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Observations of kinetic energy levels in the antarctic circumpolar current at Drake Passage
Deep-Sea Research, Vol. 28A, pp, 1 to 17 © Pergamon Press Ltd 1981. Printed in Great Britain
0198-0149/81/0101-0001 $02,00/0
Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage W . D . NOWLIN, JR,* R. DALE PILLSBURYt a n d J. BOTTEROt
(Received 26 December 1979; in revisedform 12 June 1980; accepted 15 June 1980) Abstract--Based on 31 nearly year-long records of current in Drake Passage the kinetic energy levels in the deep water (below 2000 m) across the passage and through the water column in the central passage are examined. The energy spectra show no significant temporal variability; by contrast, the spatial differences are pronounced, with more fluctuation kinetic energy (KF) in the northern than the southern passage and more above 1000 m than below. Partitioning by frequency bands shows that approx. 283/0 of the KF results from fluctuations with periods between 2 h (the Nyquist period for the sampling rate) and 2 days and that the energy level for this band is rather uniform across the passage. Large values of KF at northern passage locations result primarily from more activity in the period band between 2 and 50 days. Although that band accounts for almost half of the total KF, a large fraction (23%) of observed kinetic energy is associated with longer periods. The long-term records allowed examination of the representativeness of results---cumulative plots of K F and kinetic energy of mean motion (Ku) vs time indicate that the kinetic energy densities reach equilibrium values (for specific long-term records) only after intervals of the order of 4 months. As in the case for the Gulf Stream system, abyssal KF values in the Antarctic Circumpolar Current system at Drake Passage are one or two orders of magnitude greater than KF values in the interior of the North Atlantic subtropical gyre. Moreover, deep KF values south of Cape Horn equal deep water values beneath the Gulf Stream. Values of KM from records deeper than 2500 m increase southward from northern to central locations in Drake Passage. In the central passage KM increases upward from values near 10 cm 2 s -2 at 2700 m to over 300 cm 2 s -z at 300 m. KF increases from deep water values near 25 cm 2 s - 2 to just over 200 cm 2 s- 2 near 300 m. Relative to mid depths, ratios of K F to K M increase near the surface and near bottom in the central passage. INTRODUCTION
FROM 1975 to 1978, time series measurements of current, temperature, and pressure were made at locations stretching across the Drake Passage from Cape Horn to Livingston Island in the South Shetland Islands. The purpose of the measurements and the associated oceanographic station and XBT (expendable bathythermograph) data was to define the principal energy-containing time and space scales and the kinematic and density structure of the Antarctic Circumpolar Current (ACC) in the region. These FDRAKE measurements were used in designing the Dynamic Response and Kinematics Experiment 1979 (DRAKE 79), a field experiment of the International Southern Ocean Studies. Previous publications based on the FDRAKE data have presented much background information on the ACC. Figure 1 is a plan view of Drake Passage showing the positions of long-term current meter moorings from 1975, 1976, 1977, and 1978 in relation to the average positions of * Department of Oceanography, Texas A&M University, College Station, TX 77843, U.S.A. t School of Oceanography, Oregon State University, Corvallis, OR 97331, U.S.A. 1
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Fig. 1. Locations of FDRAKE moorings from which were obtained the time series of temperature and currents used in this study. Solid lines identify climatological boundaries between water mass zones (after WmTwoR'rH, 1980).
boundaries between water mass regimes. Moorings 2, 4, 8, 10, 12, and 14 are from 1975;
D(Diana), A(Ann), 19, 76, and E(Elizabeth) are from 1976; C(Central), N(North), E(East), W(West), and S(South) are from 1977 ; and Y(Yelcho) is from 1978. Note the proximity of moorings 2 and Diana which will be referred to as north moorings. They are north of the average position of the Subantarctic Front in the Subantarctic Surface Water regime. Moorings 14 and Elizabeth, the south moorings, are in the Subantarctic Surface Water regime north of the average position of the Continental Water Boundary. Those moorings
Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage
3
near the historical position of the Polar Front, at the southern edge of the Polar Frontal Zone, are referred to as central moorings. The 1975 experiment included 15 moorings with current and temperature recorders spaced at equal distances across the passage. Six remained in place for one year; the others were recovered after three weeks. NOWLIN,WHITWORTH, and PILLSBURY(1977) used current data from the 1975 array for the short-term period to describe the kinematic structure and estimate the total transport of the ACC. BRYDENand PILLSBURY(1977) and FANDRV and PILLSBURY(1979) used year-long current meter data from 1975 to estimate the variability of through-passage currents and thus of the barotropic transport of the ACC. PILLSBURY,WHITWORTH,NOWLIN,and SClREMAMMANO(1979) gave basic descriptions of the current and temperature records for all of the 1975 long-term instruments and discussed the flow and thermal field in the light of regional bathymetry and hydrographic conditions. BRYDEN (1979) estimated the meridional heat flux associated with the fluctuations and related these estimates to energy conversion by baroclinic instabilities. The 1976 array was deployed prior to any study of the 1975 data set and was largely a repeat of the long-term array of 1975. Five current meter moorings were set across the Drake Passage. In 1976 two additional moorings were deployed near the central passage mooring to provide a through-passage sub-array of three moorings with separations of 10, 28, and 38 m. The 1977 experiment included a cluster of five current meter moorings set near the historical location of the Polar Front. The cluster, on which currents and temperatures were measured at depths from 300 to 3500 m, was designed specifically to provide information on spatial scales of energy variation. Vertical coherence and horizontal scales of temperature and current fluctuations are presented and discussed in detail by SCIREMAMMANO, PILLSBURY,NOWLIN,and WHITWORTH(1980) on the basis of data from the 1975, 1976, and 1977 experiments. During 1978, one current meter mooring, Yelcho, was maintained in the central passage to extend our records from that location for a fourth year. Data from that mooring have not been reported previously. The FDRAKE experiments produced one of the largest sets of current meter observations in oceanography. We have studied the observations with the objective of describing kinetic energy levels in the Drake Passage. The data are adequate to describe latitudinal trends in deep water across the ACC at this location and the variations with depth at the central moorings. In this paper we first present the spatial distributions of the kinetic energy densities of the mean motions. The spatial trends and year-to-year variations of kinetic energy spectra are discussed next. Particularly energetic frequency bands are noted. Fluctuation kinetic energy is partitioned among three frequency bands for the study: 2 h to 2 days, 2 days to 50 days, and greater than 50 days. The spatial variation of fluctuation kinetic energy for each of the partitions as well as for the total unpartitioned record is shown and discussed. METHODS
The analysis has been carried out by considering three frequency bands, which we refer to as high, intermediate, and low. The high frequency band is limited above by the 1-h sampling period used throughout the experiments. The band contains motions whose periods range from 2 h (the Nyquist period) to about 2 days. The intermediate band
4
w . D, NOWLIN, JR, R. DALE PILLSBURY and J. BOTTERO
extends from 2 days to 50 days and the low frequency band from 50 days down to a lower limit determined by the record lengths. To separate the bands, two digital low-pass filters were used, one with half power at 2 days (the L L P or low low pass filter) and the other with half power at about 50 days (the VLP or very low pass filter). Both are symmetrical Cosine-Lanczos filters. Our procedure was to put each raw time series through the two filters in succession, producing first a series with data points at 6-h intervals (the L L P series, comprised of the intermediate and low frequency bands) and then a series with a 36-h interval (the VLP series). Several statistical tools were used to examine the data but variance spectra were relied on most heavily. If a time series is considered to be a realization of a stationary stochastic process, one can relate the series to that process by calculating spectral confidence limits. The confidence limits represent the degree to which the stochastic process is measured by the data. The usual view is that a spectral peak is not statistically significant if its relative height is less than the width of the confidence interval. However, a variance spectrum is an exact and faithful representation of the distribution of energy in the associated time series. Even when it is not a good source of information about the underlying stochastic process because of broad confidence limits, the spectrum does contain useful information about the time series. We have approached the F D R A K E spectra in this spirit. The kinetic energy densities are given in terms of the kinetic energy per unit mass of the mean horizontal motion (KM) and the kinetic energies per unit mass of the fluctuations (Kr). Values of K r were investigated for the unfiltered, the L L P and the VLP records. If u and v represent measured current components to the east and north, respectively, then KM -- ½(~2 + ~2), K~ = ½[(u - fi)z + (v - ~)2], where overbars are used to denote record-length time averages.
K I N E T I C ENERGY LEVELS
The F D R A K E experiments produced 31 long-term current meter records from the Drake Passage. Kinetic energy levels for these records (Table 1) include KM; K r for the unfiltered, the L L P and VLP records, and the ratios of mean-to-fluctuation kinetic energies for each of these records. For each year, the data (Table 1) are arranged by latitude; the northernmost mooring data are presented first.
