Jimmy C. Larsen

Florida Current Volume Transports from Voltage Measurements

The volume transport of the Florida Current is determined from the motionally induced voltage difference between Florida and Grand Bahama Island. Simultaneous measurements of potential differences and of volume transport by velocity profiling have a correlation of 0.97. The calibration factor is 25� 0.7 sverdrups per volt, and the root-mean-square discrepancy is 0.7 sverdrup. The induced voltage is about one-half the open-circuit value, implying that the conductance of the sediments and lithosphere is about equal to that of the water column.

Transports through the Straits of Florida

Abstract Transports were calculated for four sections of the Florida Current from Key West to Jupiter, Florida, using a moored current-meter array and voltages from cross-channel telephone cables at the western and northern ends of the Straits of Florida. In addition, moored arrays were used to estimate transport through the Northwest Providence, Santaren, and Old Bahama Channels that connect the Florida Current to the southwestern part of the North Atlantic Ocean. Transport measurements were obtained for an 11-month period from December 1990 to November 1991. Mean transports of ∼25 Sv (1 Sv ≡ 106 m3 s−1) for the flow across the western ends of the straits, which agree quite well with recent estimates of 23.8 ± 1 Sv entering the Gulf of Mexico through the Yucatan Channel, were obtained from both the Key West to Havana cable and the moored array. This estimate is about 5 Sv less than the generally accepted transport through the northern end of the straits at 27°N. This difference was partially accounted fo...

A magnetotelluric investigation of the San Andreas Fault at Carrizo Plain, California

High quality, wide-band magnetotelluric data were collected along two profiles crossing the San Andreas fault at Carrizo Plain, California for crustal imaging as part of the San Andreas Deep Drilling Project. Two-dimensional inversions of the data indicate that the upper crustal part of the San Andreas fault does not appear here as an anomalously conductive zone. Additionally, a broad resistive zone under the Temblor Mountains (east of the fault) suggests resistive crystalline or metamorphic rocks may be present here as opposed to the more conducting Franciscan assemblage, contradicting the generally-accepted geologic model.

Variations in the electrical conductivity of the upper mantle beneath North America and the Pacific Ocean

Variations in the electrical conductivity of the mantle beneath Carty Lake in the Canadian Shield, Tucson in the southwest United States, and Honolulu and Midway in the north central Pacific were determined through the inversion of long-period magnetotelluric and geomagnetic depth sounding data. Inversion of computed response functions is carried out using a minimum structure, regularized approach. The upper mantle beneath Carty Lake is approximately an order of magnitude more resistive than the upper mantle beneath Tucson and nearly 1.5 orders of magnitude more resistive than Honolulu and Midway Island. Inversions were also carried out where the minimum structure constraint was removed at known upper mantle discontinuities. These models show a jump in conductivity of ∼1.5 orders of magnitude across the 660 km discontinuity, a result that is consistent with laboratory experiments on realistic mantle assemblages. Mantle conductivity profiles at Carty Lake are significantly more resistive than those at Tucson, Honolulu, and Midway to depths of ∼300–400 km. These observations likely reflect differing thermal states, the presence (or absence) of partial melt and volatiles, and may also be related to chemical differences between depleted and undepleted upper mantle. The observed conductivity variations may be interpreted as lateral variations in temperature, partial melt, and/or dissolved hydrogen in olivine.

Subtropical Atlantic Climate Studies: Introduction

This report is an introduction to the accompanying collection of reports that present the results of a 2-year period of intensive monitoring of the Florida Current. Both direct observing systems (ship-deployed current profilers and moored current meters) and indirect observing systems (coastal tide gauge stations, bottom pressure gauge arrays, a submarine cable, acoustic arrays, and radar installations) were used to measure temperature and volume transport.

Transport measurements from in-service undersea telephone cables

The motion of the ocean through the Earths magnetic field creates a cross-stream voltage proportional to the transport of the stream. These motion-induced voltages have been observed for more than ten years by the use of an abandoned cable spanning the Florida Current. In-service undersea telephone cables, including fiber optic cables with branch lines, can also be used to determine the motion-induced voltages between the cable sea-Earth grounds because the repeaters in these cables are powered by a nearly constant electric current, the return path for this current is through the ocean via the sea-Earth grounds, and the power voltage and current are readily measured at the cable station or stations. Ocean voltages between West Palm Beach, FL, and Eight Mile Rock, Grand Bahama Island, observed since 1985 by use of an in-service cable yield monthly estimates of the Florida Current transport with an r.m.s. accuracy of 1.1 Sv when compared with the monthly voltage-derived transports between Jupiter, FL, and Settlement Point, Grand Bahama Island, observed by the use of an abandoned cable. This corresponds to an accuracy of 3% of the mean transport of 32.3 Sv. >

Tidal Transport in the Florida Current and Its Relationship to Tidal Heights and Cable Voltages

Abstract A linear relationship between tidal height (sea level of tidal frequencies) and tidal transport near 27°N in the Straits of Florida is confirmed. Transport estimates from this relationship for the O1 and M2 constituents are compared with those computed from cable voltages across the Florida Current. These estimates are independent in that the weighted tidal height model (tidal-height transport relationship) was developed using collective sets of current meter and velocity profiler data obtained at different times of the year and in different locations. The cable voltages, however, were calibrated using a quasi-synoptic sectional integration of depth-averaged profiler data. Further, a means is suggested by which changes in the cable calibration can be detected.