Dynamic Height Differences Research Articles

Regulation of South China Sea throughflow by pressure difference

AbstractSea Surface Height (SSH) data from the European Centre for Medium‐Range Weather Forecasts‐Ocean Reanalysis System 4 (ECMWF‐ORAS4) are used to determine the pressure difference in connection with variability of the South China Sea ThroughFlow (SCSTF) from 1958 to 2007. Two branches of SCSTF, the Karimata‐Sunda Strait ThroughFlow (KSSTF) and the Mindoro Strait ThroughFlow (MSTF), are examined. Using the ensemble empirical mode decomposition method (EEMD), time series of pressure difference and volume transport are decomposed into intrinsic mode functions and trend functions, with the corresponding variability on different time scales. Pressure difference agrees with the KSSTF volume transport on decadal time scale; while for the MSTF, pressure difference varies similarly with volume transport on interannual time scale. Separating the dynamic height difference into the thermal and haline terms, for the KSSTF more than half of the dynamic height difference (32 cm) is due to the thermal contributions; while the remaining dynamic height difference (23 cm) is due to the haline contributions. For the MSTF, the dynamic height difference (29 cm) is primarily due to the thermal contribution (26 cm).

Geostrophic and ageostrophic circulation of a shallow anticyclonic eddy off Cape Bojador

AbstractA shallow mesoscale anticyclonic eddy, observed south of the Canary Islands with satellite altimetry, has been intensively studied with multiparametric sampling. Hydrographic data from a CTD installed on an undulating Nu‐shuttle platform reveal the presence of a mesoscale anticyclonic eddy of ∼125 km diameter. The difference in sea level anomaly (SLA) between the interior and the edge of the eddy, as determined from altimetry, is ∼15 cm, which compares well with the maximum dynamic height differences as inferred using a very shallow reference level (130 m). Further, the associated surface geostrophic velocities, of about 35 cm s−1 in the northeast and southwest edges of the eddy, are in good agreement with direct velocity measurements from the ship. Deep rosette‐CTD casts confirm that the structure is a shallow eddy extending no deeper than 250 m before the fusion with another anticyclone. The SLA‐tendency (temporal rate of change of sea surface height) indicates a clear northwestward migration during the two first weeks of November 2008. Applying an eddy SSH‐based tracker, the eddy's velocity propagation is estimated as 4 km d−1. Use of the QG‐Omega equation diagnoses maximum downward/upward velocities of about ±2 m d−1. The instability of the Canary coastal jet appears to be the mechanism responsible for the generation of the shallow anticyclonic eddy.

Three inflow pathways of the Indonesian throughflow as seen from the simple ocean data assimilation

The SODA product is used to investigate three Indonesian throughflow (ITF) branches: the flow through the Makassar Strait; through the South China Sea; and through the eastern Indonesian basins. The results reveal strong interannual variation in the Makassar Strait and the eastern Indonesian basins throughflow. Inspection of vertically integrated dynamic height (0–1000 db), a proxy of transport function, suggests that this interannual variation can be traced to the New Guinea Coastal Current, indicative of a strong influence of the South Pacific. The vertically integrated dynamic height along the south Java coast is related to variation in the North Pacific and in particular near the east coast of Mindanao Island, whereas the vertically integrated dynamic height along the coast of West Australia is related to variation in the South Pacific, and in particular near the coast of New Guinea. The integrated dynamic height difference between the Java and New Guinea coast appears to be a good proxy of ITF transport on the interannual time scale. Regression analysis shows a phase dependence of the three ITF pathways on the Nino3.4 index. Decoupling of current anomalies between the surface and subsurface layers is identified in the developing and mature phase of El Nino, reflecting different effects of local and remote forcing through oceanic pathways at the Makassar Strait and eastern Indonesian basins.

