Equatorial Current System Research Articles

Cape Verde Frontal Zone

The term Cape Verde Frontal Zone is introduced to characterize the southeastern corner of the subtropical gyre circulation in the North Atlantic Ocean far west of the upwelling area off the Mauretanean shelf. Two water mass fronts, one overlying the other, are identified with a quasi-synoptic set of CTD-OZ and nutrient data from November 1986. In the warm water sphere we encounter North and South Atlantic Central Water (NACWISACW) superimposed on extensions of Mediterranean outflow and Antarctic Intermediate Water. The Central Water Boundary, as the separator of NACW from SACW, represents the southeastern side of the Canary/North Equatorial Current system. It acts as a barrier between the well-ventilated, nutrient-poor inner part of the basin-wide circulation of the North Atlantic and the shadow zone with its lowly oxygenated and nutrified cross-equatorial influx. Year-long current meter records, having fluctuations over typical time scales of 5(1`90 days, attest to the highly variable nature of the Cape Verde Frontal Zone. Incidentally, we observe in the data an intrathermocline eddy, called Meddy BIRGIT, which has a double maximum in the vertical salinity structure. Simultaneous Lagrangian observations by RiCHAttDSON et al. (1989, Journal of Physical Oceanography, 19, 371-383) confirm the expected anticyclonic motion of this salt lens, which must have travelled without significant mixing for at least 2500 km from its likely generation region in the Gulf of Cadiz.

The Meridional and Seasonal Structures of the Mixed-Layer Depth and its Diurnal Amplitude Observed during the Hawaii-to-Tahiti Shuttle Experiment

We describe the meridional and seasonal structures of daily mean mixed-layer depth and its diurnal amplitude and their relation to atmospheric fluxes by compositing mixed-layer depth estimates derived from density observations. The diurnal mean mixed-layer depth shows a ridge at the equator, troughs, which vary seasonally in intensity, at 10° to 15°N and 5° to 10°S, and a trough appearing just north of the equator in the second half of the year. This is in contrast to the ridge-trough structure of the top of the main thermocline, which reflects the dynamic topography associated with the equatorial current system. The diurnal amplitude is significantly different from zero for most latitudes year-round, indicating that the diurnal cycle of mixed-layer depth is a widespread phenomenon. For sufficiently strong heating, both the mixed-layer depth and its diurnal amplitude are significantly correlated with Monin-Obukhov length scales based on the mean net heat flux, mean wind stress, and mean shortwave radiation. This suggests a possible parameterization of the mixed-layer depth and diurnal amplitude in terms of the mean atmospheric fluxes for meridional scales of a few degrees and seasonal time scales.

The hydrography and ventilation of Kau Bay in Halmahera

The hydrography of Kau Bay in Halmahera was studied in 1985 during the Snellius-II Expedition. Contrary to the findings from the Snellius Expedition in 1929–1930 the deep water in the bay was not anoxic although very low oxygen concentrations were encountered in the deepest 30 m of the bay. Rough estimates of the oxygen consumption in the water column and in the sediment indicate that the deep water in the bay is ventilated probably once every year. Within one year the deep water in the bay may become anoxic. This result has been confirmed by estimates of the diffusion time scale of the observed fine structure based on tentative values of the turbulent diffusion coefficient. The flushing appears to be controlled by salinity changes in the surface water of the nearby Pacific Ocean due to shifts of the equatorial current system.

Wind Forced Wave–Mean Flow Interactions in the Equatorial Waveguide. Part I: The Kelvin Wave

A numerical model is designed to study the effects of the strong, near-surface associated with the equatorial current system on energy transmission of time-periodic equatorial waves into the deep mean. The present paper is confined to long wavelength, low-frequency Kelvin waves forced by a longitudinally confined patch of zonal wind. Energy transmission into the deep ocean is investigated as a function of mean current shear amplitude and geometry and the forcing frequency. Solutions form well-defined beams of energy that radiate energy eastward and vertically toward the deep ocean in the absence of mean flow. However, the presence of critical surfaces associated with mean currents inhibits low-frequency energy from reaching the deep ocean. For a given zonal wavenumber, longitudinal propagation through mean currents will be less inhibited as the frequency increases (phase speed increases). When the mean current amplitude is large enough, the beam encounters multiple critical surfaces (i.e., critical surfaces for different wavenumber components of the beam) where significant and momentum can take place with the men currents via Reynolds stress transfers. Work against the dominant vertical shear is the dominant wave energy loss for the case of a mean South Equatorial Current–Equatorial Undercurrent system, illustrating the need for high vertical resolution in equatorial ocean models. The model also describes the possible induction of a mean zonal acceleration as well as a mean meridional circulation. Eliassen-Palm fluxes are used to diagnose these dynamics. The presence of critical surfaces result in mean field accelerations on the equator above the core of the Equatorial Undercurrent. Implications of these results with regard to observations in the equatorial waveguide are discussed.

