The Ras al Hadd Jet: Remotely sensed and acoustic Doppler current profiler observations in 1994–1995
01 Aug 1999-Deep-sea Research Part Ii-topical Studies in Oceanography (Pergamon)-Vol. 46, Iss: 8, pp 1531-1549
TL;DR: The existence of a surface barotropic front-jet system at the confluence region off the eastern tip of Oman (Ras Al Hadd or RAH) is documented for 1994-1995 through advanced very high resolution radiometer (AVHRR) and acoustic Doppler current profiler (ADCP) observations as mentioned in this paper.
Abstract: The existence of a surface barotropic front-jet system at the confluence region off the eastern tip of Oman (Ras Al Hadd or RAH) is documented for 1994–1995 through advanced very high resolution radiometer (AVHRR) and acoustic Doppler current profiler (ADCP) observations. The thermal signature of this confluence is visible in 1995 between early May and the end of October, i.e., throughout the SW Monsoon and into the transition period between SW and NE Monsoons. The thermal characteristics are those of a NE-oriented front between cooler water of southern (upwelled) origin and warmer waters of northern Gulf of Oman origin. During the period when the thermal front is absent, ADCP data suggest that the confluence takes a more southward direction with Gulf of Oman waters passing RAH into the southeastern Oman coastal region. The thermal gradient is initially small (June–July) but later increases (August–October) into a front that exhibits small-scale instabilities. Surface current velocities within the jet, estimated by tracking these features in consecutive satellite images, are 0.5–0.7 m s−1 and in remarkable agreement with concurrent ADCP retrievals in which the seasonal maximum in velocity is 1 m s−1. ADCP observations collected during several US JGOFS cruises reveal a weakly baroclinic current in the confluence region that drives the waters into the offshore system. The fully developed jet describes a large meander that demarcates two counter-rotating eddies (cyclonic to the north and anticyclonic to the south of the jet) of approximately 150–200 km diameter. The southern eddy of this pair is resolved by the seasonally averaged, sea-level anomaly derived from TOPEX/Poseidon observations. During the SW Monsoon, the RAH Jet advects primarily cold waters along its path, but as soon as the wind system reverses with the transition to the intermonsoonal period, a warm current is rapidly established that advects the surface coastal waters of the Gulf of Oman offshore. In accordance with the interannual variation of the wind forcing phase, the reversal of the currents from NE to SW occurred earlier in 1994 than in 1995, confirming that the RAH Jet is integral part of the East Arabian Current. The transport of the Jet, estimated by combining SST information on the width with ADCP data on the velocity's vertical structure, is found to fluctuate between 2–8×106 m3 s−1 and its thickness between 150–400 m. These significant fluctuations are due to the time-variable partition of horizontal transport between eddies and the RAH Jet and are potentially important to the nutrient and phytoplankton budgets of the Arabian Sea.
Citations
More filters
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.
1,437 citations
TL;DR: In this article, the authors used an Oceanic General Circulation Model (OGCM) to simulate these currents and estimate their transports, and a 11/2-layer reduced-gravity model to investigate the processes that force them.
655 citations
TL;DR: In this article, the authors investigated the thermocline flows related to the shallow overturning circulations and estimates of subduction and upwelling in the Indian Ocean and concluded that the roll is mostly confined to the surface-mixed layer and is of little consequence for the meridional heat transport.
218 citations
TL;DR: In this paper, the authors present an overview of the dynamical response to seasonal monsoonal forcing and the characteristics of the physical environment that fundamentally drive regional biogeochemical variability.
210 citations
TL;DR: In situ measurements of phytoplankton chlorophyll in the Arabian Sea were taken largely along temporally and spatially unevenly distributed sections, especially prior to the operation of NASA's Coastal Zone Color Scanner (CZCS) as discussed by the authors.
Abstract: In situ measurements of phytoplankton chlorophyll in the Arabian Sea were taken largely along temporally and spatially unevenly distributed sections, scarce especially prior to the operation of NASA's Coastal Zone Color Scanner (CZCS). Herein, the CZCS pigment observations between late 1978 and mid-1986 north of 10°N, including the outer Gulf of Oman, are depicted for 14 subregions beyond the continental shelves as daily means, often only five days apart. To eliminate bias from electronic overshoot, the data were reprocessed with a more conservative cloud screen than used for NASA's Global Data Set. The pattern, derived from the older in situ observations, of one period with elevated chlorophyll almost everywhere during the Southwest Monsoon (SWM) and one additional late-winter bloom in the north, is confirmed. The differing nitrate silicate ratios in freshly entrained water in the central and northern Arabian Sea seem to lead to different succession and perhaps to differing vertical fluxes, and during winter favor blooms only in the north. The spatial pigment pattern in the outer Gulf of Oman is not an extension of that of the northwestern Arabian Sea. The seasonal physical forcing explains much of the timing of pigment concentration changes, but not the levels maintained over long periods. From the CZCS observations it is unclear whether the period of high phytoplankton productivity expected during the SWM in the open Arabian Sea lasts for about two or four months. During this entire season, chlorophyll values in the upper layers rarely exceed 1–2 mg m −3 outside the zone influenced by the Arabian upwelling. Near 15°N, however, fluxes into sediment traps at 3 km depth indicate an onset of high primary production very soon after the arrival of the SWM and suggest a long period of high production in the open sea. The partial temporal disconnect during the SWM between pigment changes in the upper part of the euphotic zone and of fluxes into the traps is disconcerting. For future modeling of plankton production in the open Arabian Sea, the use of two size classes of phytoplankton is recommended. The utility of satellite-derived pigment concentrations (as opposed to temporal changes of pigment) for testing such models is questioned.
