Satellite data, Empirical Orthogonal Functions, and the 1997–1998 El Niño off California

Remote Sensing of Environment 84 (2003) 234 – 254 www.elsevier.com/locate/rse

Satellite data, Empirical Orthogonal Functions, and the 1997–1998 El Nin˜o off California Nikolay P. Nezlin a,b,*, James C. McWilliams b a

b

P.P. Shirshov Institute of Oceanology RAS, Moscow, Russia Marine Science Center, University of California, Los Angeles, 621 Charles E. Young Drive South, Los Angeles, CA 90095-1606, USA Received 11 December 2001; received in revised form 5 April 2002; accepted 20 June 2002

Abstract The features of the 1997 – 1998 El Nin˜o event were analyzed by Empirical Orthogonal Functions (EOF) statistical methods applied to the remotely sensed sea surface temperature anomalies (SSTA) measured by AVHRR radiometers; anomalies of water circulation derived from sea surface height anomalies (SSHA) measured by TOPEX/Poseidon radar altimeter; and meteorological information (air temperature, upwelling index, and wind stress curl). EOF statistics demonstrated the features of an El Nin˜o event during the second half of 1997 and the first half of 1998, with sea level elevated along the coast and with SSHA gradients, indicating a retarding of both the equatorward California Current and the alongshore poleward Southern California Countercurrent. The positive SST anomaly developed first in the Southern California Bight and then in the zone of upwelling to the north of Point Conception. The anomalies of upwelling index and the wind stress curl pattern also changed during the El Nin˜o event, but these changes occurred later than hydrographic variations and were too weak to explain the observed changes in SSTA and SSHA. We conclude that off central and southern California oceanic teleconnection (i.e., the consequences of propagation northward of coastally trapped downwelling waves) was responsible for the 1997 – 1998 El Nin˜o event. D 2002 Elsevier Science Inc. All rights reserved.

1. Introduction El Nin˜o southern oscillation (ENSO) is the name of the ocean – atmospheric cycle that determines the anomalous weather conditions of global atmosphere, in particular along the entire western coast of both the North and South Americas. Its periodicity is from 3 to 7 years (Rasmusson & Wallace, 1983). The start of the El Nin˜o is related to strong westerly wind bursts over the warmer than normal equatorial waters in the western tropical Pacific. These winds generate oceanic baroclinic downwelling Kelvin waves that propagate eastward at speed 200 –250 km/day along the equator to the South American coast (Chavez et al., 1999; Kessler & McPhaden, 1995; McPhaden, 1999; McPhaden, Hayes, Mangum, & Toole, 1990; Wyrtki, 1975). These waves are then transformed into coastally trapped

* Corresponding author. Marine Science Center, University of California, Los Angeles, 621 Charles E. Young Drive South, Los Angeles, CA 90095-1606, USA. Tel.: +1-310-770-1302; fax: +1-310-206-3987. E-mail address: [email protected] (N.P. Nezlin).

waves which propagate north and south toward the poles (Chelton & Davis, 1982; Enfield & Allen, 1980; Huyer & Smith, 1985) with speed as high as 100 km/day (Fiedler, 1984) or 150– 200 km/day (Kosro, 2002; Spillane, Enfield, & Allen, 1987). In this paper we call these waves ‘‘coastal’’ rather than ‘‘Kelvin’’, since the latter refers to waves whose side boundary is vertical; the presence of topographic slopes significantly changes the propagation speed of these waves. The coastal waves deepen the thermocline and raise sea level along the American coast, and an accumulation of warm water in the upper mixed layer results in large positive sea surface temperature anomalies. The redistribution of sea surface temperature (SST) over the Pacific Ocean results in atmospheric circulation changes, reflected in an expansion of the Aleutian Low (Emery & Hamilton, 1985). The changes of local wind pattern are considered by some to be the main source of perturbations in the California Current System, ‘‘atmospheric teleconnection’’ (Breaker & Lewis, 1988; Breaker, Liu, & Torrence, 2001; Mysak, 1986; Schwing, Murphree, deWitt, & Green, 2002; Simpson, 1983, 1984); the direct influence of coastal waves, ‘‘oceanic teleconnection,’’ is emphasized by the others (Chavez,

0034-4257/02/$ - see front matter D 2002 Elsevier Science Inc. All rights reserved. PII: S 0 0 3 4 - 4 2 5 7 ( 0 2 ) 0 0 1 0 9 - 8

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1996; Rienecker & Mooers, 1986); a combination of these two effects is emphasized by Huyer and Smith (1985), Lynn, Schwing, and Hayward (1995), Ramp, McClean, Collins, Semtner, and Hays (1997), and Schwing et al. (2002). The 1997– 1998 El Nin˜o was one of the strongest in the twentieth century (Chavez et al., 1999; McPhaden, 1999). Its influence on the pelagic ecosystem off California resulted in a decrease of phytoplankton biomass (Kahru & Mitchell, 2000) and a catastrophic fall in the squid fishery (Hayward, 2000; Nezlin, Hamner, & Zeidberg, in press). Some of these ecosystem changes also were observed during the El Nin˜o events in 1982 –1983 and 1991 –1992, using different kinds of meteorological and oceanographic information including satellite observations (Fiedler, 1984; Karl et al., 1995; Strub, James, Thomas, & Abbott, 1990). These satellite observations were especially important because these remotely sensed observations provided unique methods of monitoring of the ocean. The opportunities for use of satellite observations have increased dramatically during recent years because a wider variety of remotely sensed information is collected nowadays by many different scientific satellites, and data from these satellites are provided to scientific community via the Internet. Therefore, details of the 1997– 1998 El Nin˜o event can be described and analyzed better than was previously possible.

