Southward Surface Flow Research Articles

Isotopic evidence for partial geochemical decoupling between a Jurassic epicontinental sea and the open ocean

We report stable isotope ratios (δ13C, δ18O), minor and trace elements (Mn, Fe, Sr, Mg) together with Ca concentrations from bivalve shells and belemnites from the Middle-Upper Jurassic Sundance Seaway (western United States), we compare them with coeval open-ocean Tethyan data, and reconstruct the palaeo-circulation of seaway waters. The Sundance Seaway was a 2000 km long epicontinental sea with a single entrance at mid latitudes (55–60°N), which would have fostered substantial evolution of seawater chemistry relative to its open-ocean source. Samples are distributed across the 13-million-year marine history of the seaway, and across a 540 km east-west transect spanning Wyoming. Delta13C values are in the same range as Tethyan data, and this suggests that they might record global changes in the carbon cycle, with one exception in the Oxfordian. Delta18O values from the seaway are in contrast highly depleted compared with Tethyan data (−2 to −6‰), and they indicate unrealistically high palaeotemperatures (20–40 °C), assuming an isotopic composition of seawater of −1‰, as generally used for the Jurassic. Given more realistic temperature estimates from Mg/Ca ratios of bivalve shells (10–25 °C), we explain such negative δ18O values by the southward inflow of normal-salinity, isotopically depleted (−3, −4‰), Arctic water into the seaway. Such water would become progressively more saline and denser as it flowed towards the southernmost portion of the seaway. In the Late Jurassic, characterised by wetter climate conditions, less dense Sundance waters may have instead exhibited a northward flow, reducing the southward surface flow from the Arctic. The observed partial geochemical decoupling of Sundance Seaway water masses from the open ocean strongly recommends caution in interpreting the geochemical record of ancient shallow seas, where local, regional and global drivers of change all need to be considered.

Variability and Long‐Term Trends in the Shelf Circulation Off Eastern Tasmania

AbstractThis study investigates trends and interannual variability of the marine climate across the continental shelf off eastern Tasmania for 1993–2016. This region is a hotspot for global warming and biodiversity. Eastern Tasmania lies at the boundary between two ocean currents (the East Australian Current Extension, or EAC Extension, and the Zeehan Current, ZC) leading to the local marine climate exhibiting trends and variability from both boundary currents. A numerical ocean model is used to provide high‐resolution (∼2 km) estimates of the temperature, salinity, and circulation for the region. Results indicate significant positive trends in temperature, salinity, and southward flow over the shelf, consistent with an increasing EAC Extension. These trends are particularly strong in autumn, indicating a lengthening of the warm season. The interannual variability in the EAC Extension and ZC was quantified by a simple index, based on a modal analysis of surface circulation, indicating the relative dominance of each current. Strong EAC years were related to significantly more summertime marine heat wave days. Large‐scale remote drivers of variability were considered, and we found weak but significant links with El Niño–Southern Oscillation and Tasman Sea Blocking. El Niño–Southern Oscillation was found to modulate the EAC Extension in summer with a El Niño leading to enhanced southward flow and warming over the shelf. Tasman Sea Blocking was found to drive enhanced southward surface flow, particularly in winter. Nonetheless, large‐scale forcing modes explain less than 25% of the total variability in the EAC‐ZC system indicating that most of the variability is internally generated.

Seasonal and interannual variations of surface current in the southern Taiwan Strait to the west of Taiwan Shoals

