Boundary Current Off Research Articles

Deep ocean hydrographic variability estimated from distributed geodetic sensor arrays off northern Chile

Observations of spatio-temporal variability of the deep ocean are rare and little is known about occurrence of deep ocean mesoscale dynamics. Here, we make use of 2.5 years of time series data from three distributed sensor arrays, which acquired high-resolution temperature, pressure and sound speed data of the bottom layer offshore northern Chile. Estimating salinity and density from the direct observations enable access to the full spectrum of hydrographic variability from a multi-hourly to annual time scale and with average inter-station distances of less than 1 km. Analyses revealed interannual warming over the continental slope of 0.002 °C yr−1–0.003 °C yr−1, and could trace periodic hydrographic anomalies, likely related to coastal-trapped waves, as far as to the lower continental slope. A concurrent change in the shape of the warm anomalies and the rate of deep-sea warming that occurs with the crossing of the deep-sea trench suggests that the abyssal part of the eastern boundary current system off Chile does not extend past the deep sea trench. Furthermore, the comparison of anomaly timing and shape in between stations implies southwards flow over the mid to lower continental slope, centred closer to the trench.

Seabird assemblages are linked to the major western boundary current off eastern Australia

Identifying species assemblages helps understand the relationship between organisms and their environment. Assemblages can be used to predict biological changes caused by environmental perturbations, and are thus essential surrogates to monitor biodiversity. In this study, to identify and describe seabird assemblages, we used 15 at-sea ship-based survey data sets collected over 37°of latitude off eastern Australia, from 2016 to 2021. We fitted seasonal Region of Common Profile (RCP) mixture models, for two types of data (presence–absence and abundance). RCP groups are defined as regions where the probability of encountering a particular species profile is constant within regions, but different amongst them. These groups also vary according to covariates, which in our case included oceanographic, climatic, and physiographic parameters. Results were based on 142,646 seabirds recorded from 80 species, including albatrosses, petrels, prions, shearwaters, boobies, and terns, among other taxa. All models suggested two macro-scale assemblages (‘northern’ and ‘southern’), except for the autumn presence–absence model that identified three groups. The model results consistently show a biogeographic transition at ∼34°S, near the latitude at which the East Australian Current (EAC) separates from the Australian continental slope. Sea surface temperatures or sea surface salinities were selected in all final models, further indicating a close relationship between seabird assemblages and water masses. Results from both data types, presence–absence and abundance, resulted in similar spatial and species profile patterns. RCP models clearly identified two seabird assemblages off the east coast of Australia, suggesting the persistence of these groups at seasonal and macro spatial scales. Given the ongoing poleward intensification that the EAC is experiencing, which is projected to continue over the next century, and its importance in influencing the distributions of seabirds, the methods applied in our study could be replicated to assess possible changes in seabird assemblages and how they are affected by changing environmental conditions.

Decadal changes in Atlantic overturning due to the excessive 1990s Labrador Sea convection

Changes in the Atlantic Meridional Overturning Circulation (AMOC) represent a crucial component of Northern Hemisphere climate variability. In modelling studies decadal overturning variability has been attributed to the intensity of deep winter convection in the Labrador Sea. This linkage is challenged by transport observations at sections across the subpolar gyre. Here we report simulations with an eddy-rich ocean model which captures the observed concentration of downwelling in the northeastern Atlantic and the negligible impact of interannual variations in Labrador Sea convection during the last decade. However, the exceptionally cold winters in the Labrador Sea during the first half of the 1990s induced a positive AMOC anomaly of more than 20%, mainly by augmenting the downwelling in the northeastern North Atlantic. The remote effect of excessive Labrador Sea buoyancy forcing is related to rapid spreading of mid-depth density anomalies into the Irminger Sea and their entrainment into the deep boundary current off Greenland.

