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1648 JOURNAL OF PHYSICAL VOLUME 43

Spatiotemporal Variation in Cross-Shelf Exchange across the Inner Shelf of Monterey Bay, California

C. BROCK WOODSON COBIA Lab, College of Engineering, The University of Georgia, Athens, Georgia

(Manuscript received 10 October 2011, in final form 2 April 2013)

ABSTRACT

Cross-shelf exchange resulting from wind- and wave-driven flows across the inner shelf has been the focus of a considerable body of work. This contribution extends recent analyses to the central California coastline using 5-yr of moored observations. Acoustic Doppler Current Profiler (ADCP) data from stations across the Monterey Bay (two in the northern bay and one in the southern bay), in water depths of ;20 m, showed net offshore transport throughout the year. For the northern bay sites, cross-shelf exchange was dominated by driven by along-shelf diurnal breezes during the season. Intense stratification in the northern bay leads to very shallow observed Ekman layers (;5–8 m), and consequently no overlap between bottom and surface Ekman layers within a few hundred meters of the coast. The total transport is less than predicted by theory consistent with models of shallow-water Ekman transport. The observed transport (;42% of full Ekman transport) is shown to be caused by the influence of a positive vorticity that effectively increases the parameter. Wave-driven return flow estimated from an off- shore buoy was strongly correlated with observed transport during nonupwelling conditions for the northern, outer bay site, but not for the two inner bay sites (northern and southern). In the southern bay, winds and waves have a significantly reduced effect on the cross-shelf exchange. Internal tidal bores are believed to contribute most of the observed cross-shelf exchange in this region.

1. Introduction et al. 2007; Pineda 1999; Lentz and Fewings 2012). Con- sequently, understanding the spatial and temporal vari- Wind- and wave-driven flows play important roles in ability of, and the influence of stratification on, cross-shelf cross-shelf exchange and consequently influence the exchange provides unique insights into inner-shelf circu- transport of nutrients, pollutants, larvae, and heat to and lation and dynamics. from coastal ecosystems (Lentz 1994; Kirincich et al. The inner shelf is defined as the region where surface 2005; Fewings et al. 2008; Lentz et al. 2008; Kirincich and bottom Ekman layers cannot fully develop or tend et al. 2009; Hendrickson and MacMahan 2009; Lentz to overlap effectively reducing cross-shelf transport as and Fewings 2012). These drivers of cross-shelf exchange a result of along-shelf (Ekman 1905; Lentz scale with depth and distance from shore (Fewings et al. 1994). Consequently, Lentz (1994) proposed a linear 2008). However, along-shelf variability in cross-shelf ex- modification of wind-driven Ekman transport based on change and scaling of dominant drivers as a result of depth to account for reduced upwelling in changes in coastline orientation, , winds, waves, shallow water. Kirincich et al. (2005) extended these and stratification have not yet been addressed at scales analyses and found reasonable agreement to the Lentz relevant to coastal ecosystem variability. For example, (1994) model for the Oregon coast. Such observations of stratification can interact with wind and wave forcing reduced Ekman transport in shallow water confirmed dramatically altering cross-shelf exchange and influencing a limited role of along-shelf winds in driving cross-shelf ecosystem processes in coastal habitats (McPhee-Shaw circulation that scales with distance from the coast. Closer to the coast, cross-shelf winds act to push water in the direction of the wind leading directly to upwelling or Corresponding author address: C. Brock Woodson, COBIA Lab, College of Engineering, The University of Georgia, 712H Boyd (Fewings et al. 2008). As water depth con- Graduate Studies, 200 D.W. Brooks Drive, Athens, GA 30602. tinues to decrease, the action of cross-shelf winds is to E-mail: [email protected] thoroughly mix the water column, and as a result, surface

DOI: 10.1175/JPO-D-11-0185.1

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FIG. 1. (right) Map of the Monterey Bay showing the three locations (SHB, TPT, and HMS; filled circles), the offshore NDBC 46042 buoy (triangle), and the local wind stations [Long Marine Lab (LML), Hopkins Marine Station (HMS); triangles] with corresponding monthly climatologies during 2004–09. Contours show the 10-, 20-, and 50-m isobaths. Black arrows show regional wind direction. Gray arrows show local diurnal wind direction across the bay. Axes for each mooring site are shown next to the mooring location. Along- and cross-shelf wind stress relative to outer coast orientation and significant are shown for NDBC 46042 and the observed cross-shelf transport (negative offshore) computed from ADCP records is shown for the three sites. Dashed line in each panel is the annual mean. wave–driven exchange becomes increasingly important in an earlier study (Kirincich et al. 2005). While these (Lentz 2001; Fewings et al. 2008). studies have provided a critical framework for under- In the very near shore, surface gravity waves can standing dynamics on the inner shelf, many questions substantial, subtidal offshore flows on the order of 2– still remain concerning the relative importance of wind- 2 3cms 1. Lentz et al. (2008) demonstrated that this and wave-driven cross-shelf circulation especially in the vertically sheared, surface-intensified Eulerian offshore presence of strong stratification (Lentz and Fewings transport is correlated with estimates of on-shelf trans- 2012). port resulting from during relatively low For example, Woodson et al. (2007) found a strong wind conditions using a 5-yr dataset from the Martha’s upwelling-like response to local diurnal along-shelf wind Vineyard Coastal Observatory (MVCO). This return forcing inshore of the 20-m isobath (&300 m from the flow may also confound exchange previously attributed shore) that may have been confounded by cross-shelf to wind-driven cross-shelf circulation. Consequently, winds and surface wave–driven return flows along the Fewings et al. (2008) and Kirincich et al. (2009) expanded coast of northern Monterey Bay (Fig. 1). In this region, these analyses to more variable conditions at the MVCO intense stratification leads to a two-layer system with and to the Oregon Coast, respectively. Fewings et al. warm (up to 168C, 5–8 m deep) waters overlying cold, (2008) observed that surface wave–driven return flows recently upwelled waters (;88C). During afternoon sea had a large effect on the cross-shelf circulation, although breezes, the was observed to shoal signifi- exchange was largely driven by cross-shelf wind stress. cantly and often reached the surface. The inertial period Kirincich et al. (2009) found that the surface wave–driven for this region is ;20 h, which is comparable to the di- return flow was weakly correlated with, but had little urnal wind forcing signal (24 h). Thus, resonant effects of effect on, observed transports at the same sites analyzed the diurnal wind on the cross-shelf circulation may also

