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 wintertime 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 (16 m s^?1) the winter shamal also brought cold dry air (T_a=20 °C, q_a=10 g kg^?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 q_a=15 g kg^?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 30 m. The strong wind not only strengthened the circulation in the entire Persian Gulf but also established a northwestward flowing Iranian Coastal Current (ICC, 25–30 cm s^?1) from the Strait of Hormuz to about 52°E, where it veered offshore. The strongest negative sea level of 25–40 cm was generated in the northernmost portion of the Gulf while the wind setup 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 outflow of 0.25 Sv (Sv≡10⁶ m³ s^?1) during 24 November followed by an equivalent inflow on the next day.     

Bacterivory in the northwestern Indian Ocean during the intermonsoon – northeast monsoon period

Bacterial grazing loss rates were studied by radioactive labeling of natural bacteria with L-(4,5-³H) leucine and from the rate of disappearance of bacterial cells in the northwestern Indian Ocean. Bacterivory was measured in a mixed sample that had been combined from various depths across the euphotic zone. Experiments were performed on 26 occasions at 19 stations in the Gulf of Oman and the Arabian Sea during the intermonsoon–northeast monsoon period (November–December 1994). Combined uptake of radiolabeled bacteria (ULB) in 1–8 and 8–100 μm size fractions was somewhat lower than loss of label (LBL) measured in the bacterial fraction (0.2–1.0 μm), suggesting loss of radioactivity from the grazers due to metabolism. The less sensitive rate of disappearance of bacterial cells (LBC) was on average 51% higher than LBL estimates. Results from ULB and LBL measurements revealed that bacterivory was higher in the Gulf of Oman (average loss rate 4.1% h^-1) than in the Arabian Sea where rates were slightly higher inshore (1.7% h^-1) than in the central gyre. Heterotrophic nanoflagellates in the 1–8 μm size fractions were identified as the primary bacterivores. Microzooplankton (8–100 μm) accounted for 33% of total bacterivory in the Gulf of Oman but only 16% in the central Arabian Sea. Time-course experiments conducted at two stations indicated that diel changes in bacterivory may be substantial in the northwestern Indian Ocean.     

Direct observation of subtropical mode water circulation in the western North Atlantic Ocean

Eighteen Degree Water (EDW) is the dominant subtropical mode water of the North Atlantic subtropical gyre and is hypothesized as an interannual reservoir of anomalous heat, nutrients and CO₂. Although isolated beneath the stratified upper-ocean at the end of each winter, EDW may re-emerge in subsequent years to influence mixed layer properties and consequently air–sea interaction and primary productivity. Here we report on recent quasi-Lagrangian measurements of EDW circulation and stratification in the western subtropical gyre using an array of acoustically-tracked, isotherm-following, bobbing profiling floats programmed to track and intensively sample the vertically homogenized EDW layer and directly measure velocity on the 18.5 °C isothermal surface.The majority of the CLIVAR Mode Water Dynamics Experiment (CLIMODE) bobbers drifted within the subtropical gyre for 2.5–3.5 years, many exhibiting complex looping patterns indicative of an energetic eddy field. Bobber-derived Lagrangian integral time and length scales (3 days, 68 km) associated with motion on 18.5 °C were consistent with previous measurements in the Gulf Stream extension region and fall between previous estimates at the ocean surface and thermocline depth. Several bobbers provided evidence of long-lived submesoscale coherent vortices associated with substantial EDW thickness. While the relative importance of such vortices remains to be determined, our observations indicate that these features can have a profound effect on EDW distribution. EDW thickness (defined using a vertical temperature gradient criterion) exhibits seasonal changes in opposition to a layer bounded by the 17 °C and 19 °C isotherms. In particular, EDW thickness is generally greatest in winter (as a result of buoyancy-forced convection), while the 17°–19 °C layer is thickest in summer consistent with seasonal Ekman pumping. Contrary to previous hypotheses, the bobber data suggest that a substantial fraction of subducted EDW is isolated from the atmosphere for periods of less than 24 months. Seasonal-to-biennial re-emergence (principally within the recirculation region south of the Gulf Stream) appears to be a common scenario which should be considered when assessing the climatic and biogeochemical consequences of EDW.     