Mean flow Even a cursory examination of the energy levels (Table I) reveals that the fluctuation and total kinetic energies are greatest in the northern Drake Passage. However, the kinetic energy of the mean flow has quite a different spatial distribution (Fig. 2), at least at depth. For F D R A K E records from deeper than 2500 m, the kinetic energy of the mean motion tends to increase as one proceeds from the northern to the central passage. At lesser depths there are too few F D R A K E records to establish spatial trends. However, of the seven long-term current records between 350 and 650 m in the central passage, the kinetic energy of the mean motion for six of the records falls between 240 and 300 cm 2 s - : . By comparison, the one shallow record available from the southern passage gave a value of 49 cm z s-2.
Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage
Table 1.
Mooring depth im) 1975 2, 2771 4, 2837 8, 2741 10, 1019 10, 1519 10, 2519 12, 2604 14, 2667 1976 D, 2913 76, 2791 A, 581 A, 2781 E, 461 E, 2661 1977 N, 635 N, 1145 N, 2159 E, 365 E, 1079 E, 2093 C, 282 C, 586 C, 2010 C, 3521 S, 357 S, 2086 W, 550 W, 1060 1978 Y, 416 Y, 927 Y, 1947
5
Kinetic energy levels (cm 2 s-2)from the FDRAKE current records in Drake Passage
Record length
KE of the mean
(days)
KM
291 342 351 228 352 352 254 332
0.6 5.0 7.7 42.8 24.9 17.6 10.6 15.3
180.3 29.6 26.0 53.9 38.5 21.0 21.0 26.7
159.2 19.1 17.3 41.3 25.9 13.3 13.4 19.9
310 293 253 241 341 308
5.7 13.0 80.0 10.3 48.6 9.8
141.0 43.2 164.0 40.9 49.1 19.5
311 184 300 240 258 296 273 280 187 259 287 253 171 266
247.2 127.9 37.7 288.9 112.8 37.0 365.4 249.1 46.5 14.5 254.4 27.3 236.0 96.5
187 169 182
297.0 127.1 46.4
KE of fluctuations 1Krlu.f. KF[LLP Kr'IVLP
Mean KE + Fluctuation KE KM KM KM
KFI.nC
KFILLP
KF[VLP
39.4 4.5 5.1 13.8 8.2 4.5 4.6 7.9
0.003 0.2 0.3 0.8 0.6 0.8 0.5 0.6
0.004 0.3 0.4 1.0 1.0 1.3 0.8 0.8
0.02 1.1 1.5 3.1 3.1 4.0 2.3 1.9
118.1 34.4 149.7 32.6 28.8 12.5
21.4 9.3 73.9 8.4 10.8 3.1
0.04 0.3 0.5 0.2 1.0 0.5
0.1 0.4 0.5 0.3 1.7 0.8
0.3 1.4 1.1 1.2 4.5 3.1
125.6 52.7 27.8 211.0 64.5 30.1 227.2 145.0 31.6 43.8 226.2 28.0 136.3 54.9
107.1 40.8 20.8 187.5 54.0 21.4 205.4 128.5 23.4 33.3 204.9 20.8 120.0 43.4
48.3 -7.7 70.4 21.0
2.3 3.1 1.8 1.5 2.1 1.7 1.8 1.9 2.0 0.4 1.2 1.3 2.0 2.2
5.1 -4.9 4.1 5.4
77.4 49.9 6.0 5.4 79.2 4.4 49.1 26.5
2.0 2.4 1.4 1.4 1.7 1.2 1.6 1.7 1.5 0.3 1.1 1.0 1.7 1.8
106.9 63.0 32.0
98.9 51.0 22.7
48.4 18.4 5.4
2.8 2.0 1.4
3.0 2.5 2.0
6.1 6.9 8.7
4.7 5.0 7.8 2.7 3.2 6.2 4.8 3.6
Although more data are needed to establish the variation of kinetic energy with depth at the northern and southern sides of the Drake Passage, the F D R A K E records do provide sufficient coverage in the central region. In general the kinetic energy of the mean motion is seen to decrease rapidly with increasing depth down to about 2500 m (Fig. 3). All shallow records except from moorings 10 and Elizabeth are from the central passage. With the single exception of the 500-m record at 1976 mooring A, the depth variation of K M is quite consistent for all the central moorings. The depth profiles of K M from moorings 10 and Elizabeth, well south of the mean position of the Polar Front, are distinctly different from those from the central moorings. Fluctuation kinetic energy Before discussing spatial trends, it is important to note that for the F D R A K E records spatial differences are generally greater than temporal differences based on interannual
W.D. NOWLIN, JR, R. DALE PILLSBURYand
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Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage
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Kinetic energy spectra (cm 2 s-2 CPD-t) of unfiltered current records (linearly detrended) from 1976 moorings Diana (2913 m), Ann (2781 m), and Elizabeth (2661 m).
comparisons. This is convincingly shown by comparing spectra of unfiltered current records. Spectra from the deep meters at 1976 moorings D, A, and E (Fig. 4) show that energy levels are generally higher in the northern passage and lower in the southern passage. The difference is less for the diurnal and semi-diurnal tides than for other frequencies. In fact, the tidal energy levels are almost the same in the central and southern passage and only slightly greater in the northern passage. It is also to be noted that for frequencies between semi-diurnal and Nyquist the spectra from the northern passage fall off less rapidly than those from the central and southern passage; this is true also for unfiltered temperature spectra (not shown here). Spectra based on the deep records from 1975 moorings 2, 8, and 14 (Fig. 5) show the same general relationships as do the 1976 spectra, although those from moorings 14 and 8 are more alike than are the south and central spectra from the 1976 records. Nevertheless, the kinetic energy densities at the three sites are significantly different. By contrast, spectra from deep records at the central passage location do not differ significantly from one another, even over several years. The five records at approx. 2000 m from the 1977 cluster array (Fig. 6) produced spectra virtually the same. Within that same area, we have current records from each of the four years 1975 to 1978. Again, the spectra for these deep records (Fig. 7) do not differ significantly.
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Kinetic energy spectra (cm 2 s-2 CPD-1) of unfiltered current records (linearly detrended) from 1975 moorings 2 (2771 m), 8 (2741 m), and 14 (2667 m).
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Fig. 6. Kinetic energy spectra (cm 2 s- 2 C P D - 1 ) of unfiltered current records (linearly detrended) from central passage 1977 moorings North (2159 m), East (2093 m), South (2086 m), West (2074 m), and Central (2010 m).
Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage
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Fig. 7. Kinetic energy spectra (cm 2 s-2 CPD-1) of unfiltered current records (linearly detrended) from central passage moorings 1975, 8 (2741 m); 1976, 76 (2791 m); 1977, C (2010 m); and 1978, Y (1947 m).
The latitudinal distribution of Kr for deep records (Fig. 8) shows in more detail the regional energy distribution as well as the partitioning of energy by frequency bands. The partitioning of fluctuation kinetic energy into bands is summarized in Table 2. More than 70% of the fluctuation kinetic energy is in periods greater than 2 days. Of that, almost one-third is at periods greater than 50 days. The contribution to K F due to fluctuations with periods between 2 h and 2 days (the vertical separation of the two upper curves in Fig. 8) is rather uniform in the deep water across the passage, indicating that the energy level of high frequency phenomena varies little at depth. The large increase in fluctuation kinetic energy in the northern passage is
Table 2.
