Cold Surface Water Research Articles

Sea‐Ice Induced Southern Ocean Subsurface Warming and Surface Cooling in a Warming Climate

AbstractMuch of the Southern Ocean surface south of 55° S cooled and freshened between at least the early 1980s and the early 2010s. Many processes have been proposed to explain the unexpected cooling, including increased winds or freshwater fluxes. However, these mechanisms so far failed to fully explain the surface trends and the concurrent subsurface warming (100 to 500 m). Here, we argue that these trends are predominantly caused by an increased wind‐driven northward sea‐ice transport, enhancing the extraction of freshwater near Antarctica and releasing it in the open ocean. This conclusion is based on factorial experiments with a regional ocean model. In all experiments with an enhanced northward sea‐ice transport, a strengthened salinity‐dominated stratification cools the open‐ocean surface waters between the Subantarctic Front and the sea‐ice edge. The strengthened stratification reduces the downward mixing of cold surface water and the upward heat loss of the warmer waters below, thus warming the subsurface. This sea‐ice induced subsurface warming mostly occurs around West Antarctica, where it likely enhances ice‐shelf melting. Moreover, the subsurface warming could account for about 8 ± 2% of the global ocean heat content increase between 1982 and 2011. Antarctic sea‐ice changes thereby may have contributed to the slowdown of global surface warming over this period. Our conclusions are robust across all considered sensitivity cases, although the trend magnitude is sensitive to forcing uncertainties and the model's mean state. It remains unclear whether these sea‐ice induced changes are associated with natural variability or reflect a response to anthropogenic forcing.

Paleoceanographic changes in the East China Sea during the last ~400 kyr reconstructed using planktic foraminifera

Abstract East China Sea (ECS) is greatly influenced by East Asian Summer Monsoon (EASM), precipitation discharge and differential inflow of the warm and saline Kuroshio Current (KC). Any change in surface water conditions either due to variations in ocean currents or discharge from terrestrial rivers have pronounced effects on the population abundance of planktic foraminifera; therefore, planktic foraminifera happen to be a valuable proxy to decipher paleoceanographic changes in the ECS. To understand the influence of KC and EASM at ECS, paleoceanographic changes are reconstructed for the last 400 kyr using planktic foraminiferal population abundances and Mg/Ca ratio derived Sea Surface Temperature (SST) from IODP site U1429. Nine different species/species associations of planktic foraminifera are found to be significant at this site. Q-mode factor analysis of these abundant species helped to retain four factors explaining ~91% of the data variance suggesting four different conditions prevailing at ECS over the last 400 kyr. The species association of factor 1 reflects cold surface water conditions in the ECS, factor 2 suggests the influence of the coastal water, factor 3 can be linked with the upwelling of lower thermocline water, and factor 4 is strongly associated with the strength of the KC. This study suggests that KC and EASM were stronger during the interglacial periods. The proxy that responds to KC strength (Globogerinoides ruber) and calculated Mg/Ca SST shows the presence of 23 kyr precession and 100 kyr eccentricity cycles modulated by the differential inflow of KC due to global and local factors. The comparison of SST of the western Pacific with SST of the ECS shows that KC actively transports heat to ECS from the equatorial western Pacific warm pool.

Holocene changes in sea-ice cover and polynya formation along the eastern North Greenland shelf: New insights from biomarker records

Understanding the processes controlling the natural variability of sea ice in the Arctic, one of the most dynamic components of the climate system, can help to constrain the effects of future climate change in this highly sensitive area. For the first time, a high-resolution biomarker study was carried out to reconstruct past sea-ice variability off eastern North Greenland. This area is strongly influenced by cold surface waters and drift ice transported via the East Greenland Current, meltwater pulses from the outlet glaciers of the Northeast Greenland Ice Stream and the build-up of landfast ice. The well-dated Holocene sedimentary section of Core PS93/025 provides insights into variations of the sea-ice conditions (regional and local sea-ice signal), oceanic and atmospheric circulation and the biotic response to these changes. These biomarker records show a reduced to variable sea-ice cover during the early Holocene between 10.2 and 9.3 ka, followed by a steady increase in sea-ice conditions during the mid Holocene. During the last 5–6 ka, sea-ice conditions remained more stable representing a seasonal to marginal sea-ice situation. Based on our biomarker records, stable sea-ice edge conditions, with a fully developed polynya situation occurred since the last 1 ka.

