Deep-Sea ResearchH, Vol. 40, No. 4/5, pp. 989-999, 1993. Printed in Great Britain.
0967-0645/93 $6.00 + 0.00 © 1993 Pergamon Press Ltd
A comparison of GEOSAT altimeter inferred currents and measured flow at 5400 m depth in the Argentine Basin GEORGES L. WEATHERLY,* ROBERT H. EVANSt and OTIS T. BROWNt
(Received 23 October 1990; in revised form 13 November 1991; accepted 27 January 1993) Abstract--A comparison is made between an 11 month duration current meter record obtained
near the bottom at 40°27'S,49°25'Win the mid-ArgentineBasin in water depth 5400m, and nearsurface flow inferred from GEOSAT altimeter data. The GEOSAT has a repeat path time of 17 days, and inferred near-surface flow features with periods of 34 days or less should not be detectable. Thus the current meter record was filtered with a 34 day low pass filter. Since only fluctuating flows can be inferred with the GEOSAT altimeter data, the deep recorded mean current was added to the satellite-inferred flow for comparison purposes. The deep flow and the altimeter-inferred near-surface flow agree remarkably well, suggesting that for time scales ->34 days the flowwas nearly barotropic in this region of the South Atlantic.
INTRODUCTION INFERRINGnear-surface flows from satellite data at or near the equator (CARTONand KATZ, 1980; PICAUT et al., 1990) and in the Gulf Stream and its extension (JoYCE et al., 1990; WILLEBRANDet al., 1990; TAI, 1990) is well established. One is left with the impression from these studies that near-surface flows can be predicted confidently from satellite data. However, using satellite data to infer flows at great depths has been less successful (KELLEY et al., 1982; WEATHERLYand KELLEY, 1985; IANOVet al., 1986; MULHEARNel al., 1986). While these studies indicate that whereas the abyssal flow is at times a deep extension of the surface flow, what happens at other times at great depths is not predictable from the satellite data. The more equivocal deeper flow results may be due to two factors. The comparisons were made with deep current measurements on the continental rise and margin where topographic Rossby waves are known to exist (e.g. HAMILTON, 1990). These waves are bottom trapped and whether or not they extend to the surface depends on their wavelength (ibid.). Thus abyssal flows there may be influenced at times by wave motions not detectable at the surface. The second factor is that only satellite sea surface temperatures were available for the comparison; no altimeter data were available. Certain surface thermal features are known to have shallow (about 50 m) penetration scales (e.g. HORTON and HORSLEY, 1988) and are not expected to influence flows at great depths. However, it is *Department of Oceanographyand GeophysicalFluid DynamicsInstitute, Florida State University, Tallahassee, FL 32306, U.S.A. tRosensteil Schoolof Marine and AtmosphericSciences, Universityof Miami, 4600RickenbackerCauseway, Miami, FL 33149, U.S.A. 989
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not possible to determine solely from satellite infra red data how deep surface thermal features may extend. This study is another attempt to see how well abyssal flows can be inferred from satellite data. The previously referenced comparable studies were of data from the Gulf Stream or East Australian Current regions. The present study is of data from the Brazil Current region. It differs from the previous ones in that the near-surface flow is inferred from satellite altimeter rather than sea surface temperature data, and the site is in mid-basin rather than on the continental rise or margin. METHODS
Current meter data
The near-bottom current meter record was taken as part of an ONR-sponsored study of sediment waves in the Argentine Basin (FLooD and SHOR, 1988). It was obtained 10 m above the bottom at 40°27'S, 49°25'W (water of depth 5415 m) between 29 April 1987 and 1l March 1988. This record is considered in detail by WEAXHERLV(1993; hereinafter W) and only briefly reviewed here. The site is between the mean frontal positions of the Brazil and Malvinas Current Extensions, being closer to the former, and is in a region where both currents flow to the east (Fig. 1). The mean flow is about 7 cm s -1 directed towards the east, and the abyssal eddy kinetic energy is quite high here (W). The current meter record was probably obtained in the bottom boundary layer, and W estimated that the recorded
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flow was within 5% in magnitude and 10° in direction of the flow immediately above the bottom boundary layer. Because the GEOSAT has a 17 day repeat ground track cycle, near-surface flow features detected with its altimeter with periods of 34 days or less are not expected to be detected. Thus, what is presented here is the current meter record considered in W after it has been 34 day low passed filtered (by simple block averaging). W also obtained near-bottom velocity records from four other sites in the Argentine Basin. For this preliminary study a comparison of satellite-inferred near-surface flow with measured deep flow was made at only one site. Three of the other sites are on the continental rise or margin (Fig. 1), and deep flows there are subject, as noted earlier, to contamination by topographic Rossby waves. The remaining site, located in mid-basin, has a considerably weaker eddy variability than the site considered here. The available satellite altimeter data (see below) are only suitable for estimating the fluctuating component of the flow. The site we chose to study has the highest energy level of fluctuating flow of the five sites reported in W. Satellite altimeter data GEOSAT data tapes, produced by the National Ocean Survey and distributed by the National Oceanographic Data