High-salinity Core Research Articles

Mixing Processes at the Southwestern Entrance to the Japan/East Sea

Abstract Small-scale processes at the southwestern boundary of the Ulleung Basin (UB) in the Japan/East Sea (JES) were examined using combined ship-based and moored observations along with model output. Model results show baroclinic semidiurnal tides are generated at the shelf break and corresponding slope connecting the Korea/Tsushima Strait with the UB and propagate into the UB with large barotropic-to-baroclinic energy conversion over the slope. Observations show high-frequency internal wave packets and indicate strong velocity shear and energetic turbulence associated with baroclinic tides in the stratified bottom layer. Solitary-like waves with frequencies from 0.2N to 0.5N (buoyancy frequency N) were found at the edge of the shelf break with supercritical flow. For subcritical flow, a hydraulic jump formed over the shelf break with weakly dispersive internal lee waves with frequencies varying from 0.5N to N. These high-frequency lee waves were trapped in the stratified bottom layer, with wave stress similar to the turbulent stress near the bottom. The power loss due to turbulent bottom drag can be an important factor for energy loss associated with the hydraulic jump. Turbulent kinetic energy dissipation rates of ∼10−4 W kg−1 were found. Large downward heat and salt fluxes below the high-salinity core mix warm/salty Tsushima Current Water with cold/low-salinity JES Intermediate Water. Mixing over the shelf break could be very important to the JES circulation since the calculated diapycnal upwelling (1–6 m day−1) at the shelf break and slope is substantially greater than the basin-averaged estimate from chemical tracers and modeling studies. Significant Statement The Japan/East Sea (JES) is a marginal sea, enclosed by Japan, Korea, and Russia. This study describes mixing processes over the shelf break connecting the northern Korea/Tsushima Strait (KTS) with the southern Ulleung Basin (UB), where the warm, high-salinity Kuroshio water carried by the Tsushima Current interacts with southward-flowing subsurface water masses in the JES. Our analysis suggests that the shelf break and slope between the KTS and the UB are vital areas for water-mass exchange in the southern JES. The enhanced mixing at the shelf break may impact water masses and circulation over the entire JES.

Formation, maintenance and diurnal variability of subsurface chlorophyll maximum during the summer monsoon in the southern Bay of Bengal

The vertical structure of chlorophyll in stratified oceanic waters exhibits a maximum substantially below the surface, known as the subsurface chlorophyll maximum (SCM). In the southern Bay of Bengal (BoB), a 12-day high-resolution time-series observation at 89°E and 8°N, during the 2016 summer monsoon showed that the SCM was located within the barrier layer (BL) and the euphotic layer. The strong BL (N2=3×10−4 s−2) located between 30 to 70 m, associated with a low salinity (<33.8 psu) water near the surface, maintained the stratification and the SCM. This SCM oscillated diurnally with an amplitude of 0.1 to 0.2 mg m−3. The lowest values of average chlorophyll in the SCM occurred around 06:00 hours (local time) and reached a maximum around 16:00 hours, four hours after the maximum in surface shortwave radiation. A coupled physical–biogeochemical one-dimensional model was used to examine the formation, maintenance and diurnal variability of the SCM in the southern BoB. The wind speed (<10 m s−1) that prevailed in the region during the weak phase of the monsoon was insufficient to break the BL and cause a surface bloom. The model results revealed that the surface shortwave radiation drove the diurnal cycle of the growth rate of phytoplankton at the SCM, hence the diurnal cycle of phytoplankton biomass and chlorophyll. The model’s control simulation showed the dominance of small phytoplankton, and its growth rate was limited by light and nutrients at the SCM. Whereas, in the ML, the phytoplankton growth rate was limited by nutrients. In the model, the regenerated production supported around 60% of the chl-a at the location of SCM. The subsurface high-salinity core, a prominent feature in the southern BoB, did not influence the SCM. This paper offers new insights into understanding the SCM formation, maintenance and diurnal variability in stratified tropical oceans.