Palynology, biostratigraphy, and paleoceanography of the Plio-Pleistocene at Ocean Drilling Program Site 887, Gulf of Alaska

Analyses of marine palynomorphs, including dinocysts and acritarchs, from Pliocene-Pleistocene sediments of the Ocean Drilling Program (ODP) Site 887 in the Gulf of Alaska allowed the development of a biostratigraphic scheme, which we compared with bio-events in regional diatom and radiolarian zonations. The dinocyst biostratigraphic scheme includes five biozones and four major boundaries. A first stratigraphic boundary, at 4.4 Ma, is associated with a change in productivity. The other boundaries, at 2.7 Ma, 1.7 Ma, and 0.7 Ma, correspond to the onset of the modern halocline, an intensified cooling period, and the end of the Mid-Pleistocene Transition respectively. Moreover, the analyses of dinocyst assemblages illustrate long-term changes in the surface ocean after 5.3 Ma. The occurrence of Ataxiodinium zevenboomii, Impagidinium velorum, and Impagidinium patulum suggests warm-temperate conditions until approximately 4.2 Ma. Between 4.2 and 2.7 Ma, colder and less saline events marked by an increase in cold-tolerant species, such as Habibacysta tectata, suggest regional cooling and/or lower salinity of surface water, which might be related to Alaskan glacier meltwater discharges. From 2.7 to 1.2 Ma, the presence of Impagidinium pallidum and cysts of Pentapharsodinium dalei suggests low-salinity, cold, and stratified surface waters, whereas major drops in dinocyst fluxes are linked to a decrease in productivity and harsh conditions. Progressive change from cold, stratified waters to warmer and saltier conditions occurred between 1.2 and 0.7 Ma during the Mid-Pleistocene Transition. After 0.7 Ma, dinocyst assemblages are characterized by the alternating dominance of Brigantedinium spp. and Operculodinium centrocarpum, suggesting fluctuations between nutrient-rich, low-salinity, cold waters and cool-temperate environments.

Seasonal Changes in the North Atlantic Cold Anomaly: The Influence of Cold Surface Waters From Coastal Greenland and Warming Trends Associated With Variations in Subarctic Sea Ice Cover

AbstractWorldwide sea surface temperatures (SST) have increased on average by about 1 °C since 1900 with the exception of a region of the North Atlantic subpolar gyre near 50°N which has cooled by up to 0.9 °C over the same period, generating the negative feature on temperature anomaly maps which has been colloquially described by Rahmstorf et al. (2015, https://doi.org/10.1038/nclimate2554) as the “cold blob” (abbreviated here CB). This unique long‐term surface cooling trend is most evident in February, but in August net warming is observed even at CB epicenter and the CB itself is reduced to a mere “warming hole.” These seasonal changes in the intensity of the CB are the product of two separate factors: (1) a long‐term winter cooling specific for the CB region which appears to be associated with cooling of Greenland coastal waters in autumn, plausibly linked to summer meltwater from icebergs and sea ice and (2) summer warming effects which derive from (a) dramatic reduction in summer sea ice cover in the sub‐Arctic over the last 30 years that allows enhanced absorption of sunlight by the new open water in summer and (b) an unusual period of increased summer sub‐Arctic ice cover in the early twentieth century, which lowers the SST baseline measured from 1900, thus increasing the calculated linear rate of change of SST with time. Both of these effects could contribute to the observed Arctic amplification of warming.

Tidally generated island wakes and surface water cooling over Izu Ridge

The Kuroshio current is well known for generating cold wakes behind islands over Izu Ridge in Northwestern Pacific. Observational data from the geostationary Himawari-8 satellite for 2015–2017 revealed the occurrence of cold waters during the period when the Kuroshio current flows away from the islands. With a focus on tidal currents, this study presents an investigation of dynamical processes responsible for the formation of areas with low sea surface temperature (SST) through the adoption of a high-resolution numerical ocean model for an event that happened in July 2017. Areas with cold water emerged only when tidal currents are included in the numerical model. The model results indicate the cold surface waters are formed in the vicinity of the islands because of upwelling and vertical mixing. Qualitative features of the cold water formation for each island are found to depend on its size, topography, and ambient currents. Near Kozu Island, the tidal excursion is large enough to cause eddy shedding. These shed eddies are stirred by tidal currents to extend the surface cooling effect to wider areas. Near Hachijo Island, a persistent wake is formed by the ambient northward current. Inclusion of tidal currents destabilizes the wake, and consequently leads to the formation of a low SST area, although no clear eddy shedding is detected. The flow patterns around the islands are classified using an additional non-dimensional parameter, defined as the ratio between tidal excursion and island diameter.

