XBT monitoring of a meridian section across the western Mediterranean Sea

During the Thetis-2/MAST-2 tomography experiment, T7-XBT calibrated (accuracy ∼0.05°C) probes were launched ∼28 km apart between France and Algeria, twice a month from Feb. to Sep. 1994. Combined with infrared images, altimetric data and ship drifts, they provide definite information on the structure, drift and role of the eddy-like mesoscale phenomena generated by the Algerian Current instability. When embedded in this alongslope current, these phenomena generally propagate downstream at a few km/day and are markedly asymmetrical. Because of the topography in the eastern part of the Algerian Basin, they separate from the current, become more symmetrical and follow an anticlockwise circuit in the open basin. These phenomena are deeper than ∼750 m and entrain seaward pieces of the Levantine Intermediate Water (LIW) vein flowing along the Sardinian slope, thus being responsible of the large spatial and temporal variability of the LIW distribution in the open basin. The non-existence of a LIW vein flowing westward across the Algerian Basin is definitely demonstrated. In the Gulf of Lions, new insights are provided into the formation and spreading of the Winter Intermediate Water (WIW), which is the Western Mediterranean counterpart of LIW. Considering the large amount of WIW formed during this mild winter, it is clear that this water has not received enough attention yet, and is certainly a major component of the Mediterranean outflow at Gibraltar. Finally, the XBT data account for the eastward flow of the Western Mediterranean Deep Water (WMDW) off Algeria.     

Large scale flow separation and mesoscale eddy formation in the Algerian Basin

During the ELISA/MATER experiment floats released at about 600 m depth in the Levantine Intermediate Water layer south of Sardinia in July 1997 have revealed the existence of a coherent eddy, approximately 50 km in diameter and lasting for several months. This anticyclonic eddy was first observed south-west of Sardinia in November 1997 and drifted inside the Algerian Basin during the following months until April 1998. This eddy contained Levantine Intermediate Water at intermediate level and seemed to be related to 2 main large scale features: (a) a cyclonic gyre (250 km in diameter and 3–4 months period) located in the Algerian Basin and (b) a boundary current located along the continental slope south and west of Sardinia and originating from the Sardinia–Tunisia channel. We will first describe the “Sardinian” eddy, from a kinematical point of view, and the Algerian Gyre and second, give some insights about the eddy origin and its importance for LIW large scale spreading in the Western Mediterranean Sea.     

A synthesis of the Levantine Basin circulation and hydrography, 1985–1990

The Levantine Basin circulation derived from recent data consists of a series of sub-basin-scale to mesoscale eddies interconnected by jets. The basin-scale circulation is masked by eddy variability that modulates and modifies it on seasonal and interannual time scales. Long-term qualitative changes in the circulation are reflected in the bifurcation pattterns of ther mid-basin jets, relative strengths of eddies and the hydrographic properties at the core of these eddies. Confinement within the Basin geometry strongly influences the co-evolution of the circulation features.Surface measurements, satellite images and the mass field indicate an entire range of scales of dynamical features in the region. The complexity of the circulation is consistent with the basin-wide and mesoscale heterogeneity of the hydrographic properties. The interannual variability of LIW (Levantine Intermediate Water) formation in the region appears correlated with the changes in the circulation. Wintertime convective overturning of water masses reach intermediate depths and constitute a dominant mechanism of LIW formation, especially in anticyclonic eddies and along the coasts of the northern Levantine Basin.     

The CANALES experiment (1996-1998). Interannual, seasonal, and mesoscale variability of the circulation in the Balearic Channels

Repeated hydrographic casts, mooring time series and satellite sea surface temperature collected during the CANALES experiment (1996–98) are used to describe the thermohaline circulation in the Balearic Channels (western Mediterranean) and to analyze its variability. Mass transports are estimated by inverse calculations. The role played by each channel in the meridional water exchange is clarified: the Ibiza Channel funnels southward cool, saline, northern waters whereas the Mallorca Channel appears as the preferred route for the northward progression of warm, fresh, southern waters. A neat interannual trend is revealed by the continuous decrease of the amount of Western Mediterranean Intermediate Waters (WIW) brought by the Northern Current, reflecting the increase in temperature of the winter mixed layer in the northern Mediterranean that occurred each year between 1996 and 1998. A clear seasonal signal was also seen in the transport of the Northern Current which decreased from 1 to 1.4 Sv in winter to < 0.5 Sv in summer. The current intensified again in fall. A number of mesoscale eddies, from 20 to 70 km in size, most of them anticyclonic vortex eddies were brought by the unstable Northern Current, these eddies strongly perturbed the water exchange in the Ibiza Channel forcing retroflections of northern waters back to the north-east into the Balearic Current. These eddies either stayed stalled for several months in the Gulf of Valencia to the north of the channel, or were slowly funnelled southward through the channel narrows. A decreasing trend was observed in the mesoscale activity of the Northern Current between 1996 and 1998. Conversely, large, anticyclonic eddies, 150-km diameter, progressively invaded the Algerian Basin to the south of the channels in 1997–98 and forcing northward inflows (up to 0.75 Sv) of fresh and warm waters of Atlantic origin (AW) into the Mallorca Channel. The marked interannual differences observed in both northern and southern eddy activity may be linked to the interannual variability of the large scale thermohaline circulation.     

