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Geoffrey Trivers
and
Mark Wimbush

Abstract

There is often an approximately linear relationship between various water-column integrals, in particular between surface dynamic height anomaly ΔD and acoustic round-trip travel time τ. Consequently, the record from an inverted echo sounder, which measures τ, can be interpreted in terms of ΔD. Nevertheless, the slope m of this linear relation is not everywhere well defined, and varies from place to place. This study seeks to establish where, in the extratropical North Atlantic, one can reasonably assume a linear relation between ΔD and τ, and for these regions compute m. Using climatological atlas data and historic hydrographic data, it is shown that a well-defined, linear relation exists between ΔD and τ in a region centered on the Gulf Stream and extending from the northern Sargasso Sea almost to Ireland. Where m is well defined, it is negative, and its value is usually similar to that associated with first-baroclinic-mode excitation. Its magnitude generally decreases with increasing latitude. The value of m typically ranges from −40 dyn m s−1 in the northern Sargasso Sea to −20 dyn m s−1 in the North Atlantic Current. In the Gulf Stream it is typically between −30 and −35 dyn m s−1.

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Li Li
and
Mark Wimbush

Abstract

Bottom temperature time series recorded beneath the Gulf Stream at 265 and 589 m depth off the Georgia coast are compared with simultaneous time series of main thermocline depth determined from inverted echo sounder and bottom pressure gauge records at the same sites. Bottom temperature is found to be coherent with vertical displacement of the thermocline, suggesting that bottom temperature under the Gulf Stream front is a potentially useful indicator of Gulf Stream displacement. Additional evidence is provided by the similarity of bottom temperature and thermocline depth coherences with longshore current at the shelf break. Bottom temperature at the deeper station appears to be the better indicator of Gulf Stream meandering for periods longer than five days.

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Marlene Noble
,
Bradford Butman
, and
Mark Wimbush

Abstract

Comparison of several years of current observations on the southern flank of Georges Bank with nearby wind data shows that the wind–current coupling is primarily between longshelf wind stress and longshelf current. The strongest wind–currnet coupling occurs in winter, when the water column is homogeneous. The weakest coupling is in late summer and early fall, when the water column is highly stratified. The coherence and transfer coefficient between longshelf wind and longshelf current is highest for periods between 4 and 12 days, decreasing both for longer periods (out to 56 days) and shorter periods (down to 2 days). Models of the wind–current coupling indicate that a highly damped resonance may exist on Georges Bank and that a smaller current response is expected when the water column is stratified. The observations also indicate that the wind-driven currents on Georges Bank are strongly controlled by friction. The near-surface current moves to the right of wind stress and there is a spring–neap modulation of the wind–current transfer coefficient caused by the modulation of the bottom stress associated with the spring–neap tidal cycle. The longshelf current is linearly related to wind stress and responds almost symmetrically to wind forcing.

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Charles James
,
Mark Wimbush
, and
Hiroshi Ichikawa

Abstract

An array of seven inverted echo sounders was moored along and across the Kuroshio in the East China Sea for more than one year. The data from this array show evidence of energetic meanders with periods of 7, 11, and 16 days. The respective phase velocities of these meanders are 28, 20, and 17 km day−1 downstream. The 7- and 16-day waves are intermittent, but the 11-day waves are present throughout the deployment.

The instability responsible for these waves is investigated with a spectral numerical model applied to a background state representing the Kuroshio in this region. The fastest-growing instability from the model has e-folding growth time of 2 days, period of 12 days, and phase velocity of 18 km day−1 downstream. It appears to be a close representation of the 11-day wave seen in the observational data.

Such a model has been previously used to represent meanders in the Gulf Stream at similar latitudes off the east coast of the United States. The Kuroshio meanders have approximately half the phase velocity and twice the period of the Gulf Stream meanders. To investigate the reasons for these differences, the flow and topography of the model background state were varied. The slower phase velocity and longer period of the Kuroshio meanders appear to be consequences of the deeper shelf and lower transport, with a modifying effect due to the difference in cross-shelf positioning of the current core (more over-the-shelf in the case of the Kuroshio).

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Laury Miller
,
D. Randolph Watts
, and
Mark Wimbush

Abstract

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.

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Regina R. Rodrigues
,
Lewis M. Rothstein
, and
Mark Wimbush

Abstract

In this study, a reduced-gravity, primitive equation OGCM is used to investigate the seasonal variability of the bifurcation of the South Equatorial Current (SEC) into the Brazil Current (BC) to the south and the North Brazil Undercurrent/Current (NBUC/NBC) system to the north. Annual mean meridional velocity averaged within a 2° longitude band off the South American coast shows that the SEC bifurcation occurs at about 10°–14°S near the surface, shifting poleward with increasing depth, reaching 27°S at 1000 m, in both observations and model. The bifurcation latitude reaches its southernmost position in July (∼17°S in the top 200 m) and its northernmost position in November (∼13°S in the top 200 m). The model results show that most of the seasonal variability of the bifurcation latitude in the upper thermocline is associated with changes in the local wind stress curl due to the annual north–south excursion of the marine ITCZ complex. As the SEC bifurcation latitude moves south (north) the NBUC transport increases (decreases) and the BC transport decreases (increases). The remote forcing (i.e., westward propagation of anomalies) appears to have a smaller impact on the seasonal variability of the bifurcation in the upper thermocline.

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Ho Jin Lee
,
Jae-Hun Park
,
Mark Wimbush
,
Kyung Tae Jung
,
Chan Joo Jang
,
Yang-Ki Cho
,
Young-Kyo Seo
, and
Jong Ho Nam

Abstract

Although tides are believed to be the most important source for diapycnal mixing in the ocean, few studies have directly simulated open-ocean circulation including tides. Because the East/Japan Sea (EJS) has been considered to be a “miniature ocean,” tidal effects on the intermediate water of the EJS are investigated by using an eddy-resolving ocean general circulation model that can take account of M 2 and K 1 tides as well as oceanic flows. The simulated temperature and salinity in the intermediate layer are significantly improved by including tides. The improvement results from the combined effect of two internal tides. The M 2 internal tide, propagating into the interior of the EJS, enhances vertical mixing and brings watermass characteristics closer to those observed. The K 1 internal tide, trapped along the coast, induces southwestward flow along the Russian coast in the upper layer and thereby enhances the so-called Liman Current, which transports fresh waters with density conducive to the ventilation of intermediate waters in the EJS.

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