1. Abstract – The Alaska Coastal Current (ACC) is located in a region with prevailing downwelling favorable winds, flows over a long stretch of coastline (over 1000 km), and is driven by multiple sources of freshwater discharge totaling 23000 m 3 s -1 along its length. Previous studies of wind effects on buoyancy-driven coastal currents have focused on single sources of freshwater and spatially-uniform winds. Using the Regional Ocean Modeling System (ROMS) we attempt to determine how spatially variable winds affect the downstream transport of freshwater along a long coastline with nearly continuous sources of freshwater. Our model domain is 500 km long and extends 80 km offshore with a bottom topography representative of the ACC region. Ten sources of freshwater are evenly spaced at 25 km intervals along the middle 225 km of the domain, each with a discharge of 200 m 3 s -1. This domain represents a fraction of the total length of the ACC and we use periodic boundary conditions, allowing water flowing through the downstream boundary to re-enter the domain at the upstream boundary, to mimic a continuous buoyant flow from outside of our domain. Both constant and spatially varying, predominantly downwelling favorable winds are applied over the domain. The spatial variations of wind are introduced as one period of a harmonic function. Freshwater gain in the coastal current through the buoyancy forcing region is calculated by taking a 30-day averaged difference between freshwater fluxes at the downstream and upstream edges of this buoyancy forcing region. Results from several runs are split into two categories based on this freshwater gain. Values of gain are relatively high (between 1000 m 3 s -1 and 1200 m 3 s -1 ) for all runs with moderate average wind stress (~ 0.05 Pa), regardless of its spatial variations, as well as for the case of no wind forcing. Values of gain were noticeably lower (ranging from 700 m 3 s -1 to 900 m 3 s -1 ) for runs with average light wind stresses of about 0.025 Pa, especially when wind varied spatially along the coast line. Thus light and variable downwelling favorable winds can result in substantially lower freshwater gain in downstream flux than under no wind conditions. Possible mechanisms for this reduction in freshwater gain are identified. The reversal of wind to upwelling favorable conditions over limited fraction of coastline effectively blocks the downstream freshwater transport as expected. The Effects of Spatially Variable Wind Forcing on Freshwater Transport in a Buoyancy-Driven Coastal Current J. Rogers-Cotrone, A. Yankovsky Department of Geological Sciences, University of South Carolina, Columbia, South Carolina, USA T. J. Weingartner Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, Alaska, USA 2. Model Configuration Figure 2a and b. (a) Across shelf depth-profile showing shelf slope and grid point spacing. (b) Plan view of domain showing coastline, buoyancy sources, and salinity contours at day 20 for run 16. Transect locations for calculation of fluxes are shown in green (Transect 1), black (Transect 3), blue (Transect 5), and red (10.5 km offshore). Figure 2c. Examples of wind forcing for model runs 20 (red), 22 (green), 26 (blue), and 27 (black). Negative values in this case represent wind blowing towards the downstream edge (y = 0 km) of the domain. See Table 1 for a summary of all model runs. 1.Motivational Figures The Alaska Coastal Current (top left) is a dynamically distinct buoyant flow driven by input of freshwater from multiple inland sources as well as by wind forcing, and propagates northwestward following the coast (dark red line). Variability in quantities such as temperature, salinity, and depth of the mixed layer, as well as flow rate are directly related to the seasonally-varying atmospheric forcing in this region. (Weingarten et. al. 2005) A plot of annual discharge and wind speed (bottom left) show high wind speeds (red line) in the winter months and low wind speeds in the summer months. Discharge (blue line) reaches its maximum in the fall. A plot of annual wind speeds at two different locations (bottom right) shows how wind speed varies over the course of the year as well as the significant spatial variation along the coastline throughout the year including reversal of direction in the summer months. Model Domain: 80 km Wide, 500 km long Bottom Depth: Linearly sloping from 5 m at the coast to 200 m over a horizontal distance of 10 km Resolution: 1.25 km (x-direction), 2.5 km (y-direction), 25 layers (vertical) Freshwater sources: 10 sources, 25 km apart Freshwater Discharge: 200 m 3 s -1 per source Ambient salinity: 32 psu, Ambient temperature: 4 °C, Density: 25.3 km m 3 Periodic Boundary Conditons Model Run Duration: 50 days 3. No Wind vs. Constant Wind Freshwater in our model is partitioned between offshore expansion of the plume and downstream transport. In Figure 3a we see that the freshwater plume extends further offshore than in Figure 3b where it appears that most of the freshwater is trapped near the coastline and propagates downstream. 4. Calculation of Fluxes In order to quantify the partitioning of freshwater between downstream flow and across shelf expansion of the plume, two transects were set just upstream of the first freshwater source (nearest the upstream edge of the domain) and just downstream of the last freshwater source (see Figure 2b). Freshwater fluxes through these transects were calculated using the following equation: We then take a difference between freshwater fluxes at the downstream and upstream edges (Transect 5 and Transect 1 respectively) of this buoyancy forcing region (Fig. 4c). This difference defines the gain in freshwater propagating through the forcing region and is the fraction of freshwater discharge that propagates downstream while the rest spreads offshore. Freshwater gain is a better measure of the fate of freshwater discharge than the freshwater flux itself because a high value of freshwater flux can be associated with higher alongshore velocity and not necessarily higher freshwater content. Gain is averaged over a 30-day period beginning at day 20, after which a significant amount of freshwater had emerged from the upstream boundary and propagated through the fresh water forcing region. Fig. 4a and b show that flux at Transect 1 begins to be significantly affected by buoyant water from further upstream by day 20. dx dz Where is the freshwater flux, is the sea surface height, h is water depth, W is the channel width, v is the alongshore velocity, is the background salinity (32 psu), and is the salinity. Figure 4. (a) Flux at Transect 1 (green), 3 (red), and 5 (blue) for No Wind and (b) Constant Wind (- 0.075 Pa). (c) Freshwater gain for No Wind (black) and Constant Wind (-0.075 Pa) (red). Figure 3a and b: Plan view of Run #10 – No Wind (a) and Run # 28 – Constant Wind (-0.075 Pa) (b) on day 40 showing density contours and velocity vectors at the surface. Figure 3c and d: Vertical transect of Run #10 – No Wind (c) and Run # 28 – Constant Wind (-0.075 Pa) (d) on day 40 at Transect 3 showing density contours (in color) and velocity contours (in white). Dashed contours represent velocities in the negative y-direction (out of the page).

