Establishment of a Proxy Wave Climate for Coastal Modeling in the Southern California Bight

Transcription

1 Establishment of a Proxy Wave Climate for Coastal Modeling in the Southern California Bight Douglas L. Inman Center for Coastal Studies Scripps Institution of Oceanography, UCSD La Jolla, CA phone: (858) fax: (858) dli@coast.ucsd.edu Grant # N Peter N. Adams Center for Coastal Studies Scripps Institution of Oceanography, UCSD La Jolla, CA phone: (858) fax: (858) adamsp@ucsd.edu Grant # N LONG-TERM GOAL We seek to understand the character of deep-water wave climate and the transformation of those characteristic waves from deep to shallow water in the Southern California Bight (SCB). OBJECTIVES (1) Apply statistical analyses to a deep-water wave climate hincast record to document characteristic wave heights, periods, and directions under specified ENSO/PDO climatic states. (2) Develop the procedures for calculating wave transformation over the complex Southern California Bight bathymetry. (3) Develop computer codes to calculate coastal wave energy flux, and save the output in a format that can be integrated into longshore and cross-shore sediment transport models. APPROACH We analyzed a numerical hindcast dataset for the 50-year period from a simulation using the WAVEWATCH III Version 1.18 wave model (TOLMAN, 1999) as described in GRAHAM and DIAZ (2001) and GRAHAM (2003). This dataset represents the last full cycle of decadal climate change (full PDO cycle). The hindcast domain is the North Pacific Ocean (20 N - 60 N, 150 W W) for the four winter months December-March, with 3 hourly spectra recorded in 20 frequency bins (covering the wave period range of approximately 4.5 s 26 s), and 5 degree directional resolution grouped in 72 bins. The outputs calculated from wave energy in the spectral bins, are (1) significant wave height H s

2 in deep water, (2) peak (spectrally-dominant) wave period of the significant wave height T s, and (3) peak (spectrally-dominant) wave direction α. All hindcast data are prepared for the reference deepwater location 33 N, W, a site chosen for its position west (oceanward) of the Channel Islands in the SCB. This location has the advantage of representing an open ocean wave climate signal, not subject to island sheltering, depth-induced shoaling, and the complex refraction patterns within the SCB. GRAHAM (2005) made a comparison of the 50-year hindcast record with measurements, where available, from NOAA buoys. Generally, good agreement was found except for the Pt. Arguello buoy, which showed a consistently low bias of 25-30% for hindcast wave heights from the northwest associated with poor depiction of coastal topography in the hindcast model. Treating the 50-year hindcast record as a time series, GRAHAM (2005) used empirical orthogonal functions (EOF) to show that wave height and wave energy incident to the coast increase over the 50-year period. Specific methodology utilized in this study can be divided into three categories: (1) Temporal trends in the wave height, period, and direction were determined by using a cumulative residual analysis, following the work of HURST (1951) to find intervals in the time series where data depart significantly from mean values. (2) Simple histogram analyses of subset populations of the hindcast data set were conducted. Data were separated into strong La Niña periods (when the Southern Oscillation Index, SOI, was greater than 1.0) and strong El Niño periods (when the SOI was less than -1.0), and then further separated based on occurrence during Pacific Decadal Oscillation (PDO) cool-phase ( ) or PDO warmphase ( ). The highest 5% of the data in each subset were analyzed to account for the bulk of wave energy delivered to the coast. (3) Numerical modeling of wave transformation was conducted to quantify how differences in deepwater wave climate affect coastal wave energy flux at the shore. WORK COMPLETED Progress has been made on all three objectives during FY-06. A peer-reviewed manuscript is ready for submission to the Journal of Coastal Research and an abstract for the Fall 2006 AGU Meeting has been submitted. RESULTS The cumulative residuals of monthly mean and mean annual significant wave height are computed by subtracting the mean wave height (1.68 m) and cumulatively summing the residuals to obtain the time series shown in Figure 1. Negative slopes, which dominate the record prior to 1977, indicate a belowmean trend in wave heights, whereas positive slopes, which dominate the record after 1977, indicate an above-mean trend in wave heights. This shows that, in general, wave heights were lower than average before the shift to PDO warm-phase, and higher than average since then. Likewise, peak wave period is analyzed by subtracting the mean (12.4 s) and cumulatively summing the residuals, resulting in a below-mean trend (negative slopes) in peak wave period prior to 1977, and an above-mean trend (positive slopes) in peak wave period after Peak wave directions do not show the strongly consistent monotonic slopes exhibited by the cumulative residual analysis of wave heights and periods. There are, however, several intervals displaying north-of-mean trends in peak wave directions prior to