The Kuroshio East of Taiwan: Moored Transport Observations from the WOCE PCM-1 Array

Observations from the WOCE PCM-1 moored current meter array east of Taiwan for the period September 1994 to May 1996 are used to derive estimates of the Kuroshio transport at the entrance to the East China Sea. Three different methods of calculating the Kuroshio transport are employed and compared. These methods include 1) a “direct” method that uses conventional interpolation of the measured currents and extrapolation to the surface and bottom to estimate the current structure, 2) a “dynamic height” method in which moored temperature measurements from moorings on opposite sides of the channel are used to estimate dynamic height differences across the current and spatially averaged baroclinic transport profiles, and 3) an “adjusted geostrophic” method in which all moored temperature measurements within the array are used to estimate a relative geostrophic velocity field that is referenced and adjusted by the available direct current measurements. The first two methods are largely independent and are shown to produce very similar transport results. The latter two methods are particularly useful in situations where direct current measurements may have marginal resolution for accurate transport estimates. These methods should be generally applicable in other settings and illustrate the benefits of including a dynamic height measuring capability as a backup for conventional direct transport calculations. The mean transport of the Kuroshio over the 20-month duration of the experiment ranges from 20.7 to 22.1 Sv (1 Sv ≡ 106 m3 s−1) for the three methods, or within 1.3 Sv of each other. The overall mean transport for the Kuroshio is estimated to be 21.5 Sv with an uncertainty of 2.5 Sv. All methods show a similar range of variability of ±10 Sv with dominant timescales of several months. Fluctuations in the transport are shown to have a robust vertical structure, with over 90% of the transport variance explained by a single vertical mode. The moored transports are used to determine the relationship between Kuroshio transport and sea-level difference between Taiwan and the southern Ryukyu Islands, allowing for long-term monitoring of the Kuroshio inflow to the East China Sea.

Inertial Embedded Fronts

Abstract Many recent observations have described fronts in the interior of the ocean at locations far away from any lateral boundaries. Some of these fronts are observed to be associated with considerable mass transports, which suggests that they participate importantly in setting the water mass structure of the ocean interior, and represent considerable local departures from linear Sverdrup dynamics. In this paper, a simple analytic theory of interior fronts is developed. The main features of this theory are that the fronts are highly inertial and anisotropic, and reside on the edge of a somewhat larger scale interior inertial recirculation. The recirculation is taken to be modonlike; the dynamic height difference across the edge of the recirculation supports an interior jet, which is clockwise around the edge of the recirculation and carries water from the subpolar into the subtropical gyre. Unlike in previous theories of interior fronts, all of the transports, both in the large-scale and the fronts, ar...

A shallow meddy (a smeddy) from the secondary Mediterranean salinity maximum

A smeddy, or shallow meddy with temperature and salinity core characteristics of the Secondary Mediterranean Salinity Maximum (SMSM), was found near 36.75°N, 12.5°W in March 1992, west of Cape St. Vincent and some 600 km from the Strait of Gibraltar. A detailed survey defined the temperature, salinity, nitrate, silicate, oxygen, and light transmission structure of the smeddy. The smeddy core had a maximum salinity of 36.483 psu at a depth of 775 m, and a maximum temperature of 13.55°C at a depth of 725 m. The salinity anomaly was 9 standard deviations from the mean at a depth of 600 m. The property distributions suggest that about 0.5 km of a central water column (∼500–1000 m) was traveling with the smeddy, though the dynamic influence extended from the surface to a depth of about ∼1.5 km. The velocity field was derived from dynamic height differences, the gradient equation, a drogued Argos buoy, and acoustic Doppler current profiler (ADCP) measurements. At the core depth of ∼700 m a number of well‐defined structures were found that characterized the horizontal influence of the smeddy. Surrounding an inner core (temperature > ∼ 13.25°C, salinity > ∼36.4 psu) of radius about 7 km, in near solid body rotation (with center period ∼3.7 days), there was a region of maximum azimuthal currents (∼20 cm/s) at a radius of ∼12 km. At a radius of ∼17 km, horizontal gradients of properties reached maximum values. This water mass boundary was thought to define the horizontal extent of the water actually traveling with the smeddy, giving it an aspect ratio of 1.5%. The smeddy was observed to move about 150 km southward over a period of ∼30 days (∼6 cm/s), passing from the Tagus Abyssal Plain to the Horseshoe Abyssal Plain. Remote sensing and other hydrographic data are used to suggest one route whereby water found in the core of the isolated eddy reached the Horseshoe Abyssal Plain.

Can the weak surface currents of the Cape Verde frontal zone be measured with altimetry?