Spatio-temporal variability of the equatorial front in the eastern tropical Pacific from remote sensing salinity data (2010–2015)

In the eastern Pacific, the Equatorial Front (EF) separates the relatively cold and salty Equatorial Surface Water to the South from the relatively warm and fresh Surface Tropical Water to the North. Although previous studies have succinctly described the main EF characteristics, little is known about its spatial pattern and temporal variability beyond seasonal scale. In this study, we used 5 years of remote-sensing sea surface salinity (SSS) data, acquired by SMOS in a synoptic and continuous manner, to describe the spatio-temporal variability of the haline EF (HEF). After validating the SSS satellite dataset with concomitant in situ data, we developed an algorithm based on maximum SSS gradients to automatically identify the HEF position on every SMOS SSS map. This algorithm applied to the 4-day resolution SMOS SSS product, allows us to investigate the spatio-temporal variability of the HEF between 2010 and 2015. Seasonally, west of the Galapagos Islands, the HEF is centered at ~2°N and shows relatively weak meridional displacements. In contrast, east of the Galapagos Archipelago, the HEF varies on average between ~4°S in spring and ~2°N in autumn. At intraseasonal scales, the HEF shows meridional fluctuations of ±1.25° around its average position. These intraseasonal fluctuations are partly related to Kelvin Waves, which was more noticeable since 2014. Over the 5-year studied period, the HEF position showed a northward trend of 0.3–0.4 latitudinal degrees per year between 2010 and 2015. However, the relatively short time series does not permit to investigate whether this low-frequency trend is due to long-term variability or interdecadal fluctuations.

A study on causes and recurrence of the Mid-Mediterranean Jet from 2003 to 2015 using satellite thermal and altimetry data and CTD casts

ABSTRACT The existence of the Mid-Mediterranean Jet (MMJ) as part of the pathway of the Atlantic Water (AW) in Eastern Mediterranean is still debated and the reasons for its formation are unknown. The MMJ was investigated over the 2000–2015 summers using thermal and altimetry satellite data and ‘in situ’ casts. In the Levantine basin, the AW flowing along the Libyan-Egyptian coast splits into an along-shore and an offshore branch, the latter following the northern border of the warm anti-cyclones located near the African coast (Libyan-Egyptian Eddies, LEEs). The MMJ is triggered when a surface cold water mass formed south-east of Crete, in part outflowing the Aegean Sea and in part generated within the Rhodes Gyre, moves southward and intrudes the middle Levantine basin. The surface cold water overlays the AW flowing along the northern periphery of LEEs that sinks dozens of metres below increasing its budget of anti-cyclonic vorticity and causing the formation of the jet. Satellite data show that the MMJ formed every summer from 2003 to 2015, sometimes for limited periods of time and at different latitudes, with the exception of summers 2005–2006, probably due to the weakness of the African anti-cyclones that in those years did not advect AW offshore.