3 1977, and two strongly west-of-mean trend intervals after 1977, suggesting a shift from northwesterly to westerly peak wave directions over the span of the data set. Because wave energy flux governs coastal processes and energy flux is proportional to the square of the wave height, we chose to analyze the characteristics of the highest 5% (95 th percentile and above) of waves in the numerical hindcast data set. We found that the highest 5% of waves account for 23% of the total wave energy in the hindcast record, thereby making it a useful statistic in determining the characteristics of waves of geomorphic consequence. Figure 1. Monthly and annual cumulative residuals of significant wave height from the 50-year hindcast record of GRAHAM and DIAZ (2001). The population of highest 5% of waves is markedly different in its mean characteristics as compared to the entire population of hindcast data. Mean significant wave height is more than double that of the entire population (H s,5% = 3.98 m vs. H s,all = 1.68 m), mean peak wave period is greater by 14% (T s,5% = 14.1 s vs. T s,all = 12.4 s), and peak wave direction is more westerly by 7 (α 5% = 289 vs. α all = 296 ), reflecting the importance of El Niño storm waves in the highest 5% record. Sources responsible for generating waves associated with El Niño storm events differ from those that generate La Niña storm waves. We apply simple statistical procedures to filter the hindcast data based upon SOI. The population distributions of wave characteristics for all SOI negative (El Niño) data show only slightly larger wave heights as compared to all SOI positive (La Niña) data, whereas wave characteristics of the highest 5% (95 th percentile) of SOI negative (El Niño) data show substantially higher, longer-period, and more westerly waves, as compared to the highest 5% (95 th percentile) of waves during SOI positive (La Niña) climatic conditions. The same analysis is performed in for SOI strongly positive or negative (>+1.0 or <-1.0, respectively) conditions, to gain an understanding of the distribution of wave characteristics during periods of intense La Niña or El Niño conditions. The analyses suggest that intense El Niño conditions yield storm waves that are higher and of longer period than storm waves generated by La Niña conditions. In general, population distributions for the three wave variables analyzed tend to separate into two characteristic wave types based on ENSO state. Examination of the cumulative residual analyses suggests that trend changes in the three major variables coincide with the climatic regime change from PDO cool-phase to PDO warm-phase in We analyze population distributions of all observations of significant wave height, peak wave period,

4 and peak wave direction, as separated by PDO state. In general, waves occurring during the PDO warm-phase ( ) are higher, of longer period, and come from a more westerly direction than waves occurring during the PDO cool-phase ( ). Convolving the SOI and PDO dependencies, we present the population distributions for the highest 5% of waves occurring during strongly La Niña conditions (SOI > +1.0) during the PDO cool-phase ( ) and PDO warm-phase ( ). The population distributions show that there is little difference in the wave conditions when comparing La Niñas occurring during PDO cool-phase ( ) and La Niñas occurring during the subsequent PDO warm-phase ( ). In other words, La Niña wave events (of geomorphic consequence i.e. the highest 5%) appear to be uninfluenced by state of the Pacific Decadal Oscillation. However, the same is not true for El Niño storm conditions. We perform a similar analysis on the highest 5% of waves occurring during strongly El Niño conditions (SOI < -1.0) for the PDO cool-phase ( ) and PDO warm-phase ( ) and present the results in Figure 2. PDO warm-phase El Niño waves, are higher, of longer period, and approach from a more westerly orientation than those of the PDO cool-phase. These conditions favor greater energy flux at the coast because (1) wave energy density increases as the square of wave height, and (2) increased orthogonality in wave approach angle increases the energy flux to the beach. Figure 2. Histograms of significant wave height, for highest 5% of waves for all strong El Niños, for strong El Niños during the PDO cool-phase ( ), and for strong El Niños during the PDO warm-phase conditions ( ). The statistical analysis suggests that the deep-water waves striking the Southern California coast during El Niño conditions differ substantially in their heights, periods, and directions, depending on the state of the PDO. We address the question of actual wave energy delivered to the Southern California coast by running SWAN simulations of wave transformation from deep to shallow water using typical storm wave conditions for Aleutian Low and Pineapple Express sources, respectively, as deep-water input conditions. SWAN is a third generation spectral wave transformation model that has been developed (BOOIJ et al., 1999; RIS et al., 1999) and validated in numerous recent studies (BENTLEY et al., 2002; ROGERS et al., 2003; KEEN et. al., 2004; SIGNELL et al., 2005; ZIJLEMA AND VAN DER WESTHUYSEN, 2005). Figure 3 shows the bathymetry to a depth of 300 m for the Torrey Pines subcell region, and a color map of nearshore significant wave heights calculated by SWAN for the Pineapple Express source conditions as well as alongshore distances (in km) from the northern end of the Oceanside Littoral Cell at Dana Point. Figure 4 shows the alongshore variability in wave energy flux at the 5-meter bathymetric contour for the Aleutian Low and Pineapple Express source conditions, respectively. On average, Aleutian Low nearshore wave energy fluxes are 23 kw/m, whereas Pineapple Express