Average distribution by frequency band of squared variance from FDRAKE current records distributed across the passage between 2000 and 3000 m
Original records High frequency Intermediate frequency Low frequency
Period band
% Variance
2 h to record length 2 h to 2 days 2 to 50 days 50 days to record length
100 28 49 23
W . D . NOWLIN, JR, R. DALE PILLSBURYand J. BOI"rERO
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Fig. 8. Fluctuation kinetic energy from FDRAKE records at 2000 to 3000 m in Drake Passage. Unfiltered record for periods greater than 2 h, LLP record for periods greater than 2 days, and VLP record for periods greater than 50 days.
due to longer-period phenomena; in fact practically all of the increase is associated with intermediate frequency motions with periods of 2 to 50 days. For this intermediate frequency range (the energy of which is measured by the vertical separation of the two lower curves in Fig. 8) the energy level increases slightly as one proceeds from southern to central passage locations and then increases dramatically (by about 110 cm 2 s -2) at the northern moorings. For phenomena with periods greater than 50 days, the fluctuation kinetic energy levels are uniform at 4 or 5 cm z s-2 except at the northern moorings where they increase to order 30 cm 2 s- 2. One might question to what extent the kinetic energy densities depend on record length, e.g., if the records had been one month longer or shorter, would the values for KM or Kr have differed significantly from the values presented? We have plotted as functions of time (Fig. 9) the daily cumulative averages of kinetic energy of mean motion and of fluctuations from the unfiltered records. Each day's values were obtained from the mean current for the length of record to that day and from the fluctuations relative to that mean
Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage
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Daily cumulative averages of kinetic energies of mean motion (- - - ) and of fluctuations ( ).
current over the record length to that day. Examination of the records has led us to conclude that for record lengths greater than about four months the average values of K~ and K M are relatively stable. The most extreme variations might be expected for the records from moorings 2 and D in the northern passage where the fluctuations are greatest and the record-length mean currents are smallest. However, comparison of these relations with those for records 8 and A from the central passage and for records 14 and E from the southern passage show that the cumulative average kinetic energy for the
12
W.D. NOWLIN,JR, R. DALEPILLSBURYand J. BOTTERO
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Fig. 10. Kinetic energy of fluctuations from LLP records from FDRAKE moorings at central Drake Passage.
fluctuations shows most relative variation in the central passage. This probably is due to energetic fluctuations during the winter season. The cumulative average kinetic energy of the mean motion seems most variable in the southern passage locations, where the kinetic energy of the mean motion is also largest. Although our current meter data are adequate for the examination of the variation of kinetic energy levels across the Drake Passage at depth, we have long-term observations at different depths through the water column only in the central passage location. There K r values (Fig. 10) increase upward from 20 to 30 cm 2 s -2 in the deep water to just over 200 cm 2 s-2 near 300 m.
DISCUSSION
We have noted that the depth variation of KM in the central Drake Passage varies little from mooring to mooring. By comparison the depth profile of KM from moorings 10 and Elizabeth shows less increase in mean kinetic energy with decreasing depth than the central moorings (Fig. 3). This is likely due to the fact that moorings 10 and Elizabeth were farther from the mean position of frontal zones and associated current cores (Fig. 1). NOWLIN et al. (1977) and WnITWORTH(1980) have shown that in the water masses between
Observations of kinetic energy levelsin the Antarctic Circumpolar Current at Drake Passage
13
these frontal zones the vertical shear of geostrophic currents is small relative to shears at the fronts. Our study of the vertical shear of geostrophic currents at the Polar Front from closely spaced (25 to 40 km) pairs of F D R A K E hydrographic stations showed the vertical profiles of current and associated kinetic energy to be quite similar from different years. The resulting vertical profile of kinetic energy density associated with the geostrophic current at the Polar Front shows increases of approx. 400 cm 2 s -2 between 2500 m and 1000 m and greater than 2000 cm 2 s-2 between 2500 m and the sea surface. The vertical change in K M for the central moorings is only 100 cm 2 s-2 between 2500 and 1000 m and probably less than 1000 cm 2 s -2 between 2500 m and the sea surface (Fig. 3). The fact that the vertical shear associated with the Polar Front is much greater than the mean vertical shear observed at fixed, central passage locations is not unexpected. Although the central moorings are near the mean position of the Polar Front, the Front wanders considerably in time (ScIREMAMMANOet al., 1980). At moorings where there is less frontal activity the shear of kinetic energy is likely to be even smaller than observed for the central moorings. At mooring 10, for example, the change in observed KM between 2500 and 1000 m was approx. 30 cm 2 s-2. The fluctuation kinetic energy at intermediate and low frequencies (periods greater than 2 and 50 days, respectively) increased dramatically at the northern mooring relative to other moorings in Drake Passage (Fig. 8). The increase in fluctuation kinetic energy is consistent with the idea that the northern Drake Passage is an area in which the interaction of the current system with bathymetric and boundary constraints produces energetic meso-scale oscillations (PILLSBURYet al., 1979). Our shallowest long-term observation of KF was from 282 m in the central passage (Fig. 10). Thus, we cannot accurately estimate the near-surface vertical profile of K r, and it would be unwise to extrapolate to obtain a surface value of K r from these F D R A K E records. However, WYRTKI, MAGAARD and HAGER (1976) utilized surface ship drift observations to calculate kinetic energy of the mean flow and of fluctuations for the surface layers of the world ocean based on 5-degree grid squares. For the northern Drake Passage, they show a relative high of over 800 cm 2 s- 2 for the fluctuation kinetic energy. Their values of kinetic energy of the mean flow in the northern Drake Passage are between 100 and 200 cm 2 s -2. Although their surface values of K r are not inconsistent with our data (Table 1 or Fig. 10), our values of K M for the central passage are much larger than those reported by WYRTKI et al. (1976) for the northern passage. The discrepancy is perhaps not unexpected, because (as we have shown) the kinetic energy of the mean motion is less in the northern than in the central or southern passage locations. In addition, averaging over 5-degree squares might be expected to give results different from those of point measurements. Observations of the fluctuation kinetic energy distribution for the western North Atlantic were summarized by SCHMITZ(1978). For periods greater than one day, values of K F beneath the thermocline increased from order of 1 to approx. 150 cm 2 s- 2 approaching the Gulf Stream from the interior of the subtropical gyre. In the thermocline (at 600 m) the increase was roughly from 10 to 300 cm 2 s- 2. ScHuIvz's values along 55°W are summarized in Fig. 11. In the Drake Passage, by comparison, K r values (for periods greater than 2 days) at depths greater than 2000 m are of the order of 20 cm 2 s -2 in the central and southern passage, increasing to approx. 160 cm 2 s -2 in the northern passage. The upper layers (282 to 635 m) are much more energetic at central (to over
14
W . D . NOWLIN, JR, R. DALE PILLSBURYand Jl BOTTERO
I
I
I
300
% 200
I00
I
I
I
in N. Atlantic approximately alotxj 55°W
KF
• 600m
40
38
36°N
34
32
30
28
l
I
I
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I
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Gulf Stream Axis I
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300
I
Kr in Drake Passage
,% 200
• 282-635m • below 2000m
IO0
I
I
56
58
!
60°S
I
I
62
Fig. 11. Comparison of fluctuation kinetic energy levels in Drake Passage and the North Atlantic (adapted from SCHMITZ, 1978). Vertical bars indicate range of observed values at each latitude.
200cm 2 s -2) and southern (50cm 2 s -2) locations (Fig. 11), but we do not yet have estimates in the northern passage. Although the K r values from our LLP records should be smaller than those by SCHMITZfor the same observed variability because we exclude periods between 1 and 2 days, the difference should be small. In summary, deep K r values in the Antarctic Circumpolar Current system at Drake Passage, like those in the Gulf Stream system, are one or two orders of magnitude greater than abyssal K F values reported for the interior of the North Atlantic. However, only at the northern Drake Passage location are the ACC deep K r values as large as (or slightly greater than) the deep K e values beneath the Gulf Stream. It might also be noted that the relative plateau in abyssal Kr seen in the ACC is not typical of deep K r values from the Gulf Stream although the transition area between Gulf Stream and the interior may have relatively flat Kr distributions along 55°W. Large values of Kr may be expected in regions where significant amounts of potential energy associated with large mean flow are converted by instability mechanisms to fluctuation energy (GILL GREEN and SIMMONS, 1974). Thus the large values of K u and the relatively large values of K r in the Drake Passage are consistent with the concept that this region is one in which baroclinic instability is converting potential energy of the mean flow to fluctuations. FANDRY(1979) and BRYDEN(1979) demonstrated this to be the case for the Drake Passage. Using the 1975 FDRAKE data from near 2700m, BRYDEN
Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage
15
I0000 ~:
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282
mR L:'OlO
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100
Fig. 12. Kinetic energy spectra (cm 2 s - 2 C P D - t ) of unfiltered current records (linearly detrended) from central passage 1977 mooring Central. Numbers indicate depth (m) of current meters.
obtained 1.2 x 10-* ergcm -3 s -t as a rate of conversion of available potential energy from the large scale density distribution into kinetic and potential energies of fluctuations with periods greater than 1 day. The kinetic energy of fluctuations decreases generally with depth above about 1500 m for the central Drake Passage (Fig. 10). The decrease in energy content is illustrated in spectra from the 1977 central mooring (Fig. 12), which at most frequencies show a substantial increase in kinetic energy for records above 600 m depth as compared with those at greater depths. The pattern is substantiated by records from other 1977 moorings in the central passage. There is, however, a remarkable departure from the general pattern of kinetic energy decrease with depth; namely, for periods between 1 and 30 days the deepest record (3521 m, Fig. 12) had energy levels greater than those at 2010 m (approximately equal to those in the near-surface layers, in fact). This is not inconsistent with the existence of energetic, mesoscale, bottom-trapped waves in the central Drake Passage. We have examined the depth variation of the ratio of K M to K F (Table 1). The ratio generally decreases with increasing depth below about 1000 to 1200 m. SCIREMAMMANOet al. (1980) noted an increase in the ratio below 2500 m and suggested that the increase might be due to topographic scattering or bottom intensification of the current fluctuations. It should also be noted that the ratio of K M to K e generally decreases with decreasing depth for levels above about 600 m. This might indicate near-surface fluctuations, which would explain the reported (ScIREMAMMANOet al., 1980) reduction in normalized cross correlation at zero lag for velocity components above 500 m correlated with those at mid depth.