Center, are the source of the altimeter data. These data, provided at a 10 s -1 data rate where 1 s corresponds to 6.8 km of flight distance, were processed at the University of Miami in two phases. The tapes were first scanned, and data falling in the South Atlantic in the east-west direction and within the wind-gyre in the north-south were extracted and written to disk files. After extraction of the data, all data for a given day, called a segment, were collected. Corrections were then applied to each height value (see CnENEV et al., 1987). These corrections include solid earth tide, and path delay for wet and dry troposphere and ionosphere derived from the Fleet Numerical Oceanographic Center model. If the height value exceeded 32,767 cm or the variance in height was greater than 10 cm (calculated from 1 s averages), the point was discarded. Contiguous data were interpolated to an 8 km spacing using a one-dimensional Bessel interpolation. If fewer than five values were contiguous, then all values were ignored as spurious data. The temporal mean of the bins along a repeat track were then computed for the period 8 November 1986-17 June 1989. This results in a bias-free mean curve for each repeat track. This mean curve was then subtracted from each segment, producing a residual mesoscale signal in which the geoid signal and sea surface height gradients associated with mean flows were eliminated. A quadratic trend was then removed to eliminate the remaining orbital error. All residual sea level data in each 17 day time interval were convolved with a symmetric two-dimensional guassian filter with equivalent width 1 degree latitude. The data were then interpolated to a uniform 111 × 111 km spatial grid for each 17 day interval. Inferring velocities from the altimeter data The altimeter-derived surface topography maps were made to help interpret sea surface AVHRR temperature images in the Malvinas Current and Brazil Current confluence region for a study initiated in OLSONet al. (1988). Since such surface temperature maps are
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usually available every other day (due to cloudiness), the satellite altimeter data were interpolated in time and contoured to give surface topography maps every other day fol the same region. It is from these charts that the estimates of the surface velocity at the current meter site were made. The residual sea level maps, contoured in 3 cm height increments, were displayed on a video monitor. For each the surface current at the current meter site was estimated as follows. The flow direction was assumed to be parallel to the height contour and consistent with geostrophy. The direction was read using a compass. The sea surface slope was estimated by measuring the distance between adjacent height elevation contours transverse to the flow direction. One cm of screen corresponds to about 66 km, and the distance between contours varied from about 2 to 18 mm. The magnitude of the surface current was assumed to be consistent with geostrophy. These surface current estimates were made only for the time interval in which the current m e t e r data were available. The altimeter data reduction precludes estimating mean surface flows. For comparison to the current m e t e r record, which indicated a strong mean eastward flow, the mean current recorded by the current meter was added to the surface flow estimates. While reading the surface altimeter maps we noted cases when a small shift either of the current site on the map or of the contours could result in a large change in the estimate of the surface flow direction (>90 °) and/or speed ( > a factor of two). About 30% of the images were of this type. Later we will show that the inferred surface flow estimates which are sensitive to a slight shift in position or the contours have nearly the same quality as for the other estimates. RESULTS The stick diagrams for the inferred surface current and the underlying deep current meter records are quite similar (Fig. 2). Both are characterized by periods of alternating flow, westward (one instance) and eastward (two instances). The times of transition nearly coincide with the current m e t e r record leading by about ] week. However, the time of transition for the satellite record depends on the added mean eastward velocity, and there is an uncertainty of the added value of about _+ 3 cm s - 1 (W). The cross spectrum of the east velocity components of velocity (u) indicates both series are coherent to within the 95% confidence interval for periods -> 34 days (Fig. 2) with the deep record leading the surface one by about 2 _+ 5 days. Time series of the north components of velocity (v; not shown) and their cross spectrum (Fig. 3) suggest that they are not significantly coherent. For example, the cross spectrum shows that the coherency squared does not exceed the 95% significance level for periods > 34 days. The mean u and v for the records in Fig. 2 are essentially the same [(7.4 cm s- I, 2.5 cm s -1) for the surface record and (7.3 cm s -1, 2.0 cm s - t ) for the deep record]. This is expected, since the altimeter data as processed cannot be used to infer mean flows, and the surface record in Fig. 1 has the mean of the deep record added to it. The autospectra of the two series (Fig. 4) indicate that in both, the variance in u is higher than in v. The data were separated into two categories and compared. The category designated as "sensitive" contained data in which a small shift of the current meter position on the altimeter m a p or a small shift in the contours on the m a p would result in a major change in the inferred surface current direction (->90 °) and/or magnitude (-> a factor of two). The
993
Inferred currents vs measured flow in the Argentine Basin
