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5. Conclusion 1. Introduction 2. PSIEX and Model Set-up 3. Results Modeling Coastal Upwelling Around a Small-Scale Coastline Promontory (OS23B-2018) K.A. Haas 1 , D. Cai 1 , T.M. Freismuth 2 , J.H. MacMahan 2 , S. Suanda 3 , J.A. Colosi 2 , N. Kumar 5 , E. Di Lorenzo 6 , A. Miller 3 , and C. A. Edwards 4 1 School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA, 30332 2 Naval Postgraduate School, Oceanography Department, Monterey, CA, 93943 3 Scripps Institution of Oceanography, La Jolla, CA 92093 4 University of California Santa Cruz, Santa Cruz, CA, 95064 5 University of Washington, Seattle, WA, 98105 6 School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, 30332 4. Analysis Acknowledgements Contact Fig 1. Map of the PSIEX field site. Circles indicate the location of T-stings. ADCP moorings (red diamonds) are labeled as 6, 11,15, 20, 8A, and 8B. Fig 2. Bathymetric contours of the nested four grids with nominal grid spacing of L1=600m, L2=200m, L3=66m, and L4=22m. The yellow box shows the placement of the nested child grid within the parent grid. Fig 3. (a) Depth and time averaged mean (red arrows) and principal axis (black ellipses) currents at ADCP moorings. (b). Mean surface temperature (dots) averaged over the extent of the experiment. General Observations Cross-shore Temperature Response Alongshore Temperature Response Fig 4. (a) Cross-shore (red) and alongshore (black) winds measured at NDBC buoy 46011. Positive values indicate on-shore (poleward) winds for the cross-shore (alongshore). (b) Depth-averaged alongshore currents at ADCP 20 (black) and 6 (magenta). (c) Depth-averaged cross-shore currents at ADCP 20 (black) and 6 (magenta). Subtidal temperature plotted as a function of time and depth at moorings (d) X08 and (e) X02. These moorings were co-located with ADCPs 20 and 6, respectively. The blue / dark red bars indicate periods of upwelling favorable winds / wind relaxation. Fig 5. Vertical cross-section of observed/modeled temperature (a)/(e) averaged during upwelling favorable conditions (yd 173.5 – 175.5), (b)/(f) during wind relaxation (yd 176.5 – 178.5), and (c)/(g) averaged over the entire experiment. The black-dashed contour indicates the mean isotherm during each time period, and the white dots are thermistor locations. Fig 6. (a) Alongshore temperature anomaly for observation and models with/without Mussel Pt. Fig 7. Model coastline (left) without Mussel Pt. and (right) with Mussel Pt. Shading is the time-averaged SST, contours are bathymetry and vectors are depth and time-averaged velocities. The white vector is time-mean wind during upwelling period. Fig 8. Modeled temperature, cross-shore velocity, and alongshore velocity for the 10 m isobaths without/with Mussel Pt. Red indicates (a/d) warmer temperatures, (b/e) on-shore flow, and (c/ f) poleward flow. The dashed lines represent (a/d) mean isotherm (b/e, c/f) zero velocity. Vertical dash-dot lines represent the North and South extent of Mussel Pt. Fig 10. Dominant cross-shore (upper panel) / alongshore (lower panel) momentum terms from the model for case without Mussel Pt. (a) Pressure gradient force, (b) Coriolis, (c) vertical viscosity, and (d) total advection. (e)~(h) counterparts from the model with Mussel Pt. Fig 9. Solution for Ekman surface boundary layer. Left column is velocity and right column are Coriolis (solid line) and the vertical mixing (dash line). Top row is wind in –y direction, middle row is wind in onshore (+x) direction and bottom is the combined wind ( +x and –y direction) Model Kinematics Model Dynamics Hypothesis The research was funded by the Office of Naval Research (N00014-14-1-0556). We thank PACE in Georgia Tech for the computing facilities and NOAA and NRL for the forcing data. Donghua CAI: Kevin Haas: Thomas Freismuth: The Pt. Sal Inner Shelf Experiment (PSIEX) was a 43- day field experiment aimed at measuring the temporal and spatial variability near the Pt. Sal headland on the central California coast during the summer of 2015. The flow was wind-driven with periods of upwelling favorable winds and episodes of wind relaxations. • During upwelling favorable conditions: i. Cold upwelled water was transported to the edge of the surf zone supported by a weakly stratified water column. ii. The alongshore temperature anomaly shows that a temperature gradient is set-up with warmer temperatures north of Mussel Pt(MP), a small coastal promontory and colder temperatures south of Pt. Sal. • During wind relaxations: i. Warm water was advected north to Pt. Sal. This study expands upon the flow around large headlands and shows that small coastline irregularities like Mussel Pt. can modify the local upwelling response. This hypothesis is supported with results from numerical simulations where Mussel Pt. was removed from the system. PSIEX: • Measured with 32 T-strings and 6 ADCPs • Cross-shore (X) array: 12 moorings from 5 m to 50 m water depth • Alongshore arrays: Yi, Yo, PS, and PSB arrays • 4 km alongshore and 6 km cross-shore. Model: • ROMS module of COAWST model system • COAMPS atmospheric forcings • Tidal forcing from ADCIRC tidal model • Principal axes of depth-mean subtidal flow are oriented along isobaths. • Time-mean reversal current in lee of Mussel Pt. • Time-mean north-south surface temperature gradient • Equatorward, upwelling favorable winds: year day 173.5 – 175.5 • Wind relaxation: year day 176.5 – 178.5. • During upwelling period, the mean isotherm slopes from deep h = 15 m to shallow h = 5 m water and from offshore to coast. • During wind relaxation, the mean isotherm shoaled to h= 5 m. • The experimental mean cross- shore isotherm sloped toward the shore. The temperature anomaly is calculated by subtracting the 1-hr temporal- and spatial- mean alongshore, surface temperature of the combined PS and Yi arrays from the time- mean surface temperature at each moorings. • Upwelling, surface temperatures anomalies higher north of Mussel Pt. • Relaxation, the alongshore temperature gradient reduced. • When remove Mussel Pt, no warmer anomalies north of Pt. Sal and no cooler anomalies in the lee of Mussel Pt.. Ekman Surface Boundary Layer Temperature and current observations were used to examine the flow structure around the Pt. Sal headland and a smaller promontory, Mussel Pt. The water column was continuously stratified, and upwelling occurred over the inner shelf to the edge of the surf zone. The wind-driven flow around Mussel Pt. and Pt. Sal creates a 1°C alongshore gradient over 3 km. During upwelling favorable winds, temperatures are warmer north of Mussel Pt., coldest south of Pt. Sal, and intermediate along the beach between the two promontories. Additionally, there are anomalously cold temperatures in the lee of Mussel Pt. at these times. The measurements show that small-scale (i.e., O(100 m)) promontories modify local upwelling. A modeling study was used to remove Mussel Pt. from the Pt. Sal system. The modeled kinematics and dynamics show the cross-shore component of the winds is generally decreasing the upwelling north of Pt. Sal. The orientation of the isobaths south of Mussel Pt leads to a stronger alongshore component of the wind and stronger upwelling. This supports the hypothesis that small-scale coastline irregularities impact local upwelling. • Cooler upwelled water transported on-shore. • Cross-shore component of the wind decreases upwelling north of Pt. Sal. • The isobaths’ orientation in the vicinity of Mussel Pt. i. Leads to an increased alongshore component of the wind stress in the lee of the promontory. ii. Enhances the upwelling in the lee of the small promontory. The primary momentum balance in both the alongshore and cross-shore direction: Coriolis (COR) & the vertical mixing (VM) + + − =− − ′′ − + + + =− − ′′ − (1) (2) (a) (b) (c) (d) (e) (f) (a) (b) (c) (d) (e) (f) (g) (h) (a) (b) (c) (d) (e) (f) (g) (h) (e) (f) (g) Alongshore wind (-y direction): • The classical upwelling scenario occurs. • The cross-shore velocity is directed offshore. • The alongshore velocity is in –y direction in the top layer balanced by a return flow below it. [email protected] [email protected] [email protected] Combined wind (+x and –y direction): • Net offshore transport with a small onshore layer near the surface. • Strong alongshore velocity in –y direction with only a weak return flow below. -y wind w/o Mussel Pt. w/ Mussel Pt. Temperature A fairly alongshore uniform stratification Warmer water to the north of Mussel Pt; cooler water north of Pt. Sal Cross-shore Velocity Onshore current at the surface and bottom; Offshore current in the middle On south side of MP, two layer flow, offshore on top, onshore below Alongshore Velocity Mostly southward with a small pocket of northward current at the bottom Larger pocket of northward current on south of Mussel Pt. Mussel Pt. Mussel Pt. North q W/O Mussel Pt. • Similar to Ekman with +x & -y wind stress • Dominant cross-shore balance is between COR & VM • The alongshore VM has 3 layers, balanced by a weak COR and ADV q W/ Mussel Pt. • Cross-shore has primary balance between COR&VM, with strong contributions near MP for ADV and PG • South of MP, alongshore VM is 2 layers, similar to Ekman with alongshore wind only • ADV and PG indicate the pressure drop in the center as the flow bypasses around MP The stronger upwelling in the lee of the promontory due to the orientation of the isobaths with the wind results in the cooler water patch. T(ºC) (a) (b) (c) Mussel Pt. Pt. Sal = 600m = 200 m = 66m = 22m Upwelling Relaxation Mean Velocity and SST w/o Mussel Pt Mean Velocity and SST w/ Mussel Pt +x&-y wind North w/o MP w/ MP Temperature Temperature Cross-shore Velocity Alongshore Velocity Cross-shore Velocity Alongshore Velocity w/o MP – Cross-shore Momentum w/ MP – Cross-shore Momentum PG PG COR VM ADV COR VM ADV w/o MP – Alongshore Momentum w/ MP – Alongshore Momentum PG PG COR VM ADV COR VM ADV Alongshore Distance (km) Alongshore Distance (km) Alongshore Distance (km) Alongshore Distance (km) Alongshore Distance (km) Alongshore Distance (km) Mussel Pt. Pt. Sal