Front and skeleton features based methods for tracking salinity propagation in the ocean

The Bay of Bengal (BoB) fosters several monsoon depressions and cyclones, playing a crucial role in the Asian summer and winter monsoons. The capacity of the bay to remain warm and energize such weather systems is attributed to its strong vertical stratification sustained by the large freshwater input into the bay. River runoff and rainfall into the northern bay in contrast to the high salinity water intrusion in the south creates a strong north–south salinity gradient. Here, we present a visual analysis tool to trace the path of the high salinity core (HSC) entering into the BoB from the Arabian Sea. We introduce two feature definitions that represent the movement and shape of the HSC, and algorithms to track their evolution over time. The two feature representations, namely fronts and skeletons, are based on geometric and topological analysis of the HSC. The method is validated via comparison with well established observations on the flow of the HSC in the BoB, including its entry from the Arabian Sea and its movement near Sri Lanka. Further, the visual analysis and tracking framework enable new detailed observations on forking behavior near the center of the BoB and subsequent northward movement of the HSC. The tools that we have developed offer new perspectives on the propagation of high salinity water and its mixing with the ambient low salinity waters.

Data-driven feature-oriented modeling of Southwest Monsoon Current

The multiscale synoptic circulation in the southern Bay of Bengal region is simulated using a high-resolution Feature-Oriented Regional Modeling and Simulation (FORMS) approach. First, the region’s prevalent ocean circulation features are identified and mapped onto a FORMS template using a literature survey, and analysis of in-situ and remote sensing data. These features include the Southwest Monsoon Current (SMC), Sri-Lanka Dome, Anti-cyclonic Eddy, and Subsurface Baroclinic Eddy to the east of the SMC. New data-driven feature models are developed for the SMC and Subsurface Baroclinic Eddy using observational transects collected during the Bay of Bengal Boundary Layer Experiment (BoBBLE) of July 2016. Specifically, a new elliptical feature model for the SMC’s high salinity core is developed and utilized. The developed temperature and salinity feature models are placed on the FORMS template, and pseudo observation profiles are sampled for use in an objective analysis initialization scheme. The background for the objective analysis is taken to be the NEMO global analysis model data on the day of the synoptic initialization. The FORMS derived initialization fields on 04 July 2016 are used with the MIT-MSEAS Primitive Equation modeling system to simulate the ocean circulation in the region from 04 July to 15 July 2016. The forecast skill of the model is quantified by comparison to the time-series observations collected during BoBBLE (but not used in the FORMS development) and Argo data available in the region during the simulation. Four experimental short-term synoptic simulations at a 3 km horizontal resolution and 70 vertical levels, with and without the different components of the feature models are showcased. Results are utilized to calibrate and improve the feature model, including the addition of a Subsurface Baroclinic Eddy and the development of a new surface bias-correction scheme to obtain a better match with validation datasets. The present data-driven feature modeling approach improves the representation and forecast skill for several critical dynamical features: (i) the initial high salinity core at the time-series location (ii) evolution and expansion of the sub-surface high salinity core; and (iii) salinization and freshening events at the time-series location between 3 m to 20 m. The MSEAS model simulation from the final experiment match the BoBBLE temperature and salinity data (total RMSE of 0.14 psu in salinity and 0.57 C in temperature in between 60-120 m depths and total RMSE of 0.17 psu in salinity and 0.33 C in temperature in between 3-60 m depths) and the Argo measured temperature and salinity (total RMSE of 0.16 psu in salinity and 0.66 C in temperature), showcasing the model’s success in providing excellent spatio-temporal representation of the ocean in the region of interest.

Enhanced Double-Diffusive Salt Flux from the High-Salinity Core of Arabian Sea Origin Waters to the Bay of Bengal

AbstractThe inflow of high-saline water from the Arabian Sea (AS) into the Bay of Bengal (BoB) and its subsequent mixing with the relatively fresh BoB water is vital for the north Indian Ocean salt budget. During June–September, the Summer Monsoon Current carries high-salinity water from the AS to the BoB. A time series of microstructure and hydrographic data collected from 4 to 14 July 2016 in the southern BoB (8°N, 89°E) showed the presence of a subsurface (60–150 m) high-salinity core. The high-salinity core was composed of relatively warm and saline AS water overlying the relatively cold and fresh BoB water. The lower part of the high-salinity core showed double-diffusive salt fingering instability. Salt fingering staircases with varying thickness (up to 10 m) in the temperature and salinity profiles were also observed at the base of a high-salinity core at approximately 75–150-m depth. The average downward diapycnal salt flux out of the high-salinity core due to the effect of salt fingering was 2.8 × 10−7 kg m−2 s−1, approximately one order of magnitude higher than the flux if salt fingering was neglected.

