Shelf currents,lee-trapped and transient eddies on the inshore boundary of the Agulhas Current,South Africa: their relevance to the KwaZulu-Natal sardine run

The existence and strength of the annual KwaZulu-Natal (KZN) sardine run has long been a conundrum to fishers and scientists alike ― particularly that the sardine Sardinops sagax migrate along the narrow Transkei shelf against the powerful, warm Agulhas Current. However, examination of ship-borne acoustic Doppler current profiler (S–ADCP) data collected during two research surveys in 2005 indicated that northward-flowing coastal countercurrents exist at times between the Agulhas Bank and the KZN Bight, near Port Alfred, East London, Port St Johns and Durban. The countercurrent near Port Alfred extended as far east as the Keiskamma River, within an upwelling zone known to exist there. An ADCP mooring at a depth of 32 m off Port Alfred indicated that the countercurrent typically lasted a few days, but at times remained in the same direction for as long as 10 days. Velocities ranged between 20 and 60 cm s^?1 with maximum values of ～80 cm s^?1. The S–ADCP data also highlighted the existence of cyclonic flow in the Port St Johns–Waterfall Bluff coastal inset, with a northward coastal current similarly ranging in velocity between 20 and 60 cm s^?1. CTD data indicated that this was associated with shelf-edge upwelling, with surface temperatures 2–4 °C cooler than the adjacent core temperature (24–26 °C) of the Agulhas Current. Vertical profiles of the S–ADCP data showed that the countercurrent, about 7 km wide, extends down the slope to at least 600 m, where it appeared to link with the deep Agulhas Undercurrent at 800 m. S–ADCP and sea surface temperature (SST) satellite data confirmed the existence of the semi-permanent, lee-trapped, cyclonic eddy off Durban, associated with a well-defined northward coastal current between Park Rynie and Balito Bay. Analysis of three months (May–July 2005) of satellite SST and ocean colour data showed the shoreward core-boundary of the Agulhas Current (24 °C isotherm) to commonly be close to the coast along the KZN south coast, as well as between the Kei and Mbhashe rivers on the Transkei shelf. The Port St Johns–Waterfall Bluff cyclonic eddy was also frequently visible in these satellite data. Transient cyclonic eddies, which spanned 150–200 km of shelf, appeared to move downstream in the shoreward boundary of the Agulhas Current at a frequency of about once a month. These seemed to be break-away Durban eddies. Data collected by ADCP moorings deployed off Port Edward in 2005 showed that these break-away eddies and the well-known Natal Pulse are associated with temporary northward countercurrents on the shelf, which can last up to six days. It is proposed that these countercurrents off Port Alfred, East London and Port St Johns assist sardine to swim northwards along the Transkei shelf against the Agulhas Current, but that their progress north of Waterfall Bluff is dependent on the arrival of a transient, southward-moving, break-away Durban cyclonic eddy, which apparently sheds every 4–6 weeks, or on the generation of a Natal Pulse. This passage control mechanism has been coined the ‘Waterfall Bluff gateway’ hypothesis. The sardine run survey in June–July 2005 was undertaken in the absence of a cyclonic eddy on the KZN south coast, i.e. when the ‘gate’ was closed.     

Cyclonic eddies reveal Oegopsida squid egg balloon masses in the Agulhas Current,South Africa

In 2005, 2006, 2007 and 2011, distinct Oegopsida squid egg masses were observed by scuba divers on the narrow southern KwaZulu-Natal (KZN) shelf in depths of 35–50 m off the coastal resorts of Park Rynie, Pumula and Port Edward, South Africa. In 2006, larvae in the egg balloons were sampled. DNA barcoding (i.e. cytochrome c oxidase subunit 1 sequencing) linked the larvae to the genus Lycoteuthis, a group commonly found on the continental slope of the Agulhas Bank and the west coast of South Africa. In all cases, the sightings were concomitant with low water temperatures of 14–18 °C, indicative of shelf edge upwelling. Historical ship-collected CTD data show these cooler waters to originate from a depth of 100–180 m on the KZN continental slope. Complementary satellite imagery revealed the cooler water and discoveries of the egg balloons to be coincident with cold core cyclonic eddies embedded in the shoreward boundary of the Agulhas Current. The temperature data suggest that these egg balloons, in the absence of cyclonic eddy activity, would normally be found in the current on a density surface at a depth of ～130 m where velocities are typically around 100 cm s^–1.     

