Phys. Oceanogr.,Vol.4, No. 1, pp. 53-61 (1993) @ VSP 1993.

The bottom currents in the area of abyssal hills in the north-east tropical Pacific Ocean* T. A. DEMIDOVA, E. A. KONTAR', A. V. SOKOV and A. M. BELYAEV Abstract - - The results of simultaneous measurements of the bottom (6.25 and 35 m above the bottom) currents, deep currents, and surface currents made at three points in the north-east tropical Pacific Ocean are given. The bottom intensification of the current velocity is revealed in a layer of 35-25 m above the bottom. The estimation of the thickness of the bottom boundary layer (BBL) indicates that the velocity intensification is observed over the boundary layer upper border, A 10-day long benthic storm with a

maximum measured velocityof 13 cm/s was revealed 6 m above the bottom. As was found, the origin of the benthic storm is associated with the penetration of an anticyclonic eddy down to the bottom. In recent years, ideas about the bottom circulation in the world's oceans have changed drastically. Bottom currents with velocities of 20-30 cm/s and even more than 50 cm/s have been measured at abyssal depths, and sharp intensification of the bottom currents up to several weeks long, which results in the bottom erosion (benthic storms), has been recorded. A relationship between the benthic storms and meanders (rings of jet currents), as well as synoptic eddies, has been established. However, the problem of the character of the interplay between the bottom boundary layer (BBL) and the ocean's water column has been poorly studied. In ref. 1, measurements of the bottom currents carried out during February-March 1988 on the R / V Dmitry Mendeleev (Cruise 41) in the north-east tropical Pacific Ocean have been reported. To reveal a relationship between the recorded features of the bottom currents, the deep-sea, and surface stratification, this paper involves data from complex research carried out simultaneously in the same region (Fig. 1) on the R / V Professor Khromov (Cruise 15), which encompasses measurement of the currents from the surface to the bottom, a hydrological survey of the observation area, and the meridional transect. At points 1 (14~ ' N, 131000 , W) and 2 (13056.5 ' N, 131001, W), measurements were carried out by means of bottom self-surfacing buoys (BSSB) (DS-1 and DS-2) developed at the Research Institute for the Arctic and Antarctic and the State Oceanographic Institute [2]. ATSIT meters were located 25 and 35 m above the bottom. Observations lasted 13 days (24 February-8 March) with 15 min intervals. The bottom stations were deployed close to the self-contained moorings (SCM) 1 and 2. The surface and deep currents were registered at eight levels: 100-2500 m (SCM-1) and 200-4500m (SCM-2). At point 3 (13~ 132057 ' W), observations were carried out on the self-contained bottom station DS-3 developed at the Institute of Oceanology of the USSR Academy of Sciences (a version of ADS-M) [3]. A P O T O K meter was deployed 6 m above the bottom; measurements were 35 days long (4 February-10 March). A hydrological survey of the observation area and the meridional transect along *Translated by Mikhail M. Trufanov. UDK 554.465.46.

54

T. A. Demidova

et al.

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Figure 1. (a) Observation area in the north-east tropical Pacific Ocean (hatched square). (b) Bottom topography in the observationarea and (c, d) in close vicinityto the points of measurement. Vectors of the average current velocity6 m above the bottom at DS-3, 25 m above the bottom at DS-1 and DS-2, and at the level of 4500m at SCM-2. 131 ~ W involved CTD measurements and the determination of hydrochemical parameters. Down to 2000 m, the operations were implemented by a G I D R O Z O N D complex (Special Design Bureau of the State Committee for Hydrometeorology, Obninsk). Deep-sea sampling bottles were used during sounding down to 6000 m. Table 1 shows the characteristics of the deep and bottom currents in the north-east tropical Pacific Ocean derived from the data of instrumental measurements. Figure 2 shows the distribution of the daily average velocity vectors throughout the water column. As to its bottom morphological features, this region can be considered as a zone of abyssal hills. According to bathymetric charts, there are several sea peaks 1000 m high. According to a detailed bottom survey, differences of the bottom topography in the immediate proximity of the bottom stations amount to approximately 200 m for several kilometres. At all three points, a clear-cut range of hills and valleys stretches from the north-west to the south-east. Figure 1 shows the bottom topography at the stations according to a detailed bottom survey.