On the Variability of the Circulation and Water Mass Properties in the Eastern Levantine Sea between September 2016–August 2017

The surface circulation and the thermohaline properties of the water masses of the eastern Levantine Sea (Mediterranean Sea) were monitored with mobile autonomous systems (surface drifters and gliders) during the period September 2016–August 2017. The drifters provided data for more than a year and revealed complex circulation features at scales ranging from the basin scale to the sub-mesoscale. Three drifters were captured in a semi-permanent gyre (Cyprus Eddy) allowing a quantitative study of its kinematics. During the experiment, three gliders were operated, in two different periods: September to December 2016 and February to March 2017. The autonomous instruments crossed the prevailing sub-basin structures several times. The collected in-situ observations were analyzed and interpreted in concert with remote sensing products (sea surface temperature and altimetry). The evolution of some of the prevailing features confirmed the complexity of the circulation of the basin. The Cyprus Eddy is the most persistent anticyclone, moving its geographical position and sometimes merging with the North Shikmona Eddy in a bigger structure. The gliders sampled this wide anticyclonic feature revealing its vertical structure in the two different periods. In fall, in stratified conditions, a high salinity core is evident below the thermocline. The isopycnals are characterized by an upward bending over the high salinity lens and a downward bending below it, typical of an anticyclonic modewater eddy. In winter, the core disappears following the vertical mixing that, homogenizes the upper Cyprus Eddy water down to 300 m.

Mechanisms of Barrier Layer Formation and Erosion from In Situ Observations in the Bay of Bengal

AbstractDuring the Bay of Bengal (BoB) Boundary Layer Experiment (BoBBLE) in the southern BoB, time series of microstructure measurements were obtained at 8°N, 89°E from 4 to 14 July 2016. These observations captured events of barrier layer (BL) erosion and reformation. Initially, a three-layer structure was observed: a fresh surface mixed layer (ML) of thickness 10–20 m; a BL below of 30–40-m thickness with similar temperature but higher salinity; and a high salinity core layer, associated with the Summer Monsoon Current. Each of these three layers was in relative motion to the others, leading to regions of high shear at the interfaces. However, the destabilizing influence of the shear regions was not enough to overcome the haline stratification, and the three-layer structure was preserved. A salinity budget using in situ observations suggested that during the BL erosion, differential advection brought high salinity surface waters (34.5 psu) with weak stratification to the time series location and replaced the three-layer structure with a deep ML (~60 m). The resulting weakened stratification at the time series location then allowed atmospheric wind forcing to penetrate deeper. The turbulent kinetic energy dissipation rate and eddy diffusivity showed elevated values above 10−7 W kg−1 and 10−4 m2 s−1, respectively, in the upper 60 m. Later, the surface salinity decreased again (33.8 psu) through differential horizontal advection, stratification became stronger and elevated mixing rates were confined to the upper 20 m, and the BL reformed. A 1D model experiment suggested that in the study region, differential advection of temperature–salinity characteristics is essential for the maintenance of BL and to the extent to which mixing penetrates the water column.

Experimental and Theoretical Study of Water and Solute Transport in Organic-Rich Carbonate Mudrocks