On the presence of a coastal current of Levantine intermediate water in the central Tyrrhenian sea

The hydrological structure and the seasonal variability of marine currents in the Tyrrhenian Sea, off the coasts of Latium, are analysed using a data set obtained during several cruises between February 1988 and August 1990. Of particular interest is the fact that the hydrological surveys show the intermittent presence of a current of Levantine Intermediate Water (LIW) flowing anticlockwise along the Italian slope, at 250–700 m. This current is of particular importance in inferring the pathways of the Levantine Intermediate Water in the western Mediterranean Sea and in particular in the Tyrrhenian basin, downstream of the Strait of Sicily. These phenomena remain an open problem: our observations give support to the Millot's proposed general scheme, on the existence of a general cyclonic circulation of the LIW from the Strait of Sicily to the western Mediterranean, as opposed to a direct injection of LIW towards the Algerian basin.     

A new hypothesis about the surface circulation in the eastern basin of the mediterranean sea

In time, the circulation of the Atlantic Water (AW) in the eastern basin of the Mediterranean Sea has been described differently, according to two major representations. The historical one, which began with the scheme from Nielsen in 1912 and has been refined up to the 1980s, favours a counterclockwise circulation in the whole basin, with AW flowing in its southern part as a broad flow off Libya and Egypt (from the Ionian to the Levantine subbasins), then continuing along Middle-East and Turkey before flowing back westwards. The more recent one, issued in the 1990s, favours a clockwise circulation in the northern part of the Ionian continuing offshore across the basin from the Cretan to the central part of the Levantine as the so-called “Mid-Mediterranean Jet”. This jet is depicted then as splitting both clockwise in the southeastern part of the basin and counterclockwise off Turkey (where this representation agrees with the former). Because the recent representation cannot be considered as a refinement of the historical ones, we have been interested in understanding why a given data set available to everybody is interpreted in such different ways.In the Algerian subbasin, the combined use of satellite infrared images and a significant amount of in situ data sets (hydrology and both Eulerian and Lagrangian current measurements) allowed us to solve a similar controversy. Therefore, we examined the circulation features in the eastern basin, undertaking the detailed analysis of 1000 daily and weekly composite images spanning the period 1996–2000, and of monthly composite images available since 1985. Whenever in situ observations were available, we have confronted them with the satellite thermal signatures and have shown that both are consistent. This paper focuses on the overall (basin scale) results while the detailed ones are published in an other paper. The new scheme we propose is basically a refined version of the historical ones: the circulation of AW is counterclockwise in the whole eastern basin but it is more constrained alongslope than previously thought, and the broadening historically schematised appears to be due to intense mesoscale eddies mainly generated by the instability of this circulation.     

Surface circulation of the Levantine Basin: Comparison of model results with observations

The eastern Mediterranean (Levantine Basin) hydrography and circulation are investigated by comparing the results of a high-resolution primitive equation model with observations. After a 10-year integration, the model is able to reproduce the major water masses and the circulation patterns of the eastern Mediterranean. Comparisons with the POEM hydrographical observations show good agreement. The vertical distribution of the water masses matches that of the observations quite well in terms of monthly mean. The model surface circulation is in agreement with circulation schemes derived from recent observations. Some well-known mesoscale features of the upper thermocline circulation are also realistically reproduced. In agreement with satellite observations, the model shows that high-energy mesoscale eddies dominate the upper thermocline circulation in the southern and the central parts of the Levantine Basin. Most of the Atlantic Water follows the north African coast and forms a strong coastal jet near the Libyan coast rather than forming the Mid-Mediterranean Jet described by several authors. The sub-basin circulation shows a strong seasonal signal. A strong and stable current flows along the isobaths in winter, becoming weaker and with more meanders in summer. The mesoscale eddies throughout the whole basin are more energetic in summer than in winter.     