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The goal of this paper is to analyze the changes observed in the southern part of the California Current System during the 1997 – 1998 El Nin˜o event with emphasis on the potential mechanisms of oceanic and atmospheric teleconnection. To evaluate the California Current System, we analyze remotely sensed anomalies of sea surface temperature (SSTA) and sea surface height (SSHA) using Empirical Orthogonal Functions (EOF) statistics. Pronounced seasonal variations of these parameters are typical of the region so the anomalies over time are used instead of total values, to distinguish between typical patterns of seasonal variations of these hydrographic parameters and the patterns observed during the El Nin˜o event in 1997 –1998. The Ekman wind drift (‘‘upwelling index’’) and wind stress curl are analyzed as local meteorological factors that might force the hydrographic variations.

2. Southern California coastal region The coastal region of the Pacific Ocean off central and southern California is dominated by the equatorward California Current, the eastern portion of the clockwise gyre in the North Pacific. The California Current transports cold Subarctic water from north to south throughout the year along a coast with a narrow continental shelf (Fig. 1). To the

Fig. 1. The bottom topography map and the scheme of general circulation off Southern California. Dashed arrow indicates the poleward Davidson Current occurring usually during winter season.

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north of Point Conception, permanent equatorward winds generate maximum coastal upwelling in summer and reduced upwelling in winter. The California Current is a typical broad eastern boundary current (Hickey, 1979; Lynn & Simpson, 1987), 850 –900-km-wide off the California coast and separated from the East Pacific water mass by the California Front, a southeastward continuation of the Subarctic Frontal Zone (Lynn, 1986). The California Current is not steady but migrates seasonally onshore and offshore, producing a rich eddy field (Haney, Hale, & Dietrich, 2001; Marchesiello, McWilliams, & Shchepetkin, in press; Strub & James, 2000). To the south of Point Conception the coastline turns eastward forming the basin called the Southern California Bight. This bight consists of a set of basins separated by islands and underwater ridges (Hickey, 1992). It is bordered by a narrow shelf 3- to 6-km wide, mostly shallower than 50 m. The basins between ridges are rather deep (>500 m). The stream of the California Current turns to south –southeast and passes along the continental slope. At about 32jN a branch of the California Current turns eastward and then northward along the coast of the Southern California Bight (Bray, Keyes, & Morawitz, 1999; Harms & Winant, 1998; Hickey, 1992), forming a large gyre known as the Southern California Eddy. The poleward current along the coast is called the Southern California Countercurrent (Sverdrup & Fleming, 1941) and transports warm southern water into the Santa Monica Basin and the Santa Barbara Channel. At the northwestern end of Santa Monica Basin, the poleward flow divides into two flows: one flowing northwestward through the Santa Barbara Channel, and the other flowing westward to the south of the Channel Islands (Hickey, 1992; Lynn & Simpson, 1987). The intensity of the equatorward California Current System varies seasonally (Bray et al., 1999). During spring its intensity increases compared to the poleward Southern California Countercurrent (Hickey, 1979). Its jet migrates onshore, and the eastward branches penetrate into the Southern California Bight through the Santa Barbara Channel and onward south of the Channel Islands. Later in summer, the jet of the California Current migrates offshore and remains there until winter (Bray et al., 1999; Haney et al., 2001; Reid & Mantyla, 1976), while warm southern waters penetrate further to the north and west within the Southern California Bight. The migration of the California Current offshore and weakening of southward, upwelling-favorable winds throughout the fall and winter may also result in the formation of a rather narrow, near shore (80 – 150 km) poleward surface flow north of Point Conception (Strub, Allen, Huyer, Smith, & Beardsley, 1987). This seasonal flow is referred to as the Davidson Current (Brink et al., 2000; Chelton, 1984; Chelton, Bratkovich, Bernstein, & Kosro, 1988; Hickey, 1979; McCreary, Kundu, & Chao, 1987). In spring the strengthening and onshore migration of the California Current results in a continuous southward flow along the coast. However, from time to time a pole-

ward near-shore flow has been observed during the summer season, e.g., in 1984 and 1991 (Chelton et al., 1988).

3. Methods and data used for analysis The primary focus of this study is interannual variability. Therefore, we subtract the climatological mean from all of the data (whose product we call the total anomaly), and additionally we subtract the climatological seasonal cycle from some of the data which have a pronounced unimodal component resulting from the annual solar cycle (the nonseasonal anomaly). In the latter category are SST, air temperature, and upwelling index. Climatologies are calculated on the basis of maximum available data (1981 – 1996 for SST, 1994– 2000 for air temperature, and 1967– 2000 for upwelling index). In the former category are wind stress curl (1997 –2000) and sea surface height (SSH). SSH data (1992 – 2000) are obtained from the data center in the form of total anomalies about the mean calculated during 1993– 1996, and we have not bothered to adjust the mean to match the period of availability. As an antonym of ‘‘anomalies’’ we use the term ‘‘total values’’ instead of often used ‘‘absolute values’’ (a mathematical term). 1997 – 2000 is chosen for analysis. For sea level variations (SSHA), we analyze the entire time-series of TOPEX/ Poseidon observations (1992 –2000) to emphasize the persistence of the 1997 –1998 sea level anomalies compared to other periods. The region is 32 – 37jN, 122– 117jW. In the case of the upwelling index, we expand the region to 21– 60jN in order to reveal the entire pattern of atmospheric circulation in the Northeastern Pacific during the El Nin˜o. The data used in this study is obtained, via the Internet, from centers that process and disseminate remotely sensed information. The SSHA data were obtained from the Maps of Sea Level Anomalies (MSLA)/Archiving, Validation, and Interpretation of Satellites Oceanographic Data (AVISO) scientific team of Collecte, Localisation, Satellites (CLS)/ Centre National d’E´tudes Spatiales (CNES) data center. These data correspond to Sea Level Anomaly (SLA) maps obtained from the final processing of TOPEX/Poseidon data, relative to a 3-year mean (January 1993 to January 1996). Each map covers a 10-day period. The grids have a resolution of 0.25j by 0.25j. We use data from October 1992 (the beginning of observations) to August 2000. The maps were reprocessed by AVISO TOPEX/Poseidon M-GDR (version C) data using an improved space/time objective analysis method described in Le Traon, Nadal, and Ducet (1998). The Multi-Channel Sea Surface Temperature (MCSST) data are obtained from NASA JPL PODAAC (Jet Propulsion Laboratory Physical Oceanography Distributive Active Archive Center Product 016). These data were collected by advanced very high resolution radiometers (AVHRR) aboard the NOAA-7, -9, -11, and -14 polar orbiting satellites and processed at JPL using the Multi-Channel Sea Surface Temperature algorithm (McClain, Pichel, & Walton, 1985).

