Evolution of chemical, biological, and physical water properties in the northern California Current in 2005: Remote or local wind forcing?

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L22S02, doi: /2006gl026782, 2006 Evolution of chemical, biological, and physical water properties in the northern California Current in 2005: Remote or local wind forcing? B. Hickey, 1 A. MacFadyen, 1 W. Cochlan, 2 R. Kudela, 3 K. Bruland, 3 and C. Trick 4 Received 2 May 2006; revised 28 July 2006; accepted 1 August 2006; published 6 October [1] The spring onset of persistent upwelling-favorable winds was later than usual in the northern California Current system in 2005, resulting in delayed provision of inorganic nutrients to the upper waters of the coastal ocean. This study uses water column measurements to illustrate the evolution of temperature, salinity, nitrate and chlorophyll a prior to and after the onset of persistent local upwellingfavorable winds, including recovery to typical conditions. Warm, nutrient- and chlorophyll-depleted surface conditions similar to those in an El Niño were observed from Vancouver Island to central Oregon, and extended to depths greater than 500 m. Return to typical conditions was more rapid than suggested by time-integrated local wind stress but consistent in timing with remote forcing of water properties in this region by upwelling-favorable winds off northern California. Alongshore advection also likely contributed to the observed recovery, but was much less effective than upwelling. Citation: Hickey, B., A. MacFadyen, W. Cochlan, R. Kudela, K. Bruland, and C. Trick (2006), Evolution of chemical, biological, and physical water properties in the northern California Current in 2005: Remote or local wind forcing?, Geophys. Res. Lett., 33, L22S02, doi: /2006gl Introduction [2] Seasonal water properties on the continental shelves of the California Current System (CCS) are controlled by the degree of basin scale advection from the north or south coupled with the ability of winds to raise this water to the euphotic zone [Hickey, 1979]. Whatever water is present below the shelf is upwelled onto the shelf in spring and summer; the degree of upwelling depends on the magnitude and persistence of upwelling-favorable winds (from the north) along the coast. Upwelled water on the continental shelves of the northern CCS in summer has a remarkable degree of along-coast uniformity in spite of a factor of three or more northward decrease in wind stress over the region. Also, at a given location, maximum monthly mean southward flow precedes maximum local southward wind stress by 1 2 months [Geier et al., 2006], suggesting that local 1 School of Oceanography, University of Washington, Seattle, Washington, USA. 2 Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, California, USA. 3 Ocean Sciences Department, University of California, Santa Cruz, California, USA. 4 Department of Biology, University of Western Ontario, London, Ontario, Canada. Copyright 2006 by the American Geophysical Union /06/2006GL wind stress is not solely responsible for observed alongshore currents. [3] Previous extreme conditions have been reported in the CCS e.g., El Niño and Subarctic intrusion. During most El Niños, saltier, warmer and lower nutrient water is present in the northern CCS together with more southern biota and a reduction in planktonic growth [Corwith and Wheeler, 2002]. During a Subarctic intrusion, fresher, colder, higher nutrient water is advected from the north and plankton growth is enhanced [Wheeler et al., 2003]. Here, we examine bio/chem/ physical conditions during a third type of anomaly the delayed onset of persistent local upwelling-favorable winds. 2. Data Set [4] Water property data were obtained on five cruises in spring and summer Sampling was conducted along transects off central Washington, off Vancouver Is. (only one of 6 transects is shown) and central Oregon (Figure 1). Timing of data sections relative to wind conditions is shown in Figure 1. All hydrographic profiles were collected with a calibrated SeaBird CTD. Historical CTD profiles calibrated with bottle salinity data are also used. [5] Inorganic nutrient and chlorophyll a (Chl a) samples were collected via rosette bottles. Chl a was determined using standard in vitro fluorometric analyses [Welschmeyer, 1994] after filtration onto Whatman GF/F filters (0.7 m nominal pore size). CTD fluorescence measurements were made using various in situ instruments from WETLabs, SeaTech, and Chelsea. Voltage