Near-Inertial Motions Off the Oregon Coast

Transcription

1 JOURNAL OF GEOPHYSCAL RESEARCH, VOL. 88, NO. C10, PAGES , JULY 20, 1983 Near-nertal Motons Off the Oregon Coast AN ANDERSON Unted States Coast Guard, Atlantc Area, Governors sland, New York ADRANA HUYER AND ROBERT L. SMTH School of Oceanography, Oregon State Unversty, Corvalls, Oregon Near-nertal motons were observed at all current meters n an array of fve moorngs spannng the contnental margn off central Oregon durng October 1977 to January All moorngs were between 10 and 130 km from shore, n water depths between 100 m and 2500 m. Largest near-nertal ampltudes (> 30 cm/s) were observed at the uppermost current meters of the offshore moorngs, although these were below the surface mxed layer. The near-nertal energy generally decreased wth ncreasng depth, and there was less near-nertal energy over the contnental shelf than at smlar depths offshore. The energy levels observed over the shelf were about the same as observed there durng summer 1973, and the energy levels observed offshore were comparable wth those observed n the open North Atlantc. Horzontal coherence scales were large, exceedng 115 km durng the frst half of the observatonal perod and about 60 km durng the second half; estmates of the horzontal wavelength (50 km durng the frst half and 20 km durng the second hal0 suggest that the coherence scale s of the order of three wavelengths. Although we dd not have current data n the surface mxed layer nor wnd measurements over each moorng, the avalable meteorologcal data (synoptc pressure charts and hourly wnd and pressure at Newport) suggest that most of the near-nertal energy was forced by the local wnd. NTRODUCTON Pure nertal moton on the rotatng earth s crcular, rotatng clockwse n the northern hemsphere wth a perod of 2n/f, where f s the local Corols parameter. Such pure nertal motons are seldom observed n the ocean. There s almost always some other force actng on the flud that cannot be completely neglected. However, motons that are very nearly nertal n character (called'near-nertal motons') wth very nearly crcular rotaton n the proper sense and wth a frequency near f have been observed at all depths n the oceans and large lakes of the world [Webster, 1968]. The most common characterstcs of near-nertal motons are well docu- mented [Kroll, 1975; Kundu, 1976; Fu, 1981; Thomson and Huggett, 1981]. They are hghly ntermttent and generally last only a few oscllatons wth a maxmum velocty of cm/s; the observed frequency, o, s generally slghtly greater than f; vertcal coherence scales are of the order of only a few tens of meters; and horzontal coherence scales are of the order of a few tens of klometers. Prevous studes of nertal currents have usually been conducted ether n the open ocean [e.g., Pollard, 1980; Fu, 1981] or over the contnental shelf [e.g., Johnson, 1976; Thomson and Huggett, 1981]. The few avalable studes of near-nertal motons over the Oregon shelf are based exclusvely on data collected n summer. Maxmum ampltudes observed over the Oregon shelf were about 15 cm/s durng the summer of 1973 [Johnson, 1976; Kundu, 1976], consderably lower than the open-ocean surface-layer values of about 50 cm/s at ste D n the North Atlantc [Pollard, 1980] and 35 cm/s n the Medterranean [Gonella, 1971]. Horzontal coherence scales over the Oregon shelf were between 10 and 35 km durng summer 1972 [Kndle, 1974] and about 15 km durng summer 1973 [Johnson, 1976]; these are small n comparson wth horzontal coherence Copyrght 1983 by the Amercan Geophyscal Unon. Paper number 3C /83/003 C scales of about 60 km n the North Atlantc [Pollard, 1980; Fu, 1981]. n ths paper, we nvestgate near-nertal motons n current observatons made durng late fall and wnter at fve stes spannng the contnental margn off central Oregon. We descrbe the near-nertal motons n terms of ther ampltude, coherence, and phase by usng both spectral and band-passng technques, and attempt to account for ther generaton by the local wnd. Where approprate, the results are compared wth those of prevous studes. OBSERVATONS Durng 1977 and 1978, Oregon State Unversty and the Unversty of Washngton conducted a jont feld program, the Slope Undercurrent Study (SUS), to study the crculaton along the contnental slope off central Oregon. As part of ths study, an array of current meters was moored across the contnental margn at 45ø20'N between October 1977 and January 1978 (Fgure 1). Occasonal hydrographc sectons were made along the moored array, and coastal wnds were measured contnuously at Newport, Oregon. The SUS array conssted of fve moorngs: 'Skunk,' over the 2600 m sobath just seaward of the foot of the contnental slope wth current meters at lea, 600, and 1850 m; 'Leopard,' over the 1 lea m sobath at mdslope wth current meters at 70 and 820 m; 'Ocelot,' over the 600 m sobath on the upper slope wth current meters at 141, 243, and 371 m; 'Puma,' at the 300 m sobath near the shelf break wth current meters at 64, 164 and 261 m; and 'Elephant,' over the 110 m sobath at md-shelf wth current meters at 23, 49, and 89 m. Ocelot and Elephant were mantaned by the Unversty of Washngton, and Skunk, Leopard, and Puma were mantaned by Oregon State Unversty. All moorngs were deployed n October, wth ntended recovery n January, but the orgnal Puma moorng was lost and replaced n November, and Ocelot was recovered prematurely n early December. Although there was only a very bref (390- hour) perod wth data from all fve moorngs, there are two