2. 5.Summary of Model Runs Figure 5. Average wind stress for each model run plotted against freshwater gain (green diamonds). Red and blue lines indicate the spatial variation of the wind associated with each run. Blue lines indicate wind stresses present over the sources of freshwater while red lines represent wind stresses present outside of this area. Note: Runs 19 and 20 overlap (Run 19 varies from 0 Pa at the edges to -0.1 Pa in the center while Run 20 varies from -0.075 at the edges to -0.025 in the center. Runs 18 and 21 also overlap with Run 18 having a constant windstress of -0.05 Pa and Run 21 varying from -0.025 Pa at the edges to -0.075 Pa in the center. 12100None 10 1210-0.075 C28 1170-0.1 C11 1130-0.05 C18 1130-0.05-0.025, -0.075, -0.025V21 1110-0.075-0.1, -0.05, -0.1V12 1100-0.025-0.05, 0, -0.05V16 1090-0.050, -0.1, 0V19 1090-0.05-0.075, -0.025, -0.075V20 1020-0.05-0.1, 0, -0.1V15 849-0.025 C29 823-0.010, -0.02, 0V27 801-0.0250, -0.05, 0V30 7970-0.002, 0.002, -0.002V26 692-0.01-0.02, 0, -0.02V22 Gain Avg. WindWind RangeWindRUN # Table 1: Run #: Refers to the model run # Wind: V – Variable, C - Constant, None – No Wind Wind Range: Wind stress at upstream, center, and downstream locations (Ex. -0.02, 0, -0.02 means wind stress at the downstream boundary was -0.02 Pa, 0 Pa in the center, and -0.02 Pa at the upstream boundary and plotted vs. alongshore distance as a cosine function). Units are in Pascals. Avg. Wind: The average wind stress taken over the entire domain Gain: The amount of freshwater transported through Transect 1 subtracted from the amount of freshwater passing through Transect 5. Units are in m 3 s -1 6. Possible Mechanisms Figure 6. 30-day averaged freshwater flux for Run 30 at the Transect 1 (a), Transect 5 (b), and the freshwater gain at Transect 5 (c). Plan view of salinity field for Run 30 at day 40 (d). Figure 7. 30-day averaged freshwater flux for Run 22 at Transect 1 (a), Transect 5 (b), and the freshwater gain at Transect 5 (c). Plan view of salinity field for Run 22 at day 40 (d). Figure 8. 30-day averaged depth profile of offshore freshwater flux for Run 22. The transect (red line from Fig. 3b.) runs the length of the domain at a distance of 15 km offshore. Black vertical lines indicate the locations of Transects 1, 3, and 5 where fluxes were calculated. When constant and spatially variable wind stresses were low, or in some cases reversed direction, downstream transport was hindered or completely blocked and values of freshwater gain were noticeably lower and were also substantially lower than the gain under no wind conditions. Our models runs have identified at least two possible mechanisms for a decrease in freshwater gain at the downstream location and these were best illustrated in Runs 22 and 30. In Run 30, steepening of isopycnals in the forcing region where wind stress is highest (-0.05 Pa), increases vertical shear and the tendency for instabilities to develop. These instabilities, as seen in Figure 6d, set the offshore flux of freshwater and easily overcome onshore Ekman transport. In cases of moderate downwelling wind stresses across the entire domain, onshore Ekman transport dominates this offshore flux and traps freshwater near the coast and so only in the presence of light downwelling favorable winds is onshore Ekman transport weak enough to allow instabilities to carry freshwater offshore. In Run 22, light downwelling favorable winds deepen the flow along the upstream and downstream portions of the domain. In the center where wind stress is at a minimum (0.0 Pa), the region of highest gain is relatively shallow and overlies a layer between 10 and 30 m deep where gain is negative. Convergence of the deeper freshwater flux from upstream with the slower flow under the plume in the forcing region deflects freshwater offshore as seen in Figure 8. The offshore deflection of freshwater is visible in the upstream half of the domain in Figure 8 below, and is most prominent between 300 and 400 km where it extends to a depth of 50 m. Acknowledgements: Funding for this work was provided by the National Science Foundation grant OCE-0650194. References: Weingartner, T.J., Danielson, S.L., Royer, T.C., 2005. Freshwater Variability and Predictability in the Alaska Coastal Current. Deep-Sea Research Part II: Topical Studies in Oceanography, 52, 169-191