CS2.1S'' £ftL /4V-/40MU I ESSA TR ERL 164-AOML 1 A UNITED STATES DEPARTMENT OF COMMERCE PUBLICATION ESSA Technical Report ERL 164-AOML 1 U.S. DEPARTMENT OF COMMERCE Environmental Science Services Administration Research Laboratories Correlation of Movements in the Western North Atlantic ONALD V. HANSEN BOULDER, COLO. MAY 1970 ESSA RESEARCH LABORATORIES The mission of the Research Laboratories is to study the oceans, inland waters, the lower and upper atmosphere, the space environment, and the earth, in search of the under- standing needed to provide more useful services in improving man's prospects for survival as influenced by the physical environment. Laboratories contributing to these studies are: Earth Sciences Laboratories: Geomagnetism, seismology, geodesy, and related earth sciences; earthquake processes, internal structure and accurate figure of the Earth, and distribution of the Earth's mass. Atlantic Oceanographic and Meteorological Laboratories: Oceanography, with emphasis on the geology and geophysics of ocean basins, oceanic processes, sea- air interactions, hurricane research, and weather modification (Miami, Florida). Pacific Oceanographic Laboratories: Oceanography; geology and geophysics of the Pacific Basin and margins; oceanic processes and dynamics; tsunami generation, propaga- tion, modification, detection, and monitoring (Seattle, Washington). Atmospheric Physics and Chemistry Laboratory: Cloud physics and precipitation; chem- ical composition and nucleating substances in the lower atmosphere; and laboratory and field experiments toward developing feasible methods of weather modification. Air Resources Laboratories: Diffusion, transport, and dissipation of atmospheric con- taminants; development of methods for prediction and control of atmospheric pollution (Silver Spring, Maryland). Geophysical Fluid Dynamics Laboratory: Dynamics and physics of geophysical fluid systems; development of a theoretical basis, through mathematical modeling and computer simulation, for the behavior and properties of the atmosphere and the oceans (Princeton, New Jersey). National Severe Storms Laboratory: Tornadoes, squall lines, thunderstorms, and other severe local convective phenomena toward achieving improved methods of forecasting, detecting, and providing advance warnings (Norman, Oklahoma). Space Disturbances Laboratory: Nature, behavior, and mechanisms of space disturb- ances; development and use of techniques for continuous monitoring and early detection and reporting of important disturbances. Aeronomy Laboratory: Theoretical, laboratory, rocket, and satellite studies of the physicad and chemical processes controlling the ionosphere and exosphere of the earth and other planets. Wave Propagation Laboratory: Development of new methods for remote sensing of the geophysical environment; special emphasis on propagation of sound waves, and electro- magnetic waves at millimeter, infrared, and optical frequencies. Institute for Telecommunication Sciences: Central federal agency for research and services in propagation of radio waves, radio properties of the earth and its atmosphere, nature of radio noise and interference, information transmission and antennas, and meth- ods for the more effective use of the radio spectrum for telecommunications. Research Flight Facility: Outfits and operates aircraft specially instrumented for re- search; and meets needs of ESSA and other groups for environmental measurements for aircraft (Miami, Florida). ENVIRONMENTAL SCIENCE SERVICES ADMINISTRATION BOULDER, COLORADO 80302 .^tJlENTo^ ■*"^/f«Cf StR^\^^^ U. S. DEPARTMENT OF COMMERCE Maurice H. Stans, Secretary ENVIRONMENTAL SCIENCE SERVICES ADMINISTRATION Robert M. White, Administrator RESEARCH LABORATORIES Wilmot N. Hess, Director ESSA TECHNICAL REPORT ERL 164-AOML 1 Correlation of Movements in the Western Nortli Atlantic DONALD V. HANSEN o a v^ 1 \ m ■^ '''^i I \ ^B ^ ^ mi \ 7 V o O K \ \ \ \ \ \ \ o **'i';*t*;*;*;''*'*t*;'i*?