2 . A DEgS08 ET AL.: NEAR=NERTAL MOT08 OFF OgF, OO ø N Leopard ß Skunk ß ß Oce lot Elephant 126 ø W Fg. 1. Locaton of the moorngs of the Slope Undercurrent Study. The shaded areas ndcate the locaton of pror studes of nertal currents. perods of about 55 days each wth data from four separate moorngs (Fgure 2). These two perods are lmted by the duraton of Ocelot and Puma, respectvely (Fgure 2). All of the moorngs of the SUS array were sub-surface, and Aanderaa current meters were used throughout. All nstruments recorded mean speed and nstantaneous drecton at nervals of 40 mn. Tme seres of speed were plotted for error detecton, and spurous values were replaced by lnear nterpolaton. After removal of obvous errors, the speed and drecton were used to determne the eastward (u) and northward (v) components of the current. Record gaps of 120 hours begnnng at 0900 October 10 n Ocelot 371 m and 136 hours begnnng at 0200 December 9 n Puma 261 m were were flled wth zeroes. The data were fltered wth a half power pont of 2.5 hours to suppress hgh frequency sgnals and decmated to hourly values to yeld the processedata. Vertcal profles of the square of the Brunt-V s l frequency, N 2 = -g Op/l Oz (where p s densty, g s the acceleraton due to gravty, and z s vertcally upward), were computed from the hydrographc data by usng a centered fnte dfference of 20 m. These profles (e.g., Fgure 3) show that most of the current meters n the array were below the surface mxed layer, durng both the frst (Ocelot) and second (Puma) perods. The mxed layer was farly shallow (less than 25 m) durng the frst perod. " SECOND PEROD FRST PEROD ' '1 /oo,,, Skunk, øø, 1/85o 70m '1 Leopora 'l ' 8ZO ',, Ocelot, ' l' ß! Puma ' 6/. ' ' Elephant [ [ 49 ' ß.' OCT 0 20 NOV 0 20 DEC 0 20 JAN 0 20 Fg. 2. Summary of the tme span of the current meter data. The two tme perods used for spectral analyss are ndcated.

3 ß ANDERSON ET AL.' NEAR-NERTAL MOTON OFF OREGON 126 ø W 25 ø L 0 E 124 ø ] : /o -- 5OO ct ooo 40 cph o N E FROM SHORE (km) OO o w S* ' msol P E, o! ' t ' ß )l ø -1- /'. Dec ,ooo (*26 Jan 1978) OO 15o CE FROM SHORE (km) oo Fg. 3. Profles of the Brunt-V/sl frequency, N 2, near each moorng. The only current meter possbly n the mxed layer was Elephant 23 m; the top current meter of each other moorng was n the pycnoclne (Fgure 3). Durng the second perod, the mxed layer reached almost to the bottom over the shelf, placng Elephant 23 m and 49 m n the mxed layer and Elephant 89 m n the pycnoclne; agan, the top current meter of each other moorng was n the pycnoclne. Tme seres of the eastward component of the current at all current meters (Fgure 4) show frequent epsodes wth moton that has a perod of about two-thrds of a day (.e., roughly the local nertal perod of 17 hours). Some of these epsodes appear to begn mpulsvely (e.g., at Leopard 70 m on December 13), but others show a gradual ncrease n ampltude (e.g., at Leopard 70 begnnng about October 25). The ampltude of these oscllatons appears to decrease wth depth at all moorngs (Fgure 4). Although only the upper current meters at Elephant are n the surface mxed layer, all of the current records from the SUS array contan a substantal amount of near-nertal moton. SPECTRAL ANALYSS The vector tme seres from each current meter were used to calculate rotary spectra, whch separate the clockwse- and counterclockwse-rotatng frequency components [Mooers, Pure nertal moton at ths lattude would have a clockwse-rotatng peak at cph and no counterclockwse energy. Snce the common record length for all fve moorngs was too short for meanngful spectral analyss, we used two tme perods of 1280 hours, whch both had smultaneous data

4 s oo S 600 S 1850 L 70 L : tober 1977 November December Jenuery 1978 Fg. 4a P 64 P 164 P 261 ':[ l J,.,,,,n, Al,,,,,.,,,,.,,n,,.,,A,..,,.,An,,,,,,.,,,.,....,.,,,,,,,,,,.,.,,..,,,,,,..,,,,,,,. :,..., :,,.,.,.,...,,,.,..,o[ r!y"uv"?" 'p,"' ' Qlrl 'Vl'lF"' "nrpv 'llp,: ' r,'¾" ', ","","v ' ' ",,T,, "'?l' ' ' ':..,,A,.,,,,,,,,,,,.,,,,,,.,,,A.,,&,.L, b,tljll bll dlld]a.,,[11,anlllbn,,,j,,,j,.t h,a,,,l.,q Lb A,.,,..,,,,q.,.,,...,,..,,,,,.,,t.,,,,,,,,,,11.,.,,,.u,t,,,,.,,,q October 1977 November December January 1978 Fg. 4b Fg. 4. The eastward (u) components of the currents at the current meters at (a) Skunk, Leopard, and Ocelot, and (b) Puma and Elephant.