Topographically Generated Submesoscale Shear Instabilities Associated with Brazil Current Meanders

Abstract The western boundary current system off southeastern Brazil is composed of the poleward-flowing Brazil Current (BC) in the upper 300 m and the equatorward flowing Intermediate Western Boundary Current (IWBC) underneath it, forming a first-baroclinic mode structure in the mean. Between 22° and 23°S, the BC-IWBC jet develops recurrent cyclonic meanders that grow quasi-stationarily via baroclinic instability, though their triggering mechanisms are not yet well understood. Our study, thus, aims to propose a mechanism that could initiate the formation of these mesoscale eddies by adding the submesoscale component to the hydrodynamic scenario. To address this, we perform a regional 1/50° (∼2 km) resolution numerical simulation using CROCO (Coastal and Regional Ocean Community model). Our results indicate that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instability that can trigger the meanders’ formation. Subsequently, this process generates submesoscale cyclones that contribute, along with baroclinic instability, to the meanders’ growth, resulting in a submesoscale-to-mesoscale inverse cascade. Last, as the mesoscale cyclones grow, they interact with the slope, generating inertially and symmetrically unstable anticyclonic submesoscale vortices and filaments. Significance Statement Off southeastern Brazil, the Brazil Current develops recurrent cyclonic meanders. Such meanders enhance the open-ocean primary productivity and are of societal importance as they are located in a region rich in oil and gas where oil-spill accidents have already happened. This study aims to explore the processes responsible for triggering the formation of these mesoscale eddies. We find that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instabilities that eject submesoscale filaments and vortices and can trigger the meanders’ formation. Such results show that topographically generated submesoscale instabilities can play an important role in the dynamics of mesoscale meanders off southeastern Brazil. Moreover, this may indicate that resolving the submesoscale dynamics in operational numerical models may contribute to an increase in the predictability of the regional eddies.

A Fukushima tracer perspective on four years of North Pacific mode water evolution

We present the results of a multi-platform investigation that utilizes tracer information provided by the 2011 release of radioisotopes from the Fukushima Dai-ichi Nuclear Power Plant to better understand the pathways, mixing and transport of mode waters formed in the North Pacific Ocean. The focus is on transition region mode waters and radiocesium (137Cs and 134Cs) observations obtained from the May–June 2015 GO-SHIP occupation of the 152°W line in the Northeast Pacific. Samples include profiles from the surface to 1000 m and surface/subsurface pairs that provide an average 1° of latitude spacing along 152°W. We find a clear Fukushima (134Cs) signal from the surface to 400 m. The core signal (134Cs ∼10 Bq m-3, 137Cs ∼12 Bq m-3) at 41°-43°N lies at 30–220 m where mode waters formed through deep winter mixing in 2011 outcropped in the western North Pacific. The strongest 2015 152°W Fukushima-source radiocesium signal is associated with Dense-Central Mode Waters consistent with the densest variety of these mode waters being formed off the coast of Japan 4 years earlier. The radionuclide signal transited the basin along subsurface mode water isopycnals mainly on the northern side of the subtropical gyre before outcropping at and to the east of the 152°W line. In 2015, the densest 152°W waters with 134Cs lie at ∼435 m in the bottom range of Dense-Central Mode Water at 40°N. There is a weak, but detectable, signal in the boundary current off both Kodiak and Sitka. The deepest detectable 137Cs (weapon's testing background) are found at and to the north of 45°N at 900–1000 m. With the exception of a single subsurface sample near Hawaii, as of spring 2015, the southernmost 134Cs was found above 200 m at 30°N. A total date-corrected 134Cs inventory of 11–16 PBq is estimated. Qualitative comparison to model output suggests good consistency in terms of general location, latitudinal breadth, and predicted depth of penetration, allowing discussion of the bigger picture. However, the model's 2015 152°W radiocesium signal is quantitatively weaker and the core is offset in latitude, potentially due to the lack of consideration of atmospheric deposition.