^v m K '^^i^i^^^^^^^AiJ^ ^,^ •^2^^ W" Si, • O o O >0 U P> 5 o 05 O •H ^^H nd 5^ •H Cm ^^^ tuO s CD a en -H O -H CO O 1^ w C\J Ph O O Ph -P O o CO \X) Q) in ^ ^ «o S ON • fn rH CD ^ CD -P O X:: X! O 05 -p o a ^ O ^ CD -P CO ct3 t3 •HS S=^ O CO P> ItnCm 03 CD H fH N a i+H o > d O 05 O O fH ^ o d -p CO s o u •H d CO CD -p h-px: •H 3 d-p CO P> -H O O CD O CO Dh ^ O. -H o m CD fU ?S ^-^ CD CD T3 Sh H 1 1 1 1 500 KM SEPT 11-18 PHASE PROGRESSION SEPTEMBER 1965-MAY 1966 Figure 3« Interpretation of evolution of Gulf Stream meanders, September 1965 ~ May 1966. Dashed lines denote postulated phase progression, solid lines denote phase progression supported by other evidence. JUNE 15-20 JULY 15-23 NOV 15 PHASE PROGRESSION JUNE 1966-NOVEMBER 1966 Figure h. Interpretation of evolution of Gulf Stream meanders, June -Nov ember 1966. obtained from Site D. This site (39°20»N, 70°00»W) is gener- ally 100-200 km north of the near surface core of the Gulf Stream, and low frequency fluctuations of the north current component above 200 m are shown by Thompson (1969) to have significantly greater kinetic energy density than fluctu- ations of the east component. At this level the north and east components are only poorly correlated, but the princi- pal axis of the low frequency fluctuations lies about north- south as would be the case for a Rossby wave with wave number lying nearly east-west like the Gulf Stream meanders. Possible relationships between these energetic north-south current fluctuations and the fluctuating position of the nearby Gulf Stream are of particular interest here. Observed and inferred meridional positions of the thermal front associated with the Gulf Stream on 70°W during the Explorer cruises is compared (fig. 5) with the integral of the daily mean of the north component of current metered in the upper 200 m at Site D (Webster, 1968 ). Each seg- ment of the integrated current meter record has been ad- justed meridionally for the best visual fit to the tempera- ture data. Only the gravest mode of the major meander * These and other data from the buoy project were presented in a seminar given by Dr. Webster during the Geophysical Fluid Dynamics Program, Woods Hole Oceanographic Institution, Summer, 1968. 1 < "•^ -^ H' - -^ -If z ^ + - ^.y ^' li. + -- • "^ 1 ~ -3 _ / ,±-t''' o L /» *■ 1 f V ^^— ^^ ^^^^^^^ 1"+"--.> z - < + 1 ^^ V o r "".v + ^ " O -«^N - E a> to \ O CA / - O 4 B / 4 OJ O < o E c - E O 3 ^ • -i a> CJ O ^^^^^^^ .c ^^^^^^^^^^^^^ 4- •*- + o <2 o o • -i O i| • - o to •*- o c o •*- H- O •s t o ^^^ < c o w o o « £ c o ii E -o s ■o o 0> '^^ > a> o. ? 1 k. o> ^X Liu cr a> a> ^_,^*-'^' ~ o < .E .£ 1 -3 h. 5**^ _ • + C^ o z '■•^^ ^ o *■- — _^ _ ■"> ^ , . . J . ' 1 CO r^ (O o> i+H nd ^^ CD ch ;=i c^ O o t:) -H d cc +3 Ch CO -P a o ^ CD CO O d -p -p O d CO CO Oh CD Td -P S S CO O CD in ^ O > CD o u '—^ rH S O EH CO r-1 dH p ^ O CO X ~~^ •^ ^^ T3 O Ph •H -H g CD Ph Td O -P CD -H ^ CD S IH Ch S u> CD O (0 Td S -t^ e CD (D in (J> -p x: ^ — CO -P in X! fH -H CD -P M ;5 Ch CD d d +:> Q -H O . d -H -P •H CD -P Ch -p::s CO CO Ch -H -H fH O CO O tli o l>- CO Ph d4^ in -H O CO d CO CO O CD •H+^ d >j iH d S in > CO CD CO O CO ft Ph CD X!S fi Ph Sh fH O ;:5 +:> -H >, O O CO CO Xi lOv CD - fciO o> ■iH — ^ o o ro O O CM O O features can be inferred from the temperature data; even for this the amplitude of north-south movement can only be estimated in most months, and no estimate has been attempted for the midsummer period due to the difficulty of interpre- ting evolution of the complicated thermal structures shown in figure h. However, aside from large transients in November of 1965 and 1966, the sense and magnitude of the north component of current appears to reflect meridional movements of the Gulf Stream. There is some indication that movement of the thermal front on 70°W may lead the current meter data in phase, most notably during the autumn of 1966. This phase shift is suggestive of the meander structure described by Webster (1961) from observations off Onslow Bay and reproduced in a mathematical model by Orlanski (1969) • On the other hand a phase lead on the order of 6 to 10 days is to be expected if the perturbation structure is essential- ly normal to the mean path of the stream rather than strictly north-south, but the data are insufficient to warrant further speculation about cross stream variation of phase. The mean of all current observations at this site has a negligible meridional component, which is of course also true of movement of the thermal front. A set of simultaneous current measurements was obtained from opposite sides of the Gulf Stream, at sites D and J (36°N, 70°W) in the spring of 1966. At lo m depth, fluctuation of currents with a north component on the order of 10 cm/s and a gross periodicity of about two weeks oc- ciorred essentially in phase at the two sites, and, as shown in figure 5> in phase with lateral movement of the Gulf Stream. 