Southeastward Flow Research Articles

Water flow pattern recognition and fracture characterization in near-surface chalks using time-lapse ground-penetrating radar: An experiment

Predicting subsurface water flow is inherently complex due to the heterogeneity of geological formations. Water infiltration occurs through various pathways, such as pore throats, fractures, or faults, depending on the sediment type. In an effort to enhance our understanding of water flow within heterogeneous, fractured chalks formations, we integrate ground penetrating radar (GPR), acoustic measurements, and photogrammetric surveys to examine the flow patterns. We first conduct a field experiment of injecting water into chalks and monitor the water flow through a time-lapse GPR reflection survey. Then we identify the water-affected zone by analyzing the GPR data, and correlate the water-affected zone with the identified fractures on the exposed strata. Finally, we verify the water effect through measured acoustic velocities of acquired samples. Our analysis of the GPR data encompasses direct subtraction, traveltime delay, electromagnetic (EM) velocity changes, and water saturation variations. These analyses consistently indicate a predominant southeastward flow direction. This finding is corroborated by the fractures identified through photogrammetry, which closely match the water-affected zone, suggesting that water flow is indeed fracture-driven. The measured acoustic velocities are utilized to estimate water saturation using an iso-frame rock physics model, which shows consistency with the saturation estimated through EM velocity, further validating the monitoring results. These findings advance the understanding of fluid flow and its interaction with fracture system in heterogeneous chalks, and provide a reference for future studies by demonstrating the efficacy of integrating photogrammetry-identified fractures, EM velocity, and acoustic velocity to verify monitoring outcomes.

Dynamical response of the southwestern Laurentide Ice Sheet to rapid Bølling–Allerød warming

Abstract. The shift in climate that occurred between the Last Glacial Maximum (LGM) and the Early Holocene (ca. 18–12 kyr BP) displayed rates of temperature increase similar to present-day warming trends. The most rapid recorded changes in temperature occurred during the abrupt climate oscillations known as the Bølling–Allerød interstadial (14.7–12.9 kyr BP) and the Younger Dryas stadial (12.9–11.7 kyr BP). Reconstructing ice sheet dynamics during these climate oscillations provides the opportunity to assess long-term ice sheet evolution in reaction to a rapidly changing climate. Here, we use glacial geomorphological inversion methods (flowsets) to reconstruct the ice flow dynamics and the marginal retreat pattern of the southwestern sector of the Laurentide Ice Sheet (SWLIS). We combine our reconstruction with a recently compiled regional deglaciation chronology to depict ice flow dynamics that encompass the time period from pre-LGM to the Early Holocene. Our reconstruction portrays three macroscale reorganizations in the orientation and dynamics of ice streaming followed by regional deglaciation associated with rapid warming during the Bølling–Allerød interstadial. Initial westward flow is documented, likely associated with an early set of ice streams that formed during the advance to the LGM. During the LGM ice streaming displays a dominant north to south orientation. Ice sheet thinning at ∼15 ka is associated with a macroscale reorganization in ice stream flow, with a complex of ice streams recording south-eastward flow. A second macroscale reorganization in ice flow is then observed at ∼14 ka, in which southwestern ice flow is restricted to the Hay, Peace, Athabasca, and Churchill river lowlands. Rates of ice sheet retreat then slowed considerably during the Younger Dryas stadial; at this time, the ice margin was situated north of the Canadian Shield boundary and ice flow continued to be sourced from the northeast. Resulting from these changes in ice sheet dynamics, we recognize a three-part pattern of deglacial landform zonation within the SWLIS characterized by active ice margin recession, stagnation and downwasting punctuated by local surging (terrestrial ice sheet collapse): the outer deglacial zone contains large recessional moraines aligned with the direction of active ice margin retreat; the intermediate deglacial zone contains large regions of hummocky and stagnation terrain, in some areas crosscut by the signature of local surges, reflecting punctuated stagnation and downwasting; and the inner deglacial zone contains inset recessional moraines demarcating progressive regional ice margin retreat. We attribute these macroscale changes in ice flow geometry and associated deglacial behaviour to external climatic controls during the Bølling–Allerød and Younger Dryas but also recognize the role of internal (glaciological, lithological and topographic) controls in SWLIS dynamics.