Summary The objective of this research is to determine the physicochemical processes underlying water and solute transport in organic-rich source rocks. To achieve this goal, a custom-designed experimental apparatus was constructed to conduct flow tests, founded on a high-pressure triaxial assembly. The apparatus is capable of maintaining core samples at reservoir pressure, temperature, and confining stress. We conducted several 120-day low-salinity osmotic tests in low-clay, organic-rich Eagle Ford carbonate-shale samples. Test results showed gradual, slow increase of pressure within the samples. Because this pressure behavior could not be explained properly with classical models, we formulated a mass-transport mathematical model that relies on fundamental chemical osmosis principles driving low-salinity brine into high-salinity core samples. Our mathematical model was articulated to simulate flow into the core as a 3D porous medium rather than transport across a thin, molecule-selective membrane. The model is dependent on the following principles: The low-salinity brine selectively enters the pores by diffusion mass transport, and the pre-existing, ionized dissolved salt molecules within the core are restrained by internal electrostatic forces to counterdiffuse in the direction opposite to that of the low-salinity-brine molecules entering the pore network. Critical model input data, such as permeability, porosity, and rock compressibility, were obtained from flow experiments on twin cores, and the diffusion coefficient was chosen by history matching. The strengths of the numerical simulation include reliance on mass-transport fundamental principles; not requiring the use of an ambiguously defined membrane-efficiency term; and relying on chemical-potential gradient as the driving force for the low-salinity brine to enter the high-salinity core, generating osmotic pressure within the pore network. The latter implies that osmotic pressure is the consequence of water entering the cores, not the cause. Results of this research have provided a more plausible explanation of pore-scale mass transport in organic-rich shales, and provide useful insights for design of effective enhanced-oil-recovery (EOR) processes.

The hydrographic features of anguillid spawning areas: potential signposts for migrating eels.

Catadromous anguillid eels (Genus Anguilla) migrate from their freshwater or estuarine habitats to marine spawning areas. Evidence from satellite tagging studies indicates that tropical and temperate eel species exhibit pronounced diel vertical migrations between 150 to 300 m nighttime depths to 600 to 800 m during the day. Collections of eggs and larvae of Japanese eels (A. japonica) show they may spawn at these upper nighttime migration depths. How anguillid eels navigate through the ocean and find their spawning areas remains unknown, so this study describes the salinity, temperature and geostrophic currents between 0 and 800 m depths within two confirmed and three hypothetical anguillid spawning areas during likely spawning seasons. Within all four ocean gyres many eels would encounter subducted 'Subtropical Underwater' during their nighttime ascents that could provide odor plumes as signposts. Four spawning areas are located near the western margins of where subducted water masses form cores of elevated salinities (~35.0 to 36.8) around 150 m depths, while one is found near the center of subduction. Low salinity surface waters and fronts are present in some of the areas above the high-salinity cores. Spawning may occur at temperatures between 16 to 24°C where the thermocline locally deepens. At spawning depths, weak westward currents (~0 to 0.1 m s-1) prevail, and eastward surface countercurrents are present. Anguillid eels possess acute sensory capabilities to detect these hydrographic features as potential signposts guiding them to where they spawn.

The subsurface water in the North Pacific tropical gyre

The subsurface water (22.5–25.5 σθ) in the North Pacific tropical gyre (NPTG; 130°E–150°W, 5°–10°N), addressed as the North Pacific tropical subsurface water (TSSW), features a lateral salinity minimum and vertical salinity maximum in the western and central Pacific Ocean. In this study this water body is investigated using Argo float profiles and Word Ocean Atlas 2009 (WOA09) dataset. Comparing with the North Pacific Tropical water (NPTW), the TSSW is of lower salinity, lower oxygen, higher nutrient concentration, and denser vertical salinity maximum Smax. Subtropical ventilation and local ocean dynamics are both important in setting up its unique characteristics. Our analysis shows that its properties, structure, and seasonal variations are diverse at different longitudes. In the western Pacific, the TSSW is mainly of North Pacific water origin. Its high salinity core is formed by the southward intrusion of the NPTW. Diapycnal mixing with the surface-layer fresh water reduces its salinity and lowers its Smax to denser isopycnals. The TSSW in the western Pacific can be regarded as a diluted portion of the NPTW. In the central Pacific, advection of salty water masses from both hemispheres and westward invasion of the eastern Pacific fresh water are all important in regulating its characteristics. The TSSW in the central Pacific is a mixture of various water sources formed under highly variable flow pattern and intensive mixing. The TSSW should be regarded as an important subsurface water body connecting North/South Pacific thermocline water, although it might not be a water mass by traditional definition.

