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Internal Tides and Mixing in a Submarine Canyon with Time-Varying Stratification

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Internal Tides and Mixing in a Submarine Canyon with Time-Varying Stratification DECEMBER 2012 Z H A O E T A L . 2121 Internal Tides and Mixing in a Submarine Canyon with Time-Varying Stratification ZHONGXIANG ZHAO Applied Physics Laboratory, University of Washington, Seattle, Washington MATTHEW H. ALFORD,REN-CHIEH LIEN, AND MICHAEL C. GREGG Applied Physics Laboratory and School of Oceanography, University of Washington, Seattle, Washington GLENN S. CARTER Department of Oceanography, University of Hawaii at Manoa, Honolulu, Hawaii (Manuscript received 7 March 2012, in final form 25 May 2012) ABSTRACT The time variability of the energetics and turbulent dissipation of internal tides in the upper Monterey Submarine Canyon (MSC) is examined with three moored profilers and five ADCP moorings spanning February–April 2009. Highly resolved time series of velocity, energy, and energy flux are all dominated by the semidiurnal internal tide and show pronounced spring-neap cycles. However, the onset of springtime up- welling winds significantly alters the stratification during the record, causing the thermocline depth to shoal from about 100 to 40 m. The time-variable stratification must be accounted for because it significantly affects the energy, energy flux, the vertical modal structures, and the energy distribution among the modes. The internal tide changes from a partly horizontally standing wave to a more freely propagating wave when the thermocline shoals, suggesting more reflection from up canyon early in the observational record. Turbulence, computed from Thorpe scales, is greatest in the bottom 50–150 m and shows a spring-neap cycle. Depth- 2 integrated dissipation is 3 times greater toward the end of the record, reaching 60 mW m 2 during the last 2 2 spring tide. Dissipation near a submarine ridge is strongly tidally modulated, reaching 10 5 Wkg 1 (10–15-m overturns) during spring tide and appears to be due to breaking lee waves. However, the phasing of the breaking is also affected by the changing stratification, occurring when isopycnals are deflected downward early in the record and upward toward the end. 1. Introduction as well as primary productivity and particle transport along the coast (e.g., Shea and Broenkow 1982; Hunkins Submarine canyons of various shapes and sizes are 1988; Gardner 1989; Paull et al. 2005; Lee et al. 2009). common features on continental shelves and slopes, Observations in submarine canyons indeed find diapycnal occupying as much as 40% of continental slopes on the 22 2 21 diffusivities Kr up to 10 m s , three orders of magni- west coast of the United States by some measures 2 2 tude greater than the open ocean value of O(10 5 m2 s 1) (Hickey 1995). Because of their ability to focus internal (e.g., Lueck and Osborn 1985; Carter and Gregg 2002; waves (Gordon and Marshall 1976; Wunsch and Webb Carter et al. 2005; Gregg et al. 2005). However, parame- 1979; Hotchkiss and Wunsch 1982), they have long been terizations of the mixing in terms of the usual internal wave identified as potential sites of intense internal wave ac- cascades underpredict the measured levels by two orders tivity and elevated turbulent mixing, and thus are likely of magnitude (Kunze et al. 2002), indicating different important in processes such as the large-scale circulation and/or additional mixing processes are likely at play in canyons than in the ocean interior. The energy source for the turbulence is assumed to Corresponding author address: Zhongxiang Zhao, Applied Physics Laboratory, University of Washington, 1013 NE 40th Street, Seattle, be the internal tides propagating in through their mouth WA 98105. or ocean end (though the possible additional role of E-mail: [email protected] baroclinic conversion within them has not been ruled DOI: 10.1175/JPO-D-12-045.1 Ó 2012 American Meteorological Society Unauthenticated | Downloaded 10/11/21 10:22 AM UTC 2122 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 42 FIG. 1. (a) Map of the upper MSC. The red line denotes the canyon’s thalweg, or deepest path. The along-thalweg distance is marked with black dots at 1-km intervals and labeled at 5-km intervals. Isobaths are shown at 50-m intervals, with contours at 200-m intervals bold and labeled at left. The Wain et al. (2012, manuscript submitted to J. Geophys. Res.) SWIMS3 tracks are shown in light yellow. At each mooring, the depth-integrated semidiurnal energy and flux are shown with circles and arrows, respectively. Green and blue colors indicate time averages over the first spring- neap cycle (yearday 48–62) and the successive three (yearday 62–106), respectively. (b) Map of Monterey Bay and adjacent region. The green box shows the upper MSC as shown in (a). out; Wain et al. 2012, manuscript submitted to J. Geophys. at the canyon mouth; specifically, that the Sur Plateau