versão On-line ISSN 0717-6538
Gayana (Concepc.) v.68 n.2 supl.TIProc Concepción 2004
Gayana 68(2) supl. t.I. Proc. : 48-53, 2004 ISSN 0717-652X
FEATURING ENSO 1997-2000 IN CENTRAL CHILE
Mónica Bello1, Manuel Castillo1, Jenny Maturana1, Claudia Valenzuela1 & M Angela Barbieri2,3
1. Servicio Hidrográfico y Oceanográfico de la Armada, email: firstname.lastname@example.org
Using satellite images of the area between 32-36°S and 71-75°W, from October 1997 to September 1999, temporal and spatial variability of sea surface temperature (SST) was analyzed. During this period of time, one of the most intense episode of the last century of the warm phase (El Niño) and the beginning of the cold phase (La Niña) of the ENSO cycle were registered, allowing to identify differences in the thermal surface structure. During the warm phase, cold water isotherms in the coastal region (upwelling) warmer than usual were presented, and temperature gradients offshore were smaller than in the cold phase.
Several reports suggest that a large part of the Eastern Ocean Pacific Boundary variability (particularly at low frequencies) has equatorial origin. This research shows that part of the observed fluctuations in sea level at Callao, are due to intraseasonal oscillations likely modulated by perturbations of equatorial origin propagating as coastal trapped waves at the eastern boundary of the Pacific Ocean, having a signature in the currents off Valparaiso as a poleward flow, whose core exhibits higher salinity and low oxygen located at 180 m depth. On the other hand, geostrophic currents off Valparaiso consists of alongshore currents and countercurrents, with increasing speeds at fall.
In the period 1980 - 2000, the Pacific Ocean experienced 4 round ENSO phases (warm-cold). Within this period occurred the warm events 1982-83 and 1997-98, which were the most intense in the last 50 years. Also, the most intense cold phases happened between 1988-89 and 1998-2000. The high variability of ENSO events appears at comparing the shortness of the cold phase 1997-98 (around 1 year long) with the unusually long time of the following cold phase (1998-2000) (McPhaden, 1999).
The understanding of ENSO cycle is currently related to equatorial waves propagation such as Kelvin waves and long Rossby waves (McPhaden, 2001). The propagation of this waves along the equatorial Pacific causes the rises and falls of the equatorial thermocline (tens of meters), with a surface signal in the sea level, as well as a change in intensity and direction of ocean currents (Clarke and Van Gorder, 1994; Shaffer et al., 1997).
This dynamics has been studied by several authors, who indicate that most of the sea surface temperature and currents variability along the coasts of Peru and Chile has intraseasonal fluctuations remotely forced (Huyer et al., 1991; Shaffer et al., 1997; Pizarro, 1999).
Due to the importance that ENSO 1997-2000 effects had in Chile, mostly reflected by damage in agriculture, road infrastructure, fisheries, among others, the Servicio Hidrográfico y Oceanográfico de la Armada (SHOA) implemented since 1999, a monitoring of the oceanographic characteristics off central Chile (Valparaiso), through oceanographic cruises made every 6 months called "ENSO cruises".
This research has the goal of identifying in central Chile (32-36°S), through satellite images obtained between 1997 and 1999, differences in the surface thermal structure between the warm and cold phases of ENSO 1997-2000. On the other hand, through analysis of sea level at coastal stations in Peru (Callao) and Chile (Arica and Valparaiso), to evaluate the occurrence of possible intraseasonal fluctuations remotely forced, and to describe oceanographic conditions off Valparaiso during the cold phase of this phenomenon.
SST Satellite Images
A number of 357 daily sea surface temperature (SST) satellite images of the Advanced Very High Resolution Radiometer (AVHRR) from NOAA's satellite (National Oceanic and Atmospheric Administration), obtained at the Remote Sensing Laboratory Pontificia Universidad Catolica de Valparaiso (PUCV) were analyzed in this research. Analysis were conducted in two periods, the first one from october 1997 to september 1998 and the second one from october 1998 to september 1999. SST images covered an area ranging from 32°S to 36°S and 71W to 75°W, gridded every 1 nautical mile.
