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Terrence M. Joyce

Abstract

Using an inverse fast Fourier transform technique, numerical calculations have been made in order to study the contamination of moored temperature measurements of internal waves by passive temperature fine-structure. Vertical displacements and fine-structure spectra typical of the central North Atlantic have been modelled. Results indicate general agreement with the theory of Garrett and Munk for contamination of temperature autospectra with the amount of signal degradation depending upon the square of the Cox number. Studies of coherence loss with vertical sensor separation indicate no significant dependence for separations greater than several meters. Attempts at “decontaminating” autospectral and coherence calculations post facto given sufficient fine-structure statistics appear promising. Thus, it would seem that for central ocean work, fine-structure contamination, when important at all, can be partially removed during the spectral analysis of temperature time series.

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Terrence M. Joyce

Abstract

Methods for in situ calibration of acoustic-Doppler current profiles (ADCPs) are considered for measurement of absolute current profiles from a moving ship. Errors are of two types: sensitivity and alignment. Least square error estimates are given for experimental determination of both factors, for use in the “water track” or “bottom tracking” mode. Errors in the estimation of either factor may lead to large errors in derived water velocities, although the major contributions of the two factors arise from different sources and are approximately orthogonal to one another.

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Terrence M. Joyce

Abstract

The zonally-integrated curl of the wind stress across the equatorial Atlantic Ocean results in a southward transport of 10 × 106 m3 s−1 of water into the South Atlantic, which must be returned in a western boundary current Historical hydrographic and wind data have been used together with a simple steady model to calculate the vertical and horizontal structure of the southward Sverdrup transport. In contrast to the Pacific Ocean, the meridional currents are southward over most of the equatorial Atlantic with strongest flow in the central Atlantic near the surface; the major exception to the pattern is between 31°–39°W where near surface currants are northward. Estimates of the meridional heat transport associated with this steady wind-driven circulation are 0.6–0.8 × 1015 W. Climatological data also reveal an extraordinary correlation (0.86) between seasonally varying meridional wind stress and meridional sea surface slope in the central and western equatorial Atlantic, as if the ocean were responding in a quasi-steady manner to the seasonal changes in the winds.

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Terrence M. Joyce

Abstract

A scheme for calculation of cross-equatorial flow is presented which permits an estimation of meridional velocity at the equator from hydrographic station data and surface wind stress. It is offered to rationalize the observations that surface winds are neither zonal nor spatially uniform at the equator and that large-scale patterns exist in the meridional slope of the dynamic height field at the equator. Using historical data in the equatorial Pacific for surface wind stress and dynamic height, a large-scale estimate of meridional velocity is presented for the upper 2000 m with a zonal resolution of 10° of longitude. The flow across much of the central equatorial Pacific is northward in the upper 200 m and southward at greater depth. Southward near-surface currents are estimated east of 120°W, in agreement with direct current measurements at 110°W. The frictional component to the flow, although determined only in the vertically integrated sense, is included assuming an exponential decay from the surface. Over much of the basin the pattern of northward surface, southward subsurface flow is responsible for an overall net positive heat transport across the equatorial Pacific Ocean of 0.5–1.1 (×1015 W).

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Terrence M. Joyce

Abstract

An integral relationship is derived expressing the total dissipation of thermal variance by oceanic microstructure in terms of the large-scale forcing at the ocean surface by air/sea heat exchange. The net heat gain by the ocean over warm water and heat loss over cold water is evaluated using zonal averages of annual oceanic heat fluxes and temperatures between 60°N and 60°S. If thermal dissipation occurs in the upper ocean, with a scale depth of 600 m, the average dissipation χ is estimated to be 10−7 °C2 s−1. This value compares favorably with published observations of oceanic microstructure dissipation. The prediction is independent of any dynamical model of turbulent cascade from large to small scales in the ocean.

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Terrence M. Joyce

Abstract

The role of medium-scale interleaving of temperature and salinity in frontal regions is investigated and a model is presented in which a statistical equilibrium of the medium scale is achieved. Small-scale diffusion across intrusions, causing an attenuation of their T/S characteristics, is balanced by horizontal advection of heat and salt by the medium-scale motions. The “energy” source for the balance is the lateral variation in the temperature/salinity field associated with water mass transitions. Estimates of the cross frontal heat or sole exchange can he made based upon the intensity of the interleaving T/S fields. The lateral transfer is directly proportional to the vertical transports across intrusion boundaries by microscale processes. The same general principle for the enhancement of the cross frontal heat transfer by interleaving is similar to that achieved in automobile cooling systems by a radiator. The model, in effect, attempts to quantify our ignorance of lateral mixing of water masses. It is also shown to be a generalized statistical extension of longitudinal dispersion in pipes suggested by Taylor (1953).

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Terrence M. Joyce

Abstract

Interannual anomalies of climate variability in the eastern United States for the past 100+  yr have been studied for their spatial EOF structure, long-term changes, and the covariability with several climate indices: the Southern Oscillation index (SOI), North Pacific index (NPI), and North Atlantic Oscillation (NAO) index. Especially for air temperature, wintertime (December–February) variability is much more pronounced than summertime (June–August). The leading principal component (PC) of wintertime air temperature, which explains 70% of the interannual variance, is significantly correlated with the NAO, while the leading PC of wintertime precipitation correlates with the SOI. The spatial structure of the leading EOFs have a similar spatial character when compared to the correlation between the data and the climate indices, suggesting that the EOFs can be thought of as proxies for mapping the effects of climate indices upon the eastern United States. The effects of the SOI and NPI are generally the same; however, these two climate indices are not independent. The long-term sensitivity of the eastern U.S. climate to the Pacific indices seems only weakly dependent with time, whereas the NAO has grown considerably in importance with time since the beginning of the twentieth century. Surrogate temperature data from New Haven, Connecticut, has been used to extend this 100+  yr analysis back into the previous century, and the apparent long-term trend in the sensitivity to the NAO completely disappeared in the latter part of the nineteenth century. If a measure of potential predictability is the degree to which interannual climate covaries with these climate indices, the recent period (post 1960) may overestimate this predictability based on the long-term changes observed in sensitivity.