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The data have been collected since November 1981, averaged weekly, and interpolated (without missing values) over global equal-angle grids of spatial resolution of 2048/360 pixels ( f 18.5 km) per degree of longitude and latitude. The nominal accuracy is 0.3 jC. We use only the data collected during night-time (descending pass) in order to avoid the short-period SST variations resulting from heating the thin surface layer. To obtain the ‘‘climatic’’ seasonal cycles of SST, data are averaged for each 8-day period over the entire period of observations (November 1981 – December 1996). ‘‘Climatic’’ seasonal variations of SST are then subtracted from the data collected during the period January 1997 –December 2000 to analyze SST variations in terms of anomalies rather than total values. The daily air temperature data in the Southern California Bight is obtained from the NOAA National Climatic Data Center (NCDC) Internet site. For air temperature, climatic seasonal variations are calculated over the period 1994 – 2000 and subtracted from the data for 1997– 2000 to obtain air temperature anomalies. The resulting time-series is smoothed using a 5-day running average. Only the anomalies evident after smoothing are taken into account to avoid considering short-term variations. The values of the ‘‘upwelling index’’ (i.e., the Ekman offshore and alongshore drifts calculated from 6-h fields of atmospheric pressure; Bakun, 1973) for 15 locations along the US Pacific Coast (21jN – 60jN)) are obtained from the Pacific Fisheries Environmental Laboratory (PFEL) Internet site and averaged over 8-day time periods. The climatic seasonal variations are averaged over 34 years of observa-

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tions (1967 –2000) for each location. These climatic seasonal variations are smoothed using a five-point running average. The anomalies of offshore and alongshore drifts are then estimated for the 1997– 2000 period by subtracting the climatic variations from the data observed in each 8-day period of 1997 –2000. For wind stress curl we use meteorological information from the National Center for Environmental Prediction (NCEP) and supplied by the NASA Goddard Space Flight Center Distributive Active Archive Center (GSFC DAAC) as ancillary information for SeaWiFS users. These files contain regular grids of zonal and meridional wind speeds at 10 m above sea level interpolated on an equidistant cylindrical projection of 1j spatial resolution and 6-h temporal resolution (12-h during some periods in 1998 and 1999). These data are averaged over 8-day periods and recalculated to wind stress (kg m 1 s 2) using the conventional equation s = Cd
qa
AUA
U, where U is wind speed (m s 1) at 10 m, qa is air density (1.2 kg m 3), and Cd is the dimensionless ‘‘drag coefficient’’ (0.0013). The wind stress is then recalculated to wind stress curl. The El Nin˜o period is revealed using NINO3 index (SSTA averaged over the region 5jS – 5jN; 150jW – 90jW) obtained from the International Research Institute for Climate Prediction.

4. Empirical Orthogonal Functions method Although Empirical Orthogonal Functions analysis has not been used extensively in relation to the California

Fig. 2. Variability (standard deviation, mm) of sea surface height anomalies during October 1992 – August 2000. Right axis—distance in kilometers. Dotted lines and digits indicate TOPEX/Poseidon tracks.

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Fig. 3. (A) Contribution of the first EOF mode of sea surface height anomalies (SSHA) to the circulation pattern off Southern California. Shaded areas indicate negative SSHA. Arrows indicate the directions and velocity of geostrophic flow resulting from variations of sea surface height, size of each arrow being proportional to the slope of SSHA; (B) temporal variations of the first EOF mode; (C) NINO3 index; (D) spectral density of the first mode.

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Fig. 4. (A) Contribution of the second EOF mode of sea surface height anomalies to the circulation pattern off Southern California (see caption to Fig. 3A); (B) temporal variations of the second mode; (C) NINO3 index; (D) spectral density of the second mode.

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Current System, this statistical approach is a convenient method for analysis of successive images of data distributed in space. The method has been used for different kinds of remotely sensed data analysis, including some areas off California (Abbott & Barksdale, 1991; Eslinger, O’Brien, & Iverson, 1989; Kelly, 1985; Lagerloff & Bernstein, 1988). The Empirical Orthogonal Function (EOF) method decomposes space- and time-distributed data into modes ranked by their temporal variance. The methodology is described in Priesendorfer (1988), also in Lagerloff and Bernstein (1988). In this study, we use neither spatially nor temporally ranked EOFs, because we analyze either total values (wind stress curl) or anomalies (SSTA, SSHA, and upwelling index) derived from the differences between observations obtained over recent years and climatic means estimated over different time periods

(Section 3). The resulting means are not equal to zero; therefore, we do not analyze pure spatial or temporal variance but the joint space – time data variance (Priesendorfer, 1988). Each grid of observations (wind curl or anomalies of SST, SSH, and upwelling index) is converted into a vector of the matrix T with dimension M
N, where M is the number of spatially distributed points, i.e., the number of grid nodes excluding land, and N is the number of observations over time (i.e., grids). The matrix T was then decomposed into two additional matrices as follows: T = A
B, where A is M
I matrix and B is I
N matrix, with I being the number of nonzero EOF modes. Taking into account the percentage of explained variance, modes with eigenvalues >1 are considered significant. Then each vector of matrix A is converted into a grid representing the contribution of this mode into different areas of the

Fig. 5. Mean (A) and standard deviation (B) of the sea surface temperature anomalies during 1997 – 2000.

