On the Mechanisms of Episodic Salinity Outflow Events in the Strait of Hormuz

Observations in the Strait of Hormuz (26.26 degrees N, 56.08 degrees E) during 1997-98 showed substantial velocity fluctuations, accompanied by episodic changes in the salinity outflow events with amplitude varying between 1 and 2 psu on time scales of several days to a few weeks. These events are characterized by a rapid increase in salinity followed by an abrupt decline. The mechanisms behind these strong pulses of salinity events are investigated with a whigh-resolution (similar to 1 km) Hybrid Coordinate Ocean Model (HYCOM) with particular reference to the year 2005. In accordance with the observations, the simulated salinity events are characterized by strong coherence between the enhanced flows in zonal and meridional directions. It is inferred that most of the simulated and observed outflow variability is associated with the continuous formation of strong mesoscale cyclonic eddies, whose origin can be traced upstream to around 26 degrees N, 55.5 degrees E. These cyclonic eddies have a diameter of about 63 km and have a remnant of Persian Gulf water (PGW) in their cores, which is eroded by lateral mixing as the eddies propagate downstream at a translation speed of 4.1 cm s(-1). The primary process that acts to generate mesoscale cyclones results from the barotropic instability of the exchange circulation through the Strait of Hormuz induced by fluctuations in the wind stress forcing. The lack of salinity events and cyclogenesis in a model experiment with no wind stress forcing further confirms the essential ingredients required for the development of strong cyclones and the associated outflow variability

Persian Gulf response to a wintertime shamal wind event

The results from a~1 km resolution Hybrid Coordinate Ocean Model (HYCOM),forced by 1/2° Navy Operational Global Atmospheric Prediction System (NOGAPS) atmospheric data, were used in order to study the dynamic response of the Persian Gulf to winter time shamal forcing. Shamal winds are strong northwesterly winds that occur in the Persian Gulf area behind southeast moving cold fronts. The period from 20 November to 5 December 2004 included a well-defined shamal event that lasted 4–5 days. In addition to strong winds (16ms_1) the winter shamal also brought cold dry air(Ta=20 °C, qa=10 gkg-1) which led to a net heat loss in excess of 1000 W m-2 by increasing the latent heat flux. This resulted in SST cooling of up to 10°C most notably in the northern and shallower shelf regions. A sensitivity experiment with a constant specific humidity of qa= 15 gkg-1 confirmed that about 38% of net heat loss was due to the air– sea humidity differences. The time integral of SST cooling closely followed the air–sea heat loss, indicating an approximate one-dimensional vertical heat balance. It was found that the shamal induced convective vertical mixing provided a direct mechanism for the erosion of stratification and deepening of the mixed layer by 30m. The strong wind not only strengthened the circulation in the entire Persian Gulf but also established a northwestward flowing Iranian Coastal Current (ICC,25–30cms-1) from the Strait of Hormuz to about 52°E, where it veered off shore. The strongest negative sea level of 25–40cm was generated in the northern most portion of the Gulf while the wind set up against the coast of the United Arab Emirates established a positive sea level of 15–30 cm. The transport through the Strait of Hormuz at 56.2°E indicated an enhanced out flow of 0.25Sv (Sv=106 m3 s-1) during 24 November followed by an equivalent in flow on the next day

Enhanced phytoplankton bloom triggered by atmospheric high-pressure systems over the Northern Arabian Sea

Abstract During winter, the dry, cool air brought by prevailing northeasterly trade winds leads to surface ocean heat loss and convective mixing in the northern Arabian Sea. The current paradigm is that the convective mixing process leads to the injection of nutrients up into the surface waters and exert a dominant control on winter productivity. By combining a variety of observations, atmospheric reanalysis and model simulations, we unraveled the processes responsible for the observed year-to-year chlorophyll-a variations in the northern Arabian Sea. Our findings suggest that the atmospheric high-pressure systems that traverse the northern Arabian Sea every winter and spring disrupt winter convective mixing and create an array of environmental conditions conducive to trigger phytoplankton blooms. The arrival of an atmospheric high with the anticyclonic flow in the northern Arabia Sea sets the stage for a sequence of events culminating in intermittent mixed-layer restratification due to buoyancy gain aided by increased specific humidity, supplemented with abundant sunlight due to clear skies, and suppressed turbulent mixing owing to weak winds. These combined with the mixed layer that is shallower than the euphotic zone and the influx of nutrients into the euphotic zone brought by convective mixing between the calm periods, caused unprecedented high concentrations of chlorophyll-a in the northern Arabian Sea

Physical and Biological Responses to Hurricane Katrina (2005) in a 1/25 Degrees Nested Gulf of Mexico HYCOM