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Leif N. Thomas
and
Terrence M. Joyce

Abstract

Sections of temperature, salinity, dissolved oxygen, and velocity were made crossing the Gulf Stream in late January 2006 to investigate the role of frontal processes in the formation of Eighteen Degree Water (EDW), the Subtropical Mode Water of the North Atlantic. The sections were nominally perpendicular to the stream and measured in a Lagrangian frame by following a floating spar buoy drifting in the Gulf Stream’s warm core. During the survey, EDW was isolated from the mixed layer by the stratified seasonal pycnocline, suggesting that EDW was not yet actively being formed at this time in the season and at the longitudes over which the survey was conducted (64°–70°W). However, in two of the sections, the seasonal pycnocline in the core of the Gulf Stream was broken by an intrusion of cold, fresh, weakly stratified water, nearly saturated in oxygen, that appears to have been subducted from the surface mixed layer north of the stream. The intrusion was identified in three of the sections in profiles with a nearly identical temperature–salinity relation. From the western-to-easternmost sections, where the intrusion was observed, the depth of the intrusion’s salinity minimum descended by ∼90 m in the 71 h it took to complete this part of the survey. This apparent subduction occurred primarily on the upstream side of a meander trough, where the cross-stream velocity was confluent and frontogenetic. Using a variant of the omega equation, the vertical velocity driven by the confluent flow was inferred and yielded downwelling in the vicinity of the intrusion spanning 10–40 m day−1, a range of values consistent with the intrusion’s observed descent, suggesting that frontal subduction was responsible for the formation of the intrusion. In the easternmost section located downstream of the meander trough, the flow was diffluent, driving an inferred vertical circulation that was of the opposite sense to that in the section upstream of the trough. In transiting the two sides of the trough, the intrusion was observed to move toward the center of the stream between the downwelling branches of the opposing vertical circulations, resulting in a downward Lagrangian mean vertical velocity and net subduction. Hydrographic evidence of the subduction of weakly stratified surface waters was seen in the southern flank of the Gulf Stream as well. The solution of the omega equation suggests that this subduction was associated with a relatively shallow vertical circulation confined to the upper 200 m of the water column in the proximity of the front marking the southern edge of the warm core.

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Helen E. Phillips
and
Terrence M. Joyce

Abstract

This paper describes the oceanic variability at Bermuda between 1989 and 1999, recorded in two overlapping hydrographic time series. Station S and Bermuda Atlantic Time Series Study (BATS), which are 60 km apart, both show that a multidecadal trend of deep warming has reversed, likely as a result of the increased production of Labrador Sea Water since the early 1980s. In addition to recording similar changes in watermass properties, the two time series show similar mean vertical structure and variance as a function of pressure for temperature, salinity, and density above 1500 dbar. The seasonal cycles of these water properties at the two sites are statistically indistinguishable. The time series differ in the individual eddy events they record and in their variability below 1500 dbar. The two time series are used to investigate the propagation of eddy features. Coherence and phase calculated from the low-mode variability of density show westward propagation at ∼3 cm s−1 of wavelengths around 300–500 km. Satellite altimeter data are used to provide a broader spatial view of the eddy (or wave) field near Bermuda.

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Jiayan Yang
and
Terrence M. Joyce

Abstract

The seasonal variation of the North Equatorial Countercurrent (NECC) in the tropical Atlantic Ocean is investigated by using a linear, one-layer reduced-gravity ocean model and by analyzing sea surface height (SSH) data from Ocean Topography Experiment (TOPEX)/Poseidon (T/P) altimeters. The T/P data indicate that the seasonal variability of the NECC geostrophic transport, between 3° and 10°N, is dominated by SSH changes in the southern flank of the current. Since the southern boundary of the NECC is located partially within the equatorial waveguide, the SSH variation there can be influenced considerably by the equatorial dynamics. Therefore, it is hypothesized that the wind stress forcing along the equator is the leading driver for the seasonal cycle of the NECC transport. The wind stress curl in the NECC region is an important but smaller contributor. This hypothesis is tested by several sensitivity experiments that are designed to separate the two forcing mechanisms. In the first sensitivity run, a wind stress field that has a zero curl is used to force the ocean model. The result shows that the NECC geostrophic transport retains most of its seasonal variability. The same happens in another experiment in which the seasonal wind stress is applied only within a narrow band along the equator outside the NECC range. To further demonstrate the role of equatorial waves, another experiment was run in which the wind stress in the Southern Hemisphere is altered so that the model excludes hemispherically symmetrical waves (Kelvin waves and odd-numbered meridional modes of equatorial Rossby waves) and instead excites only the antisymmetrical equatorial Rossby modes. The circulation in the northern tropical ocean, including the NECC, is affected considerably even though the local wind stress there remains unchanged. All these appear to support the hypothesis presented in this paper.

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