Bruce A. Taft

Dynamic Heights and Zonal Geostrophic Transports in the Central Tropical Pacific during 1979–84

Abstract Dynamic height is calculated from XBT and surface salinity data in the central Pacific using a mean temperature–salinity (T–S) relation in the usual way below the thermocline but assuming isohaline water in the upper layer where the temperatures are isothermal. This scheme produces a better estimate of dynamic height than the use of a mean T–S relationship alone and produces significant improvements near the equator where small pressure gradients imply large geostrophic currents. During the El Nino of 1982–83, water of very low surface salinity was observed spanning the equator; this event is attributed both to extreme local rainfall and anomalous advection from the western Pacific. Geostrophic transports of the major surface currants are estimated for the period January 1979 through December 1984. The North and South Equatorial countercurrents are found to have the largest annual fluctuations, and the vertical displacements of the thermocline associated with these fluctuations are qualitatively ...

Dynamics of Seasonal and Intraseasonal Variability in the Eastern Equatorial Pacific

Abstract Time series measurements from surface moored buoys in the eastern equatorial Pacific are analyzed for the period 1983–86. The data, collected as part of the EPOCS and TROPIC HEAT programs, consist of currents, temperatures, and winds on the equator at 110°, 124.5° and 140°W. The purpose is to examine the dynamics of seasonal and intraseasonal variability in the upper 250 m from a diagnosis of the depth integrated zonal momentum (i.e., transport) equation. The principal conclusions of this paper are that 1) there is an approximate balance between mean zonal wind stress and depth integrated pressure gradient; nonlinear advection is significantly nonzero however and leads to an enhancement of eastward transport along the equator, 2) there is an interannual change in zonal wind stress and pressure gradient in which both approximately double over the record length; 3) at the annual cycle, zonal wind stress and depth integrated pressure gradient tend to balance, though the uncertainties are large and o...

The Local Dynamics of Eddies in the Western North Atlantic

The local dynamics of oceanic mesoscale eddies is a subject of enormous scope and detail. The scope of this paper, on the other hand, is restricted to a particular experiment, the POLYMODE Local Dynamics Experiment (LDE). The LDE is unique among mesoscale experiments to date in its high sampling density and variety of measurements, and thus is well suited to considerations of eddy dynamics in one locale. This uniqueness is, of course, one of degree not kind. Other mesoscale mapping experiments are POLYGON (USSR), MODE, Tourbillon (France), and the USSR component of POLYMODE. We, the authors, are the experimentalists and analysts of the LDE.

Variations of zonal currents in the central tropical Pacific during 1970 to 1987: Sea level and dynamic height measurements

The annual and interannual variations of the major zonal currents of the central tropical Pacific for the period 1970–1987 are described by analyzing two data sets: sea level data from a few selected island stations and ship-of-opportunity expendable bathythermograph (XBT) observations. The annual cycles of geostrophic surface current indices (dynamic height or sea level difference between the ridge and the trough defining the current) in the North Equatorial Current (NEC) and North Equatorial Countercurrent (NECC) are found to be in phase (maxima in November), to be strong, and to explain about 50% of the total variance. The annual cycle of volume transport relative to 400 dbar in the NECC is also strong, but NEC volume transport has a weak annual cycle. The discrepancy between the NEC surface current and the transport is due to the variation of the surface current being larger than that of the deep currents. The annual cycle of the transport of the South Equatorial Current (SEC) is 6 months out of phase with that of the NEC and NECC. Phases of the annual cycles of the sea level and dynamic height indices of flow agree well in general, but there are significant differences between the annual amplitudes of the two types of measurements. These differences arise from a variety of sources: dynamic height reference level assumptions, longitudinal spacing of sea level gauges, meridional resolution of sea level array, and different signal to noise ratios for the two sets of indices. The NECC shows marked flow maxima during the peaks of the strong Ninos of 1972 and 1982, which were followed by near disappearance of the current the following year; these fluctuations are clearly present in both the surface current indices and the volume transport. In 1976 and 1986–1987 (moderate Ninos) the same pattern is not marked. The El Nino signal in NEC transport is weak. There was a strong rise of equatorial sea level during the early phases of the 1972 and 1982 Ninos followed by a fall the next year. This resulted in a decrease of the transport of the SEC in the first year followed by an increase to a maximum during the second year; the maximum coincides with a minimum in NECC transport.

Systematic errors in expendable bathythermograph (XBT) profiles

Abstract Intercomparison of 306 casts in the tropical Pacific of Sippican T-4 (∼450-m depth) and 139 casts in the Sargasso Sea of Sippican T-7 (∼750 m depth) expendable bathythermographs (XBTs) and calibrated Neil Brown conductivity-temperature-depth (CTD) profiles has been performed to detect errors in XBT temperature and depth. Also included are results from previous XBT-CTD intercomparisons. Systematic temperature errors were determined in depth ranges where the vertical temperature gradient was small and the contribution from depth errors would be small. In all cases the XBT temperatures were systematically higher than CTD temperatures. Isotherm depths from XBTs were systematically less than CTD depth below an intermediate depth. The depth error dependence is a characteristic of the instrument, and for the T-7 XBT it is repeatable in at least two regions of differing thermal structure. The standard deviation of the depth error is approximately 10 m for both T-4 and T-7 instruments. The accuracy of XBT data can be significantly improved by periodic XBT-CTD intercomparison. An algorithm is given to correct the T-7 XBT systematic depth error.