Antarctic Circumpolar Current Dynamics at the Pacific Entrance to the Drake Passage Over the Past 1.3 Million Years

AbstractThe Antarctic Circumpolar Current (ACC) is the world's largest current system connecting all three major basins of the global ocean. Our knowledge of glacial‐interglacial changes in ACC dynamics in the southeast Pacific is not well constrained and presently only based on reconstructions covering the last glacial cycle. Here we use a combination of mean sortable silt grain size of the terrigenous sediment fraction (10–63 μm, ) and X‐ray fluorescence scanner‐derived Zr/Rb ratios as flow strength proxies to examine ACC variations at the Pacific entrance to the Drake Passage (DP) in the vicinity of the Subantarctic Front. Our results indicate that at the DP entrance, ACC strength varied by ~6–16% on glacial‐interglacial time scales, yielding higher current speeds during interglacial times and reduced current speeds during glacials. We provide evidence that previous observations of a reduction in DP throughflow during the last glacial period are part of a consistent pattern extending for at least the last 1.3 Ma. The orbital‐scale cyclicity follows well‐known global climate changes from prevailing ca. 41‐kyr cycles in the early part of the record (1.3 Ma to 850 ka; marine isotope stage 21) across the mid‐Pleistocene transition into the middle and late Pleistocene 100‐kyr world. A comparison to a bottom water flow record from the deep western boundary current off New Zealand (Ocean Drilling Program Site 1123) reveals anti‐phased changes between the two sites. The enhanced supply of deep water along the DP and into the Atlantic Ocean during interglacials corresponds to a weakened flow of the SW Pacific deep western boundary current.

Marine debris pathways across the southern Indian Ocean

The distribution of marine debris both with the open oceans and along coastlines is largely governed by surface currents and thereby dependent upon seasonal circulation patterns and annual variability inherent in the major current systems. The recovery of a satellite-tracked current drifter on a South African beach some 22 months after release off Western Australia was an indicator of the ocean-wide processes operating in the southern Indian Ocean gyre. Using this single drift trajectory in combination with data from other current drifters released along the Western Australian coast between 2008 and 2013, from earlier satellite-tracked drifters and a selection of historical drift bottle returns, drift patterns across the southern Indian Ocean between Western Australia and east Africa were analysed to quantify inter-continental debris transport rates via the South Equatorial Current (northern east-west pathway) and the South Indian Ocean Current (southern west-east pathway). The northern east-west trans-ocean pathway can be considered in three segments: the complex and highly variable Leeuwin Current eddy system in the east, which can retain drifting debris for up to a year, the comparatively uniform westward travel in the South Equatorial Current, and the eddying components in the western boundary current system off southern Africa. A simple Eddy Activity Index is derived here to clearly distinguish the Leeuwin Current eddy region from the South Equatorial Current. The southern route to Australia is simpler via the Agulhas Return Current and the South Indian Ocean Current.

Seawater acidification and anthropogenic carbon distribution on the continental shelf and slope of the western South Atlantic Ocean

Abstract Increases in atmospheric carbon dioxide (CO2) concentrations since the beginning of the Industrial Revolution have led to increases in the flux of CO2 into the oceans. A large fraction of this increased flux occurs due to anthropogenic carbon (Cant), i.e., the excess carbon released by human activities. The oceans take up large amounts of Cant, and this process likely affects calcifying organisms and the formation and dissolution of calcium carbonate. In this study, we measured hydrographic and carbonate system parameters in the western South Atlantic Ocean. This region is key for understanding climate issues; however, it remains understudied, especially regarding the drivers of ocean acidification and its effects on marine organisms and ecosystems. Using novel data collected in this undersampled area of the global ocean, we (i) determined values of the marine carbonate system parameters and (ii) characterized the seawater acidification state along the western boundary current off the southeastern and southern coasts of Brazil. Specifically, we measured total alkalinity (AT) and pH and applied the TrOCA approach (to derive the Cant storage in the water masses) in the study area. Our results show that (i) AT and the dissolved inorganic carbon distribution in the study region are governed by calcification/dissolution and dilution/evaporation processes and that (ii) Cant is present in the central and intermediate water masses at an average concentration (± method precision) of 75.0 ± 6.6 μmol kg−1 and at depths of up to ~1000 m. The highest concentrations are observed in the lower levels of the Tropical Water, although we cannot exclude an overestimated Cant concentration in this layer, and the South Atlantic Central Water (SACW). These findings suggest that cross-shelf processes likely contribute to increased Cant along western boundary current systems. The storage of Cant in the SACW and the Antarctic Intermediate Water has led to long-term pH decreases of −0.17 ± 0.07 and −0.10 ± 0.06 pH units, respectively.