N.P. Nezlin, J.C. McWilliams / Remote Sensing of Environment 84 (2003) 234–254 Table 1 The percentage of total variance of sea surface height anomalies (SSHA), sea surface temperature anomalies (SSTA), upwelling index anomalies, and wind stress curl explained by first five EOF modes Mode

1 2 3 4 5

Percent of variance (%) SSHA

SSTA

Upwelling index anomalies

Wind stress curl

17.43 12.04 10.78 10.02 7.23

27.43 20.09 8.34 4.54 4.00

18.88 17.05 12.22 11.89 8.84

44.91 33.42 6.99 5.68 3.49

region. The corresponding vector of B matrix is analyzed by a Fourier time-series method to reveal typical periods of temporal variability.

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5. Sea surface height anomalies Fig. 2 illustrates the general pattern of variation of sea surface height anomalies (SSHA) over October 1992 – August 2000. The SSHA variations appear higher along the tracks of the TOPEX/Poseidon satellite, but this is an artifact of the algorithm of objective analysis used to construct the SSHA maps (Le Traon et al., 1998). Thus, the validity of SSHA values interpolated to different grid nodes depends on the position of each one in relation to the satellite tracks. With the EOF analysis the SSHA grids are processed treating all nodes as if they are of equal significance; however, all features of water circulation derived from SSHA located within the spaces between the tracks of satellite orbits (i.e., where there are no observations) should be considered doubtful.

Fig. 6. Contribution of the first and second EOF modes to the variations of sea surface temperature anomalies off Southern California. Thicker contour line indicates zero.

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The standard deviation of SSHA is markedly higher about 100– 150 km offshore along the track data, as compared to the near-shore zone (Fig. 2). This feature was noted by Lynn and Simpson (1987) on the basis of CalCOFI hydrographic data and attributed to the transitional zone between the coastal and offshore hydrographic regimes, resulting in an increased amount of mesoscale eddies and meanders. A similar feature was indicated on the basis of surface drifters and TOPEX/Poseidon altimeter data (Haney et al., 2001; Kelly et al., 1998; Marchesiello et al., in press). Table 1 gives the percentage of total variance of SSHA explained by the first five modes. The first mode explains >17% and the second mode about 12% of the total variance. The third and fourth modes each explain about 10 –11%, but their temporal variation did not reveal explainable patterns and are not presented here. Figs. 3A and 4A illustrate the distribution of the first and second EOF modes. High and low values of SSHA are designated by arrows indicating the direction of anomalies of geostrophic flow resulting from the SSH variations. The size of each arrow indicates the slope of the SSHA map, i.e., high values of modes at Figs. 3B and 4B correspond to strengthening of water circulation indicated by the arrows at Figs. 3A and 4A, and vice versa. The first mode corresponds to seasonal variations of the California Current System. The spectrum of its time-series (Fig. 3D) reveals a single maximum with a periodicity of precisely 1 year. The amplitude of this first mode increases during summer and decreases during winter (Fig. 3B). Fig. 3A illustrates that during summer the equatorward California Current strengthens in the northern part of the region and migrates offshore in its southern part. The poleward Southern California Countercurrent also strengthens during summer. This pattern appears to be in accordance with the wellknown regularities of seasonal variation of the California Current System (Bray et al., 1999; Brink et al., 2000; Chelton, 1984; Hickey, 1979; Strub & James, 2000). During summer, the rise of the slope of the SSH, with highest values offshore, can be also attributed to seasonal changes in dynamic height resulting from the heating and cooling cycle (Lynn & Simpson, 1987; Wyrtki, 1975). The pattern of the first SSHA EOF mode changed in 1997 – 1998 during the El Nin˜o event (Fig. 3C). In addition to the seasonal late winter negative extreme, an unseasonable negative spike occurred in August 1997 and another long one in November 1997 –January 1998. These periods coincide with two pulses of poleward propagation of the El Nin˜o signal, revealed by Strub and James (2002) on altimetric data analyzed over the Eastern Pacific Ocean and corroborated by hydrographic observations (Lynn & Bogard, 2002). The time-series of the second mode reveal evident interannual variations rather than seasonal ones (Fig. 4B,D). High values of the second mode indicate a retarding of both the southward jet of the California Current and the alongshore poleward Southern California Countercurrent

(Fig. 4A) at the same time that the California Current migrates offshore. The southeastward direction of the arrows in the central part of the Southern California Bight indicates that during the strengthening of the second mode, the jet of the Southern California Countercurrent does not weaken but is restricted to narrow near-shore flow. The influence of the second mode on the circulation pattern in the study region was most pronounced from early August 1997 until mid-May 1998, i.e., during the El Nin˜o event (Fig. 4B,C). Thus, the remotely sensed observations of sea surface topography revealed an obvious signature of the El Nin˜o: the rise of the sea level alongshore and a weakening of the equatorward California Current. The rise of the dynamic height inshore the Southern California Bight from July 1997 until April 1998 was described by Bogard and Lynn (2001). Similar anomalous poleward water transport, correlated with the rise of the sea level along the coast during the El Nin˜o periods, was discussed by Chelton, Bernal, and McGowan (1982). It is worth mentioning that the pattern of the 1997 – 1998 El Nin˜o event revealed by the SSHA variations over the central and southern California coastal ocean by EOF analysis persisted until May 1998 (Fig. 4B). At the same time, the sea-level anomalies measured by the coastal tide-gauge stations indicated the abrupt end of the El Nin˜o event as soon as February 1998 (Kosro, 2002; Ryan & Noble, 2002). Thus, the satellite

Fig. 7. Temporal variations (A) and spectral density (C) of the first EOF mode of sea surface temperature anomalies. Dashed line in (A) is a fivepoint running average. (B) NINO3 index.