From daily SST images, monthly averages were computed and temperature profiles obtained at 3, 10, 30 and 60 miles offshore to the coast. This signal was high frequency filtered and visualized in space and time in order to identify cold and warm areas and their corresponding extension offshore.
Daily averages of sea level at Callao (12 03'S, 077 09'W), Arica (18 29'S, 070 18'W) and Valparaiso (33 02'S, 071 38'W) were used. Sea level at Callao was obtained from Sea Level Center, University of Hawaii web site (http://www.soest.hawaii.edu/kilonsky/uhslc.html), and sea level at Arica and Valparaiso were provided by Servicio Hidrográfico y Oceanográfico de la Armada de Chile. Information from the chilean coasts were adjusted by atmospheric pressure using data from National Center for Environmental Prediction (NCEP) and from National Center for Atmospheric Research (NCAR)1.
To analyze intraseasonal fluctuations a 361 weights Lanczos cosine log-pass filter was used, allowing the pass of 20 and 120 days. Filtered series will be denoted as sea level hereinafter . To calculate phase speed of sea level fluctuations, a cross correlation analysis was performed among tide gauge stations at Callao (CLL), Arica (ARI) and Valparaiso (VAL). By knowing distance between stations and maximum correlation lag phase speed was calculated. Another way to obtain phase speed, in frequency domain, is through adjusting significant phases spectrum, so coherence and phase spectrums were computed.
Remote origin of low frequency fluctuations was detected through variability of the 20C (ISO20) isotherm depth at 110W and its relationship with sea level perturbations at the south American coasts. This information was obtained from TOGA-TAO Project Office, NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington (http://www.pmel.noaa.gov/tao/data_deliv/deliv-nojava.html) which is a result of linear interpolation of eulerian temperature observations.
From 1999 SHOA started a periodic monitoring program consisting of oceanographic observations off Valparaiso, called "ENSO cruises" (Table 1). These observations were originated due to the unusually cold conditions at the tropical Pacific. Cruises extended 200 nautical miles (nm) offshore with a total of 22 stations each one. Temperature, salinity and dissolved oxygen measurements at 1000 m depth were obtained with a CTD-SBE19. Results are presented as temperature, salinity, dissolved oxygen and geostrophic velocity sections (calculated at a reference level of 1000 db).
The time series of mean temperatures from satellites clearly identify the dominance of the annual signal, with maximum temperatures occurring during the summer season. In the second and first periods a difference of 1,8°C was observed, mainly due to the influence of El Niño 1997-98 and La Niña 1998-99.
In Fig. 1, the latitudinal and -temporal variability of monthly SST, 3 nm offshore it is shown, being clear the annual signal previously indicated. Highest temperatures occurred between january and may 1998 in the area 32S to 34S, ranging from 16C to 19C, associated to the development of El Niño 1997-98. In the second period maximum temperatures (16°C) occurred in January and February (effects of La Niña 1998-2000). When monthly SST profiles at 3, 10, 30 and 60 nm offshore are compared among them, a similar pattern in terms of warmer and colder periods appears. Likewise, a gradual increase in temperature from 3 to 60 nm offshore is observed.
Temperatures of 15C and 13°C were the typical values in both periods. Using the latitudinal distribution of the characteristic isotherms of each period and its occurrence frequency at 3, 10 and 30 nm offshore, the three most frequent areas of colder water were determined This information was useful to identify upwelling areas: Valparaiso (32,5-33,1°S), whose core is located at punta Curaumilla; south of San Antonio (33,5-34,2°S), whose core is at punta Topocalma, and south of Constitucion (34,9-35,9°S) whose core is at cabo Carranza and punta Nugurne. In both periods, the least observed area was at the northern part. The second upwelling area, showed similar frequencies of observation than the third area during the first period, while in the second period, the area located at the south showed higher frequencies.
Figure. 1: Temporal series of SST derived from
Equatorial thermocline variability
The ISO20 series, showed high variability during 1997 (Fig. 2) with displacements of about 50 meters around the average, with peaks in February, May and July. This activity tends to diminish at the beginning of 1998. Nevertheless, in the second half of that year important thermocline falls also appear, particularly in june and july. During 1999, four 90 days period waves were observed, with amplitudes of 20 to 30 meters. Fewer perturbations in the isotherm were present during the first half of 2000 than in 1997-99 period, nevertheless, descents of ISO20 still are present during January-February, May and June. The second half of 2000 showed an increase in the activity of intraseasonal perturbations, with evident falls of the ISO20 in September, October and mid December.