Oceanic bottom mixed layer in the Clarion-Clipperton Zone: potential influence on deep-seabed mining plume dispersal

The oceanic bottom mixed layer (BML) is a well mixed, weakly stratified, turbulent boundary layer. Adjacent to the seabed, the BML is of intrinsic importance for studying ocean mixing, energy dissipation, particle cycling and sediment-water interactions. While deep-seabed mining of polymetallic nodules is anticipated to commence in the Clarion-Clipperton Zone (CCZ) of the northeastern tropical Pacific Ocean, knowledge gaps regarding the form of the BML and its potentially key influence on the dispersal of sediment plumes generated by deep-seabed mining activities are yet to be addressed. Here, we report recent field observations from the German mining licence area in the CCZ that characterise the structure and variability of the BML locally. Quasi-uniform profiles of potential temperature extending from the seafloor reveal the presence of a spatially and temporally variable BML with an average local thickness of approximately 250 m. Deep horizontal currents in the region have a mean speed of 3.5 cm s-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-1}$$\\end{document} and a maximum speed of 12 cm s-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-1}$$\\end{document} at 18.63 ms above bottom over an 11 month record. The near-bottom currents initially have a net southeastward flow, followed by westward and southward flows with the development of complex, anticyclonic flow patterns. Theoretical predictions and historical data show broad consistency with mean BML thickness but cannot explain the observed heterogeneity of local BML thickness. We postulate that deep pressure anomalies induced by passing surface mesoscale eddies and abyssal thermal fronts could affect BML thickness, in addition to local topographic effects. A simplified transport model is then used to study the influence of the BML on the interplay between turbulent diffusion and sediment settling in the transport of deep-seabed mining induced sediment plumes. Over a range of realistic parameter values, the effects of BML on plume evolution can vary significantly, highlighting that resolving the BML will be a crucial step for accurate numerical modelling of plume dispersal.

Wind-driven retreat of cold water pool and abrupt sea temperature rise off the southwest coast of Korea in summer 2017

Variations in seawater temperature have a critical influence on marine ecosystems and aquaculture industries in coastal regions. From July 25 to August 5, 2017, the sea surface temperature off the southwestern coast of Korea rose rapidly from 17.1 °C to 27.3 °C, which induced environmental stress in marine organisms such as farmed abalone. This study investigated the cause of this abrupt sea temperature rise. Westerly winds from July 1 to 25 were favorable for the upwelling of cold subsurface water along the bottom slope in this coastal region, helping maintain cool surface water temperatures. As the wind changed to easterly, the cold subsurface water moved away and warmer, fresher surface water moved into the coastal region, where the surface currents changed from a southeastward flow to a northeastward flow from July 25 to August 5. Thus, to maintain stable, cool seawater temperatures during summer, both cold water supply and upwelling wind conditions are essential. The analysis of water masses indicated that the cold water was formed by the mixing of the Tsushima Warm Current Water and Yellow Sea Bottom Water in the northern East China Sea. The cold water advected eastward and formed a cold water pool in the intermediate depths (30–80 m) of the northern Jeju Strait, providing cold water to the coastal region during the upwelling period. The westerly winds were disrupted by the approach of a typhoon, which discontinued the supply of bottom cold water. The physical processes identified in this study will help predict short-term increases in water temperature and can assist in the development of countermeasures for the aquaculture industry against the negative effects of abrupt environmental changes.