Structure of potential upwelling areas in the Philippines

The three-dimensional structure of two potential mesoscale upwelling areas that are located in the external waters of the Philippine archipelago (i.e. northwest of Luzon and east of Mindanao) were constructed by analysis of historical data. A unique characteristic of both upwelling sites is that they can be identified by their anomalously cold subsurface temperatures rather than sea surface temperature distributions. As such, they cannot be observed in sea surface temperature fields derived by satellite imagery. The data used in the analysis were obtained from the National Oceanographic Data Center hydrographic database. Objective analysis was performed to produce monthly temperature fields at several standard depths within the upper 500 m of the region 0–30°N and 100–140°E with a horizontal grid resolution of 0.5°. The extent and timing of these upwelling areas are described. A review of existing hypothesis on the mechanisms for their evolution and seasonal modulation are presented. The change in heat content during the upwelling season is greater than 300 W m⁻² in both areas. Based on the excursion of isotherms, vertical velocities of 83 cm day⁻¹ and 26 cm day⁻¹ were obtained for upwelling northwest of Luzon and east of Mindanao, respectively.     

Structure and mechanisms of the Arabian Sea variability during the winter monsoon

In the southern Arabian Sea (between the Equator and 10°N), the shoaling of isotherms at subsurface levels (20 °C isotherm depth is located at ∼90 m) leads to cooling at 100 m by 2–3 °C relative to surrounding waters during the winter monsoon. The annual and interannual variations of this upwelling zone, which we call the Arabian Sea dome (ASD), are studied using results from an eddy-permitting ocean general circulation model in conjunction with hydrography and TOPEX/ERS altimeter data. The ASD first appears in the southeastern Arabian Sea during September–October, maturing during November–December to extend across the entire southern Arabian Sea (along ∼5°N). It begins to weaken in January and dissipates by March in the southwestern Arabian Sea. From the analysis of heat-budget balance terms and a pair of model control experiments, it is shown that the local Ekman upwelling induced by the positive wind-stress curl of the winter monsoon generates the ASD in the southeastern Arabian Sea. The ASD decays due to the weakening of the cyclonic curl of the wind and the westward penetration of warm water from the east (Southern Arabian Sea High). The interannual variation of the ASD is governed by variations in the Ekman upwelling induced by the cyclonic wind-stress curl. Associated with the unusual winds during 1994–1995 and 1997–1998 Indian Ocean dipole (IOD) periods, the ASD failed to develop. In the absence of the ASD during the IOD events, the 20 °C isotherm depth was 20–30 m deeper than normal in the southern Arabian Sea resulting in a temperature increase at 97 m of 4–5 °C. An implication is that the SST evolution in the southern Arabian Sea during the winter monsoon is primarily controlled by advective cooling: the shoaling of isotherms associated with the ASD leads to SST cooling.     

Quantifying the potential impact of land cover changes due to sea-level rise on storm surge on lower Texas coast bays

In this study we investigated the impacts of potential changes of land cover due to sea-level rise (SLR) on storm surge (i.e., the rise of water above normal sea level, namely mean-sea level and the astronomical tide, caused by hurricane winds and pressure) response inside bays on the lower Texas coast. We applied a hydrodynamic and wave model (ADCIRC + SWAN) forced by hurricane wind and pressure fields to quantify the importance of SLR-induced land cover changes, considering its impacts by changing bottom friction and the transfer of wind momentum to the water column, on the peak surge inside coastal bays. The SLR increments considered, 0.5 m to 2.0 m, significantly impacted the surge response inside the bays. The contribution of land cover changes due to SLR to the surge response, on average, ranged from a mean surge increase of 2% (SLR of 0.5 m) to 15% (SLR of 2.0 m), in addition to the SLR increments. The increase in surge response strongly depended on storm condition, with larger increases for more intense storms, and geographical location. Although land cover changes had little impact on the surge increase for SLR increments lower than 1.0 m, intense storms resulted in surge increase of up to 10% even for SLR below 1.0 m, but in most cases, the geometry changes were the major factor impacting the surge response due to SLR. We also found a strong relationship between changes in bottom friction and the surge response intensification; demonstrating the importance of considering land cover changes in coastal regions that are highly susceptible to SLR when planning for climate change.     