Anticyclonic Eddy Revealing Low Sea Surface Temperature in the Sea South of Japan: Case Study of the Eddy Observed in 1999–2000

Various kinds of datasets, such as satellite-derived sea surface temperature (SST), sea surface height, surface velocity produced by combining surface drifter and satellite altimeter data, and hydrographic data, led to the discovery of an anticyclonic eddy with lower SST than those of surrounding waters in the Kuroshio recirculation region south of Shikoku, as if the eddy were cyclonic. This anticyclonic eddy was formed east of Kyushu in late August to early September 1999 from the merger of two anticyclonic eddies which had migrated in the recirculation region to the sea south of Japan from the east. After the merger, the anticyclonic eddy strengthened abruptly and began to exhibit the low SST. In October, this eddy coalesced with the Kuroshio and moved swiftly eastward, accompanied by an amplitude growth of the Kuroshio meander. In mid November, off the Kii Peninsula, the eddy detached from the meandering Kuroshio. It then moved southwestward and again slowly propagated westward along the 30°N line. During this period, at least from late October 1999 to January 2000, SSTs over the anticyclonic eddy were found to be continuously lower than those of surrounding waters. This case tells us that we have to pay careful attention to the interpretation of mesoscale SST distributions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Low Salinity, Cool-Core Cyclonic Eddy Detected Northwest of Luzon during the South China Sea Monsoon Experiment (SCSMEX) in July 1998

To detect eddies, intensive surveys of the northeast South China Sea (SCS) (114°30′–121°30′ E, 17°–22°N) were conducted in July 1998 during the international SCS Monsoon Experiment (SCSMEX), the U.S. Navy using Airborne Expendable Bathythermograph and Conductivity-Temperature-Depth sensors (AXBT/AXCTD), and the Chinese Academy of Sciences using Acoustic Doppler Current Profilers (ADCP). The hydrographic survey included 307 AXBT and 9 AXCTD stations, distributed uniformly throughout the survey area. The ADCP survey had two sections. The velocity field inverted from the AXBT/AXCTD data and analyzed from the ADCP data confirm the existence of a low salinity, cool-core cyclonic eddy located northwest of Luzon Island (i.e., the Northwest Luzon Eddy). The radius of this eddy is approximately 150 km. The horizontal temperature gradient of the eddy increases with depth from the surface to 100 m and then decreases with depth below 100 m. The cool core was evident from the surface to 300 m depth, being 1°–2°C cooler inside the eddy than outside. The tangential velocity of the eddy is around 30–40 cm/s above 50 m and decreases with depth. At 300 m depth, it becomes less than 5 cm/s. This revised version was published online in August 2006 with corrections to the Cover Date.     

Observed Variability of Thermohaline Fields,Currents and Eddies in the Western Bay of Bengal During BOBMEX-99

Field measurements during the Bay of Bengal Monsoon Experiment (BOBMEX-99), data from a deep sea moored buoy, and satellite altimeter were used to describe variability in the hydrographic and meso-scale features in the Bay of Bengal (BoB) during the summer monsoon of 1999. The thermohaline fields showed two regions of upsloping of isopleths centered at 82°E and 84.75°E, ～110 km and 450 km away from the coast, respectively, followed by downsloping. The upsloping/downsloping of isopleths and the alternating currents was part of cyclonic and anti-cyclonic circulation patterns in the western BoB. In this region, both wind and current were important in the dynamics of coastal upwelling. The observations showed a relationship between the propagating waves and eddy on variability of thermohaline fields. On an annual cycle, four Kelvin waves were observed in the BoB, but only the downwelling Kelvin wave formed during October entered the Arabian Sea. During the monsoon season, four eddies were formed in the western BoB, of which the anticyclonic eddy centered at 15°N, 84°E and the cyclonic eddy centered at 17.5°N, 84.5°E were prominent. The baroclinic instability caused by the opposing currents along the east coast and the wind stress curl favored the formation of eddies. Okhubo-Weiss and Isern-Fontanet parameter confirmed the presence of eddies in the BoB.     