Unauthenticated | Downloaded 10/09/21 08:26 PM UTC 1650 JOURNAL OF VOLUME 43 have influenced these observations. A further study on 2. Data and methods the interior shelf of northern Monterey Bay found that Location and data closure of the heat budget was achieved through a com- bination of cross-shelf Ekman transport and along-shelf The Monterey Bay is a semienclosed embayment lo- advection (Suanda et al. 2011). Cross-shelf Ekman cated along the eastern Pacific between 36.68 and transport from the one month record during 2007 was 37.28N (Fig. 1). This region of central California is within estimated to be approximately 76% of the theoretical the upwelling system and is domi- values consistent with the Lentz (1994) model for this site nated by along-shelf winds through much of the year. in northern Monterey Bay. In contrast, Hendrickson Offshore winds are oriented along shelf toward the and MacMahan (2009) found that cross-shelf winds equator (northwesterly winds, black arrow; Fig. 1) with and wave-driven flows drive cross-shelf exchange in the brief periods of relaxation or reversal. Strong diurnal south-central portion of Monterey Bay, along a section of winds develop within the bay during regional upwelling westward-facing coastline with a smooth, linear bathym- conditions (Banta et al. 1993). These sea breezes (gray etry (Fig. 1). These observed differences in cross-shelf arrows,Fig.1)occurwhenairtemperaturesoverthe exchange across a relatively small region raise questions Salinas Valley to the southeast warm significantly cre- about the spatial and temporal variability in cross-shelf ating a strong gradient between the Monterey exchange along the coast. Bay and the adjacent valley. This contribution extends the analyses of Lentz et al. During active regional upwelling, a southward up- (2008) and others (e.g., Fewings et al. 2008; Kirincich welling jet originates near Point Ano~ Nuevo and travels et al. 2009; Hendrickson and MacMahan 2009) to addi- across the mouth of the bay (Rosenfeld et al. 1994) and tional sites along the coast of Monterey Bay [including surface waters within the bay warm considerably. This the site used in Suanda et al. (2011)] using a set of long- process creates a 5–8-m-deep warm lens of less-dense term (5 yr) ADCP and observations to water over the shelf within the bay. Temperature dif- address the role of wind- and wave-driven transports on ferences between surface waters within the bay and cross-shelf exchange (Woodson et al. 2007, 2009). Sig- upwelling jet/deeper waters within the bay can be as high nificant correlations were found between along-shelf as 88C (Woodson et al. 2009). Salinity within the bay varies wind stress and cross-shelf currents in the northern bay minimally during active upwelling because of minimal during the upwelling season (April–October) and be- freshwater runoff and because waters within the bay typ- tween cross-shelf currents and surface gravity waves ically originate from the same source (Point Ano~ Nuevo). during nonupwelling periods on the outer coast. During The northern bay is exposed to a wide range of and the upwelling season, observed Ekman flows in the wind conditions throughout the annual cycle. In contrast, northern bay caused by a strong diurnal sea breeze fol- the southern bay is largely shielded from dominant swell low the Lentz (1994) model for shallow water depths. and winds especially during the summer upwelling season. It is further shown that 1) the surface is During nonupwelling months (November–March), circu- fully developed, 2) the surface and bottom Ekman lation within and around the bay is more variable and layers do not (or extremely rarely) overlap as a result surface waves tend to be larger in magnitude (Breaker of strong stratification present during upwelling, and and Broenkow 1994). This contrast between upwelling 3) the bulk Richardson number is consistently large. A and nonupwelling conditions and the gradient in expo- net positive subtidal vorticity resulting from a strong sure to winds and waves across the bay provide a natu- horizontal (cross shelf) shear in the along-shelf flow yx in ral platform to evaluate cross-shelf exchange related to the northern bay acts to reduce the theoretical Ekman wind- and wave-driven flows. transport and spinup. Thus, the region inside the 20-m Time series (5 yr) of depth-dependent currents are isobath (located within a few hundred meters of the taken from three monitoring sites within Monterey Bay coast at the study sites) in the Monterey Bay is rarely maintained by the Partnership for the Interdisciplinary within the inner shelf as defined in Lentz and Fewings Studies of Coastal (PISCO). Sandhill Bluff (SHB) (2012). and Terrace Point (TPT) are separated by approximately Data collection methods and analyses are outlined in 10 km along a stretch of coastline where the mean orien- section 2. Results are then presented for both correlation tation changes from 3058 (southeast–northwest) to 2708 analysis between depth-dependent flows and forcing (east–west, Fig. 1; Table 1). SHB is along the outer coast variables and EOFs of the complex velocity in section 3. and is exposed to higher wind and wave forcing than TPT The influence of vorticity on the cross-shelf circulation is located within the Monterey Bay. Both sites are strongly discussed in section 4. Finally, a discussion and summary influenced by the diurnal winds that develop during the are presented in sections 5 and 6, respectively. upwelling season. In the southern bay, the Hopkins

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TABLE 1. Local conditions at the three mooring locations. 1994). All data cover the period from 1 January 2004 to 31 December 2009, were recorded at 2–5-min sampling Principal axis Site Depth (m) (8; positive y axis) Data gaps (days) rates, and were subsequently filtered to create hourly- averaged data for analyses. Hourly-averaged current data SHB 21 305 15, 49, 47, 30, 20, 38 TPT 19 270 8, 22, 9,14 were then filtered using either a 40- (regional analyses) or HMS 20 132 12, 87, 70 a 20-h low-pass filter (local analyses). The 20-h low-pass filter (12 pole, Butterworth) attenuated less than 5% of

the diurnal band variability. Here, K1 tidal currents for Marine Station (HMS) site is located on the inner side the 20-h-filtered data were removed using T- of the Monterey Peninsula (Fig. 1; Table 1). This site (Pawlowicz et al. 2002) for each bin depth to account for is largely protected from southern and western swell. the bottom boundary layer. For analysis of the cross-shelf Along-shelf coordinates are determined from the average velocity and exchange, the current profiles are separated orientation of local isobaths within 5 km of each mooring into a depth-averaged and depth-dependent component: (Fig. 1; Table 1). Dominant currents follow isobaths at all ^ 5 2 three locations as determined from principal axes (Drake u(z, t) u(z, t) uda(t), (1) et al. 2005). Acoustic Doppler Current Profilers (ADCPs) recor- where ded depth-dependent currents using 45-ping ensembles ð 0 recorded every 2 min during 2004–09. Nearby moorings 5 1 uda(t) u(z, t) dz. (2) recorded temperature (Hobo Tidbits) every 2 min at 0-, H 2H 4-, 11-, and 19-m depth. All data gaps longer than 12 h s were discarded from the analysis. SHB had 5 significant The cross-shelf transport U is then computed from data gaps of 15, 49, 47, 30, 20, and 38 days (Table 1). TPT the velocity profiles by either assuming the velocity in had 4 significant data gaps of 8, 22, 9, and 14 days. HMS the top-most ADCP bin extended to the surface, or by had 3 significant data gaps of 12, 87, and 70 days. Addi- linearly extrapolating the top-most bins of the ADCP to tional data gaps of less than 12 h were filled using linear the surface and integrating as interpolation. Meteorological and swell data (signifi- ð 0 cant wave height and direction) were obtained from the Us 5 u^(z, t) dz, (3)