Picophytoplankton dynamics and production in the Arabian Sea during the 1995 Southwest Monsoon

Phytoplankton community structure is expected to shift to larger cells (e.g., diatoms) with monsoonal forcing in the Arabian Sea, but recent studies suggest that small primary producers remain active and important, even in areas strongly influenced by coastal upwelling. To better understand the role of smaller phytoplankton in such systems, we investigated growth and grazing rates of picophytoplankton populations and their contributions to phytoplankton community biomass and primary productivity during the 1995 Southwest Monsoon (August–September). Environmental conditions at six study stations varied broadly from open-ocean oligotrophic to coastal eutrophic, with mixed-layer nitrate and chlorophyll concentrations ranging from 0.01 to 11.5 μM NO₃ and 0.16 to 1.5 μg Chl a. Picophytoplankton comprised up to 92% of phytoplankton carbon at the oceanic stations, 35% in the diatom-dominated coastal zone, and 26% in a declining Phaeocystis bloom. Concurrent in situ dilution and ¹⁴C-uptake experiments gave comparable ranges of community growth rates (0.53–1.05 d⁻¹ and 0.44–1.17 d⁻¹, to the 1% light level), but uncertainties in C:Chl a confounded agreement at individual stations. Microzooplankton grazing utilized 81% of community phytoplankton growth at the oligotrophic stations and 54% at high-nutrient coastal stations. Prochlorococcus (PRO) was present at two oligotrophic stations, where its maximum growth approached 1.4 d⁻¹ (two doublings per day) and depth-integrated growth varied from 0.2 to 0.8 d⁻¹. Synechococcus (SYN) growth ranged from 0.5 to 1.1 d⁻¹ at offshore stations and 0.6 to 0.7 d⁻¹ at coastal sites. Except for the most oligotrophic stations, growth rates of picoeukaryotic algae (PEUK) exceeded PRO and SYN, reaching 1.3 d⁻¹ offshore and decreasing to 0.8 d⁻¹ at the most coastal station. Microzooplankton grazing impact averaged 90, 70, and 86% of growth for PRO, SYN, and PEUK, respectively. Picoplankton as a group accounted for 64% of estimated gross carbon production for all stations, and 50% at high-nutrient, upwelling stations. Prokaryotes (PRO and SYN) contributed disproportionately to production relative to biomass at the most oligotrophic station, while PEUK were more important at the coastal stations. Even during intense monsoonal forcing in the Arabian Sea, picoeukaryotic algae appear to account for a large portion of primary production in the coastal upwelling regions, supporting an active community of protistan grazers and a high rate of carbon cycling in these areas.     

Monsoonal influence on the distribution of phytoplankton pigments in the Arabian Sea