5 nearshore wave energy fluxes are 74 kw/m. The mean difference in wave energy flux between Aleutian Low conditions and Pineapple Express conditions is approximately 51 kw/meter shoreline, or ~320%. Figure 3. Nested SWAN model results for nearshore waves within Torrey Pines subcell region of Oceanside littoral cell, Southern California. Color map of nearshore wave heights for Pineapple Express conditions. Bathymetric contours to 300 m depth (contour interval = 25 m). White circles mark alongshore distance (km) from northern limit of Oceanside littoral cell at Dana Point. Figure 4. Longshore variation in wave energy flux along 5-meter bathymetric contour. Blue asterisks represent output from Aleutian Low conditions. Red asterisks represent output from Pineapple Express conditions. IMPACT/APPLICATIONS This work aids our understanding of the far-field climatic forcing of changes in coastal morphology. Coastal processes are driven by the magnitudes and directions of wave energy flux, whose value is proportional to the square of the wave height. El Niño storm waves during PDO warm-phase intervals (conditions similar to the Pineapple Express type discussed above) deliver the most energy to the coast. If wave approach direction is sufficiently high, these conditions may result in rapid rates of sediment transport within the littoral cells. Where there is a prolonged negative divergence of littoral drift, we expect to see a systematic decrease in beach sediment with time, resulting in exposure of the coastal bedrock (sea cliffs) to wave attack. When this occurs, the natural protection of the beach is gone, and the only defense available to sea cliffs is their inherent lithologic strength, which depends on rock type. Much of the developed, heavily-populated, cliffed coast of California is composed of weakly-

6 consolidated sedimentary rock, underscoring the potentially catastrophic consequences of prolonged exposure to westerly storm waves. Combining wave transformation modeling with recent sea level history and the aforementioned climate proxy records may provide insight on the locations of erosional hotspots within the SCB in the past. Likewise, the combination of wave modeling, our current understanding of ENSO/PDO climatic cyclicity, and projections of future sea level rise might provide valuable estimates on the location and magnitude of future coastal erosion. RELATED PROJECTS This work represents the oceanic zone module of a larger effort to model coastal evolution. Results from the proxy wave climate analysis will be used to calculate the potential longshore transport of sediment on the Southern California coast under proscribed climatic conditions. This work will be integrated with previous work, supported by ONR, wherein a model was developed to calculate beach profiles based on thermodynamic principles (Jenkins and Inman, 2005). We have successfully run the FORTRAN codes necessary to compute beach profiles, and look forward to integrating these codes into our ongoing MATLAB modeling architecture. REFERENCES BENTLEY, S.J., T.R. KEEN, C.A. BLAIN, and W.C. VAUGHAN, The origin and preservation of a major hurricane event bed in northern Gulf of Mexico: Hurricane Camille, Marine Geology, 186, BOOIJ, N., R.C. RIS, and L.H. HOLTHUIJSEN, A Third-Generation Wave Model for Coastal Regions - 1. Model Description and Validation. Journal of Geophysical Research-Oceans, 104, GRAHAM, N.E., Variability in the Wave Climate of the North Pacific: Links to Inter-Annual and Inter-Decadal Variability. Oceans 2003 Conference. San Diego. Marine Technology Society, GRAHAM, N.E., Coastal Impacts of North Pacific Winter Wave Climate Variability: The Southern California Bight and The Gulf of the Farallones. Scripps Institution of Oceanography, for the California Energy Commission, PIER Energy Related Environmental Research. CEC GRAHAM, N.E., and H.F. DIAZ, Evidence for Intensification of North Pacific Winter Cyclones since Bulletin of the American Meteorological Society, 82, HURST, H.E., Long-Term Storage Capacity of Reservoirs. Transactions of the American Society of Civil Engineers, 116, KEEN, T.R., S.J. BENTLEY, W.C. VAUGHAN, and C.A. BLAIN, The generation and preservation of multiple hurricane beds in the northern Gulf of Mexico. Marine Geology, 210,

7 RIS, R.C., L.H. HOLTHUIJSEN, and N. BOOIJ, A Third-Generation Wave Model for Coastal Regions 2. Verification, Journal of Geophysical Research-Oceans, 104, ROGERS, W.E., P.A. HWANG, and D.W. WANG, Investigation of Wave Growth and Decay in the SWAN Model: Three Regional Scale Applications, Journal of Physical Oceanography, 33, SIGNELL, R.P., S. CARNIEL, L. CAVALERI, J. CHIGGIATO, J.D. DOYLE, J. PULLEN, and M. SCLAVO, Assessment of wind quality for oceanographic modeling in semi-enclosed basins. Journal of Marine Systems, 53, TOLMAN, H.L., User Manual and System Documentation of Wavewatch-Iii Version 1.18: NOAA / NWS / NCEP / OMB Technical Note 166, 110 p. ZIJLEMA, M. and A.J. VAN DER WESTHUYSEN, On convergence behaviour and numerical accuracy in stationary SWAN simulations of nearshore wind wave spectra. Coastal Engineering, 52, PUBLICATIONS Adams, P.N., D.L. Inman, and N.E. Graham, submitted, Characterization of Deep-Water Wave Climate from a 50 Year Hindcast, Southern California: Evidence of ENSO and PDO. Journal of Coastal Research. Adams, P. N., D. L. Inman, and N. Graham, 2006, Modeling Coastal Response to Wave Energy Flux in the Southern California Bight Using a Proxy Wave Climate. EOS, Trans., American Geophysical Union, 2006 Fall Mtg.