Interdecadal variability of the thermocline along the west coast of South America

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L20307, doi: /2004gl020998, 2004 Interdecadal variability of the thermocline along the west coast of South America Oscar Pizarro Departamento de Física de la Atmósfera y del Océano, COPAS & PROFC, Universidad de Concepción, Concepción, Chile Aldo Montecinos Departamento de Oceanografía, COPAS & PROFC, Universidad de Concepción, Concepción, Chile Received 13 July 2004; accepted 21 September 2004; published 20 October [1] Using gridded MBT and XBT data for the period , we analyze the interdecadal variability of the thermocline along the western coast of South America between the equator and 32 S. Results show a relatively small, but coherent, interdecadal oscillation of the depth of different isotherms representing the thermocline along the region. This oscillation is well correlated with interdecadal SST anomalies. At the end of the 1960s the thermocline was on average 10 m shallower than during the beginning of the 1980s, while SST anomalies changed from 0.3 C to0.5 C during the same period. The transition from shallow to deep thermocline depths during the mid-1970s is consistent with the change from cold to warm conditions observed at the surface. Changes in the depth of the thermocline base may modify the properties of the subsurface water that feeds the coastal upwelling, with further consequences for the ecosystem of the region. INDEX TERMS: 4215 Oceanography: General: Climate and interannual variability (3309); 4223 Oceanography: General: Descriptive and regional oceanography; 4522 Oceanography: Physical: El Nino. Citation: Pizarro, O., and A. Montecinos (2004), Interdecadal variability of the thermocline along the west coast of South America, Geophys. Res. Lett., 31, L20307, doi: / 2004GL Introduction [2] Sea surface temperature (SST) shows significant interannual and interdecadal variability along the western coast of North and South America [e.g., Montecinos et al., 2003]. While interannual SST oscillations along these regions have been directly related to ENSO dynamics, the origin of decadal and longer timescale variability in the Pacific Ocean is unclear and remains an issue of debate and controversy. One of the most plausible hypothesis to explain SST fluctuations at interdecadal timescales, point out that low-frequency disturbances of the thermocline in the eastern equatorial Pacific modulate the thermocline depth along the coast of North and South America, and these changes are, in turn, related to changes in SST [e.g., Clarke and Lebedev, 1999]. Due to coastal upwelling, SST variability off Peru and Chile may be very sensitive to subsurface changes of the thermocline depth, which may also affect surface salinity, concentration of gasses like oxygen, nitrous oxide and carbon dioxide and may modulate the nutrient supply Copyright 2004 by the American Geophysical Union /04/2004GL to the surface layer impacting biological productivity. Coastal upwelling variability is directly induced by fluctuations of the local alongshore wind stress. However, low frequency changes of the thermocline depth may modify the characteristics of the upwelled water, and thereby the properties of the water in the surface layer. This phenomenon has been recognized to be important off Peru [Barber and Chavez, 1983; Huyer et al., 1987] and off northern Chile [e.g., Morales et al., 1999; Blanco et al., 2002] during El Niño episodes. Similarly, at decadal and longer timescales, thermocline fluctuations may also modulate the variability of surface waters and can be associated with major changes in fisheries and plankton observed during the 1970s in this region [e.g., Yañez et al., 2001; Alheit and Ñiquen, 2004]. However, such long-period fluctuations of the thermocline have been poorly documented along this emblematic upwelling region. Here, based on gridded expandable bathythermographs (XBT) and mechanical bathythermographs (MBT) observations we analyze the interdecadal variability of the thermocline and its relation to SST along the western coast of South America in the period Data and Methods [3] We analyzed temperature profiles along the west coast of South America from 1955 to The data are monthly gridded values (2-degree latitude by 5-degree longitude) at standard levels prepared by the Joint Environmental Data Analysis Center (JEDAC) at Scripps Institution of Oceanography (courtesy of Dr. Warren White). These data are based on XBT and MBT observations compiled by the National Oceanographic Data Center. Details of the quality control and the objective interpolation methods used can be found at In addition, we use SST, sea level and wind data from Antofagasta ( S). Atmospheric pressure was added to the sea level records, using a scale factor of 1 cm per hpa, to form adjusted sea level (ASL) values. Time series of thermocline depth were calculated for 16 grids off the west coast of South America between the equator and 32 S. Typically, grids extend from 1 to 5 degrees of longitude offshore. Note that information is not available for the grid centered at 30 S. [4] To characterize the thermocline depth we selected two specific isotherms representing the mid-thermocline and the thermocline base for each grid point. Then, monthly anomalies of these isotherm depths were used to compute annual mean anomalies. These annual time series were then L of5