Rate of production, dissolution and accumulation of biogenic solids in the ocean

The equatorial current system, by its response to global circulation changes, provides a unique recording mechanism for long range climatic oscillations. A permanent record of the changes in rate of upwelling and organic production is generated in the equatorial deep sea sediments, particularly by such biogenic components which are unaffected by secondary dissolution. In order to determine the rates of accumulation of various sedimentary components, a reliable differential measurement of age of the strata must be obtained. Various approaches to this problem are reviewed, and sources of error discussed. Secondary dissolution of calcium carbonate introduces a substantial and variable difference between the dissolution-modified, and hence a priori unknown, rate of deposition on one hand and the rate of accumulation, derivable from the observed concentration, on the other. The cause and magnitude of these variations are of importance, particularly since some current dating schemes are based on assumed constancy in the rate of accumulation of this and, in some cases, also all other sedimentary components. The concepts used in rate evaluation are discussed with emphasis on the difference between the state of dissolution, an observable property of the sediment, and the rate of dissolution, a parameter that requires deduction of the carbonate fraction dissolved, and of the time differential. As a most likely cause of the enhanced state of dissolution of the interglacial carbonate sediments is proposed the lowered rates of biogenic production and deposition, which cause longer exposure of the carbonate microfossils to corrosion in the bioturbated surface layer of the sediment. Historical perspective is included in the discussion in view of the dedication of the Symposium to Hans Petterson, the leader of the Swedish Deep Sea Expedition 1947–1948, an undertaking that opened a new era in deep sea research and planetary dynamics.

Major features and variability of the Kuroshio in the East China Sea

This is a brief introduction of the Kuroshio in the East China Sea (ECS). The main results of the study for this part of the Kuroshio system in recent years are reviewed and presented with emphases placed on the major features of the current structure, annual and inter-annual variations of the velocity and volume transport of the Kuroshio in ECS, and the relation between the variation of the Kuroshio in ECS and that of the Kuroshio south of Japan. And finally, an indirect relation between the variation of the Kuroshio in ECS and that of the North Equatorial Current system is suggested. It is shown that the fluctuation of the Kuroshio in ECS is also correlated with that of the North Equatorial Current and North Equatorial Counter-current. Ties of the above relation are the wind stress curl field over the tropical and subtropical belts.

Upper Ocean Water Masses and Transports in the Western Tropical Pacific (165°E)

Abstract As part of the international TOGA program, the ORSTOM Center in Noumea (New Caledonia) initiated in January 1984 a series of semi-annual cruises along the 165°E meridian from 20°S to 10°N, across the equatorial current system of the western Pacific. This paper presents an analysis of the first six hydrographic (0-1000 m) and current (0-600 m) sections. A detailed description of “typical” January 1986 vertical structures of temperature, salinity and zonal measured velocity is offered. Differences are noted with structures previously obtained in the tropical Pacific. Compared to the central and eastern Pacific, the 165°E dataset evidences a much weaker equatorial upwelling and deeper surface isothermal layer and subsurface currents. Compared to the few western Pacific measurements, the two speed cores of the Equatorial Undercurrent (EUC) previously reported at 100 and 200 m are not observed here. Special attention is given to the eastward equatorial jet (2°S-2°N; 0-75 m) measured in January 1985 wh...

Variability of dynamic topography and equatorial currents in relation to hydroclimatic conditions of the western Pacific : Donguy, J.-R., Gérard Eldin, Alain Morlière and J.-P. Rébert, 1984. Océanographie trop., ORSTOM, 19(2):155–160

From hydrographical data obtained between 1956 and 1977 in the Western Pacifie, composite transequatorial cruises have been assembled with special references to the hydroclimatic conditions. Geostrophic culculations show interannual variability of the equatorial current system from 100 N to 200 S connected to the appearance of El Nino. Dynamic heights, large before El Nino, become small during post El Nino conditions. The stronger Ninos show a larger decrease. North of 30 N, the dynamic slope is mach stronger in post El Nino than in pre El Nino and this feature corresponds to a larger eastward transport of the North Equatorial Counter Current.