5 5964 ANDERSON ET AL.' NEAR-NERTAL MOT O OFF OREGO Leopard 70 Elephant 23 Elephant 49 clockwse F rst Perod ' o n- 01 z uj o 00 0 Second Perod 0 0 ø o 04 o 08 cph Fg. 5. Rotary spectra of selected current meterecords for both the frst (Oct. 13 to Dec. 5) and second (Nov. 20 to Jan. 12) tme perods. The local nertal frequency f, and the semdurnal frequency M 2 are ndcated. from four of the moorngs (Fgure 2). The frst tme perod, wth data from Skunk, Leopard, Ocelot, and Elephant, began at 0400 UT October 13 and ended at 1200 UT December 5; ths perod ncluded an epsode of strong near-nertal moton at the end of October. The second perod started at 0400 UT November 20, ended 1200 UT January 12, and ncluded data from Skunk, Leopard, Puma, and Elephant. We allowed the two perods to overlap by 15 days to use as much of the avalable data as possble. Spectra were obtaned for each tme perod by ensemble-averagng fve perodograms calculated from 256- hour record segments. The resultng spectra have 10 degrees of freedom and a spectral bandwdth of cph, about 6% off. The rotary autospectra for the frst tme perod (e.g., Fgure 5) all show a promnent clockwse-rotatng peak near the local nertal frequency and very lttle counterclockwse energy n that frequency band. n all cases, the near-nertal energy s equal to or greater than the sem-durnal energy. The peak frequency of the near-nertal moton vares slghtly among the dfferent current meters; n most cases, t s somewhat greater thanf. Smlar frequency shfts have been observed elsewhere n the ocean [e.g., Fu, 1981; Gonella, 1971; Kundu, 1976]. The peak energy decreases wth ncreasng depth at each locaton except over the contnental shelf (Table 1, Fgure 6). The rato between the major and mnor axes computed from the rotary TABLE 1. Characterstcs of the Clockwse Near-nertal Peak n the Rotary Spectrum for Each Current Meter Durng Both Tme Perods Frst Perod (Oct. 13 to Dec. 5) Second Perod (Nov. 20 to Jan. 12) Peak Peak Peak Peak Current Energy, Frequency, Wdth Major Axs/ Energy, Frequency, Wdth Meter ctn 2/S 2 cph cph Mnor Axs ctn 2/S 2 cph cph S 100 m S 600 m S 1850 m L 70 m L 820 m Major Axs/ Mnor Axs O 141 m O 243 m O 371 m P 64 m P 164 m P 261 m E 23 m E 49 m E 89 m

6 ANDERSON ET AL.: NEAR-NERTAL MOT O 4 OFF OREC, O w oo oo 0,000 / SPECTRAL ENERGY at cph (cm2/sec z) //// /// E // // // /,//, SPECTRAL ENERGY ot cph (cm2/sec ).O O OO OOO,,, '//' ' //.//. E // /// /,/ L ///,// p / // // / SECOND PEROD / /S // (20 Nov-12 Jan) / / / / /,,4, Fg. 6. Log-log plot of the clockwse spectral energy versus depth for both tme perods. Along the sold lne energy would vary nversely wth depth. The separaton between the dashed lnes ndcates the 95% confdence nterval for each spectral estmate. spectra [Mooers, 1973] shows that the near-nertal moton s very nearly crcular offshore, wth slght ellptcty over the contnental shelf (Table 1). The bandwdth of the near-nertal peak, defned as the dfference between frequences where the energy falls to one-half of the peak value [Fu, 1981], s about cph (twce the spectral bandwdth) offshore, and somewhat greater over the contnental shelf (Table 1). The bandwdth of the peak s nversely proportonal to the persstence of near-nertal motons [Munk and Phllps, 1968]; the persstence seems to be about 120 hours offshore and 100 hours over the shelf. Durng the second perod, there were promnent near-nertal peaks at all current meters of the three offshore moorngs (Skunk, Leopard, and Puma), smlar to those observed durng the frst tme perod, except that the energy was generally lower and the bandwdth was generally greater (Table 1). Over the contnental shelf, the nertal energy was much smaller durng the second tme perod than durng the frst (Table 1, Fgure 5). The spectrum of the top current meter at Elephant (Fgure 5) has two near-nertal peaks, centered at cph (very near f) and cph (about 14% below f), respectvely. A smlar splt was observed at Puma 64, near the shelf edge (Table 1). Elephant 49 m has a relatvely broad, flat peak centered at cph, about 7% belowf All other current meters have a sngle peak frequency that s very nearly equal to for somewhat greater thanf (Table 1). The downward frequency shfts may be a result of Doppler shftng of the near-nertal moton n the presence of a mean flow [Thomson and Huggett, 1981]' There was strong mean alongshore flow, about cm/s, at Elephant 23 m, Elephant 49 m, and Puma 64 durng the second perod; elsewhere the mean flow was much weaker. Durng the frst perod, mean currents were weak everywhere. Durng both tme perods, the near-nertal energy decreases monotoncally wth depth at each of the offshore moorngs (Table 1). A log-log plot of the energy versus depth at the most common peak frequency (Fgure 6) shows that the ponts from Skunk, Leopard, and Ocelot le roughly along a straght lne correspondng to an nverse relatonshp between energy and depth. Durng both tme perods the current meters at Elephant (.e., over the shelf) have much less near-nertal energy than offshore