Bottom currents bt the north-east tropical Pacific Ocean

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Figure 3. Verticaldistributionof oceanographiccharacteristicsin the regionof measurement: (a) soundings; (b) meridional transect along 131~W; (1) isotherms; (2) isohalines. Hydrological conditions in the deep oceanic layer in the region of the measurements are formed under the influence of the Antarctic bottom water mass (ABWM) propagating generally north-eastwards, according to the macrobottom topography of the northeastern trench in the Pacific Ocean, namely the extent of the Clarion and Clipperton fracture zones [4]. A benthic thermocline (BT) is found in the vertical distribution of oceanographic characteristics at the levels 3650-3750 m down to 4000 m (Fig. 3). A BT is defined by vertical gradients of oceanographic characteristics which are several times

Bottom currents in the north-east tropical Pacific Ocean

57

higher compared with the layers above and below (2-4x 10-4 grad/m). A BT represents a transition zone between the ABWM and the deep water mass in the Pacific Ocean. A quasi-isothermal distribution of temperature with depth is observed below the BT. The bottom currents measured during 25 February-8 March at three stations were mainly southward with an average velocity of 1-2.5 cm/s. T h e periods of weak currents alternated with periods of sharply increasing velocities, having maximum values of 10, 9, and 13 cm/s for DS-1, DS-2, and DS-3, respectively. At points 1 and 2, a vertical correlation of the bottom currents is obvious in the diagrams showing a time dependence of the modulus, direction, and the zonal and latitudinal components of the velocity, as well as in the curves showing a difference in the velocity moduli and directions between the upper and lower instruments for each station (Fig.2). The first 4 days of measurements at DS-1 were an exception when westward currents prevailed in the upper layer, south-westward ones in the lower, and the rotation of the velocity vector exceeded 30 ~ The velocities of currents 35 m above the bottom were lower than those at 25 m: the average values for DS-1 were lower by 0.5-1 cm/s, and those for DS-2 by 1-2cm/s. The increase of the currents towards the bottom was accompanied by rotation of the velocity vector leftwards (counter-clockwise). The mean rotations of the vectors in this direction were 20 ~ and 17~ for DS-1 and DS-2, respectively. As the velocity increased, the rotation value decreased (except for the first 4 days at DS-1). The bottom intensification of currents (close to our observation area) has also been revealed by Hayes [5, 6]. According to his measurements, at the point 14038, N, 125~ the velocity increased between 200 and 30 m above the bottom and then decreased between 30 and 6 m above the bottom. The velocity vector rotation between the levels 30 and 6m was 9~ counter-clockwise. At 9~ 151~ W, the velocity increased within the layer of 500-50 m above the bottom, decreasing further downwards. The estimated thickness of Ekman's layer and the rotation of the velocity vector measured allowed the conclusion that the weakening of the current below 30 m in the first case and below 50 m in the second is related to the effect of the boundary layer. In both cases, we did not manage to find a reason for the increasing velocity above this layer. The expression for the thickness of the turbulent Ekman layer, used in refs 5 and 6 for a non-stratified fluid, has the following form: he = k o u . / f ,

