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Christopher S. Meinen

Abstract

Altimetric observations of sea surface height anomaly (SSHA) from the TOPEX/Poseidon and ERS satellites, hydrography, and the ECMWF and Florida State University wind products are used to track warm water (≥20°C) as it is exchanged between the equatorial Pacific Ocean and the higher latitudes during 1993–2003. The large El Niño event of 1997–98 resulted in a significant discharge of warm water toward the higher latitudes within the interior of the Pacific Ocean. The exchange of anomalous warm water volume with the Northern Hemisphere appears to be blocked under the intertropical convergence zone, consistent with most current ideas on the time-mean tropical–subtropical exchange. Little of the warm water discharged northward across 5° and 8°N during the 1997–98 El Niño event could be traced as far as 10°N. To the south, however, these anomalous volumes of warm water were visible at least as far as 20°S, primarily in the longitudes around 130°–160°W. In both hemispheres most of the warm water appeared to flow westward before returning to the Tropics during the recharge phase of the El Niño–La Niña cycle. The buildup of warm water in the Tropics before the 1997–98 El Niño is shown to be fed primarily by warm water drawn from the region in the western Pacific within 5°S–15°N. The exchange cycle between the equatorial band and the higher latitudes north of the equator leads the cycle in the south by 6–8 months. These results are found in all three datasets used herein, hydrography, altimetric observations of SSHA, and Sverdrup transports calculated from multiple wind products, which demonstrates the robustness of the results.

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Christopher S. Meinen

Abstract

Two years of observations from an array of 16 inverted echo sounders deployed south of Australia near 51°S, 143.5°E are combined with hydrographic observations from the region to estimate the differences in baroclinic transport, as well as temperature and velocity structure, that result from trying to estimate the true mean using a limited number of snapshot sections. The results of a Monte Carlo–type simulation suggest that over a 350-km distance, completely spanning the Subantarctic Front (SAF) at most times, a minimum of six temporally independent sections would be required to determine the baroclinic transport mean (surface to 4000 db) of the observed 2-yr period to within an accuracy of 10% when the sections are averaged in either an Eulerian (geographic) or stream coordinates manner. However, even with 10 sections during a 2-yr period the details of the mean velocity and temperature structures obtained can be quite different than the “true” 2-yr mean structure, regardless of whether the sections are averaged in either Eulerian or stream coordinates. It is estimated that at least 20 independent sections would be required during a 2-yr period in order to determine the baroclinic velocity structure to within an accuracy of 10%, irrespective of whether they are averaged in Eulerian or stream coordinates. Implications for future sampling strategies and for inverse modeling analyses are discussed.

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Christopher S. Meinen

Abstract

Moored temperature sensors, whether fixed or profiling, routinely need to be corrected to remove the signals associated with the vertical motion of the sensors when the moorings “blow over” in strong flow events (for profiling sensors the problems occur only at the upper end of the profiling range). Hydrographic data are used to estimate the accuracy with which moored temperature sensors in the Gulf Stream can be corrected for mooring motion aliasing using standard correction techniques, and the implications for other ocean regions are discussed. Comparison with hydrographic data and coincident inverted echo sounder (IES) data from the Synoptic Ocean Prediction Experiment (SYNOP) shows that the errors inherent in mooring motion corrected temperatures during significant pressure deflections are potentially 2–3 times as large as previous estimates based on a smaller dataset of observations in the Kuroshio at approximately the same latitude in the Pacific. For sensors with a nominal level of 400 dbar and a typical root-mean-square pressure deflection of 150 dbar, accuracy limits of up to 0.7°C on the “corrected” temperatures are applicable. Deeper sensors typically have smaller accuracy bounds. There is a suggestion that the presence of a mode water layer near the nominal depth of the shallowest sensor can result in much higher errors in mooring motion corrected temperature data. The accuracy estimates derived herein should apply not only to moorings deployed in the Gulf Stream but also to all currents that exhibit similar velocity amplitudes and thermal gradients such as the Agulhas or Kuroshio.