2 captured by this data set, as it is shown when a comparison with other variables (see below) is made. Figure 1. Monthly temperature time series for 3 selected grids along the western coast of South America, between the equator and 32 S, based on JEDAC XBT and MBT data (left panels). Right panels show the long-term mean temperature profiles. Circles, asterisks, and triangles indicate the mean depth of the isotherms that are representatives of the mid thermocline (Z m, maximum gradient), the base of the upper thermocline (Z b ), and the minimum temperature gradient (Z p ) within the upper 400 m, respectively. The lines in the left panels show the depth of these selected isotherms. smoothed using a 7-year running mean applied twice to obtain the interdecadal time series [cf. Montecinos et al., 2003]. The same filter was applied to other variables. Finally, interdecadal time series at 30 S were obtained by linear interpolation of the time series at the neighbor grids. Note that positive (negative) thermocline-depth anomalies correspond to shallower (deeper) than normal thermocline levels. [5] Monthly temperature time series and their mean profiles for 3 selected grids are shown in Figure 1. Based on the mean profiles we selected isotherms to represent the middle and the base of the thermocline for each grid point. For the mid-thermocline we chose the isotherm with the largest vertical temperature gradient (denoted here by Z m ). When the isotherm representing the mid-thermocline reached the surface, we assumed Z m to be equal to zero. The isotherm representative of the thermocline base was defined as the shallower isotherm (below the surface mixed layer) where the gradient was smaller than 0.03 C m 1 (denoted by Z b ). On the other hand, below the upper thermocline near m depth, the vertical gradient of temperature decreases, creating a local minimum. This is a common feature in eastern ocean boundaries, related to a downward bending of the subsurface isotherms near the coast in connection with an alongshore poleward subsurface flow. Consequently, we also considered an isotherm representative of this minimum gradient observed below the surface layer for each grid point (denoted by Z p ). Lack of data during a number of years in some of the coastal grids is an important limitation time series in Figure 1 illustrate the typical data density along the study region, but we consider that interdecadal variability is relatively well 3. Interdecadal Thermocline Variability [6] Most of the time series of Z m, Z b, and Z p show positive trends, indicating that the depth of the thermocline has increased along the western coast of South America during the last 50 years. The mean Z b trend for the whole study region is 0.34 m yr 1 (17 m per 50 years). However, the trends at different latitudes differ considerably, varying from 0.60 m year 1 (30 m in 50 years) at 6 S, to 0.12 m year 1 (6 m in 50 years) at 12 S. Thus, to analyze interdecadal fluctuations we detrended the annual time series of Z m,z b and Z p. We use the standard EOF analysis to obtain a common mode of variability of the thermocline in the study region. [7] The first principal components (PC1s) of Z m,z b, and Z p explain 52%, 73%, and 60% of the total interdecadal variance, respectively (Figure 2a). The three PC1s are relatively well correlated (r = 0.89 and r = 0.90 for PC1 Z m -PC1 Z b, and PC1 Z b -PC1 Z p respectively, and r = 0.66 for PC1 Z m -PC1 Z p ). Figure 2a shows that the thermocline was shallower than average during the period and after 1988, while it was deeper between 1976 and The correlation between the PC1 and the original interdecadal time series as a function of latitude is shown in Figure 2b. The spatial pattern shows that the PC1 Z b is relatively well correlated with the original interdecadal time series, presenting significant values along the whole region (correlations higher than 0.55 are significantly different Figure 2. First principal components (PC1) of the interdecadal time series of the depth of three selected isotherms (Z m,z b, and Z p see Figure 1) from coastal grids along the western coast of South America (a). Alongshore structure of the first EOF mode (b), presented as correlation between the PC1 and the respective time series. Standard deviation of the observed time series for each grid point (c). In all figures the circles, asterisks, and triangles are associated with Z m,z b, and Z p time series, respectively. 2of5

3 from zero at 95% confidence, using an estimated integral timescale of 2.75 years). PC1 Z p presents larger correlations near the equator, decreasing southward, while PC1 Z m is well correlated at subtropical latitudes, decreasing equatorward. On the other hand, the interdecadal variability of Z b remains rather constant along the coast with a standard deviation of about 5 m (Figure 2c), while the amplitude of Z m is small near the equator increasing to about one-half the amplitude of Z b off the equatorial region. In contrast, the Z p variability tends to decrease from equatorial to subtropical latitudes, except in the last grid, where it shows the largest standard deviation. Therefore, the spatial pattern of the first EOF mode and the relatively constant interdecadal standard deviation of Z b along the coast indicate that the PC1 Z b is the best index representing the common interdecadal variability of the thermocline along the tropical and subtropical western coast of South America. [8] Long records of data along the Chilean coast are scarce. However, near