonal, interannual) and long-term analyses.
Abstract: . PISCES-v2 (Pelagic Interactions Scheme for Carbon and Ecosystem Studies volume 2) is a biogeochemical model which simulates the lower trophic levels of marine ecosystems (phytoplankton, microzooplankton and mesozooplankton) and the biogeochemical cycles of carbon and of the main nutrients (P, N, Fe, and Si). The model is intended to be used for both regional and global configurations at high or low spatial resolutions as well as for short-term (seasonal, interannual) and long-term (climate change, paleoceanography) analyses. There are 24 prognostic variables (tracers) including two phytoplankton compartments (diatoms and nanophytoplankton), two zooplankton size classes (microzooplankton and mesozooplankton) and a description of the carbonate chemistry. Formulations in PISCES-v2 are based on a mixed Monod–quota formalism. On the one hand, stoichiometry of C / N / P is fixed and growth rate of phytoplankton is limited by the external availability in N, P and Si. On the other hand, the iron and silicon quotas are variable and the growth rate of phytoplankton is limited by the internal availability in Fe. Various parameterizations can be activated in PISCES-v2, setting, for instance, the complexity of iron chemistry or the description of particulate organic materials. So far, PISCES-v2 has been coupled to the Nucleus for European Modelling of the Ocean (NEMO) and Regional Ocean Modeling System (ROMS) systems. A full description of PISCES-v2 and of its optional functionalities is provided here. The results of a quasi-steady-state simulation are presented and evaluated against diverse observational and satellite-derived data. Finally, some of the new functionalities of PISCES-v2 are tested in a series of sensitivity experiments.
466 citations
TL;DR: The Bay of Bengal is traditionally considered to be a less productive basin compared to the Arabian Sea as discussed by the authors, and the authors explore the reasons for this in the central Bay during summer and show that abundant rainfall and river water freshen the upper layers of the Bay by 3-7 psu during summer, and SST was warmer by 15-2°C than the central Arabian Sea.
Abstract: [1] The Bay of Bengal is traditionally considered to be a less productive basin compared to the Arabian Sea We explore the reasons for this in the central Bay during summer Copious rainfall and river water freshen the upper layers of the Bay by 3–7 psu during summer, and SST was warmer by 15–2°C than in the central Arabian Sea This leads to a strongly stratified surface layer The weaker winds over the Bay are unable to erode the strongly stratified surface layer, thereby restricting the turbulent wind-driven vertical mixing to a shallow depth of <20 m This inhibits introduction of nutrients from below, situated close to the mixed layer bottom, into the upper layers While advection of nutrients rich water into the euphotic zone makes the Arabian Sea highly productive, this process is unlikely in the Bay of Bengal
423 citations
TL;DR: The Modular Ocean Data Assimilation System (MODAS) is used by the U.S. Navy for depiction of three-dimensional fields of temperature and salinity over the global ocean as mentioned in this paper.
Abstract: The Modular Ocean Data Assimilation System (MODAS) is used by the U.S. Navy for depiction of three- dimensional fields of temperature and salinity over the global ocean. MODAS includes both a static climatology and a dynamic climatology. While the static climatology represents the historical averages, the dynamic cli- matology assimilates near-real-time observations of sea surface height and sea surface temperature and provides improved temperature and salinity fields. The methodology for the construction of the MODAS climatology is described here. MODAS is compared with Levitus and Generalized Digital Environmental Model climatologies and with temperature and salinity profiles measured by SeaSoar in the Japan/East Sea to illustrate MODAS capabilities. MODAS with assimilated remotely sensed data is able to portray time-varying dynamical features that cannot be represented by static climatologies.
310 citations
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...SeaSoar measurements were processed in a manner similar to that described in Lee et al. (2000) to yield vertical profiles with approximately 3-km horizontal resolution and 4-m vertical resolution....
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2,364 citations
TL;DR: In this article, a technique for the objective analysis of oceanic data has been developed and used on simulated data, which is based on a standard statistical result, the Gauss-Markov Theorem, which gives an expression for the least square error linear estimate of some physical variable given measurements at a limited number of data points, the statistics of the field being estimated in the form of space-time spectra, and the measurement errors.
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TL;DR: In this article, a 212-layer, thermodynamic numerical model is used to study the dynamics, thermodynamics and mixed-layer physics of Indian Ocean circulation, and a surface mixed layer of temperature Tm is imbedded in the upper layer of the model, and entrainment and detrainment in the mixed layer are determined by wind stirring and surface cooling.
659 citations
TL;DR: The high-energy flow, in the form of low-level southerly jet streams which have been reported earlier over Kenya, is only one part of a much more extensive current of air which flows rapidly around the western half of the Indian Ocean during the northern summer as mentioned in this paper.
Abstract: It is demonstrated that high-energy flow, in the form of low-level southerly jet streams which have been reported earlier over Kenya, is only one part of a much more extensive current of air which flows rapidly around the western half of the Indian Ocean during the northern summer. The high speeds are associated with the concentration of the cross-equatorial airflow into the zone from longitude 38 deg E to about 55 deg E instead of being rather evenly distributed from 40 deg to 60 deg or 80 deg E as illustrated in many climatological atlases and charts of mean flow during the season. The high-speed current is shown to flow intermittently from the vicinity of Mauritius through Madagascar, Kenya, eastern Ethiopia, Somalia and thence across the Indian Ocean to the west coast of India and beyond. The stream is occasionally reinforced by northward flow through the Mozambique Channel.
The high-speed air current, or system of low-level jet streams, is closely associated with the Intertropical Convergence Zone over the Arabian Sea and western India, and variations in the strength of the stream over Kenya during a two month period were related to the rainfall which western India received from the south-west monsoon.
598 citations
TL;DR: In this paper, spatial and temporal variability in concentration and optical properties of colored dissolved organic matter (CDOM) were examined in the Arabian Sea during the 1995 Southwest (Summer) Monsoon on NRL/Seasoar Cruise, 21 June-12 July 1995.
Abstract: Spatial and temporal variability in concentration and optical properties of colored dissolved organic matter (CDOM) were examined in the Arabian Sea during the 1995 Southwest (Summer) Monsoon on NRL/Seasoar Cruise, 21 June–12 July 1995. Classical upwelling patterns were observed near the coast, with strong fronts characterized by cold water, high chlorophyll a, and high nutrients. CDOM concentration showed a negative correlation with temperature in the surface waters throughout the study region, reflecting upwelling as the major source of CDOM to surface waters. Three water mass types were identified and found to have distinctive excitation–emission matrix (EEM) fingerprints. Persian Gulf Water had two fluorescence maxima due to UV and visible humic substance fluorescence. Maximum CDOM concentrations were found in the core of this water mass. Upwelling water had five or six fluorescence peaks, due to new and old CDOM. Elevated CDOM concentrations at the depth of the chlorophyll a maximum indicated production of new CDOM associated with the phytoplankton bloom. Oligotrophic water had extremely low CDOM concentrations, and only UV-absorbing and fluorescing compounds were found. This is consistent with CDOM which has been subjected to extensive exposure to solar irradiation at the sea surface. A new fluorophore with an Ex/Em maximum at 280/340 nm is described from highly productive waters. Two new EEM fingerprints are presented, one for oligotrophic blue waters showing photoresistant DOM, and one for upwelling water that may be of biological origin.
589 citations