Seasonal and interannual variations of surface currents in the southern Taiwan Strait to the west of Taiwan Shoals were investigated by using measurements of high frequency (HF) ground wave radars from January 2006 to April 2009. The results demonstrate that surface currents in the water channel to the west of Taiwan Shoals are composed of a significant, seasonally fluctuating component and a relatively stable northward component. Forced by the East Asia monsoon, the annual variation of the surface longshore current is linearly correlated with the longshore wind. Behind the seasonal signal is a stable northward flow with speeds of O (10 cm/s). The observations also show that surface currents in the area are subject to distinct interannual variation, and the southward surface flow was more profound in winter 2007/2008 than in other winters. Observations from bottom-mounted ADCP also indicated that, in that winter, longshore currents in the west side of the Taiwan Strait are very different from the previous winter. The northward flow appeared much weaker, the currents were southward in most layers, and the interannual differences were observed throughout the water column. Hydrographic observations in the same period suggest that the large-scale southward intrusion of the Zhe-Min coastal water is the direct cause of the interannual difference. The enhancement of the East Asian monsoon during the 2007/2008 La Nina event is considered as the main reason for the winter current anomalies in the study area.

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.

A Zonal Momentum Balance on Density Layers for the Central and Eastern Equatorial Pacific

Abstract Brown et al. analyzed the kinematics of flow in the equatorial Pacific Ocean, along time-varying isopycnals in a three-dimensional eddy-permitting model. Here the dynamics of these flows is explored in the same model via the zonal momentum equation (ZME). Previous work has shown that the dominant terms of the ZME, on and near the equator, are the pressure gradient, wind stress, and Coriolis term. In one model study, the nonlinear and friction terms were significant but negated each other. In this study, with a higher-resolution model and more realistic friction scheme it is shown that the nonlinear term is important along and north of the equator, while the explicit friction term is negligible. The part of the nonlinear term derived from high-frequency eddy flows acts like a friction on the Equatorial Undercurrent, while the remaining part of the nonlinear term from smooth flows enhances it. In density coordinates, meridional tropical cells lie on either side of the equator in the first half of the year (January–June) as expected. In July–December, a continuous southward surface flow appears from 4°N into the Southern Hemisphere and arises from variations in the geostrophic flow and the nonlinear term. Variations in the geostrophic flow are due to both seasonal variability in the thermocline and a surface bolus effect arising from baroclinic instability. The nonlinear term increases in the surface layers at the same time assisting the southward flow, most likely because of tropical instability waves.

Water and sediment dynamics in the Red River mouth and adjacent coastal zone

The coastline of the Red River Delta is characterized by alternating patterns of rapid accretion and severe erosion. The main branch of the Red River, the Ba Lat, is presently expanding seaward with a main depositional area several km downstream and offshore the Ba Lat River mouth. Sediment deposition rates are approximately 6 m in the past 50 years. Field measurements were done to determine the processes that regulate marine dispersal and deposition of sediment supplied by the Ba Lat. These measurements reveal that the waters surrounding the Ba Lat delta are strongly stratified with a pronounced southward-flowing surface layer. This southward-flowing surface layer is a coastal current which is generated by river plumes that flow into the coastal zone north of the Ba Lat. However, outflow of turbid river water is not continuous and most sediment enters the coastal zone when the alongshore surface velocities are low. As a consequence, most sediment settles from suspension close to the river mouth. In addition to the southward surface flow, the southward near-bottom currents are also stronger than northward currents. Contrasting with the residual flow near-surface, this southward flow component near-bottom is caused by tidal asymmetry. Because most sediment is supplied by the Ba Lat when wave heights are low, sediment is able to consolidate and therefore the long-term deposition is southward of, but still close to, the Ba Lat mouth.

Dispersal of suspended sediments in the turbid and highly stratified Red River plume