3. RELATION TO MOVEMENT OF CURRENT EDDIES At the outset of the stream path monitoring project, two cyclonic current rings or geostrophic eddies were discovered freshly separated on the south side of the Gulf Stream and were observed (Fuglister, 1967) for the next several months. Figure 6 shows the meridional movement of these eddies 200 to 300 km to the south compared with the local meridional movement of the 15°0 isotherm inferred from figure 3» Observations of the eddy position suffer from the same lack of resolution in time as the sequence of pathlines, and the already imprecise indication of possible relationships between their positions is further obscured by zonal movement of the eddies. Particularly at 66°W, it can be argued that the sequence of eddy latitudes is just as well represented by a random distribution about a linear trend. On the other hand, there is indication of a systematic relation to me- ridional movement of the nearby Gulf Stream, in that although meridional movements of the eddies are small, in every case where a clear judgment can be made, they are in 10 Latitude of I5*C isotherm @ 200 m Latitude of eddies Figure 6 Meridional movement of detached eddies (open circles), and observed (dark points) and inferred (dashed lines) meridional movement of Gulf Stream at (a) 66°W and (b) 63°W. 11 the same direction as the inferred frontal movement. At 63°W both the sense and magnitude of the eddy movements are in reasonable agreement with those of the stream movements. h. RELATION TO SWALLOW FLOAT MEASUREMENTS If the positions of the thermal front summarized in figure 3 are approximated by 0T = 0ni(x)+0p(x)cos k(x-ct) , (l) where 0t denotes the latitude of an isotherm in the main front as a function of eastward distance x, and time t, 0ni is the time average latitude, and 0p is the amplitude of a progressive wave disturbance of the mean, the few suitable observations suggest the existence of an identifiable meridional flow that, aside from a possible ambiguity of phase, behaves as u = a_0r]n = ck0^ Sin k(x-ct) . (2) 6 1 While no direct evidence is available to indicate how far from the stream this disturbance may be felt, Iselin (1961) has suggested an association with transient currents revealed by the well known Swallow float measurements near Bermuda. As summarized by Crease (1962), these were a predominantly meridional motion with an apparent time scale 12 of 10 to 100 days and an rms amplitude of approximately 6 cm/s. Between 65°W and 70°W, the longitude of the float measurements, meander amplitudes are on the order of 100 km. The period and rms amplitude of the meridional movement described by (2) are therefore 2lL-^6days, and £_£ pill cm/s , ck y 2 both in order of magnitude agreement with values cited by Crease. Crease also computed a current structure function, [ u(x)-u(x+^) ] , for simultaneous float measurements separated a distance^, the overbar denoting time or ensemble average. For the flow specified by equation (2) the normal- ized form of this function is. Sjt = 2 [ 1- cos(kicose) ] . (3) If Z is taken normal to the current structure, then S/ is simply, S = 2[ 1-cos ki] . (^) I judge however from Crease's figure 1 that the separation of his float pairs was more or less randomly oriented, in which case (3 ) becomes , S - 2[ l-JoCk/)] , (5) where Jn is the conventional notation for Bessel's function. 'o 13 His values normalized by the mean square velocity fluctuation and a mean meander wave-length of 320 km, are plotted for comparison with these estimates (figure 7)» The small amount of data available from the float measure- ments as well as the meander observations prohibits confi- dence, but it is clear that the float measurements are quantitatively suggestive of an attenuated effect of Gulf Stream meanders. 5. DISCUSSION The evidence offered here is for the existence of meander scale current structures outside the stream proper, rather like the radiated geostrophic wave obtained in a mathematical model of Gulf Stream meanders by Robinson and Niiler (1967 )• They estimate cross stream current components on the order of 10 cm/s for a stationary meander pattern, but differing by 180° from the phase relation between meridional currents and frontal movement suggested by figures 5 and 6. An alternative interpretation is that Gulf Stream meanders may in fact be a manifestation of quasi-geostrophic oscil- lations arising in mid-ocean independently of the stream, as has been suggested by Phillips (1966), as a result of fluctu- ating wind stress. The amplitudes of both the meridional movements of the Gulf Stream and the transients sampled by the Swallow floats are well above those computed by Phillips 1^ Figure 7. Structure function computed from {k) (solid line), (5) (dashed line), and float measurements (dark points). 