Tracing Atlantic Waters Using 129I and 236U in the Fram Strait in 2016

AbstractIn this study 129I and 236U concentrations in seawater samples collected onboard R/V Polarstern during the PS100 expedition in the Fram Strait in 2016 are presented. The overall aim of the study was to investigate the distribution of these long‐lived radionuclides along the transect located at 79°N. The combination of both radionuclides was used for the first time in the Fram Strait to trace ocean circulation pathways of Atlantic waters. Results show that both 129I and 236U concentrations as well as 236U/238U ratios are about two times higher (> 600 × 107 at kg(−1), > 20 × 106 at kg(−1), and 2.8 × 10−9, respectively) in the cold and fresh outflowing surface waters from the Arctic Ocean (Polar Surface Water, PSW) compared to inflowing Atlantic origin waters (300 × 107 at kg(−1) 129I, 12 × 106 at kg(−1) 236U, and 1.4 × 10−9 236U/238U). A comparison with the different 129I and 236U input functions for the Atlantic branches entering the Arctic Ocean reveals that the middepth Atlantic origin waters outflowing the Arctic Ocean show more influence of the Barents Sea Branch Water than the Fram Strait Branch Water. The high radionuclide concentrations observed in the PSW indicate substantial influence of the Norwegian Coastal Current. This current carries a significantly larger proportion of 129I and 236U releases from European reprocessing plants than the aforementioned Atlantic branches. We estimate surface water transit times from the northern Norwegian Coast through the Arctic to the PSW of 12–19 years, less than for the middepth Barents Sea Branch Water (16–23 years).

Alkanes in a number of hydrothermal systems of the Russian Far East

The article presents the results of research dedicated to saturated hydrocarbons in the hydrothermal systems of the Russian Far East. The research has revealed significant differences in molecular-mass distribution of alkanes in the hot springs with different temperatures and origin. Alkanes, contained in the most high-temperature fluid from deep wells in Kamchatka peninsula , have the thermogenic genesis. At the same time alkanes, contained in the continental hot springs, are formed as a result of chemical re-synthesis of organic residues. In the least hot waters, as well as in cold underground and surface waters, hydrocarbons with bacterial and vegetable genesis have been revealed.

SST variability in the eastern intertropical Indian Ocean – On the search for trigger mechanisms of IOD events

This study focusses on the variability of sea surface temperature in the eastern intertropical Indian Ocean off the coasts of Sumatra and Java - a region that is central to the Indian Ocean Dipole (IOD). Using multiplatform satellite and ARGO data, here we show that the appearance of vast areas of anomalously cold surface water off the southwest Sumatran coast during positive IOD (pIOD) events is preconditioned by upwelling favourable winds along Sumatra's central west coast (1–3°S). A simple theory of coastal upwelling along coasts of finite horizontal extent, developed in this work, can explain this SST response. While this theory is consistent with the pIOD events of the years 1994, 1997 and 2006, it uncovers the existence of another strong upwelling event in the year 2011 that is missed when calculating average SST anomalies in the wider region (i.e. the Dipole Mode Index). In addition, we also derived the surface circulation establishing in the study region during pIOD events. This circulation pattern indicates that equatorward transport of colder water by a classical upwelling jet (that we call the South West Sumatra Current) reinforces negative (cold) SST anomalies off southwest Sumatra.

The summer upwelling of the Eastern Aegean Sea detected from MODIS SST scenes from 2003 to 2015

ABSTRACTThe summer upwelling of the Eastern Aegean Sea is investigated by daily night-time MODerate resolution Imaging Spectroradiometer Sea Surface Temperature scenes in the periods from June to November (extended summer) 2003–2015. The analysis is supported by wind data downloaded from the European Centre for Medium-Range Weather Forecasts Re-Analysis archive. On average over these summers, upwelling and Etesian wind occurred 69% and 78% of time, respectively. The highest correlation between upwelling and wind was found from June to September, with the beginning of the upwelling events shifted between 24 and 48 h after the start of the Etesian wind. The upwelling always begins in the same areas of the Eastern Aegean coast with the only differences that these areas tesian wind pattern. The largest upwelling events occurred in summers 2003–2009 and not in the following years of the study period. The upwelled cold surface water, still pushed by the persistent Etesian wind, fills part of South-Eastern Aegean and eventually outflows the Aegean Sea through Kassos and Karpathos Straits of the eastern Cretan Arc.