Vertical distribution of dissolved organic carbon (DOC) in the Western Mediterranean Sea in relation to the hydrological characteristics

Vertical profiles of dissolved organic carbon (DOC) from eight hydrological stations in the Tyrrhenian Sea, Sardinia Channel and Algerian Sea, are reported. DOC exhibits concentrations ranging from 58 to 88 μM in surface water, 43–57 μM in the intermediate layer and 49–63 μM in deep waters. The assessment of the hydrological characteristics allows different water masses in the study area to be identified; moreover, different hydrological processes are observed in the Tyrrhenian and Algerian basins. DOC exhibits different values in the different water masses. The lowest DOC concentrations (43–46 μM) were found in the Tyrrhenian Levantine Intermediate Water (LIW). Correlations between DOC and apparent oxygen utilization (AOU), investigated within each water mass, exhibit different behaviors in the intermediate and deep waters, suggesting the occurrence of different processes of oxygen consumption in the different water masses.     

Analysis of mesoscale phenomena in the Algerian basin observed with drifting buoys and infrared images

During the autumn–winter of 1996–1997, drifting buoy trajectories and infrared satellite images provided new information on the characteristics of several mesoscale phenomena generated by the Algerian Current (AC) in the western Mediterranean Sea. A mesoscale event, as defined by previous studies, consists of a meander of the current associated with a surface anticyclonic eddy inside its crest, a transitory surface cyclonic eddy (E_c) upstream from the crest, and a deep anticyclonic eddy just below the meander. Most events propagate eastward along the coast at a few km per day until they are forced, mainly by the topography at the entrance to the channel of Sardinia, to detach from the coast and propagate seaward. They thus become open-sea anticyclonic eddies and generally complete an anticlockwise circuit in the Algerian basin. Surface buoys were launched upstream from an event and across it near 1°E. They made it possible to characterise the anticyclonic and cyclonic surface eddy features, and for the first time clearly showed the meander, which is in general not well depicted with images. It has thus been definitely demonstrated that most of the AC (speeds of several tens of cm/s) crosses the relatively slowly propagating events. As usual, the event we sampled reached a mature stage characterised by a vanishing of the E_c, and increased up to ∼100 km. Its arrest and decrease before it reached the channel of Sardinia, which is not so usual, was contemporaneous to the reappearance of the E_c and could be related to the growing of another coastal eddy upstream. At the entrance to the channel of Sardinia (near 7–8°E), the trajectories and images also documented another event which was larger (up to ∼120 km) and in the phase of detachment. Since the buoys drifted alternately to the west and to the east between this event and the coast, it is clear that an event can detach only temporarily and allow part of the AC to flow eastward directly. As indicated by infrared images, the definitive detachment occurred after all the buoys escaped from the event. The whole in situ and satellite data set is fully consistent with all the previous observations of the AC mesoscale variability, and quantitatively supports the proposed hypotheses for the event structure. It is consistent with laboratory experiments and some results of numerical models of coastal instability processes.     

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.     

