Bottom Temperature Fluctuations on the Continental Slope of the Northwest Atlantic Ocean

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HOWARD W. BROEK St. Charles, Illinois

(Manuscript received 9 September 2003, in final form 31 August 2004)

ABSTRACT Temperature fluctuations on the western continental slopes of the Atlantic Ocean have been measured at three stations on the ocean bottom at depths from 1400 to 2100 m and from 17° to 35°N. All three stations were in or near the deep western boundary current. Daily readings were taken for four to six years. Maximum-to-minimum fluctuations of the isotherms were 800 m off Cape Hatteras, with average temperature of 3.80°C. Temperature appeared episode-driven, with minima consistently near 3.60°C and maxima near 4.10°C. Large swings in temperature had a duration of 10–30 days. Data from Antigua also appeared episode-driven but showed fewer large shifts in temperature, but the maximum-to-minimum fluctuations of the isotherms were 400 m. Hence interesting motions may exist in the seldom-studied region near the bend in the Antilles Islands chain. These two stations are vastly different from previously reported data from the Blake Plateau in which temperature increases are consistently more rapid than decreases and suggest frequent overflow currents. Annual oscillations were seen at the Antigua and Blake Plateau stations, but no semiannual effect was seen.

1. Introduction 1984). The DWBC was shown to be continuous from Abaco (26.5°N) to Barbados (13°N) by Fine and Mo- Stommel (1958) predicted a deep western boundary linari (1988). Lee et al. (1990) give a detailed history of current (DWBC) below 1000-m depth in the North At- DWBC studies; east of Abaco they found no annual lantic Ocean. The DWBC transports water southward effect at any depth, but an “event-dominated record, from the Labrador Sea and from overflow from the with events occurring on the average every 100 days.” Norwegian Sea in the opposite direction to the Gulf Lee et al. (1996) discovered that the DWBC meanders Stream. Swallow and Worthington (1961) made the sideways; sometimes it touches the continental slope first observation of the DWBC and placed the level of and sometimes it is over 120 km away from the slope. no motion at 1900 m off the Blake Plateau. But several The continental slope east of Antigua may be af- observers found that the Gulf Stream extends to the fected by variability of the North Brazil Current bottom, even in the deep ocean (Fuglister 1963). The (NBC). Johns et al. (1990) found that circa 400-km ed- question of how the currents cross was answered by dies propagate westward from the NBC. But the Richardson (1977), saying that the DWBC goes under DWBC is at 4300-m depth, far below the thermistors in the Gulf Stream “except in brief current reversals.” this report (Johns et al. 1993). Time scales of 60–90 days However, the shallow part of the DWBC can some- are found below the surface, as well as “large anticy- times cross the Gulf Stream: meanders of the Gulf clonic eddies” that propagate northwestward (Johns Stream allow the DWBC to continue south between the et al. 1998). continental slope and the Gulf Stream and then to (oc- Although many papers address fluctuations in hori- casionally) push through the Gulf Stream (Bower and zontal currents in the deep ocean, rather few reports Hunt 2000). consider years of temperature fluctuations on the con- Off the Blake Plateau deep horizontal motions are tinental slopes. Northeast Pacific Ocean data off Cape found (Riser, Freeland, and Rossby 1978): eddies with Mendocino (Broek 1969a) showed sizable aperiodic de- Ϫ radius near 40 km and orbital speeds near 40 cm s 1. clines in 1962, a fortnightly tide, three diurnal tidal The fluctuations in the DWBC were soon found to be components, and one semidiurnal tidal component. A as great as the current itself (Mills and Rhines 1979; Lai strong periodicity at 49 days was identified with the Madden–Julian oscillation (Madden and Julian 1971, 1972, 1994). On the other hand, temperature fluctua- Corresponding author address: Howard W. Broek, 57 White tions from 2000 m on the northwest Atlantic slope near Oak Circle, St. Charles, IL 60174-4164. the Blake Plateau show no spectral peaks at all (except E-mail: [email protected] possibly near 50 days periodicity), but increases in tem-