16
w . D . NOWLIN,JR, R. DALEPILLSBURYand J. BOTTERO
MCWILLIAMS,HOLLAND and Crlow (1978) presented results from numerical models of the Antarctic Circumpolar Current as a two-layer, wind-driven, quasi-geostrophic ocean in a channel with bottom and lateral friction. Their calculations showed that a partial barrier to zonal flow and a topographic bump significantly affect the resultant solutions, in terms of the equilibrium balances between eddies and mean currents and their spatial distributions. Readers are cautioned that our values for kinetic energy densities in Drake Passage, which is both a lateral barrier and a topographic obstacle to zonal flow, may not be representative of the ACC as a whole. They are further cautioned that the northern passage location of moorings Diana and 2 lie within the Subantarctic Water Mass regime and may not be representative of the Antarctic Circumpolar Current. SUMMARY
Between January 1975 and the end of 1978, 31 records of current and temperature of approx. 6 to 12 month duration were obtained at locations in the Drake Passage. The data are sufficient to characterize kinetic energy levels across the passage at depths below 2000 m and from 300 m to the bottom at central passage locations near the historical position of the Polar Front. We have considered kinetic energy of the mean motion (K~) and the kinetic energy of the fluctuations (Kr) from the mean motion. Study of KM and K r as functions of record length has convinced us that equilibrium values generally are not reached for record lengths less than about 4 months. However, when the complete record lengths are considered the energy levels seem quite stable. Kinetic energy spectra from almost yearlong records evidence no significant year-to-year differences for records from the same locality. Kinetic energy spectra from the deep, long-term records do show significant spatial variations from north to south across the passage and from near bottom to the shallowest depths sampled ( ~ 300 m) in the central passage. The kinetic energy of the year-long mean flow recorded at depths greater than 2500 m increases southward in the Drake Passage. Values of K u increase from an average of approx. 3 cm z s -2 in the northern passage (near 57°S) to values nearer 10 to 15 cm 2 s -2 in the central (,-, 59°S) and southern passage. In the central passage K M increases upward from near 10cm 2 s -2 at 2700m to approx, l l 0 c m 2 s -2 at 1000m and to over 3 0 0 c m 2 s -2 at 300m. At moorings more distant from the historical positions of the fronts, the vertical shear of KM is expected to be considerably less than at the central moorings, which are influenced by the large vertical current shear associated with the Polar Front. Filters with half-power points at 2 and at 50 days were applied to the current records to partition the fluctuation kinetic energy into three period bands: 2 h (the Nyquist period for these records) to 2 days, 2 to 50 days, and greater than 50 days. Based on partitioned records from depths between 2000 and 3000 m the division of K r between the high, intermediate, and low frequency bands was 28, 49, and 23~, respectively. Values of K r at depth due to high frequency fluctuations are almost constant across the Drake Passage. The K r due to motions with periods between 2 and 50 days increases somewhat from southern (10 cm 2 s-2) to central (15 cm 2 s-2) passage locations and then greatly increases to approx. 110 cm 2 s-2 at the northern moorings. For phenomena in the low frequency band the deep K r values are uniform at 4 or 5 cm 2 s-2 except at the northern moorings, where the average value is 30 cm 2 s- 2.
Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage
17
Fluctuation kinetic energy at the central moorings increases upward from 20 to 30 cm 2 s - 2 below 2000 m to over 200 cm 2 s - 2 near 300 m. Relative to mid-depth values ratios of KF/KM generally increase downward below 1000 or 1200 m (more so below 2500 m) and upward above 600 or 700 m. This may indicate the increasing importance of fluctuations near the bottom and near the surface. The K r values from the Antarctic Circumpolar Current system at Drake Passage are much greater than Kr values from the interior of the North Atlantic. Values of K r in the ACC are of the same order as those in the Gulf Stream system (including the transition between the stream and the interior), although only at the northern moorings are the Kr values actually as large as reported Kr values directly beneath the Gulf Stream. Acknowledgements--Support for the FDRAKE work was provided as a part of the International Southern Ocean Studies by the Office for the International Decade of Ocean Exploration of the National Science Foundation. The fine measurements of current and temperature result from the dedication of the current meter group at Oregon State University, especially to the careful preparations and supervision by Mr ROBERT STILL. We appreciate constructive comments and suggestions from Drs BRUCE WARREN, THOMAS WHITWORTH and CHRIS FANDRY. REFERENCES BRYDENH. L. (1979) Poleward heat flux and conversion of available potential energy in Drake Passage. Journal of Marine Research, 37, 1-22. BP,YDEN H. L. and R. D. PILLSB~V (1977 Velocity of deep flow in the Drake Passage. Journal of Physical Oceanography, 7, 803-810. FANDRY C. (1979) Baroclinic instability of the Antarctic Circumpolar Current in the Drake Passage. Ocean Modelling, 22, 8-9. FANDRY C. and R. D. PILLSBURY(1979) On the estimation of absolute geostrophic volume transport applied to the Antarctic Circumpolar Current. Journal of Physical Oceanography, 9, 449-455. GILL A. E., J. S. A. GREEN and A. J. SIMMONS(1974) Energy partition in the large-scale ocean circulation and the production of mid-ocean eddies. Deep-Sea Research, 21,499-528. McWILLIAMS J. C., W. R. HOLLANDand J, H. S. CHOW (1978) A description of numerical Antarctic Circumpolar currents. Dynamics of Atmospheres and Oceans, 2, 213-291. NOWLIN W. D., JR, T. WHI'rV4ORTH 1II and R. D. PILLSBURY (1977) Structure and transport of the Antarctic Circumpolar Current at Drake Passage from short-term measurements. Journal of Physical Oceanography,,7, 788-802. PILLSBURYR. D., T. WHITWORTHIII, W. D. NOWLIN, JR and F. SCI~MAMMANO(1979) Currents and temperatures as observed in Drake Passage during 1975. Journal of Physical Oceanography, 9, 469-482. SCHMITZ W. J., JR (1978) Observations of the vertical distribution of low frequency kinetic energy in the western North Atlantic. Journal of Marine Research, 36, 295-310. SCIREMAMMANOF., R. D. PILLSBURY,W. D. NOWLIN, JR and T. WHITWORTHIII (1980) Spatial scales of temperature and flow in Drake Passage. Journal of Geophysical Research (in press). WHITWORTH T. Ili (1980) Zonation and geostrophic flow of the Antarctic Circumpolar Current at Drake Passage. Deep-Sea Research, 27, 497-507. WYRTKI K., U MAGAARDand J. HAGER (1976) Eddy energy in the oceans. Journal of Geophysical Research, 81, 2641-2646.
