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Tangdong Qu and Eric J. Lindstrom

Abstract

Time-averaged circulation is examined using historical hydrographic data near the Australia and Papua New Guinea coast in the Pacific. By averaging the data along isopycnal surfaces in a 0.5° × 0.5° grid, the authors are able to show many detailed phenomena associated with the narrow western boundary currents, including the vertical structure of the bifurcation latitude of the South Equatorial Current (SEC) and the connection between the Solomon and Coral Seas. The bifurcation latitude of the SEC is found to move southward from about 15°S near the surface to south of 22°S in the intermediate layers. The origin of the Great Barrier Reef Undercurrent (GBRUC) is identified to be at about 22°S. Farther to the north, the GBRUC intensifies underlying the surface East Australian Current, and merges with the North Queensland Current (NQC) at about 15°S. The NQC turns eastward to flow along the Papua New Guinea coast and feeds into the New Guinea Coastal Undercurrent (NGCUC) through the Louisiade Archipelago. Further analysis shows that there is a strong water property connection between the Coral and Solomon Seas, confirming the earlier speculation on the water mass origins of the NGCUC.

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Tangdong Qu and Eric J. Lindstrom

Abstract

The northward intrusion of Antarctic Intermediate Water (AAIW) is examined using historical data combined with synoptic observations from a repeated hydrographic section in the western Pacific Ocean. The results of this analysis suggest that AAIW is traced as a salinity minimum to only about 15°N via the New Guinea Coastal Undercurrent and the Mindanao Undercurrent. There is no northward extension of AAIW farther to the north along the western boundary. Although relatively high oxygen water does exist in the Okinawa Trough, it is connected with high-oxygen water in the South China Sea (SCS) through the Luzon Strait but not from the south as an extension of AAIW. Local circulation seems to play an essential role in localizing the oxygen maximum in the SCS. Evidence exists to suggest that high-oxygen water enters the SCS as part of the Pacific deep water around the still depth (∼2000 m) of the Luzon Strait; from there, part of it upwells and is entrained into shallower isopycnal surfaces by vertical mixing and eventually flows back to the Pacific through the Luzon Strait at depths of AAIW.

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Curtis C. Ebbesmeyer and Eric J. Lindstrom

Abstract

Two distinct types of 18°C water (Subtropical Mode Water) were observed during the POLYMODE Local Dynamics Experiment (LDE; May–July 1978; 31.0°N, 69.5°W). These were revealed on isopyncals by salinity histograms which were bimodal. Salinity was highly correlated with oxygen, vortex stretching, and 17.5°–18.5°C thickness. The correlations are positive between salinity and both oxygen and thickness and negative between salinity and vortex stretching. The origins of the two water types are deduced using a variety of measurements in the Sargasso Sea including apparent oxygen utilization, vortex stretching and salinity. It is found that the modes were formed approximately 16 months prior to the LDE during the severe winter of 1976/77. Sharp horizontal salinity gradients between the two LDE water types are comparable to those observed more than a year earlier, and the spatial scale (∼100 km) of the regions of saline mode water is smaller in the LDE than immediately after the 1976/77 winter (∼200 km). Thew observations suggest that the characteristics of newly formed 18°C water may persist for several years despite strong mesoscale stirring in the Gulf Stream Recirculation Zone.

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Eric J. Lindstrom and Bruce A. Taft

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 water types at their core. Based on the characteristics described here, McDowell has deduced origins for some of the eddies in the eastern basin of the North Atlantic, the southern North Atlantic and Labrador Sea. The distant sources suggest that they may have lifetimes of several years. The dynamic characteristics of the eddies as described here are used in an assessment of theoretical implications of sub-mesoscale vortices by McWilliams.

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John R. Bennett and Eric J. Lindstrom

Abstract

An empirical forced wave model of currents and thermocline displacements in the coastal zone of Lake Ontario is derived from data from the International Field Year for the Great Lakes (1972). The model consists of three linear wave equations for predicting the depth of the thermocline, its slope and the longshore volume transport from the wind. The empirical phase speeds are consistent with internal Kelvin wave and topographic wave theory and the response to a unit longshore wind stress is consistent with cross-section models of long lakes.

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Eric Gilleland, Johan Lindström, and Finn Lindgren

Abstract

Image warping for spatial forecast verification is applied to the test cases employed by the Spatial Forecast Verification Intercomparison Project (ICP), which includes both real and contrived cases. A larger set of cases is also used to investigate aggregating results for summarizing forecast performance over a long record of forecasts. The technique handles the geometric and perturbed cases with nearly exact precision, as would be expected. A statistic, dubbed here the IWS for image warp statistic, is proposed for ranking multiple forecasts and tested on the perturbed cases. IWS rankings for perturbed and real test cases are found to be sensible and physically interpretable. A powerful result of this study is that the image warp can be employed using a relatively sparse, preset regular grid without having to first identify features.