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internal part of the Southern California Bight between Santa Monica and San Diego (Fig. 5A). Therefore, negative SSTA were observed in the zone of upwelling and positive values occurred in the zone of maximum influence of the warm Southern California Countercurrent. In other words, the cold zone was colder, and the warm zone was warmer than normal during 1997 – 2000. This phenomenon could be explained by the intensification of the California Current System in transporting cold water equatorward and warm water poleward (Fig. 1). However, an alternative explanation is that the number of AVHRR satellite observations has gradually been increasing during the past two decades. During the study period (1997 – 2000), the 8-day composites used for analysis were based on valid data rather than interpolation, as in previous years, and, because of this, the contrasts of SST distribution may have increased. The maximum variation in standard deviation of SSTA was observed in the near-shore zones of the Southern California Bight, Santa Monica Basin, and Santa Barbara Channel (Fig. 5B). These areas are under the influence of both the cold equatorward California Current and the warm poleward Southern California Countercurrent. Moreover, this zone is rather shallow and sensitive to heating and cooling from atmospheric influences. Hence, better spatial resolution of remote sensing should also increase the SST variability in this zone. Fig. 8. Temporal variations (A) and spectral density (C) of the second EOF mode of sea surface temperature anomalies. Dashed line in (A) is a fivepoint running average. (B) NINO3 index.

altimetry provides a wider range of information than coastal observations of sea level, and the revealed pattern better corresponds to the hydrographic signature of the 1997 – 1998 El Nin˜o event, which persisted until mid-summer 1998 off Oregon and northern California (Huyer, Smith, & Fleischbein, 2002) and off Baja California (Durazo & Baumgartner, 2002). Lynn and Bogard (2002), analyzing the dynamic evolution of hydrographic properties in the southern California Current System, indicated westward propagation of the low dynamic height signal during spring 1998. Therefore, after February 1998 the anomalies of sea level along the coast retreated to a zero level, but the hydrographic state offshore was far from normal.

6. Sea surface temperature anomalies Fig. 5 indicates the distribution of the means and standard deviations of sea surface temperature anomalies (SSTA). The SSTA observed during four years (1997 –2000) were referenced to climatic seasonal SST variations averaged over the previous 15-year period (1981 – 1996), which is why SSTA averaged over 1997 – 2000 is not zero. The minimum values (about 0.5 jC) occurred near central California between Monterey Bay and Point Conception, whereas maximum (about + 0.5 jC) were observed in the

Fig. 9. Variations of air temperature at San Nicholas Island during 1997 – 2000 referred to the period 1994 – 2000. Dashed line is a five-point running average. Temporal variations (A) and spectral density (B).

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The first two modes in SSTA variation explain about onehalf (>47%) of the total variance (Table 1). High values of the first mode indicate the pronounced increase of surface temperature in the Southern California Bight and its slight decrease in the rest of the study area (Fig. 6). The second mode, in contrast, indicates warming in the upwelling region and equatorward transport of cold water and slight cooling of offshore waters and the near-shore zone of the Southern California Bight between San Diego and the Santa Monica Basin. The maximum of spectrum of the first mode corresponds again to a periodicity of precisely 1 year (Fig. 7C). High values of the first mode were observed during summer seasons (Fig. 7A); the minima were evident during winter periods, except the winter of 1997 – 1998. The maximum of spectrum of the second mode had an interannual period of temporal variations (Fig. 8C). Maximum

values of the second mode were observed during winter (November – February) of 1997 – 1998 and the minimum during the winter season of 1998 – 1999 (Fig. 8A). The first mode reveals residual seasonal variations of the intensity of the California Current System not removed by subtracting the 1981 – 1996 climatic seasonal averages from the SST data observed during 1997 – 2000. The decrease of SST in the zone of the California Current and simultaneous increase of SST in the near-shore zone of the Southern California Bight result from variations of intensity of both the cold equatorward California Current and warm Southern California Countercurrent. This pattern is more typical of summer than winter, excluding the period of the 1997 – 1998 El Nin˜o event (Fig. 7A,B). The second mode manifests the warming of the zone of cold equatorward California Current and coastal upwelling

Fig. 10. Cross-correlation functions between the first and second EOF SST modes, and air temperature (Tair). The horizontal dashed lines indicate the 5% confidence level of correlation coefficients.

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during winter 1997 –1998 and its cooling during the next winter season (Fig. 8A). There are two possible explanations for the SSTA variations: the atmosphere –sea heat flux and the hydrographic variations resulting from horizontal and vertical advection of water. Therefore, we next compare variations of SST with variations of air temperature anomalies observed at San Nicolas Island located in the middle of the Southern California Bight (Fig. 9). The spectral analysis of the air temperature anomalies reveals the dominance of interannual variations. Three periods of high air temperature were observed in 1997 –1998: May – June 1997, September –October 1997, and August – September 1998. All three of these peaks correspond to high values of the first SSTA EOF mode (Fig. 7A), which means that these three peaks were related to variations of heat flux between ocean and the atmosphere. The third peak (August –September 1998) corresponds to high values of both the first and the second EOF modes, i.e., in August – September 1998, high air temperature corresponded to warming of both Southern California Bight and the upwelling zone. These correlations stress the close interaction between the ocean and atmosphere; this is why the periods of extremes from both EOF modes that are not explained by air temperature variations are of particular interest. These periods are June – July 1997 in the first EOF mode and November 1997 –February 1998 in the

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second EOF mode (slightly manifested in the first EOF mode as well). Both periods indicate warming of some part of the region due to water circulation rather than local heat flux. The first period shows warming of the Southern California Bight in mid-summer 1997; the second period shows warming of the upwelling zone during the winter 1997 – 1998. Hence, the warming occurred first in the southern part of the study region and then in its northern part. The time lag between the periods of warming in different parts of the study region suggests that coastal waves are propagating poleward. The coastally trapped waves permanently propagate poleward along the US Pacific coast (Enfield & Allen, 1980); a salient feature of the El Nin˜o event is the significantly increased magnitude of these waves. To test the hypothesis of the northward propagation, we calculated the time-lagged correlation coefficients between the first and the second EOF SSTA modes and the anomalies of air temperature (Fig. 10). The crosscorrelation analysis revealed that both EOF modes were positively correlated within a wide range of time lags. Nevertheless, the maximum correlation occurred when the first EOF mode preceded the second one with a time lag of about 2 weeks. The correlations between the air temperature anomalies and both SSTA EOF modes manifest similar regularities. The variations of the air temperature at San Nicholas Island (Tair) preceded the first SSTA EOF mode,

Fig. 11. Climatically averaged seasonal variations of offshore (A) and alongshore equatorward (B) Ekman wind drift (‘‘upwelling index’’) along the coast of North America from 21jN and 60jN (data from PFEL). Thicker contour line indicates zero.