0198-0149/81/0101-0001 $02,00/0
Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage W . D . NOWLIN, JR,* R. DALE PILLSBURYt a n d J. BOTTEROt
(Received 26 December 1979; in revisedform 12 June 1980; accepted 15 June 1980) Abstract--Based on 31 nearly year-long records of current in Drake Passage the kinetic energy levels in the deep water (below 2000 m) across the passage and through the water column in the central passage are examined. The energy spectra show no significant temporal variability; by contrast, the spatial differences are pronounced, with more fluctuation kinetic energy (KF) in the northern than the southern passage and more above 1000 m than below. Partitioning by frequency bands shows that approx. 283/0 of the KF results from fluctuations with periods between 2 h (the Nyquist period for the sampling rate) and 2 days and that the energy level for this band is rather uniform across the passage. Large values of KF at northern passage locations result primarily from more activity in the period band between 2 and 50 days. Although that band accounts for almost half of the total KF, a large fraction (23%) of observed kinetic energy is associated with longer periods. The long-term records allowed examination of the representativeness of results---cumulative plots of K F and kinetic energy of mean motion (Ku) vs time indicate that the kinetic energy densities reach equilibrium values (for specific long-term records) only after intervals of the order of 4 months. As in the case for the Gulf Stream system, abyssal KF values in the Antarctic Circumpolar Current system at Drake Passage are one or two orders of magnitude greater than KF values in the interior of the North Atlantic subtropical gyre. Moreover, deep KF values south of Cape Horn equal deep water values beneath the Gulf Stream. Values of KM from records deeper than 2500 m increase southward from northern to central locations in Drake Passage. In the central passage KM increases upward from values near 10 cm 2 s -2 at 2700 m to over 300 cm 2 s -z at 300 m. KF increases from deep water values near 25 cm 2 s - 2 to just over 200 cm 2 s- 2 near 300 m. Relative to mid depths, ratios of K F to K M increase near the surface and near bottom in the central passage. INTRODUCTION
FROM 1975 to 1978, time series measurements of current, temperature, and pressure were made at locations stretching across the Drake Passage from Cape Horn to Livingston Island in the South Shetland Islands. The purpose of the measurements and the associated oceanographic station and XBT (expendable bathythermograph) data was to define the principal energy-containing time and space scales and the kinematic and density structure of the Antarctic Circumpolar Current (ACC) in the region. These FDRAKE measurements were used in designing the Dynamic Response and Kinematics Experiment 1979 (DRAKE 79), a field experiment of the International Southern Ocean Studies. Previous publications based on the FDRAKE data have presented much background information on the ACC. Figure 1 is a plan view of Drake Passage showing the positions of long-term current meter moorings from 1975, 1976, 1977, and 1978 in relation to the average positions of * Department of Oceanography, Texas A&M University, College Station, TX 77843, U.S.A. t School of Oceanography, Oregon State University, Corvallis, OR 97331, U.S.A. 1
2
w . D . NOWLIN,JR, R. DALEPILLSBURYand J. BOTTERO
,
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Fig. 1. Locations of FDRAKE moorings from which were obtained the time series of temperature and currents used in this study. Solid lines identify climatological boundaries between water mass zones (after WmTwoR'rH, 1980).
boundaries between water mass regimes. Moorings 2, 4, 8, 10, 12, and 14 are from 1975;
D(Diana), A(Ann), 19, 76, and E(Elizabeth) are from 1976; C(Central), N(North), E(East), W(West), and S(South) are from 1977 ; and Y(Yelcho) is from 1978. Note the proximity of moorings 2 and Diana which will be referred to as north moorings. They are north of the average position of the Subantarctic Front in the Subantarctic Surface Water regime. Moorings 14 and Elizabeth, the south moorings, are in the Subantarctic Surface Water regime north of the average position of the Continental Water Boundary. Those moorings
Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage
3
near the historical position of the Polar Front, at the southern edge of the Polar Frontal Zone, are referred to as central moorings. The 1975 experiment included 15 moorings with current and temperature recorders spaced at equal distances across the passage. Six remained in place for one year; the others were recovered after three weeks. NOWLIN,WHITWORTH, and PILLSBURY(1977) used current data from the 1975 array for the short-term period to describe the kinematic structure and estimate the total transport of the ACC. BRYDENand PILLSBURY(1977) and FANDRV and PILLSBURY(1979) used year-long current meter data from 1975 to estimate the variability of through-passage currents and thus of the barotropic transport of the ACC. PILLSBURY,WHITWORTH,NOWLIN,and SClREMAMMANO(1979) gave basic descriptions of the current and temperature records for all of the 1975 long-term instruments and discussed the flow and thermal field in the light of regional bathymetry and hydrographic conditions. BRYDEN (1979) estimated the meridional heat flux associated with the fluctuations and related these estimates to energy conversion by baroclinic instabilities. The 1976 array was deployed prior to any study of the 1975 data set and was largely a repeat of the long-term array of 1975. Five current meter moorings were set across the Drake Passage. In 1976 two additional moorings were deployed near the central passage mooring to provide a through-passage sub-array of three moorings with separations of 10, 28, and 38 m. The 1977 experiment included a cluster of five current meter moorings set near the historical location of the Polar Front. The cluster, on which currents and temperatures were measured at depths from 300 to 3500 m, was designed specifically to provide information on spatial scales of energy variation. Vertical coherence and horizontal scales of temperature and current fluctuations are presented and discussed in detail by SCIREMAMMANO, PILLSBURY,NOWLIN,and WHITWORTH(1980) on the basis of data from the 1975, 1976, and 1977 experiments. During 1978, one current meter mooring, Yelcho, was maintained in the central passage to extend our records from that location for a fourth year. Data from that mooring have not been reported previously. The FDRAKE experiments produced one of the largest sets of current meter observations in oceanography. We have studied the observations with the objective of describing kinetic energy levels in the Drake Passage. The data are adequate to describe latitudinal trends in deep water across the ACC at this location and the variations with depth at the central moorings. In this paper we first present the spatial distributions of the kinetic energy densities of the mean motions. The spatial trends and year-to-year variations of kinetic energy spectra are discussed next. Particularly energetic frequency bands are noted. Fluctuation kinetic energy is partitioned among three frequency bands for the study: 2 h to 2 days, 2 days to 50 days, and greater than 50 days. The spatial variation of fluctuation kinetic energy for each of the partitions as well as for the total unpartitioned record is shown and discussed. METHODS
The analysis has been carried out by considering three frequency bands, which we refer to as high, intermediate, and low. The high frequency band is limited above by the 1-h sampling period used throughout the experiments. The band contains motions whose periods range from 2 h (the Nyquist period) to about 2 days. The intermediate band
4
w . D, NOWLIN, JR, R. DALE PILLSBURY and J. BOTTERO
extends from 2 days to 50 days and the low frequency band from 50 days down to a lower limit determined by the record lengths. To separate the bands, two digital low-pass filters were used, one with half power at 2 days (the L L P or low low pass filter) and the other with half power at about 50 days (the VLP or very low pass filter). Both are symmetrical Cosine-Lanczos filters. Our procedure was to put each raw time series through the two filters in succession, producing first a series with data points at 6-h intervals (the L L P series, comprised of the intermediate and low frequency bands) and then a series with a 36-h interval (the VLP series). Several statistical tools were used to examine the data but variance spectra were relied on most heavily. If a time series is considered to be a realization of a stationary stochastic process, one can relate the series to that process by calculating spectral confidence limits. The confidence limits represent the degree to which the stochastic process is measured by the data. The usual view is that a spectral peak is not statistically significant if its relative height is less than the width of the confidence interval. However, a variance spectrum is an exact and faithful representation of the distribution of energy in the associated time series. Even when it is not a good source of information about the underlying stochastic process because of broad confidence limits, the spectrum does contain useful information about the time series. We have approached the F D R A K E spectra in this spirit. The kinetic energy densities are given in terms of the kinetic energy per unit mass of the mean horizontal motion (KM) and the kinetic energies per unit mass of the fluctuations (Kr). Values of K r were investigated for the unfiltered, the L L P and the VLP records. If u and v represent measured current components to the east and north, respectively, then KM -- ½(~2 + ~2), K~ = ½[(u - fi)z + (v - ~)2], where overbars are used to denote record-length time averages.
K I N E T I C ENERGY LEVELS
The F D R A K E experiments produced 31 long-term current meter records from the Drake Passage. Kinetic energy levels for these records (Table 1) include KM; K r for the unfiltered, the L L P and VLP records, and the ratios of mean-to-fluctuation kinetic energies for each of these records. For each year, the data (Table 1) are arranged by latitude; the northernmost mooring data are presented first.
Mean flow Even a cursory examination of the energy levels (Table I) reveals that the fluctuation and total kinetic energies are greatest in the northern Drake Passage. However, the kinetic energy of the mean flow has quite a different spatial distribution (Fig. 2), at least at depth. For F D R A K E records from deeper than 2500 m, the kinetic energy of the mean motion tends to increase as one proceeds from the northern to the central passage. At lesser depths there are too few F D R A K E records to establish spatial trends. However, of the seven long-term current records between 350 and 650 m in the central passage, the kinetic energy of the mean motion for six of the records falls between 240 and 300 cm 2 s - : . By comparison, the one shallow record available from the southern passage gave a value of 49 cm z s-2.
Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage
Table 1.