National Oceanic and Atmospheric Administration zcross (NOAA)–National Data Buoy Center (NDBC) 46042 mooring located approximately 30 km offshore along the where zcross is the first observed zero crossing of the central axis of the Monterey Bay (Fig. 1). Swell height velocity profile u^(z, t). Both methods yielded similar and direction from the offshore mooring were compared results, and results from the linear extrapolation method with data available from local Coastal Data Information are reported here. For all analyses, we adopt a west coast Program (CDIP; http://cdip.ucsd.edu/) moorings in the coordinate system, where x is the cross-shelf direction Monterey Bay, which provide nearshore hourly direc- (positive onshore) and y is the along-shelf direction tional wave spectra. These comparisons showed minimal (positive poleward). refraction between the offshore and the 20-m isobath at all locations. Swell direction was rotated into a frame of 3. Correlation and EOF analysis reference with the positive y oriented poleward and positive x oriented onshore for each site. Local winds The objective of this section is to identify the relative were obtained from either Long Marine Laboratory importance of primary contributors to cross-shelf ex- (LML; University of California, Santa Cruz, Santa Cruz, change in the Monterey Bay with a particular focus on California; Fig. 1) or from Hopkins Marine Station winds, surface waves, and bottom boundary layer dy- (HMS; Stanford University, Pacific Grove, California, namics. Climatological monthly means for both compo- Fig. 1). Both components (along and cross shelf) of the nents of the wind stress and surface wave height are wind stress were computed from both the local (LML and computed from NDBC 46042 (Fig. 1). Cross-shelf wind HMS) and regional (NDBC 46042) wind data using stress is minimal throughout the year, although it can Large and Pond (1981). Estimates of bottom boundary be episodically high during winter storm events. Along- layer Ekman transport were estimated from the along- shelf wind stress is negative throughout the year and shelf velocity in the ADCP bin closest to the bed using the stronger during upwelling months (March–August). Sig- By 5 B linearized formulation t /ro rdy , and setting nificant wave height is higher and more variable during 24 21 rd 5 5 3 10 ms for a smooth sandy bottom (Lentz the winter (nonupwelling) months as a result of winter

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s W storms and decreases to a minimum in late summer (July– U 5 t y /r f . (5) September). o All mooring locations show a net offshore transport On the inner shelf, water depths that are less than the with stronger and more variable transport in the north- Ekman layer depth or considerable overlap of the surface ern bay, which is subject to more intense wind and wave and bottom Ekman layers lead to wind-driven transport forcing (Fig. 1). Peaks in cross-shelf transport in the in the direction of the wind (Fewings et al. 2008). Cross- northern bay shift from February to March and June shelf wind stress in this region that acts toward (away to July at SHB (outer bay), corresponding to an increase from) the shore acts to raise (lower) near the in the net along-shelf wind stress, to August–October for coast with a downwelling (upwelling) type response of theinnerbaysite(TPT).Incontrast,thesouthernbay the mean circulation. site (HMS) shows low net cross-shelf transport and minimal seasonal variation. These patterns are likely b. Surface –driven flows due to a combination of wind and wave forcing that Surface waves transport fluid onshore between the varies not only temporally (Fig. 1) but spatially as well. crests and troughs via Stokes drift. Because of this net To address the relative contributions of winds and onshore transport, a return flow develops below the surface waves to cross-shelf exchange in Monterey Bay, surface waves. The wave-driven, subtidal circulation can the theoretical underpinnings of each are outlined in be estimated from the onshore Stokes transport as the following subsections. gH2 a. Wind-forced flows U ’ sig cosu (6) w 16c w Along-shelf wind stress leads to a net cross-shelf transport to the right (left) in the Northern (Southern) for time scales much longer than the wave period (Lentz

Hemisphere. Ekman (1905) estimated the net transport et al. 2008). Here, uw is the orientation of the waves to resulting from a surface wind stress for a linear coastline the coast. In a Lagrangian reference frame, the Stokes and general assumptions about the flow. These results can velocity becomes depth dependent as be obtained by integrating the along-shelf momentum 2 balance over the surface layer: H vk cosh2k(z 1 h) u (z) 5 sig cosu . (7) ð ST 16 sinh2kH w 0 P y 1 uy 1 yy 1 wy 1 y 1 fu dz t x y z In this formulation, v is the wave frequency, k is the 2d ro s wavenumber, and H is the local water depth. Outside of W t y 2 ty(d 5 0) 5 s , (4) the , the Coriolis parameter becomes impor- ro tant, and an Eulerian return flow directly opposes this onshore transport (Hasselmann 1970; Monismith et al. where u, y, and w are the cross-shelf, along-shelf, and 2007; Lentz et al. 2008) as a result of an along-crest wave vertical velocity components with subscripts indicating stress, often called the Coriolis–Stokes force (Polton differentiation. The along-shelf pressure gradient is Py, et al. 2005): and ds is the depth of the surface Ekman layer, which is 2 taken as zcross following previous observational studies dtwave H vk cosh2k(z 1 h) 52r sig (Lentz 2001; Kirincich et al. 2005; Fewings et al. 2008). of 2 . (8) 2 2 dz 16 sinh kH The Coriolis parameter f is taken to be 0.875 3 10 4 s 1 corresponding to a latitude of 378N. The effect of the Combining (7) and (8) yields the theoretical Eulerian ; Coriolis term for the depth-averaged flow (sfuda ds return flow uH that directly opposes the Stokes trans- : 3 27 21 6 13 10 ms ) is considered negligible relative to port uST, when the Coriolis term balances the Coriolis– 25 21 the surface flow term (sfUs ; 2:78 3 10 ms )over Stokes forcing. This condition exists when the fluid is long time periods as computed from the ADCP records considered inviscid or when the Stokes layer dST is much (estimates given). However, fudads can be episodically greater than the Ekman layer depth ds. These theoretical important owing to the formation of coastal eddies and results have been observed in laboratory flume studies filaments. For a steady, uniform flow with no along- (Monismith et al. 2007) and in the field (Lentz et al. shelf pressure gradient, (4) reduces to the classical 2008; Kirincich et al. 2009). Field observations however Ekman transport equation when flow is linear and stress are mixed and observations of no net transport have at base of the surface layer approaches zero (Ekman been reported for the Monterey Bay shelf (Rosman 1905): et al. 2007).