The Red River, annually transporting 100 million tons of sediment, flows into a shallow shelf sea where it rapidly deposits most of its sediment on a prograding delta front. Oceanographic cruises were carried out in February–March and July–August 2000 to determine the vertical structure of the Ba Lat river plume and sediment transport patterns on the delta front. The surface waters in the coastal zone were strongly stratified with a low density and high sediment concentration during the larger part of the wet season, caused by low mixing rates of river plumes with ambient water. The river plume is advected to the south by a well-developed coastal current which originates from the river plumes that enter the Gulf of Tonkin North of the Ba Lat and are deflected southward by the Coriolis force. Sediment predominantly leaves the surface plume by settling from suspension and less by mixing of fresh and marine water. A one-dimensional model for plume deposition valid for fair weather conditions indicates that most sediment is deposited within 10 km and southward of the river mouth. Of prime importance for this depositional pattern is the phase relation between river outflow and tidal currents, in combination with the southward surface flow; alongshore advection is very low during outflow of the turbid river plume. The agreement of modeled plume sedimentation patterns with long-term bathymetric changes strongly suggests that fair weather depositional processes determine delta front development. This may be related to the fact that reworking of sediment mainly occurs several months after the peak deposition period; in the meantime sediment compaction and consolidation have increased the shear strength of deposited sediments.

Water, heat and freshwater flux out of the northern Indian Ocean in September–October 1995

Abstract World Ocean Circulation Experiment (WOCE) Transindian Hydrographic Section I1 (I1) is the northernmost of the zonal sections carried out during the WOCE Indian Ocean Expedition of 1994–1995. It crosses the southern boundaries of both the Bay of Bengal (I1e) in the east and the Arabian Sea (I1w) in the west. From I1, heat, freshwater and water-mass budgets are computed for the Arabian Sea and Bay of Bengal. Unfortunately, unlike the flow in the Atlantic and Pacific, the flow through I1 experiences considerable seasonal variability due to the annual reversal of the monsoonal winds. Therefore, at best we can expect to compute a “snapshot” of the heat and freshwater flux at the end of the SW Monsoon. But at least the timing of this section was chosen to coincide in the period where the mean circulation is most like the “normal” subtropical gyres found at mid-latitudes in the other oceans. During WOCE I1 both the Arabian Sea and the Bay of Bengal acted as heat sources. The mechanisms of the heat exportation in these two basins differed slightly from each other with the deep-ocean flow playing an important role in exporting heat from the Arabian Sea. The total heat transport out of the Arabian Sea was 0.60±0.27 PW. Of the 0.60 PW heat transport, a total of 0.28 PW was exported below 2000 m. The monsoonally driven southward surface flow accounted for the remaining 50% of the total heat export. Meanwhile, the Bay of Bengal was exporting heat at a rate of 0.63±0.16 PW, with half of the heat export due to surface flow and the other half due to meridional overturning at mid-depths. Meanwhile, the Arabian Sea was importing freshwater at a rate of 0.38±0.09×106 m3 s−1 while the Bay of Bengal was exporting freshwater at a rate of 0.38±0.08×106 m3 s−1. The mechanisms for the freshwater transport from the two basins were fundamentally different. In the Arabian Sea, vertical recirculation cells in the upper and deep ocean contributed to the freshwater import across I1w with the deep cell accounting for ∼25% of the total freshwater transport. In the Bay of Bengal, most of the freshwater export occurred in the surface layer because of strong southward Ekman surface flow and fresh surface waters from river runoff and monsoon rainfall. The role the horizontal circulation plays in the heat and freshwater transport across I1 was different in the Arabian Sea and Bay of Bengal. The horizontal circulation contributed 0.06 PW of the total heat transport in contrast to −0.60 PW of the total heat transport crossing I1w and ∼30% of the freshwater transport across I1w in the Arabian Sea. In the Bay of Bengal, the horizontal circulation contributed ∼20% heat transport and ∼45% of the freshwater transport across I1e. The difference in horizontal circulation between the two basins is predominately due to the role of the Somali Current in the Arabian Sea.

