Eli Joel Katz

The forced annual reversal of the Atlantic North Equatorial Countercurrent

Abstract We analyze the variability of the thermal structure associated with the Atlantic North Equatorial Countercurrent (NECC) and its relation to the seasonally varying winds. The analysis performed allows us to establish the period of time and the region where the NECC reverses direction. West of 25°W, the depth of the thermocline in the southern side of the NECC (4 to 7°N) annually oscillates 180° out of phase to the oscillations on the northern side (7 to 10°N). East of 25°W the thermocline rises and falls nearly in phase across the countercurrent. If the flow can be uniquely presumed from the thermal structure, the NECC disappears during the boreal spring in the western basin. The reversal of phase from north to south over one part of the ocean, but no the other, is shown to be mirrored in the annual variation of the curl of the stress induced at the surface by the winds. In order to quantify this result, the different terms of the vorticity equation are calculated from the data. We conclude that t...

An Interannual Study of the Atlantic North Equatorial Countercurrent

Abstract Beginning in 1983, the dynamic height difference at the sea surface defining the countercurrent trough at 38°W was continuously observed by Inverted Echo Sounders at 3°N and 9°N for six years and ten months. The amplitude of the average annual signal for that time period was 36 cm, and the range of the interannual variability was 25 cm. Thus, the annual signal dominates as expected, but the interannual variability is of comparable magnitude. Two years, 1983 and 1987, are noted as years of sharp increase in the interannual signal (periods immediately following El Nino events in the Pacific). From a derived relationship for geostrophic volume transport, it is estimated that every year the North Equatorial Countercurrent attained a transport of 20 Sv (≡ 106 m3 s−1) for at least one month, while in the cited years it sustained this value for over half the year. The output of an unbounded reduced-gravity model, forced by monthly averaged wind stress, is compared to the observations. The model can be t...

Vertical Coherence of the Internal Wave Field from Towed Sensors

Abstract Constant depth and isopycnal‐following tows are used to estimate the, towed vertical coherence of the internal wave field, at vertical separations of 8.5, 18, 28 and 70 m. The depths of the tows are ∼750 m at the maximum of the buoyancy frequency in the main thermocline of the Sargasso Sea, and near 350 m in the buoyancy frequency minimum between the main and seasonal thermoclines. The towed spectra and towed vertical coherence are compared with three model spectra (GM75, GM76 and IWEX): at 750 m the agreement between data and models is very good, with IWEX being slightly better. At 350 m several of the measured towed vertical coherence spectra are more complex than the spectra from the deeper tows, there are anomalously high coherences in a band from 0.7 to 2 cycles per kilometer that are not predictable by the models. We suggest this coherence bump may be evidence of Eckart resonance, i.e., modes tunneling between the two thermoclines into the region of low buoyancy frequency.

Waves along the Equator in the Atlantic

Abstract Spectra of two hundred days of data from five inverted echo sounders deployed along the equator in 1983–84 and a thousand days of 30 crossings of the equator by the TOPEX/POSEIDON altimeter in 1993–95 are both found to have enhanced variance of sea surface displacement in two frequency bands centered nominally about 50−1 and 25−1 cycles/day. Complex empirical orthogonal function analysis of the two bands finds much of the variance being accounted for by eastward and westward propagating waves, respectively, that are coherent across the basin. The eastward propagating wave is identifiable as a first-mode Kelvin wave with a period of 54 (49) days in the sounder (altimeter) dataset, a basinwide wavelength and a speed of 2.1 (1.8) m s−1. It is suggested that the westward propagating wave is a surface expression of the meridional oscillation of the Equatorial Undercurrent about the equator, presumably induced by a tropical instability wave, with a period of 24 (28) days, a wavelength of 600 (580) km, ...