Variations in the distribution of chemotaxonomic pigments were monitored in the Arabian Sea and the Gulf of Oman at the end of the SW monsoon in September 1994 and during the inter-monsoon period in November/December 1994 to determine the seasonal changes in phytoplankton composition. The Gulf of Oman was characterized by sub-surface chlorophyll maxima at 20-40 m during both seasons, and low levels of divinyl chlorophyll a indicated that prochlorophytes did not contribute significantly to the total chlorophyll a. Prymnesiophytes (19′-hexanoyloxyfucoxanthin), diatoms (fucoxanthin) and chlorophyll b containing organisms accounted for most of the phytoplankton biomass in September, while prymnesiophytes dominated in November/December. In the Arabian Sea in September, high total chlorophyll a concentrations up to 1742 ng l^-1 were measured in the coastal upwelling region and a progressive decline was monitored along the 1670 km offshore transect to oligotrophic waters at 8°N. Divinyl chlorophyll a was not detected along this transect except at the two most southerly stations where prochlorophytes were estimated to contribute 25–30% to the total chlorophyll a. Inshore, the dominance of fucoxanthin and/or hexanoyloxyfucoxanthin indicated that diatoms and prymnesiophytes generally dominated the patchy phytoplankton community, with zeaxanthin-containing Synechococcus also being important, especially in surface waters. At the southern oligotrophic localities, Synechococcus and prochlorophytes dominated the upper 40 m and prymnesiophytes were the most prominent at the deep chlorophyll maximum. During the inter-monsoon season, total chlorophyll a concentrations were generally half those measured in September and highest levels were found on the shelf (1170 ng l^-1). Divinyl chlorophyll a was detected at all stations along the Arabian Sea transect, and we estimated that prochlorophytes contributed between 3 and 28% to the total chlorophyll a, while at the two oligotrophic stations this proportion increased to 51–52%. While procaryotes were more important in November/December than September, eucaryotes still accounted for >50% of the total chlorophyll a. Pigment/total chlorophyll a ratios indicated that 19′-hexanoyloxyfucoxanthin-containing prymnesiophytes were the dominant group, although procaryotes accounted for 65% at the two southerly oligotrophic stations.     

The influence of the south-west monsoon upon the nutrient biogeochemistry of the Arabian Sea

Variations in the nutrient concentrations were studied during two cruises to the Arabian Sea. The situation towards the end of the southwest monsoon season (September/October 1994) was compared with the inter-monsoonal season during November and December 1994. Underway surface transects showed the influence of an upwelling system during the first cruise with deep, colder, nutrient-rich water being advected into the surface mixed layer. During the southwesterly monsoon there was an area of coastal Ekman upwelling, bringing colder water (24.2°C) into the surface waters of the coastal margin. Further offshore at about 350 km there was an area of Ekman upwelling, as a result of wind-stress curl, north of the Findlater Jet axis; this area also had cooler surface water (24.6°C). Further offshore (>1000 km) the average surface temperatures increased to >27°C. These waters were oligotrophic with no evidence of the upwelling effects observed further inshore. In the upwelling regions nutrient concentrations in the close inshore coastal zone were elevated (NO₃=18 μmol l^-1, PO₄=1.48 μmol l^-1); higher concentrations also were measured at the region of offshore upwelling off the shelf, with a maximum nitrate concentration of 12.5 μmol l^-1 and a maximum phosphate concentration of 1.2 μmol l^-1. Nitrate and phosphate concentrations decreased with increasing distance offshore to the oligotrophic waters beyond 1400 km, where typical nitrate concentrations were 35.0 nmol l^-1 (0.035 μmol l^-1) in the surface mixed layer. A CTD section from the coastal shelf, to 1650 km offshore to the oligotrophic waters, clearly showed that during the monsoon season, upwelling is one of the major influences upon the nutrient concentrations in the surface waters of the Arabian Sea off the coast of Oman. Productivity of the water column was enhanced to a distance of over 800 km offshore. During the intermonsoon period a stable surface mixed layer was established, with a well-defined thermocline and nitracline. Surface temperature was between 26.8 and 27.4°C for the entire transect from the coast to 1650 km offshore. Nitrate concentrations were typically between 2.0 and 0.4 μmol l^-1 for the transect, to about 1200 km where the waters became oligotrophic, and nitrate concentrations were then typically 8–12 nmol l^-1. Ammonia concentrations for the oligotrophic waters were typically 130 nmol l^-1, and are reported for the first time in the Indian Ocean. The nitrogen/phosphorus (N/P) ratios suggest that phytoplankton production was potentially nitrogen-limited in all the surface waters of the Arabian Sea, with the greatest nitrogen limitation during the intermonsoon period.     