Intercomparison of the Performance of Four Data Assimilation Schemes in a Limited‐Area Model on Forecasts of an Extreme Rainfall Event Over the Uttarakhand in Himalayas

AbstractThe present study compares the performance of four data assimilation (DA) systems: Ensemble Adjustment Kalman Filter (EAKF), Variational (3DVAR/4DVAR), and Hybrid ensemble‐3DVAR (HYBRID) in the Weather Research and Forecast (WRF) model. A heavy rainfall event that caused notorious floods in the Uttarakhand over the Himalayan region is considered. Observations are assimilated at every 6 h interval and all the conventional observations including cloud tracked‐wind from the satellite available from Global Telecommunication System are used. The forecasts initialized from the analysis of four DA systems at different lead times are evaluated. Additionally, a non‐cycled nested assimilation strategy that provides advantages of increased resolution in the DA system is tested. The results indicate that 4DVAR experiments produce more skillful forecasts for wind while both 4DVAR and EAKF experiments show improvement for upper tropospheric temperature forecasts as compared to the other experiments. The evaluation of rainfall forecast depicts that the 4DVAR DA system has outperformed the other DA systems when the effect of high‐resolution assimilation is mimicked in the system using the nested assimilation strategy. Further analysis on the dynamics of the event indicates that an early merging of the southward protruding trough with the westward‐moving monsoon depression has resulted in stronger southeastward flow in EAKF and HYBRID experiments, which is suggested as a potential reason for enhanced precipitation over the Uttarakhand in both the experiments.

A case study of coastal currents in relation with tides and winds in a tropical coastal waters of Vengurla, West Coast of India

Coastal currents consist of wind-driven, tidal, surface wave and geostrophic flows. These currents have a major role for nearshore sediment transport, but the measured data availability on coastal currents for the Indian coast is relatively less. This study analyze the variability of coastal currents along with the tides at nearshore and offshore waters during two different seasons (fag end of summer monsoon and initial stage of pre-monsoon) at Vengurla, west coast of India. Results of this study exhibited that the tides at Vengurla are mixed semi-diurnal dominated by solar constituent during summer monsoon (September to October); whereas the lunar constituents dominates during pre-monsoon (February to March). The nearshore current was dominated by the westward flow (sluggish current) during monsoon and south eastward flow during pre-monsoon, whereas the offshore flow (strong current) showed seasonality. At offshore, southerly current dominated during summer monsoon, whereas the northerly current played a major role during pre-monsoon. The current profile analysis at offshore revealed that the gradation of strong current at the surface to the sluggish nature of currents at the bottom and were comparatively higher during summer monsoon than pre-monsoon periods. This study revealed a complex pattern among winds, tides and currents at offshore.

Atmospheric vorticity sets the basin-scale circulation in Hudson Bay

Hudson Bay of northern Canada receives upward of 700 km3 of river discharge annually. Cyclonic water circulation in Hudson Bay transports this massive volume of riverine water along the coast toward Hudson Strait and into the Labrador Sea. However, synoptic, seasonal and interannual variability of the freshwater transport in Hudson Bay remains unclear. Using yearlong observations of current velocity profiles, collected from oceanographic moorings deployed in western Hudson Bay from September 2016 to September/October 2017, we examined the role of atmospheric forcing on circulation and freshwater transport in the Bay. Our analysis reveals that the along-shore southeastward current through western Hudson Bay was amplified through the entire water column in response to winds generated by cyclones passing over Hudson Bay toward Baffin Bay and/or the Labrador Sea. An atmospheric vorticity index was used to describe the atmospheric forcing and found to correlate with sea surface height and along-shore currents. We showed that a surface Ekman on-shore transport increases sea surface heights along the coast, producing a cross-slope pressure gradient that drives an along-shore southeastward flow, in the same direction as the wind. Expanding our observations to the bay-wide scale, we confirmed this process of wind-driven water dynamics with (1) satellite altimetry measurements and (2) ocean model simulations. Ultimately, we find that cyclonic wind forcing amplifies cyclonic water circulation in Hudson Bay facilitating the along-shore freshwater transport to Hudson Strait. During periods of positive atmospheric vorticity, this forcing can reduce the residence time of riverine water in Hudson Bay.