Oscillations of Dynamic Topography in the Eastern Equatorial Pacific

For 14 months in 1980–81, surface dynamic height was monitored with inverted echo sounders at five sites from 0 to 9°N along 110°W. These records show that the SEC/NECC equatorial current system was well-developed during the boreal summer and fall, but weak and irregular during winter and spring when westward flow associated with the NEC extended as far south as 6°N. Superimposed on the mean dynamic topography of this region are energetic 20-to-80-day oscillations, longer periods being associated with higher latitudes. Near the equatorial ridge (∼5°N), these oscillations have predominantly monthly periods, and amplitude of ∼10 dyn cm comparable to the mean dynamic-height difference across the NECC. The broad in-phase meridional extent of these monthly oscillations implies that the principal mode of ridge variation is vertical undulation rather than meridional meandering, producing large in-phase monthly modulations in transport of the SEC and NECC. Oscillations or the equatorial ridge are correlated with propagating ∼1000-km wavelength sea surface temperature (SST) wave patterns observed in satellite infrared imagery. Passage of a northerly SST crest on the equatorial front at 110°W corresponds to a dynamic height minimum on the equatorial ridge. The relative phase and trochoidal shape of these crests is explained kinematically by superposition of the observed mean and oscillatory dynamic-height fields.

Fine‐structure variability observed in CTD measurements from the central equatorial Pacific

Conductivity, temperature, and depth data from the 15 quasi‐monthly cruises of the Hawaii‐Tahiti Shuttle Experiment are used to document fine‐structure variability in the upper 400 m of the central equatorial Pacific. The analysis reveals the existence of anomalously strong temperature and density variations on 0(1–10 m) vertical scales concentrated in the high mean shear zones of the equatorial current system and in the thermostad. These are regions where microscale measurements also indicate intense turbulent activity. The implication of this spatial correspondence between large‐, fine‐, and microscale signals is that there may be a cascade of energy from the largest to the smallest scales of variability that is mediated by fine‐structure processes. The highly structured fine‐scale signal suggests that the cascade would be most pronounced and persistent on certain density surfaces that lie within the regions of highest mean shear. Other processes, such as wind stirring, air‐sea buoyancy exchange, and intrusions, also contribute to the generation of fine‐structure variability and mixing at equatorial latitudes. There is evidence, for example, that intrusions across the sharp equatorial salinity front in the upper 150–200 m account for 10%–20% of the fine‐structure temperature variance and more than 50% of the fine‐structure salinity variance. The dominant vertical scale of these intrusions appears to be ≲10 m and the cross‐frontal scale < 100 km. A simple model calculation of cross‐frontal mixing indicates that they are important in transferring heat and salt laterally across the front. Intrusions in the vicinity of the Equatorial Undercurrent core may support salt fingering; hence they may contribute to vertical transports of salt and heat in a region where turbulent mixing is weak.

A Model of the Equatorial Sea Surface Temperature Field and Associated Circulation Dynamics

Abstract A linear, continuously stratified analytical model of the equatorial ocean is developed and used to simulate the major features of the equatorial sea surface temperature (SST) field and current system. The model is bounded by meridional barriers to flow in the east and west and is forced by an idealized southeast trade wind field. The steady state response is studied here, although the model is time periodic and capable of simulating the annual cycle or a more general time dependence. The surface heat flux forcing is set to zero so that the contribution of the equatorial upwelling field in maintaining the cool equatorial surface water can be isolated. Many of the observed features of the equatorial SST field and current system are simulated. The model generates cool SST anomalies an the equator in the eastern half of the basin. The cool water core is located to the south of the equator in the easternmost regions of the basin, with the coldest water along the eastern boundary. The latitudinal and ...

Detection of radiation from a heated and modulated equatorial electrojet current system

In May 1983, ionospheric heating experiments were conducted using the Jicamarca very high frequency (VHF) radar facility located ∼11 km east of Lima, Peru. Experiments involving high frequency (HF) heating of the lower D-region of the ionosphere were successfully conducted during 1982 using the Arecibo, Puerto Rico HF transmitter located 5km east at Islote, Puerto Rico. These local experiments had characterized the signal radiated from a heated and modulated ionospheric current system near the mid-latitudes. A long-path signal had also been received in September 1982 at Salinas, Puerto Rico from a mid-day equatorial electrojet, heated and modulated by the Jicamarca facility. We have investigated the characteristics of the local signal that would be radiated from a strong equatorial electrojet when heated and modulated, and report here that at the geomagnetic equator they were similar to, but less intense than, those observed at Arecibo, Puerto Rico due to parameter differences. This radiation is believed to be the first detected from a heated and modulated equatorial electrojet current system in the Western Hemisphere.