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Christopher S. Meinen and Douglas S. Luther

Abstract

In the presence of a strong current, such as the Gulf Stream or the North Atlantic Current, current meter moorings are known to “blow over” due to drag from the moving water. This dipping of the current meters, which has been documented to exceed 500 m in some cases, can significantly affect estimates of fluxes on level surfaces. Pressure measurements made by sensors collocated along the mooring near each current meter are commonly used to correct for this mooring motion. Data from a current meter mooring near 42°N, 45°W are used to demonstrate that, in cases where there is a failure of the pressure sensors, measurements from an inverted echo sounder near the current meter mooring can be combined with the mooring temperature records and historical hydrography to produce “synthetic” pressure records for current meters within the main thermocline depth range. Pressures at other current meters on the mooring can then be determined using mooring design parameters. This technique allows corrections for mooring motion when they would otherwise be impossible due to the loss of the directly measured pressure records. Comparison to directly measured pressures in the main thermocline from a mooring near the North Atlantic Current demonstrates that this technique can determine synthetic pressure records to within a root-mean-square difference of about 46 dbar for an instrument with observed mooring motion related pressure dips of 200–500 dbar. The technique is also applied to a number of other current meters in the North Atlantic Current region as well as instruments that were moored in the Subantarctic Front near 143°E to demonstrate where the technique will and will not work.

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Christopher S. Meinen and Michael J. McPhaden

Abstract

Previous studies have indicated that the volume of warm water (WWV) in the equatorial Pacific is intrinsically related to the dynamics of the El Niño–Southern Oscillation (ENSO) cycle. A gridded subsurface temperature dataset, which incorporates temperature measurements from expendable bathythermograph profiles as well as measurements from the moorings in the Tropical Atmosphere and Ocean Array, was used along with historical hydrography to quantify the variability of the WWV (with temperatures greater than 20°C) within the region 8°S–8°N, 156°E–95°W during 1993–99. These data were also used to estimate geostrophic transports of warm water relative to a level of no motion at 1000 dbar (1 dbar = 104 Pa). Ekman transports were estimated using wind data from gridded observations, assimilating model output, and satellite scatterometer measurements. The results indicate that the buildup of WWV between the weak 1994–95 El Niño event and the onset of the strong 1997–98 El Niño event resulted primarily from an anomalous decrease in the net southward transport (geostrophic plus Ekman) across 8°S in the western and central Pacific combined with an anomalous increase in eastward flow across 156°E. Afterward, beginning in April 1997, WWV in the equatorial region was reduced by about 26% coincident with the 1997–98 El Niño. Approximately half of the warm water lost during this period is accounted for by poleward transport across a wide range of longitudes, while the remaining half is accounted for by vertical water mass transformations. The implications of the results for understanding and modeling the ENSO cycle are discussed.

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Rigoberto F. Garcia and Christopher S. Meinen

Abstract

For more than 30 years, the volume transport of the Florida Current at 27°N has been regularly estimated both via voltage measurements on a submarine cable and using ship-based measurements of horizontal velocity at nine historical stations across the Florida Straits. A comparison of three different observational systems is presented, including a detailed evaluation of observational accuracy and precision. The three systems examined are dropsonde (free-falling float), lowered acoustic Doppler current profiler (LADCP), and submarine cable. The accuracy of the Florida Current transport calculation from dropsonde sections, which can be determined from first principles with existing data, is shown to be 0.8 Sv (1 Sv ≡ 106 m3 s−1). Side-by-side comparisons between dropsonde and LADCP measurements are used to show that the LADCP-based transport estimates are accurate to within 1.3 Sv. Dropsonde data are often used to set the absolute mean cable transport estimate, so some care is required in establishing the absolute accuracy of the cable measurements. Used together, the dropsonde and LADCP sections can be used to evaluate the absolute accuracy and precision of the cable measurements. These comparisons suggest the daily cable observations are accurate to within 1.7 Sv, and analysis of the decorrelation time scales for the errors suggests that annual transport averages from the cable are accurate to within 0.3 Sv. The implications of these accuracy estimates for long-term observation of the Florida Current are discussed in the context of maintaining this key climate record.