current meters at the same depth (Fgure 6). The current meters at Puma, at the edge of the contnental shelf, have less near-nertal energy than those n the deep water but more than those over the shelf (Fgure 6). Data from prevous studes show a smlar varaton of near-nertal energy wth depth. The energy levels observed n the open North Atlantc [Pollard, 1980; Fu, 1981] seem to be comparable to those at Skunk, Leopard, and Ocelot (.e., these data seem roughly to obey the same nverse relatonshp between energy and depth). The near-nertal energy observed over the Oregon shelf n summer 1973 [Kundu, 1976] are smlar to those at Elephant, and energy levels from the vcnty of the 200-m sobath n Queen Charlotte Sound [Thomson and Huggett, 1981] are comparable to those at Puma. Thus, t may be generally true that there s less near-nertal energy over the contnental shelf than at smlar depths n the open ocean. Coherence and phase spectra were computed for all current meter pars and both tme perods. Wth 10 degrees of freedom, coherence squared values of 0.53, 0.58, and 0.68 are sgnfcantly dfferent from zero at the 95, 97, and 99% levels, respectvely [Thompson, 1979]. There was sgnfcant coherence at the 95% level at a clockwse-rotatng frequency between and cph between 23 out of 55 current meter pars durng the frst perod and between 19 pars durng the second perod. Durng both perods, there was generally hgher coherence between horzontally or dagonally separated current meters than between vertcally separated current meters (Fgure 7). The horzontal coherence scale exceeded the extent of the array (115 km) durng the frst perod and was at least half the extent of the array durng the second perod (Fgure 8). The large horzontal coherence scale of the frst perod exceeds those observed n the open ocean at comparable depths [Fu, 1981] and s comparable to the coherence scales observed n the surface mxed layer at ste D n the North Atlantc [Pollard, 1980] and n the relatvely shallow waters of Queen Charlotte Sound off Brtsh Columba [Thomson and Huggett, 1981]. Phase spectra were also computed for both tme perods, and confdence lmts for phase were determned by the procedure descrbed by Koopmans [1974, p ]. Ths method of computng confdence ntervals s conservatve, snce t assumes that the tme seres data have a Gaussan dstrbuton; f one assumes nstead that the data conssts of a coherent sgnal plus ncoherent nose, the phase estmates would be exact [Schott and Dng, 1976]. The phase dfferences between current meter pars can be used to determne the drecton of phase propagaton and the wavelength of the moton, f the separaton between current meters (horzontally or vertcally) s small compared wth the wavelength of the near-nertal motons. For example, Kundu [1976] used data from 11 current meters separated vertcally by 4-20 m to estmate upward phase propagaton of cm s- and vertcal wavelengths of m. n our case, most of the current meter separatons are dagonal, makng t dffcult to estmate ether the vertcal or horzontal propagaton speeds. To add to the dffculty, most of the sepa-

7 ß ß ß ß. ß ANDERSON ET AL.' NEAR-NERTAL MOTON OFF OREGON 126 ø W S 1 5oL 0 oo OO 000 FRST PEROD (13 Oct-5 Dec) ß DSTANCE FROM SHORE (krn) oo ' W, 5o L P E 124* o oo OO OOO ß.. SECOND PEROD ß (20 Nov-12 Jan) 1500 ß. DSTANCE FROM SHORE (km) 200O Fg. 7. Dstrbuton of current meter pars coherent n the clockwse rotatng nertal frequency band of cph. Pars connected by sold lnes have a squared coherence of at least 0.58, the 97% sgnfcance level. ratons (both vertcal and horzontal) arc larger than the ex- dcncc nterval of km. Smlarly, the phase dfference of pected vertcal and horzontal wavelengths. Nevertheless, wc 380 ø (20 ø ø) between Puma and Elephant (19.7 km apart) used the phase spectra to estmate the horzontal east-west yelds a wavelength of 19 km (the 80% confdence nterval s (cross-shelf) wavelength by makng the followng assumptons: km). By addng ntegral numbers of 360 ø to the phase (1) phase propagaton was from easto west (.e., energy propa- dfferences between the other pars, wc were able to obtan gated onshore); (2) the vertcal separaton (maxmum of 71 m) consstent estmates of the cross-shelf wavelength for each tme between Elephant 89 m, Puma 64 m, Ocelot 141 m, Leopard 70 perod (Table 2). The average wavelength obtaned was 50 km m, and Skunk 100 m was nsgnfcant when compared wth the for the frst perod and 20 km for the second perod. The 50-km horzontal separaton; (3) durng the frst perod, the horzontal wavelength for the frst perod s consstent wth the large wavelength was larger than the smallest horzontal separaton; coherence scale observed then: Only three wavelength span and (4) durng the second perod, the horzontal wavelength the entre array. Durng the second perod, both the estmated was slgttly smaller than the smallest separaton. Wth these wavelength and the coherence scale arc shorter. n both assumptons, the phase dfference of 115 ø between Leopard 70 perods, there s sgnfcant coherence between current meters m and Ocelot 141 m (13.9 km apart) durng the frst perod separated by two horzontal wavelengths or less. yelds an cast-west wavelength of 44 km wth an 80% conf: Wc also consdered estmatng the vertcal wavelength, but