Dynamics of the Kuroshio/Oyashio current system using eddy‐resolving models of the North Pacific Ocean

A set of numerical simulations is used to investigate the Pacific Ocean circulation north of 20°S, with emphasis on the Kuroshio/Oyashio current system. The primitive equation models used for these simulations have a free surface and realistic geometry that includes the deep marginal seas, such as the Sea of Japan. Most of the simulations have 1/8° resolution for each variable but range from 1/2°, 1.5‐layer reduced gravity to 1/16°, six layer with realistic bottom topography. These are used to investigate the dynamics of the Kuroshio/Oyashio current system and to identify the processes that contribute most to the realism of the simulations. This is done by model‐data comparisons, by using the modularity of layered ocean models to include/exclude certain dynamical processes, by varying the model geometry and bottom topography, and by varying model parameters, such as horizontal grid resolution, layer structure, and eddy viscosity. In comparison with observational data, the simulations show that the barotropic mode, at least one internal mode, nonlinearity, high “horizontal” resolution (1/8° or finer), the regional bottom topography, and the wind forcing are critical for realistic simulations. The first four are important for baroclinic instability (eddy‐mean energetics actually show mixed barotropic‐baroclinic instability), the wind curl pattern for the formation and basic placement of the current system, and the bottom topography for the distribution of the instability and for influences on the pathways of the mean flow. Both the Hellerman and Rosenstein (1983) (HR) monthly wind stress climatology and 1000‐mbar winds from the European Centre for Medium‐Range Weather Forecasts (ECMWF) have been used to drive the model. East of about 150°E, they give a mean latitude for the Kuroshio Extension that differs by about 3°, approximately 34°N for HR, 37°N for ECMWF, and 35°N observed. The subarctic front is the northern boundary of the subtropical gyre. It is associated with the annual and April–September mean zero wind stress curl lines (which are similar), while the Kuroshio Extension is associated with wintertime zero wind stress curl. This means that part of the flow from the Kuroshio must pass north of the Kuroshio Extension and connect with the Oyashio and subarctic front. Realistic routes for this connection are flow through the Sea of Japan, a nonlinear route separated from the east coast of Japan, and bifurcation of the Kuroshio at the Shatsky Rise. In addition, the six‐layer simulations show a 3‐Sv meridional overturning cell with southward surface flow and northward return flow centered near 400 m depth. Baroclinic instability plays a critical role in coupling the shallow and abyssal layer circulations and in allowing the bottom topography to strongly influence the shallow circulation. By this means, the Izu Ridge and Trench and seamounts upstream and downstream of these have profound influence on (1) the mean path of the Kuroshio and its mean meanders south and east of Japan and (2) on separating the northward flow connecting the Kuroshio and the Oyashio/subarctic front from the east coast of Japan. Without the topographic influence, the models show an unrealistic northward current along the east coast of Japan. In essence, the topography regulates the location and strength of the baroclinic instability. The baroclinic instability gives eddy‐driven deep mean flows that follow the f/h contours (where f is the Coriolis parameter and h is the depth of the water column) of the bottom topography. These abyssal currents then strongly influence the pathway for subtropical gyre flow north of the Kuroshio Extension and steer the mean meanders in the Kuroshio south and east of Japan. This is corroborated by current meter data from the Kuroshio Extension Regional Experiment (World Ocean Circulation Experiment line PCM 7). The meander path south of Japan depends on the occurrence of baroclinic instability west of the Izu Ridge; otherwise, a straight path occurs. The pathway shows little sensitivity to the Tokara Strait transport over the range simulated (36–72 Sv in yearly means). However, interannual increases in wind forcing or Tokara Strait transport give rise to a predominant meander path, while decreases yield a predominant straight path. Resolution of 1/8° in an ocean model is comparable to the 2.5° resolution used in atmospheric forecast models in the early 1980s based on the first internal mode Rossby radius of deformation. Model comparisons at 1/8° and 1/16° resolution and comparisons with current meter data and Geosat altimeter data show that 1/16° resolution is needed for adequate eastward penetration of the high eddy kinetic energy associated with the Kuroshio Extension.