First Detailed Study of Circulation off Angola

New data give scientists insight into the eastern boundary current off Angola, helping them to evaluate and assess why simulations create sea surface temperature biases in the region.

From interannual to decadal: 17 years of boundary current transports at the exit of the Labrador Sea

AbstractOver the past 17 years, the western boundary current system of the Labrador Sea has been closely observed by maintaining the 53°N observatory (moorings and shipboard station data) measuring the top‐to‐bottom flow field offshore from the Labrador shelf break. Volume transports for the North Atlantic Deep Water (NADW) components were calculated using different methods, including gap filling procedures for deployment periods with suboptimal instrument coverage. On average the Deep Western Boundary Current (DWBC) carries 30.2 ± 6.6 Sv of NADW southward, which are almost equally partitioned between Labrador Sea Water (LSW, 14.9 ± 3.9 Sv) and Lower North Atlantic Deep Water (LNADW, 15.3 ± 3.8 Sv). The transport variability ranges from days to decades, with the most prominent multiyear fluctuations at interannual to near decadal time scales (±5 Sv) in the LNADW overflow water mass. These long‐term fluctuations appear to be in phase with the NAO‐modulated wind fluctuations. The boundary current system off Labrador occurs as a conglomerate of nearly independent components, namely, the shallow Labrador Current, the weakly sheared LSW range, and the deep baroclinic, bottom‐intensified current core of the LNADW, all of which are part of the cyclonic Labrador Sea circulation. This structure is relatively stable over time, and the 120 km wide boundary current is constrained seaward by a weak counterflow which reduces the deep water export by 10–15%.

Characteristics of the Nordic Seas overflows in a set of Norwegian Earth System Model experiments

Global ocean models with an isopycnic vertical coordinate are advantageous in representing overflows, as they do not suffer from topography-induced spurious numerical mixing commonly seen in geopotential coordinate models. In this paper, we present a quantitative diagnosis of the Nordic Seas overflows in four configurations of the Norwegian Earth System Model (NorESM) family that features an isopycnic ocean model. For intercomparison, two coupled ocean-sea ice and two fully coupled (atmosphere-land-ocean-sea ice) experiments are considered. Each pair consists of a (non-eddying) 1° and a (eddy-permitting) 1/4° horizontal resolution ocean model. In all experiments, overflow waters remain dense and descend to the deep basins, entraining ambient water en route. Results from the 1/4° pair show similar behavior in the overflows, whereas the 1° pair show distinct differences, including temperature/salinity properties, volume transport (Q), and large scale features such as the strength of the Atlantic Meridional Overturning Circulation (AMOC). The volume transport of the overflows and degree of entrainment are underestimated in the 1° experiments, whereas in the 1/4° experiments, there is a two-fold downstream increase in Q, which matches observations well. In contrast to the 1/4° experiments, the coarse 1° experiments do not capture the inclined isopycnals of the overflows or the western boundary current off the Flemish Cap. In all experiments, the pathway of the Iceland-Scotland Overflow Water is misrepresented: a major fraction of the overflow proceeds southward into the West European Basin, instead of turning westward into the Irminger Sea. This discrepancy is attributed to excessive production of Labrador Sea Water in the model. The mean state and variability of the Nordic Seas overflows have significant consequences on the response of the AMOC, hence their correct representations are of vital importance in global ocean and climate modelling.