Determining the origin, circulation path and residence time of geothermal groundwater using multiple isotopic techniques in the Heyuan Fault Zone of Southern China

Medium–low temperature geothermal resources are abundant in Southern China, but their heat source and link to fault zones is poorly understood. Consequently, geothermal energy is only used at small scale. In order to broaden the footprint of geothermal energy use in Southern China, it is first necessary to understand and track the geothermal groundwater circulation pattern associated with fault zones. The Heyuan Fault Zone serves as a typical medium–low temperature geothermal system of South China. Here we show that the geothermal groundwater circulation pattern can be traced by using stable hydrogen and oxygen isotopes, helium and neon isotopes, carbon-13 and carbon-14 as well as hydrochemical parameters. The results show that: in the Heyuan Fault Zone, the main hydrochemical type of the hot springs is HCO3-Na + K, while the shallow cold groundwater is mainly enriched in HCO3-Ca. The results of the helium isotope and neon isotope indicate that the geothermal groundwater of this area is derived from the crust, thus excluding the possibility of extremely deep paths of groundwater tapping into mantle helium sources. Furthermore, the characteristics of the stable hydrogen and oxygen isotopes imply that geothermal groundwater is of local meteoric origin, and the recharge area located at the hilly area of the hanging wall of the Heyuan main fault, with the recharge elevations ranging from about 440 m to 670 m. The hot spring geothermometer shows that the highest reservoir temperature is about 157 °C, and the deepest circulation depth is about 6500 m. Carbon-14 isotope age dating suggests that the geothermal groundwater ages are mostly from 9.9 kyr BP to 12.3 kyr BP. According to the geological structural characteristics of the study area, the main upward channel for geothermal groundwater is the Heyuan main fault. Generally, the hot springs in this area are mixed with shallow cold groundwater and surface water, which raises the ratio of the Ca2+ in water and dilutes the heavy hydrogen and oxygen isotopes.

Evidence for a volcanic underpinning of the Atlantic multidecadal oscillation

The Atlantic multidecadal oscillation (AMO) is a 60–70 year pattern of sea-surface temperature (SST) variability in the North Atlantic commonly ascribed to internal ocean dynamics and changes in northward heat transport. Recent modeling studies, however, suggest that SSTs fluctuate primarily in response to major volcanic eruptions and changes in atmospheric circulation. Here, we utilize historical SST, atmospheric reanalysis, and stratospheric aerosol optical depth data to examine the basic evidence supporting a volcanic link. We find that cool intervals across the North Atlantic coincide with two distinct episodes of explosive volcanic activity (1880s–1920s and 1960s–1990s), where key eruptions include 1883 Krakatau, 1902 Santa María, 1912 Novarupta, 1963 Agung, 1982 El Chichón, and 1991 Pinatubo. Cool SST patterns develop in association with an increased prevalence of North Atlantic Oscillation (NAO)+ atmospheric patterns caused by stratospheric aerosol loading and a steepened poleward temperature gradient. NAO+ patterns promote wind-driven advection, evaporative cooling, and increased albedo from enhanced Saharan dust transport and anthropogenic aerosols. SSTs across the subpolar gyre are regulated by strength of low pressure near Iceland and the associated wind-driven advection of cold surface water from the Labrador Sea. This is contrary to an interpretation that subpolar SSTs are driven by changes in ocean overturning circulation. We also find that North Pacific and global mean SST declines can be readily associated with the same volcanic triggers that affect the North Atlantic. Thus, external forcing from volcanic aerosols appears to underpin multi-decade SST variability observed in the historical record.

Last Glacial Maximum surface water properties and circulation over Laurentian Fan, western North Atlantic

Millennial scale events marked by the contribution of detrital sand are recorded in North Atlantic sediments during the Last Glacial Maximum (LGM), between Heinrich events (HE) 1 and 2, and left their imprint on Laurentian Fan (LF – 43°N) sediments off eastern Canada. The LF counterpart of the well-known detrital events consist of glacial red-brick sediments resulting from subglacial flows separated by olive-grey sediments appears at ∼21.4–19.9 and ∼19.5–18.65 cal kyr BP. High-resolution analyses of diatom assemblages and lithic grains coupled with planktonic oxygen isotopic records reveal that while the red sediment is almost barren of diatoms, foraminifera and lithics (>150 μm), they are abundant in the olive-grey sediment. Diatom assemblages reveal three phases during these events: (1) initial relatively warm/temperate conditions followed by (2) very cold surface water and drifting ice, and (3) a final phase characterized by relatively warmer waters and the appearance of detrital carbonate. Although these events possibly reflect the variability specific to the slope water region, they are likely the response to Atlantic Meridional Overturning Circulation perturbations and ice-sheet instability. Through a chain of mechanisms, meltwater inputs into the North Atlantic led ultimately to an increased volume of tropical waters and part of the heat stored in the subsurface was flushed by a brief convective episode that was not sustained, accounting for the return of cold conditions after the events. The sequence of mechanisms deduced from the paleo data here and elsewhere is consistent with previous modeling results. These data suggest that the detrital events between Heinrich event 1 and 2 may be synchronous across the North Atlantic, and that the LGM was probably not a time of prolonged steady state in the climate system.