Detection, Structure, and Origin of Extreme Anomalies in a Western Atlantic Oceanographic Section

Abstract Ten anomalous water parcels were detected in the water column of a western Atlantic oceanographic section (0–5550 m; 70°W; 23–33°N). The parcels had extreme properties lying either two standard deviations from historical mean values, or estimated origins lying 2000 km from the oceanographic section. Detection, structure, and origin of the parcels were determined from analyses on isobaric and isopycnal surfaces using eight kinds of measurements temperature, salinity, oxygen, light scattering, silicate, phosphate, nitrate, and tritium. The parcels originated from seven of the major water masses comprising the North Atlantic Ocean. As observed along the oceanographic section the parcels had the following average (range) characteristics vertical pressure extent = 650 db (100–150 db); horizontal extent = 130 km (30–260 km); and distance to origin = 2300 km (400–5200 km). A population density equaling approximately one parcel per 100 km was obtained as the number of parcels (10) divided by the length o...

Water Mass Structure during the POLYMODE Local Dynamics Experiment

Abstract The seven intensive hydrographic surveys of temperature, salinity and oxygen of the POLYMODE local Dynamics Experiment (LDE) (15 May–15 July 1978) were carded out in a 200 km-wide octagonal region centered at 31°05′N, 69°30′W; this location was within the southern portion of the Gulf Stream Recirculation. The vertical structure of the mean and standard deviations of water properties and physical parameters for the experiment are presented. Comparisons of LDE and climatological statistics at 31°N, 70°W showed that the profiles of mean quantities were not markedly different; however, LDE eddy potential energy in the water column above 2000 m was roughly one-half to one-third the long-term climatological values. There were peaks in variability of salinity on potential density surfaces in the 18°C-water, the midthermocline (850 db) and the lower thermocline (1400 db). The 18°C-peak was associated with a marked bimodal salinity distribution, the midthermocline peak with extreme outliers (positive and ...

Small Water-Property Transporting Eddies: Statistical Outliers in the Hydrographic Data of the POLYMODE Local Dynamics Experiment

Abstract Hydrographic measurements from the POLYMODE Local Dynamics Experiment (LDE; 31°N, 69.5°W; May–July 1978) are examined for highly anomalous, outlying observations of salinity, oxygen and vortex stretching on isopycnal surfaces. The methodology which defines outliers in the observations is based on examination of gaps in histograms of these water properties on each isopycnal. These outliers are indicative of vertically confined (over 20–2000 m), sub-mesoscale eddies (diameters of 25–50 km) at the LDE site. Evidence for 31 eddies is found at the LDE site over the two months of the experiment. These features are described individually and characterized by their core water-properly signal and dynamical structure. Eighteen features are found to be anticyclonic lenslike structure, eight have no detectable dynamic signal and only four are characterized by cyclonic circulation. The work establishes that sub-mesoscale eddies in a small portion of the western North Atlantic exhibit a variety of distinct wat...

The Mesoscale Spatial Structure and Evolution of Dynamical and Scalar Properties Observed in the Northwestern Atlantic Ocean during the POLYMODE Local Dynamics Experiment

Abstract Dynamical properties examined in this paper are the dynamic height and the pressure on an isopycnal. Scalars are the salinity and the oxygen concentration on an isopycnal. The sea surface temperature and salinity are also examined. These properties are obtained from the spatially uniform and densely sampled hydrographic surveys conducted during a two-month period of the POLYMODE Local Dynamics Experiment in 1978 near 31°N, 69.5°W. The spatial maps of the dynamic height and the pressure of an isopycnal show that the baroclinic current in this area sometimes intensifies to a jetlike flow and at other times has the shape of elongated eddies. The current flows primarily in the NE–SW direction. Westward propagation occurs but varies in time and with depth. Eddies that transport water properties are also observed in this area. The salinity and the oxygen on an isopycnal are found to be correlated with itself and with each other from the surface to the 18°C water layer, within a 200-db range in the ther...

Absolute Geostrophic Velocity Determination from Historical Hydrographic Data in the Western North Atlantic

Abstract Beta-spiral calculations are presented using averaged western North Atlantic hydrographic data from the period 1914–73. Profiles of long-term mean geostrophic flow relative to ISM db along 70°W from 26 to 32°N are shown. Absolute reference velocities at 1500 db are obtained using two beta-spiral methods and data from three depth ranges. Results indicate a dependence of the reference velocity on the depth range used in the calculation. Northeastward reference velocities are found from upper and middle thermocline data while near-zero velocities are obtained using data from the lower thermocline and below. Evidence that mesoscale variability plays an important role in the vorticity dynamics of the upper kilometer of the western North Atlantic is discussed. It is concluded that only the velocities derived from beta-spiral analyses at the deeper levels are valid in terms of the model assumptions. The resulting absolute velocity profiles are compared with field observations, the geophysical inverse me...