The Phenology of Coastal Upwelling in the California Current

Steven J. Bograd and Daniel M. Palacios NOAA Southwest Fisheries Science Center, Environmental Research Division

William J. Sydeman PRBO Conservation Science

Scott R. Benson NOAA Southwest Fisheries Science Center, Protected Resources Division

Mark H. Carr and Mark D. Readdie Long Marine Laboratory, University of California, Santa Cruz

Letter of Intent for FY2007 funding under Fisheries and the Environment (FATE)

BACKGROUND

Many marine organisms have life histories adapted to seasonal events in the environment. Changes in the amplitude or phase of seasonal events can therefore significantly affect the productivity and community structure of marine ecosystems, from primary producers to fish stocks to apex predators (Cushing, 1990; Beare and McKenzie, 1999; Bograd et al., 2002; Logerwell et al., 2003). Such phenological effects are potentially more disruptive than those associated with interannual climate events and decadal climate shifts. Thus, any set of indicators of ecosystem state would need to include those that quantify changes in the seasonal cycle of dominant physical processes. Phenology plays a particularly critical role in the California Current System (CCS), in which ecosystem productivity and structure is driven largely by the seasonal cycle of coastal upwelling. The impact of an anomalous seasonal cycle was particularly evident in 2005, when the onset of coastal upwelling was delayed by several weeks in the northern CCS (Schwing et al., 2006),resulting in anomalously warm sea surface temperatures (Kosro et al., 2006; Pierce et al., 2006),low surface chlorophyll levels (Thomas and Brickley, 2006), a spatial redistribution of zooplankton species (Mackas et al., 2006), record low rockfish recruitment and a lack of forage species (Brodeur et al., 2006), total breeding failure of a dominant planktivorous marine bird (Sydeman et al., 2006), and changes in California sea lion foraging strategies (Wiese et al.,2006). Our objective is to develop simple indicators that quantify the timing of the onset of coastal upwelling in the California Current (i.e., the date of the “spring transition”), the temporal evolution and overall intensity of upwelling, and the total duration of the upwelling season, as well as the spatial variations of these properties along the West Coast between Baja California and Vancouver Island. Once developed, these indicators will be applied in two contexts. First, we will validate their usefulness through correlations with historical biological time series, including time series of seabirds, salmon, rockfish, and leatherback turtles. Second, we will develop and test (in spring 2007) an operational spring transition index that could provide an early warning signal to resource managers of the probability of a disruption to the California Current ecosystem.

DATA SETS

Physical Data The coastal upwelling index (UI; Bakun, 1973; Schwing et al., 1996) has been a workhorse of fisheries oceanography for more than three decades, greatly contributing to our understanding of physical-biological coupling in eastern boundary current systems. The UI represents the magnitude of the offshore component of Ekman transport, which approximates the amount of water upwelled from the base of the Ekman layer. Positive values are the result of equatorward wind stress; negative values imply downwelling, the onshore advection of surface waters accompanied by a downward displacement of water. Upwelling indices are computed from the 6-hourly sea level pressure (SLP) fields obtained from the U.S. Navy Fleet Numerical Meteorology and Oceanography Center (FNMOC). We will use the 40-year time series (1967-2006) of daily UI at six locations along the U.S. West Coast, from 33°N to 48°N, separated by 3° latitude, to develop indicators of upwelling conditions. In addition, we will use complementary surface wind and coastal sea level data from NOAA buoys and shore stations, respectively, to contribute to indicator development. NOAA buoys closest to the UI locations will be used.

Figure 1. Seabird reproductive phenolgy from the Farallon Islands.

