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Thomas A. Rago and H. Thomas Rossby

Abstract

Direct velocity and temperature measurements obtained on a bimonthly basis from September 1980 to May 1983 in the Gulf Stream at 73°W longitude were used in combination with historical hydrographic and current meter data across the remainder of the ocean to obtain an annual mean estimate, as well as seasonally averaged estimates, of poleward heat transport in the North Atlantic Ocean at 32°N latitude. The annual mean heat transport was found to be northward across 32°N with a magnitude of (1.38 ± 0.19) × 1015 W and with an annual cycle of abut 0.4 × 1015 W from its minimum in the first half of the year to its maximum in the second half. The main contribution to the annual cycle comes from the Gulf Stream.

The heat transport at 32°N was also examined in terms of the mass transport of different temperature classes. This analysis shows that the heat transport into the North Atlantic basin is due to a diagonal cell in which a net amount of warm water flows northward in the upper layers in the western boundary current, and is balanced by a southward return flow at deeper levels in the central ocean. Our results are consistent with those of Hall and Bryden, who have estimated the mean meridional Atlantic heat transport at 24.5°N.

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H. Thomas Rossby and Thomas B. Sanford

Abstract

A time series of velocity profiles derived from three methods are used to describe the variations of current in time and in the vertical. Absolute velocity profiles were Obtained by acoustically tracking a falling probe; relative profiles were derived from motional electric fields (EM method) measured by a second free-fall instrument and from density observations using the dynamic method. The two free-fall profile methods agree within 0.01 m s−1 rms averaged over depth intervals in which the observations were separated in time by less than 10 min. Although the rms differences between profiles increases to about 0.02 m s−1, due to the fact that one device falls at one-third the speed of the other, the agreement between methods was sufficiently good that the eight acoustic profiles and six EM profiles were combined to yield a time series lasting 4 days. These profiles, taken near Bermuda In May 1971, were divided into two sets having a mean time separation of 2 days. Each set of profiles was fitted to a time-mean or steady profile and a rotary component of inertial frequency. Using lagged correlation and vector spectral analysis, it is shown that the inertial energy propagates downward at a group velocity having a vertical component of about 0.5 mm s−1. These results suggest a surface or near-surface energy source and a lack of modal structure to the inertial currents. The steady component agrees within 0.02 m s−1 rms with the geostrophic profile computed every 200 m and both have the same shear over the interval 200–1200 m.

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Ping-Tung Shaw and H. Thomas Rossby

Abstract

Downstream velocity relative to the axis of the Gulf Stream is examined through the use of data from SOFAR floats. The speed calculated from the position of the floats along constant pressure surfaces is expressed in terms of a transformed cross-stream coordinate given by temperature, which is telemetered from the floats. The result is a distribution of downstream velocity unaffected by meanders from Cape Hatteras to 46°W. The speed at 700 m is about 75 cm s−1 west of 57°W and decreases sharply to 40 cm s−1 to the east. In the deep water from 1300 to 2200 m, the core speed is 35 cm s−1 between 65° and 50°W, if it is present. The flow in the Gulf Stream may be disturbed by local processes, which are frequently observed in satellite imagery. Examples am shingles, ring formation and meanders.

Although SOFAR floats are quasi-Lagrangian (isobaric) devices, the float data can give a Lagrangian description of the Gulf Stream. Above the main thermocline, a current coinciding with the tilting isotherms from Cape Hatteras to 46°W implies that water is efficiently transported downstream. In the deep ocean, water is accelerated by the surface Stream off Cape Hatteras and is at times transported downstream by the deep flow thus formed. The New England Seamounts can block this deep flow. There is little evidence of a deep current and thus, water transport east of the Seamounts.

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H. Thomas Rossby and Robert J. Serafin

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Edward J. Kearns and H. Thomas Rossby

Abstract

The glass-pipe technology used for RAFOS floats is applied to the monitoring of convection in deep mixed layers. The velocity of a vertical current is estimated from the relationship between the drag force exerted on a float by the vertical current and the buoyancy force induced by the float's resultant displacement from hydrostatic equilibrium. Tests conducted in the winters of 1990 and 1991 in the 18°C waters of the northwestern Sargasso Sea reveal definite convective events. Vertical velocities of both upwelling and downwelling plumes are estimated to approach maxima nearing 0.05 m s−1, with durations of up to 2 h. One float that crossed the Gulf Stream and entered the Newfoundland Basin showed evidence of very active vertical currents in the near-surface waters with maximum velocities greater than 0.09 m s−1.