Numerical Study of the Upper-Layer Circulation in the South China Sea

Upper-layer circulation in the South China Sea has been investigated using a three-dimensional primitive equation eddy-resolving model. The model domain covers the region from 99° to 122°E and from 3° to 23°N. The model is forced by the monthly averaged European Centre for Medium-Range Weather Forecasts (ECMWF) model winds and the climatological monthly sea surface temperature data from National Oceanographic Data Center (NODC). Inflow and outflow through the Taiwan Strait and the Sunda shelf are prescribed monthly from the Wyrtki estimates. Inflow of the Kuroshio branch current in the Luzon Strait is assumed to have a constant volume transport of 12 Sv (1 Sv = 10⁶ m³/s), and the outflow from the open boundary to the east of Taiwan is adjusted to ensure the net volume transport through all open boundaries is zero at any instant. The model reveals that a cyclonic circulation exists all year round in the northern South China Sea. During the winter time this cyclonic eddy is located off the northwest of Luzon, coinciding with the region of positive wind stress curl in this season. This cyclonic eddy moves northward in spring due to the weakening of the northeast winds. The cyclonic circulation becomes weak and stays in the continental slope region in the northern South China Sea in the summer period. The southwest wind can raise the water level along the west coast of Luzon, but there is no anticyclonic circulation in the northern South China Sea. After the onset of the northeast monsoon winds in fall, the cyclonic eddy moves back to the region off the west coast of Luzon. In the southern South China Sea and off the Vietnam coast, the model predicts a similar flow structure as in the previous related studies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Three dimensional diagnostic study of the circulation in the South China Sea during winter 1998

On the basis of hydrographic data obtained from 28 November to 27 December, 1998, the three-dimensional structure of circulation in the South China Sea (SCS) is computed using a three-dimensional diagnostic model. The combination of sea surface height anomaly from altimeter data and numerical results provides a consistent circulation pattern for the SCS, and main circulation features can be summarized as follows: in the northern SCS there are a cold and cyclonic circulation C1 with two cores C1-1 and C1-2 northwest of Luzon and an anticyclonic eddy (W1) near Dongsha Islands. In the central SCS there is a stronger cyclonic circulation C2 with two cores C2-1 and C2-2 east of Vietnam and a weaker anticyclonic eddy W2 northwest of Palawan Island. A stronger coastal southward jet presents west of the eddy C2 and turns to the southeast in the region southwest of eddy C2-2, and it then turns to flow eastward in the region south of eddy C2-2. In the southern SCS there are a weak cyclonic eddy C3 northwest of Borneo and an anti-cyclonic circulation W3 in the subsurface layer. The net westward volume transport through section CD at 119.125°E from 18.975° to 21.725°N is about 10.3 × 10⁶ m³s⁻¹ in the layer above 400 m level. The most important dynamic mechanism generating the circulation in the SCS is a joint effect of the baroclinicity and relief (JEBAR), and the second dynamical mechanism is an interaction between the wind stress and relief (IBWSR). The strong upwelling occurs off northwest Luzon.     

Effects of deep-sea eddies on the northern KwaZulu-Natal shelf,South Africa

This paper describes the westward movement of a cyclonic eddy across the Mozambique Channel and the subsequent south-westward propagation of the eddy along the east coast of South Africa and its interaction with the shelf. A hydrographic survey on 13 September 2006 off Nine-mile Reef (NMR), Sodwana Bay, showed a well-developed Agulhas Current along the continental shelf inshore of a cyclonic eddy flanked by two anti-cyclonic eddies, further offshore. A satellite-tracked drifter and complementary altimetry data confirmed the dimensions of the eddy and tracked its movement towards the coast. Shelf-edge upwelling was measured at NMR by an underwater temperature recorder (UTR) when the cyclonic eddy first came into contact with the shelf and again when the cyclonic eddy interacted with the leading edge of the anti-cyclonic eddy moving onto the shelf. Further shelf–eddy interactions off Aliwal Shoal, south of Durban, and consequent upwelling were similarly caused by the same cyclonic eddy as it progressed south-westward along the east coast. Analysis of UTR data between 2004 and 2006 indicated that between two and five cyclonic eddies impact the shelf off NMR per year.     