Res.). Therefore, understanding the manner in which re- (Fig. 1b) is the primary source of internal tides into MSC motely incident internal tides propagate inside canyons, (e.g., Jachec et al. 2006; Hall and Carter 2011; Johnston navigate around their bends, and reflect off their steep et al. 2011; Kang and Fringer 2012). At the mouth of the walls is key to determining the total dissipation within canyon, the modeled cross-section up-canyon energy flux canyons and its horizontal and depth distribution, both of is about 9 MW (Hall and Carter 2011; Kang and Fringer which are necessary for determining the buoyancy flux. 2012), agreeing well with measurements by Kunze et al. Monterey Submarine Canyon (MSC), the largest sub- (2002). In the lower MSC, the canyon is usually subcritical marine canyon on the US west coast, has been the site of (i.e., less steep than semidiurnal internal-wave character- a number of observational and modeling studies in recent istics) in the along-thalweg direction so that semidiurnal years (e.g., Petruncio et al. 1998; Kunze et al. 2002; Carter internal tides are topographically steered around the gen- and Gregg 2002; Jachec et al. 2006; Hall and Carter 2011). tler Monterey and San Gregorio meanders (Petruncio et al. MSC runs across the continental shelf of Monterey Bay, 2002; Jachec et al. 2006; Hall and Carter 2011). However, with its head just off Moss Landing, California (Fig. 1). It the internal tide does not appear to follow the sharper bend features a winding thalweg (red), in contrast to some other at Gooseneck Meander (Hall and Carter 2011; Wain et al. canyons that are relatively straight (e.g., Ascension and 2012, manuscript submitted to J. Geophys. Res.). As a re- Kaoping, Gregg et al. 2011; Lee et al. 2009). Starting at the sult, the baroclinic velocities lead to flow perpendicular to canyon mouth, there are three sharp bends along the canyon the ridge near the bend, leading to a breaking lee wave that thalweg (Carter 2010): the San Gregorio, Monterey, and dominates the dissipation in the upper canyon (Wain et al. Gooseneck meanders (Fig. 1b), the latter of which (Fig. 1a) 2012, manuscript submitted to J. Geophys. Res.). is studied here in detail with a set of 8 moorings. Simulta- Recently, the role of low-frequency flows and strati- neous shipboard surveys are reported separately by Wain fication changes has become recognized in modulating et al. (2012, manuscript submitted to J. Geophys. Res.). the generation and propagation of internal tides in the Model simulations and field observations confirm open ocean (Alford and Zhao 2007a; Kelly et al. 2012), a substantial incident semidiurnal baroclinic energy flux their reflection from continental margins (Klymak et al. Unauthenticated | Downloaded 10/11/21 10:22 AM UTC DECEMBER 2012 Z H A O E T A L . 2123 2011), their propagation on continental shelves Canyon experiment (Gregg et al. 2011), and the other (MacKinnon and Gregg 2003a,b; Kurapov et al. 2003), seven moorings were recovered on 16–17 April. Detailed and the interference patterns that arise from multiple instrument configurations of the moorings are listed in waves (Alford et al. 2006). In MSC, Petruncio et al. (1998) Table 1. observed progressive and standing waves in field experi- The moorings ranged from ;19 km (LR1) to ;2km ments conducted in April and October 1994, respectively, (WH1) from the canyon head, with the water depth ranging and they attributed the difference to changes in stratification from 604 m at LR1 to 153 m at WH1 (Fig. 1a). The ADCP between the two experiments. In the work presented here, moorings (LR1–LR4 and WH1) were deployed along the we demonstrate that the shoaling of the thermocline asso- canyon’s thalweg (Fig. 1a, red), whereas the MP moor- ciated with springtime upwelling-favorable winds markedly ings (MP1–MP3) were on the canyon’s southern flank. affects the patterns of velocity, displacement, energy, and The along- and cross-canyon structure of the energy energy flux, supporting model predictions by Hall and flux and dissipation rate were investigated on the second Davies (2007), Kurapov et al. (2010), and Osborne et al. cruise using SWIMS3, a towed profiler (Gregg et al. 2011; (2011), and observations by Petruncio et al. (1998). In our Wain et al. 2012, manuscript submitted to J. Geophys. case, the stratification changes are substantial enough to Res.), along 16 cross-canyon sections (Fig. 1a, light yellow necessitate their incorporation into the energy flux and lines). We focus here on mooring observations, referring modal structure calculations; use of a time-mean stratifica- interested readers to Wain et al. (2012, manuscript sub- tion results in substantial errors. The deeper thermocline mitted to J. Geophys. Res.) for the spatial features re- during the first period results in greater energy but similar vealed with the SWIMS3 surveys. net up-canyon energy flux, implying greater flux from b. MP moorings further up canyon.
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