(1)

where k0 is Karman's constant, equal to 0.4; f is the Coriolis parameter; and u. is the dynamic velocity determined as u. -- (To~p) 1/2, where 7-ois, in its turn, the shear stress at the bottom and p is the seawater density. The dynamic velocity for the uniform bottom is preset in the following form: u, = 0.03U, (2) where U is the current velocity over the boundary layer [5, 6]. Substituting the maximum daily average velocity of 4.5 cm/s at DS-1 into equations (1) and (2) as U, we find that the thickness of the bottom boundary layer is 16 m. This estimation of the thickness of the turbulent Ekman layer allows the conclusion that the meters at DS-1 and DS-2 were outside the bottom boundary layer. The latter agrees with the recorded increase of the velocity within the layer 35-25 m above the bottom and the sharp rotation of the velocity vector. As to DS-3, the thickness of the Ekman layer was 30m at the maximum daily average velocity of 8 cm/s in the place of deployment. Thus, a POTOK meter, located 6 m above the bottom, was deployed within the BBL.

58

T. A. Demidova

et al.

The bottom currents at DS-3 are considered in detail in ref. 1. Two periods, which vary drastically by the velocity value and the direction of currents, are typical of these currents measured from 5 February to 11 March. During the first half of the measurements (5-24 February), the velocities were extremely low (1-2 cm/s), but beginning from 29 February they increased and reached 13 cm/s on 2-3 March. Then the velocity decreased gradually, remaining considerably higher than during the first half of the measurements. In ref. 1, it is suggested that this interval of a steady velocity increase should be considered as an abyssal bottom storm. The development of the storm was accompanied by a reversal of the current (during 18-19 February the vector veered practically by 180~ from the north-west to the south-east). Spectral analysis of the current velocity fields was carried out through computation of the spectrograms of the kinetic energy density and filtration of the time series. The spectra were computed by the fast Fourier transformation algorithm [7]. To provide steady results at the admissible frequency resolution, the spectra were averaged over samplings and frequencies. Confidence intervals were determined on the basis of the X2 test. The Tukey parameters of the series filtration were selected according to harmonics singled out of the spectra. Analysis of the energy spectra for DS-1, DS-2, and DS-3 (Fig. 4) reveals significant peaks at the tidal (diurnal and semi-diurnal) and inertial frequencies (the period of inertial oscillations is 48 h at the latitude of the measurements). This is pronounced in the spectra of DS-3. Note that the spectra computed over the entire series and over the segment corresponding to the observation interval at all other stations are identical. A small, although pronounced, energy peak at the period 6 h is found in the same place. An analogous peak was well pronounced in the spectrum of the instrument's recordings 6 m above the bottom during the experiment done by Hayes [5]. This is similar to the doubled tidal frequency induced by the bottom 'straightening' which is described in ref. 8. The velocity spectra of the bottom currents at DS-1 are similar. In addition to the dominant tidal and inertial variability, a peak is also present in the low-frequency spectral part. However, owing to the insufficient length of the series under analysis, it cannot be treated as significant. For DS-2, the energy spectrum is essentially of another form: the peak at the inertial frequency is absent, and the energy of the tidal oscillations is essentially lower than that for DS-1 and DS-3. The absence of a peak at the inertial frequency can be related to the well-known stratification and the unsteady character of these oscillations [9, 10]. In the upper layer, the overshoots of the inertial energy of the currents often correlate with the wind [9, 11]. Observations carried out in the oceanic water column [12, 13] indicate the propagation of inertial oscillations from the top downwards. The inertial components in the bottom layer can result from further downward propagation of these oscillations. Thus, in ref. 5, the propagation of inertial oscillations downwards in the lower 200 m layer with a group velocity of 0.04 cm/s is revealed. This is accompanied by a sharp decrease in their energy in the BBL. Inertial oscillations can also be generated during the interaction between the low-frequency flow and the bottom topography. If the bottom Ekman layer is developed, the generation of inertial oscillations can be analogous to their generation at the surface. A considerable increase in the energy of the inertial oscillations towards the bottom is reported in ref. 6. Analogous results indicative of the generation of inertial oscillations at the bottom were also obtained by Saunders [14] during measurements of the bottom currents on the Madeira Abyssal Plain in the North Atlantic Ocean. The results of the spectral analysis of the bottom current velocities at DS-1 indicate increasing energy