Surface Equatorial Current System along 23° W (July 1982‐January 1984)

Oceanographic observations were made during the first seven cruises of the FOCAL program along 23°W from July 1982 to January 1984. Using both direct current measurements and the dynamical method the equatorial current system is described. The seasonal variations of the North Equatorial Countercurrent and South Equatorial Current are related to the wind field. A weak eastward surface current near 2°‐3°S seems to be a permanent feature during the period of observation.

Mean Water and Current Structure during the Hawaii-to-Tahiti Shuttle Experiment

Abstract Mean sections based on data from a discrete one-year period during the Hawaii-to-Tahiti Shuttle Experiment are used to describe and analyse the equatorial current system, its water masses and the transverse circulation in the central Pacific Ocean. Computations of mean geostrophic transports for the individual currents identify the south equatorial current as the strongest flow, transporting 55 × 106 m3 s−1. Information derived from the water mass and nutrient distributions is used to develop a consistent picture of the transverse circulation and of the role of the different divergences and convergences.

Global mesoscale variability from collinear tracks of SEASAT altimeter data

In 1978, during the last 25 days of the SEASAT mission, the satellite ground tracks repeated within 2.5 km every 3 days. This yielded eight to nine sets of global collinear altimeter data with a cross‐track grid spacing of approximately 900 km at the equator and 600 km at mid‐latitude. Because the geoid is time‐invariant, such observations can reveal sea surface height variability due to dynamic ocean phenomena. We have solved for variations due to deep‐ocean mesoscale features by eliminating the longer wavelength deviations. Modeled tidal heights were first subtracted from the altimeter data, and linear trends were then removed from collinear segments approximately 2000 km in length. This second step eliminates relative orbit error together with any residual tide model errors. The resulting sea height profiles have a precision of a few centimeters, and the global variability map constructed from these data reveals a strikingly realistic view of mesoscale energetics. Maximum values of 20–40 cm rms variability are generated by meanders and eddies of five major current systems: Gulf Stream, Kuroshio, Agulhas, Antarctic Circumpolar, and Falkland/Brazil confluence. Perhaps a more significant discovery was the dominance of exceedingly small variability over vast regions of the oceans; approximately 70% of all global values were less than 5 cm. This category included the eastern North Pacific, eastern South Pacific, and almost the entire South Atlantic where values as small as 1–2 cm were common. With such a low level of background noise, even some currents with relatively small sea height signatures could be detected. In both the Atlantic and the Pacific, for example, the North Equatorial Current systems were clearly expressed as zonal bands of higher variability.

Spatial dependence of day-time vertical polarisation of Pc 3–5 magnetic pulsations in Sri Lanka

The rate of change of the horizontal and vertical components of magnetic pulsations in the period range 20–600 s have been recorded at four stations in Sri Lanka, namely, Vavuniya, Maradankadawela, Maho and Colombo. An analysis of the records shows that the horizontal polarisation of the pulsations is predominantly along the magnetic meridian at all four stations. The vertical polarisation as measured by the ratio ΔZ/ΔH increases with increase in period and for signals in a given periodic band, except for those recorded at Maho ; there is also an increase of vertical polarisation with distance of the recording station from the magnetic equator. At Maho, there is a local decrease of the vertical polarisation at all periods probably due to anomalous electrical conductivity in this region. Although the spatial and period dependence of the vertical polarisation of the pulsations recorded at the other three stations can be explained in terms of an oscillating ionospheric current band of half-width about 105 km flowing east-west in the neighbourhood of the magnetic equator, and its image current in a uniform conducting earth, such a model may not be realistic in that it neglects possible effects due to induced currents in the ocean deflected round the coasts of Sri Lanka. It is suggested that the observed day-time polarisation may represent the effect of a wider equatorial ionospheric current system, such as the equatorial electrojet and additional effects due to induced currents in the ocean.