was converted to Chl a by regression of discrete bottle samples and coincident fluorometer data for each cruise (r 2 ranged from 0.72 to 0.81). Samples were analyzed for nitrate plus nitrite (NO 3 +NO 2 ; hereafter referred to as nitrate or N ) using a flow-injection Lachat autoanalyzer with the procedure of Smith and Bogren [2001]. [6] Hourly wind speed and direction were obtained from the National Data Buoy Center buoys off central Washington (B46041; Figure 1) and northern California (B46014; N, W). These data were rotated to the coastline direction and alongshore wind stress was computed using the drag coefficient of Large and Pond [1981]. The data were filtered using a cosine-lanczos filter with a half power point of 46 hr. 3. Results 3.1. How Anomalous were Conditions in 2005? Comparison with Other Years [7] The spring transition to upwelling conditions, which is associated with the onset of persistent upwelling-favorable L22S02 1of5

2 Figure 3. Temperature-salinity relationships on the Washington mid shelf (100 m) in early summer for 2005 and selected other years. Figure 1. Locations of sampling sections and wind buoy (B41). Alongshore wind is also shown (positive to right of axis), with section timing indicated on the axis. winds, was delayed in 2005 from its average date in March or April [Strub and James, 1988] until about May 24 [Kosro et al., 2006] (Figure 1). In the following we use the year 2003 as a typical year (defined as having no El Niño and no Subarctic intrusion) for comparison to Physical, chemical and biological characteristics for 2003 are similar to long term summer averages for Oregon and Washington as shown by Landry et al. [1989]. In 2005, stronger than average upwelling-favorable winds began on about July 14, and full recovery to typical physical, biological and chemical conditions was observed by early August as shown below. [8] Upper water column nitrate in late May 2005 was significantly lower than in other years (Figure 2, a comparison with 2003; also see Landry et al. [1989, Figure 1.17], long term average surface values >5 M at mid shelf in May and June for both Washington and Oregon). In 2003, nitrate was greatest nearshore (>10 M) and decreased offshore such that no detectable nitrate (detection limit M) was observed in the upper 10 m of the water column seaward of the 50 m isobath. In contrast, during May 2005, the depth of undetectable nitrate extended from the surface to depths >10 30 m across the entire shelf region, with the deepest depletion at mid shelf. [9] Temperature-salinity (T-S) relationships at mid shelf (100 m water depth) show that water properties in 2005 were similar to those in the summer of the greatest El Niñoof the last 30 years, 1983 (Figure 3). Overall, the T-S properties Figure 2. Concentration of nitrate across the Washington shelf and slope in early summer 2005 and Arrows on top indicate station locations. On this and subsequent figures, dots indicate bottle sample depths. Figure 4. Deep hydrographic vertical structure in early summer 2005 (GHR) and 2003 (CB). 2of5

3 near the shelf break (Figure 4). Warmer, saltier water near the shelf break ( spicier water) is consistent with conditions frequently observed during El Niño [Huyer et al., 2002]. For a given temperature, water was saltier, indicating a difference in water mass as opposed to reduced upwelling. Fresher surface water over the shelf in 2005 is due to a northward plume from the Columbia River (Figure 1), as typically occurs during periods of summer downwelling-favorable wind [Hickey et al., 2005]. Figure 5. Nitrate off Vancouver Is. (LD) and central Oregon (NH) in June/July indicate a large range of natural variability, with 2005 near the warm extreme for salinities >32 psu Depth Structure [10] Water was warmer from the sea surface to at least 500 m in 2005 compared to 2003, and more saline at depths Alongshore Extent [11] Comparison of data across 13 sections over a distance of 450 km demonstrates that warmer, fresher surface waters and nutrient depletion extended from mid Vancouver Island to central Oregon with nitrate deficits occurring across the entire shelf and slope (most northern and most southern transects, nitrate only, shown in Figure 5). The Oregon pattern is typical of late May and early June patterns on the other sampling lines (not shown). Corresponding Chl a sections (not shown) showed remarkably low concentrations Figure 6. Seasonal evolution of water properties in 2005 off the Washington coast. 3of5