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TABLE 1. Locations of thermistors on the sea bottom. Lat Lon Depth Length Location (°N) (°W) (m) (yr) East of Cape Hatteras, NC 35 74 1400 6 Northeast of Eleuthera 26 76 2000 6.25 (Blake Plateau), Bahamas East of Antigua, West Indies, 17 61 1700 4 two thermistors, 2 km apart perature are consistently more sudden than decreases (Broek 1969b). Tides are also found near 3400 m on the continental slope of the northeast Atlantic (Thorpe 1987). Temperature fluctuations on the continental slope off central Chile have been reported (Shaffer et al. 1999), and temperature fluctuations off Bermuda at depths to 2000 m have also been reported (Frankignoul 1981). Data from seven thermistors on the continental slope of the northeast Pacific showed a strong semiannual effect in 1962, various tidal oscillations, the 50-day Madden–Julian oscillation, and a temperature decline in March, when the surface current reverses (Broek 2000). The present paper is only the second report of long-duration temperature fluctuations on the western continental slope of the North Atlantic, the other being Broek (1969b). Temperature fluctuations on the Hatteras Abyssal FIG. 1. Map with locations of thermistors marked by X. Plain are small, about 0.05°C, with long stretches (months) of nearly constant temperature (Brown et al. 1975). Temperature fluctuations on the abyssal plains same time of day. The thermistors were in the middle of in the eastern North Atlantic are so small as to be al- a long telephone cable. The thermistors were not most undetectable (Saunders and Cherriman 1983). moored or buoyed, but were not necessarily on the Davis et al. (2003) have the longest temperature series bottom. If the local bottom was pockmarked or rough, of all, 40 years long, which show large fluctuations in the thermistor might be slightly above the bottom. Con- the western North Atlantic (Labrador Sea) but nearly versely, if the thermistor hit a soft bottom, it might be constant temperature at abyssal depths in the central buried in sediment, or subsequent overflow currents or North Atlantic and eastern North Pacific. slumping might bury it. (Results presented below sug- South Atlantic data show fluctuations up to 0.1°Cin gest that the Blake Plateau thermistor may have be- the Argentine Basin (Coles et al. 1996), in the Vema come buried.) Locations are given in Table 1 and Fig. 1. Channel (Hogg and Zenk 1997), and in the Drake Pas- sage (Rubython et al. 2001). b. Data In general, temperature fluctuations are often very small at abyssal depths and greater near continental Data from near Cape Hatteras were available for the margins. six calendar years 1961–66. Data from the Blake Pla- teau—that is, Eleuthera, reported by Broek (1969b)— 2. Method covered 1 January 1960–1 April 1966. Data from Anti- gua were available for the calendar years 1961–62 and a. Experiment 1965–66. Antigua data were taken on two thermistors separated horizontally by 2 km; these provided almost Temperature measurements were made by use of identical data. Occasional missing data were supplied thermistors placed between pairs of wires in telephone by interpolation. A few badly inconsistent measure- cables that were lowered to the ocean bottom to depths ments were replaced with interpolated values. Plots of of 1400–2000 m (Table 1). The thermistors were placed data reveal gradual temperature changes on top of at three locations: east of Cape Hatteras, North Caro- large excursions in temperature (Figs. 2–4). lina, east of Eleuthera, Bahamas, and east of Antigua, West Indies. The Antigua experiment had two ther- c. Spectrum analysis mistors, 2 km apart, which recorded almost the same temperatures. Temperatures were recorded manually, Techniques of spectrum analysis were about the by balancing a galvanometer once a day, usually at the same as in earlier work (Broek 2000). The data were

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ing. The strongest semimonthly tides are at 13.660 and 13.633 days, or 0.0732 and 0.0734 cycles per day. Sampling once per day introduces the possibility of aliases of the diurnal and semidiurnal tides. The semidiurnal and diurnal tides are expected to produce their strongest alias peaks at 14.765 and 14.191 days, or 0.0677 and 0.0705 cycles per day. Diurnal tides can alias to one cycle per year, and the semidiurnal tide can alias to two cycles per year.