A plume-modified lithospheric barrier to the southeastward flow of partially molten Tibetan crust inferred from magnetotelluric data

Broadband and long-period magnetotelluric (MT) data were used to obtain a continuous image of the lithospheric electrical conductivity structure across the southeastern Tibetan Plateau margin, where long-wavelength crustal thickening and surface uplift have been attributed to the influx of weak crustal material from Tibet into the adjacent Yangtze Block. The resulting resistivity model reveals two major conductive features in the middle–lower crust beneath southeastern Tibet and the Chuxiong basin, both of which are consistent with the presence of fluids and likely enhanced by shear deformation along large faults. The horizontally extensive conductor beneath southeastern Tibet could be interpreted as a partially molten layer that represents topography-driven crustal flow from central Tibet, whereas the anomalously high conductivities observed beneath the Chuxiong basin require the addition of substantial amounts of saline aqueous fluids. These two conductors are separated by a lithospheric-scale, high-resistivity body that situated beneath the geologically inferred core of the Emeishan large igneous province (ELIP). Together with other geophysical and geological evidence, we interpret this anomaly as remnants of a plume-modified lithosphere that originally formed during the Permian Emeishan volcanism, and speculate it acts as an obstruction to the southeastward crustal flow from Tibet since Late Cenozoic. These results indicate that topography-driven crustal flow may contribute to crustal thickening and uplift in southeastern Tibet, but not in the adjacent foreland. Instead, magmatic underplating associated with the Permian plume activity could account for both the elevated topography and thickened crust of the adjacent Yangtze Block without the need for crustal flow.

Flow patterns in the Chukchi Sea based on an ocean reanalysis, June through October 1979–2014

The oceanography of the Arctic is changing, with the potential to restructure the function and production of its ecosystems. The physical oceanographic conditions that have occurred on the Chukchi Sea shelf during June through October of the years 1979–2014 were investigated using the ORAS4 ocean reanalysis product. Time series of vertically integrated temperatures (especially during September and October) indicate greater warming in the first half of the 36-year record. This change in the long-term temperature trend may be in part due to trends in the mean currents, which in Herald Canyon and off the coast of Northwest Alaska tended to be slightly less poleward after 2000. A k-means cluster analysis of monthly mean sea surface height anomaly distributions was used to describe five distinct patterns of flow. Two of the five patterns (clusters 2 and 4) relate to the strength of the Alaskan Coastal Current (ACC). Another pair of patterns (clusters 3 and 5) had their strongest expressions in the northwest Chukchi Sea and relate to periods of weak southeastward versus strong northwestward flow in this region associated with the presence or absence of the Siberian Coastal Current. The fifth pattern (cluster 1) was defined by weak southeastward flow anomalies in the western Chukchi and a slightly suppressed ACC relative to the mean in the eastern Chukchi Sea. The composite sea surface height anomaly patterns of the five cluster types correspond closely with the mean sea level pressure anomaly distributions during the months constituting each cluster type. Our findings for the Chukchi Sea region provide a long-term context for previous field observations and may be useful for interpretation of past ecosystem variations.

Current variability by wave propagation in Todos Santos Bay, Baja California, Mexico

The temperature, velocity, and sea level fields for a summer climatological month (August) were analyzed using tridimensional and baroclinic model outputs in Todos Santos Bay (TSB), Baja California, Mexico. The numerical model was forced with wind (nonstationary), heat flux, and California Current System climatology on the open boundary. The 3- to 5-day current variability is related to a baroclinic wave traveling towards the northwest of the bay. Wave travel periodicity was due to the release of accumulated water from the Ensenada–Punta Banda Estuary (E–PBE) region. Local wind stress causes southeastward water flow and, given the TSB geometry, water accumulates in the E–PBE region. Weakening wind stress was the main cause of water release. In addition, complex empirical orthogonal function analysis found that outer TSB disturbances cause sea level variability.