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Surface waves are often generated by either remote or sites, although there is still a contribution from surface local winds. If generated locally, the resulting cross-shelf wave–driven circulation at the outer bay site (SHB) be- transport can then be attributed to a time-lagged wind cause of the periodic arrival of remotely generated south response. I classify this response as wave driven because swells. Correlations for all variables were weak and it is still fundamentally a result of surface waves as op- nonsignificant throughout both periods in the southern posed to other forms of cross-shelf transport described bay. Similarly, correlations with the bottom stress were above. Analysis of local wind forcing as a driver of short- also weak and nonsignificant at both northern bay sites. period waves that contribute to cross-shelf exchange is During the upwelling season, the Monterey Bay is beyond the scope of the current study. However, during forced by strong diurnal sea breezes (Fig. 3; Banta et al. the upwelling season, local winds are the source of much 1993; Woodson et al. 2007, 2009). For the outer portions of the surface wave energy, and consequently, the di- of the bay, these winds are along shelf as opposed to rection of wave travel is roughly along shelf (mean wave typical cross-shelf sea breezes (Fig. 1). The diurnal direction at CDIP mooring in 20 m of water located sea breeze, combined with local stratification drives an 300 m from the shore during 2004–09: 319.68610.78; upwelling-like signal on the inner shelf. Correlation

Table 1). Therefore, because uw is ;908 during the up- analysis of the primary forcing mechanisms using the welling season for these sites, locally generated short- local wind observations show a strong correlation be- period surface waves are not expected to contribute tween cross-shelf transport and the along-shelf local di- significantly to cross-shelf exchange. urnal wind signal at both SHB and TPT during the upwelling season (Fig. 4). The correlations in Fig. 2 result c. Bottom boundary layer dynamics from a correlation between the low-pass-filtered NDBC Along-shelf flows driven by remote forcing, pressure wind stress and the low-pass-filtered local wind stress gradients, or can also force considerable cross-shelf because the diurnal wind signal is only present during exchange through bottom stress–driven Ekman transport active regional upwelling (Woodson et al. 2007). This (Brink 1997). In this case, the bottom stress is dependent correlation is not present during nonupwelling months on the flow velocity and the bed roughness that generates because of the absence of a persistent along-shelf the frictional bed stress. Along the Monterey Bay coast, wind signal near the coast. Slopes of the regression fit ^5 Wy 1 a poleward (equatorward) along-shelf current will create [u a(t /rof ) b] over the depth are relatively small, an equatorward (poleward) bed stress and consequently but integrated over the surface layer yield coefficients an offshore (onshore) bottom layer transport. Through that agree reasonably well with the Lentz (1994) model. continuity these flows can drive an upwelling (equator- This results from (3) and taking b 5 0: ward current) or downwelling (poleward current) response ð ð on the inner shelf that may contribute significantly to ob- Wy 0 Wy 0 t ^ t served cross-shelf exchange. C 5 u(z, t) dz 5 a(z) dz, (9) r f 2 r f 2 o ds o ds d. Multiple regression analysis To address the above forcing mechanisms, I con- where C is the proportion of full Ekman transport from ducted a multiple regression with each bin depth of the the Lentz model, yielding ADCP record as the response variable, and the expected ð 0 contributors to cross-shelf exchange, regional, or local C 5 a(z) dz. (10) 2 winds (along and cross shelf), surface waves, and bottom ds stress (Fewings et al. 2008). For each mooring, winds were rotated into local coordinate axes and converted to scaled Wx,y wind stress t /rof . Bottom stress was also scaled as Cross-shelf velocity profiles during weak wave con- B t /rof for the multiple regression. Surface wave transport ditions illustrate the contribution of poleward along- was converted to 2Uw in order to account for changes in shelf currents and the resulting negative bed stress in coastline orientation. driving a weak offshore transport in the bottom Ekman Correlation analysis of the low-pass-filtered ADCP layer in the northern bay (Fig. 5). Estimates of bottom and NDBC records suggests that surface wave–driven layer Ekman transport [m 5 9:5 3 1025 m2 s21, s2 5 EB EB circulation contributes to the observed cross-shelf trans- 2:89 3 1022 (m2 s21)2] were at least an order of magni- port during both the upwelling and nonupwelling season tude less than surface Ekman layer transport (Fig. 3). at the outer bay site (SHB), but not at the inner bay sites During high along-shelf wind stress, offshore Ekman (Fig. 2). During the upwelling season, cross-shelf trans- transport with an interior (nonfrictional) return flow is port is driven by along-shelf winds at the northern bay observed (uH; solid black line in Fig. 5). The offshore

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FIG. 2. Correlation coefficient r as a function of ADCP bin depth for the three sites [(a),(b) SHB; (c),(d) TPT; and (e),(f) HMS], of low-pass-filtered cross-shelf velocity u^ in each ADCP bin with along-shelf wind stress tWy (circles), Wy By cross-shelf wind stress t (diamonds), bottom stress t , and wave forcing 2Uw (squares) at zero lag, assuming one independent point every 24 h, during the nonupwelling season (October–March) in (a),(c),(e) and upwelling season (April–September) in (b),(d),(f). Solid (open) symbols show correlations that are (are not) significant at the 95% confidence level.

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FIG. 3. Ekman transport estimated for along-shelf wind stress and bottom stress at TPT during upwelling season of 2007. Wind stress computed from Long Marine Laboratory using Large and Pond (1981). Bottom stress computed from bottom ADCP bin. The x axis is in Julian days.

flow in the bottom Ekman layer is also reversed during either masked by the strong diurnal wind responses or periods of high along-shelf wind stress. During both areminimalduetopredominantwavedirectionduring periods, the along-shelf flow is consistently poleward with the upwelling season (Figs. 6 and 7). Previous studies in minimal shear in the upper layer (3–7-m depth). These the region of northern Monterey Bay found similar profiles further support a dominant role of the along- results during the upwelling season with no evidence of shelf wind in driving cross-shelf exchange in northern a sheared return flow (e.g., Rosman et al. 2007). Monterey Bay. e. EOF analysis of complex velocity Cross-shelf exchange driven by surface waves can be a signal of a lagged wind response. Correlation between The first two modes of the real part (cross shelf) of the wind stress and surface waves give an initial indication of EOF accounted for .86% of the variability in the cross- the indirect response of the inner-shelf circulation to shelf velocity for both SHB and TPT (Figs. 9a,b). The local/regional wind forcing (Fig. 6a). However, during first mode of the EOF for the cross-shelf flow had the the upwelling season (tWy , 0), when along-shelf winds same characteristic shape as the correlation analysis are the largest contributor to cross-shelf exchange, sur- with along-shelf wind stress for SHB (Fig. 9a), was sig- face gravity waves are most often oriented nearly parallel nificantly coherent (g ’ 0.78 for upper and lower layers)