Recent studies indicated sea surface temperature (SST) cooling of 6-7 degrees C and a phytoplankton bloom of 3 mg chlorophyll-a m(-3) during the passage of Hurricane Katrina (23-30 August 2005) in a region from 23.5 degrees to 25.5 degrees N and 85 degrees to 83 degrees W in the Gulf of Mexico (GoM). Employing a 20-layer. 1/25 degrees horizontal resolution nested CoM HYbrid Coordinate Ocean Model (HYCOM), the evolving three-dimensional ocean response to Hurricane Katrina in the GoM was examined. During the passage of Hurricane Katrina, analysis of model surface and subsurface dynamics in this region revealed strong upwelling/downwelling of 1.5-2 x 10(-4) m s(-1), wind-driven currents dominating the surface circulation, and near-inertial oscillations following Hurricane Katrina. Associated with the storm, the 26 degrees C isotherm was raised by 28 m, generating SST cooling of 3-4 degrees C and salinity freshening of 0.1-0.2 in less than 24 h. Comparison of model-simulated SSTs with in situ buoy data and satellite observations revealed that model SSTs were cooler by 1-2 degrees C and had a greater spatial extent of cooling. Analysis of heat budget terms in the mixed layer (20 m) indicated that surface heat flux accounted for pre-storm temperature changes, and wind-driven mixing (-3375 W m(-2)) dominated net upper-ocean cooling (-2464 W m(-2)) during Hurricane Katrina. At 50 m depth, temperature changes were largely due to vertical advection associated with upwelling and downwelling processes. A temperature-nitrate relationship was derived to illustrate the potential contribution that nitrate influx had upon the satellite-observed phytoplankton bloom associated with Hurricane Katrina. Comparison of calculated nitrate agreed reasonably well with in situ nitrate profiles in the interest region. Nitrate concentrations of 3.7 mu M were entrained from 30 m depth during hurricane passage. An approximate nitrate to chlorophyll-a ratio provided a chlorophyll-a value of 3 mg m(-3), which was consistent with that derived from satellite. Thus, the elevated chlorophyll-a concentration following the passage of Katrina was greatly influenced by nitrate entrainment into the surface layer through vertical mixing and Ekman divergence. (C) 2009 Elsevier B.V. All rights reserved

Blooms of \u3ci\u3eNoctiluca miliaris\u3c/i\u3e in the Arabian Sea - An In Situ and Satellite Study

Phytoplankton cell density, chlorophyll a (chl a) concentration and pigment data collected during a series of five cruises in the northern Arabian Sea in the Northeast Monsoon (NEM, Nov-Jan) and the Spring Intermonsoon (SIM, Mar-May) since 2003 contradicted the established notion that winter blooms mainly consist of diatom communities. Recent data show that following the NEM and well into the SIM, phytoplankton populations are dominated by the dinoflagellate Noctiluca miliaris Suriray (synonym Noctiluca scintillans Macartney). In the SIM they were often in association with the well-known blooms of the diazotroph Trichodesmium sp. Large blooms of N. miliaris have also begun making their appearance annually in the Gulf of Oman and off the coast of Oman. This study uses NASA\u27s recently developed product of merged SeaWiFS and Aqua-MODIS chl a data to investigate the temporal evolution and spatial extent of these taxonomically validated blooms. Satellite chl a in relation to Aqua-MODIS SST and altimetry data suggest that mesoscale eddies that populate the western Arabian Sea during the NEM contribute to the genesis and dispersal of these blooms from the Gulf of Oman into the central Arabian Sea. (c) 2008 Elsevier Ltd. All rights reserved

Data assimilation considerations for improved ocean predictability during the Gulf of Mexico Grand Lagrangian Deployment (GLAD)

•Extensive drifter observations allow new understanding to data assimilation.•Background error covariance is the point at which assumptions have historically been placed.•Components of background error covariance are tested to determine impact.•Amplitude of background error covariance is a critical factor.•Time correlation in background errors must be considered in 3DVar and 4DVar.Ocean prediction systems rely on an array of assumptions to optimize their data assimilation schemes. Many of these remain untested, especially at smaller scales, because sufficiently dense observations are very rare. A set of 295 drifters deployed in July 2012 in the north-eastern Gulf of Mexico provides a unique opportunity to test these systems down to scales previously unobtainable. In this study, background error covariance assumptions in the 3DVar assimilation process are perturbed to understand the effect on the solution relative to the withheld dense drifter data. Results show that the amplitude of the background error covariance is an important factor as expected, and a proposed new formulation provides added skill. In addition, the background error covariance time correlation is important to allow satellite observations to affect the results over a period longer than one daily assimilation cycle. The results show the new background error covariance formulations provide more accurate placement of frontal positions, directions of currents and velocity magnitudes. These conclusions have implications for the implementation of 3DVar systems as well as the analysis interval of 4DVar systems