Interannual to decadal changes in the western boundary circulation in the Atlantic at 11°S

AbstractThe western boundary current system off Brazil is a key region for diagnosing variations of the Atlantic meridional overturning circulation (AMOC) and the southern subtropical cell. In July 2013 a mooring array was installed off the coast at 11°S similar to an array installed between 2000 and 2004 at the same location. Here we present results from two research cruises and the first 10.5 months of moored observations in comparison to the observations a decade ago. Average transports of the North Brazil Undercurrent and the Deep Western Boundary Current (DWBC) have not changed between the observational periods. DWBC eddies that are predicted to disappear with a weakening AMOC are still present. Upper layer changes in salinity and oxygen within the last decade are consistent with an increased Agulhas leakage, while at depths water mass changes are likely related to changes in the North Atlantic as well as tropical circulation changes.

South Pacific dissolved Nd isotope compositions and rare earth element distributions: Water mass mixing versus biogeochemical cycling

Despite its enormous extent and importance for global climate, the South Pacific has been poorly investigated in comparison to other regions with respect to chemical oceanography. Here we present the first detailed analysis of dissolved radiogenic Nd isotopes (εNd) and rare earth elements (REEs) in intermediate and deep waters of the mid-latitude (∼40°S) South Pacific along a meridional transect between South America and New Zealand. The goal of our study is to gain better insight into the distribution and mixing of water masses in the South Pacific and to evaluate the validity of Nd isotopes as a water mass tracer in this remote region of the ocean. The results demonstrate that biogeochemical cycling (scavenging processes in the Eastern Equatorial Pacific) and release of LREEs from the sediment clearly influence the distribution of the dissolved REE concentrations at certain locations. Nevertheless, the Nd isotope signatures clearly trace water masses including AAIW (Antarctic Intermediate Water) (average εNd=−8.2±0.3), LCDW (Lower Circumpolar Deep Water) (average εNd=−8.3±0.3), NPDW (North Pacific Deep Water) (average εNd=−5.9±0.3), and the remnants of NADW (North Atlantic Deep Water) (average εNd=−9.7±0.3). Filtered water samples taken from the sediment–water interface under the deep western boundary current off New Zealand suggest that boundary exchange processes are limited at this location and highlight the spatial and temporal variability of this process.These data will serve as a basis for the paleoceanographic application of Nd isotopes in the South Pacific.

Evaluating Cenozoic equatorial sediment deposition anomalies for potential paleoceanographic and Pacific plate motion applications

If equatorial sediments form characteristic deposits around the equator, they may help to resolve the amount of northwards drift of the Pacific tectonic plate. Relevant to this issue, it has been shown that 230Th has been accumulating on the equatorial seabed faster than its production from radioactive decay in the overlying water column during the Holocene (Marcantonio et al. in Paleoceanography 16:260–267, 2001). Some researchers have argued that this reflects the deposition of particles with adsorbed 230Th carried by bottom currents towards the equator (“focusing”). If correct, this effect may combine with high pelagic productivity, which is also centered on the equator, to yield a characteristic signature of high accumulation rates marking the paleoequator in older deposits. Here we evaluate potential evidence that such an equatorial feature existed in the geological past. Seismic reflection data from seven meridional transects suggest that a band of equatorially enhanced accumulation of restricted latitude was variably developed, both spatially and temporally. It is absent in the interval 14.25–20.1 Ma but is well developed for the interval 8.55–14.25 Ma. We also examined eolian dust accumulation rate histories generated from scientific drilling data. A dust accumulation rate anomaly near the modern equator, which is not obviously related to the inter-tropical convergence zone, is interpreted as caused by focusing. Accumulation rates of Ba and P2O5 (proxies of export production) reveal a static equatorial signature, which suggests that the movement of the Pacific plate over the period 10–25 Ma was modest. The general transition from missing to well-developed focusing signatures around 14.25 Ma in the seismic data coincides with the mid-Miocene development of the western boundary current off New Zealand. This current supplies the Pacific with deep water from Antarctica, and could therefore imply a potential paleoceanographic or paleoclimatic origin. At 10.05–14.25 Ma, the latitudes of the seismic anomalies are up to ~2° different from the paleoequator predicted by Pacific plate-hotspot models, suggesting potentially a small change in the hotspot latitudes relative to the present day (although this inference depends on the precise form of the deposition around the equator). The Ba and P2O5 anomalies, on the other hand, are broadly compatible with plate models predicting slow northward plate movement over 10–25 Ma.