Major shift in the phenology of crustacean biomass in western Lake Superior associated with temperature anomaly

Abstract Variable weather patterns during the early months of 2014 and 2015 resulted in differences between years in spring and summer surface water temperatures in the offshore areas of western Lake Superior. Zooplankton were collected in western Lake Superior during several cruises from late spring to early fall in 2014 and 2015 to test the hypotheses that colder summer water temperatures in 2014 were correlated with reduced zooplankton biomass, later peaks in zooplankton biomass, and a smaller contribution of warm-water taxa to the zooplankton assemblage. The total amount of zooplankton biomass from early June through early October did not differ greatly between years. Of the taxonomic subcategories (large-bodied calanoids, small-bodied calanoids, cyclopoids, nauplii, and herbivorous cladocerans) however, cyclopoid and cladoceran biomass was somewhat smaller in 2014 compared to 2015, providing some support for the hypothesis that warm-water taxa contribute less to offshore zooplankton biomass in colder years. The timing of peak biomass for cladocerans, small-bodied calanoids, and cyclopoids did not differ between years, but peaks in the biomass of copepod nauplii and large-bodied calanoids (primarily Limnocalanus macrurus) occurred several weeks later in 2014 than 2015 which was evidence for phenological delay during a year with unusually cold spring and summer surface water temperatures. Though this study only evaluates the role of temperature in driving zooplankton biomass and phenology in Lake Superior, it does provide insight into the potential effects of climate variability on the Lake Superior food web.

Cadmium cycling in the water column of the Kuroshio-Oyashio Extension region: Insights from dissolved and particulate isotopic composition

We measured dissolved and particulate Cd isotopic composition in the water column of a meridional transect across the Kuroshio-Oyashio Extension region in a Japanese GEOTRACES cruise to investigate the relative influence of physical and biogeochemical processes on Cd cycling in the Northwestern Pacific Ocean. Located at 30–50°N along 165°E, the transect across the extension region possesses dramatic hydrographic contrast. Cold surface water and a relatively narrow and shallow thermocline characterizes the Oyashio Extension region in contrast to a relatively warm and highly stratified surface water and thermocline in the Kuroshio Extension region. The contrasting hydrographic distinction at the study site provides us with an ideal platform to investigate the spatial variations of Cd isotope fractionation systems in the ocean. Particulate samples demonstrated biologically preferential uptake of light Cd isotopes, and the fractionation effect varied dramatically in the surface water of the two regions, with relatively large fractionation factors in the Oyashio region. Based on the relationship of dissolved Cd concentrations and isotopic composition, we found that a closed system fractionation model can reasonably explain the relationship in the Kuroshio region. However, using dissolved Cd isotopic data, either a closed system or steady-state open system fractionation model may explain the relationship in the surface water of the Oyashio region. Particulate δ114/110Cd data further supports that the surface water of the Oyashio region matches a steady-state open system model more closely. Contrary to the surface water, the distribution of potential density exhibits comparable patterns with Cd elemental and isotopic composition in the thermocline and deep water in the two extension regions, showing that physical processes are the dominant forcing controlling Cd cycling in the deep waters. The results demonstrate that Cd isotope fractionation can match either a closed or open system Rayleigh fractionation model, depending on the relative contribution of physical and biogeochemical processes on its cycling.