3. Results a. Temperature data Daily temperature data from the Cape Hatteras station are shown in Fig. 2. Averaged temperature from the Cape Hatteras station is shown in Fig. 3. Small fluctuations are superimposed on occasional large sudden processes. Excursions to low temperature consistently reach a minimum near 3.60°C while upward swings FIG. 2. Temperature data from the Cape Hatteras station for 1966. peak near 4.00°–4.08°C. The Blake Plateau data have a different appearance: “slow decreases in temperature (over seven days or averaged by the S4S5S6 filter (summing four adjacent more) followed by a much more rapid increase” (Broek data, then five of the result, then six of that result), then 1969b). decimated by keeping every fourth datum of the aver- Antigua data are similar to that that from Cape Hat- aged data. Spectrum analyses were performed on the teras, and both appear different from the Blake Plateau decimated data. Spectrum estimates have a 90% chance data. Averaged temperature data from Antigua for of being within a factor of 2.23 of the true value. 1961, 1962, 1965, and 1966 are shown in Fig. 4. As at Cape Hatteras, small temperature fluctuations are overwhelmed by occasional large, sudden changes. d. Tides and tidal aliasing Downswings usually reach a minimum at about 3.70°C, The periodicities and potentials of the tides were while upswings may peak between 3.94° and 4.08°C. computed exhaustively by Doodson (1922) and by February 1966 has a unique pattern: an upward step Wunsch (1967). The semiannual tide has a potential 6.3 function followed by a damped oscillation. times as great as that of the annual tide, but annual The major departures in temperature have a duration oscillations may be greatly increased by seasonal heat- of roughly 10–30 days at both stations, but the Cape

FIG. 3. Low-pass time-averaged temperature data from the Cape Hatteras station (a) for 1961–63 and (b) for 1964–66.

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FIG. 5. Spectrum of temperature fluctuations from an Antigua FIG. 4. Low-pass time-averaged temperature data from an station. Antigua station for 1961, 1962, 1965, and 1966. middle of the 8-day interval gave only a random scatter pattern. Density of temperature change in 8 days is Hatteras station has more jagged curves (more large symmetric about zero temperature change, but falls off swings in temperature). like an exponential, and not like the sum of two or more Gaussians. The median absolute value of temperature b. Temperature densities change at the Cape Hatteras station is 0.045°C versus only 0.026°C at the Antigua station. Likewise the 90% The data from the Cape Hatteras station can be fitted point: the absolute value of the 8-day temperature by a Gaussian with mean of 3.80°C and standard de- change at the Cape Hatteras station is above 0.127°Cin viation of 0.098°C. Depth is about 1400 m. only 10% of observations. The corresponding number In the Blake Plateau data, temperature has an aver- at the Antigua station is 0.077°C. age of 3.28°C. Temperatures above average are fitted by a Gaussian with standard deviation of 0.07°C, but at c. Temperature spectra low temperature the density lies above the Gaussian. On 10 occasions temperatures as low as 2.80°C were The spectra from Cape Hatteras and Antigua decline as frequency to the Ϫ1.5 power (Fig. 5). The Blake observed (Broek 1969b). Ϫ The data from the Antigua station are fitted by a Plateau spectrum declines as frequency to the 2.5 Gaussian with mean of 3.82°C and standard deviation power (Broek 1969b). of 0.082°C. Depth was about 1700 m. A tidal peak at 14.8-days periodicity in the Antigua These standard deviations are only slightly larger spectrum was also prominent in the Pacific data. This than those observed on the continental slope of the peak may be either a fortnightly tide or an alias of the northeast Pacific (Broek 2000), which were in the in- largest tide (the M2 tide, two cycles per lunar day). The terval 0.067–0.097°C. The extent of variation, from low- Blake Plateau spectra has a large annual peak and a est observed temperature to highest, was 0.508°Catthe possible broad 50-day peak (Broek 1969b). Cape Hatteras station and 0.404°C at the Antigua sta- d. Coherency tion. These ranges are similar to Pacific data, but temperature gradients versus depth are greater at the At- Coherency between the two thermistors off Antigua lantic stations. Temperature gradients derived from the separated by 2 km falls from unity at zero frequency to atlas compiled by Fuglister (1960) imply that isotherms 0.6 at the Nyquist frequency (0.5 cycles per day), as at the Cape Hatteras station can move roughly 400 m in shown in Fig. 6. The phase difference is usually less either direction and 200 m at the Antigua station. than 30°. No coherency is seen between any other pair Data from the Antigua station suggest a possible of stations. In this paper coherency is defined by the semiannual effect only for 1962. Higher-than-average root-mean-square formula. temperature prevailed around 10 March and 10 Sep- tember, and low temperature around 10 January and 10 4. Discussion July. a. Time domain Temperature densities at the Cape Hatteras and An- tigua stations are not far from Gaussian. Scatterplots of The DWBC, which is normally offshore from the temperature change in 8 days versus temperature at the continental slope, can meander toward the slope or