Biological versus physical processes as drivers of large oscillations of the air–sea CO2 flux in the Antarctic marginal ice zone during summer

The fugacity of CO₂ and abundance of chlorophyll a (Chla) were determined in two long transects from the Polar Front to the Antarctic Continent in austral summer, December 1995–January 1996. Large undersaturations of CO₂ in the surface water were observed coinciding with high Chla content. In the major hydrographic regions the mean air–sea fluxes were found to range from −3 to +7 mmol m⁻² d⁻¹ making these regions act as a sink as well as a source for CO₂. In the total 40-d period, the summation of the several strong source and sink regions revealed an overall modest net source of 0.3 mmol m⁻² d⁻¹, this based on the Wanninkhof (J. Geophys. Res. 97 (1992) 7373) quadratic relationship at in situ windspeed. A simple budget approach was used to quantify the role of phytoplankton blooms in the inorganic carbonate system of the Antarctic seas in a time frame spanning several weeks. The major controlling physical factors such as air–sea flux, Ekman pumping and upwelling are included. Net community production varies between −9 and +7 mmol m⁻² d⁻¹, because of the large oscillations in the dominance of autotrophic (CO₂ fixation) versus heterotrophic (CO₂ respiration) activity. Here the mixed layer depth is the major controlling factor. When integrated over time the gross influx and efflux of CO₂ from air to sea is large, but the net residual air/sea exchange is a modest efflux from sea to atmosphere.     

Spatial and temporal variation in surfclam (Spisula solidissima) larval supply and settlement on the New Jersey inner shelf during summer upwelling and downwelling

To examine the relationship between near-bottom larval surfclam concentrations and surfclam settlement at an inner continental shelf site off New Jersey (USA), four consecutive sets of settlement experiments were carried out at three stations at the Long-term Ecosystem Observatory (LEO-15) from 14 to 31 July 1997 during upwelling and downwelling. Two inshore stations were on the landward and seaward sides of Beach Haven Ridge at ∼12 m depth, and a third station was 8 km further offshore at ∼20 m depth. In each experiment, four replicate trays of azoic sand from Beach Haven Ridge were placed flush with the seafloor and exposed for 3–7 days. Larval surfclam concentrations were measured every 4 h at 1 m above the bottom (mab) using Moored, Automated, Serial Zooplankton Pumps at the three stations. At all three stations, larval surfclam concentrations (1 mab) were low during upwelling, and higher during and after downwelling. Pulses of highest larval surfclam concentrations coincided with the initial arrival of downwelled warm water. In addition, larval surfclam concentrations were higher at the two inshore stations than at the offshore station. Larval surfclam settlement in the trays was higher during and following downwelling than during upwelling at one inshore station and at the offshore station. At the other inshore station (landward of Beach Haven Ridge), surfclam settlement did not increase during and following downwelling. Overall, surfclam settlement was higher inshore than offshore. The results indicate that spatial and temporal variation in larval surfclam supply was controlled by upwelling and downwelling circulation and that surfclam settlement was influenced by larval supply. Bottom flows across Beach Haven Ridge during a storm may have reduced larval surfclam settlement on the upcurrent side of the ridge, affecting initial densities on a small (∼1 km) scale.     

Long-term hydrographic changes at 52 and 66°W in the North Atlantic Subtropical Gyre & Caribbean