Oceanographic drivers of sablefish recruitment in the California Current

AbstractOceanographic processes and ecological interactions can strongly influence recruitment success in marine fishes. Here, we develop an environmental index of sablefish recruitment with the goal of elucidating recruitment‐environment relationships and informing stock assessment. We start with a conceptual life‐history model for sablefish Anoplopoma fimbria on the US west coast to generate stage‐ and spatio‐temporally‐specific hypotheses regarding the oceanographic and biological variables likely influencing sablefish recruitment. Our model includes seven stages from pre‐spawn female condition through benthic recruitment (age‐0 fish) for the northern portion of the west coast U.S. sablefish stock (40°N–50°N). We then fit linear models and use model comparison to select predictors. We use residuals from the stock‐recruitment relationship in the 2015 sablefish assessment as the dependent variable (thus removing the effect of spawning stock biomass). Predictor variables were drawn primarily from ROMS model outputs for the California Current Ecosystem. We also include indices of prey and predator abundance and freshwater input. Five variables explained 57% of the variation in recruitment not accounted for by the stock‐recruitment relationship in the sablefish assessment. Recruitment deviations were positively correlated with (i) colder conditions during the spawner preconditioning period, (ii) warmer water temperatures during the egg stage, (iii) stronger cross‐shelf transport to near‐shore nursery habitats during the egg stage, (iv) stronger long‐shore transport to the north during the yolk‐sac stage, and (v) cold surface water temperatures during the larval stage. This result suggests that multiple mechanisms likely affect sablefish recruitment at different points in their life history.

Seasonal Variation of Barrier Layer in the Southern Ocean

AbstractThe seasonal variability of barrier layer (BL) and its formation mechanism in the Southern Ocean are investigated using the most recent Argo data. The results reveal that the BL is a persistent feature in the Southern Ocean with a strong seasonal cycle. The thickest BL appears in winter with the maximum amplitude exceeding 250 m while it dramatically decreases to less than 50 m in summer. The spatial distribution of BL is zonally oriented in the Pacific and Indian Ocean sectors, which is in agreement with that of the mixed layer depth (MLD) and the isothermal layer depth (ILD). Two areas with the most prominent BL are identified. One is located south of Australia and the other in the southeastern Pacific. The BL formation in both areas is generally attributed to a shallow mixed layer controlled by surface freshwater intrusion and a deep isothermal layer modulated by seasonal vertical convection. In the former region, the cold and fresh Antarctic Surface Water (ASW) is transported northward across the Subantarctic Front (SAF) by the Ekman effect and overlies the warm Subantarctic Mode Water (SAMW). The resulting inverse temperature structure facilitates the development of thick BLs. In the latter region, the BL emerges in the ventilation area where the shallow Surface Salinity Minimum Water (SSMW) coming from north leans against the deep vertical isotherms. In summer, positive surface heat flux into the ocean overwhelms other thermodynamic effects in the mixed layer heat budget. The MLD and ILD coincide and thus the BL is destroyed.

Life history and morphology of Eel Larvae in the Gulf of Guinea of western Africa: revisiting Jacques Blache’s research (1960–1977) 40 years later

Eel larvae (leptocephali) are rarely studied extensively both spatially and temporally, and detailed illustrations of most species are limited. This study uses the unique research reported in the monograph of Blache (Leptocephales des poissons anguilliformes dans la zone sud du golfe de Guinee. ORSTOM Faune Tropicale 10:1–381, 1977, in French) to describe and evaluate the species composition, abundance, life history characteristics and morphology of 10,284 anguilliform leptocephali collected throughout the year during 15 ichthyoplankton surveys (1960–1971) in relation to regional oceanography. Leptocephali of 70 species of 7 families were described, with Ophichthidae (26 species), Muraenidae (13), and Congridae (13) being the most diverse, and local spawning indicated by ≥ 34 species. Larvae of biogeographically restricted Heterencheylidae eels (mud eels) were abundant along the continental shelf and 5 species comprised 35% of total catches. Their larval distributions may reflect adult depth-segregation from nearshore/estuaries to the outer shelf and slope and larval retention. Nettastomatid leptocephali of Hoplunnis punctata were the most abundant species, and Rhynchoconger sp., Uroconger syringinus, Chlopsis olokun, and Dalophis boulengeri were also abundant. Small leptocephali distributions indicated spawning occurred over or near the continental shelf, and length-frequency data indicated most spawning was during the November–May warm-water season. Detailed morphology illustrations showed the characteristics of all stages of larvae. The Gulf of Guinea eel fauna is not diverse compared to the Indo-Pacific possibly due to phylogeography and a lack of coral reef habitats and the unusual low-latitude seasonal influx of cold surface waters, but is unique in being the worldwide center of distribution of the burrowing eels of the Heterencheylidae.