Evidence for secondary circulation associated with a shelfbreak front is obtained from a high‐resolution, cross‐shelf section of hydrographic, optical and velocity fields. Convergence in the bottom boundary layer on the inshore side of the front and subsequent upwelling into the interior is evident by a mid‐water region of suspended bottom material emanating from the foot of the front and extending to within 35 m of the surface, 80 m above bottom. Downwelling on the offshore side of the front in the upper water column is inferred from a 20‐m downward bend of the subsurface phytoplankton layer. These observations are in agreement with recent model predictions for secondary circulation near an idealized shelfbreak front. Convergence in measured cross‐shelf velocity at the foot of the front is consistent with upwelling of bottom material detected there. An estimate of 9±2 m day⁻¹ of upwelling on the inshore side of the shelfbreak front is obtained, implying a transit time from the bottom to the surface of 10–16 days

Barth, John A.

Bogucki, Darek

Kosro, P. Michael, 1951-

Pierce, Stephen D.

Undetermined

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GEOPHYSICAL RESEARCH LETTERS, VOL. 25, NO.15, PAGES 2761-2764, AUGUST 1, 1998 , Secondary circulation associated with a shelfbreak front John A. Barth, Darek Bogucki, Stephen D. Pierce, and P. Michael Kosro College of Oceanic •; Atmospheric Sciences, Oregon State University, Corvallis, Oregon Abstract. Evidence for secondary circulation associ- ated with a shelfbreak front is obtained from a high- resolution, cross-shelf section of hydrographic, optical and velocity fields. Convergence in the bottom bound- ary layer on the inshore side of the front and subsequent upwelling into the interior is evident by a mid-water re- gion of suspended bottom material emanating from the foot of the front and extending to within 35 m of the surface, 80 m above bottom. Downwelling on the off- shore side of the front in the upper water column is inferred from a 20-m downward bend of the subsurface phytoplankton layer. These observations are in agree- ment with recent model predictions for secondary circu- lation near an idealized shelfbreak front. Convergence in measured cross-shelf velocity at the foot of the front is consistent with upwelling of bottom material detected there. An estimate of 9:t:2 m day-1 of upwelling on the inshore side of the shelfbreak front is obtained, implying a transit time from the bottom to the surface of 10-16 days. Introduction In the Middle Atlantic Bight (MAB), a shelfbreak front is often found separating cold, fresh shelf water from warm, saline slope water offshore. The associated westward alongshore jet, the primary circulation, is an order of magnitude stronger than any secondary cir- culation that may occur in a cross-shelf, vertical plane. This disparity makes measurement of the secondary cir- culation challenging. Another way to "measure" the secondary circulation is to infer it from distributions of passive tracers present in the flow field. In this paper, two naturally occurring tracers, phytoplankton and sus- pended bottom material, are used to infer the secondary circulation associated with the shelfbreak front in the MAB. The physical, optical and velocity distributions pre- sented here and the secondary circulation inferred from them will be compared with idealized model predic- tions. Gawarkiewicz and Chapman (1992) and Chap- man and Lentz (1994) (hereafter CL) have presented models for the secondary circulation associated with an idealized coastal density front. To reach the steady Copyright 1998 by the American Geophysical Union. Paper number 98GL02104. 0094-8534/98/98GL-02104505.00 state predicted by CL, offshore buoyancy flux in the bottom boundary layer (BBL) occurs until the foot of the front is deep enough such that the vertical shear within the front (in thermal wind balance with the rel- atively time-independent, cross-shelf density gradient) causes a reversal in the near-bottom cross-shelf flow. This results in convergence in the BBL on the inshore side of the density front and upwelling into the inte- rior. Near the surface on the offshore side of the front, convergence also occurs but here leads to downwelling. Data Vertical profiles of water properties were made across the continental shelf and slope south of Cape Cod, Mas- sachusetts using instruments aboard SeaSoar, a towed, undulating measurement platform. The vehicle cycled through the water column between the surface and to within 5-10 m of the bottom while being towed at 7- 8 knots (3.6-4.1 m s-l). Complete cycles to 105(55) m were accomplished in 210(75) s at the offshore (onshore) end of a cross-shelf