Antofagasta a time series of coastal temperature and sea level, as well as meteorological data from the commercial airport, are long enough to address low frequency variability. Additionally, given the importance of pelagic fisheries during the last decades, the region off northern Chile is one of the best-sampled regions of the eastern South Pacific in terms of hydrography; thereby we have there a relatively independent data set to compare our results. Figure 3 shows the interdecadal time series of meridional (near alongshore) wind, ASL and coastal SST anomalies observed at Antofagasta during part of the second half of the last century. The time series of Z b in the grid centered at 24 S is also shown, together with the PC1 Z b.to us, the most striking feature of the figure is that interdecadal SST and ASL anomalies apparently do not change in response to the upwelling favorable wind. In fact, the maximum equatorward wind observed at the beginning of the 1980s coincides with a maximum in SST and ASL. On the other hand, SST and ASL anomalies are (inversely) well correlated with the depth of the thermocline base anomalies (r = 0.86 and r = 0.78 for SST and Z b, and for ASL and Z b, respectively). At this location, the interdecadal Z m and Z b time series are well represented by the PC1s, as derived from the correlation shown in Figure 2b. This is also the case for SST changes along the west coast of South America, as shown by Montecinos et al. [2003]. These authors studied the interdecadal variability of SST by mean of EOF analysis applied to data from 9 coastal stations between 5 S and 37 S. A linear regression between SST at Antofagasta and Z b at 24 S showed that for interdecadal timescales an increment of the depth of the thermocline base in 1.1 m involves an increment of the SST in 0.1 C(r= 0.90). Since both SST [Montecinos et al., 2003] and Z b interdecadal time series show relatively constant variance and EOF structure along the coast, a similar relation between these variables should be valid at other latitudes in our study region. [9] We also analyzed the zonal wind stress along the equatorial Pacific as a possible forcing of the interdecadal variability observed along the west coast of South America. Wind stress was estimated using the difference of the sea level pressure (SLP) between the eastern and the central equatorial Pacific, according to the methodology proposed by Clarke and Lebedev [1997]. The SLP at each region was Figure 3. Interdecadal time series of meridional wind (a), sea level and SST (b) anomalies near 23.5 S, and interdecadal time series of Z b for the coastal grid centered at 24 S (c). Thin line in (c) is the reconstructed Z b for this location using the first EOF mode. Bottom panel (d) shows a simulated thermocline depth in the eastern equatorial Pacific based on zonal wind stress anomalies estimated from the zonal sea level pressure gradient between the eastern and the central equatorial Pacific (see text for details). computed by averaging the Smith and Reynolds [2004]- reconstructed SLP data inside boxes delimited by 2 N and 2 S, and 170 E and the date line (central box), and 100 W and 90 W (eastern box). Based on the estimated wind stress, we computed changes of the thermocline depth in the eastern tropical Pacific (Z eq ) using the relationship proposed by Clarke and Lebedev [1999]. According to this relation, long timescale fluctuations of Z eq are proportional to the integral of the zonal wind stress along the equatorial Pacific. Effectively, Z eq presents interdecadal changes with a minimum depth during the mid-1970s and a maximum at the beginning of the 1980s that are notably similar to those observed in coastal SST (with opposite sign) and Z b (Figure 3d). [10] Hydrographic data from off northern Chile (19 24 S, W) were also used to estimate interdecadal changes of the base of the thermocline (Z bh ). Large gaps in the data prevented the possibility of having a continuous interdecadal time series. Thus, based on our previous results, we calculated average of Z bh on these data for different periods (Figure 4). These results are consistent with those based on the JEDAC data shown in Figure 2a. 4. Discussion and Conclusions [11] During the second half of the last century, the thermocline along the west coast of South America presented relatively small but significant, and spatially coherent interdecadal variability. At this timescale, the 3of5

4 depth of the thermocline base changed about 10 m, thus, during the early 1980s Z b was about 5 m deeper than average, coinciding with a period of positive SST anomalies of about 0.6 C. In contrast, during the end of the 1960s, the thermocline was shallower and SST presented anomalies of about 0.3 C. The transition from positive (negative) to negative (positive) anomalies of the thermocline depth (SST) occurred during mid-1970s, while an inverse transition seems to have occurred during the second half of the 1980s. The mid-1970s change has been widely documented, and is frequently identified as a climate regime shift, while a most recent change to cold conditions during the 1990s is a matter of present investigation [e.g., Chavez et al., 2003]. Such a transition in the eastern South Pacific is also suggested by our results. [12] At interdecadal timescales, an increasing of the coastal SST in 0.1 C corresponds to a deepening of the thermocline of about 1.1 m within a coastal region extending a few hundred kilometers offshore. Such a