Interactions of A Paleocene River, A Rising Fold, and Early-Diagenetic Concretions

Abstract: The relative rates of sediment accumulation, erosion, and structural uplift determine whether a growing fold develops positive topographic relief, is beveled by antecedent streams, or is buried under thick growth strata. When folds rise in subsiding basins, upward, convergent flow of groundwater through the permeable growth strata that underlie antecedent streams enhances the flux of ions required for concretion growth. Early diagenetic concretions that grow in such alluvial strata may constitute the only clasts larger than sand size available for transport when antecedent streams become erosive. The first reworked concretions deposited by these streams should accurately mark the transition from aggradation to erosion as folds rise into the paths of streams. In this situation, the ability to differentiate reworked from in situ concretions is crucial. The west-vergent Simpson Ridge anticline, a N–S-trending, thick-skinned Laramide structure in east-central Wyoming, separates the larger Hanna basin from the Carbon basin. Near the north nose of this anticline, in situ iron-oxide-rich concretions are abundant in folded Paleocene strata (Ferris Formation) and, just to the east, large, reworked , iron-rich concretions are abundant in younger, more gently dipping conglomerates in the basal Hanna Formation of the backlimb. Smaller reworked concretions are also present near the base of the Hanna Formation at least 7 km south of the anticlinal nose and just east of the fold9s axis. At the anticlinal nose, in situ (non-reworked) concretions up to 3 m × 1 m × 1 m are abundant at the top of an ∼ 7-km-thick sequence of sandstones and siltstones that constitute the Late Cretaceous–early Paleocene Ferris Formation. Reworked concretions are absent in the strata hosting these in situ concretions, but reworked concretionary clasts up to 2 m in diameter are present in exposures of conglomerates in the lowermost Hanna Formation that lie just above the in situ Ferris concretions and southeast of the anticlinal nose. These early-diagenetic concretions were originally cemented by siderite (FeCO 3 ). Oxidation of some small, rinded siderite-cemented clasts took place after their fluvial transport into the Hanna Formation, but abundant angular, un-rinded, iron-oxide-cemented clasts indicate that many large, in situ siderite concretions had resided in the vadose zone before they were entrained. The distribution of reworked concretions and the orientations of crossbeds show that antecedent Hanna streams eroded a swath at least 5 km wide across the rising structure. These streams transported Ferris Formation concretions southeastward into the Carbon basin, and deposited them in a conglomeratic sandstone body in the Hanna Formation. Large calcite-cemented concretions, many with a pipe-like morphology, then grew within Hanna crossbeds. In many cases, these in situ concretions enclose transported, iron-rich concretions, but there is no evidence any calcite-cemented concretions were reworked. The NW–SE alignment of the pipes record southeastward flow of groundwater and thus also provide evidence (together with the orientation of the crossbeds) of the original continuity of the Hanna Formation across the anticline. Reworked concretions reveal the interplay of deposition, diagenesis, and erosion. Due to convergent groundwater flow over growing anticlines, early diagenetic concretions, both in situ and as reworked clasts, are especially likely to be found in growth strata.

Coupling of the spatial–temporal distributions of nutrients and physical conditions in the southern Yellow Sea