[cos(uw) ; 0] to the coast leading to minimal net transport with the local diurnal wind signal using coherence spec- Uw (Figs. 6b,c). Although refraction of waves as a result tral analysis, and accounted for .65% of the variance in of the sloping bottom typically yields waves that are close the cross-shelf exchange. The second mode at SHB was to perpendicular to the coast, the particular bathymetry the barotropic tidal signal and was significantly co- of Monterey Bay (relatively flat with steep slope close to herent (g ’ 0.63) with K1 and M2 tides. However, at TPT shore) does not allow for significant refraction (Fig. 7). (Fig. 9b), the first mode was coherent with the barotropic Therefore waves tend to impinge the coast close to their tidal signal (g ’ 0.72), and the second mode (first baro- offshore direction of travel. clinic mode) was coherent (g ’ 0.54) with the local di- Surface wave orientation relative to the coast supports urnal winds. The third mode at these northern bay sites much higher cross-shelf transport during winter storm appears related to activity given the broad events where winds and waves tend to be episodically spectral peak across the 10–30-min period evident in the stronger and cross shelf (tWy . 0, Fig. 6c; Breaker and power spectrum of the PC loading time series. This mode Broenkow 1994). Surface wave–driven return flows were accounts for ;7% of the total variance in the cross-shelf observed during winter at the SHB site (Fig. 8). During flow. these times, periods of increased swell are more frequent, In the southern bay, the first three modes of the real and upwelling-favorable wind conditions are more in- part of the EOF account for more than 77% of the cross- frequent and variable. Evidence of the Coriolis–Stokes shelf exchange (Fig. 9c). The first mode (;52% of the return flow during winter months suggests that wave- variance) of the EOF is strongly bottom enhanced and driven flows contribute significantly to cross-shelf ex- is coherent with the tides (g ’ 0.73). The second mode change in and around the Monterey Bay region, but are accounts for ;20% of the variance and appears to be

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FIG. 4. (a),(b) Correlation coefficient r and (c),(d) regression coefficient a as a function of ADCP bin depth for northern bay inner-shelf sites [SHB in (a),(c); TPT in (b),(d)] of band-pass-filtered (20–2-h period) cross-shelf ve- locity u^ in each ADCP bin with along-shelf wind stress tWy (circles), cross-shelf wind stress tWx (diamonds), bottom By stress t (triangles), and wave forcing 2Uw (squares) at zero lag, assuming one independent point every 24 h, during the upwelling season only (April–September). Wind stress computed from local wind observations from LML. Solid (open) symbols show correlations that are (are not) significant at the 95% confidence level. representative of warm front relaxation and associated to full theoretical values (Fig. 10a, gray circles) when high-frequency internal waves that are not resolved in comparing theoretical to observed transport as a result these long-term observations (Walter et al. 2012). of local winds. These values are lower than those esti- mated by Suanda et al. (2011) likely because of the longer record used in this analysis, but they agree well 4. Influence of background vorticity with the estimates from the correlation analysis (Fig. 4). Lentz (1994) showed a linear relationship between Us Lentz (1994) proposed four potential causes for the re- Wy 5 1 and t /rof of the form y Cx d, where C [(9) and duced Ekman transport in shallow depths: 1) the surface (10)] is defined as the fraction of full Ekman transport Ekman layer cannot fully develop (ds * H), 2) surface and d is dependent on local conditions such as depth and and bottom Ekman layers overlap (ds 1 db * H), 3) bathymetric variability. The SHB and TPT sites follow when the interior stress computed at z 52ds is com- the reduced Ekman transport model proposed by Lentz parable to the surface or bottom stress [e.g., when the (1994) with C 5 0.42 at TPT (0.39 at SHB) with respect bulk Richardson number, Ri 52(gdr/dz)2/(du/dz)2,is

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FIG. 5. Mean along- and cross-shelf velocity profiles during low Wy wave conditions (Hsig , 0.75 m) at TPT for high (uH, yH; jt j . 22 Wy 22 0.03 N m ) and low (uL, yL; jt j , 0.03 N m ) along-shelf wind stress. low], or 4) nonlinear terms in the along-shelf momentum equation are important. In the presence of stratification, the depth of the 21/2 Ekman layer scales as ds ; u(Nf) (Ralph and Niiler 1999). This formulation is used to estimate the thickness of the bottom Ekman layer where the stratification is approximately linear. The surface Ekman layer is taken as the depth of the thermocline or as zcross as computed above, whichever is deeper. The two estimates of ds, from scaling theory (Ralph and Niiler 1999) and zcross, were within 1 m of each other for all cases. Although not the focus here, this is an interesting observation given that the above equation is a scaling and not an equality. The surface and bottom Ekman layer thicknesses, esti- mated from the thermocline depth using the scaling given above respectively, rarely overlap due to the strong near- surface thermocline (Fig. 10b). Estimates of vertical shear FIG. 6. (a) Significant wave height Hsig, (b) wave direction, and (c) estimated Stokes transport Uw for each mooring location as in the horizontal velocity and density stratification be- a function of the regional along-shelf wind stress (negative up- * tween surface and bottom suggest RiB 100 within the welling favorable) from the NDBC 46042 mooring. In (b), dashed stratified region of the inner shelf. These observations lines show along-shelf orientation of three moorings [cos(uw) ; 0]. rule out the first three mechanisms for reduced transport In (c), symbols are for SHB (black circles), TPT (gray squares), and proposed by Lentz (1994), and leaves only the nonlinear HMS (dark gray triangles).

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FIG. 7. (top) Histogram of wave direction during upwelling season measured at 20-m depth approximately 300 m from coast and (bottom) typical cross-shelf depth profile for sites in Monterey Bay. In the top panel, the solid line shows coast orientation. advection terms in the along-shelf momentum balance as a mechanism for the observed reduction in Ekman transport. a. Along-shelf momentum balance The objective of this section is to provide an order of magnitude, first-order analysis of the along-shelf mo- FIG. 8. Mean cross-shelf velocity profiles at SHB during low mentum balance, in particular the nonlinear advection 2 jtWyj . 2 terms. Many of these terms are difficult to accurately along-shelf wind conditions ( 0.03 N m ; October–March) for periods of low (H , 0.75 m), middle (0.75 , H , 2 m), and estimate from field observations; however, to leading sig sig high (Hsig . 2 m) swell. Dashed lines are the theoretical wave- order, such estimates can provide insights into the dy- driven return flow profile computed from (7) for Hsig 5 1.5 and namics of inner-shelf circulation and Ekman transport. 2.5 m, the mean values of Hsig for the respective bins. Separating the pressure gradient into barotropic and baroclinic components and integrating, (4) becomes was computed as the centered difference of the hourly ð ADCP currents following Lentz (2001). The horizontal, 0 1 1 1 1 1 s 1 1 2 nonlinear convective acceleration terms (uyx, yyy) were (yt uyx yyy wyz) dz fU gdshy gds ry 2 2r ds o estimated from HF radar combined with ADCP veloc- W ities. Cross-shelf horizontal shear y was estimated as the t y 2 ty(z 52d ) x 5 s . difference between the velocity averaged over the sur- r o face layer from the ADCP and several nearby HF radar (11) nodes (Fig. 11) obtained from the Central and Northern Each of the terms in the along-shelf momentum bal- California Ocean Observing System (CeNCOOS; Paduan ance was then estimated using a combination of ADCP, and Cook 1997), as HF radar, and local wind data for each site. Moving from y 5 [yADCP(t) 2 yHF(t)]/L, (12) left to right, the unsteady, or local acceleration term yt, x