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i.e., the variations of SST in the inshore part of Southern California Bight, by about 2 weeks; Tair preceded the second SSTA EOF mode, i.e., the variations of SST to the north of Pont Conception by about 2 months. These correlations suggest that SSTA propagate poleward up the Southern California coast. This is only a qualitative analysis because we cannot say anything about the propagation speed.

7. Upwelling index The studies of atmospheric teleconnection imply the analysis of wind pattern over wide area. This is the reason we analyzed seasonal and interannual variations of upwelling index not only in the region (32 – 37jN, 122 –117jW), but over the entire region where the upwelling index was calculated and disseminated by PFEL: 15 locations along the Pacific coast of North America from 21jN to 60jN. Fig.

11 illustrates seasonal variations of offshore and alongshore upwelling index averaged over 1967– 2000. The general features correspond to the well-known pattern of seasonal variations of the wind field along the west coast of North America (Halliwell & Allen, 1987; Strub, Allen, Huyer, & Smith, 1987; Strub, Allen, Huyer, Smith, & Beardsley, 1987). To the south of approximately 50jN, the offshore upwelling index has an evident seasonal cycle: poleward winds in winter change to equatorward winds in summer, the phenomenon called ‘‘the spring transition’’ (Strub, Allen, Huyer, & Smith, 1987). Offshore the Ekman drift resulting from equatorward wind is most pronounced between 30jN and 40jN, i.e., in the region off Central and Southern California. The period of most intensive wind drift is from April to August, when offshore water transport reaches 200 m3/s per 100 m of coastline. At the same time, the absolute values and seasonal variability of alongshore water transport are evidently

Fig. 12. Seasonal variations of anomalies of offshore (A) and alongshore equatorward (B) Ekman wind drift (‘‘upwelling index’’) along the coast of North America from 21jN and 60jN in 1997 – 2000 (data from PFEL). The ticks at the right axis indicate the study region. Arrows indicate the periods discussed in the text.

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Fig. 13. Contribution of positive values of the first (A) and second (B) EOF modes of upwelling index anomalies to offshore and alongshore water transport along the coast of North America from 21jN and 60jN. Rectangle indicates the study region (32jN – 37jN; 122jW – 117jW).

lower, especially between 30jN and 40jN. To the north of approximately 37jN, alongshore water transport is equatorward in winter and poleward in summer; between 32jN and 35jN, i.e., in the Southern California Bight, the wind-

driven water transport is poleward all year. The anomalies of upwelling index were analyzed using the EOF method. Fig. 12 illustrates the variations of upwelling index anomalies during 1997 –2000. Negative anomalies of offshore

Fig. 14. Temporal variations (A) and spectral density (C) of the first EOF mode of upwelling index anomalies. (B) NINO3 index. Dashed line in (A) is a five-point running average.

Fig. 15. Temporal variations (A) and spectral density (C) of the second EOF mode of upwelling index anomalies. (B) NINO3 index. Dashed line in (A) is a five-point running average.

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Ekman transport (Fig. 12A, arrow 1) dominated during the whole El Nin˜o period (second half of 1997 and the beginning of 1998), especially between 40jN and 50jN; in the region off Southern California (32jN–37jN) negative anomalies of offshore Ekman transport were observed only in the summer of 1997. The period of increased offshore water transport was evident off Southern California (32jN – 37jN) in the second half of 1998 and the first half of 1999 (La Nin˜a period). Two short-period anomalies of both offshore and alongshore Ekman transport were evident during the 1997 –1998 El Nin˜o event. In August – September 1997, a sharp negative anomaly occurred about 24jN, near the tip of Baja California, which could be related to the early stage of the El Nin˜o event (Fig. 12A,B, arrow 2). The second anomalous period occurred within 27jN – 45jN in February 1998. The negative anomaly of the offshore upwelling index coincided with a positive anomaly of equatorward water transport (Fig. 12A,B,

arrow 3). However, these variations were too short to cause the long-period variations of sea level and SST associated with the El Nin˜o event. The percentages of total variance of the upwelling index anomalies explained by the first five EOF modes are given in Table 1. The first mode explains about 19% of total variance, and the second mode explains 17%. High values of the first mode imply the increase of both offshore and poleward water transport between 40jN and 50jN (Fig. 13A). To the south of Point Conception, the increase of the first EOF mode means a slight weakening of offshore transport. The spectrum of the first EOF mode has a maximum with a one-year period (Fig. 14C). The seasonal pattern of the first EOF mode of upwelling index anomalies indicates that seasonal variations during 1997– 2000 were more pronounced than regular climatic variations. These residual seasonal variations show the abrupt drop of the Ekman drift during the winter periods of February 1998 and

Fig. 16. Mean (A) and standard deviation (B) of wind stress (10

6

kg m

2

s

2

) curl during 1997 – 2000.