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Christopher S. Meinen and D. Randolph Watts

Abstract

The addition of an accurate pressure sensor to the inverted echo sounder (IES) has allowed for the development of a new method for calibrating the IES’s acoustic travel-time record without the need for coincident conductivity–temperature–depth (CTD) or expendable bathythermograph profiles. Using this method, the round-trip travel-time measurement of the IES can be calibrated into various dynamic quantities with better accuracy than was possible with previous methods. For a set of four IES records from the Newfoundland Basin, the estimate of the accuracy of the geopotential height anomaly (integrated between 100 and 4000 db) calibrated from the IES measurements was reduced from 0.65 to 0.52 m2 s−2, which is a substantial reduction toward the intrinsic scatter of the geopotential height anomaly versus travel-time relationship for this region (0.42 m2 s−2). The addition of the pressure sensor to the IES results in reduced errors and eliminates the need for coincident CTD measurements. Moreover, the pressure sensor provides a complementary dataset recording the changes of the barotropic pressure field.

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Christopher S. Meinen and Michael J. McPhaden

Abstract

This paper describes observed changes in surface winds, sea surface temperature (SST), and the volume of water warmer than 20°C (WWV) in the equatorial Pacific Ocean for the period 1980–99. The purpose is to test recent hypotheses about the relationship between variations in WWV and the El Niño–Southern Oscillation (ENSO) cycle. The results confirm inferences based on theory, models, and previous empirical analyses using proxy data (namely sea level) that ENSO involves a recharge and discharge of WWV along the equator and that the cyclic nature of ENSO results from a disequilibrium between zonal winds and zonal mean thermocline depth. The authors also find that the magnitude of ENSO SST anomalies is directly related to the magnitude of zonal mean WWV anomalies. Furthermore, for a given change in equatorial WWV, the corresponding warm El Niño SST anomalies are larger than the corresponding cold La Niña anomalies. This asymmetry between the warm and cold phases of the ENSO cycle implies differences in the relative importance of physical processes controlling SST during El Niño and La Niña events.

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Alan D. Chave, Douglas S. Luther, and Christopher S. Meinen

Abstract

Interactions between motional electric fields and lateral gradients in electrical conductivity (e.g., seafloor topography) produce boundary electric charges and galvanic (i.e., noninductive) secondary electric fields that result in frequency-independent changes in the electric field direction and amplitude that are specific to a single location. In this paper, the theory of galvanic distortion of the motional electric field is developed from first principles and a procedure to correct for it is then derived. The algorithm is based on estimation of intersite transfer tensors for the horizontal electric fields at the high frequencies where external (ionospheric and magnetospheric) sources, not oceanic motionally induced electric fields, dominate. A decomposition of each measured tensor is derived that expresses it as the product of a set of distortion tensors and the underlying, undistorted transfer tensor. The algorithm may be applied simultaneously to a set of sites and assessed statistically, yielding the undistorted electric field uniquely at each site except for a single site-dependent multiplicative scalar, which must be obtained from other data. Because the distortion is frequency independent, the same tensors may be used to undistort the low-frequency, motional induction components that are of interest in oceanography. This procedure is illustrated using an electric field dataset collected in the Southern Ocean in 1995–97, which is significantly distorted by galvanic processes.

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Christopher S. Meinen, Michael J. McPhaden, and Gregory C. Johnson

Abstract

Geostrophic and Ekman transports calculated from observations of subsurface thermal structure and surface winds are used to determine vertical velocities and transports as a function of time, depth, and longitude in the equatorial Pacific within 5°S–5°N, 165°E–95°W during 1993–99 via a box volume balance. The vertical transports are determined in boxes of 10° of latitude by generally 15° in longitude. The corresponding vertical velocities represent a spatial average over these regions. Both the total vertical velocity and the cross-isopycnal component of the vertical velocity (approximated by the cross-isothermal component) are calculated on seasonal and interannual timescales. For the eastern equatorial Pacific (5°S–5°N, 155°–95°W) the mean vertical transport across 50 m is (24 ± 3) × 106 m3 s−1. Variability in the vertical velocity is large relative to the mean. On interannual timescales this variability is well correlated with the local winds in the western portion of the study area, while the correspondence is weaker in the east where wind variability is much smaller. At seasonal timescales there is good correspondence between the vertical velocity and the local winds throughout the study region. The cross-isothermal vertical velocity is significantly smaller than the total vertical velocity, and the means of both compare well with the few historical estimates available.

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