59

Bottom currents in the noah-east tropical Pacific Ocean

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of the inertial oscillations towards the bottom and do not contradict the hypothesis about the generation of inertial oscillations near the bottom. On the horizontal correlation of the bottom currents The horizontal scale of variability of the bottom currents on the abyssal plains is 35 km according to Saunder's data [14], and 27 km according to Vangriesheim [15]. In the zone of the abyssal hills they can be even lower. Hayes [6] did not find a correlation of the bottom currents at a distance of 10 km. He found a good correlation only at stations spaced apart by 2km. In our case, DS-1 and DS-2 were spaced apart by 36km along the meridian, and DS-3 was located 200 km west-south-west from DS-2. A combined

60

T. A. Demidova et al.

analysis of the results of measurements of the current velocity demonstrated that nearly the same direction of the bottom currents is typical of all points. A simultaneous increase of velocity (with a slight phase shift) was recorded during 29 February to 2 and 3 March. On 25 February, a general local maximum of the velocity was observed. Pronounced tidal oscillations (especially semi-diurnal) are common for the spectral variability. The spectral features of the energy characteristics are largely different for various points in the domain of inertial oscillations and on the synoptic scale. One can note a synoptic minimum approximately at the same periods (,,~ 4 - 5 days) on the spectra at points 1 and 3. On the bottom storm

One of the open issues of BBL dynamics consists in elucidation of the reasons for the sharp steady increase of the bottom current velocity which resembles the so-called benthic storm registered at DS-3 [1, 16-18]. In our case, enhancement of the bottom current corresponds to the southern transport throughout the entire water column, which is supported by measurements made at two SCMs at levels from 100 to 4500 m (Fig. 2). According to the results of the hydrological survey of the area in the region of the stations and the section along 131~ W, the dynamic conditions were determined by the anticyclonic eddy travelling north-westwards. According to the data of the microsurvey, the diameter of the dosed isolines is 60 km. The eddy is traced well for dynamic heights at least down to 2000m (the depth of the majority of the stations in the microobservation area). The eddy centre is located at 14~ N, 131~ W. SCM-1 appeared to be located to the north of the centre and SCM-2 to the south, which is supported by the difference in the directions of the currents measured at the closely spaced points. At SCM-1, southward transport prevailed between 100 and 2500m, at SCM-2 westward and south-westward, and only at the level of 4500 did southward transport dominate. The dynamical computations made using data of the deep-sea transect along 131~ also indicate that an anticyclonic eddy with its centre at 14~176 exists at the northern periphery of the north equatorial current. The meridional extent of this eddy is approximately 250 km, and the eddy found at the micro-observation area designates it centre. The temperature distribution at the meridional transect (its deep part is shown in Fig. 3b) is characterized by the bending of the isotherms at 13~ ~ N. The bending of isohalines and iso-oxygens also corresponds to the bending of isotherms. Downwelling of warm, less saline, and oxygen-impoverished seawater from the above layers reflects the conditions of vertical circulation in the anticyclonic eddy. The largest bending of the isotherms is observed at the levels 400-500 m (the upper part of the transect is not given here), where the velocity maximum is located. It is not pronounced in the thermocline and is preserved in deeper layers down to 4500 m. The bending of isolines at the section along 131~ is an anomalous phenomenon, since the long-term observations carried out in this section [4] demonstrate the gradual elevation of isolines from the south northwards without any bending in the observation area. Summing up data on the anomalous distribution of oceanographic characteristics in the deep sea and on the currents measured at SCMs, we can conclude that the eddy penetrates down to the bottom which results in a benthic storm. Previously, a relationship was established between benthic storms and meanders and rings of the Gulf Stream [16-18] and the East Australian current [19], as well as with synoptic eddies in the North Atlantic Ocean [20]. For the abyssal regions of the Pacific Ocean, the penetration of the eddy down to the bottom and its relationship with benthic storms has been found for the first time.