4 Figure 7. (left) Distance from the coast of the nose of 8 C water and (right) thickness of the nose at mid WA shelf relative to local (WA), local upwelling-favorable only [WA(uf)] and remote (CA) alongshore wind stress. The black symbol on the right panel is the value interpolated from stations on either side of the 100 m station, whose data appeared to be biased by a small scale anomaly. in these regions, consistent with satellite-derived distributions given by Thomas and Brickley [2006] Seasonal Evolution of Ocean Properties [12] In a section off central Washington on April 19 (not shown) 8 and 9 C isotherms tended downward toward the coast over the slope, consistent with average winter conditions rather than average April conditions [see Huyer et al., 2002]. Water colder than 7 C was deeper than 225 m, indicating that upwelling had not yet begun. Upwelling of this deep water from the slope to the shelf occurred between April 19 and May 31 (Figure 6), likely associated with the May 24 spring transition. Water in the ranges C and psu progressed shoreward over the shelf bottom as the season advanced. This water reached the inner shelf (bottom depth of 30 m) by July 17 (Figure 6), following the onset of strong local upwelling-favorable winds on July 14. The upward slope of temperature and salinity isopleths in the upper 50 m increased and the region of upward slope expanded offshore as the season advanced. The thickness of the layer of colder water also increased. [13] The salty, cold tongue along the bottom brought with it higher nitrate (between 25 and 30 M). However nitrate in the upper layers began to increase even before the cold tongue reached the inner shelf (e.g., July 9). This suggests that vertical mixing or three dimensional effects also play a role in nitrate supply. Note that differences in layer thickness in the contoured nitrate sections are due in part to the different sampling density between surveys (black dots indicate bottle locations). [14] Chl a was generally low at the beginning of the season (<2 gl 1 ) and concentration increased as the season progressed, exceeding 15 g l 1 by August 5, a high value for this period [Landry et al., 1989; see also R. M. Kudela et al., 2006]. Elevated Chl a concentrations on July 9 are related to the plume from the Columbia River, which supports elevated biomass under some conditions [Hickey, 2006]. 4. Discussion [15] Data presented here show that both Chl a and nitrate were suppressed from central Oregon to Vancouver Is. in early summer relative to typical conditions. Recovery to average surface concentrations on the inner shelf did not occur until early August, following the onset of strong local upwelling-favorable wind stress. However, physical and chemical conditions below the surface layer were progressing steadily toward normal conditions much earlier in the season (Figure 6). This progression is evaluated with two metrics: first, the distance of the nose of 8 C water from the coast, and second, the thickness of the water <8 C at mid shelf (100 m depth). Both are compared to time-integrated alongshore wind stress assuming linearized equations of motion, and a two dimensional, frictional (surface and bottom stress equal) mass balance with constant density. If we also depth average velocity over a bottom boundary layer with assumed constant thickness H, then distance D across the shelf bottom can be expressed as D = R dt, where is Hf alongshore wind stress, is density, t is time and f is the Coriolis parameter. Thus the location of the nose of the upwelled water might be expected to be at least roughly related to time-integrated local wind stress. For local winds we use both total integrated wind stress (WA) and upwelling-favorable integrated stress only [WA(uf)]. Use of the latter may be more reasonable because alongshore currents on the mid to outer shelf after the spring transition do not reverse under downwelling wind conditions [MacFadyen et al., 2005]. Thus transport is always onshore in the bottom boundary layer in those regions until an undercurrent develops. The integration was started on April 15. [16] Cold water advanced up the shelf in May and June more persistently than suggested by the rate of change of local time-integrated upwelling-favorable wind stress 4of5