section; horizontal separation at the surface and bottom of the sawtooth-shaped SeaSoar trace was 800(300) m. From August 14 to September l, 1996, repeated cross-shelf sections were made at several alongshore locations near 40.5øN, 70.5øW. Temperature, conductivity and pressure were mea- sured at 24 Hz using a Seabird 911+ instrument with the sensors mounted pointing forward through Sea- Soar's nose. Salinity and density were obtained follow- ing procedures described in Barth et al. (1998) and vertical sections were made by averaging over 1.2 km horizontally and 2-db in the vertical. Light attenuation and absorption at nine wavelengths were measured at 6 Hz using a WETLabs ac9 instru- ment mounted on top of SeaSoar. Water pumped through the optical measurement tubes without filter- ing was obtained from an inlet adjacent to the Seabird sensors in the nose of SeaSoar. A time lag was applied to the optical data to align it with the hydrographic data so that light absorption could be corrected for temper- ature dependence. Vertical sections were made from 1-sec averaged data. Vertical profiles of velocity were obtained using an RDInstruments 300-kHz, hull-mounted acoustic Doppler current profiler (ADCP). The ADCP had vertical bins and pulse lengths of 4 m, an ensemble averaging time of 2.5 min, and a depth range of 10-200 m. Details of the ADCP data processing follow those described by Pierce 2761 lO0 lO0 Distance (krn) 70 60 50 40 30 20 10 0 I • I I ! I 70 T (C) S (psu) c• k,m '• 40 6 40.5 40.4 40.3 40.2 40.1 40.0 Latitude (øN) Distance (km) 40 30 , , -- __ 40.6 '• 40.3 40.2 "• Latitude (øN) / 50 41 ß 40 71 70 10 40.1 4O.0 Figure 1. Cross-shelf ections f temperature, salinity and ensity along 70.48øW from 13:01 to19:35 UTC on August 21, 1996. Gridded data locations are hown as light dots. The location f the cross-shelf ection is indicated by a thick line on the map. lO0 lO0 0.25 - . 0.2- 0.15 - 0.05 - . 400 Distance (km) 70 60 50 40 30 20 10 0 I ! Jl• I i ! , , chlorop. hyti :, orl3tton - 40.6 40.5 40.4 40.3 40.2 40.1 Latitude (øN) absorption (m -q) ,•,..,-,•'•"' chlorophyll peak '" ,-. ..... ,..... ,,,. .... .. ..... .. -- : atten, uation (•-•* 0 2) . ' " '1 ..... ' i 500 6OO 7O0 wavelength (nm) 4O 0 et al. (1997). Short-term inherent random errors for an ensemble are at most 0.015 m s -1 and estimated rms errors in absolute velocity are 0.01 m s -1 while bottom tracking (water depth • 200 m) and 0.03 m s -1 when using differential G PS navigation. The ADCP veloci- ties were derided by removing depth-averaged ti al cur- rents estimated from harmonic analysis following Can- dela et al. (1992). M2 current amplitudes vary between 0.02 m s -[ over the slope to 0.15 m s -[ near the shallow end of the cross-shelf transect. Results Vertical sections of temperature, salinity and den- sity measured across the continental shelf and slope are shown in Figure 1. A warm surface layer exists over the entire section separated from colder water below by - Figure 2. Cross-shelf sections of chlorophyll ab- sorption and 650-nm light attenuation measured with _ a nine-wavelength absorption and attenuation instru- _ ment on the towed, undulating vehicle SeaSoar along . 70.48øW from 13:01 to 19:35 UTC on August 21, 1996. _ The thick white curve is the at - 25.8 kg m -3 isopyc- . nal; data locations are indicated by light dots. On the bottom are spectra of light absorption (solid curves) and attenuation (dashed curves) from a depth of 50 m at 40.13øN (thick curves) and at 40.21øN (thin curves). BARTH ET AL.: SECONDARY CIRCULATION NEAR A SHELFBREAK FRONT 2763 a strong thermocline between 10 and 30 m. The ther- mocline bends downward near 40.1øN so that a region of warm water separates cold water inshore, the "cold pool", from that found offshore in the mid-water col- umn. Warm, salty slope water is present offshore at depth. The salinity field is relatively uniform over the Vertical sections of ADCP velocity show a strong, surface-intensified alongshore jet in thermal wind bal- ance with the sloping shelfbreak front (Figure 3). Cross- shelf velocities how offshore flow (v < 0) near the foot of the front (40.3øN) with a region of onshore flow near the bottom (40.15øN) just offshore. The implied con- shelf, separated from offshore water by a strong surface-  vergence near the foot of the front (40.2-40.25øN) is ev- to-bottom front located between 40.1 and 40.35øN. A strong pycnocline 10-20 m above bottom (mab) is found where the salinity front intersects the bottom at 85 m depth; this defines the "foot" of the shelfbreak front. In the upper half of the water column associated with the salinity front are several mesoscale (5-10 km) features. The density field shows a strong pycnocline between 10 