Bottom currents in the north-east tropical Pacific Ocean

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REFERENCES

1. Demidova, T. A. and Kontar', E. A. On the bottom currents in the area of development of ferrous manganese nodules. Dokl. Akad. Nauk SSSR (1989) 2, 468-472. 2. Balakin, R. A., Zograf, D. A., Pershin, A. A. and Nikitin, O. P. (Eds). A bottom self-surfacing buoy for studying the currents at abyssal depths. In: Complex Studies of the Tropical Pacific Ocean. Moscow (1991), pp. 8-17. 3. Kontar', E. A. Self-surfacing Systems for Geological and Geophysical Research in the Ocean. Moscow: Nauka (1984). 4. Sokov, A. V., Lezhnev, V. V., Lyashenko, A. F. Nikitin, O. P. and German, V. Kh. (Eds). Propagation of the Antarctic bottom water in the central and north-east trenches of the Pacific Ocean. In: Studies of the Oceanographic Processes 02 the Tropical Pacific Ocean. Moscow (1989), pp. 167-177. 5. Hayes, S. P. Benthic current observations at DOMES sites A, B, and C in the tropical North Pacific Ocean. In: Marine Geology and Oceanography of the Central Pacific Manganese Nodule Province, Bischoff, J. L. and Piper, D. Z. (Eds). (1979), pp. 83--112. 6. Hayes, S. P. The bottom boundary layer in the eastern tropical Pacific. Z Phys. Oceanogr. (1980) 10, 315-329. 7. Otnes, P. and Enoxon, L. Applied Analysis of Temporal Series. Moscow: Mir (1982). 8. Wimbusch, M. and Munk, W. The benthic boundary layer. In: The Sea, Maxwell, A. E. (Ed.) New York (1970), Vol. 4, Part 1, pp. 730-758. 9. Halpern, D. Observations of the deepening of the wind-mixed layer in the northeast Pacific Ocean. Z Phys. Oceanogr. (1974) 4, 454-466. 10. Webster, F. Observations of inertial period motions in the sea. Rev. Geophys. (1968) 6, 472-490. 11. Pollard, R. T. and Millard, R. C. Comparisons between observed and simulated wind-generated inertial oscillations. Deep-Sea Res. (1970) 17, 813-821. 12. Frankignoul, C. L. Preliminary observations of inertial wave energy flux in frequency, depth-space. Deep-Sea Res. (1974) 21, 895-910. 13. Leaman, K. D. and Sanford, T. B. Vertical energy propagation of inertial waves: a vector spectral analysis of velocity profiles. Z Geophys. Res. (1975) 80, 1975-1978. 14. Saunders, P. M. Benthic observations on the Madeira Abyssal Plain: current and dispersion. Z Phys. Oceanogr. (1983) 13, 1416--1429. 15. Vangriesheim, A. Deep layer variability in the eastern North Atlantic: the EDYLOC experiment. Oceanologica Acta (1988) 11, 149-158. 16. Hullistcr, Ch. D., Howell, A. R. M. and Jumars, P. A. Restless depths. In tlze World of Science (1984), 3-16. 17. Kelley, E, A. and Weatherly, G. L. Abyssal eddies near the Gulf Stream. Z Geophys. Res. (1985) 90, 3151-3159. 18. Weatherly, G. L. and Kolley, E. A. Storm and flow reversals at the HEBBLE site. Mar. Geol. (1985) 66, 205-218. 19. Mulhearn, P. J., Filloux, J. H., Lilley, F. E. M., Bindoff, N. L. and Fergnson, I. L. Abyssal currents during the formation and passage of a warm-core ring in the East Australian current. Deep-Sea Res. (1986) 33, 1563-1580. 20. Klein, H. Benthic storms, vortices and particle dispersion in the deep west European basin. Dtsch. Hydrogr. Z. (1987) 40, 87-102.