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FIG. 9. Cross-shelf (real) component of the EOFs of the complex velocity for (a) SHB, (b) TPT, and (c) HMS showing variance explained. where L is defined as the distance between the mooring Contamination of current data during high winds is not location and the centered location of the HF radar a significant issue for offshore HF radar in the region measurement window (Fig. 11). Horizontal velocities because currents predominantly travel in the direction were then averaged over the depth of the surface layer of the wind (Paduan and Cook 1997). Averaging over to remove noise in near surface bins. Although HF radar the surface layer had less than a 3% effect on calculations only provides surface velocities, it does give a reason- of the convective acceleration terms because along-shelf able estimate over the depth of the surface layer during vertical velocity shear yz was minimal over this depth low wind conditions (5–8 m deep; Fig. 12, left panel). (Fig. 5). Here, yy was estimated from adjacent along-shelf Here, yx was computed between the ADCP and HF radar HF radar locations during low wind conditions only and because diurnal winds tended to contaminate the HF compared with estimates from ADCP data using a Taylor radar during high wind conditions (Fig. 12, right panel). advection scheme (y 5 yt). Both methods yielded

FIG. 10. Observed (local wind) vs theoretical mass transport binned by wind stress from classical Ekman theory at (a) TPT, (b) surface and bottom Ekman depths, and (c) correlation results (correlation and regression coefficients) for local winds including nx.In(a),cross-shelf 5 Wy 5 Wy 1 transport is computed as UE t /rof (gray circles, dashed gray line), and with adjustment for large-scale vorticity as UE t /ro(f z) (black circles, dashed black line). Solid black line in (a) shows 1:1 fit of observed data to theoretical estimates. Mean error estimates (std dev) 21/2 from binned values are shown next to reported statistics. In (b), ds and db computed as the depth of zcross or u(Nf) , respectively. Mean (circle), first (bars) and second (tails) std dev shown. In (c), significant correlations are filled circles.

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gradient was allowed to develop, illustrate that this term may not significantly affect Ekman transport in the presence of horizontal shear or at short time scales (di- urnal) relative to large-scale adjustments in circulation (from days to weeks). The local baroclinic pressure gra- dient was estimated from the difference in the mean water column density (constant salinity 5 33.5) between SHB and TPT. The wind stress term was calculated from the local wind data using Large and Pond (1981) as de- scribed in section 2. Finally, the interfacial stress at the base of the surface layer should be small given that the stress divergence in the stratified interior at the base the surface boundary layer should also be small. Canon- FIG. 11. Map of (top left) northern and (bottom right) southern ical day estimates of the along-shelf momentum were regions of Monterey Bay showing locations moorings (open cir- computed by centering the peak wind and averaging from cles), HF radar nodes (dots), and mean location (stars) of HF radar 2 1 nodes (larger dots) used in vorticity computation. Here, L is the 15 to 9 h before and after. The along-shelf momentum distance used in computation of yx. balance analyses were also performed for both the bot- tom layer and the full water column. quantitatively similar results. The vertical convective The along-shelf and vertical advection terms (yyy, wyz) acceleration term wyz was estimated from ADCP- are several orders of magnitude smaller than the cross- s computed vertical shear and low-pass-filtered (2-h cutoff) shelf advection, Coriolis, and wind stress terms (uyx, fU , isotherm displacements computed from each mooring to and tw/ro) in the surface layer. The pressure gradient 2 remove high-frequency motions associated with internal terms [gdshy, (1/2ro)gds ry]arenotsmallandplayim- waves. The Coriolis term is computed using Us from the portant roles in the momentum balance as will be shown cross-shelf transport estimate described above and taking later. For now, I assume they are small relative to the 2 2 f 5 0.875 3 10 4 s 1. leading terms. The remaining terms are decomposed into For the pressure gradient terms, the surface displace- multiple components (e.g., u 5 usubt 1 u~1 u0)basedon ment hy is assumed to be much less than the depth of the the temporal scale of variability. The components are subt surface layer (hy ds). Because all other terms in the defined as low-frequency or subtidal (u , .28-h pe- balance can be estimated, the barotropic alongshore riod), diurnal (u~, 20–28-h period), and high-frequency pressure gradient was assumed to be the residual of the (u0, ,20-h period) variability, respectively. Comparison sum of all other terms. Results of Brink (1997) for a of the components in the cross-shelf advection term uyx bottom boundary layer, where the along-shelf pressure suggests that the terms involving the cross-shelf subtidal

FIG. 12. Comparison of ADCP and HF radar-derived along-shelf velocities from (left) TPT and (right) comparable estimates of yx for high and low wind conditions.

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W FIG. 13. Time series of (top) 40-h low-pass-filtered regional along-shelf wind stress t y from NDBC 46042 and (bottom) vorticity z computed over northern bay from high-frequency radar and depth-averaged along-shelf 40-h low-pass-filtered ADCP. Coriolis parameter f shown in bottom panel (dashed gray line). Shaded regions indicate relaxation periods. velocity are not important because usubt ; 0 u~ as esti- b. Vorticity and diurnal upwelling 21 mated from the ADCP record (s subt 520:0013 m s ; u During periods of active regional upwelling, the up- s 520:034 m s21). In the diurnal band (20–28 h), y u~ x welling jet that originates at Point Ano~ Nuevo and the ranges from a minimum when diurnal winds are strong, poleward return flow within the bay lead to positive which acts to slow the nearshore poleward current to vorticity that can be on the order of f (Fig. 13). This vor- a maximum equal to ysubt when diurnal winds are weak x ticity contributes to the along-shelf momentum balance and there is no modification to the poleward current. (Fig. 14) and modifies the effective Coriolis parameter. Therefore, it is assumed that ~y ysubt.Throughdepth x x This improves the fit between local winds and theoretical integration and substituting in (3), uy can then be re- x the Ekman transport (Fig. 10a, black circles). The addi- placed with Usy . Consequently, the decomposed along- x tion of the vorticity term improves the fit and brings the shelf momentum balance then becomes regression slope to approximately 1 (a 5 0.96 6 0.008) s y suggesting agreement between observations and theory. ~s subt 1 ~ s 5 t U yx f U . (13) Temporal evolution of the dominant terms in the mo- ro mentum balance therefore suggests that the wind stress is ~s Large-scale vorticity acts on a fluid in the form of largely balanced by (f 1 z)U starting around peak wind a Coriolis parameter of the form f 1 z, where z is the (Fig. 14a). Accounting for the additional vorticity in the vorticity through the upper water column. This condi- system also increases the correlation between wind stress subt tion arises when the nonlinear advection terms (uyx , and cross-shelf velocity such that the integral of the re- subt subt subt 5 6 yyy , uux , and yuy ) are of the same magnitude as gression coefficients is also approximately 1 (a 1.03 the Coriolis parameter. In coastal regions, water mass 0.06; Fig. 10c). interfaces (e.g., fronts) are associated with along-shelf The residual of the along-shelf momentum balance (along front) flows that can be in opposing directions between the surface and bottom layers is not significantly leading to underresolved estimates of the cross-shelf ad- different. It should be noted that the residual in the bot- ~ subt vection of momentum (uyx )whileothertermsremain tom layer is of the same magnitude as the leading terms, subt 5 subt 2 subt small. The vorticity (z yx uy ) in this case can a condition that lends some uncertainty to the estimates be of comparable magnitude to f at midlatitudes. From of the bottom layer momentum balance. However, for (13), the classical Ekman transport equation becomes the following discussion, the estimates of the terms in the (Stern 1965; Niiler 1969; Brink 1987; Thomas and Lee bottom layer momentum balance are considered rea- 2005; Thomas and Ferrari 2008) sonable because the residual is comparable to the surface