This study investigated the coupling of the spatial–temporal variations in nutrient distributions and physical conditions in the southern Yellow Sea (SYS) using data compiled from annual-cycle surveys conducted in 2006–2007 as well as satellite-derived sea-surface temperature (SST) images. The influence of physical dynamics on the distribution and transport of nutrients varied spatially and seasonally in the SYS. The Changjiang Diluted Water (CDW) plume (in summertime), the Subei Coastal Water (SCW) (year-round), and the Lubei Coastal Current (LCC) (in wintertime) served as important sources of nutrients in the inshore area in a dynamic environment. The saline Taiwan Warm Current (TWC) might transport nutrients to the northeast region of the Changjiang Estuary in the summer, and this nutrient source began to increase from spring to summer and decrease when autumn arrived. Three types of nutrient fronts, i.e., estuarine, offshore, and coastal, were identified. A circular nutrient front caused by cross-shelf transport of SCW in the southeast shelf bank area in the winter and spring was observed. The southeastward flow of western coastal cold water in the SYS might be an important conduit for cross-shelf nutrient exchange between the SYS and the East China Sea (ECS). The tongue-shaped low-nutrient region in the western study area in the wintertime was driven by the interaction of the southward Yellow Sea Western Coastal Current (YSWCC) and the biological activity. The vertically variable SCM (subsurface Chl-a maximum) in the central SYS was controlled by coupled physical–chemical processes that involved stratification and associated nutricline. The average nutrient fluxes into the euphotic zone due to upwelling near the frontal zone of the Yellow Sea Cold Water Mass (YSCWM) in the summer are estimated here for the first time: 1.4±0.9×103μmol/m2/d, 0.1±0.1×103μmol/m2/d, and 2.0±1.3×103μmol/m2/d for DIN, PO4–P, and SiO3–Si, respectively. The depletion of nutrients in the central SYS and the upwelled transport in the boundary of the YSCWM resulted in a spatial transfer of the high Chl-a zone, varying generally from the central SYS to the boundary of the YSCWM from spring to summer, and the nutrient flux associated with this upwelling could contribute significantly to local primary production. This study deepens our understanding of the mechanisms influencing the distribution and transport of nutrients in the SYS.

Continental breakup and the dynamics of rifting in back-arc basins: The Gulf of Lion margin

Deep seismic profiles and subsidence history of the Gulf of Lion margin reveal an intense stretching of the distal margin and strong post-rift subsidence, despite weak extension of the onshore and shallow offshore portions of the margin. We revisit this evolution from the geological interpretation of an unpublished multi-channel seismic profile and other published geophysical data. We show that an 80 km-wide domain of thin lower continental crust, the “Gulf of Lion metamorphic core complex”, is present in the ocean-continent transition zone and exhumed mantle makes the transition with oceanic crust. The exhumed lower continental crust is bounded upward and downward by shallow north-dipping detachments. The presence of exhumed lower crust in the deep margin explains the discrepancy between the amount of extension deduced from normal faults in the upper crust and total extension. We discuss the mechanism responsible for exhumation and present two scenarios, a first one involving a simple coupling between mantle extension due to slab retreat and crustal extension, and a second one involving extraction of the lower crust and mantle from below the margin by the south-eastward flow of hot asthenosphere in the back-arc region during slab rollback. In both scenarios, the combination of Eocene crustal thickening related to the Pyrenees, the nearby volcanic arc and a shallow lithosphere-asthenosphere boundary weakened the upper mantle and lower crust enough to make them flow south-eastward. The overall hot geodynamic environment also explains the subaerial conditions during most of the rifting stage and the delayed subsidence after breakup.

A method for predicting the occurrence of paralytic shellfish poisoning along the coast of Hokkaido in the Okhotsk Sea in summer

To design a method for predicting outbreaks of paralytic shellfish poisoning (PSP) in scallop fishing grounds, the relationship between the distribution of the toxic dinoflagellate Alexandrium tamarense and the dynamics of the Soya Warm Current (SWC) was examined in the Okhotsk Sea off Hokkaido. Surveys were conducted from May to June to clarify the transportation mechanism of A. tamarense from the oceanic area to the coastal area. The sea-level difference (SLD) between Wakkanai and Abashiri was monitored as an index of the strength of the SWC southeastward flow in an alongshore belt to examine the possible occurrence of A. tamarense in the coastal area during temporal weakening of the SWC. A bottom-mounted acoustic Doppler current profiler (ADCP) was used for direct observations of the SWC. The results indicated that PSP occurred when low-salinity water contaminated with A. tamarense extended to the coast during temporal weakening of the SWC due to a decrease of the SLD. Our results strongly indicate that predictions can be realized by monitoring the decrease of SLD as an index of temporal weakening of the SWC after surveys of the distribution of A. tamarense in the oceanic area before the period of PSP occurrence.