Circulation of shelf waters in the KwaZulu-Natal Bight,South Africa

Ship-based acoustic Doppler current profiler (S-ADCP) technology, used in survey mode, has enabled near- synoptic views of the in situ 3-D current field in the KwaZulu-Natal (KZN) Bight to be elucidated for the first time. Data acquired by the research vessels RS Africana and RS Algoa in June 2005, September 2007, March 2009 and July 2010 are presented. Each S-ADCP dataset showed similar circulation characteristics whereby the continental slope and outer shelf of the KZN Bight were strongly influenced by the south-westward flowing Agulhas Current. This was particularly evident in the extreme north between Cape St Lucia and Richards Bay where the shelf is narrowest and velocities exceeded 200 cm s?1. The widening of the bight to the south moves the Agulhas Current further from the coast, resulting in a diminishing velocity gradient on the outer shelf which terminates around the midshelf axis. The southern region of the bight was mostly influenced by the Durban cyclonic eddy (Durban Eddy), and in June 2005 and September 2007, by a cyclonic ‘swirl’ that occupied the entire southern half of the KZN Bight, the latter identified by a combination of S-ADCP-, satellite-derived SST- and ocean colour data. Satellite data showed low-chlorophyll offshore water to move into this swirl and northwards along the inner- and midshelf, reaching the Thukela River. Inner-shelf circulation north of the Thukela River was weak (<20 cm s?1) and highly variable. Satellite-tracked surface drogues deployed in the Durban Eddy found their way into the northward coastal current in the KZN Bight, with velocities exceeding 90 cm s?1 at times. The drogues also highlighted the strong influence of wind, especially in the northern bight between Durnford Point and Cape St Lucia, with residence times on the shelf exceeding 14 days, suggesting this region to be of biological importance particularly for recruitment.     

北太平洋中尺度涡时空特征分析

利用1993~2011年19 a的AVISO卫星高度计资料研究了北太平洋(10°~60°N,120°E~100°W)中尺度涡的时空分布特征,结果表明:北太平洋每年约产生1 800余个涡旋,其中气旋涡稍多。北太平洋东部沿岸、西北沿岸、黑潮延伸体北侧、副热带逆流区是中尺度涡的高发区,春、冬季是涡旋的高发季节。涡极性分布以35°N为界,北部多反气旋涡,南部多气旋涡。涡旋半径以100 km左右为主,并且基本随纬度升高而减小,涡旋数量随着周期增长而急剧下降。反气旋涡的平均半径和周期均大于气旋涡。利用Argo浮标剖面资料分析的6个个例涡旋的垂直结构显示,每个涡旋都有其独特的冷暖核结构,深度不同。研究结果对于分析北太平洋涡动能分布及传输具有一定的参考价值。     

Biogeochemical responses associated with the passage of a cyclonic eddy based on shipboard observations in the western North Pacific

A shipboard high-resolution hydrographic survey in the subtropical region of the western North Pacific conducted from October to November 2008 detected part of a cyclonic eddy around 30°N, 145°E. This eddy had propagated westward in the region south of the Kuroshio extension for at least 6 months as a wavelike disturbance. Within this eddy, isopycnals shallowed between a depth of 600 m and just below the surface mixed layer. In addition, maximal dissolved oxygen concentrations were observed in the subsurface layer between depths of 50 and 100 m. Nitrate was depleted within this subsurface maximal oxygen layer. These results suggest that nutrients in the deeper layers were supplied into the euphotic layer as a result of the uplift of isopycnals in the eddy, fueling the photosynthesis of phytoplankton in the subsurface and emitting an excess of oxygen due to new production. Compared with the outside of the eddy, the enhancement of oxygen and the decrease of nitrate in the center of the eddy were estimated to be 2.7 mol O₂ m^?2 and 0.22 mol N m^?2, respectively. The primary productivity calculated using the eddy transition speed of 5.1 km day^?1 was 548 mg C m^?2 day^?1 at the center of the eddy. The enhanced primary productivity due to the passage of the eddy is likely to have an important role in the ecosystem and on material cycling in the subtropical region.     