8 ANDERSON ET AL..' NEAR-NERTAL MOTON OFF OREGON '-' "' 0.6 ). 0 o (n 0.4 F rst Perod z rr o A km 02 od Ax Fg. 8. The squared coherence at cph between pars of current meters wth roughly horzontal separaton: Skunk 100 m, opard 70 m, Ocelot 141 m, Puma 64 m, and Elephant 89 m. The 90%, 95%, and 97% sgnfcance levels are 0.44, 0.53, and 0.58, respectvely. there was sgnfcant coherence between only a few vertcally separated pars. The spacng of the current meters was too large to resolve vertcal wavelengths of the magntude ( m) generally observed n the ocean. Although we could have obtaned estmates by assumng partcular values of ntegral wavelengths between current meters, the choce would have been completely arbtrary, and, hence, the resultng estmates would be meanngless. BAND-PASSED DATA To examne the tme varablty of the near-nertal moton, we followed Kundu's [1976] example and used the technque of band-passng rather than complex demodulaton. (Whle the latter has the advantage of usng both components of the orgnal vector tme seres, the former has the advantage of showng the current varablty more clearly. n a case lke ours, where the moton n the near-nertal frequency band s essentally crcular and clockwse, the two technques gve equvalent results.) The current records were band passed by usng a narrow flter centered at cph, the most common frequency of the near-nertal peak n the autospectra. The procedure was to determne the Fourer components of each current record by usng a Sande-Tukey fast Fourer transform, pass 100% of the energy n the frequency band between and cph, and suppress all of the energy at frequences less than cph and greater than cph, taperng lnearly wthn the edges (0.002 cph wde) of the flter. The band passed by the flter les entrely between the semdurnal and the durnal tdal frequences. For computatonal effcency, we fltered only 2048 hours of the long records, 1408 hours of the Ocelot records, and 1280 hours of the Puma records. n each case, there were at least 32 Fourer components wthn the band passed by the flter. The possblty of peak smearng due to the sharp flterng was tested by usng a synthetc 1024-hour seres wth 5, 10, or 15 cycles of moton at cph, wth the remander of the seres set to zero. Lke Kundu [1976], we found that peak smearng s less pronounced when the number of cycles was greater: When the nput seres contaned more than 10 cycles, the ampltude of the band-passed output dd not vary greatly from the nput seres. The phase of the near-nertal moton was not altered by the band-pass flter, regardless of the number of cycles. The tme seres of the eastward component of the bandpassed current at all current meters (Fgure 9) shows there s some near-nertal moton throughout most of each record. The energy seems to occur predomnantly n bursts or 'events' of several days duraton. Some of these events appear to begn mpulsvely and decay gradually, but others ncrease gradually and persst for more than a week. The maxmum value (39 cm/s) of the band-passe data occurred at Leopard 70 at the end of October. At each current meter, the band-passed northward current s very smlar to the eastward component, except that t s 90 ø out of phase (e.g., Fgure 10). The tme seres of the band-passe data (Fgure 9) demonstrate explctly some of the conclusons drawn from the spectral analyss: The strongest near-nertal currents occur at the offshore moorngs, and at each locaton the ampltude generally decreases wth depth. Durng the frst perod, October 13 to December 5, the near-nertal moton s very persstent at the offshore moorngs and less persstent over the contnental shelf. At most current meters, the largest ampltudes occur durng the frst perod rather than the second. Durng the second tme perod, November 20 to January 12, there s very lttle nearnertal energy over the contnental shelf. Durng each perod of hgh near-nertal energy, the phase dfference between the band-passed current vectors at Skunk 100 m and Leopard 70 m, and between Leopard 70, and Ocelot 141 m, remaned farly constant (Fgure 11). Durng the strong near-nertal event of October 27 to November 13, Skunk 100 TABLE 2. Horzontal East-West (Cross-Shell) Wavelengths Calculated From Phase Dfferences at cph, for Current Meter Pars Wth Nearly Horzontal Separatons. Frst Perod (Oct. 13 to Dec. 5) Second Perod (Nov. 20 to Jan. 12) Current Separaton Meter Dstance, Coherence Phase Wavelength, Coherence Phase Wavelength, Pars km Squared Dfference N km Squared Dfference N km S 100-L ø q- 21 ø S 100-O ø q- 25 ø 1 55 S 100-P m S 100-E ø+ 17 ø L ø + 25 ø 0 J 44 L 70-P m 0.48 L 70-E ø q- 22 ø O 141-E ø + 20 ø 1 46 P 64-E ø q- 49 ø ø q- 31 ø ø q- 25 ø 1 19 The 80% confdence nterval s shown for phase' N s the assumed number of ntegral wavelengths between each par.