118 citations
References
More filters
Book•
01 Jan 1994
TL;DR: In this article, what drives the ocean currents? temperature, salinity, density and the oceanic pressure field the Coriolis force, geostropy, Rossby waves and the westwood intensification Ekman layer transports, Ekman pumping and the Sverdrup balance water mass formation, subduction and oceanic heat budget Antarctic oceanography Arctic oceanography.
Abstract: Introduction - what drives the ocean currents? temperature, salinity, density and the oceanic pressure field the Coriolis force, geostropy, Rossby waves and the westwood intensification Ekman layer transports, Ekman pumping and the Sverdrup balance water mass formation, subduction and the oceanic heat budget Antarctic oceanography Arctic oceanography -the path of North Atlantic Deep Water the Pacific Ocean hydrology of the Pacific Ocean adjacent seas of the Pacific Ocean the Indian Ocean hydrology of the Indian Ocean adjacent seas of the Indian Ocean and the Australasian Mediterranean Sea the Atlantic Ocean hydrology of the Atlantic Ocean aspects of advanced regional oceanography the oceans and the world's mean climate El Nino and the Southern oscillation (ENSO) the ocean and climate change.
1,261 citations
TL;DR: In this paper, an algorithm to detect fronts in satellite-derived sea surface temperature fields is presented, which relies on a combination of methods and operates at the picture, the window, and the local level.
Abstract: An algorithm to detect fronts in satellite-derived sea surface temperature fields is presented. Although edge detection is the main focus, the problem of cloud detection is also addressed since unidentified clouds can lead to erroneous edge detection. The algorithm relies on a combination of methods and it operates at the picture, the window, and the local level. The resulting edge detection is not based on the absolute strength of the front, but on the relative strength depending on the context, thus, making the edge detection temperature-scale invariant. The performance of this algorithm is shown to be superior to that of simpler algorithms commonly used to locate edges in satellite-derived SST images. This evaluation was performed through a careful comparison between the location of the fronts obtained by applying the various methods to the SST images and the in situ measures of the Gulf Stream position.
301 citations
TL;DR: In this paper, the development of the split window approach for correcting satellite measurement of radiance for atmospheric attenuation is reviewed, and the theoretical results are compared to results from actual measurements which consist of satellite measurements in the three infrared windows of the AVHRR.
Abstract: The development of the “split window” approach for correcting satellite measurement of radiance for atmospheric attenuation is reviewed. Then the theoretical results are compared to results from actual measurements which consist of satellite measurements in the three infrared windows of the AVHRR. Ground truth for the comparisons comes from buoys. The satellite measurements were screened for clouds, and the remaining ones were used in the analysis. Using this data set, several statistical analyses were performed. These showed that, when the two channels that are truly a split window are used, the result of the statistical model agrees with the one derived from theoretical considerations. When the 3.8-μm channel is combined with one in the 10–12 μm region, the result of the statistical model does not take the split window form. Results show that the method is capable of producing sea surface temperatures with a standard deviation of 1 K or less.
257 citations
TL;DR: In this paper, the authors analyzed the data collected by the US JGOFS Arabian Sea Process Experiment (JGOFS) during all seasons in the northern Arabian Sea and found that the region exhibits considerable mesoscale variability resulting in extremely variable temperature-salinity (TS) distributions in the upper 1000db.