Primary Study of the Mechanism of Eddy Shedding from the Kuroshio Bend in Luzon Strait

The mechanism of the anticyclonic eddy's shedding from the Kuroshio bend in Luzon Strait has been studied using a nonlinear 2 1/2 layer model, in a domain including the North Pacific and South China Sea. The model is forced by steady zonal wind in the North Pacific. Energy analysis is adopted to detect the mechanism of the eddy shedding. Twelve experiments with unique changes of wind forcing speed (to obtain different Kuroshio transports at Luzon Strait) were performed to examine the relationship between the Kuroshio transport (KT) and the eddy shedding events. In the reference experiment with KT of 22.7 Sv (forced with zonal wind idealized from the annual mean wind stress from the COADS data set), the interval of eddy shedding is 70 days and the shed eddy centers at (20°N, 117.5°E). When the Kuroshio bend extends westward, the southern cyclonic perturbation grows so rapidly as to form a cyclonic eddy (18.5°N, 120.5°E) because of the frontal instability in the south of the Kuroshio bend. In the evolution of the cyclonic eddy, it cleaves the Kuroshio bend and triggers the separation of the anticyclonic eddy. In statistical terms, anticyclonic eddy shedding occurs only when KT fluctuates within a moderate range, between 21 Sv and 28 Sv. When the KT is larger than 28 Sv, a stronger frontal instability south of the Kuroshio bend tends to generate a cyclonic eddy of size similar to the width of the Luzon Strait. The bigger cyclonic eddy prevents the Kuroshio bend from extending into the SCS and does not lead to eddy shedding. On the other hand, when the KT decreases to less than 21 Sv, the frontal instability south of the Kuroshio bend is so weak that the size of corresponding cyclonic eddy is smaller than half the width of the Luzon Strait. The cyclonic eddy, lacking power, fails to cleave the Kuroshio bend and cause separation of an anticyclonic eddy; as a result, no eddy shedding occurred then, either.     

Dynamics and role of the Durban cyclonic eddy in the KwaZulu-Natal Bight ecosystem

The semi-permanent Durban Eddy is a mesoscale, lee-trapped, cold-core cyclonic circulation that occurs off the east coast of South Africa between Durban in the north and Sezela, some 70 km to the south. When present, strong north-eastward countercurrents reaching 100 cm s^–1 are found inshore. It is hypothesised that the cyclone is driven by the strong south-westward flowing Agulhas Current offshore of the regressing shelf edge near Durban. Analysis of ADCP data and satellite imagery shows the eddy to be present off Durban approximately 55% of the time, with an average lifespan of 8.6 days, and inter-eddy periods of 4 to 8 days. After spin-up the eddy breaks loose from its lee position and propagates downstream on the inshore boundary of the Agulhas Current. The eddy is highly variable in occurrence, strength and downstream propagation speeds. There is no detectable seasonal cycle in eddy occurrence, with the Natal Pulse causing more variability than any seasonal signal. A thermistor array deployed in the eddy centre, together with ship CTD data, indicates upward doming of the thermal structure in the eddy core associated with cooler water and nutrients being moved higher in the water column, stimulating primary production. Together with the use of satellite imagery, our findings indicate a second mechanism of upwelling, viz. divergent upwelling in the northern limb of the eddy. Satellite-tracked surface drifters released in the eddy demonstrated the potential for nutrient-rich eddy water to be transported northwards along the inshore regions of the KwaZulu-Natal (KZN) Bight, thus contributing to the functioning of the bight ecosystem, as well as southwards along the KZN and Transkei coasts – both by the eddy migrating downstream and by eddy water being recirculated into the inshore boundary of the Agulhas Current itself.     

Spectral wave characteristics off Gangavaram,Bay of Bengal

Spectral wave characteristics were studied based on waves measured for 1 year during 2010 off Gangavaram, Bay of Bengal. Maximum wave height of 5.2 m was observed on 19 May 2010 due to the influence of cyclonic storm LAILA. The wave spectrum was single-peaked during 57 % of the time and the double-peaked spectrum observed was mainly swell-dominated. Low-frequency waves (0.05–0.15 Hz) were predominantly from 150° to 180°, whereas high-frequency waves (>0.15 Hz) during November–January were mainly from 90° to 120°, and during July and August from 180° to 210°. Annual average significant wave height was similar to the value (1 m) observed in the eastern Arabian Sea.     