Spatial Change of Salinity at Kuroshio in East China Sea with Seasons

Kuroshio influences hydrological-climate environment of East China Sea by exchanging heat and salinity. It has great influence on climate-environment in East China. Understanding of seasonal variability of salinity at Kuroshio in the East China Sea (ECS Kuroshio) can provide a scientific basis to reveal the climate-environment change in East China. In this paper, Using salinity database of World Ocean Atlas 2009(WOA09) issued by NOAA in 2010, the change of salinity distribution and the high-salt areas at ECS Kuroshio with seasons have been analyzed, the results show that: upper low-salinity water between 0~125m is obviously changing with seasons. From spring to winter, the horizontal stratification at upper low-salinity water first increases and then decreases, but the phenomenon of vertical mixing is opposite with the horizontal stratification. The horizontal stratification is the most obvious in summer, it forms 0.33/m of halocline strength above 10m; the vertical mixing is the strongest in winter, upper low-salinity water mixes with higher-salinity water in Okinawa trough, and salinity is homogenous in vertical above 250m. There are seasonal variations of depth and range of the high-salt areas in different segments along ECS Kuroshio, and there are three high salinity cores in the high-salt areas: two of the high salinity cores appear in the sea area east of Taiwan in autumn and winter, the third core appears in the sea area northwest of Miyako-jima in spring.

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.

Intrathermocline eddies in the Southern Indian Ocean

In 2001, two relatively saline intrathermocline eddies (ITEs) were observed southeast of Madagascar at 200 m depth. They are characterized by a subsurface salinity maximum of over 35.8 at potential temperatures between 18° and 22°C. The oxygen concentrations within the high salinity cores are slightly elevated compared with those of the surrounding water. Their horizontal extent is about 180 km, several times the Rossby deformation radius, while their thickness is about 150 m. The observed circulation around the ITEs is anticyclonic and maximum velocities of 20 to 30 cm/s are observed at 200 m depth. In these cores the potential density anomaly (25.0 &lt; γ &lt; 25.9 kg/m3) has a relatively low vertical gradient and therefore a low planetary potential vorticity. The hydrographic properties of these ITEs are distinctly different from those of the surrounding thermocline water, and especially from the much fresher water mass in the East Madagascar Current. Strong evidence has been found that the distant formation area of the water mass in the ITEs is the subtropical Southern Indian Ocean east of 90°E and south of 25°S, where Subtropical Underwater (STUW) is formed with similar characteristics. Similar high‐salinity cores as the ITEs are also found in the thermocline around 200 m depth along an almost zonal section between Madagascar and 100°E. Differences between the hydrographic properties of these cores and the ITEs near Madagascar may partly be explained by interannual variations in the temperature and salinity of the surface mixed layer water in the possible formation area.

Hydrography around Dokdo

CTD data taken in the Ulleung Basin between 1996 and 2001 are analyzed to understand the hydrography around Dokdo. Major features occurring in the Ulleung Basin such as the path variability of the East Korean Warm Current (EKWC), the location and size of the Ulleung Warm Eddy (UWE) and the position of the Offshore Branch along the Japanese coast all influence the hydrography around Dokdo. The Dokdo area frequently lies in the eastern part of the meandering EKWC and the UWE that results in a filting of isolines sloping upwards to Dokdo in the Ulleung Interplain Gap (UIG) between Ulleungdo and Dokdo. Subsurface water near Dokdo then becomes colder and less saline than water near Ulleungdo. Two cases that are opposite to this general trend are also identified when the Dokdo area is directly affected by the EKWC and by a small scale eddy ffd by the Offshore Branch. High salinity cores and warm waters are then found near Dokdo with isolines sloping upwards to Ulleungdo. Freshening of the East Sea Intermediate Water was observed in the UIG when neither the EKWC nor the UWE was developed in the Ulleung Basin during June-November 2000.

The use of the salinity maximum of the equatorial undercurrent for estimating nutrient enrichment and primary production in the Gulf of Guinea

In the Atlantic Ocean the summer equatorial nutrient enrichment seems due to the increased vertical mixing induced by the vertical shear between the westward South Equatorial Current and the eastward Equatorial Undercurrent. The high salinity core of the undercurrent is a good indicator of the intensity of vertical mixing: its salinity decreases as vertical mixing increases. There is a linear relationship between the salinity of the core and the nitrate enrichment of the surface layer, and at times when nitrate is limiting, the salinity maximum of the Equatorial Undercurrent can be used as a measure of the primary production.