Does the vorticity flux from Agulhas rings control the zonal pathway of NADW across the South Atlantic?

As part of the global thermohaline circulation, some North Atlantic Deep Water (NADW) exits the Atlantic basin to the south of Africa. Observations have shown that there is a quasi‐zonal pathway centered at 25°S carrying NADW eastward, connecting the Deep Western Boundary Current to the Cape Basin. However, it has been unclear what sets this pathway. In particular, waters must move southward through the Cape Basin, thereby crossing isolines of planetary vorticity, in order to exit the basin. Here, we find that an eddy thickness flux induced by Agulhas rings moving northwestward forces a circulation of NADW through the Cape Basin. The pathway at 25°S feeds the southeastward flow of this circulation while conserving potential vorticity. Using Lagrangian floats advected for 300 years in a 1/10° resolution ocean model, we show that the most common pathway for NADW in our model lies directly below the Agulhas ring corridor. By analyzing the velocity and density fields in the model, we find that the decay of these rings, and their forward tilt with depth, results in a southward velocity, across isolines of planetary vorticity, of 1 to 2 cm/s in the deep waters. The associated stream function pattern yields a deep circulation transporting 4 Sv of NADW from the Deep Western Boundary Current at 25°S to the southern tip of Africa.

Southward surface flow in the central South Pacific

A large-scale surface flow with a southward component is proposed for the central South Pacific Ocean based on an interpretation of existing closely spaced and accurately measured temperatures and salinities along two latitudes in two different southern hemisphere winters: 28o S (Scorpio) and five degrees south of that (WOCE). Such a southward flow is not predicted from theory nor is it shown on current charts and globes. The observed longitudinal maximum in surface temperature along 28o S is centered around 130o W and has an amplitude of at least 5o C and an east/west range of about 60o of longitude. This striking feature is most easily explained by horizontal transport from latitudes closer to the equator. Since temperature atlases show that equatorial surface temperatures are always highest in the west, the origin of the warm water probably is toward the western side of the ocean as well. Thus the surface flow surrounding the longitudinal temperature maximum should be directed to the southeast. Where the surface temperatures are maximum the mixed layer depths are relatively large in a convex downward lens with maximum depths of 100 m; a correlation that is consistent with warm water moving south and being cooled from above. Salinities are maximum near the temperature maximum, also suggesting that the source of the surface flow is at low latitudes, where evaporation is usually expected to exceed precipitation. It is conjectured that the large-scale southeastward flow of the South Pacific is the analogue of the northeastward wide warm current off California documented over 30 years ago.

Formation and transportation of high-salinity water produced in polynyas south of the St. Lawrence Island

The authors studied variations of temperature and salinity in seawater under sea ice using hydrologic data collected from polynyas south of the St. Lawrence Island during March of 2008 and 2009. The results indicate that the high-salinity water found during the cruises of 2008 and 2009 was due to the formation of polynyas. The salinity observed in 2008 was higher than that in 2009 as a result of higher salt production in 2008. The spatial distributions of high-salinity cores differed between the two cruises. In March 2008, a southeastward flow was formed under the persistent northerly wind in the observation region, which transported the high-salinity water produced by the polynyas to the southeast. The similar flow, however, did not exist in March 2009 because the northerly wind over the study area was interrupted by a southerly wind. Accordingly, the polynyas and the high-salinity water produced by them existed for a short time. As a result, the high-salinity water in 2009 did not spread very far, and stayed within the polynyas. In addition, during the 2009 cruise, two stages of observations in the polynyas showed the core of high-salinity water was shifted to the southwest of the St. Lawrence Island. This result suggested that a southwestward flow might have existed in the area at the onset of the northerly wind, which was consistent with the alongshore and/or offshore flows caused by the northerly wind.