The flow field off southwest India at 8N during the southwest monsoon of August 1993

The flow field off the southwest coast of India at 8N was investigated during RV Sonne cruise 89 in August 1993 by direct velocity observations from shipboard- and lowered-ADCP and geostrophic computations from CTD stations. The upper ocean between 75E and 76°52'E near the South Indian shelf was governed by a northward flow with a subsurface velocity maximum of 25 cm s -1 at about 100 m depth. This flow, organized as a poleward undercurrent hugging the continental slope, is typical for the southwest monsoon season. The northward transport in August 1993 was 4.7 Sv (1 Sverdrup = 10 6 m 3 s -1 ) for the upper 300 m from the shipboard ADCP. Earlier geostrophic observations showed southward surface flow above the poleward undercurrent, but in August 1993 the northward flow reached to the surface and in the geostrophy calculations, i.e. without the southward Ekman flow near the surface, there was even no clear subsurface core. The T-S characteristics show that Bay of Bengal Water (BBW) was carried with this flow, and low wind conditions seemed to be connected to the flow of BBW from the southern tip of Sri Lanka toward the southwest coast of India. Further offshore, two meridional current bands were identified in the upper 300 m of the ocean. West of the coastal undercurrent a band of southward flow existed with velocities up to 35 cm s -1 above and to the east of the Chagos-Laccadive Ridge, from 72°10'E to 75E. The associated geostrophic transport in the upper 500 m was 5.2 Sv. As the T-S relation was different from that of the northward flow, this current band was not a local recirculation of the poleward undercurrent. Further west, the flow was weak, but intensified toward the central Arabian Sea, between 66E and 69°20'E, where another southward current band was found with velocities of up to 20 cm s -1 and a total geostrophic transport for the upper 300 m of -7.2 Sv.

Dynamical simulations of filament formation and evolution in the Coastal Transition Zone

Using the semispectral primitive equation model of Haidvogel et al. (1991), the evolution of a forced, surface‐intensified, eastern boundary current is studied in the presence of both finite‐amplitude topography and irregular coastline geometry. The model domain is 1000 km in alongshore length, and extends on average 700 km in the cross‐shelf direction. A representative cape, as well as smoothed continental shelf‐slope topography, are included. The model is forced by inclusion of nudging terms in the equations of motion which relax the fluid system back to a prescribed reference state on a time scale of 45 days. The reference state chosen is a broad, geostrophically balanced, equatorward flow having a maximum current at the surface of 0.45 m s−1 and a transport of approximately 10 Sv. No explicit wind forcing is included. Initialized with the smooth surface current, the model quickly approaches a turbulent, time‐dependent equilibrium featuring an intense, meandering alongshore jet with local velocities of 0.8–1.0 m s−1. A deep, poleward undercurrent also forms. Subsequent interaction with the protruding cape geometry causes an offshore deflection in the steepening frontal meanders, some of which produce elongated filaments which penetrate significant distances (400–500 km) offshore. The emerging filaments are characterized by a strong downwelling signal (maximum vertical velocities of 30–40 m d−1). The simulated filaments ultimately pinch off, typically within 40–50 days, to form a corotating pair of detached eddies. The existence of the cape geometry, as well as the southward surface flow, appears to be necessary in this model to produce filament generation; removal of the irregular coastal geometry or reversal of the sense of the surface circulation is shown to inhibit filament formation. Detailed analysis of both instantaneous and time‐mean momentum balances show dynamical similarity to observations taken during the Coastal Transition Zone experiment and elucidate the eddy transport mechanisms responsible for the formation of the poleward undercurrent in these experiments. Offshore transport of heat by the filaments, found to be O(0.06) PW, is substantial.

Seasonal variation of the alongshore velocity field ovei the continental shelf off Oregon1

The seasonal variation in the alongshore velocity field is inferred from direct current observations made over the Oregon continental shelf at various locations and irregular intervals since 1965. Monthly mean currents are computed and compared with earlier studies to give a description of the seasonal variation in the alongshore currents. In winter, the alongshore flow is generally northward and independent of depth. In spring, flow is southward at all depths but stronger near the surface. In summer, surface flow is southward and deep flow is northward; the southward surface flow forms a coastal jet and the deep northward velocity increases with distance offshore.