other category contained the remainder of the data. This was done as a test of the sensitivity of the results to locating the current meter site on the satellite maps, and of whether the altimeter data contouring yielded better (or worse) results in certain conditions. The variance of the sensitive values is somewhat greater (cf. Figs 5 and 6). Since the current magnitudes for the sensitive values are somewhat smaller (cf. Figs 5 and 6), the vectors with more uncertainty (primarily in direction) contribute less to the coherence estimates in Fig. 3. DISCUSSION
It appears obvious, particularly from Fig. 1, that the near-surface current fluctuations with periods > 1 month in this region of the Argentine Basin are highly barotropic. From results obtained in comparable regions of the western North Atlantic (e. g. R~C~ARDSON, 1983; WEAXHERLY,1984) and western North Pacific (e.g. SCHMITZ,1984), we would expect
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Fig. 2. Upper: Stick plot of the near-surface flow at 40°27'S, 49°25'W inferred from SEASAT altimeter data. The mean flow for the record in the lower plot has been added. Lower: Stick plot of the measured flow 10 m above the bottom at the same site with values plotted every other day as for the near-surface flow. Water depth is 5415 m. Note that in each plot east is up and the velocity scale is the same.
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the intensity of the near-bottom current fluctuations to be much less than those at the surface. Here they appear to be comparable in that the abyysal eddy kinetic energy was 58 cm 2 s -2 and the estimated surface eddy kinetic energy was 40 cm 2 s -2. We find this to be remarkable in light of the above noted findings for the Gulf Stream and Kuroshio Extensions. If the surface fluctuating flow is essentially barotropic, as indicated in this study, then the best that can be expected from altimeter-inferred surface currents is that they be equal to the near-bottom current. Any smoothing of the altimeter data will lead to a reduction in the magnitude of the near-surface current estimates. Thus we do not find the slight (order 20%) underestimation of the near-surface current to be very significant. However, future studies are required to identify the source of the discrepancy. We noted that, unlike the u velocity component, the v fluctuations were not highly correlated when their time series were compared and their cross spectrum was examined. However, the stick plots in Fig. 2 suggest some correlation. The sense of rotation of the vectors is determined by the v fluctuations. Often when the vectors of one series rotate clockwise with time the vectors of the other do so as well (e.g. June 1987 in Fig. 2), as is the case and similarly for counterclockwise rotation (e.g. July-August 1987 in Fig. 2). One might question if the good agreement in Fig. 2 results partly from bias on the part of the reader. The near-bottom flow vector was displayed on the monitor while the altimeter data was being read, and this may have lead to a bias. What was displayed on the screen was not the 34 day low pass record in Fig. 2 but the daily averaged vectors which are considerably more noisy (Fig. 7). The impression the reader (GW) had while reading the altimeter maps was similar to his impression of earlier similar studies which used satellite thermal images instead, that is, that at times the agreement was good, at other times it was not good, and that it was not possible to determine apriori which condition would apply. It
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was only after looking at the 34 day averaged deep current record, formed after the altimeter data were read, and comparing it to the near-surface record with the mean abyssalflow added that the rather surprising agreement became apparent. We would not expect as good agreement if a similar comparison had been made with the three shoreward sites. These sites have strong evidence of topographic Rossby wave activity (W). However, if these motions are predominantly locally forced (due to the .~5 30
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proximity of these other sites to the Brazil and Malvinas Current Extensions) and extend to the surface, some agreement would be expected. The remaining site (site 5 in Fig. 1) is a mid-basin one and might yield a promising comparison. However, the signal there (i.e. the level of the fluctuating flows) is much smaller. Nonetheless, the comparison of these sites' deep current observations with satellite altimeter-inferred near-surface flow, determined in a more objective manner than in this study, is the subject of ongoing work. The primitive manner in which the altimeter data was read (by compass and ruler from a display monitor) is a result of this being done at the end of two research programs, each funded for other purposes. The results presented here are those obtained from the one and only reading we made of the altimeter record; no attempt was made to re-read sections to attempt to get better agreement. A smoother near-surface stick plot probably would result if modern methods were to be used to infer the sea surface slope from the altimeter data. However, this preliminary study indicates that satellite altimeter data can be a useful quantitative tool for inferring abyssal flow. Acknowledgements--During the course of this study we benefited from comments and discussions with P. Niiler and D. Olson. We want to acknowledge James Brown and Steven Emmerson for the data processing and the analysis software, and Reinard Harkema and John Ritch for help with several of the figures. Support was provided by the Office of Naval Research under grants N00014-87-J-115 (GW) and N000-1489-J-1144 (RE and OB).
