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Seasonal and Interannual Variability of Satellite-Derived Chlorophyll Pigment, Surface Height, and Temperature Off Baja California T

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Seasonal and Interannual Variability of Satellite-Derived Chlorophyll Pigment, Surface Height, and Temperature Off Baja California T JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, C03039, doi:10.1029/2003JC002105, 2004 Seasonal and interannual variability of satellite-derived chlorophyll pigment, surface height, and temperature off Baja California T. Leticia Espinosa-Carreon,1 Centro de Investigacio´n Cientı´fica y de Educacio´n Superior de Ensenada, Baja California, Mexico P. T. Strub College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA Emilio Beier,1 Francisco Ocampo-Torres, and Gilberto Gaxiola-Castro Centro de Investigacio´n Cientı´fica y de Educacio´n Superior de Ensenada, Baja California, Mexico Received 22 August 2003; revised 19 December 2003; accepted 23 January 2004; published 24 March 2004. [1] Mean fields, seasonal cycles, and interannual variability are examined for fields of satellite-derived chlorophyll pigment concentrations (CHL), sea surface height (SSH), and sea surface temperature (SST) during 1997–2002. The analyses help to identify three dynamic regions: an upwelling zone next to the coast, the Ensenada Front in the north, and regions of repeated meanders and/or eddy variability west and southwest of Point Eugenia. High values of CHL are found in the upwelling zone, diminishing offshore. The exception is the area north of 31°N (the Ensenada Front), where higher CHL are found about 150 km offshore. South of 31°N, the long-term mean dynamic topography decreases next to the coast, creating isopleths of height parallel to the coastline, consistent with southward geostrophic flow. North of 31°N the mean flow is toward the east, consistent with the presence of the Ensenada Front. The mean SST reveals a more north-south gradient, reflecting latitudinal differences in surface heating due to solar radiation. Harmonic analyses and EOFs reveal the seasonal and interannual patterns, including the region of repeated eddy activity to the west and southwest of Point Eugenia. A maximum CHL occurs in spring in most of the inshore regions, reflecting the growth of phytoplankton in response to the seasonal maximum in upwelling-favorable winds. SST and SSH anomalies are negative in the coastal upwelling zone in spring, also consistent with a response to the seasonal maximum in upwelling. When the seasonal cycle is removed, the strongest signal in the EOF time series is the response to the strong 1997– 1998 El Nin˜o, with a weaker signal representing La Nin˜a (1998–1999) conditions. El Nin˜o conditions consist of low chlorophyll, high SSH, and high SST, with opposite conditions during La Nin˜a. INDEX TERMS: 4223 Oceanography: General: Descriptive and regional oceanography; 4227 Oceanography: General: Diurnal, seasonal, and annual cycles; 4275 Oceanography: General: Remote sensing and electromagnetic processes (0689); KEYWORDS: chlorophyll, El Nin˜o, La Nin˜a, Baja California, California Current Citation: Espinosa-Carreon, T. L., P. T. Strub, E. Beier, F. Ocampo-Torres, and G. Gaxiola-Castro (2004), Seasonal and interannual variability of satellite-derived chlorophyll pigment, surface height, and temperature off Baja California, J. Geophys. Res., 109, C03039, doi:10.1029/2003JC002105. 1. Introduction delimited at its northern boundary by the eastward North Pacific Current, which divides the Subtropical Gyre from the [2] In this paper, we examine the links between physical Subarctic Alaska Gyre. At its southern extreme, the CCS forcing and the lower trophic levels off Baja California, in the flows into the westward North Equatorial Current [Pare´s- southern part of the California Current System (CCS). Sierra et al., 1997]. At the most basic level, the flow of the The mean flow in the CCS is equatorward next to the coast California Current is controlled by the equatorward current in the Northeast Pacific Ocean [Hickey, 1979, 1998]. It is needed to complete the Subtropical Gyre and by the predom- 1 inantly equatorward winds. This area is considered to be an Also at College of Oceanic and Atmospheric Sciences, Oregon State oceanographic transitional zone between midlatitude and University, Corvallis, Oregon, USA. tropical ocean conditions [Durazo and Baumgartner, 2002]. Copyright 2004 by the American Geophysical Union. [3] The CCS is often depicted as a prototype eastern 0148-0227/04/2003JC002105$09.00 boundary current (EBC), with a 2-D upwelling structure C03039 1of20 C03039 ESPINOSA-CARREON ET AL.: BAJA CALIFORNIA CHLOROPHYLL IN 1997–2002 C03039 consisting of offshore Ekman transport at the surface and fields of Levitus et al. [1998] (Figure 2b) and added to the subsurface onshore return flow. An equatorward jet forms spatial EOF patterns for altimeter SSH. There is some at the offshore edge of the upwelled water, which is cold, question about the spatial ‘‘resolution’’ of SSH fields salty, and rich in nutrients. These nutrients