9 5968 ANDERSON ET AL.' NEAR-NERTAL MOTON OFF OREGON 0 cm/sec -.., S 00 S 600 S 1850,AAA... AAAAAAAA... A..,.AAA... A... AA,,A,,,,AAAAAA,,,,,,AA... A,......, AAAAAAAAAAAAAA,,,., o...vv... -::-' -,,..wvvvv,.-v,,vvv-,,,,-v-v-.-vvvvv--vvvvvvv,,.v.,... v... v... v...,,-vvwv vvvul/vvv,,vvv-vvvvvvv... '... vvvvvvv'- :-' %%-:' %%':--: O... :... AAAAA... --v,a#,,.,,,a. v--v v--vvv...,,v,,---,, -v,, vvvvvvvvvvvvv,,.,,-vvvvvvvvv- ' AAA. AAAAAAAA AAAAAAAAA v M: v va L 7O L o o- ;VVVVVVVVVVVVV"Vo"VV"'v"'... ',vvvvvvvvv"... '... o..:#::-: --..AA A,,,,,AA AAAAAAAA.AAAA,,.AAAAAAAAAAAA.,,,A,,,,AAAAAAAAAAAAA,,,..AAAAAAA... vvvv '"vvvvllvvl/gg"vgvv"vvvvvvvvl/vv '""'vvvvgvvvv. vv"" 'WVV1/VVVVVVVV': October 1977 November December January 1978 Fg. 9a P 64 P 164 P [... vvvvvvv...,.vv AAAAA,,AAA.... wv.vvvgl/vvvgg AAA,, ^AAAAAAAAAAA v..., A.,A vvv---v... vv-... 'vv :: ::;:... vvvv ^AA... v,- vvg AAAA v.,ray 0... q½-:' ';:--:-:::# E 23.,,.,AAA...AAAhAAAAAAA.,..,,.AAhA.AA,,AAA,,,,.,,..,,AAAA,,,AAA... A,,^.....AAA,AAA,...vvv.vvv,VV,V g...,. v,,,...,v,... E 49 E 89,o o o,,o o, K 0 0, ;0 October 1977 November December January 19 Fg. 9b Fg. 9. The band-passed tme seres of the eastward component of the current at the current meters at (a) Skunk, Leopard, and Ocelot, and (b) Puma and Elephant.

10 ANDERSON ET AL.' NEAR-NERTAL MOTON OFF OREOON T Leopard 70m Band Passed l J, ]',,,. Fg. 10. A 10 day (Oct. 27 to Nov. 6) segment of the band-passed eastward (u) and northward (p) components of current at Leopard 70 m. The approxmately equal ampltude of the components and the 90 ø lag of u behnd p ndcate clockwse crcular moton. m lags behnd Leopard 70 m by about 330 ø, and Leopard 70 m lags behnd Ocelot 141 m by about 120ø; these numbers agree very well wth the phase spectra for the frst perod (Table 2). Thus, the horzontal wavelength estmate of 50 km seems to be vald throughout the 20-day duraton of strong near-nertal moton begnnng about October 27. Several current meters showed hgh near-nertal energy durng December (Fgure 9). At Leopard 70 m, there seem to be two separate near-nertal events that begn about December 13 and December 24; the near-nertal energy at Skunk 100 m seems to be marc nearly contnuous. The phase dfference between Skunk 100 m and Leopard 70 m vares consderably durng ths perod: t s about 180 ø on December 14-16, about 75 ø on December 18-22, and about 270 ø on December 24-28; only the frst of these agrees well wth phase spectra for the second perod (Table 2). Thus, the near-nertal phase dfference between a par of current meters seems to bc well defned and farly constant durng a partcular nearnertal event but vares consderably from one event to another. To the extent that dfferences n phase reflect dfferent ø t -2 $60 L : : : : :... L 7O -40 $60 S 00-L *' ) 0 20 ) October November December January 2o S 00 Fg. 11. The hourly dfference n drecton between the Leopard 70 m and the Skunk 100 m or Ocelot 141 band-passed current vectors.