Biological Data The biological time series available for analysis with the phenological indices include ~35years of nesting dates for seabirds of the Farallon Islands (Figure 1a). The seabird dataset includes information on the phenology and productivity of planktivorous auklets (Ptychoramphus aleuticus) and piscivorous murres (Uria aalge). In general, the timing of nesting is predictive of murre and auklet breeding success (Abraham and Sydeman 2004). However, the factors affecting seabirds timing of breeding are not well understood. Preliminary analyses indicate that mean annual auklet timing of egg-laying varies by upwards of 60 days between years, and is related to wintertime SST in a non-linear fashion (Figure 1b). A similar pattern has been found for murres. Auklet nesting is highly synchronous in years of early upwelling and cold SST. When upwelling is delayed, egg- laying is more spread out and less peaked (Sydeman, unpublished data). The leatherback aerial survey data include information on distribution and abundance in shelf waters (<90m depth) between Point Conception and the California/Oregon border during 1990-2003, and similar fine-scale survey data between Point Piños and the Golden Gate Bridge during 2002-2006, which also included qualitative data on leatherback prey, jellyfish. Leatherbacks migrate to the west coast of North America to forage on seasonal aggregations of jellyfish prey, although the factors affecting the development and location of prey patches are poorly understood. Recent telemetry studies indicate that leatherback arrival at the California Current generally occurs during late spring through early summer, with peak densities occurring during September relaxation events when jellyfish abundance tends to be greatest; however, temporal and spatial variation in this pattern is common. Previous analyses have established a link between leatherback abundance off California and the Northern Oscillation Index (NOI) during1990-2003 (Benson et al., in press). More recently, leatherback and jellyfish densities were observed to be very low during 2005-2006, when the onset of upwelling was delayed. These patterns suggest that coastal upwelling is central to the occurrence of jellyfish and leatherbacks, but more detailed data on the timing, duration and intensity of upwelling are required to elucidate the physical processes involved. In this project, we will examine interannual patterns of leatherback arrival, distribution and abundance relative to the timing and duration of upwelling/relaxation events during 1990- 2006. Indices will be validated against aerial surveys conducted within the same regions during July-October 2007. The PISCO subtidal monitoring program conducts annual community surveys of rocky reefs along a section of the California coast from Santa Cruz to the Channel Islands. Initiated in 1999,the surveys monitor aspects of fish (density and size), invertebrate and algal community structure, rockfish recruitment and small scale physical oceanography (temperature, currents). Biological data are used to investigate large-scale biogeographic patterns and to understand long-term variability in kelp forest communities. Physical aspects of the coastline (relief, substrate, swell exposure), in addition to the derived upwelling indicators, will be used in the analysis to understand their effects on patterns of community structure. Local rockfish recruitment dynamics are analyzed in conjunction with the pelagic juvenile rockfish trawls done by SWFSC-FED.

APPROACH

Development of Upwelling Indices This project will build upon and expand the utility of the historical Upwelling Index (UI; Bakun, 1973).Since upwelling has a cumulative effect on ecosystem productivity and structure, we focus on the cumulative upwelling index (CUI),based on integrating the mean daily upwelling indices at six locations along the U.S. West Coast (Schwing et al., 2006). The st integration begins on January 1 , and continues to the end of the year. The climatological mean (1967- 2005) CUI at the six locations are shown in Figure 2, yielding a mean start (when CUI reaches its annual minimum) and end date (when CUI reaches its annual maximum) of the upwelling season at each latitude. Note that the climatological upwelling season is nearly year-round (though weaker) off southern California and Baja, but gets progressively shorter with northward latitude. We define the start date of the upwelling season (i.e., the date of spring transition) as the st date on which the CUI, integrated from January 1 , reaches its minimum value. This is the date after which positive UI (upwelling) prevails. Similarly, the end date of the upwelling season is defined as the date on which the CUI reaches its maximum value. That is, downwelling prevails after this date. We will determine the date of spring transition, the intensity of upwelling, and the total duration of the upwelling season from the 40-year CUI series at each location as described below:

Spring Transition Index (STI): This is determined as the date at which the minimum value st of CUI is achieved, following an integration beginning on January 1 . This is the same definition used by Pierce et al. (2006) and Schwing et al. (2006). This index will be quantified as the Julian date of the spring transition (STI) and the anomaly from the ). climatological mean spring transition date (STIanom The spring transition dates developed here will also be compared todates derived from other data sources, including coastal sea level.

3 Figure 3. CUI (m /s/100m) integrated over climatological upwelling season at 39°N, 125°W. Arrow marks the time of maximum climatological upwelling. Julian day of start (SD), end (ED) and maximum upwelling (MD) are given. Mean and standard deviation (black solid and dashed curves, respectively), 1967-2003 individual years (gray curves) and 2004-2006 (color curves) are shown.

Total Upwelling Magnitude Index (TUMI): This is the total CUI integrated from the observed start date (STI) to the observed end date of the upwelling season. This is a measure of the total intensity of coastal upwelling over the entire length of the upwelling season (see

Figure 3). Both actual and anomaly (TUMIanom; difference between observed and climatological mean TUMI) will be developed. In addition, a separate TUMI will be developed to relate the onset and magnitude of upwelling during the early portion of the upwelling season to central California rockfish recruitment time series. This index, TUMIrock, st st will be the CUI integrated over the period March 1 to May 31 . Upwelling Punctuality Index (UPI): This is the same as TUMI, but the CUI integration is performed from the observed start date (STI) to the date of climatological maximum upwelling (date of maximum climatological CUI slope). In conjunction with TUMI, the UPI relates how much of the total seasonal upwelling occurred in the early vs. the latter half of the season. Both actual and anomaly (UPIanom; diffrence between observed and climatological mean UPI) will be developed.