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gua station. The peak at 14.8 days is likely to be an alias

of the M2 tide, since the M2 tide is strong at the nearby Jungfern–Grappler Sill (MacCready et al. 1999). Spectra from the Blake Plateau station showed a very large peak at one cycle per year, and a possible broad peak at 50-day periodicity (Broek 1969b).

c. Caribbean Deep Water The Antigua stations at 1700-m depth are 300 km away from the Jungfern–Grappler Sill at 1815 m depth. This sill is a source of Caribbean Deep Water (Mac- Cready et al., 1999). The episodic fluctuations in temperature have a superficial resemblance to episodic current fluctuations at the Jungfern–Grappler Sill, as reported in Figs. 4a and 20 of the paper by MacCready et al. (1999). Both processes appear to have upper and lower limits, like shifts between two quasi-stable states. Hence the temperature fluctuations off Antigua may be part of a large process that results in the episodic over-

FIG. 6. Coherency between temperatures at two thermistors off flow of North Atlantic Deep Water into the Caribbean. Antigua separated by 2 km. d. Sea surface height away from it (Lee et al. 1996). When the DWBC is Reversing the sign of the temperature fluctuations touching the continental slope, below-average tempera- gives a quantity that is proportional to the height of the tures would be expected. This process might explain the isotherms. This quantity can then be compared with sea occasional appearance of very low temperature (non- surface height observed at nearby shore stations. No Gaussian density) at the Blake Plateau station (Broek semiannual effect is found in the temperature data from 1969b), but the station at the Blake Plateau shows “a the Atlantic stations, except that Antigua data have tendency for slow decreases in temperature (over seven maxima in October 1961 and April and December days or more) followed by a much more rapid in- 1962, close but not quite semiannual. crease.” Conversely a rapid increase may be followed by a slow decrease. Such an effect on the steep slopes e. Comparison with northeast Pacific data around the Blake Plateau might be caused by episodes of overflow currents (often called turbidity currents). In the Pacific data (Fig. 1 of Broek 2000), the tem- The duration of the observed uptrends in temperature perature had a minimum near 1 May and 1 December is consistent with simulations of overflow currents 1962. The spectra showed a semiannual peak, and the (Jiang and Garwood 1996; Kaempf and Backhaus semiannual tide has a potential 6.3 times as great as that 1999). Possibly both overflow currents and DWBC me- of the annual tide. But the tidal explanation is dubious anders are in operation. for three reasons: the temperature minima are sharp At the stations off Cape Hatteras and Antigua tem- and not sinusoidal; the minima are 7 months apart; and perature increases as fast as it decreases, and no appar- the “semiannual” process is not seen in most of the ent overflow currents are seen. Off Cape Hatteras the other years. fluctuations are larger and more frequent than off An- Sea surface height data from the Pacific (Fig. 2 of tigua, possibly due to the head-on meeting and crossing Chelton and Davis 1982) have maxima near February, of the Gulf Stream and the DWBC near Cape Hatteras. May, and October 1962 from Unalaska to Los Angeles. The Gulf Stream splits the DWBC in two (Richardson Hence sea surface height fluctuations can resemble and Knauss 1971), and the part of the DWBC next to fluctuations in temperature on the continental slope the continental slope slides down the slope to conserve (with reversed sign), but the resemblance may be coin- potential vorticity (Hogg and Stommel 1985). This part cidental. The semiannual fluctuations do not appear to of the DWBC occasionally pushes the Gulf Stream out be caused by an astronomical tide. of its way (Bower and Hunt 2000). A tidal peak at 14.8 days periodicity in the Antigua spectrum was also prominent in the Pacific data. This peak may be either a fortnightly tide or an alias of the b. Frequency domain largest tide (the M2 tide, two cycles per lunar day). No spectrum peaks were seen in data from the Cape Analysis of the Pacific data advanced the fortnightly Hatteras station, but two peaks were seen at the Anti- tide as the more likely explanation (Broek 1969a).