Seasonal variations of shelf circulation in Hidaka Bay, Hokkaido, Japan, with an interpretation of the migration route of juvenile walleye pollock

The outer shelf of Funka Bay, located at the bay head of Hidaka Bay, is a recognised main winter spawning ground for walleye pollock (Theragra chalcogramma), whose newly hatched juveniles migrate eastward along the Hidaka shelf to the nursery ground located in the Doto area. To examine the seasonal change of the coastal current along this migration route, four current moorings were deployed along the shelf in Hidaka Bay from April, 2004 to June, 2006. Since these mooring sites were close to the coast, the circulation was estimated after removal of the wind-driven component. It was found that the winter coastal flow, forced by a north-westerly monsoon wind, is clockwise along the Hidaka shelf. However, this flow is weak due to the superimposition of the opposite Coastal Oyashio flow trapped on the shelf. In summer, a bifurcation of the coastal current occurs along the north-eastern Hidaka shelf with a south-eastward flow, associated with the Tsugaru Gyre, and a north-westward flow, consisting of a branch from this gyre. Our results provide a new understanding of the migration of juvenile walleye pollock: (1) very slow transport of juveniles along the Hidaka shelf from winter to spring, and (2) selection of earlier spawning survivors due to the bifurcated flow in early summer.

Seasonal and interannual variability of the surface circulation in the eastern Gulf of Cadiz (SW Iberia)

An 11‐year (1996–2007) time series of current meter observations representative of the open sea circulation; a 4‐year (2001–2005) time series of current meter records over the continental shelf and in situ data during different seasons have been compared in order to study the seasonal and interannual variability of the surface circulation in the eastern Gulf of Cadiz. The open sea velocity observations indicate southeastward flow along the northern continental slope of the Gulf of Cadiz, compatible with anticyclonic circulation, during most of the year and more intense during summer months. Flow reversals (northwestward circulation) at seasonal time scales in late autumn and early winter (preferably December and January) are a rather recurrent feature with variable intensity depending on the year. Anticyclonic circulation is associated with westerlies, whereas flow reversals usually take place under easterly episodes, suggesting wind‐driven circulation. Negative North Atlantic Oscillation indices (indicative of southward displacement of the Azores high) are also linked to the reversals. Changes in this mainly wind‐driven large‐scale surface circulation are echoed by the shelf circulation: the coastal countercurrent that closes the mesoscale cyclonic cell over the eastern shelf in spring‐summer (upwelling season) is replaced by and eastward current in autumn and winter.

Tongue‐shaped frontal structure and warm water intrusion in the southern Yellow Sea in winter

In winter, a thermohaline front forms at the Yellow Sea (YS) entrance where the warm and saline Cheju Warm Current (CWC) water meets cold coastal water. The frontal structure, as well as the northwestward intrusion of the warm water, was investigated by analyzing conductivity‐temperature‐depth (CTD) data, tracks of drifting floats, moored current data, and satellite images. The CWC water advances westward as a tongue with its tip heading westward across the YS entrance, not intruding northward along the deepest part of the YS trough, whereas a secondary warm water band is developed northwestward along the western flank of the trough. Floats in the warm tongue moved northwestward across the western frontal zone where the frontal structure was weak, reflecting the northwestward intrusion along the 50–70 m isobaths through the frontal zone. Currents in the trough do not always meet the upwind flow theory that, in an elongated basin, the wind‐driven flow is upwind in the deep channel, but rather show the predominance of the southward downwind flow. Model experiments show that on the western flank of the trough, both the southeastward tide‐induced residual flow and the southeastward flow due to the nonlinear effect between tide and wind are generated at the same time. Thus the intermittent intrusion of the CWC across the western frontal zone may be closely associated with the predominance of the upwind flow over the southeastward nonlinear interacted flow.