Nearshore waves in southern California: hindcast, and modeled historical and 21st-century projected time series

Abstract: This data release presents modeled time series of nearshore waves along the southern California coast, from Point Conception to the Mexican border, hindcasted for 1980-2010 and projected using global climate model forcing for 1975-2005 and 2012-2100. Details: As part of the Coastal Storm Modeling System (CoSMoS), time series of hindcast, historical, and 21st-century nearshore wave parameters (wave height, period, and direction) were simulated for the southern California coast from Point Conception to the Mexican border. Changes in deep-water wave conditions directly regulate the energy driving coastal processes. However, a number of physical processes, for example, refraction on continental shelves and/or diffraction by islands, transform deep-water waves as they propagate to the nearshore, which complicates large-scale modeling efforts. In this work, a hindcast of nearshore waves was simulated by forcing a numerical wave model with hindcasted intermediate-water waves and reanalysis winds. A lookup table was created by relating corresponding offshore winds and waves with nearshore wave conditions. Using the lookup table, historical and 21st-century nearshore-wave time series were generated for global climate model-forced offshore winds and waves. Three-hourly wave parameters from the U.S. Army Corps of Engineers Wave Information Studies (WIS; http://wis.usace.army.mil/) and near-surface winds (10 m above ground) from the California Reanalysis Downscaling at 10 km (CaRD10; Kanamitsu and Kanamaru, 2007) were used to force the Simulating Waves Nearshore (SWAN) numerical model in stationary mode over a curvilinear grid extending along the coast from Point Conception to the Mexican border and from the shoreline to approximately 25 km offshore to hindcast the time period 1980-2010. The offshore extent of the model domain was defined by the locations of WIS stations used for forcing. Horizontal-grid resolution varies largely depending on bathymetry and shoreline curvature, ranging from 24 to 543 m in the along- and across-shore directions. Bathymetry data are from the 2013 Coastal California TopoBathy Merge Project (National Oceanic and Atmospheric Administration, 2013). Wave spectra were computed with a JONSWAP shape, 10-degree directional resolution, and 34 frequency bands ranging logarithmically from 0.0418 to 1 Hz. Three-hourly nearshore wave parameters (significant wave height [Hs], mean wave period [Tm], peak wave period [Tp], mean wave direction [Dm], and peak wave direction [Dp]) were output from the simulations at the 10-m bathymetric contour approximately every 100 m in the alongshore direction at a total of 4,802 locations in the nearshore and at an additional 23 locations coincident with California Data Information Program (CDIP; Scripps Institute of Oceanography; http://cdip.ucsd.edu) wave buoys. A lookup table was generated by relating offshore wind and deep-water wave conditions at a single offshore point and nearshore wave conditions simulated by the wave hindcast. The open boundary of the SWAN simulation does not represent deep-water wave conditions, as it is located in intermediate water and shoreward of the Channel Islands. Therefore, the NOAA WW3 Climate Forecast System Reanalysis Reforecast (CFSRR; Chawla and others, 2012) wave time series at a single point (CDIP buoy 067, equivalent to National Data Buoy Center station 46219) defined the deep-water end member. The lookup table was based on binning CFSRR deep-water wave parameters (Hs, Tp, Dp) and CaRD10 wind speed (U) at CDIP 067. Significant wave height was binned from 0.5 to 10.25 m at 0.25-m intervals; peak wave period was binned from 3 to 24 s at 3-s intervals; peak wave direction was binned from 5 to 360 degrees at 5-degree intervals; and wind speed was binned from 0 to 24 m/s at 6-m/s intervals. Interval sizes for Hs and Tp were based on the average RMSE for each variable. For each combination of deep-water Hs, Tp, Dp, and U, time indices falling into each bin were identified. For each nearshore location, median Hs, Tp, Tm, Dp, and Dm corresponding to all time indices of a given set of deep-water binned conditions were computed to complete the lookup table. Because swell travel time from offshore to nearshore is on the order of 1.5 h (assuming an average depth of 100 m and Tp of 15 s over a distance of about 120 km) and the model outputs are at three-hourly intervals, we assume no time lag between deep water and nearshore conditions. Historical (1976-2005) and 21st-century (2012-2100) deep-water wave time series at CDIP 067 were derived from the WaveWatch3 wave model over global (1.25 deg x 1.25 deg) and nested eastern North Pacific regional (0.25 deg x 0.25 deg) grids forced by three-hourly near-surface wind fields from a global climate model (GCM; GFDL-ESM2M RCP 4.5). Wind (CaRD10 and GFDL-ESM2M at CDIP 067) and coincident deep-water wave time series were passed through the lookup table to generate historical and 21st-century nearshore wave conditions. Wind and wave conditions that were not present in the lookup table or that had not occurred in the hindcast were filled using quantile relationships. Outputs include: southern California three-hourly, nearshore wave parameters (Hs, Tp, Dp, Tm, Dm) for 4,802 locations approximately 100 m apart along the 10-m bathymetric contour from Point Conception to the Mexican border and for an additional 23 points collocated with CDIP wave buoys. Wave parameters are available for three periods: 1) a validated hindcast (1980-2010) period derived from reanalysis data, 2) a historical (1976-2005) projection derived from GFDL-ESM2M (GCM-historical), and 3) a 21st-century (2012-2100) projection also derived from GFDL-ESM2M. Data are available as NetCDF files packaged by region, with each file containing the time series for roughly 600 locations. The points collocated with wave buoys are within one separate file. References: Chawla, A., Spindler, D., and Tolman, H., 2012, 30 Year Wave Hindcasts using WAVEWATCH III with CFSR winds--Phase 1: National Oceanic and Atmospheric Administration, National Weather Service, Environmental Modeling Center, Marine Modeling and Analysis Branch, Technical note, MMAB Contribution n. 302, 12 p. with Appendices. Kanamitsu, M., and Kanamaru, H., 2007, 57-Year California Reanalysis Downscaling at 10km (CaRD10) Part 1--System Detail and Validation with Observations: Journal of Climate, v. 20, p. 5,527-5,552. National Oceanic and Atmospheric Administration, 2013, 2013 NOAA Coastal California TopoBathy Merge Project, National Oceanic and Atmospheric Administration, National Centers for Environmental Information database, accessed February 28, 2015 at https://coast.noaa.gov/dataviewer/#/lidar/search/where:ID=2612.