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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

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

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

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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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

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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Bo Qiu and Terrence M. Joyce

Abstract

Twenty-two years (1967–88) of hydrographic data collected by the Japan Meteorological Agency along the 137°E meridian and surface wind data compiled by Florida State University (FSU) were analyzed to study the interannual variability in the western North Pacific.

In the midlatitude region north of 22°N, the dominant signal in the dynamic height field was the interannual path variations of the Kuroshio. Whereas the eastward transport of the Kuroshio itself had no significant changes between the straight-path and meander-path years, the net transport of the Kuroshio system including recirculations showed a 30% increase during the meander-path years. In the straight-path years when the net transport was small, the Kuroshio tended to take a straight path with a strong recirculation developed to the south. The interannual path variations of the Kuroshio strongly influenced the water-mass movement in the midlatitudes. During the Kuroshio meander years, we found that a significant portion of the North Pacific Intermediate Water east of the Kuroshio meander was blocked from subducting farther westward. In the middepth layer of 1500–2500 m, analysis of the θ–S relation revealed a water-mass movement negatively correlated to the upper-layer Kuroshio path changes, implying a possible compensating flow in the middepth layer for the cold-core eddy emerging north of the Kuroshio.

In the low-latitude region along 137°E, fluctuations in the surface height anomaly field had a meridionally coherent structure, and large surface height drops coincided with the ENSO events in the tropical Pacific. Accompanying the surface height drops in the ENSO years was an increase in the transport of both the North Equatorial Current (NEC) and the North Equatorial Countercurrent (NECC) and a southward shift in the boundary of the NEC and NECC. Based on the FSU surface wind data, we found that these interannual fluctuations of the NEC and the NECC were highly correlated to the Sverdrup transport fluctuations estimated from the basinwide wind-stress curl field. Using a reduced-gravity model and simplified patterns of wind forcing, we showed that this high correlation came about because the center of the interannual signal of the wind-stress curl field is close to the western Pacific (near the date line) and because the thermocline tilt in the NEC region attenuates the strong latitude dependence of the phase speed of the long baroclinic Rossby wave.

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Terrence M. Joyce and Paul Robbins

Abstract

A long, by oceanographic standards, time series of hydrographic observations at Bermuda was begun in 1954 and continues to the present. Analysis of this dataset has shown the temperature and salinity variations on interannual timescales to be largely independent in the surface layer (0–200 dbar), where integral timescales for salinity are 50% longer than for temperature and surface salinity changes are correlated with decadal changes in the 18°C Water production. Temperature/salinity anomalies are highly correlated in the thermocline where the interannual variability at that level is accountable to either vertical oscillations of the thermocline with amplitudes of ±50 m or meridional oscillations of the horizontal gradient set up at the southern edge of the recirculation gyre of ±300 kim. Salinity changes in the deepest layer observed at Bermuda station “S” (1500–2500 dbar) are uncorrelated with temperature, masked by measurement errors in the early years of the time series. Inclusion of earlier data of opportunity near Bermuda has permitted the time series of temperature to be extended backward 32 years to 1922. Over the entire 73-yr period, temperature is characterized by decadal timescale changes at all depths, but notably the deepest layer (1500–2500 dbar) also shows a long-term, secular warming trend with a rate of increase of about 0.5°C/century. This rate of temperature rise amounts to a sequestration of 0.7 W m−2 of “excess” heat flux in the deep water and, due to the thermal expansion of seawater, should produce a local rise in steric sea level of approximately 7 cm/century. A comparison of two meridional sections taken through the western subtropical gyre passing just to the west of Bermuda during the IGY and 1985 shows that this long-term temperature increase is widespread, with spatially averaged values comparable to the long-term climatological trend at Bermuda, and is associated with a salinity increase in the upper deep water.

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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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