Seasonal variability in the recruitment of macrofouling community in Kudankulam waters, east coast of India

The seasonal variability in fouling community recruitment on submerged artificial substratum was studied in Kudankulam coastal water, Gulf of Mannar, East coast of India for a period of two years, from May 2003 to April 2005. The results indicated that the fouling community recruitment occurred throughout the year with varying intensities. Barnacles, ascidians, polychaetes, bivalves and seaweeds were the major fouling groups observed from the test panels. Maximum fouling biomass of 9.17 g dm⁻² was observed during August 2004 and a minimum value of 0.233 g dm⁻² in February 2004. The biomass build-up on test panels was relatively high during the premonsoon season and low during the postmonsoon months. The number of barnacles settled on the panels varied from 1 to 4460 no. dm⁻². The maximum percentage of the ascidian coverage (72%) on test panels was observed during March 2005. In general, July–December was the period of intense recruitment for barnacles and March–May was the period for ascidians.     

Path Transition of the Kuroshio Due to Mesoscale Eddies: A Two-Layer, Wind-Driven Experiment

We have executed numerical experiments using a two-layer, wind-driven ocean model with simplified coastal geometry and bottom topography to investigate the possibility of the Kuroshio path transition due to mesoscale eddies. A straight path easily changes into a meandering path due to the eddy action. For this transition, an anticyclonic eddy is preferable to a cyclonic one when imposed in the beginning region of the Kuroshio (east of Taiwan). When imposed southeast of Kyushu, on the other hand, a cyclonic eddy is more effective than an anticyclonic one. The reverse transition, from a meandering to a straight path, did not occur at all in this experiment. This revised version was published online in August 2006 with corrections to the Cover Date.     

Seasonal characteristics of the near-surface circulation in the northern South China Sea obtained from satellite-tracked drifters

The surface circulation of northern South China Sea (hereafter SCS) for the period 1987–2005 was studied using the data of more than 500 satellite-tracked drifters and wind data from QuikSCAT. The mean flow directions in the northern SCS except the Luzon Strait (hereafter LS) during the periods October~March was southwestward, and April~September northeastward. A strong northwestward intrusion of the Kuroshio through the LS appears during the October~March period of northeasterly wind, but the intrusion became weak between April and September. When the strong intrusion occurred, the eddy kinetic energy (EKE) in the LS was 388 cm²/s² which was almost 2 times higher than that during the weak-intrusion season. The volume transport of the Kuroshio in the east of the Philippines shows an inverse relationship to that of the LS. There is a six-month phase shift between the two seasonal phenomena. The volume transport in the east of the Philippines shows its peak sis-month earlier faster than that of the LS. The strong Kuroshio intrusion is found to be also related to the seasonal variation of the wind stress curl generated by the northeasterly wind. The negative wind stress curl in the northern part of LS induces an anticyclonic flow, while the positive wind stress curl in the southern part of LS induces a cyclonic flow. The northwestward Kuroshio intrusion in the northern part of LS happened with larger negative wind stress curl, while the westward intrusion along 20.5°N in the center of the LS occurred with weaker negative wind stress curl.     

Drifter Observations of Anticyclonic Eddies near Bussol' Strait, the Kuril Islands

