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

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Bottom Temperature Fluctuations on the Continental Slope of the Northwest Atlantic Ocean 388 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 35 Bottom Temperature Fluctuations on the Continental Slope of the Northwest Atlantic Ocean 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 tempera- ture 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- © 2005 American Meteorological Society Unauthenticated | Downloaded 10/03/21 12:28 AM UTC JPO2695 MARCH 2005 BROEK 389 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 semian- nual 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 Unauthenticated | Downloaded 10/03/21 12:28 AM UTC 390 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 35 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 semidi- urnal 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 sta- tion are shown in Fig. 2. Averaged temperature from the Cape Hatteras station is shown in Fig. 3. Small fluc- tuations 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).
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