Mooring depth im) 1975 2, 2771 4, 2837 8, 2741 10, 1019 10, 1519 10, 2519 12, 2604 14, 2667 1976 D, 2913 76, 2791 A, 581 A, 2781 E, 461 E, 2661 1977 N, 635 N, 1145 N, 2159 E, 365 E, 1079 E, 2093 C, 282 C, 586 C, 2010 C, 3521 S, 357 S, 2086 W, 550 W, 1060 1978 Y, 416 Y, 927 Y, 1947
5
Kinetic energy levels (cm 2 s-2)from the FDRAKE current records in Drake Passage
Record length
KE of the mean
(days)
KM
291 342 351 228 352 352 254 332
0.6 5.0 7.7 42.8 24.9 17.6 10.6 15.3
180.3 29.6 26.0 53.9 38.5 21.0 21.0 26.7
159.2 19.1 17.3 41.3 25.9 13.3 13.4 19.9
310 293 253 241 341 308
5.7 13.0 80.0 10.3 48.6 9.8
141.0 43.2 164.0 40.9 49.1 19.5
311 184 300 240 258 296 273 280 187 259 287 253 171 266
247.2 127.9 37.7 288.9 112.8 37.0 365.4 249.1 46.5 14.5 254.4 27.3 236.0 96.5
187 169 182
297.0 127.1 46.4
KE of fluctuations 1Krlu.f. KF[LLP Kr'IVLP
Mean KE + Fluctuation KE KM KM KM
KFI.nC
KFILLP
KF[VLP
39.4 4.5 5.1 13.8 8.2 4.5 4.6 7.9
0.003 0.2 0.3 0.8 0.6 0.8 0.5 0.6
0.004 0.3 0.4 1.0 1.0 1.3 0.8 0.8
0.02 1.1 1.5 3.1 3.1 4.0 2.3 1.9
118.1 34.4 149.7 32.6 28.8 12.5
21.4 9.3 73.9 8.4 10.8 3.1
0.04 0.3 0.5 0.2 1.0 0.5
0.1 0.4 0.5 0.3 1.7 0.8
0.3 1.4 1.1 1.2 4.5 3.1
125.6 52.7 27.8 211.0 64.5 30.1 227.2 145.0 31.6 43.8 226.2 28.0 136.3 54.9
107.1 40.8 20.8 187.5 54.0 21.4 205.4 128.5 23.4 33.3 204.9 20.8 120.0 43.4
48.3 -7.7 70.4 21.0
2.3 3.1 1.8 1.5 2.1 1.7 1.8 1.9 2.0 0.4 1.2 1.3 2.0 2.2
5.1 -4.9 4.1 5.4
77.4 49.9 6.0 5.4 79.2 4.4 49.1 26.5
2.0 2.4 1.4 1.4 1.7 1.2 1.6 1.7 1.5 0.3 1.1 1.0 1.7 1.8
106.9 63.0 32.0
98.9 51.0 22.7
48.4 18.4 5.4
2.8 2.0 1.4
3.0 2.5 2.0
6.1 6.9 8.7
4.7 5.0 7.8 2.7 3.2 6.2 4.8 3.6
Although more data are needed to establish the variation of kinetic energy with depth at the northern and southern sides of the Drake Passage, the F D R A K E records do provide sufficient coverage in the central region. In general the kinetic energy of the mean motion is seen to decrease rapidly with increasing depth down to about 2500 m (Fig. 3). All shallow records except from moorings 10 and Elizabeth are from the central passage. With the single exception of the 500-m record at 1976 mooring A, the depth variation of K M is quite consistent for all the central moorings. The depth profiles of K M from moorings 10 and Elizabeth, well south of the mean position of the Polar Front, are distinctly different from those from the central moorings. Fluctuation kinetic energy Before discussing spatial trends, it is important to note that for the F D R A K E records spatial differences are generally greater than temporal differences based on interannual
W.D. NOWLIN, JR, R. DALE PILLSBURYand
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Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage
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FREQUENCY, CYCLES/DAY Fig. 4.
Kinetic energy spectra (cm 2 s-2 CPD-t) of unfiltered current records (linearly detrended) from 1976 moorings Diana (2913 m), Ann (2781 m), and Elizabeth (2661 m).
comparisons. This is convincingly shown by comparing spectra of unfiltered current records. Spectra from the deep meters at 1976 moorings D, A, and E (Fig. 4) show that energy levels are generally higher in the northern passage and lower in the southern passage. The difference is less for the diurnal and semi-diurnal tides than for other frequencies. In fact, the tidal energy levels are almost the same in the central and southern passage and only slightly greater in the northern passage. It is also to be noted that for frequencies between semi-diurnal and Nyquist the spectra from the northern passage fall off less rapidly than those from the central and southern passage; this is true also for unfiltered temperature spectra (not shown here). Spectra based on the deep records from 1975 moorings 2, 8, and 14 (Fig. 5) show the same general relationships as do the 1976 spectra, although those from moorings 14 and 8 are more alike than are the south and central spectra from the 1976 records. Nevertheless, the kinetic energy densities at the three sites are significantly different. By contrast, spectra from deep records at the central passage location do not differ significantly from one another, even over several years. The five records at approx. 2000 m from the 1977 cluster array (Fig. 6) produced spectra virtually the same. Within that same area, we have current records from each of the four years 1975 to 1978. Again, the spectra for these deep records (Fig. 7) do not differ significantly.
I0000
........ l
iO00
........ I
........ ,
j
NORTH
2 ~ 8
CENTRAL
~14
SOUTH
....... :
I00
I0 >-
z w n W
0.1
001 ,------- 9 5 %
0001
........ I 001
........
0.I
,
. . . . . . . ,J
I
I0
, , I00
FREQUENCY, C Y C L E S / D A Y Fig. 5.
Kinetic energy spectra (cm 2 s-2 CPD-1) of unfiltered current records (linearly detrended) from 1975 moorings 2 (2771 m), 8 (2741 m), and 14 (2667 m).
IO~D
I
I I I|ll|lJ
I IllUlJ
I ttllln]
I [ I Ill:
I00
I0 >.. z w
.l
tO
.01
I
,~1
I I Ilalll|
Ol
~_~---~95%
! Illlllll
.l
FREQUENCY,
I llllllJl
I
1 Illll
I0
I00
CYCLES/DAY
Fig. 6. Kinetic energy spectra (cm 2 s- 2 C P D - 1 ) of unfiltered current records (linearly detrended) from central passage 1977 moorings North (2159 m), East (2093 m), South (2086 m), West (2074 m), and Central (2010 m).
Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage
I0000
........
I
........
,
........
,
9
.......
I000
I00
IO >.I03 z
"'
I
£3
UJ ,./
0.1
0.01 95%
0.001 0.01
........ ' 0.1
........ I I
FREQUENCY,
........ I I0
..... I00
CYCLES/DAY
Fig. 7. Kinetic energy spectra (cm 2 s-2 CPD-1) of unfiltered current records (linearly detrended) from central passage moorings 1975, 8 (2741 m); 1976, 76 (2791 m); 1977, C (2010 m); and 1978, Y (1947 m).
The latitudinal distribution of Kr for deep records (Fig. 8) shows in more detail the regional energy distribution as well as the partitioning of energy by frequency bands. The partitioning of fluctuation kinetic energy into bands is summarized in Table 2. More than 70% of the fluctuation kinetic energy is in periods greater than 2 days. Of that, almost one-third is at periods greater than 50 days. The contribution to K F due to fluctuations with periods between 2 h and 2 days (the vertical separation of the two upper curves in Fig. 8) is rather uniform in the deep water across the passage, indicating that the energy level of high frequency phenomena varies little at depth. The large increase in fluctuation kinetic energy in the northern passage is
Table 2.
Average distribution by frequency band of squared variance from FDRAKE current records distributed across the passage between 2000 and 3000 m
Original records High frequency Intermediate frequency Low frequency
Period band
% Variance
2 h to record length 2 h to 2 days 2 to 50 days 50 days to record length
100 28 49 23
W . D . NOWLIN, JR, R. DALE PILLSBURYand J. BOI"rERO
10
200
•
|
I
I
t
190 180 170 160 150 140 m
130 120 O~
I10 I00
I--
9O
.,_1 =,
80
>L9 n.t~ Z LO I-Lgl
• Unfiltered Current
70
• LLP Current
60
• V L P Current
5O •U
40
_z 30
•
im
20 I0 a
0
I
57 °
I
58 °
|
59 =
,
|
60*
r 61 °
SOUTH LATITUDE
Fig. 8. Fluctuation kinetic energy from FDRAKE records at 2000 to 3000 m in Drake Passage. Unfiltered record for periods greater than 2 h, LLP record for periods greater than 2 days, and VLP record for periods greater than 50 days.
due to longer-period phenomena; in fact practically all of the increase is associated with intermediate frequency motions with periods of 2 to 50 days. For this intermediate frequency range (the energy of which is measured by the vertical separation of the two lower curves in Fig. 8) the energy level increases slightly as one proceeds from southern to central passage locations and then increases dramatically (by about 110 cm 2 s -2) at the northern moorings. For phenomena with periods greater than 50 days, the fluctuation kinetic energy levels are uniform at 4 or 5 cm z s-2 except at the northern moorings where they increase to order 30 cm 2 s- 2. One might question to what extent the kinetic energy densities depend on record length, e.g., if the records had been one month longer or shorter, would the values for KM or Kr have differed significantly from the values presented? We have plotted as functions of time (Fig. 9) the daily cumulative averages of kinetic energy of mean motion and of fluctuations from the unfiltered records. Each day's values were obtained from the mean current for the length of record to that day and from the fluctuations relative to that mean
Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage
I
I
I
f
r
11
I
200 180 160 140 120 I00 2700m AT MOORING 2, 1975 AND MOORING D, 1976
80 60 40 20 e k/~
pjJ~
0 40 Lg
30 W Z kO I-" W
20
MOORING A, 1976
_z LO
_>
10
\ ,-'~-\
..,
-J
0
i
=
,
t
i
i
=
i
=
,
=
=
i
J
J
i
,
i
|
|
J
30
20
I0 j " / IMARL 1975
2700m AT MOORING 14, 1975 AND MOORING E, 1976 =MAY I
'JUL I
*SEP '
'NOV'
=J A N I 1976
=MAR i
MAY j
'JUL j
'SEP =
'NOV'
'JAN j 1977
TIME months
Fig. 9.