Hydrographic surveys and satellite imaging reveal that mesoscale anticyclonic (AC) eddies are common features of the area south of Bussol' Strait, the deepest of the Kuril straits connecting the western North Pacific and Sea of Okhotsk. To examine the velocity structure of these eddies, we deployed groups of 15-m drogued satellite-tracked surface drifters over the Kuril-Kamchatka Trench in the fall of 1990 and late summer of 1993. Drifters in both groups entered large AC eddies centered over the axis of the trench seaward of Bussol' Strait and subsequently underwent a slow northeastward translation. One drifter (Drifter 1315) deployed near the center of the “Bussol' eddy” in 1990, remained in the eddy for roughly 45 days and made five loops at successively greater distances from the eddy center. Large-amplitude (80–100 cm/s) storm-generated inertial oscillations were observed during the first two loops. The vorticity field associated with the eddy resulted in a Doppler “red-shift” of inertial frequency motions such that the “effective” inertial period of 21 hours was roughly 4 hours greater than the nominal inertial period for the drifter latitude (45°N). In 1993, a second drifter (Drifter 15371) was retained in the Bussol' eddy for about 40 days. This eddy had characteristics similar to those of the 1990 eddy but was devoid of significant high-frequency motions until the drifter's final half loop. The observed spatial scales, persistence, and slow poleward translation of the eddies suggests that they play an important role in the dynamics of the East Kamchatka and Oyashio current systems. This revised version was published online in August 2006 with corrections to the Cover Date.     

Physical-biological coupling in the Algerian Basin (SW Mediterranean): Influence of mesoscale instabilities on the biomass and production of phytoplankton and bacterioplankton

The biomass and production of phytoplankton and bacterioplankton was investigated in relation to the mesoscale structures found in the Algerian Current during the ALGERS'96 cruise (October 1996). Biological determinations were carried out in three transects between 0° and 2°E aimed at crossing a so-called event, formed by a coastal anticyclonic eddy associated with an offshore cyclonic eddy to the west. The concentration of chlorophyll a (Chl) was maximum (>1.2 mg m⁻³) within the cyclonic eddy and at the frontal zones between the Modified Atlantic Water (MAW) of the Algerian Current and the Mediterranean waters further north. Chl (total and >2 μm) was significantly correlated with proxies of nutrient flux into the upper layers. Autotrophic picoplankton and heterotrophic bacterial abundance and production presented clear differences between MAW and Mediterranean water, with higher values at those stations under the influence of the Algerian Current. In general, greater differences were observed in production than in biomass variables. The photosynthetic parameters (derived from P–E relationships) and integrated primary production (range 189–645 mg m⁻² d⁻¹) responded greatly to the different hydrological conditions. The mesoscale phenomena inducing fertilization caused a 2 to 3-fold increase in primary production rates. The relatively high values found within the cyclonic eddy suggest that, although short-lived in comparison with anticyclonic eddies, these eddies may produce episodic increases of biological production not accounted for in previous surveys in the region.     

An offshore eddy in the California current system part II: Surface manifestation

Ship and satellite observations taken over the last thirty years show that mesoscale patterns of sea surface temperature (SST) in the California Current System are consistently found throughout the year and usually occur in approximately the same geographical locations. Typically, these patterns are more pronounced in fall/winter than in spring/summer. The temporal and spatial characteristics of these persistent feature were examined with satellite infrared (IR) measurements during winter 1980–1981. In January 1981, a ship surveyed the vertical structure of several physical, chemical, and biological parameters beneath one of these SST features centered near 32°N, 124°W. The surface IR pattern had a length scale of 200 km and a time scale of about 100 days. It disintegrated following the first two storms of the winter season. Motion studies of the pattern in late October indicated an anticyclonic rotation with maximum velocities of 50 cm s^?1 at 50 km from the axis of rotation. As a unit, the pattern advected southward with an average speed of 1 cm s^?1. Thermal fronts, determined from the satellite imagery, were strongest (0.4°C km^?1) along the rim of the pattern and were advected anticyclonically with the pattern; their length scales were 20–30 km in the along-front direction and less than 10 km wide. The hydrographic data revealed a three-layer structure beneath the surface pattern; a 75 m deep surface layer, a cold-core region from 75 to 200 m depth, and a warm-core eddy extending from 250 to 1450 m. The anticyclonic motion of the surface layer was caused by a geostrophic adjustment to the surface dynamic height anomaly produced by the subsurface warm-core eddy. The IR pattern observed from space reflects the horizontal structure of the surface layer and is consistent with a theoretical model of a mean horizontal SST gradient perturbed by a subsurface density anomaly. Ship of opportunity SST observations collected by the National Marine Fisheries are shown to resolve mesoscale patterns. For December 1980, the SST pattern near 32°N, 124°W represented a 2°C warm anomaly compared with the 20-year mean monthly SST pattern.