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February 1999; but no seasonal winter minimum was observed in 2000. High values of the second EOF mode indicate onshore water transport between 30jN and 40jN, poleward flow between 45jN and 50jN, and equatorward flow between 30jN and 40jN (Fig. 13B). These processes were pronounced in November 1997, February 1998, and February 2000 (Fig. 15A). Negative values of the second EOF mode (i.e., offshore transport along the California coast) occurred in March 1997, from December 1998 to March 1999, and in January 2000. The spectrum of the second EOF mode indicates an interannual period of variations (Fig. 15C). These patterns of spatial and temporal variations of upwelling index anomalies do not reveal features, which distinguish the period of the 1997– 1998 El Nin˜o event. High values of the second EOF mode, observed during winter 1997 – 1998, showed two separated peaks, which occurred few months after the onset of the El Nin˜o event evident from hydrographic data (see Sections 5 and 6).

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Furthermore, the second EOF mode shows strengthening of the equatorward drift off California in winter –spring 1998; it is also evident from alongshore upwelling index (Fig. 12B). The latter contradicts the general pattern of El Nin˜o along North America coast associated with poleward flow of warm waters (Chelton et al., 1988).

8. Wind stress curl Fig. 16A illustrates the distribution of wind stress curl averaged over March 1997 – December 2000. Maximum values of wind stress curl occurred near Point Conception and to the south of Santa Barbara Channel. About 200 km offshore, the sign of wind stress curl is negative, resulting from maximum wind stress there. A similar pattern of distribution of wind stress curl was previously computed from direct wind observations (Abbott & Barksdale, 1991; Hickey, 1979; Winant & Dorman, 1997). The variability of

Fig. 17. Contribution of the first (A) and second (B) EOF modes to wind stress curl variations off Southern California. The zone of negative influence of EOF modes is shaded. Thicker contour line indicates zero.

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autumn and winter of 1997) and spring (April – May). The increase of positive wind stress curl in spring and early summer was mentioned by Abbott and Barksdale (1991). In spring 1998 the maximum was less evident as compared with other years (Fig. 18A). Maximum values of the second EOF mode (Fig. 17B) occurred along the boundary between positive and negative wind stress curl values. The maximum of spectrum has period about 86 days (Fig. 19C). The second EOF mode seems to be mesoscale variations resulting in pulsation of the width of the alongshore area of positive wind stress curl (Fig. 16A). High values of the second mode indicate spreading of the zone of positive wind stress curl offshore and vise versa. Breaker and Lewis (1988) and Breaker et al. (2001) observed quasi-periodic oscillations of zonal wind stress of a 30 – 70-day period along the central California coast. The period of oscillations we observed (about 86 days) is slightly longer but comparable. During the 1997– 1998 El Nin˜o event (Fig. 19B), no changes in the second EOF mode are observed. The minimum observed in January– February 1998 was comparable with similar seasonal minima observed in 1999 and 2000; other deviations from zero in the second half of 1997 and the first half of 1998 did not differ in magnitude and duration from the oscillations observed later. Fig. 18. Temporal variations (A) and spectral density (C) of the first EOF mode of wind stress curl. (B) NINO3 index. Dashed line in (A) is a fivepoint running average.

wind stress curl is maximum in the zone of its positive values along the shore, near Point Conception, and in the outer area of the Southern California Bight (Fig. 16B). The offshore zone, where wind stress curl is negative, has the lowest level of variability. Therefore, in the upwelling zone and in the Southern California Bight we expect a direct influence of positive wind stress curl on the hydrographic state through pumping of deep water into the upper mixed layer. Temporal variations of wind stress curl did not manifest a seasonal pattern (see below); this is the reason the input data for the EOF method were the total values of wind stress curl rather than its anomalies. The first EOF mode explains almost 45% of the total variance; the second mode explains 33.4% (Table 1); hence, only these two modes are worth attention. The positive values of the first EOF mode occurred within the offshore zone of 100– 150-km width and over the Southern California Bight (Fig. 17A), i.e., in the zone of positive wind stress curl (compare with Fig. 16A). In other words, the first EOF mode appeared to be associated with the values of wind stress curl. The temporal variations of the first mode have a smoothed spectrum maximum at 1 year (Fig. 18C). However, the first mode has no unimodal seasonal pattern (Fig. 18A): two annual maxima occurred in November – December (except late

Fig. 19. Temporal variations (A) and spectral density (C) of the second EOF mode of wind stress curl. (B) NINO3 index. Dashed line in (A) is a fivepoint running average.

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9. Discussion The mechanism of influence of El Nin˜o on the coastal ecosystem off California implies two processes: a deepening of the pycnocline within wide coastal zone resulting from propagating poleward coastally trapped waves (oceanic teleconnection) and a weakening of southward California Current, coastal upwelling, and offshore water transport resulting from change of local wind patterns (atmospheric teleconnection). During the 1997 – 1998 El Nin˜o event, hydrographic variations were noticeably more pronounced than those of the local wind. Moreover, it is implausible that the wind pattern observed in 1997 – 1998 was responsible for the changes of both the circulation derived from SSH and the SST distribution. Hydrographic variations occurred as early as the summer of 1997 and persisted until summer 1998 (Fig. 4B). In contrast, wind forcing was not evident until the beginning of 1998, and these changes were not related to weakening of southward current and coastal upwelling. Wind stress curl decreased in the end of 1997 in the Southern California Bight and according to Sverdrup theory, it should have resulted in weakening of the poleward current, but the observed variations in SSH reveal the opposite pattern. It is worth mentioning, however, that Sverdrup theory is a prediction for the depth-averaged flow not the surface flow, in contrast to the patterns derived from the SSH anomalies. The southward and onshore wind drift strengthened between 33jN and 39jN in February 1998 (the second EOF mode of upwelling index anomalies; Figs. 13B and 15A). Simultaneously, the low salinity waters of the California Current moved onshore (Castro et al., 2002; Collins et al., 2002), indicating a weakening of the California Undercurrent (Chavez et al., 2002). Our observations indicate that the main cause of changes in hydrographic variations during the 1997– 1998 El Nin˜o event was coastal waves propagating poleward along the coast, resulting in deepening of the pycnocline and accumulation of heat in the upper mixed layer. The resulting changes in SST distribution altered the pattern of wind circulation over the whole west coast of North America, but these altered wind patterns did not reinforce the El Nin˜o influence off the central and southern California. Previously published materials agree with this concept. Strub, Allen, Huyer, Smith, and Beardsley (1987) described the 1982 – 1983 El Nin˜o event off central California using the data on alongshore wind stress, sea level, and SSTA. They noted that the rise of sea level was more pronounced in the southern part of the California region as compared to its northern part, and wind stress anomalies were too weak to be responsible for the observed hydrographic variations. Chelton et al. (1982) and Chelton (1984) revealed poor correlation between the wind stress and the flow of the California Current. They noted that the maximum correlation between the flow and wind occurred when sea level

