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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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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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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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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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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 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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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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Eric J. Lindstrom, David W. Behringer, Bruce A. Taft, and Curtis C. Ebbesmeyer

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 method results of Wunsch (1978), and the Worthington (1976) circulation pattern.

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

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 negative) on the frequency distribution and the lower thermocline with a positively skewed frequency distribution indicating Mediterranean Sea Water influence. Peaks in oxygen variability were observed at the same levels as for salinity. Examples of small anomalous water masses (outliers) are shown in vertical and horizontal sections. The dynamical fields (pressure of density surfaces, dynamic height) were highly anisotropic with the preferred orientation being in the NE/SW direction. There was evidence of northwest translation of dynamical features at a speed of 4 km d−1; the long horizontal scale of the dynamical features (ridges and troughs or elongated eddies) were not resolved by the experiment. Measurements adjacent to the LDE region indicate that the scale in the NE/SW direction was in excess of 400 km. The last four surveys showed the development of a very strong baroclinic feature (velocity shear > 50 cm s−1 between 700 and 1500 db). This jetlike structure, which extended across the survey region, had a gradient (NW/SE direction) with a horizontal able of 100 km. Because of the development of the Strong flow during the period of hydrographic surveys, the results are not typical of the whole period of the experiment; mooring measurements showed that two such events occurred in a 15-month period. There are three major differences between MODE and LDE results: the eddy potential and eddy kinetic energy was 3 to 4 times higher in the LDE; the LDE dynamic height fields were more anisotropic; and them were a number of intense small-scale (horizontal and vertical) property anomalies observed in the LDE that did not have a counterpart in MODE.

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