Does the poleward boundary current off Western Australia exert a dominant influence on coastal chaetognaths and siphonophores?

A transect that extended 40 km offshore across the continental shelf off Perth, Western Australia, was sampled monthly during 1997 and 1998. Zooplankton was sampled at 5 km intervals with a 300 micron-mesh bongo net deployed vertically to within 3 m of the bottom, or to a maximum depth of 70 m. Numbers of species of chaetognaths and siphonores were quantified, as were abundances of the common species from these groups and of the hydromedusae Auglaura hemistoma. The potential influences of four environmental variables (sea-level, sea surface temperature, salinity and chlorophyll concentration) on variability in diversity and abundance were assessed using generalized additive modeling. A combination of factors were found to influence the seasonal and spatial biological variability and, of these factors, non-linear relationships always contributed to the best fitting models. In all but one case, each of the environmental variables was included in the final model. The seasonally variable Leeuwin Current, whose strength is measured as variations in local sea-level, is the dominant mesoscale oceanographic feature in the study region but was not found to have an overriding influence on the shelf zooplankton. This contrasts a previous hypothesis that subjectively attributed seasonal variability of the same taxa examined in this study to seasonal variations in the Leeuwin Current. There remains a poor understanding of shelf zooplankton off Western Australia and, in particular, of the processes that influence seasonal and spatial variability. A more complete understanding of potential causative influences of the Leeuwin Current on the shelf plankton community of south-western Australia must be cognizant of a range of biophysical factors operating at both the broader mesoscale and at smaller scales within the shelf pelagic ecosystem.

Decadal variability of subpolar gyre transport and its reverberation in the North Atlantic overturning

Analyses of sea surface height (SSH) records based on satellite altimeter data and hydrographic properties have suggested a considerable weakening of the North Atlantic subpolar gyre during the 1990s. Here we report hindcast simulations with high‐resolution ocean circulation models that demonstrate a close correspondence of the SSH changes with the volume transport of the boundary current system in the Labrador Sea. The 1990s‐decline, of about 15% of the long‐term mean, appears as part of a decadal variability of the gyre transport driven by changes in both heat flux and wind stress associated with the North Atlantic Oscillation (NAO). The changes in the subpolar gyre, as manifested in the deep western boundary current off Labrador, reverberate in the strength of the meridional overturning circulation (MOC) in the subtropical North Atlantic, suggesting the potential of a subpolar transport index as an element of a MOC monitoring system.

On the annual cycle of coastal and open ocean satellite chlorophyll off Chile (18°–40°S)

High resolution SeaWiFS (Sea‐viewing Wide Field‐of‐view Sensor) chlorophyll and satellite wind data (1997–2004) are used to analyze the coastal and oceanic behavior of sea surface chlorophyll (CHL) concentration in the eastern boundary current system off Chile (18°–40°S). Two contrasting regimes are evident; a coastal regime, where CHL maximum occurs during summer, coinciding with the maximum upwelling‐favorable wind stress and an offshore regime, where the CHL maximum is in winter and is not related to the maximum wind (stress, curl). Moreover, our analysis reveals that the annual coastal CHL cycle is strong along the entire latitudinal extension, contrary to previous observations that showed a weak seasonal cycle off northern Chile. Averaging ocean‐color data across spatial domains where different forcings and processes operate will lead to unreliable retrieval of pigment biomass. Proper characterization of biological processes in upwelling and coastal regions requires deriving statistics over the relevant spatial scales.