In July–September 1997 two hydrographic lines were done in the western N. Atlantic along longitudes of 52 and 66°W as part of the WOCE one-time hydrographic survey of the oceans. Each of these two lines approximately repeated earlier ones done during the International Geophysical Year(s) (IGY) and the mid-1980s. Because of this repeated sampling, long-term hydrographic changes in the water masses can be examined. In this report, we focus on temperature and salinity changes within the subtropical gyre mainly between latitudes of 20 and 35°N and compare our results to those presented by Bryden et al. (1996), who examined changes along a zonal line at 24°N, most recently occupied in 1992. Since this most recent 24°N section in 1992, substantial changes have occurred in the western part of the subtropical gyre at the depths of the Labrador Sea Water (LSW). In particular, we see clear evidence for colder, fresher Labrador Sea Water throughout the gyre on our two recent sections that was not yet present in 1992 at similar longitudes along 24°N. At shallower depths inhabited by waters that are an admixture of Mediterranean (MW) and Antarctic Intermediate Waters (AAIW), our recent survey shows an increase in salinity, which can only be attributed to changes in water masses on potential temperature or neutral density surfaces. Furthermore, waters above the MW/AAIW layer and into the deeper part of the main pycnocline have continued to become saltier and warmer throughout the 40-year period spanned by our sections. These latter changes have been dominantly due to a vertical sinking of density surfaces as T/S changes in density surfaces are small, but depths of individual T/S horizons have increased with time. The net change since the IGY shows a mean temperature increase between 800 and 2500 m depth at a rate of 0.57°C/century with a corresponding steric sea level rise of 1 mm/yr, and a net downward heave with small values near the top and bottom, and a maximum rate of −2.7 m/yr at 1800 m depth. Changes in the deep Caribbean indicate a warming since the IGY due to temperature increases of the inflowing source waters in the subtropical gyre at 1800m depth, but no significant change in the deep salinity.     

Analyzing sea level rise and tide characteristics change driven by coastal construction at Mokpo Coastal Zone in Korea

This study investigates a construction-induced sea level rise and tide characteristics change, using a regression analysis to separate the local construction effect such as sea-dike/seawalls and global warming from total sea level change. The study also makes it clear why and how the extreme high water level has risen just after constructions at Mokpo harbor in Korea. As a result of the regression analysis, it is found that the high water level rise for the period of 1960–2006 is ～60 cm, which is summation of four components: ～23 cm for Youngsan River sea-dike (1981), ～15 cm for Youngam seawall (1991), ～8 cm for Geumho seawall (1994), and ～14 cm for gradual rise (due mainly to global warming). Then, a numerical simulation at Mokpo coastal zone is performed to identify each component, and the results support the premise that the tidal amplification caused by constructions is due mainly to the extinguishment of the tidal choking effect at outer Mokpogu. The tidal flat effect makes the amplification greater at spring tide or extraordinary high tide, which would result in the increase of inundation risk at the Mokpo coastal zone. Frequency distribution of observed high water level data shows increasing trend for both maximum value of astronomical tide component (simulated high water level) and meteorological tide component (surge height) after the coastal constructions. A frequency analysis presents that the high water level for 50 year return period, which is often used for design in practice, is 474 cm before the construction, and while that is 553 cm after the construction. Furthermore, design height might steadily be elevated considering future global sea level rise.     

Formation rates of subtropical underwater in the Pacific Ocean

Water mass formation rates were calculated for subtropical underwater (STUW) in the North and South Pacific by two partially independent methods. One is based on the World Ocean Circulation Experiment (WOCE)/TOGA drifter array over two periods: 1988–1992, and 1992–1996. Drifter velocities were used to calculate two components of the subduction rate, lateral induction and vertical pumping. The second method used CFC-12 data (1987–1994) from WOCE and Pacific Marine Environmental Laboratory to calculate ages on σ_θ surfaces. Subduction rates were estimated from the inverse age gradient. The two subduction rate methods are independent, but they share a common identification of STUW formation area based on satellite-derived surface temperature maps. Using both methods, one can put bounds on the formation rates: 4–5 Sv in the North and 6–7 Sv in the South Pacific. The drifter calculated STUW subduction rates for 1988–1992 and 1992–1996 are 21 and 13 m/yr in the North Pacific and 25 and 40 m/yr in the South. The CFC-12 calculated STUW subduction rate in the North Pacific is 26 m/yr, and 32 m/yr in the South. The South Pacific rates exceed those in the North Pacific. Consistent differences between the two methods support earlier studies, they conclude that mixing contributes to STUW formation in addition to the larger-scale circulation effects. The drifter and tracer rates agree well quantitatively, within 22%, except for the second period in the North Pacific and there are some differences in spatial patterns. Tracer rates integrate over time, and drifters allow analysis of interannual variability. The decrease in subduction rate between periods in the North Pacific is due to negative lateral induction entraining STUW into the mixed layer. The increase in the South Pacific rate is due to an increase in the vertical pumping. Although Ekman pumping is in phase in the North and South, the subduction rate is out of phase. These results confirm that subduction depends on the large-scale circulation and a combination of the outcrop pattern and air–sea fluxes. Temporal differences in rates and partitioning between the hemispheres are consistent with interannual changes in gyre intensity and current positions.     