15 from wind stress of plausible magnitude. Phillips suggests that the oscillations generated by fluctuating wind stress interact with and extract energy from the Gulf Stream as part of the Gulf Stream meander inducing mechanism. It is difficult to reconcile the westward phase speed of about 8 cm/s expected of the mid-ocean oscillations with the eastward phase speed of the same magnitude observed in the meanders. Unfortunately, there are no observations to indicate phase speeds of the fluctuations in the float measurements. It appears feasible to test the existence of the suggested relation of currents outside the stream to Gulf Stream meandering by means of shallow Swallow floats or parachute drogue observations while monitoring the position of the Gulf Stream. One such experiment using a single parachute drogue was attempted near 71°W on 30 March 1967* Figure 5 shows that the currents at Site D changed from southerly to northerly at about this time. Unfortunately the anticyclonic section of meander whose leading (eastern) edge was selected for the experiment site did not develop sufficient amplitude to provide a useful result. The drogue, set initially in slope water, moved south across the edge of the stream becoming entrained in the stream, and thereafter acquiring a predominantly eastward motion. The drogue must have been influenced to some extent 16 by northerly winds of 25" knots. In view of the relatively weak (~10 cm/s) expected of the systematic flow, it is to be expected that a number of such experiments would be required for a clear result to show above the random variations. 6, ACKNOWLEDGMENTS The correlations attempted in this note have drawn upon the efforts of a considerable number of people. I particu- larly wish to acknowledge the contributions of Dr. Ferris Webster and Mr. Frederick Fuglister of Woods Hole Oceano- graphic Institution who kindly provided the current meter and eddy position data, and Mr. R. L. Pickett and Mr. J. C. Wilkerson of the U. S. Naval Oceanographic Office whose special efforts to obtain ART data along the stream were most helpful in interpreting some of the month to month variations of the stream path. Finally, the labors and long days at sea required of the officers and men of the U. S. Coast and Geodetic Survey vessels Explorer , Pierce , and Whiting to obtain the Gulf Stream paths is acknowledged. 17 7. REFERENCES Crease, J. (1962), Velocity measurements in the deep water of the western North Atlantic, J. Geophys. Res. 62 (8), 3173-3176. Fofonoff, N. P. (1968), Current measurements from moored buoys, 1959-1965? Summary Report, Woods Hole Oceano- graphic Institution, Ref . 69-30 (Unpublished manu- script . } Fuglister, F. C. (1967), Cyclonic eddies formed from meanders of the Gulf Stream, Abstract of paper pre- sented before Am. Geophys. Un. in April 1967* Trans. Am. Geophys. Un. }+8, (l), 123- Hansen, D. V. (1970), Gulf Stream meanders between Cape Hatteras and the Grand Banks, Deep-Sea Res. (in press). Iselin, C. O'D. (196I), An interpretation of the deep current measurements, Oceanus 2. 0)> 9. Orlanski, I. (1969), The influence of bottom topography on the stability of jets in a baroclinic fluid, J. Atmos- pheric Sci. 26 (6), 1216-1232. Phillips, N. (1966), Large scale eddy motion in the Western Atlantic, J. Geophys. Res. 71 (I6), 3833-3892. 18 Robinson, A. R. and P. P. Niiler, (196?) , The theory of free inertial currents, I. Path and structure ^ Tellus XIV (2), 269-291. Thompson, R. (1969), The search for topographic Rossby waves in the gappy current records at Site D, Technical Rpt., Woods Hole Oceanographic Institution Ref . 69-67. (Unpublished manuscript.) Webster, F. (1961), A description of Gulf Stream meanders off Onslow Bay, Deep-Sea Res. 8, 130-1^3. Webster, F. (1968), Observations of time dependent ocean currents, ^ course lectures and abstracts of seminars, 1968 Summer Study Program in Geophysical Fluid Dynamics, Woods Hole Oceanographic Institution Ref. No. 68-72, 115-122. Webster, F. (1969), Vertical profiles of horizontal ocean currents, Deep-Sea Res. 16, 85-98. 19 ''^lir'.PJi*.!^ UNIVERSITY LIBRARIES llllllliilili