and 30 m over the region due mostly to tempera- ture over the shelf and to both temperature and salin- ity offshore. The shelfbreak density front rises from the seafloor near 40.35øN to within 35 m of the surface be- fore bending down offshore. This bending down offshore may be due to horizontal meandering of the front. Vertical sections of light absorption in the chloro- phyll band, an indicator of chlorophyll concentration, and light attenuation at 650 nm are shown in Figure 2. Chlorophyll absorption is obtained by computing the area under the 676-nm peak above the background ab- sorption level: a676- (a650 + a715)/2. Chlorophyll concentration is high near the base of the pycnocline across the entire shelf but bends down abruptly (from 30 to 50 m) in the region of the shelfbreak front (40.1øN). Offshore of this region of deep chlorophyll, elevated val- ues are found following the density surface defining the shelfbreak front (at = 25.8 kg m-3). Light attenuation at 650 nm, a measure of the total suspended particulate matter, is elevated within the chlorophyll features just described, but is also high in a bottom nepheloid layer. A warm, salty mesoscale surface lens near 40.075øN is clear compared with the rest of the surface waters; low light attenuation here is consistent with low chloro- phyll. Offshore at depth the water is clear and there is relatively less light attenuation in the mid-water col- umn over the shelf between the pycnocline chlorophyll maximum and the BBL. A noticeable feature is the tongue of high light attenuation extending up through the water column from the foot of the shelfbreak front (40.35øN), along the inshore side of the frontal bound- ary (at = 25.8 kg m -3) and reaching to within 35 m of the surface just inshore of the downward dipping chloro- phyll maximum. Between approximately 35 and 50 m, the two maxima, chlorophyll offshore and light atten- uation inshore, are separated horizontally. Also shown in Figure 2 are spectra of light absorption and attenua- tion at 50 m from within the two maxima. The absorp- tion spectra have been scaled so that a650 is the same in both samples to highlight differences in the 676-nm chlorophyll peaks. The material in the offshore sample is high in chlorophyll and has lower light attenuation at all wavelengths than that found in the inshore sample, which is presumably rich in suspended sediment. ident in a plot of the cross-shelf derivative of cross-shelf velocity, i.e. horizontal divergence assuming alongshore uniformity. There is also convergence in the mid-water column near 40.1-40.15øN and divergence in the upper- water column near 40.2-40.25øN and at depth offshore of the front (40.05-40.1øN). Here the positive cross-shelf direction is due north, which maximizes the alignment between the coordinate system and the front/jet, thus yielding the strongest secondary circulation signal. Distance (km) 70 60 50 40 30 20 10 0 0 , I , I , I , I • I • I , I , •.•:•. :•;•:•:•A•;• • .;•:',•' •, ,• •:•: • • :• ....... -: ..... •:• ::•:: ........ • :;.•..•.,•: ::•{•D•;•.•;• ß :•.•.•2•5•4•,4-:•.....• ;.:.•,..;   •  ::•;:,;: :.•: ...........::, • -•,:•::,::::• ................... ,•,•:•,•'  ..... ,•A• :• :•::.•:•:•:;;• ., ,• . .•.. :,....,, :• •:•:...:• ..... o . . . -,•:•;, .:•;•: :;.•,,,,• .,•.•...... ,.... .......... • ..... :• ...... , ......... •........ 100 ' I ' I ' I ' I ' I ' I ' I , I , I , I , I , I , I , I 5o ': "'m: .......... :::'--: '?:'::%:.. ...... .. . • . •oo 40.6 40.5 40.4 40.3 40.2 40. • 4• Lo katitude Figure 3. Cross-shelf sections of alongshore (u) and cross-shelf (v) velocity, measured with a shipboard acoustic Doppler current profiler, and the cross-shelf derivative of v (dv/dy). Regions with negative values are shaded and the thick curve is the at = 25.8 kg m -3 isopycnal; gridded data locations are indicated by light dots. 2764 BARTH ET AL ß SECONDARY CIRCULATION NEAR A SHELFBREAK FRONT Discussion The region of elevated light attenuation emanating from the BBL and extending upward through the wa- ter column along the inshore side of the shelfbreak front is consistent with model predictions of Gawarkiewicz and Chapman (1992) and CL. In CL, neutrally buoy- ant particles released near the bottom and shoreward of the density front were advected up along the frontal boundary. The same pattern is observed here as evident by upwelled bottom material. The downward bend from 30 to 50 m of the sub- surface chlorophyll maximum associated with the shelf- break front may indicate downwelling. This pattern is consistent with the downwelling predicted by CL in the upper part of the water column near the offshore side of the shelfbreak front. In fact, the four maxima visible in ADCP divergence (Figure 3), divergence above con- vergence on the inshore side of the front, convergence above divergence on the offshore side of the front, are consistent with CL's results. Assuming that near-bottom cross-shelf convergence is balanced by upwelling, the ADCP convergence field (Figure 3) can be integrated over a region 30 mab and 15 km across to yield an upwelling estimate of 9q-2 m day -1. Thus, a water parcel could travel from the bottom along the front to the surface (a distance of 115 m) in 10-16 days. Two additional long, cross-shelf SeaSoar/ADCP sec- tions were obtained to the west of the data presented here. A section 13 km to the west shows a similar bending down of the chlorophyll maximum where the shelfbreak front approaches the surface. A region of elevated light attenuation at depth is found following the inshore side of the front and separating clearer wa- ter above and below. This region extends 30 km along the sloping front and is up to 20 m in height. A sec- tion 25 km to the west shows a similar downward turn in the upper-water column chlorophyll maximum, but only a small region (• 5 km wide, 20 m high) of elevated attenuation near the foot of the front. Therefore, the implied convergence and upwelling near the foot of the front is occurring over at least 25 km in the alongfront direction, but there is alongfront variation in either the strength of the circulation or the efficiency with which it resuspends bottom material. Complimentary evidence for convergence near the foot of the shelfbreak front and subsequent upwelling was found by Houghton and Visbeck (1998) using a purposeful dye release in the BBL. They estimate an equivalent upwelling velocity of 4-7 m day-l, consistent with results presented here. Conclusions By using a high-resolution, cross-shelf section of phys- ical, optical and velocity fields from the Middle At- lantic Bight, the secondary circulation associated with a shelfbreak front has been documented. Convergence near the foot of the shelfbreak front and subsequent upwelling into the interior and downwelling on the off- shore side of the front in the upper part of the water column were established from optical property distribu- tions. Both elements of this secondary circulation are consistent with idealized model predictions. Cross-shelf velocities measured with shipboard ADCP were shown to be consistent with the convergence implied by the distribution of suspended bottom material. By inte- grating the measured divergence fields, an estimate of 9q-2 m day-1 of upwelling was made, implying atransit time from the seafloor to the surface of 10-16 days. This secondary circulation has important implications for the biogeochemistry of shelfbreak fronts through upwelling of nutrients into the euphotic zone and concentration of material on the offshore side of the front. Acknowledgments. We thank M. Willis, R. O'Malley and L. Fayler for the successful SeaSoar operations. The officers and crew of the R/V Endeavor performed superbly. Conversations with R. Houghton (LDEO) and G. Gawark- iewicz (WHOI) proved valuable. This work was funded by the Office of Naval Research Grant N0014-95-1-0382. References Barth, J. A., S. D. Pierce and R. L. Smith, A separating coastal upwelling jet at Cape Blanco, Oregon and its con- nection to the California Current System, Deep-Sea Res., in press, 1998. Candela, R. C. Beardsley and R. Limeburner, Separation of tidal and subtidal currents in ship-mounted acoustic Doppler current profiler observations, J. Geophys. Res., 97, 769-788, 1992. Chapman, D. C. and S. J. Lentz, Trapping of a coastal density front by the bottom boundary layer, J. Phys. Oceanogr., 24, 1464-1479, 1994. Gawarkiewicz, G. and D.C. Chapman, The role of strati- fication in the formation and maintenance of shelfbreak fronts, J. Phys. Oceanogr., 22, 753-772, 1992. Houghton, R. W. and M. Visbeck, Upwelling and conver- gence in the Middle Atlantic Bight shelfbreak front, Geo- phys. Res. Left., in this issue, 1998. Pierce, S. D., J. A. Barth and R. L. Smith, Acoustic Doppler Current Profiler observations during the Coastal Jet Sepa- ration project on R/V Wecoma, 17-27 August 1995, Data Report 166, Reft 97-J, College of Oceanic and Atmo- spheric Sciences, Oregon State University, 1997. John A. Barth, Darek Bogucki, Stephen D. Pierce and P. Michael Kosro, College of Oceanic & Atmospheric Sciences, Oregon State University, Corvallis, OR 97331-5503. (e-maih barth@oce.orst.edu) (Received April 21, 1998; accepted May 27, 1998.) 

Secondary circulation associated with a shelfbreak front

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Secondary circulation associated with a shelfbreak front

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