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(used as a measure of water transport) leads the winds by 3 months. Abbott and Barksdale (1991) noted no significant differences between the wind curl pattern during the 1983 El Nin˜o period compared to other years. Brink and Muench (1986) described weak correlation between the wind and sea level variability during summer period off Point Conception. Breaker and Lewis (1988) and Breaker et al. (2001) indicated the absence of consistent relationship between wind stress and sea level off central California. Spillane et al. (1987) revealed no correlation between wind stress and sea level along the US West Coast. Schwing, Moore, Ralston, and Sakuma (2000) revealed no correlation between upwelling driving winds and SST during the 1997 – 1998 El Nin˜o event, in contrast to the following 1998 – 1999 La Nin˜a period, when negative SST anomalies were significantly correlated with the upwelling index at 39jN. Dever and Winant (2002) indicated that near Point Conception SST anomalies preceded by 30 days the anomalies in the local wind stress. The observed variations of SSTA (Figs. 6– 8) illustrate that the warming of the upper mixed layer in the Southern California Bight precedes warming of the waters north of Point Conception by 2 – 3 months, as expected by propagation of coastal waves poleward. We speculate that the warm water constituting the upper mixed layer accumulated first in the Southern California Bight and then flowed north beyond the Point Conception. Chapman (1987) has demonstrated that coastal sea level fluctuations in the Southern California Bight are not strongly related to those north of the Bight, i.e., if coastal trapped waves are responsible for a significant part of the fluctuations in the flow field within the Bight, they do not necessarily propagate around Point Conception. Hickey (1992) indicated that long-period waves enter the internal part of the Southern California Bight from the south and only a portion of these waves propagates over the western sill of the Santa Monica Basin. The phase speeds of these waves are reduced in the Southern California Bight by almost an order of magnitude due to complex bottom topography. Hayward (2000) reports that both the increase of SST and the rise of sea level off San Diego preceded similar changes off central California by 2 –3 months. Some consider coastally trapped waves of minor importance as compared with local wind forcing (Mysak, 1986; Simpson, 1983, 1984). Norton and McLain (1994) speculate that coastally trapped wave propagation influences waters of about 300-m depth, whereas the upper mixed layer is more influenced by local wind forcing. However, the remotely sensed data analyzed in this study refer to the upper layer of the ocean rather than the deep part of the water column. The changes in the biological components of pelagic ecosystem corroborate the significance of remote oceanic teleconnection in the onset and maintaining of the El Nin˜o events off California. The remotely sensed phytoplankton biomass significantly decreased during the summer; chlorophyll minimum indicating higher level of nutrient limitation resulting from the deepening of the pycnocline induced

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by downwelling coastal waves (Chavez, 1996; Lynn et al., 1995; Nezlin et al., in press). An interesting example of the influence of local atmospheric forcing on the pelagic ecosystem in the absence of oceanic teleconnection during the 1982 – 1983 El Nin˜o events was observed in the western Caribbean Sea (Gonzalez et al., 2000). Coastally trapped waves do not penetrate beyond the Central America Isthmus, but the changes in the wind pattern over that region induced by El Nin˜o exert a similar influence on the pelagic ecosystem both west and east of Central America. These winds eroded the thermocline and evidently increased the CZCS-measured phytoplankton biomass in the western part of Intra-Americas Sea. This effect appears to be the opposite of that expected from atmospheric teleconnection. The results of our study do not completely reject the idea that the changes in wind pattern exert influence on the hydrographic and biological state along the Pacific Coast of North America during the El Nin˜o events. It is likely that further to the north of Central California the ENSO influence decreases (Lluch-Cota, Wooster, & Hare, 2001), and the role of atmospheric teleconnection increases when compared with oceanic teleconnection (Ramp et al., 1997), in particular, on an oceanic-wide scale (Schwing et al., 2002). However, the analysis of remotely sensed observations indicates that during the 1997 – 1998 El Nin˜o event off California, the mechanism of oceanic teleconnection (poleward propagation of the coastally trapped downwelling waves) dominated over local wind pattern induced by atmospheric teleconnection. Acknowledgements The authors would like to thank the NASA Physical Oceanography Distributed Active Archive Center at the Jet Propulsion Laboratory, California Institute of Technology for sea surface temperature data (JPL PODAAC Product 016). We also thank the CLS Space Oceanography Division for the altimeter products produced as part of the Environment and Climate EC AGORA (ENV4-CT9560113) and DUACS (ENV4-CT96-0357) projects. We thank the SeaWiFS Project (Code 970.2) and the Distributed Active Archive Center (Code 902) at the Goddard Space Flight Center, Greenbelt, MD 20771, for the distribution of NCEP wind data. We thank Dr. William M. Hamner for his critical, and extremely helpful, review of this manuscript. Two anonymous reviewers provided helpful comments and their input is greatly appreciated. The study was supported by California Seagrant (#R/CZ-156), the UCLA Institute of the Environment. References Abbott, M. R., & Barksdale, B. (1991). Phytoplankton pigment patterns and wind forcing off central California. Journal of Geophysical Research, 96, 14649 – 14667.

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