Reconstruction of Late-Quaternary sea- and lake-level changes in a tectonically active marginal basin using seismic stratigraphy: The Gulf of Cariaco,NE Venezuela

The Gulf of Cariaco is a marginal basin located between the Cariaco Basin and the Paria Gulf, offshore NE Venezuela, along a system of active right-lateral strike-slip faults. It is connected to the Caribbean Sea via a shallow 58-m-deep sill implying that the gulf was disconnected from the global ocean during eustatic lowstands. A dense grid of high-resolution reflection seismic profiles has been used to determine the overall tectonic structure of the gulf and to establish the seismic stratigraphy of its sedimentary infill. Six unconformity-bounded seismic–stratigraphic units were identified in the upper ~ 200 m of the sedimentary infill. Detailed seismic–stratigraphic and seismic-facies analysis allowed defining a series of sedimentary features that can be used as indicators of past sea or lake level in the Gulf of Cariaco: i) delta offlap breaks, ii) evaporites, and iii) erosional unconformities. Using accurate measurements of these various indicators at several locations in the gulf and a simple total subsidence model, a relative sea/lake-level history encompassing the last 130 kyr could be reconstructed. In periods of connection with the open ocean, reconstructed relative sea level correlates well with eustatic sea level. In times of disconnection, distinct lake-level fluctuations occurred, which sometimes resulted in total dessication of the gulf. Lake-level fluctuations appear to correlate with major Heinrich Events, stadials and interstadials. MIS 4, the LGM and the Younger Dryas were thus identified in the Gulf of Cariaco sedimentary record. The last reconnection to the Caribbean Sea occurred during MWP1b (around 11.5 kyr). The very good fit of the Cariaco sea/lake-level curve with the eustatic sea-level curves (both in terms of amplitude and of timing) underscores potential for future paleoclimate research of the sedimentary record contained in this marginal basin, despite its active tectonic setting.     

Continuous measurement of an artificial upwelling of deep sea water induced by the perpetual salt fountain

A perpetual salt fountain can draw up deep sea water without any artificial energy source. This is one of the ideal ways to draw up deep sea water for nutrient enhancement application. A perpetual salt fountain experiment was carried out in May 2004. The estimated average velocity of the upwelling after 48 h from the start of the experiment was about 7 mm/s, and it continued for about 3 days. The flow rate with a single pipe was estimated as approximately 45 m³/day. The amount of upwelled deep sea water by this single pipe seems too small to be of benefit, but we can easily increase the amount by deploying a number of pipes.     

Interannual to decadal Gulf Stream variability in an eddy-resolving ocean model

Meridional shifts of the Gulf Stream (GS) jet on interannual to decadal timescales and the corresponding oceanic changes around the GS are investigated using a near global eddy-resolving ocean model hindcast from 1960 to 2003. The simulated variability in the shifts of the GS jet axis shows good agreement with observations, and lags atmospheric fluctuations characterized by the North Atlantic Oscillation by about 2 years. This lagged response of the GS jet to the atmospheric variations is attributed to the westward propagation of the undulation of the jet axis from 45°W to 75°W, which has a wavelength of about 4000 km and a displacement of 0.5°. The propagation direction and phase speed of about 2.8 cm s⁻¹ are consistent with the thin-jet theory. The shifts of the jet axis in the downstream region are likely induced by wind fluctuations through Ekman convergence over the central North Atlantic. Associated with the northward (southward) shift of the jet axis, sea surface temperature is warming (cooling) around and north of the jet, and the former warming has a deep and meridionally narrow subsurface structure, consistent with the northward shift of the jet. The meridional shifts of the jet accompany coherent meridional shifts of energetic eddy activity regions around the GS. Our numerical results suggest that the GS jet brings the atmospheric signals from the central to the western North Atlantic, and the resultant meridional shift of the jet induces the notable oceanic changes around the GS.     

