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Date: Fri, 6 Oct 1995 05:53:02 +0000 (GMT)

Greeting from the THOMPSON SeaSoar Mid Cruise Report TTN-051 - Oct 3, 1995 The SW monsoon 1995 appears to be over! For those of us on the June July August legs this is welcome relief as calm seas and balmy nights currently prevail. The first half of the seasoar cruise saw the last hiccups of Monsoon winds and the seas have dramatically calmed. This is in strong contrast to our June Seasoar cruise which felt the brunt of the monsoon. Besides the noted improvement in "life aboard ship", we are observing obvious bio-physical changes from June. Our departure from Muscat on 19 Sept, lead us on a similar cruise track as June, with SeaSoar towed along Omani coastal waters to the southern JGOFS line; continuous underway surface measurements were also collected. Towing continued out the JGOFS line and through the first radiator (see June cruise report). A hydrographic section was conducted through leg 5 of the nearshore radiator, based on Seasoar and AVHRR imagery which indicated a frontal feature running parallel to the JGOFS line. Seasoar towing continued along the JGOFS line through the bowtie, and onto the second radiator, now nearly completed. I'm pleased to report that all operations are going quite well without problems. Certainly, the calm sea and efficient THOMPSON crew have contributed to this. Near real-time reception of the FNOC 12 hourly winds each day permits monitoring of the basin -wide wind system during the highly variable inter-monsoon period. Since the end of Process Cruise 5, two large scale intensifications of the northern Arabian Sea surface winds have occurred- the first just prior to the start of the cruise and the second as we neared the southern JGOFS line. The latter event produced maximum winds of approximately 16-20 knots from the SW for several days. The very light winds we are currently experiencing appear to result from a basin -wide relaxation of the entire SW monsoon system. Real time AVHRR imagery collected aboard ship shows a decrease in the intensity of filaments extending offshore from the Omani coast. This contrasts with the stronger alongshore currents and offshore filaments of upwelled coastal waters observed in June. The development of a near shore eddy field is observed extending approximately 150 km off the coast and is most likely attributed to relaxation of the winds and dissipation of the coastal current. However, north of Ra's Al Hadd, a NE flow extends from the cape along a strong front and is associated with a persistent anticyclonic gyre in the southern Gulf of Oman. SeaSoar surveys suggest that the large (100 km) horizontal scales dominate upper ocean velocity, temperature, and salinity variability during the period following the southwest monsoon. The small-scale interleaving features seen during the inter- and northeast monsoon periods were not present. Patterns of near shore physical and biological variability were more diffuse than those observed during the SW monsoon. In particular, the strong coastal filaments previously observed have weakened considerably. Although the filaments appeared more diffuse than in early July, there are still sharp fronts that show dramatic increases in nutrient and chlorophyll fluorescence and beam c. Despite their being more diffuse, the influence of the upwelled water on the characteristics of the alongshore SeaSoar transect appear much greater. Along the coastal transect, ocean temperatures were generally cooler and of lower salinity than that observed in July. This contrasts with the intermingling of filaments and warm salty water intruding from offshore that was observed in July. The result of this diffuse structure is that both nutrients and phytoplankton biomass remain quite high in the coastal regions. Along this transect from Ra's Al Hadd to the southern JGOFS line, surface nitrate concentration varied from as low as 2 uM to as much as 12 uM. The higher concentrations were found where filaments extended offshore from the coast. At the inshore radiator SeaSoar sampled the southern portion of a shallow warm core eddy (MLD=40 m), which was also discernable in the satellite imagery. The southern edge of this eddy showed a thermosaline front running parallel to the JGOFS line with temperature and salinity changes ranging from >0.5 C (28.0 max) and <0.3 psu (36.3 max), respectively. The hydrographic section through this front indicated cooler frontal waters to the south which appeared to have been advected (200 km) from the coast, as indicated in AVHRR SST imagery. These waters have higher nutrient concentration and are associated with less salty (<36 psu) water and higher chlorophyll fluorescence and beam attenuation coefficients. Total chlorophyll concentrations (chl+phaeo) were >1.0 ug/L south of the front and <0.5 ug/L north of the front. Across this front nitrate decreased from 2-3 uM along the JGOFS line to .1-.2 uM to the north. These cooler waters extended as far east as the northern corner of the mooring array. Net phytoplankton samples obtained during the station transect across the first radiator were dominated by Phaeocystis and the heterotrophic dinoflagellate Protoperidinium on the cool side of the temperature front, and by Rhizosolenia and a diverse dinoflagellate assemblage on the warm side of the front. Phaeocystis concentrations were highest at the front, and dropped off rapidly as temperature and salinity increased north of the front. Salps were also extremely abundant in samples collected on the warm side of the front. The bowtie pattern around the mooring positions showed less pronounced temperature and salinity variations which ranged from 0.6 C (28.2 max) and <0.2 psu (36.05 max) with total chlorophyll in the range of 1.0 ug/l near the main JGOFS line to 0.3 ug/l in the central region. Further offshore in the second radiator, the survey reveals patchy distribution of surface phytoplankton. This contrasts sharply with conditions during the SW monsoon, when phytoplankton biomass (inferred from fluorescence and beam c) remained low offshore of the mooring array. Underway sample analyses showed larger variations in temperature >0.8 C (28.7 max) but with little variation in salinity or chlorophyll, 36 psu and 0.5 ug/l respectively. Upper ocean density profiles often reveal 25-50 m deep mixed layers over a thin layer of weakly stratified fluid, beneath which lies another unstratified layer extending as deep as 100 m. Strong surface heating and variable winds during the post SW monsoon period may initiate restratification, and could produce density profiles similar to those observed. As we progressed offshore along the JGOFS line, increases in surface nitrate were often accompanied by decreases in silicate. Usually the presence of elevated nutrient concentration in the upper layer was accompanied by an increase in chlorophyll fluorescence and a transition from a subsurface chlorophyll maximum to an upper layer maximum. Oxygen concentration of 1-2 ml/l penetrated to about 100 m along most of the JGOFS line. deepening toward the east. Between 100 and 150 m there was a gradient to very low concentrations. This contrasts with the NE monsoon observations when low oxygen water was found as shallow as 50 m occasionally. Maximum productivity normalized to chlorophyll concentration, PBmax (obtained from P vs. I curves), during underway sampling has ranged from 1.6 to 35 ug/C/l/hr/chl. Generally, values of PBmax for the <200 um size fraction have been around 5.0. Bioluminescence measured with a new sensor on SeaSoar showed discrete layers of high intensity in the surface mixed layer. These layers ranged from 3 to 5 nmi long and were only occasionally coincident with layers of high fluorescence. Bioluminescence layers were offset with fluorescence by 1-3 nmi. The abundance of chroococcoid cyanobacteria in surface waters is much less variable on this cruise than on either the June or February periods. Throughout the cruise, chroococcoid cyanobacteria abundance has ranged between 8 and 25 X 104 cells ml-1 in surface waters, with most observations of lower or higher abundances occurring in coastal waters. Summary : While at several stations, we were greeted by large numbers of flying fish.. The dinner table was set and on came the sharks and squid. We were entertained by a feeding frenzy which has been the best show yet .. Oh yea.. The squid were enjoyed the next day at the barbeque. Chief Scientist TTN-051 -- Bob Arnone bob arnone is