11 5970 ANDERSON ET AL.' NEAR-NERTAL MOTON OFF OREGON 2 dynes/cm W nd Stress Magntude Nov Dec Jan Elephant 23m... JAAAAAfAAAfAA... naa.aaahaa.aaa, naaa.....,lb,.. Leopard 70rn Fg. 12. The magntude of the Newport wnd stress, and the magntude and castward component of the Pollard and Mllard model, and castward components of the band-passed current at Elephant 23 m (n the mxed layer), and Leopard 70 m (about 30 m below the mxed layer). wavelengths, these varatons mply that the wavelength of near-nertal motons vares consderably between dfferent energetc epsodes. energetc near-nertal moton observed at the three offshore moorngs n late October and early November. Snce the model s very senstve to small-scale varatons n the wnd feld WND FORCNG [Pollard, 1980], we mght expect poor agreement between observed currents and model results whenever there are strong The generaton of near-nertal moton n the mxed layer by atmospherc fronts n the regon. local wnd forcng at ste D n the North Atlantc was modeled Frontal passages were very common off Oregon durng late successfully by Pollard and Mllard [1970]. Although most of October and early November (Fgure 13): between October 23 our current observatons were well below the mxed layer and and November 15, 17 fronts assocated wth low pressure centhe nearest wnd observatons were made on the Newport ters of varyng sze and ntensty passed eastward over the SUS south jetty 85 km south of the array, we used the Pollard and array. Many of these frontal passages were recorded n the Mllard model to obtan a qualtatve estmate of the currents Newport pressure data and caused clockwse rotaton of the that mght have been generated by the observed wnds. The wnd at Newport (Fgure 13). Snce the extent of the SUS array model equatons are was 115 km, the arrval tme of each front would not generally u,--fv = :"/(pz)- cu (1) be the same tme at all moorngs. Wherever the rotaton of the near-nertal moton caused by each frontal passage was n v, + fu = :'/(pz)- cv (2) phase wth the pre-exstng near-nertal moton, there would be constructve nterference [Pollard, 1970] whch would ncrease where f s the local nertal frequency, u and v are eastward and the near-nertal energy. To look for phase shfts n the obnorthward components of the current, c s the dampng coef- served near-nertal moton, we calculated the phase dfference fcent, p s the water densty, x0:", :Y) s the wnd stress, and Z s between the band-passed current vectors and a reference vector the mxed layer depth. The wnd stress, x = Cap,, JVJV, was rotatng clockwse at cph: vectors wth no phase shfts computed from the Newport hourly vector, V, the densty of wll have a constant phase dfference they have the same ar, p = 1.25 x 10-3 gm/cm 3, and a drag coeffcent, Ca = 1.5 frequency as the reference vector or a lnearly ncreasng and x We assumed a constant mxed layer depth of 25 m. We decreasng phase dfference ther frequency s slghtly dfferused a smple fnte dfference form wth a 4-s tme step to ent. Tme seres of ths phase dfference for Leopard 70 m and compute hourly values of the model current. Elephant 23 m (Fgure 13) show that no shfts n the phase of The Pollard and Mllard model shows a strong near-nertal the near-nertal moton occurred at Leopard 70 m durng late response to the wnd stress peaks of December 14 and January October/early November, whle several phase shfts occurred at 4 (Fgure 12), and a weaker response to some of the weaker Elephant 23 durng the same perod. Ths suggests that most of storms between November 7 and December 10. Near-nertal the wnd-generated near-nertal moton at Leopard n late moton was observed more frequently at Leopard 70 m than n October/early November was nearly n phase, causng the the Pollard and Mllard model results (Fgure 12). Near-nertal strong persstent near-nertal currents there. On the other moton occurred frequently at Elephant 23 m from md- hand, some of the near-nertal moton generated Elephant October to early December, but not mmedately after the may have been out of phase wth pre-exstng near-nertal strong storms of December 14 and January 4, although both of motons, causng destructve nterference on about October 27 these storms seemed to generate near-nertal motons at Leo- and agan on November 4 and 6. Thus, the observatons pard 70 m. The model faled to account for the persstent and strongly suggesthat the strong and persstent near-nertal

12 ANDERSON ET AL.' NEAR-N TAL MOTO8 OFF OREC, O ' N /000rob ' 40 ø "W 125 ø Oct 130øW 125" 12øNøo v...!. : o"w 125, : o w 1: 5 o ODec / / ".' oooo / / 130øW 125' 010 NEWPORT Barometrc Pressure 980 mb Oct. Nov. Dec. Jon. N WPORT Low-Poss Wnd Drecton t... /L/ 360 ø' ld ::::::::::::::::::::::::: :... LEOPARD 70rn Phose Dfference 40 ELEPHANT 23m 360 ø ::- ELEPHANT 23m Phose Dfference Dec, Jor Fg.!3. Examples of the synoptc atmospherc pressur e charts from the perod of the near-nertal 'event' of late October/early November and tme seres of atmospherc pressure and the drecton of the low-passed (< cph) wnd at Newport, wth the eastward components and the phase dfferences of the band-passed current vectors at Leopard 70 m and Elephant 23 m relatve to a reference vectorotatng clockwse at cph.