Another description of the Mediterranean Sea outflow

Papers about the outflow in the Strait of Gibraltar assume that (i) it is composed of only two Mediterranean Waters (MWs), the Levantine Intermediate Water (LIW) and the Western Mediterranean Deep Water (WMDW) from the eastern and western basins, respectively, (ii) both MWs are mixed near 6°W, hence producing a homogeneous outflow that is then split into veins, due to its cascading along different paths and to different mixing conditions with the Atlantic Water (AW).A re-analysis of 1985–1986 CTD profiles (Gibraltar Experiment) indicates two other MWs, the Winter Intermediate Water (WIW) from the western basin and the Tyrrhenian Dense Water (TDW) basically originated from the eastern basin. In the central Alboran subbasin, these four MWs are clearly differentiated, roughly lying one above the other in proportions varying from north to south. Proportions also vary with time, so that the outflow can be mostly of either eastern or western origin. While progressing westward, the MWs can still be differentiated and associated isopycnals tilt up southward as much as being, in the sill surroundings, roughly parallel to the Moroccan continental slope where the densest MWs are. The MWs at the sill are thus juxtaposed and they all mix with AW, leading to an outflow that is horizontally heterogeneous just after the sill (5°45′W) before progressively becoming vertically heterogeneous as soon as 6°15′W. There can be little LIW and/or no WMDW outflowing for a while.An analysis of new 2003–2008 time series from two CTDs moored (CIESM Hydro-Changes Programme) at the sill (270 m) and on the Moroccan shelf (80 m) confirms the juxtaposition of the MWs, their individual and generally intense mixing with AW, as well as the large temporal variability of the outflow composition. Only LIW and TDW were indicated at the sill while, on the shelf, only LIW, TDW sometimes denser there than 200 m below, and WMDW were indicated; but none of the MWs has been permanently outflowing at one or the other place.The available data can be analyzed coherently. Intermediate and deep MWs are formed in both basins in amounts that, although variable from year to year, allow their tracing up to the strait. Four major MWs circulate alongslope counterclockwise as density currents and as long as they are not trapped within a basin, which is necessarily the case for the deep MWs. In the Alboran, the intermediate MWs (WIW, LIW and upper-TDW) circulate in the north while the deep MWs (lower-TDW and WMDW) are uplifted, hence relatively motionless and mainly pushed away in the south. Since both the intermediate and deep MWs outflow at the sill, they are considered as light and dense MWs, the light–dense MWs interface possibly intersecting the AW–MWs interface in the sill surroundings. Considering an outflow east of the sill composed of only two (light–dense) homogeneous layers gives significant results. Across the whole strait, the outflow has spatial and temporal variabilities much larger than previously assumed. The MWs are superposed in the sea and lead at the sill to juxtaposed and vertically stratified suboutflows that will cascade independently before forming superposed veins in the ocean. These veins can have similar densities and hydrographic characteristics even if associated with different MWs, which accounts for the features permanency assumed up to now. The outflow structure downstream of the sill depends on its composition upstream and, more importantly, on that of AW in the sill surroundings where fortnightly and seasonal signals are imposed on the whole outflow.     

New Development of Eastern Mediterranean Circulation based on Hydrological Observations and Current Measurements

Abstract. A number of recent studies based on hydrographic observations and modelling simulations have dealt with the major climatic shift that occurred in the deep circulation of the Eastern Mediterranean. This work presents hydrographic observations and current measurements conducted from 1997 to 1999, which reveal strong modifications in the dynamics of the upper, intermediate and deep layers, as well as an evolution of the thermohaline characteristics of the deep Aegean outflow since 1995. The reversal of the circulation in the upper layer of the north/central Ionian is worthy of note. The observations indicate a reduction of Atlantic Water in the northern Ionian with an increase on the eastern side of the basin. In the intermediate layer, the dispersal path of the Levantine Intermediate Water (LIW) is altered. Highly saline (>39.0) and well-oxygenated intermediate waters were found near the Western Cretan Arc Straits. They flow out from the Aegean, thus interrupting the traditional path of the LIW, and spread prevalently northwards into the Adriatic Sea. In the deep layer, dense waters, exiting from the Adriatic (σ_ø−29.18 kg · m⁻³), flow against the western continental margin in the Ionian Sea at a depth of between 1000–1500 m. Dense waters of Aegean origin (> 29.20 kg · m⁻³), discharged into the central region of the Eastern Mediterranean during the early stages of the transient, propagate prevalently to the east in the Levantine basin and to the west in the northern Ionian Sea. Near-bottom current measurements conducted in the Ionian Sea reveal unforeseen aspects of deep dynamics, suggesting a new configuration of the internal thermohaline conveyor belt of the Eastern Mediterranean.     

Variability in the Canary Islands area of filament-eddy exchanges

The physical background to a suite of biological studies carried out in the Canary Islands upwelling region is presented. The area is unique in that the coastal transition zone is spanned by an archipelago of islands that shed mesoscale eddies of diameter 50–100 km into the alongshore flow. A recurrent filament and eddy system was sampled intensively to study the changing properties of waters as they are advected towards the open ocean in the filament and to investigate the exchanges between filament and eddies. The system was more complex than previously revealed. In early August, a single filament extended offshore from near Cape Juby. Two weeks later, a second filament had developed slightly farther north and extended offshore to merge with the first at 100 km offshore. The merged filament was entrained around a recurrent, topographically trapped cyclonic eddy and interacted with transient cyclonic and anticyclonic eddies shed from the island of Gran Canaria. Between the two filaments and the coast, a pair of counter-rotating eddies re-circulated water parcels for several weeks. Surface layer drifters cycled around this near-shore re-circulation several times before following convoluted paths that demonstrate significant exchange between continental shelf and open ocean waters.     