Abstract: Between September 1994 and December 1995, the US JGOFS Arabian Sea Process Experiment collected extensive, high quality hydrographic data (temperature, salinity, dissolved oxygen and nutrients) during all seasons in the northern Arabian Sea. An analysis of this unique data suite suggests the presence of many features that are described in the canonical literature, but these new data provided the following insights. 1. Although the seasonal evolution of mixed-layer depths was in general agreement with previous descriptions, the deepest mixed-layer depths in our data occurred during the late NE Monsoon instead of the SW Monsoon. 2. The region exhibits considerable mesoscale variability resulting in extremely variable temperature-salinity (TS) distributions in the upper 1000 db. This mesoscale variability is readily observed in satellite imaging, in the high resolution data taken by a companion ONR funded project, and in underway ADCP data. 3. The densest water reaching the sea surface during coastal upwelling appeared to have maximum offshore depths of ∼150 m and σ θ ’s close to the core value (∼25) for the saline Arabian Sea Water (ASW), but salinities in these upwelling waters were relatively low. The densest water found at the sea surface during late NE Monsoon conditions has σ θ ’s>24.8 and relatively high salinities, suggesting that they are a source for the ASW salinity maximum. 4. Persian Gulf Water (PGW) with a core σ θ of 26.6 forms a widespread salinity maximum. Despite the considerable extent of this feature, Persian Gulf outflow water, with a salinity (4) of ∼39 at its source, can only be a minor contributor. Within the standard US JGOFS sampling grid, maximum salinities on this surface are ∼36.8 at stations near the Gulf, falling to values as low as ∼35.3 at the stations farthest removed from its influence. Even at our standard stations closest to the Gulf (N-1 and N-2), the high-salinity, low-nutrient Persian Gulf water has only a modest direct effect on nutrient concentrations. This PGW salinity maximum is associated with the suboxic portions of the Arabian Sea’s oxygen minimum zone. 5. The salinity maximum associated with Red Sea Water (RSW, core σ θ =27.2) in the JGOFS study region is clearly evident at the southermost sampling site at 10′N (S-15). Elsewhere, this signal is weak or absent and salinity on the 27.2 σ θ surface tends to increase towards the Persian Gulf, suggesting that the disappearance of this salinity maximum is due, at least in part, to the influence of the Persian Gulf outflow. 6. Inorganic nitrogen-to-phosphate ratios were lower (frequently much lower) than the standard Redfield ratio of 15/1–16/1 (by atoms) at all times and all depths suggesting that inorganic nitrogen was more important than phosphate as a limiting nutrient for phytoplankton growth, and that the effects of denitrification dominated the effects of nitrogen fixation. 7. The water upwelling off the Omani coast during the SW Monsoon has inorganic nitrogen to silicate ratios that were higher (∼2/1) than the ∼1/1 ratio often assumed as the ratio of uptake during diatom growth. 8. The temporal evolution of inorganic nitrogen-to-silicate ratios suggests major alteration by diatom uptake only during the late SW Monsoon cruise (TN050) in August–September 1995. 9. Widespread moderate surface layer nutrient concentrations occurred during the late NE Monsoon. 10. A zone of high offshore nutrient concentrations was encountered during the SW Monsoon, but instead of being associated with offshore upwelling it may represent offshore advection from the coastal upwelling zone, the influence of an eddy, or both. 11. Although our data do not contradict previous suggestions that the volume of subtoxic water may be reduced the SW Monsoon, they suggest a weaker re-oxygenation than indicated by some previous work. Similarly, they do not confirm results suggesting that secondary nitrite maxima may be common in waters with oxygen concentrations >5 μM.
257 citations
TL;DR: In this article, a year-long time series of winds, incoming shortwave and longwave radiation, air and sea temperatures, relative humidity, barometric pressure, and precipitation were collected from a surface mooring deployed off the coast of Oman along the climatological axis of the Findlater Jet from October 1994 to October 1995.
Abstract: Accurate, year-long time series of winds, incoming shortwave and longwave radiation, air and sea temperatures, relative humidity, barometric pressure, and precipitation were collected from a surface mooring deployed off the coast of Oman along the climatological axis of the Findlater Jet from October 1994 to October 1995. Wind stress, heat flux, and freshwater flux were computed using bulk formulae. The Northeast Monsoon was characterized by steady but moderate winds, clear skies, relatively dry air, and two months, December and January, in which the ocean, on average, lost 45 W m-2 to the atmosphere. The Southwest Monsoon had strong winds, cloudy skies, and moist air. Because of reduced latent and longwave heat loss, it was accompanied by sustained oceanic heat gain, with the strongest monthly mean warming, 147 W m-2, in August.
Large differences are found between the observations and older climatologies. Recent climatologies agree better with the observations. The means of the Southampton Oceanography Center climatology for 1980–1995 are close to the buoy monthly means. Monthly means from that climatology show that 1994–1995 was in general a typical year, with surface meteorology and air–sea fluxes within one standard deviation of the long term means. Concurrent data from the NCEP, ECMWF, and FNMOC show that the models provide realistic surface winds. FNMOC winds show that the timing and character of the onset of the Southwest Monsoon in 1995 differed from 1994 and 1996 when variability within one month is resolved. The models fail to replicate other observed surface meteorology and to produce realistic heat fluxes. Annual and monsoonal mean net heat fluxes from the models differed from those of the buoy by 50 to 80 W m-2. Because of these differences, some care is warranted in selecting and using air-sea flux fields in studies of the Arabian Sea.
233 citations