REFERENCES CARTONJ. A. and E. J. KATZ (1990) Estimates of the zonal slope and seasonal transport of the Atlantic North Equatorial Countercurrent. Journal of Geophysical Research, 95, 3091-3100. CHENEY R. E., B. C. DOUGLAS,R. W. AGREEN, L. MILLER, D. L. PORTERand N. S, DOYLE(1987) GEOSAT Altimeter Geophysical Data Record User Handbook, NOAA Technical Memorandum NOS NGS-46. FLOODR. and A. SHOR(1988) Project Mudwaves: a coordinated study Of abyssal bedforms and sedimentation in the Argentine Basin. Transactions of the American Geophysical Union, 69, 1258. HAMILTONP. (1990) D e e p currents in the Gulf of Mexico. Journal of Physical Oceanography, 20, 1087-1104.
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HARKEMAR. and G. L. WEATHERLY(1989) A compilation of moored current meter data in the Argentine Basin April 25, 1987-March 14, 1988. Department of Oceanography, Florida State University, Report No. CMF89-01, 64 pp. HORTON C. W. and L. E. HORSLEY(1988) Variability in Gulf Stream surface-subsurface frontal separation: the unimportance of Ekman advection. Journal of Geophysical Research, 93, 3519-3528. IONOVV. V., G. L. WEATHERLYand R. HARKEMA(1986) On the temporal variability of the surface Gulf Stream and near-bottom flow. Journal of Geophysical Research, 91, 83-97. JovcE T. M., K. A. KELLY,D. M. SCHUaERTand M. J. CARUSO.(1990) Shipboard and altimetric studies of rapid Gulf Stream variability between Cape Cod and Bermuda. Deep-Sea Research, 37,897-910. KELLEY,E. A. Jr. and G. L. WEATHERLY(1985) Abyssal eddies near the Gulf Stream. Journal of Geophysical Research, 90, 3151-3160. KELLEYE. G., G. WEATHERLYand J. EVANS(1982) Correlations between surface Gulf Stream and bottom flow near 5000 meters depth. Journal of Physical Oceanography, 12, 1150-1153. MULHEARNP. J., J. H. FILLOUX,F. E. M. LILLEY, N. L. BINDOFFand I. J. FERGUSON(1986) Abyssal currents during the formation and pasage of a warm-core ring in the east Australian Current. Deep-Sea Research, 33, 1563-1576. OLSON D. P., G. P. PODESTA,R. H. EVANSand O. T. BROWN(1988) Temporal variation in the separation of the Brazil and Malvinas Currents. Deep-Sea Research, 35, 1971-1990. PICAUTJ., A. J. BUSALACCHI,M. J. MCPHADENand B. CAMUSAT(1990) Validation of the geostrophic method for estimating zonal currents at the equator. Journal of Geophysical Research, 95, 3015-3024. RICHARDSONP. L. (1983) Eddy kinetic energy in the North Atlantic from surface drifters. Journal of Geophysical Research, 88, 4355-4367. SCHMITZ W. J. Jr. (1984) Observations of the vertical structure of the eddy field in the Kuroshio Extension. Journal of Geophysical Research, 89, 6355--6364. TAI C.-K. (1990) Estimating the surface transport of meandering oceanic jet streams from satellite altimetry: surface transport estimates for the Gulf Stream and Kuroshio Extension. Journal of Physical Oceanography, 20, 860-879. WILLEBRANDJ., R. H. KASE,D. STAMMER,H. H. HINRICHSENand W. KRAUSS(1990) Verification of Geosat sea surface topography in the Gulf Stream Extension with drifting buoys and hydrographic measurements. Journal of Geophysical Research, 95, 3007-3014. WEATHERLY G. L. (1984) An estimate of bottom frictional dissipation by Gulf Stream fluctuations. Journal of Marine Research, 42,289-301. WEATHERLY G. L. (1993). On deep-current and hydrographic observations from a mudwave region and elsewhere in the Argentine Basin. Deep-Sea Research II, 40, 939-961.