Profile of an Isopycnal Surface in the Main Thermocline of the Sargasso Sea

Abstract The vertical displacement and slope of an isopycnal surface (by which is meant a density surface of constant σt) are described spatially. The data were acquired by towing pressure, temperature and conductivity sensors in the main thermocline of the Sargassso Sea south of Bermuda at depths between 550 and 7oo m. Four tows are discussed, the longest being 580 km and two consisting of repeated tracks. The mean slopes of the surfaces lie between 2 × 10−4 and 3 × 10−4 rad and can extend for hundreds of kilometers. The power spectrum of vertical displacement is computed over a bandwidth of 0.02 to 30 cycles km−1. The effective internal wave bandwidth is suggested to extend from about 0.05 to 1 cycle km−1, and its spectrum has a −1.55 wavenumber dependency. Above 1 cycle km−1, it falls off more rapidly. The spectrum is repeatable and, weighted by the Vaisala frequency, agrees with the few previous reports in widely different locales.

Observations of Inertia-Gravity Waves in the Atlantic from Inverted Echo Sounders during FGGE

Abstract The study is based on observations from inverted echo sounders anchored near the equator in the western Atlantic Ocean. It appears to be a good method of detecting inertia-gravity waves with large vertical scale. The spectral analysis of the records shows variable spectral density levels in the band of 2–5 days. Three of the four Inverted Echo Sounder (IES) records show an increase in the variance centered at 3.75 days. In addition, two sounders deployed during the latter half of the year (stronger winds) also show a peak in the variance at 2.14 and 2.31 days. The most likely interpretation of the spectral peak at 3.75 days is an atmospherically forced, vertically propagating inertia-gravity wave with a meridional structure given by the n=3 mode. The corresponding equivalent depth is h=0.7 m, and the vertical wavelength is ∼730 m. The energy density, estimated from the variance in the IES records, is E = 0.7 × 103 J m−2.

A synthesis of the first GARP Globa Experiment (FGGE) in the equatorial Atlantic Ocean

Abstract A synthesis of near-surface oceanographic and surface meteorological data collected during the First GARP Global Experiment, FGGE, is presented to portray the oceanic response to the seasonal wind forcing for the period December 1978 to November 1979, inclusive. Major wind events during FGGE are in phase with events given in climatology. In particular, the February–March–April relaxation and May enhancement of equatorial winds occurs within one month of the mean event. Accordingly, the oceanic responses, such as the May, June, July appearance of an equatorial cold water tongue, the acceleration of the South Equatorial Current (SEC) and the vertical displacement of the equatorial thermocline occur at the average time. Furthermore, the curl distribution in the vicinity of the North Equatorial Countercurrent (NECC) during 1979 is similar to the climatological distribution in terms of phase and amplitude, except for a westward displacement in the position of the maximum curl. As predicted from linear theory, the 1979 thermocline response across the NECC is in phase with the climatological response with a westward displacement of the maximum thermocline movement. Deeper than average equatorial thermoclines and a weaker SEC may, in part, be responsible for the anomalously warm sea-surface temperatures observed on the equator between 10°W and 30°W from June to November.

Near Surface Temperature Observations Obtained in the Equatorial Atlantic Ocean During Fgge (1979)

Temperature and surface wind data collected across the equatorial Atlantic Ocean during the First GARP Global Experiment (FGGE), 1979, have been compiled to study the seasonal evolution of the near-surface temperature field. The development of the large-scale, sea-surface temperature (SST) field is characterized by the appearance during boreal summer of a tongue of cold water which extends from the eastern to western basin, on and south of the equator. During 1979, the cold water first appears at 4°W and 28°W during early May and at 22°W some four weeks later. East of about 20°W, the thermocline rises and the mixed layer becomes shallower simultaneously with the lowering of SSTs. West of about 30°W, the thermocline and mixed layer deepen at this time. Below average temperatures are observed through October, as the thermocline redeepens in the east and continues to deepen in the west. The 1979 SST and thermocline distributions along the equator have been compared to climatological distributions derived from historical data. The FGGE year fields are qualitatively similar to the climatological fields. Surface wind data collected during FGGE indicate that the surface cooling occurs within several days of an increase in both components of wind at 4°W and 28°W and within several weeks of the wind increase at 22°W.