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H. Thomas Rossby and Robert J. Serafin

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No abstract available.

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Gunnar I. Roden and H. Thomas Rossby

Nils Gissler (1715–71) was a remarkable Swedish scientist and physician who appears to be the first to describe the inverted barometer effect, based on joint observations of sea level and atmospheric pressure at Härnösand, a small town on the Gulf of Bothnia. He not only observed that when the atmospheric pressure increased, the sea level dropped, but that the degree of sea level decrease depended upon weather conditions. He also looked at ancient rocky beach terraces and pondered about their age and method of formation. The present article looks at the life and times of this remarkable man, gives a full translation of his original 1747 article, and points out his legacy.

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D. Randolph Watts and H. Thomas Rossby

Abstract

Inverted Echo Sounders (IES) were deployed during MODE at seven ocean bottom stations to acoustically monitor depth variations of the main thermocline. The IES transmits pulses of 10 kHZ sound and records the time τ for the echo to return from the ocean surface; τ varies by a few milliseconds in response to vertical displacements of the temperature and salinity profiles in the water column. The acoustic travel time is inherently an integral measurement, which is insensitive to finestructure in the vertical but is dominantly influenced by vertical displacements which are coherent throughout the water column. Thus the IES performs as a natural “matched filter” for the most fundamental internal displacement mode. A perturbation analysis on the dynamic height (D), the total heat content (Q) and the acoustic travel time (τ) integrals shows that all three are dominated by displacements of the main thermocline. The proportionality is unique when a single mode of internal displacements is dominant.

Comparisons with MODE hydrographic data near each instrument show that the measured travel times may be rescaled into dynamic height (ΔD) records with an uncertainty of only ±1 dyn cm, which is comparable to the best of hydrographic measurements. Time series of τ show that internal waves on the main thermocline in this “mid-ocean” location have larger amplitude than is generally appreciated: ΔD can change by 2–3 dyn cm in 2–3 h, thereby aliasing a measurement taken at a single instant in time. Differences between the low-pass filtered IES dynamic height records from pairs of sites are compared, via the thermal wind relationship, with the observed current shear across the main thermocline, as determined from current meter and SOFAR float records; the agreement is good within the limitations imposed an estimating the current streamfunctions from a sparse network of current meters. Thus the IES records can be used to extend the mapping of the baroclinic velocity field.

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Edward R. Levine, D. N. Connors, Peter C. Cornillon, and H. Thomas Rossby

Abstract

No abstract available.

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Stephan D. Howden, D. Randolph Watts, Karen L. Tracey, and H. Thomas Rossby

Abstract

From August 1989 until August 1990, a simple acoustic telemetry system was used for obtaining real-time data from four inverted echo sounders (IESs) deployed in the Synoptic Ocean Prediction Experiment (SYNOP) inlet array in the Gulf Stream east of Cape Hatteras. The telemetry system is based on encoding data as a time-delayed broadcast acoustic signal: the delay of the time of broadcast of the signal, with respect to a reference time, is proportional to the data value. The changes in delay time, from one broadcast signal to the next, are recorded at a remote receiving station.

Moored near the sea floor, IESs are designed to emit high-frequency (10 kHz) acoustic pings toward the sea surface and receive the reflected signal. In the Gulf Stream region, the round-trip travel time of the emitted signal is proportional to the depth of the main thermocline. As the Gulf Stream meanders back and forth over the instrument, its position can be tracked with the thermocline depth changes.

Every 24 h, each AES calculated a representative travel time from a set of 48 bursts of measurements (τ), and telemetered that value to a listening station on Bermuda. From the received data, a daily time series of the depth of the 12°C isotherm (our proxy for main thermocline depth) at each IES was calculated. The position of the Gulf Stream north wall through the IES array was calculated on a daily basis from the thermocline depth information at each IES site.

Three of the four IESs were recovered in August 1990. Although the IES at site B2 was not recovered, its telemetered data was received at Bermuda. The rms agreement between thermocline depths, as calculated from the data on tape from the recovered IESs and as calculated from the received telemetry data, is 20 m. This compares favorably with the 19-m uncertainty in calibrating the τ's as a measure of the thermocline depth. The rms agreement between the position of the Gulf Stream path through the IESs as calculated from the tape data and the telemetry data is 5 km.

This telemetry system is not IES specific. It could be used with other appropriately modified oceanographic instruments, such as current meters and pressure sensors.

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