s layer residual. The residuals in the surface and bottom s 5 y 1 subt U t /ro(f z ). (14) momentum suggest a mean barotropic pressure gradient

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FIG. 14. Canonical day estimates of along-shelf momentum balance terms for (a) surface layer, (b) bottom layer, and (c) total, along with (d) momentum balance residual for TPT (2gHhy) during the upwelling season. The uyyds and wyzds are not shown because of low values. Shaded bounds are 95% confidence intervals. Shaded region at top shows period of northward front propagation typically crossing mooring location.

(only remaining term not resolved) of about 25 3 yields a balance between the wind stress and the along- 2 2 10 5 m2 s 2 that builds with the diurnal wind forcing shelf pressure gradients (barotropic and baroclinic). and drops to near zero during the frontal crossing period The vorticity has important implications for the tem- (Fig. 14b). During periods of weak winds, it appears poral and spatial scales of Ekman dynamics. Spinup of that the local acceleration and pressure gradient terms Ekman transport to a steady balance between wind are roughly in balance. During the frontal passage period stress and the scales as 1/f as derived from (shaded region at top of figure), the balance appears to the unsteady Ekman problem. Using this scaling for close, which is due either to the baroclinic pressure gra- northern Monterey Bay, the inertial period (2p/f)is dient being under resolved using the methods here or 19.94 h and therefore, an Ekman spinup of approximately the along-shelf pressure gradient being negligible after 3.2 h. However, Ekman spinup will also be influenced by front passage (e.g., Woodson et al. 2009). A larger baro- the vorticity in the flow, which changes the scaling to clinic term near the front would bring the barotropic 1/(f 1 z). Adjusting for the vorticity yields Ekman spin term closer to the overall mean because the two terms up periods on the order of 1.5 h consistent with pre- are opposing in this region (positive baroclinic pressure vious observations of spinup as a result of diurnal winds gradient, negative barotropic pressure gradient). In the (Woodson et al. 2007). A total spinup (-down) of less than integration of the along-shelf momentum in the bottom 6 h is well within the inertial and diurnal periods allowing layer (Fig. 14d), the onshore return flow appears to be this diurnal upwelling process to operate with minimal balanced by a combination of the barotropic pressure feedback. The canonical balance in these terms over the gradient and the bottom stress. Integrating the components diurnal cycle supports this scaling (Fig. 14). The added of the along-shelf momentum over the entire water column vorticity therefore makes the approximation of the steady

Unauthenticated | Downloaded 10/09/21 08:26 PM UTC AUGUST 2013 W O O D S O N 1663 balance for the diurnal wind a reasonable assumption for However, the agreement with theory (Fig. 10) suggests this region during active upwelling and minimizes the that the important physics is captured by this leading- potential for resonant effects. order estimate of the vorticity. These results lead to in- The southern bay site (HMS) has similar stratification teresting dynamic considerations about how the surface during upwelling months, but it does not exhibit a strong vorticity in the presence of a coastal barrier may influence diurnal Ekman (downwelling in the case of HMS) signal the stability and generation of nearshore fronts as has (Fig. 2). This may be because of the lack of a large-scale been observed and modeled in the open ocean (Thomas background vorticity in the southern bay. These obser- and Lee 2005; Thomas and Ferrari 2008). vations suggest that either 1) the vorticity in the northern Another factor in the dynamics of the inner shelf of bay allows for Ekman transport, but not in the southern northern Monterey Bay is the presence of strong posi- bay, 2) diurnal wind forcing leads to different inner-shelf tive wind stress curl (Wang et al. 2011). During periods dynamics, or 3) other dynamics are larger contributors to of strong regional upwelling, a persistent gradient in cross-shelf exchange in the southern bay. wind stress develops along the outer edge of the bay near SHB. The wind stress curl likely has important effects in the development and persistence of the upwelling front, 5. Discussion but does not appear to affect the inner-shelf dynamics Cross-shelf exchange along the central coast of Cal- within the upwelling shadow extensively. However, fur- ifornia is observed to be largely due to along-shelf winds ther study is needed to evaluate the contribution of the with surface gravity waves playing important episodic wind stress curl to inner-shelf dynamics in this region. roles especially during winter months. During the up- Surface wave–driven flows appear to be the dominant welling season, strong diurnal sea breezes along north- mechanism of cross-shelf exchange during periods of weak ern Monterey Bay lead to cross-shelf Ekman transport. and variable upwelling (October–March). During these In the southwestern portion of Monterey Bay, a weak periods, the inner shelf is typically not stratified, and large net offshore transport is consistent throughout the year swells are more frequent because of winter storms in the and is believed to be caused by internal borelike fea- northern Pacific. The 5-yr current records used in this tures or the local interaction of tides with nearshore study were sufficiently long to reasonably resolve the re- stratification (Walter et al. 2012). Cross-shelf winds and turn flow velocity profile in this region supporting the surface waves also provide significant contributions in assertion that longer records are capable of addressing the southeastern quadrant of the bay (Hendrickson and wave-driven circulation along the eastern Pacific (Kirincich MacMahan 2009). et al. 2009). Consistent with previous studies in the region, Large-scale vorticity and intense stratification over we found little or no evidence for a wave-driven return flow the northern bay provides a constraint on the transport during the upwelling season (Rosman et al. 2007). and spinup of Ekman circulation. The interplay of the No evidence for wave-driven return flow was seen in large-scale vorticity imposed by the Ano~ Nuevo up- southern Monterey Bay likely because of the unique welling jet and the diurnal sea breeze allows full theo- geometry of the mooring location. The presence of a retical Ekman transport to occur very close to the coast headland (Point Pinos)~ may allow development of a large effectively eliminating the inner shelf from this region gradient during large swell conditions during the upwelling season. The surface vorticity esti- that drives a strong along-shelf flow (e.g., Lentz et al. mates may be influenced by the coarse scale of the 1999) and may also contribute to cross-shelf exchange. high-frequency radar estimates (Paduan and Cook 1997) Internal tidal bores and surface tide interactions with relative to the true flow patterns. The grid scale in north- nearshore stratification also appear to be significant ern Monterey Bay is on the order of 2 km, yet observations contributors to cross-shelf exchange in this region (Walter of current shear across the upwelling front suggest a full et al. 2012). Both of these phenomena however could not transition of less than 1 km (Woodson et al. 2009). Addi- be addressed with the data used in this study. Conse- tionally, I have assumed that the surface currents map- quently, a more thorough examination of the effects of ped by the high-frequency radar extend to the depth of wave-driven circulation and internal dynamics in the the ADCP measurements. Although vertical shear in the southern portion of the bay is warranted. nearshore poleward flow may also affect the Ekman balance (Fig. 5), the vertical shear is minimal over the 6. Summary and conclusions depth of the surface Ekman layer. Refined estimates of the surface vorticity from cross-shelf ADCP deploy- Cross-shelf exchange across the inner shelf along the ments would help resolve the vorticity in the region and central California coast exhibits considerable variability more accurately constrain the Ekman transport model. that is most closely aligned with the seasonal upwelling