Daily cumulative averages of kinetic energies of mean motion (- - - ) and of fluctuations ( ).
current over the record length to that day. Examination of the records has led us to conclude that for record lengths greater than about four months the average values of K~ and K M are relatively stable. The most extreme variations might be expected for the records from moorings 2 and D in the northern passage where the fluctuations are greatest and the record-length mean currents are smallest. However, comparison of these relations with those for records 8 and A from the central passage and for records 14 and E from the southern passage show that the cumulative average kinetic energy for the
12
W.D. NOWLIN,JR, R. DALEPILLSBURYand J. BOTTERO
1
1
1
1
I
I
1
1
l
}
'
I
'
I
I
'
1
I
l
I
1
1
1
'
*
,
J
I
1
l
I
I
1
*
*
,
*
.C
•
500
.A
.%
r o 'S
"N c IO°
I000
WeeeEast N
olO
1500
2000
dCEast N
nt-D_ I.d 2500
.10 A~76
5000
.c
5500 i
o
*
i
i
I
50
*
l
i
i
I
I00
,
*
*
*
I
i
150
,
*
i
I
,
200
250
I
500
i
*
|
*
1
i
*
350
J
,
4 O0
KF cmz sec-2
Fig. 10. Kinetic energy of fluctuations from LLP records from FDRAKE moorings at central Drake Passage.
fluctuations shows most relative variation in the central passage. This probably is due to energetic fluctuations during the winter season. The cumulative average kinetic energy of the mean motion seems most variable in the southern passage locations, where the kinetic energy of the mean motion is also largest. Although our current meter data are adequate for the examination of the variation of kinetic energy levels across the Drake Passage at depth, we have long-term observations at different depths through the water column only in the central passage location. There K r values (Fig. 10) increase upward from 20 to 30 cm 2 s -2 in the deep water to just over 200 cm 2 s-2 near 300 m.
DISCUSSION
We have noted that the depth variation of KM in the central Drake Passage varies little from mooring to mooring. By comparison the depth profile of KM from moorings 10 and Elizabeth shows less increase in mean kinetic energy with decreasing depth than the central moorings (Fig. 3). This is likely due to the fact that moorings 10 and Elizabeth were farther from the mean position of frontal zones and associated current cores (Fig. 1). NOWLIN et al. (1977) and WnITWORTH(1980) have shown that in the water masses between
Observations of kinetic energy levelsin the Antarctic Circumpolar Current at Drake Passage
13
these frontal zones the vertical shear of geostrophic currents is small relative to shears at the fronts. Our study of the vertical shear of geostrophic currents at the Polar Front from closely spaced (25 to 40 km) pairs of F D R A K E hydrographic stations showed the vertical profiles of current and associated kinetic energy to be quite similar from different years. The resulting vertical profile of kinetic energy density associated with the geostrophic current at the Polar Front shows increases of approx. 400 cm 2 s -2 between 2500 m and 1000 m and greater than 2000 cm 2 s-2 between 2500 m and the sea surface. The vertical change in K M for the central moorings is only 100 cm 2 s-2 between 2500 and 1000 m and probably less than 1000 cm 2 s -2 between 2500 m and the sea surface (Fig. 3). The fact that the vertical shear associated with the Polar Front is much greater than the mean vertical shear observed at fixed, central passage locations is not unexpected. Although the central moorings are near the mean position of the Polar Front, the Front wanders considerably in time (ScIREMAMMANOet al., 1980). At moorings where there is less frontal activity the shear of kinetic energy is likely to be even smaller than observed for the central moorings. At mooring 10, for example, the change in observed KM between 2500 and 1000 m was approx. 30 cm 2 s-2. The fluctuation kinetic energy at intermediate and low frequencies (periods greater than 2 and 50 days, respectively) increased dramatically at the northern mooring relative to other moorings in Drake Passage (Fig. 8). The increase in fluctuation kinetic energy is consistent with the idea that the northern Drake Passage is an area in which the interaction of the current system with bathymetric and boundary constraints produces energetic meso-scale oscillations (PILLSBURYet al., 1979). Our shallowest long-term observation of KF was from 282 m in the central passage (Fig. 10). Thus, we cannot accurately estimate the near-surface vertical profile of K r, and it would be unwise to extrapolate to obtain a surface value of K r from these F D R A K E records. However, WYRTKI, MAGAARD and HAGER (1976) utilized surface ship drift observations to calculate kinetic energy of the mean flow and of fluctuations for the surface layers of the world ocean based on 5-degree grid squares. For the northern Drake Passage, they show a relative high of over 800 cm 2 s- 2 for the fluctuation kinetic energy. Their values of kinetic energy of the mean flow in the northern Drake Passage are between 100 and 200 cm 2 s -2. Although their surface values of K r are not inconsistent with our data (Table 1 or Fig. 10), our values of K M for the central passage are much larger than those reported by WYRTKI et al. (1976) for the northern passage. The discrepancy is perhaps not unexpected, because (as we have shown) the kinetic energy of the mean motion is less in the northern than in the central or southern passage locations. In addition, averaging over 5-degree squares might be expected to give results different from those of point measurements. Observations of the fluctuation kinetic energy distribution for the western North Atlantic were summarized by SCHMITZ(1978). For periods greater than one day, values of K F beneath the thermocline increased from order of 1 to approx. 150 cm 2 s- 2 approaching the Gulf Stream from the interior of the subtropical gyre. In the thermocline (at 600 m) the increase was roughly from 10 to 300 cm 2 s- 2. ScHuIvz's values along 55°W are summarized in Fig. 11. In the Drake Passage, by comparison, K r values (for periods greater than 2 days) at depths greater than 2000 m are of the order of 20 cm 2 s -2 in the central and southern passage, increasing to approx. 160 cm 2 s -2 in the northern passage. The upper layers (282 to 635 m) are much more energetic at central (to over
14
W . D . NOWLIN, JR, R. DALE PILLSBURYand Jl BOTTERO
I
I
I
300
% 200
I00
I
I
I
in N. Atlantic approximately alotxj 55°W
KF
• 600m
40
38
36°N
34
32
30
28
l
I
I
I
I
•lllllli
Gulf Stream Axis I
I
I
300
I
Kr in Drake Passage
,% 200
• 282-635m • below 2000m
IO0
I
I
56
58
!
60°S
I
I
62
Fig. 11. Comparison of fluctuation kinetic energy levels in Drake Passage and the North Atlantic (adapted from SCHMITZ, 1978). Vertical bars indicate range of observed values at each latitude.