Annual and semiannual variations of the geomagnetic field at equatorial locations

For a year of quiet solar-activity level, geomagnetic records from American hemisphere observatories located between about 0° and 30° north geomagnetic latitude were used to compare the annual and semiannual variations of the geomagnetic field associated with three separate contributions: (a) the quiet-day midnight level, MDT; (b) the solar-quiet daily variation, Sq; (c) the quiet-time lunar semidiurnal tidal variation, L(12). Four Fourier spectral constituents (24, 12, 8, 6 h periods) of Sq were individually treated. All three orthogonal elements ( H, D and Z) were included in the study. The MDT changes show a dominant semiannual variation having a range of about 7 gammas in H and a dominant annual variation in Z having a range of over 8 gammas. These changes seem to be a seasonal response to the nightside distortions by magnetospheric currents. There is a slow decrease in MDT amplitudes with increasing latitude. The Sq changes follow the patterns expected from an equatorial ionospheric dynamo electrojet current system. The dominant seasonal variations occur in H having a range of over 21 gammas for the 24 h period and over 12 gammas for the 12 h period spectral components. The higher-order components are relatively smaller in size. The Sq( H) amplitudes decrease rapidly with increasing latitude. Magnetospheric contributions to the equatorial Sq must be less than a few per cent of the observed magnitude. The L(12) variation shows the ionospheric electrojet features by the dominance of H and the rapid decrease in amplitude with latitude away from the equator. However, the seasonal variation range of over 7 gammas has a maximum in early February and minimum in late June that is not presently explainable by the known ionospheric conductivity and tidal behavior.

Exchange times in the Pacific Equatorial Current system

Tritium data from the GEOSECS eastern and western track stations sampled in the thermocline of the North Equatorial Counter Current (NECC) showed high values, which are indicative of the contribution of the North Equatorial Current (NEC) to the equatorial region. A tritium front at 6°N is evidence of the penetration of northern latitude water into the NECC. The results of a two-☐ model show no net flux between NEC and NECC and a lower bound estimate of five years for an exchange time scale between the NEC and NECC in the thermocline; this is consistent with a horizontal diffusivity of 5 × 10 7 cm 2/s.

Non-Linear Theory of an Equatorial Flow. II

(1) A simple model was considered for the equatorial current system on the assumption that pressure gradients and current velocity do not change in the east-west direction. (2) A balance between the Coriolis forces, pressure gradients and field accelerations was assumed, -w(∂u/∂z) and -w(∂v/∂z) being neglected as small compared with -v(∂u/∂y) and -v(∂v/∂y) respectively. The friction due to mixing and thermodynamic process were disregarded. (3) The solution was carried out for a dynamic meridional section, first computing the velocity components at the equator, then extending calculation to higher latitudes of both hemispheres by different procedures. (4) An attempt was made to interpret the significance of Hidaka-Tsuchiya's formula of computing the east-west equatorial velocity. (5) The equatorial undercurrent can be located as an equatorial jet stream of a speed about 85cm/sec confind to a narrow band between 1°S to 1.5°N. (6) The location and size of equatorial countercurrent agree with observations quite well. (7) It can be shown theoretically that the east-west component of velocity is approximately geostrophic even in the 1° intervals next to the equator. (8) Upwelling velocities were determined and found to be an order of -10-3cm/sec. (9) Vertical circulation in a meridional section was determined by constructing the stream function between v and w. (10) A brief discussion was made as to the merit of the inertia terms in hydrodynamical equations of an equational flow.

Non-Linear Theory of an Equatorial Flow. I

(1) A simple model was considered for the equatorial current system on the assumption that pressure gradients and current velocity do not change in the east-west direction. (2) A balance between the Coriolis forces, pressure gradients and field accelerations was assumed, -w(∂u/∂z) and -w(∂v/∂z) being neglected as small compared with -v(∂u/∂y) and -v(∂v/∂y) respectively. The friction due to mixing and thermodynamic process were disregarded. (3) The solution was carried out for a dynamic meridional section, first computing the velocity components at the equator, then extending calculation to higher latitudes of both hemispheres by different procedures. (4) An attempt was made to interpret the significance of Hidaka-Tsuchiya's formula of computing the east-west equatorial velocity. (5) The equatorial undercurrent can be located as an equatorial jet stream of a speed about 85cm/sec confind to a narrow band between 1°S to 1.5°N. (6) The location and size of equatorial countercurrent agree with observations quite well. (7) It can be shown theoretically that the east-west component of velocity is approximately geostrophic even in the 1° intervals next to the equator. (8) Upwelling velocities were determined and found to be an order of -10-3cm/sec. (9) Vertical circulation in a meridional section was determined by constructing the stream function between v and w. (10) A brief discussion was made as to the merit of the inertia terms in hydrodynamical equations of an equational flow.