Three data types are compared in the low‐current‐velocity regime in the southeastern North Atlantic, between 12°N and 30°N, 29°W and 18°W: Geosat altimetric sea level and derived surface geostrophic velocities, shallow current meter velocities, and dynamic heights derived from hydrographic data from cruises 4, 6, and 9 of the research vessel Meteor. The four current meter daily time series, at depths around 200 m, were smoothed over 1 month; the altimetric geostrophic velocities were computed from sea surface slopes over 142 km every 17 days. The correlation coefficients between the current meter and altimetric geostrophic velocities range between 0.64 and 0.90 for the moorings near 29°N but between 0.32 and 0.71 for the two around 21°N; the associated rms discrepancies between the two measurement types range between 1.5 and 4.4 cm/s, which is 49% to 127% of the rms of the respective current meter time series. Dynamic heights relative to 1950 dbar for the months of November 1986 (dM4), November 1987 (dM6), and February 1989 (dM9) were computed from Meteor cruises 4, 6, and 9. Both dynamic heights and altimetric heights (hM4, hM6, hM9) were averaged over 1° boxes for the duration of each cruise. Differences dM4 − dM6 and dM9 − dM6 were computed only at bins where at least one station from both cruises existed. Assuming that dynamic heights d in dynamic centimeters are equivalent to sea level h in centimeters, the standard deviation σ of the differences ((hM4 − hM6) − (dM4 − dM6)) and corresponding M9 − M6 values was 2.1 cm. This value (squared) is only 13% of the (5.8 cm)2 variance of the dynamic height differences and is indistinguishable from the 2.7‐ to 5.6‐cm natural variability of sea level in the area expected between the times when the ship and the satellite sampled the ocean. The areally averaged discrepancy for M9 − M6 was only 0.7 cm, but the corresponding value for M4 − M6 was 5.2 cm. A systematic difference between the water vapor corrections used before and after July 1987 is responsible for the M4 − M6 difference. The average M4 − M6 discrepancy is only 0.1 cm using the Fleet Numerical Oceanography Center correction, with a standard deviation of 3.1 cm. In spite of the underlying differences in sampling and physics, including unknown barotropic components not included in our hydrographic dynamic heights, and in data errors, including water vapor, ionospheric, and orbital effects on the altimetry, consistent interannual changes of the mean sea level from the independently obtained altimetric and hydrographic data sets are obtained, and correlated seasonal changes in surface currents are observed with both altimetry and current meters.

An Interannual Study of the Atlantic North Equatorial Countercurrent

Abstract Beginning in 1983, the dynamic height difference at the sea surface defining the countercurrent trough at 38°W was continuously observed by Inverted Echo Sounders at 3°N and 9°N for six years and ten months. The amplitude of the average annual signal for that time period was 36 cm, and the range of the interannual variability was 25 cm. Thus, the annual signal dominates as expected, but the interannual variability is of comparable magnitude. Two years, 1983 and 1987, are noted as years of sharp increase in the interannual signal (periods immediately following El Nino events in the Pacific). From a derived relationship for geostrophic volume transport, it is estimated that every year the North Equatorial Countercurrent attained a transport of 20 Sv (≡ 106 m3 s−1) for at least one month, while in the cited years it sustained this value for over half the year. The output of an unbounded reduced-gravity model, forced by monthly averaged wind stress, is compared to the observations. The model can be t...

Baroclinic transport of the Kuroshio Extension between 141°E and 165°E

Using a water‐mass‐based climatology of temperature and salinity, geostrophic transports derived from dynamic height differences across the width of the Kuroshio Extension are presented. The dynamic height differences are obtained by calculating dynamic heights in the warmer water mass south of the Kuroshio and the cooler water mass north of the Kuroshio. The mean baroclinic transport is examined between 141°E and 165°E relative to several pressure levels. The baroclinic transports are sensitive to the selection of the northern and southern boundaries. The baroclinic transports increase from 141°E to about 145°E and decline east of 145°E. The computed seasonal variation of the longitudinally averaged baroclinic transport shows a minimum during spring and a broad maximum from July to November.

Monitoring the strength of equatorial currents from XBT sections and sea level

Transequatorial expendable bathythermograph (XBT) sections in the central Pacific are used to compute dynamic height profiles by means of temperature‐sigma t relations. Dynamic height differences and geostrophic transport indices are derived, and it is demonstrated that they correlate extremely well for each of the four equatorial currents, making surface slope an excellent indicator of the geostrophic transport. This is due to the strong baroclinity of the equatorial current system. Indices of the strength of equatorial currents derived from computations of dynamic height, from sea level observations, and from a two‐layer approximation agree very well, which allows the use of each method for monitoring. It is concluded that low‐frequency variations of the equatorial current system may be suitably monitored by such simple methods as repeated XBT sections or sea level observations.