lead to an formed from combinations such as this. We take the scale increase in primary production, which provides the basis of resolution to be approximately 100–200 km, but in for a productive ecosystem. analyses such as these, using temporal means, harmonic [4] Unlike the simple 2-D patterns depicted in most analyses and EOFs, small-scale noise is reduced and only textbooks, however, synoptic circulation patterns observed the dominant larger-scale features are preserved. in the CCS and other EBCs are characterized by complex mesoscale eddies, fronts, and meanders, with horizontal 2.3. Sea Surface Temperature (SST) scales from tens to hundreds of kilometers [Pelae´z and [7] The several clearest images from each week were McGowan, 1986]. This structure develops seasonally used to form ‘‘warmest-pixel’’ fields of SST from the and varies on interannual and longer scales [Pelae´z NOAA-14 Advanced Very High Resolution Radiometer and McGowan, 1986; Strub et al., 1991]. In this paper, (AVHRR), between January 1997 and May 2002. Monthly we use satellite observations of chlorophyll pigment means were calculated from these weekly fields. Nominal concentration, sea surface height, and temperature to spatial resolution of the remapped SST data is 1.2 km. illustrate the space and time relationships between bio- Multichannel SST algorithms were similar to those used by logical and physical processes off Baja California. This the NOAA-NASA Pathfinder project. region has been previously described by Go´mez-Valde´s 2.4. Wind Stress and the Coastal Upwelling Index and Ve´lez-Mun˜oz [1982], Pare´s-Sierra et al. [1997] and Durazo and Baumgartner [2002]. The analysis in this [8] Temporal variability in the upwelling intensity over paper also builds on previous work using in situ and Baja California is quantified by the Coastal Upwelling satellite chlorophyll estimations from Bernal [1981], Index (CUI), produced by the NOAA/NMFS Pacific Fish- Pela´ez and Guan [1982], Pela´ez and McGowan [1986], eries Environmental Laboratory in Monterey, California Strub et al. [1990], Thomas and Strub [1990], Fargion et [Bakun, 1975; Schwing et al., 1996]. We use the CUI time al. [1993], Thomas et al. [1994], Hayward et al. [1999], series centered on 27°N and 116°W (Figure 1) as represen- Kahru and Mitchell [2000, 2001, 2002], Bograd et al. tative of the Baja California region. The period for the data [2000], and Durazo et al. [2001]. is from January 1997 to May 2002. To characterize the spatial patterns of seasonal wind stress, we use winds from the European Centre for Medium-Weather Forecasts 2. Data and Methods (ECMWF), from January 1986 to January 1998. 2.1. Chlorophyll 2.5. Seasonal Cycle [5] SeaWiFS (Sea Viewing Wide Field of View Sensor) [9] Seasonal cycles of chlorophyll, SSH, SST, wind ocean color data were provided by A. Thomas at the stress, and the CUI are calculated by fitting the time series University of Maine. After initial processing [Barnes et to a mean plus annual and semiannual harmonics, al., 1994], 8-day and monthly composites were remapped with 4-km resolution. The period chosen for study extends from September 1997 (the beginning of the available data) FðÞ¼x; t A0ðÞþx A1ðÞx cosðÞþwt À j1 A2ðÞx cosðÞ 2wt À j2 to May 2002; the region is 22°N–33°N and 112°W–120°W ð1Þ (Figure 1). where A0, A1, and A2 are the temporal mean, annual 2.2. Sea Surface Height (SSH) amplitude, and semiannual amplitude for each time series, 2p 6 [ ] SSH data were formed from a combination of TOPEX respectively; w = 365:25 is the annual radian frequency; and ERS-2 altimeters, using data from January 1997 to j1 and j2 are the phases of annual and semiannual November 2001 (when the TOPEX satellite was moved to a harmonic respectively; and t is the time (as year-day). new orbit), over the same region as used for SeaWiFS Mean fields were obtained for all data, except for SSH, chlorophyll pigment concentrations. These data were pro- where the long-term mean of the altimeter SSH must be cessed and made available by the NOAA-NASA Pathfinder removed to eliminate the marine geoid. The long-term mean Project [Strub and James, 2002], using standard atmospheric of dynamic height is derived from the Levitus et al. [1998] corrections. In order to combine the data from the two climatology with reference of 500 dbar (Figure 2b). altimeters, the spatial mean over the domain of interest for [10] In the analysis below, we refer to ‘‘seasonal anoma- each altimeter is first removed for each month. This lies’’ as the sum of the second and third terms of the right- removes offsets between the two altimeters caused by hand of equation (1). ‘‘Nonseasonal anomalies’’ are the time residual orbit errors, and it also removes the dominant series after removing the temporal mean, annual and semi- seasonal cycle, which is the rise and fall of SSH caused by annual cycles as in equation (1). Empirical Orthogonal the seasonal heating cycle. What remains is the temporal Functions (EOFs) are presented for both seasonal and and spatial variability in SSH gradients, which is the nonseasonal anomalies.
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