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Isotherms at the Cape Hatteras station and at the Gas hydrates are found below the seafloors of the Blake Plateau station can move roughly 400 m in either margins of all oceans, and also below permafrost (Ma- direction, and 200 m at the Antigua station. These mo- zurenko and Soloviev 2003). These hydrates often emit tions are greater than the 86–160 m observed in the methane into the ocean in either gradual or violent Pacific data (Broek 2000). releases (Judd 2003), but it is not known if such processes have a detectable effect on ocean-bottom temperatures. 5. Summary Acknowledgments. I am grateful to Charles F. Wie- The station at 1400 m off Cape Hatteras is often in busch and Warren A. Tyrrell for suggesting this project. the DWBC, composed of Labrador Sea Water. The It is a pleasure to recall conversations with John C. average temperature is 3.80°C with standard deviation Beckerle, R. H. (Nick) Nichols, and R. D. (Bob) Wor- of 0.098°C. Data range between 3.55° and 4.10°C. No ley concerning ocean data. I thank J. A. Pecon for work periodicities are seen. on the instrumentation for this project. Thanks are Data from off the Blake Plateau at 2000-m depth are given to Mrs. J. E. Larkin for the computer program- vastly different (Broek 1969b). Temperature rises are ming. I thank Christopher John Broek for a fine job of consistently faster than declines. Average current is be- producing the illustrations. I acknowledge fine coop- lieved to be small, but occasionally the DWBC may eration from the librarians at the University of Wiscon- move westward to the continental slope. Motion of the sin, Nova Southeastern University, Northern Illinois DWBC and overflow currents from the shelf is a pos- University, and Florida Atlantic University. sible explanation for the observed temperature fluctuations. Spectra show a large annual peak and a possible REFERENCES broad peak near 50-day periodicity. Bower, A. S., and H. D. Hunt, 2000: Lagrangian observations of The station near Antigua at 1700-m depth had aver- the deep western boundary current in the North Atlantic age temperature of 3.82°C and standard deviation of Ocean. Part II: The Gulf Stream–deep western boundary cur- 0.082°C. The data resemble those from the Cape Hat- rent crossover. J. Phys. Oceanogr., 30, 784–804. teras station except that the latter have more large fluc- Broek, H. W., 1969a: Determination of the aliasing band corre- tuations, possibly from the opposition between the Gulf sponding to each spectrum peak in ocean-bottom temperature spectra. J. Geophys. Res., 74, 5439–5448. Stream and the DWBC. Even though data from the ——, 1969b: Fluctuations in bottom temperature at 2000-meter Antigua station have less fluctuations, the similarity be- depth off the Blake Plateau. J. Geophys. Res., 74, 5449–5452. tween data from the two stations (and the fact that their ——, 2000: Bottom temperature fluctuations in the northeast Pa- standard deviations are comparable) suggest that pos- cific Ocean at depth 920 to 1140 m. J. Phys. Oceanogr., 30, 1910–1916. sible complex current fluctuations may exist near the Brown, W., W. Munk, F. Snodgrass, H. Mofjeld, and B. Zetler, “Barbuda right angle” in the Antilles chain. 