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Eric J. Lindstrom, Curtis C. Ebbesmeyer, and W. Brechner Owens

Abstract

In the POLYMODE Local Dynamics Experiment (31°N; 69.5°W) a small cyclonic eddy was discovered and mapped. The eddy was characterized by an upward doming of isotherms in the upper part of the main thermocline. The dynamical signal extended from the surface downward to a pressure of 800 db and outwards to a radius of 25–30 km. There was no deep-water property signal associated with the eddy, but a layer (∼10m thick) of very low salinity water was observed at the sea surface immediately above the feature. The salinities in the surface layer were lower than any reported in the historical hydrographic data for the region. Rainfall is discounted as a possible source of this signal because of the layer's thickness and horizontal scale. Alternatively the closest source for the low salinity is in the Slope Water, some 500 km away. The eddy may have originated by the splitting near several Gulf Stream rings. This mechanism could also account for the unusually low surface salinities since rings may trap Slope Water and transport it into the Sargasso Sea.

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Colin Y. Shen, James C. Mcwilliams, Bruce A. Taft, Curtis C. Ebbesmeyer, and Eric J. Lindstrom

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 thermocline and an ∼400-db range centered near 1500 db. There are periods during which the salinity and the oxygen as well as the sea surface properties are correlated with the near-surface current structure; the correlation is not found at deeper depths. These scalar properties, which are interpreted as tracers, are more vigorously “stirred” in and above the thermocline than below, where the motion is more waveline. The net movement of water mass indicated by these tracers is due west through most of the water column during the period of the experiment.

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Tangdong Qu, Shan Gao, Ichiro Fukumori, Rana A. Fine, and Eric J. Lindstrom

Abstract

The origin and pathway of the thermostad water in the eastern equatorial Pacific Ocean, often referred to as the equatorial 13°C Water, are investigated using a simulated passive tracer and its adjoint, based on circulation estimates of a global general circulation model. Results demonstrate that the source region of the 13°C Water lies well outside the tropics. In the South Pacific, some 13°C Water is formed northeast of New Zealand, confirming an earlier hypothesis on the water’s origin. The South Pacific origin of the 13°C Water is also related to the formation of the Eastern Subtropical Mode Water (ESTMW) and the Sub-Antarctic Mode Water (SAMW). The portion of the ESTMW and SAMW that eventually enters the density range of the 13°C Water (25.8 < σθ < 26.6 kg m−3) does so largely by mixing. Water formed in the subtropics enters the equatorial region predominantly through the western boundary, while its interior transport is relatively small. The fresher North Pacific ESTMW and Central Mode Water (CMW) are also important sources of the 13°C Water. The ratio of the southern versus the northern origins of the water mass is about 2 to 1 and tends to increase with time elapsed from its origin. Of the total volume of initially tracer-tagged water in the eastern equatorial Pacific, approximately 47.5% originates from depths above σθ = 25.8 kg m−3 and 34.6% from depths below σθ = 26.6 kg m−3, indicative of a dramatic impact of mixing on the route of subtropical water to becoming the 13°C Water. Still only a small portion of the water formed in the subtropics reaches the equatorial region, because most of the water is trapped and recirculates in the subtropical gyre.

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Tangdong Qu, Shan Gao, Ichiro Fukumori, Rana A. Fine, and Eric J. Lindstrom

Abstract

The obduction of equatorial 13°C Water in the Pacific is investigated using a simulated passive tracer of the Consortium for Estimating the Circulation and Climate of the Ocean (ECCO). The result shows that the 13°C Water initialized in the region 8°N–8°S, 130°–90°W enters the surface mixed layer in the eastern tropical Pacific, mainly through upwelling near the equator, in the Costa Rica Dome, and along the coast of Peru. Approximately two-thirds of this obduction occurs within 10 years after the 13°C Water being initialized, with the upper portion of the water mass reaching the surface mixed layer in only about a month. The obduction of the 13°C Water helps to maintain a cool sea surface temperature year-round, equivalent to a surface heat flux of about −6.0 W m−2 averaged over the eastern tropical Pacific (15°S–15°N, 130°W–eastern boundary) for the period of integration (1993–2006). During El Niño years, when the thermocline deepens as a consequence of the easterly wind weakening, the obduction of the 13°C Water is suppressed, and the reduced vertical entrainment generates a warming anomaly of up to 10 W m−2 in the eastern tropical Pacific and in particular along the coast of Peru, providing explanations for the warming of sea surface temperature that cannot be accounted for by local winds alone. The situation is reversed during La Niña years.

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