Length of Upwelling Season Index (LUSI): This is defined as the total number of days between the observed start date (STI) and observed end date (date of maximum CUI) of the upwelling season. Both actual and anomaly (LUSIanom; difference between observed and climatological mean LUSI) will be developed.

Lasker Event Index (LEI) and Relaxation Event Index (REI): While the indices described above provide an annual characterization of the timing and intensity of each upwelling season, it is important to quantify the timing and variability of upwelling/relaxation events within an upwelling season. Following Lasker (1978), we define a Lasker Event Index (LEI) as the total number of days over the period of LUSI in which buoy-derived daily averaged surface winds were weaker than 4 m/s. Similarly, we will derive the Relaxation Event Index (REI) as the total number of relaxation events over the period of LUSI. Relaxation events will be defined as periods of downwelling-favorable alongshore wind stress over a minimum period of 3consecutive days that follows a similarly long upwelling period.

Application to Historical Biological Time Series As part of this project, we will examine the within-season distributional properties of auklet and murre egg-laying dates, develop indices of these distributions, and relate indices to the within-season upwelling indices. We will perform regression analyses of the upwelling indicators against the timing of egg-laying of common murre and Cassin’s auklet (1972-present). We will also look at the skewness and kurtosis of the breeding time series in relation to upwelling conditions. The arrival date of leatherback turtles at the California Current (from telemetry data), and distribution and abundance within the coastal upwelling zone during 1990-2006 (from aerial surveys) will be related via regression techniques to the developed indices representing the timing and strength of coastal upwelling and subsequent relaxation events. Patterns of jellyfish distribution, relative abundance, and species composition from the 2002- 2006 fine-scale aerial surveys will be analyzed relative to the indices to identify processes that are favorable to jellyfish patch formation, and the temporal and spatial scales involved. Timing of upwelling/relaxation events (start, duration, and frequency) and magnitude of upwelling from indices for central California will be of particular interest to determine how asynchronous upwelling may affect distribution of leatherbacks and jellyfish prey. Results indicating a response by jellyfish and leatherbacks to upwelling indices in this analysis will be incorporated into predictive models of jellyfish and leatherback occurrence along the North American coast. The ultimate goals of these models is to 1) increase our understanding of foraging habitat use by endangered leatherbacks, and 2) predict where and when fisheries and leatherbacks may overlap, to mitigate potential impacts. Test of an Operational Spring Transition Index The derived indicators will be based on the daily upwelling index values at the six nominal locations. For the 2007 upwelling season, we will use real-time wind products to compute a CUI, updated daily, and to monitor the transition to upwelling conditions throughout the CCS. This will allow real-time computations of CUI and STI, which can be compared to large-scale indicators of coastal surface conditions derived from real-time satellite imagery (SST anomalies from Pathfinder and surface chlorophyll anomalies from SeaWiFS). Based on the STI and evolution of CUI during the early portions of the upwelling season, as well our derived relationships between these indicators and biological time series, we will make a qualitative projection of the overall productivity of the California Current ecosystem for summer-autumn2007. This will provide an initial test of the prospect of developing operational upwelling indicators for the CCS.

DELIVERABLES

A set of historical indices that quantify the timing of onset, evolution, total intensity, and duration of coastal upwelling at six locations in the California Current over the period1967-2006, as well as the number of within-season Lasker and relaxation events. All derived indicators will be posted on the FATE website; Analyses to test the utility of these indicators for describing variability in relevant biological time series (seabird reproductive success, rockfish recruitment, leatherback distribution and abundance); A peer-reviewed manuscript describing the upwelling indicators, their spatial and temporal variability, and their value in explaining variability in relevant biological time series; Incorporation of the upwelling indices into a California Current Integrated Ecosystem Assessment, along with a projection of the overall productivity of the California Current in summer 2007 based on real-time estimates of STI and UPI; Integration of the acquired information for implementation into stock-specific and ecosystem-based assessment models, in collaboration with stock assessment scientists.

BENEFITS

We will build on the foundation of the historical upwelling index, which has been applied effectively for years in fisheries oceanography research. The set of physical indicators developed here will be the first to address the critical issue of phenology in the California Current, by quantifying the timing, evolution, intensity, and duration of coastal upwelling. Furthermore, this project will facilitate unique collaborations between physical oceanographers, fisheries biologists, seabird ecologists, and stock assessment scientists to improve our understanding of climate-ecosystem interactions in the CCS. The information obtained will be of benefit to managers tasked with assessing future changes in commercially important fish and protected turtle populations and in the overall ecosystem health of the CCS, since global and regional climate models project significant variability in the timing and magnitude of coastal upwelling accompanying various climate change scenarios (Snyder et al., 2003; Diffenbaugh et al., 2004). REFERENCES

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