13 5972 ANDERSON ET AL.' NEAR-NERTAL MOTON OFF OREGON motons observed at the offshore moorngs n late October and early November were generated by the local wnd. Wthout drect wnd observatons over the moorngs, ths tentatve concluson cannot be verfed. Near-nertal CONCLUSONS motons were observed at all current meters n an array of fve moorngs whch spanned the contnental margn off Oregon between October 1977 and January The current meters were between 23 and 1850 m deep, n water depths between 100 and 2500 m, at locatons between 10 and 130 km from shore. Although almost all of the current meters were below the mxed layer, large ampltude (~ 20 cm/s) nearnertal motons were observed on several occasons. The largest ampltude (> 30 cm/s) near-nertal motons were observed at the shallowest current meters of the offshore moorngs; these current meters were n the pycnoclne at depths between 70 and 141 m. Although there were current meters at shallower depths (23 and 49 m) wthn the surface mxed layer over the contnental shelf, there was much less near-nertal energy there. The amount of near-nertal energy observed over the shelf was smlar to that observed there n the summer of 1972, and the amount of energy n the deep water was comparable to that observed n the North Atlantc. Most of the near-nertal moton had a frequency slghtly greater than the local nertal frequency, but some of t had a frequency less than the local nertal frequency. The downward frequency shft occurred n the presence of a strong northward current. The near-nertal moton at some current meters was very persstent (n one case t perssted for more than 15 days). n ths case, the ampltude seemed to ncrease gradually before t decayed. n other cases, the ampltude ncreased mpulsvely wth gradual decay, as usually observed elsewhere. Spectral analyss showed that the horzontal coherence scale exceeded the extent of the array (115 km) n October and November and was about half the extent of,the array (.e., 60 km) n December and early January. These coherence scales are much larger than those prevously observed over the Oregon shelf n summer. Estmates of the horzontal wavelength of the moton changed from 50 km durng the frst half of the records to about 20 km durng the second. These estmatesuggest that there s sgnfcant coherence between current meters separated by two wavelengths or less. The near-nertal phase dfference between a par of current meters vared consderably between separate near-nertal events, even though the stratfcaton remaned much the same throughout the perod of observaton. Ths suggests that the wavelength s the result of the partcular crcumstances generatng the moton, rather than of the oceanc envronment. Because of the lack of current measurements n the mxed layer and the absence of drect wnd measurements over each moorng, we could not rgorously test the hypothess of local generaton of the near-nertal motons. However, the smple Pollard and Mllard model of local forcng, usng the Newport wnd observatons (85 km south of the array) as the forcng functon, ndcates that some of the near-nertal moton observed below the mxed layer (e.g., at Leopard 70 m) was generated mpulsvely by strong peaks n the wnd stress (e.g., December 14 and January 4). The strong and persstent perod of near-nertal moton n late October and early November may have been the result of a sequence of fronts that happened to pass over the regon n the proper phase for constructve nterference. Thus, our data are consstent wth the hypothess that the local wnd can force strong near-nertal motons at depths well below the mxed layer. Acknowledgments. We wsh to thank Barbara Hckey of the Unversty of Washngton for provdng the data at Ocelot and Elephant. We also thank Rck Romea and Jm Rchman for ther comments on earler drafts and Joseph Bottero and Wllam Glbert for assstance n the analyss. Ths study was completed whle the frst author was studyng for a degree at Oregon State Unversty. Fnancal support was provded by the Unted States Coast Guard and by the Natonal Scence Foundaton through grants OCE and OCE REFERENCES Fu, L., Observatons and models of nertal waves n the deep ocean, Rev. Geophys. Space Phys., 19, , Gonella, J., A local study of nertal oscllatons n the upper layers of the ocean, Deep Sea Res., 18, , Johnson, W., Cyclesonde measurements n the upwellng regon off Oregon, Tech. Rep. 76-1, Rosental School of Mar. and Atmos. Sc., Unv. of Mam Florda, Kndle, J. C., The horzontal coherence of nertal oscllatons n a coastal regon, Geophys. Res. Lett., 1, , Koopmans, L. 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