Further Studies of a Cold Eddy on the Eastern Side of the Gulf Stream Using Satellite Data and Ship Data

Abstract A cold eddy was observed in late March and early April 1975 to encounter the eastern boundary of the Gulf Stream and entrain warm Gulf Stream water into its outer fringes. Available evidence indicated that the entrainment of Gulf Stream water in the western and southern portions of the eddy enhanced the density stratification creating available potential energy which led to an increase in kinetic energy in those regions. A secondary, cold perturbation with a high-salinity core was detected south of the center of the cold eddy. It was not clear from the data whether the conversion of available potential energy into kinetic energy increased the kinetic energy of the cold eddy, produced the secondary perturbation, or both.

On the barium data from the Atlantic GEOSECS expedition

Barium profiles at the Atlantic stations of the Geochemical Ocean Sections (GEOSECS) Program are remarkably similar to those for silicate and alkalinity. In nutrient depleted surface waters barium ranges between 36 and 44 nm kg −1: the Arctic overflows have values between 44 and 49 nm kg −1 and the Antarctic Bottom Water 100 to 106 nm kg −1. Both barium and silica behave non-conservatively in basins with restricted circulation—the Greenland and Norwegian seas and the Angola Basin—with anomalies of up to 1, 2 and 7 nm Ba kg −1, respectively. There is extensive in situ production of silica and barium in the bottom waters of the Weddell Sea Basin, the silica anomaly ranging up to 35 μm kg −1 and that for barium to 15 nm kg −1. Barium behaves uniquely only in the upper waters. At tropical and temperate latitudes there is commonly a minimum of several nannomoles per kilogram in the uppermost thermocline, often associated with the high salinity core of the Sub-Tropical Underwater. In the Circumpolar Current the depletion in surface silica is associated with only a minor reduction in barium. The values at <5 μm Si kg −1 are about 30 nm kg −1 higher than those at lower latitudes. The important question of the removal mechanism is not resolved from the profile data.

Observation of an oceanic front in the Ionian Sea during early winter 1970

Detailed observations of an oceanic front during early winter in the Ionian Sea are described. The front ran north-south and was located over deep water. Midoceanic shallow upwelling was established. A high-salinity core was located in the center of the frontal zone.

Meanders and long waves in the equatorial Atlantic

Observations from the GATE Equatorial Oceanographic Experiment are presented. They reveal large scale meandering of the westward flowing South Equatorial Current and of the eastward flowing Equatorial Undercurrent with time scales of 2–3 weeks. Meandering of the flow pattern was found to be related to corresponding displacements of the high salinity core of the undercurrent. The observations tend to support the assumption of a long wave propagating westward with a phase speed of 2.3 m s−1 and a wavelength of 3,200 km. A possible explanation may be given in terms of unstable waves caused by large scale horizontal shear in the Equatorial Current System.

Equatorial undercurrent and deeper flows in the central Pacific

In April, 1971, absolute current velocity was measured down to 1000 m at 1°N, 0° and 1°S, along 150°W. Two velocity time series were obtained: a 25-day record from the Equatorial Undercurrent (102 m) at 1°N and a 152-day record from the deep water (3100 m) at 1°S. The profiles show eastward flow at the surface, a strong Equatorial Undercurrent, a secondary maximum of eastward flow at the bottom of the thermostad, and a westward flow below 500 m. The surface current was counter to the trade winds. Although there was a high-salinity core south and a low-salinity tongue north of the equator, usually interpreted as indirect evidence of equatorward flow at the level of the Undercurrent, the flow at 1°N (at 102 m) was poleward for all 25 days. There was an accelerating eastward and northward current at 1°N; the data are consistent with a northward translation of the Undercurrent at a speed of 7 cm s −1. The winds at Christmas Island (2°N, 157°W) and the Undercurrent velocity at 1°N, 150°W, showed evidence of a 3- to 4-day period; it appears that at this time the currents in the thermocline may have been responding directly to the winds. The meter moored in deep water at 1°S showed a relatively steady eastward velocity component with a mean of 4 cm s −1. Below the Undercurrent in the central Pacific at least two reversals are indicated in the east-west current profile.