Modeling of mesoscale coupled ocean–atmosphere interaction and its feedback to ocean in the western Arabian Sea

Observations of the western Arabian Sea over the last decade have revealed a rich filamentary eddy structure, with large horizontal SST gradients in the ocean, developing in response to the southwest monsoon winds. This summertime oceanic condition triggers an intense mesoscale coupled interaction, whose overall influence on the longer-term properties of this ocean remains uncertain. In this study, a high-resolution regional coupled model is employed to explore this feedback effect on the long-term dynamical and thermodynamical structure of the ocean.The observed relationship between the near-surface winds and mesoscale SSTs generate Ekman pumping velocities at the scale of the cold filaments, whose magnitude is the order of 1 m/day in both the model and observations. This additional Ekman-driven velocity, induced by the wind-eddy interaction, accounts for approximately 10–20% of oceanic vertical velocity of the cold filaments. This implies that Ekman pumping arising from the mesoscale coupled feedback makes a non-trivial contribution to the vertical structure of the upper ocean and the evolution of mesoscale eddies, with obvious implications for marine ecosystem and biogeochemical variability.Furthermore, SST features associated with cold filaments substantially reduce the latent heat loss. The long-term latent heat flux change due to eddies in the model is approximately 10–15 W/m² over the cold filaments, which is consistent with previous estimates based on short-term in situ measurements. Given the shallow mixed layer, this additional surface heat flux warms the cold filament at the rate of 0.3–0.4 °C/month over a season with strong eddy activity, and 0.1–0.2 °C/month over the 12-year mean, rendering overall low-frequency modulation of SST feasible. This long-term mixed layer heating by the surface flux is approximately ±10% of the lateral heat flux by the eddies, yet it can be comparable to the vertical heat flux. Potential dynamic and thermodynamic impacts of this observed air–sea interaction on the monsoons and regional climate are yet to be quantified given the strong correlation between the Somalia upwelling SST and the Indian summer monsoons.     

Comparison between Eulerian diagnostics and finite-size Lyapunov exponents computed from altimetry in the Algerian basin

Transport and mixing properties of surface currents can be detected from altimetric data by both Eulerian and Lagrangian diagnostics. In contrast with Eulerian diagnostics, Lagrangian tools like the local Lyapunov exponents have the advantage of exploiting both spatial and temporal variability of the velocity field and are in principle able to unveil subgrid filaments generated by chaotic stirring. However, one may wonder whether this theoretical advantage is of practical interest in real-data, mesoscale and sub-mesoscale analysis, because of the uncertainties and resolution of altimetric products, and the non-passive nature of biogeochemical tracers. Here we compare the ability of standard Eulerian diagnostics and the finite-size Lyapunov exponent in detecting instantaneous and climatological transport and mixing properties in the south-western Mediterranean. By comparing with sea-surface temperature patterns, we find that the two approaches provide similar results for slowly evolving eddies like the first Alboran gyre. However, the Lyapunov exponent is also able to predict the (sub-)mesoscale filamentary processes occurring along the Algerian current and above the Balearic Abyssal Plain. Such filaments are also observed, with some mismatch, in sea-surface temperature patterns. Climatologies of Lyapunov exponents do not show any compact relation with other Eulerian diagnostics, unveiling a different structure even at the basin scale. We conclude that filamentation dynamics can be detected by reprocessing available altimetric data with Lagrangian tools, giving insight into (sub-)mesoscale stirring processes relevant to tracer observations and complementing traditional Eulerian diagnostics.     

Statistics of mesoscale cyclonic Eddies over the Black Sea

On the basis of the results obtained by using the PRECIS regional climate model with high space resolution (25 km), we study the mesoscale specific features of the atmospheric circulation in the Black-Sea region for a period of 30 yr. To separate and trace the subsynoptic eddies, we use a method based on the Okubo–Weiss criterion. Several types of cyclonic eddies are selected and described: winter Caucasian coastal eddies, summer Caucasian separated eddies, cyclonic eddies of the open sea, and seldom quasitropical cyclones. For various types of eddies, we present the statistics of their lifetime and intensity and the histograms of diurnal and seasonal cycles.     

