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  • Author or Editor: Christopher S. Meinen x
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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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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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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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Renellys C. Perez
,
Silvia L. Garzoli
,
Christopher S. Meinen
, and
Ricardo P. Matano

Abstract

Two ocean general circulation models are used to test the ability of geostrophic velocity measurement systems to observe the meridional overturning circulation (MOC) and meridional heat transport (MHT) in the South Atlantic. Model sampling experiments are conducted at five latitudes (between 15° and 34.5°S) spanning the range of extratropical current regimes in the South Atlantic. Two methods of estimating geopotential height anomalies and geostrophic velocities are tested, simulating dynamic height moorings (TS array) and current and pressure recording inverted echo sounders (CPIES) deployed within the models. The TS array accurately reproduces the MOC variability with a slight preference for higher latitudes, while the CPIES array has skill only at higher latitudes resulting from the increased geopotential height anomaly signal. Whether direct model velocities or geostrophic velocities are used, MHT and the MOC are strongly correlated, and successful reconstruction of MHT only occurs when there is skill in the MOC reconstructions. The geopotential height anomaly signal is concentrated near the boundaries along 34.5°S, suggesting that this is an advantageous latitude for deployment of an in situ array. Four reduced arrays that build upon the sites from two existing pilot arrays along 34.5°S were examined. For these realistically sized arrays, the MOC and MHT reconstructions from the TS and CPIES arrays have comparable skill, and an array of approximately 20 instruments can be effectively used to reproduce the temporal evolution and vertical structure of the MOC and MHT.

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Christopher S. Meinen
,
Douglas S. Luther
,
D. Randolph Watts
,
Karen L. Tracey
,
Alan D. Chave
, and
James Richman

Abstract

Profiles of absolute velocity are difficult to obtain in the ocean, especially over long periods of time at the same location. This paper presents a method of estimating full water column absolute horizontal velocity profiles as a function of time by combining historical hydrography with the measurements from two separate instruments, the inverted echo sounder (IES) and the horizontal electric field recorder (HEFR). Hydrography is used to construct temperature, salinity, and specific volume anomaly characteristics as functions of the independent variables pressure and seafloor-to-sea-surface round-trip acoustic travel time (τ). Each IES measured τ is combined with these two-dimensional characteristics to estimate the profile of specific volume anomaly, which then is integrated vertically to obtain profiles of geopotential height anomaly (Δϕ). Profiles of Δϕ from adjacent IES sites are differenced to yield vertical profiles of relative geostrophic velocity. Horizontal electric fields arising from the vertically averaged horizontal water velocity provide the requisite referencing of the IES-derived relative velocities. Comparisons are presented between HEFR+IES absolute velocities in the Southern Ocean near 51°S, 143.5°E and absolute velocities determined via hydrography, acoustic Doppler current profiler, and current meter.

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