Seasonal and spatial variability in plankton production and respiration in the Subtropical Gyres of the Atlantic Ocean

Euphotic zone plankton production (P) and respiration (R) were determined from the in vitro flux of dissolved oxygen during six latitudinal transects of the Atlantic Ocean, as part of the Atlantic Meridional Transect (AMT) programme. The transects traversed the North and South Atlantic Subtropical Gyres (N gyre, 18–38°N; S gyre, 11–35°S) in April–June and September–November 2003–2005. The route and timing of the cruises enabled the assessment of the seasonal variability of P, R and P/R in the N and S gyres, and the comparison of the previously unsampled N gyre centre with the more frequently sampled eastern edge of the gyre. Mean euphotic zone integrated rates (±SE) were P=63±23 (n=31), R=69±22 (n=30) mmol O₂ m⁻² d⁻¹ in the N gyre; and P=58±26 (n=30), R=62±24 (n=30) mmol O₂ m⁻² d⁻¹ in the S gyre. Overall, the N gyre was heterotrophic (R>P) and it was more heterotrophic than the S gyre, but the metabolic balance of both gyres changed with season. Both gyres were net heterotrophic in autumn, and balanced in spring. This seasonal contrast was most pronounced for the S gyre, because it was more autotrophic than the N gyre during spring. This may have arisen from differences in nitrate availability, because spring sampling in the S gyre coincided with periods of deep mixing to the nitracline, more frequently than spring sampling within the N gyre. Our results indicate that the N gyre is less heterotrophic than previous estimates suggested, and that there is an apparent decrease in R from the eastern edge to the centre of the N gyre, possibly indicative of an allochthonous organic carbon source to the east of the gyre.     

Drowned coralline algal dominated deposits off Lanai,Hawaii; carbonate accretion and vertical tectonics over the last 30 ka

We present detailed bathymetry, remotely operated vehicle (ROV) and submersible observations, and sedimentary and radiocarbon age data from carbonate deposits recovered from two submerged terraces at − 150 m (T1) and − 230 m (T2) off Lanai, Hawaii. The tops of the terraces are veneered by relatively thin (< 5 m) in situ accumulations of coralline algal nodule, coralgal nodule, Halimeda and a derived oolitic facies deposited in intermediate (30–60 m) to deep fore-reef slope settings (60–120 m). The data are used to develop a sedimentary facies model that is consistent with eustatic sea-level variations over the last 30 ka. Both nodule facies on T1 and T2 initiated growth 30–29 ka following a fall in sea level of ∼50 m and increase in bottom currents during the transition from Marine Isotope Stage 3 to 2. The nodules accreted slowly throughout the Last Glacial Maximum when sea-level was relatively stable. Drowning occurred during the early deglaciation (17–16 ka) and was marked by the complete drowning of coralline algal nodules facies on T2 and incipient drowning of coralgal facies on T1. Abrupt sea-level rise during the middle deglaciation, perhaps associated with global meltwater pulse 1A (14–15 ka), finally drowned the coralgal facies on T1, which in turn was overlain by a deep-water Halimeda facies or an oolitic facies derived from upslope. Our data indicates that Lanai has experienced relatively little vertical tectonic movement over the last 30 ka. Using paleobathymetric data derived from the sedimentary facies, age vs. depth relationships, and published sea-level curves, we estimate that Lanai could be either slowly uplifting or subsiding, but at rates < 0.1 m/kyr (uplift) or < 0.4 m/kyr (subsidence) over this 30 kyr period.