change is in agreement with the variability of the eastern equatorial Pacific thermocline depth, estimated from the zonal wind anomalies in the equatorial Pacific. In contrast, changes of the local wind at Antofagasta (Figure 3a) and the integral of the alongshore wind stress from the equator to 24 S (based on COADS data, not shown) are not able to reproduce the observed interdecadal thermocline fluctuations. These results support the idea [e.g., Clarke and Lebedev, 1999] that long time oscillations of the thermocline depth, as well as SST, along the Pacific coast of the Americas are controlled by changes in the equatorial zonal wind. [13] Montecinos et al. [2003] showed that the relative importance of the interdecadal SST signal increases with latitude in the subtropical region, reaching similar values as those for the interannual SST signal. Here, the variability (standard deviation) of Z b and Z p between the equator and 10 S at interannual timescales is about 4 5 times larger than that observed at interdecadal timescales. Farther south, this variability is reduced to about 2 3 times the interdecadal one. It is worth noting that the correlation between the PC1 SST and PC1 Zb at interannual timescales is also highly significant (r = 0.85). [14] If the observed interdecadal fluctuations of the thermocline depth were responsible for a coherent and persistent change of the SST in the region, then these Figure 4. Annual mean values of the thermocline base off northern Chile estimated from hydrographic data collected within a grid that extends from 19 S to23 Sand from 72 W and 73 W. Horizontal lines represent the mean value for different periods. The thick curve shows the PC1 Z b estimated using JEDAC XBT and MBT data as in Figure 2. Figure 5. Annual landing of anchovy (solid line) and sardine (dashed line) from northern Chile between about 18.5 S and 24 S [from Yañez et al., 2001]. Low-pass filtered time series for the same species are shown by the smooth lines. changes may also have an impact on the nutrient upwelling and on other variables that regulate biological productivity and plankton distribution, and so modulate the abundance and distribution of commercial fisheries. Figure 5 shows the annual landings of anchovy and sardine in northern Chile. These fisheries have experienced long time changes during the last 50 years. A comparison of the interdecadal time series of the anchovy and sardine landings with Z b shows that a shallower thermocline depth is related to an increase of anchovy catches, while the sardine catches seem to fluctuate with an opposite phase. These results are consistent with the basin-wide ecosystem response presented by Chavez et al. [2003]. [15] Here we have quantified the magnitude of the interdecadal variability of the thermocline in the eastern South Pacific, and showed that these changes are of the order of a few meters. They are rather small compared with the interannual oscillations observed commonly in this region which are of the order of several tens of meters, but still they have a significant imprint on the SST. It will be exciting to search for the influence of these small, but long-lasting, changes of the thermocline depth on the regional fishery fluctuations. [16] Acknowledgments. Gridded data were obtained from the Joint Environmental Data Analysis Center. The Hydrographic and Oceanographic Service of the Chilean Navy (SHOA) and the Chilean Meteorological Service (DMC) provided coastal data. This work was supported by grants from the Chilean National Research Council (FONDECYT , FIP and FONDAP COPAS) and Fundación Andes, Chile. AM was supported by a MECESUP/UCO-0002 graduate scholarship. References Alheit, J., and M. Ñiquen (2004), Regime shifts in the Humboldt Current ecosystem, Prog. Oceanogr., 60, Barber, R. T., and F. P. Chavez (1983), Biological consequences of El Niño, Science, 222, Blanco, J. L., M.-E. Carr, A. C. Thomas, and P. T. Strub (2002), Hydrographic conditions off northern Chile during the La Niña and El Niño events, J. Geophys. Res., 107(C3), 3017, doi: /2001jc Chavez, F. P., J. Ryan, S. E. Luch-Cota, and M. Ñiquen (2003), From anchovies to sardines and back: Multidecadal change in the Pacific Ocean, Science, 299, Clarke, A. J., and A. 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5 Huyer, A., R. L. Smith, and T. Paluszkiewicz (1987), Coastal upwelling off Peru during normal and El Niño times, J. Geophys. Res., 92, 14,297 14,307. Montecinos, A., S. Purca, and O. Pizarro (2003), Interanual-to-interdecadal sea surface temperature variability along the western coast of South America, Geophys. Res. Lett., 30(11), 1570, doi: / 2003GL Morales, C. M., S. Hormazabal, and J. L. Blanco (1999), Interannual variability in the mesoscale distribution of the depth of the upper boundary of the oxygen minimum layer off northern Chile (18 24 S): Implications for the pelagic system and biogeochemical cycling, J. Mar. Res., 57, Smith, T. M., and R. W. Reynolds (2004), Extended reconstruction of oceanic sea-level pressure based on COADS and station data ( ), J. Atmos. Oceanic Technol., 21, Yañez, E., M. A. Barbieri, C. Silva et al. (2001), Climate variability and pelagic fisheries in northern Chile, Prog. Oceanogr., 49, A. Montecinos, Departamento de Oceanografía, COPAS & PROFC, Universidad de Concepción, Concepción, Chile. O. Pizarro, Departamento de Física de la Atmósfera y del Océano, COPAS & PROFC, Universidad de Concepción, Concepción, Chile. (orpa@profc.udec.cl) 5of5