1975: MODE Bottom Experiment. J. Phys. Oceanogr., 5, 75– The Antigua station is at nearly the same depth as 85. the Jungfern–Grappler Sill (through which flows water Chelton, D. B., and R. E. Davis, 1982: Monthly mean sea-level that becomes Caribbean Deep Water). Temperature variability along the west coast of North America. J. Phys. Oceanogr., 12, 757–768. fluctuation episodes at the Antigua station show a su- Coles, V. J., M. S. McCartney, D. B. Olson, and W. M. Smethie, perficial resemblance to current fluctuations reported 1996: Changes in Antarctic Bottom Water properties: The by MacCready et al. (1999) at the Jungfern–Grappler western South Atlantic in the late 1980s. J. Geophys. Res., Sill. 101, 8957–8970. Davis, E. E., K. Wang, K. Becker, R. E. Thomson, and I. The only tide observed was a small 14.8-day spec- Yashayaev, 2003: Deep-ocean temperature variations and trum peak from the Antigua station. In general, the implications for errors in seafloor heat flow determinations. time domain seems more instructive than the frequency J. Geophys. Res., 108, 2034, doi:10.1029/2001JB001695. domain. The data and the references point toward the Doodson, A. T., 1922: Harmonic development of the tide- existence of large episodic currents in the deep waters generating potential. Proc. Roy. Soc. London, A100, 305– 329. of the northwest Atlantic. These episodic curents may Fine, R. A., and R. L. Molinari, 1988: A continuous deep western be related to the “periodic surges of cold bottom wa- boundary current between Abaco (26.5°N) and Barbados ters” reported by Johns et al. (1993), or they may be (13°N). Deep-Sea Res., 35, 1441–1450. related to the “large anticyclonic eddies” that propa- Frankignoul, C., 1981: Low-frequency temperature fluctuations gate northwestward from the North Brazil Current off Bermuda. J. Geophys. Res., 86, 6522–6528. Fuglister, F. C., 1960: Atlantic Ocean Atlas of Temperature and (Johns et al. 1998). Salinity Profiles and Data from the International Geophysical Temperature fluctuations on the continental slope Year of 1957–1958. Woods Hole Oceanographic Institution often resemble (with reversed sign) fluctuations in sea Atlas Series, 1, 209 pp. surface height; however, the resemblances may be co- ——, 1963: Gulf Stream 60. Progress in Oceanography, Vol. 1, Pergamon, 265–383. incidental. “Semiannual” temperature fluctuations ob- Hogg, N. G., and H. Stommel, 1985: On the relation between the served in 1962 are not attributable to astronomical deep circulation and the Gulf Stream. Deep-Sea Res., 32, tides. 1181–1193.