Mixing estimates from a large-scale hydrographic survey in the North Atlantic

[1] A large-scale hydrographic survey from 2003 (RV Meteor cruise M59/2) is used to map turbulent mixing by means of eddy diffusivities from finescale shear and strain in the subpolar North Atlantic, covering various hydrographic and bathymetric conditions. Results from the analysis indicate small (O(10−5) m2s−1) diffusivities below the Labrador Sea Water layer at mid-depth in the open ocean. Higher values exceeding 10−4 m2s−1 are found in the abyss close to rough topography and over the flanks of the Mid-Atlantic Ridge. The strongest mixing with diffusivities of 10−3 m2s−1 occurs in the Deep Western Boundary Current off Flemish Cap. The results of this study convey the need to map mixing globally from existing sets of CTD/LADCP data as well as its feasibility: the similarities and differences in mixing patterns as compared to other studies indicate that large scale patterns of small scale processes are still not well known and understood.

Circulation and variability in the Chile Basin

Current observations at several depths between 250 and 3750m are reported from a 30°S, deep sea site 150km off the Chile coast for the period July 1993–June 2001. These results are used with current observations from a nearby slope site, satellite altimeter data, and hydrographic and Acoustic Doppler Current Profiler data from the WOCE P6E section along 32.5°S to address mean and variable flow in the Chile Basin. Strong current variability in the upper ocean for periods more than 100 days is explained in terms of remotely forced Rossby waves, local baroclinic instabilities of coastal currents and variable wind forcing. Intraseasonal to seasonal current variability at the deep ocean site was greatest during La Niña events while intraseasonal variability at the slope site was greatest during the 1997–1998 El Niño event. Mean westward and poleward flow was observed at all depths at the deep sea site but upper ocean means were not significantly different from zero. There was a well-defined, mean poleward flow of 0.6±0.3cms−1 at 2450m depth there. Geostrophic current calculations are presented for the P6E section with levels of no motion based on our current observations and other constraints. These reference choices yield reasonable and consistent results for the steady-state heat balance of the deep Chile and Peru Basins. Results show a deep equatorward flow of 3–4Sv on the eastern flank of the East Pacific Rise and a deep poleward boundary flow of about 10Sv within 1500km of the Chile coast. Up to half of the total mid-depth outflow of the South Pacific may take place east of the Rise. Thus the deep poleward boundary current off Chile is a major component of the deep circulation of the global ocean but the dynamics of this current remain a puzzle.

Phytoplankton community response to a manipulation of bioavailable iron in HNLC waters of the subtropical Pacific Ocean

Studies in high nutrient, low chlorophyll (HNLC) regions have demonstrated that increased Fe availability results in an increase in phytoplankton biomass and changes in community composition. Here we present experiments in which the availability of iron (Fe) was increased or reduced to monitor the response of individual groups of phytoplankton (large eukaryotes, pico- eukaryotes and cyanobacteria) by flow cytometry. Additions (0.5 to 5.0 nM Fe) and reductions in available Fe (through addition of 1 to 10 nM of the fungal siderophore desferrioxamine B) were made to enclosed communities from the South American eastern boundary current off Peru, where ambi- ent dissolved Fe concentrations were <100 pM. As predicted, chlorophyll concentrations increased in the added Fe treatments relative to the control, indicative of Fe limitation. Flow cytometry demon- strated that this was due to increases in the abundance of large eukaryotes that are Fe-starved under ambient conditions. Cyanobacterial abundance increased and decreased linearly with Fe availabil- ity, suggesting that cyanobacteria were Fe-limited but not Fe-starved. In contrast, picoeukaryote cell abundance increased with decreasing Fe availability, although chlorophyll cell -1 in this group responded in an inverse manner. The results demonstrate that members of the marine phytoplankton community respond differently to Fe availability, which may influence the outcome of biological competition among organisms in Fe-limited environments.