基于拉格朗日分析法的中尺度涡精细三维结构研究及其误差估计

在前人的工作中,拉格朗日分析法被用来演示大尺度环流,同时拉格朗日拟序结构可以较好的演示中尺度涡两维结构的发展过程。然而,很少研究关注怎么利用拉格朗日分析法针对中尺度涡三维结构进行演示。与以往利用欧拉方法研究中尺度涡三维结构的工作不同,我们利用拉格朗日分析法,从另一个视角来研究涡旋结构。我们在海山上方模拟出一个理想的气旋涡,涡旋内的下沉流和涡旋旁的上升流形成一个闭合的环流。这种结构很难从欧拉角度来演示。然而,粒子的运动轨迹很好地展示了整个循环:流体在涡旋中旋转下沉,汇聚到底层的上升流区,并通过上升流返回到海表面。我们也将拉格朗日分析法应用于真实的模拟结果中。作为中国南海的一个重要现象,靠近越南中部的海域中的偶极子（反气旋涡/气旋涡）,关于其结构的研究已经比较成熟了,但这些研究主要关注的是海面过程。通过拉格朗日分析,我们很好的演示了偶极子的三维结构:流体在反气旋涡(气旋涡)内部旋转上升(下沉)。更重要的是,粒子的轨迹表明,这两个涡旋之间不存在水团交换,因为强边界急流将它们彼此分开。以上结论均得到了计算误差估计的可信度支持。尽管在强辐散流和强垂直扩散流中,计算误差逐渐增大,但是在一定的时间步长和积分周期内,计算误差始终保持在一个较小的值。     

卫星跟踪浮标和卫星遥感海面高度中的南海涡旋结构

选择4个南海卫星跟踪Argos漂流浮标及同期的TOPEX／Poseidon卫星遥感海面高度资料，研究了南海海域涡旋的活动及空间结构。这4个Argos漂流浮标的轨迹除了基本符合各季节海盆尺度环流趋势外，分别在菲律 宾以西、越南外海、南海中部等海域呈现出中尺度旋转轨迹。这些尺度涡旋现象在同期的TOPEX卫星遥感海面高度异常（SSHA）分布中得到了准确印证，并在诊断得到的地转流场中对应了一系列瞬变的中尺度涡旋运动。     

Mesoscale eddies dominate surface phytoplankton in northern Gulf of Alaska

The HNLC waters of the Gulf of Alaska normally receive too little iron for primary productivity to draw down silicate and nitrate in surface waters, even in spring and summer. Our observations of chlorophyll sensed by SeaWiFS north of 54°N in pelagic waters (>500 m depth) of the gulf found that, on average, more than half of all surface chlorophyll was inside the 4 cm contours of anticyclonic mesoscale eddies (the ratio approaches 80% in spring months), yet these contours enclosed only 10% of the total surface area of pelagic waters in the gulf. Therefore, eddies dominate the chlorophyll and phytoplankton distribution in surface pelagic waters. We outline several eddy processes that enhance primary productivity. Eddies near the continental margin entrain nutrient - (and Fe) - rich and chlorophyll-rich coastal waters into their outer rings, advecting these waters into the basin interior to directly increase phytoplankton populations there. In addition, eddies carry excess nutrients and iron in their core waters into pelagic regions as they propagate away from the continental margin. As these anticyclonic eddies decay, their depressed isopycnals relax upward, injecting nutrients up toward the surface layer. We propose that this transport brings iron and macro-nutrients toward the surface mixed layer, where they are available for wind-forced mixing to bring them to surface. These mesoscale eddies decay slowly, but steadily, perhaps providing a relatively regular upward supply of macro-nutrients and iron toward euphotic layers. They might behave as isolated oases of enhanced marine productivity in an otherwise iron-poor basin. We note that much of this productivity might be near or just below the base of the surface mixed layer, and therefore poorly sampled by colour-sensing satellites. It is possible, then, that eddies enrich phytoplankton populations to a greater extent than noted from satellite surface observations only.     

Physical oceanographic conditions around the S1 mooring site

We describe physical oceanographic conditions around the S1 biogeochemical mooring site (30°N, 145°E) between February 2010 and July 2013. At the S1 mooring site, there is a clear seasonal variability of the mixed layer depth, wind forcing as well as horizontal kinetic energy in a near-inertial band. Interannual variability of the winter mixed layer was observed. The winter mixed layer depth was shallower in early 2010 and became deeper afterwards. Several mesoscale eddies and typhoons passed by the S1 mooring sites every year. Based on observed events, we suggest that those physical processes possibly affected biogeochemical properties around the S1 mooring site.