5 (Figure 7). Both the cold water advance and the increase in thickness of the cold layer have a temporal structure (i.e., changes in slope are similar) more related to time-integrated wind stress off California than off Washington (Figure 7; r 2 = 0.98 vs. 0.91, respectively). The correlation with local wind stress is not improved (0.84) if the early season data are removed from the Washington dataset, suggesting that changes are more strongly related to remote than local wind stress throughout the season. [17] Bottom velocity estimated from the advance of the cold water nose is an order of magnitude less (0.5 cm s 1 ) than that calculated from the expression above. However, we note that observed cross-shelf bottom velocities in this region are frequently below 1 cm s 1, in part because the majority of the upwelling return flow occurs above the bottom boundary layer [Hickey, 1989]. The failure of simple local stress balance and the better statistical relationship with temporal changes in time-integrated alongshore wind stress from northern California suggests that remote wind forcing and subsequent internal Kelvin wave propagation play a significant role in seasonal setup of the summertime density field, as suggested by model studies [McCreary et al., 1987]. [18] Both cold water advancement along the shelf bottom and the thickness of the cold water layer also likely reflect some contribution from alongshore advection of Subarctic = v, where T is temperature, v velocity and y is alongshore distance). To estimate the order of magnitude of this effect we use an average alongshore temperature gradient of 2 C per 1000 km [Huyer, 2003] and velocities near 50, 74 and 100 m over both the mid and outer shelf in June and July (maximum velocity 10, 5 and 2.5 cm s 1, respectively [from Hickey, 1989; Geier et al., 2006]). The advective temperature contributions are 1.2, 0.6 and 0.3 C at mid shelf, much less than the observed differences of 4.0, 2.5 and 1.0 C at mid shelf and about half the values observed on the outer shelf. This result, combined with a salinity increase rather than the decrease expected from alongshore advection ( 0.2 psu for a velocity of 10 cm s 1 ) suggest that upwelling dominates the observed seasonal changes. [19] Thus we conclude that the delicate balance between upwelling-favorable winds, nutrient supply and plankton growth in the northern CCS likely depends not only on local winds but also on remote forcing by winds along the northern California coast. These latter winds and the internal Kelvin waves they generate appear critical for uplifting deeper, nutrient-rich water to the shelf and transporting it from the shelf break to the inner shelf, where it can be upwelled to the sea surface by the local winds. The flow that accompanies the resulting density field transports upwelled water southward along the coast and likely also contributes to the seasonal water property evolution. [20] Acknowledgments. Water property data collection was supported by the National Science Foundation (NSF) as part of the CoOP RISE Program (OCE to B. Hickey and OCE to K. Bruland and R. Kudela) as well as by the Coastal Ocean Program of the National Atmospheric and Oceanic Administration (NOAA) (NA17OP2789 to B. Hickey) and NSF (OCE to B. Hickey and W. Cochlan) as part of ECOHAB PNW, and to B. Hickey as part of ORHAB (NA07OA0310). Analysis was supported by these grants as well as by the GLOBEC Northeast Pacific CCS program (OCE to B. Hickey). This is contribution 9 of the ECOHAB PNW program, 188 of the ECOHAB program and 315 of the U.S. GLOBEC program. The findings and conclusions are those of the authors and do not necessarily reflect those of NOAA or the Department of Commerce. Our many thanks to the seagoing RISE, ECOHAB PNW and ORHAB teams for their critical contribution to the data collection. References Corwith, H. L., and P. A. Wheeler (2002), El Niño related variations in nutrient and chlorophyll distributions off Oregon, Prog. Oceanogr., 54, Geier, S. L., B. M. Hickey, S. R. Ramp, P. M. Kosro, N. B. Kachel, and F. Bahr (2006), Interannual variability in water properties and velocity in the U.S. Pacific Northwest coastal zone, Eos Trans. AGU, 87(36), Ocean Sci. Meet. Suppl., Abstract 36D-23. Hickey, B. M. (1979), The California current system Hypotheses and facts, Prog. Oceanogr., 8, Hickey, B. M. (1989), Patterns and processes of circulation over the Washington shelf and slope, in Coastal Oceanography of Washington and Oregon, edited by M. R. Landry, and B. M. Hickey pp , Elsevier, New York. Hickey, B. M. (2006), Complexity of a large freshwater plume, Eos Trans. AGU, 87(36), Ocean Sci. Meet. Suppl., Abstract Hickey, B., S. Geier, N. Kachel, and A. 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