200cm 2 s -2) and southern (50cm 2 s -2) locations (Fig. 11), but we do not yet have estimates in the northern passage. Although the K r values from our LLP records should be smaller than those by SCHMITZfor the same observed variability because we exclude periods between 1 and 2 days, the difference should be small. In summary, deep K r values in the Antarctic Circumpolar Current system at Drake Passage, like those in the Gulf Stream system, are one or two orders of magnitude greater than abyssal K F values reported for the interior of the North Atlantic. However, only at the northern Drake Passage location are the ACC deep K r values as large as (or slightly greater than) the deep K e values beneath the Gulf Stream. It might also be noted that the relative plateau in abyssal Kr seen in the ACC is not typical of deep K r values from the Gulf Stream although the transition area between Gulf Stream and the interior may have relatively flat Kr distributions along 55°W. Large values of Kr may be expected in regions where significant amounts of potential energy associated with large mean flow are converted by instability mechanisms to fluctuation energy (GILL GREEN and SIMMONS, 1974). Thus the large values of K u and the relatively large values of K r in the Drake Passage are consistent with the concept that this region is one in which baroclinic instability is converting potential energy of the mean flow to fluctuations. FANDRY(1979) and BRYDEN(1979) demonstrated this to be the case for the Drake Passage. Using the 1975 FDRAKE data from near 2700m, BRYDEN
Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage
15
I0000 ~:
tO00
282
mR L:'OlO
Io0 I0 >... I--
zILl
\,i
i,,
I
r'~ hi ".1
0,1 0.01 L 0.001 0.01
~95% Ol I I0 FREQUENCY, CYCLES/DAY
100
Fig. 12. Kinetic energy spectra (cm 2 s - 2 C P D - t ) of unfiltered current records (linearly detrended) from central passage 1977 mooring Central. Numbers indicate depth (m) of current meters.
obtained 1.2 x 10-* ergcm -3 s -t as a rate of conversion of available potential energy from the large scale density distribution into kinetic and potential energies of fluctuations with periods greater than 1 day. The kinetic energy of fluctuations decreases generally with depth above about 1500 m for the central Drake Passage (Fig. 10). The decrease in energy content is illustrated in spectra from the 1977 central mooring (Fig. 12), which at most frequencies show a substantial increase in kinetic energy for records above 600 m depth as compared with those at greater depths. The pattern is substantiated by records from other 1977 moorings in the central passage. There is, however, a remarkable departure from the general pattern of kinetic energy decrease with depth; namely, for periods between 1 and 30 days the deepest record (3521 m, Fig. 12) had energy levels greater than those at 2010 m (approximately equal to those in the near-surface layers, in fact). This is not inconsistent with the existence of energetic, mesoscale, bottom-trapped waves in the central Drake Passage. We have examined the depth variation of the ratio of K M to K F (Table 1). The ratio generally decreases with increasing depth below about 1000 to 1200 m. SCIREMAMMANOet al. (1980) noted an increase in the ratio below 2500 m and suggested that the increase might be due to topographic scattering or bottom intensification of the current fluctuations. It should also be noted that the ratio of K M to K e generally decreases with decreasing depth for levels above about 600 m. This might indicate near-surface fluctuations, which would explain the reported (ScIREMAMMANOet al., 1980) reduction in normalized cross correlation at zero lag for velocity components above 500 m correlated with those at mid depth.
16
w . D . NOWLIN,JR, R. DALEPILLSBURYand J. BOTTERO
MCWILLIAMS,HOLLAND and Crlow (1978) presented results from numerical models of the Antarctic Circumpolar Current as a two-layer, wind-driven, quasi-geostrophic ocean in a channel with bottom and lateral friction. Their calculations showed that a partial barrier to zonal flow and a topographic bump significantly affect the resultant solutions, in terms of the equilibrium balances between eddies and mean currents and their spatial distributions. Readers are cautioned that our values for kinetic energy densities in Drake Passage, which is both a lateral barrier and a topographic obstacle to zonal flow, may not be representative of the ACC as a whole. They are further cautioned that the northern passage location of moorings Diana and 2 lie within the Subantarctic Water Mass regime and may not be representative of the Antarctic Circumpolar Current. SUMMARY
Between January 1975 and the end of 1978, 31 records of current and temperature of approx. 6 to 12 month duration were obtained at locations in the Drake Passage. The data are sufficient to characterize kinetic energy levels across the passage at depths below 2000 m and from 300 m to the bottom at central passage locations near the historical position of the Polar Front. We have considered kinetic energy of the mean motion (K~) and the kinetic energy of the fluctuations (Kr) from the mean motion. Study of KM and K r as functions of record length has convinced us that equilibrium values generally are not reached for record lengths less than about 4 months. However, when the complete record lengths are considered the energy levels seem quite stable. Kinetic energy spectra from almost yearlong records evidence no significant year-to-year differences for records from the same locality. Kinetic energy spectra from the deep, long-term records do show significant spatial variations from north to south across the passage and from near bottom to the shallowest depths sampled ( ~ 300 m) in the central passage. The kinetic energy of the year-long mean flow recorded at depths greater than 2500 m increases southward in the Drake Passage. Values of K u increase from an average of approx. 3 cm z s -2 in the northern passage (near 57°S) to values nearer 10 to 15 cm 2 s -2 in the central (,-, 59°S) and southern passage. In the central passage K M increases upward from near 10cm 2 s -2 at 2700m to approx, l l 0 c m 2 s -2 at 1000m and to over 3 0 0 c m 2 s -2 at 300m. At moorings more distant from the historical positions of the fronts, the vertical shear of KM is expected to be considerably less than at the central moorings, which are influenced by the large vertical current shear associated with the Polar Front. Filters with half-power points at 2 and at 50 days were applied to the current records to partition the fluctuation kinetic energy into three period bands: 2 h (the Nyquist period for these records) to 2 days, 2 to 50 days, and greater than 50 days. Based on partitioned records from depths between 2000 and 3000 m the division of K r between the high, intermediate, and low frequency bands was 28, 49, and 23~, respectively. Values of K r at depth due to high frequency fluctuations are almost constant across the Drake Passage. The K r due to motions with periods between 2 and 50 days increases somewhat from southern (10 cm 2 s-2) to central (15 cm 2 s-2) passage locations and then greatly increases to approx. 110 cm 2 s-2 at the northern moorings. For phenomena in the low frequency band the deep K r values are uniform at 4 or 5 cm 2 s-2 except at the northern moorings, where the average value is 30 cm 2 s- 2.
Observations of kinetic energy levels in the Antarctic Circumpolar Current at Drake Passage
17
Fluctuation kinetic energy at the central moorings increases upward from 20 to 30 cm 2 s - 2 below 2000 m to over 200 cm 2 s - 2 near 300 m. Relative to mid-depth values ratios of KF/KM generally increase downward below 1000 or 1200 m (more so below 2500 m) and upward above 600 or 700 m. This may indicate the increasing importance of fluctuations near the bottom and near the surface. The K r values from the Antarctic Circumpolar Current system at Drake Passage are much greater than Kr values from the interior of the North Atlantic. Values of K r in the ACC are of the same order as those in the Gulf Stream system (including the transition between the stream and the interior), although only at the northern moorings are the Kr values actually as large as reported Kr values directly beneath the Gulf Stream. Acknowledgements--Support for the FDRAKE work was provided as a part of the International Southern Ocean Studies by the Office for the International Decade of Ocean Exploration of the National Science Foundation. The fine measurements of current and temperature result from the dedication of the current meter group at Oregon State University, especially to the careful preparations and supervision by Mr ROBERT STILL. We appreciate constructive comments and suggestions from Drs BRUCE WARREN, THOMAS WHITWORTH and CHRIS FANDRY. REFERENCES BRYDENH. L. (1979) Poleward heat flux and conversion of available potential energy in Drake Passage. Journal of Marine Research, 37, 1-22. BP,YDEN H. L. and R. D. PILLSB~V (1977 Velocity of deep flow in the Drake Passage. Journal of Physical Oceanography, 7, 803-810. FANDRY C. (1979) Baroclinic instability of the Antarctic Circumpolar Current in the Drake Passage. Ocean Modelling, 22, 8-9. FANDRY C. and R. D. PILLSBURY(1979) On the estimation of absolute geostrophic volume transport applied to the Antarctic Circumpolar Current. Journal of Physical Oceanography, 9, 449-455. GILL A. E., J. S. A. GREEN and A. J. SIMMONS(1974) Energy partition in the large-scale ocean circulation and the production of mid-ocean eddies. Deep-Sea Research, 21,499-528. McWILLIAMS J. C., W. R. HOLLANDand J, H. S. CHOW (1978) A description of numerical Antarctic Circumpolar currents. Dynamics of Atmospheres and Oceans, 2, 213-291. NOWLIN W. D., JR, T. WHI'rV4ORTH 1II and R. D. PILLSBURY (1977) Structure and transport of the Antarctic Circumpolar Current at Drake Passage from short-term measurements. Journal of Physical Oceanography,,7, 788-802. PILLSBURYR. D., T. WHITWORTHIII, W. D. NOWLIN, JR and F. SCI~MAMMANO(1979) Currents and temperatures as observed in Drake Passage during 1975. Journal of Physical Oceanography, 9, 469-482. SCHMITZ W. J., JR (1978) Observations of the vertical distribution of low frequency kinetic energy in the western North Atlantic. Journal of Marine Research, 36, 295-310. SCIREMAMMANOF., R. D. PILLSBURY,W. D. NOWLIN, JR and T. WHITWORTHIII (1980) Spatial scales of temperature and flow in Drake Passage. Journal of Geophysical Research (in press). WHITWORTH T. Ili (1980) Zonation and geostrophic flow of the Antarctic Circumpolar Current at Drake Passage. Deep-Sea Research, 27, 497-507. WYRTKI K., U MAGAARDand J. HAGER (1976) Eddy energy in the oceans. Journal of Geophysical Research, 81, 2641-2646.