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——, and W. Zenk, 1997: Long-period changes in the bottom in the Tropics with a 40–50 day period. J. Atmos. Sci., 29, water flowing through Vema Channel. J. Geophys. Res., 102, 1109–1123. 15 639–15 646. ——, and ——, 1994: Observations of the 40–50 day tropical os- Jiang, L., and R. W. Garwood Jr., 1996: Three-dimensional simu- cillation—A review. Mon. Wea. Rev., 122, 814–837. lations of overflows on continental slopes. J. Phys. Oceanogr., Mazurenko, L. L., and V. A. Soloviev, 2003: Worldwide distribu- 26, 1214–1233. tion of deep-water fluid venting and potential occurrences of Johns, W. E., T. N. Lee, F. A. Schott, R. J. Zantopp, and R. H. gas hydrate accumulations. Geo-Mar. Lett., 23, 162–176. Evans, 1990: The North Brazil Current retroflection: Sea- Mills, C. A., and P. Rhines, 1979: The Deep Western Boundary sonal structure and eddy variability. J. Geophys. Res., 95, Current at the Blake–Bahama Outer Ridge: Current meter 22 103–22 120. and temperature observations, 1977–78. WHOI Tech. Rep. ——, D. M. Fratantoni, and R. J. Zantoff, 1993: Deep Western 79–85, 77 pp. Boundary Current variability off northeastern Brazil. Deep- Richardson, P. L., 1977: On the crossover between the Gulf Sea Res., 40, 293–310. Stream and the western boundary undercurrent. Deep-Sea ——, T. N. Lee, R. C. Beardsley, J. Candela, R. Limeburner, and Res., 24, 139–159. B. Castro, 1998: Annual cycle and variability of the North ——, and J. A. Knauss, 1971: Gulf Stream and western boundary Brazil Current. J. Phys. Oceanogr., 28, 103–128. undercurrent observations at Cape Hatteras. Deep-Sea Res., Judd, A. G., 2003: The global importance and context of methane 18, 1089–1109. escape from the seabed. Geo-Mar. Lett., 23, 147–154. Riser, S. C., H. Freeland, and H. T. Rossby, 1978: Mesoscale motions near the deep western boundary of the North At- Kaempf, J., and J. O. Backhaus, 1999: Sediment-induced slope lantic. Deep-Sea Res., 25, 1179–1191. convection: Two-dimensional numerical case. J. Geophys. Rubython, K. E., K. J. Heywood, and J. Vassie, 2001: Interannual Res., 104 (C9), 20 509–20 522. variability of bottom water temperatures in Drake Passage. J. Lai, D. Y., 1984: Mean flow and variabilities in the Deep Western Geophys. Res., 106, 2779–2793. Boundary Current. J. Phys. Oceanogr., 14, 1488–1498. Saunders, P. M., and J. W. Cherriman, 1983: Abyssal temperature Lee, T. N., W. Johns, F. Schott, and R. Zantopp, 1990: Western measurements with Aanderaa current meters. Deep-Sea Res., boundary current structure and variability east of Abaco, Ba- 30, 663–667. hamas at 26.5°N. J. Phys. Oceanogr., 20, 446–466. Shaffer, G., S. Hormazabat, O. Pizzari, and S. Salinas, 1999: Sea- ——, W. E. Johns, R. J. Zantopp, and E. R. Fillenbaum, 1996: sonal and interannual variability of currents and temperature Moored observations of western boundary current variability off central Chile. J. Geophys. Res., 104, 29 951–29 961. and thermocline circulation at 26.5°N in the subtropical Stommel, H., 1958: The abyssal circulation. Deep-Sea Res., 5, 80– North Atlantic. J. Phys. Oceanogr., 26, 962–983. 82. MacCready, P., W. E. Johns, C. G. Rooth, D. M. Fratantoni, and Swallow, J. C., and L. Worthington, 1961: An observation of a R. A. Watlington, 1999: Overflow into the deep Caribbean: deep countercurrent in the western North Atlantic. Deep-Sea Effects of plume variability. J. Geophys. Res., 104 (C11), Res., 8, 1–19. 25 913–25 935. Thorpe, S. A., 1987: Current and temperature variability on the Madden, R. A., and P. R. Julian, 1971: Detection of a 40–50 day continental slope. Philos. Trans. Roy. Soc. London, A323, oscillation in the zonal wind in the tropical Pacific. J. Atmos. 471–517. Sci., 28, 702–708. Wunsch, C., 1967: The long-period tides. Rev. Geophys., 5447– ——, and ——, 1972: Description of global-scale circulation cells 5474.

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