Unauthenticated | Downloaded 10/09/21 08:26 PM UTC 1664 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 43 cycle of the region and coastline orientation. During the Drake, P. T., M. A. McManus, and C. D. Storlazzi, 2005: Local wind spring/summer upwelling season, cross-shelf exchange forcing of the Monterey Bay area inner shelf. Cont. Shelf Res., in the northern bay is dominated by Ekman transport 25, 397–417. Ekman, V. W., 1905: On the influence of the earth’s rotation on caused by strong diurnal, along-shelf winds. The pres- ocean currents. Ark. Mat., Astron. Fys., 2, 1–52. ence of positive vorticity across the northern bay con- Fewings, M., S. J. Lentz, and J. Fredericks, 2008: Observations of strains the Ekman spinup to period much less than the cross-shelf flow driven by cross-shelf winds on the inner con- inertial period for the region. This constraint allows for tinental shelf. J. Phys. Oceanogr., 38, 2358–2378. a dynamic diurnal upwelling circulation to occur. This Hasselmann, K., 1970: Wave-driven inertial oscillations. Geophys. Fluid Dyn., 1, 463–502. dynamic condition also suggests that the inner shelf, as Hendrickson, J., and J. MacMahan, 2009: Diurnal sea breeze ef- defined in Fewings et al. (2008) and Lentz and Fewings fects on inner-shelf cross-shore exchange. Cont. Shelf Res., 29, (2012), rarely exists in the northern bay during active 2195–2206. upwelling. In contrast, the existence of an inner shelf has Kirincich, A. R., J. A. Barth, B. A. Grantham, B. A. Menge, and been documented for the southeastern region of the J. Lubchenco, 2005: Wind-driven inner-shelf circulation off central Oregon during summer. J. Geophy. Res., 110, C10S03, Monterey Bay (Hendrickson and MacMahan 2009). In doi:10.1029/2004JC002611. the southwestern part of the bay, cross-shelf exchange is ——, S. J. Lentz, and J. A. Barth, 2009: Wave-driven inner-shelf largely attributed to internal tidal bores and surface tide motions on the oregon coast. J. Phys. Oceanogr., 39, 2942– interactions with nearshore stratification (Walter et al. 2956. 2012). During fall/winter months, surface wave–driven Large, W. G., and S. Pond, 1981: Open ocean momentum flux measurements in moderate to strong winds. J. Phys. Ocean- flows associated with storm events appear responsible ogr., 24, 324–336. for most of the observed cross-shelf exchange. Lentz, S. J., 1994: Current dynamics over the Northern California The prevalence of positive vorticity near coastal Inner Shelf. J. Phys. Oceanogr., 24, 2461–2478. boundaries resulting from upwelling, tidal mixing fronts, ——, 2001: The influence of stratification on the wind-driven cross- and coastal boundary layers along with stratification on shelf circulation over the North Carolina Shelf. J. Phys. Ocean- the inner shelf suggests that horizontal shear in along- ogr., 31, 2749–2760. ——, and M. Fewings, 2012: The wind- and wave-driven inner shelf shelf flows likely has important implications for cross- circulation. Annu. Rev. Mar. Sci., 4, 317–343. shelf exchange in coastal regions worldwide. Continued ——, R. T. Guza, S. Elgar, F. Feddersen, and T. H. C. Herbers, research on the effects of vorticity on upwelling and front 1999: Momentum balances on the North Carolina Inner Shelf. dynamics in the coastal zone, and on the influence of in- J. Geophys. Res., 104 (C8), 18 205–18 226. teractions between surface tides and internal tidal bores ——, M. Fewings, P. Howd, J. Fredericks, and K. Hathaway, 2008: Observations and a model of over the inner conti- with nearshore stratification on cross-shelf exchange is nental shelf. J. Phys. Oceanogr., 38, 2341–2357. warranted to fully understand the dynamics of cross-shelf McPhee-Shaw, E. E., D. A. Siegel, L. Washburn, M. A. Brzezinski, circulation on continental shelves. J. L. Jones, A. Leydecker, and J. Melack, 2007: Observations and modeling of coastal internal waves driven by a diurnal sea Acknowledgments. The author would like to thank breeze. Limnol. Oceanogr., 52, 1748–1766. D. A. Fong, S. G. Monismith, J. A. Barth, and L. Washburn Monismith, S. G., E. Cowen, H. Nepf, J. Magnaudet, and L. Thais, 2007: Laboratory observations of mean flows under surface for comments and suggestions on earlier versions of this gravity waves. J. Fluid Mech., 573, 131–147. manuscript. The author is also thankful for the detailed Niiler, P., 1969: On the Ekman divergence in an oceanic jet. J. Geo- comments of two anonymous reviewers that greatly phys. Res., 74, 7048–7052. improved the paper. The author was supported by the Paduan, J. D., and M. Cook, 1997: Mapping surface currents in Center for Ocean (David and Lucille Packard Monterey Bay with codar-type HF radar. Oceanography (Washington D.C.), 10, 49–52. Foundation) and NSF Awards 0824972 and 0926738 Pawlowicz, R., R. Beardsley, and S. Lentz, 2002: Classical tidal during this work. harmonic analysis including error estimates in MATLAB us- ing T-TIDE. Comput. Geosci., 28, 929–937. 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