Sea level low frequency propagation
Intraseasonal sea level series (Fig. 2) showed perturbations of increased amplitude during 1997 and beginning 1998 (maximum ranges of 0.1 to 0.2 m). During 1998 and 1999 important perturbations were not registered (not as the previously described), but the activity of these oscillations increased by the middle of 2000. It is noteworthy that not all the sea level fluctuations observed in Callao and Valparaiso are shown in Arica, which is consistent with the teleconnection phenomenon described by Hormazábal et al. (2002) for the same region.
To establish the phase speeds of these oscillations, cross correlations were computed for the series CLL-VAL and ARI-VAL, where correlations were significant at 99% (Davis, 1976). This analysis showed a 4 days lag for CLL-VAL and 10 days lag for ARI-VAL, considering the distance between these stations, velocities of 256.9 Km d-1 between CLL-VAL and 265.0 Km d-1 for ARI-VAL were obtained.
Complementary, since correlation analysis considers all frequencies, phase speeds were calculated through phase spectrum with only significant coherences, for the same series used in the correlation analysis, obtaining velocities of 221.6 Km d-1 (CLL-VAL) and 221.00 Km d-1 (ARI-VAL).
Figure. 2: Time series of 20C isotherm depth at 110W in the equator and low-frequency elevation of the sea level in Callao, Arica and Valparaiso during the 1997-2000 period.
Oceanographic conditions off Valparaiso
The geostrophic velocities obtained for the cruises conducted during autumn (ENSO 1 and 3) showed an alongshore currents and countercurrents system off Valparaiso (Fig. 3). In ENSO 1 there was a poleward flow at the coast in the first 40 nm, with maximum surface speeds of 16 cm s-1. At ENSO 3 this flow was located between 40 to 80 mn from the coast, with maximum speeds of 16 cm s-1 at 150 meter depth. Farther from this poleward flow, there is an equatorward flow, that in ENSO 1 is located between 45 and 65 mn, until 50 meter depth, with maximum surface speeds of 4 cm s-1. During ENSO 3 this flow was located between 80 and 150 mn, extending until 300 m depth approximately, and with surface speeds of 20 cm s-1.
The geostrophic velocities calculated for spring (ENSO 2 and 4) showed different patterns than those observed in autumn, because in the 80 nm closer to the coast a northward flow was present in both cruises. ENSO 4 was characterized by an increase of the flow, with maximums of 22 cm s-1 until 200 meter depth. In ENSO, a poleward flow was registered between 80 and 120 nm, with velocities ranging between 2 and 4 cm s-1, until 400 meter depth. From 130 and 200 nm offshore an equatorward flow with speed over 8 cm s-1, reached a depth of 250 m.
Figure. 3: Vertical distribution of geostrophic velocity field in front of Valparaíso Bay (red means northward and blue southward).
From October 1997 to May 1998, surface thermal structure was warmer than a normal year, due to the effects of El Niño 1997-98. This warm event was described by McPhaden (1999) as one of the most intense of the century, with global scale climate implications. The intensity of this event began to diminish in June 1998, stepping into a surface structure colder than normal, which prevailed until the end of this research period. La Niña 1998-2000 was more important because of its duration than due to its intensity (Fedorov and Philander, 2000).
Monthly mean temperature distribution indicated a difference of 2°C approximately between both periods, where the first one was the warmest (warm event). This difference is coherent with the selection of representative isotherms for every period (15C and 13°C) allowing the identification of three upwelling centers in the research area.
The first upwelling area included an important upwelling center in punta Curaumilla (Valparaiso, 33°S) which was recognized by Fonseca and Farías (1987). In both periods this area reported fewer frequencies than the one identified at the south. During the cold phase, an increased frequency of the representative isotherm until 10 nm offshore was observed.
From the south of San Antonio (33°30'S) until punta Topocalma (34°07'S) the second upwelling area was identified (Fonseca and Farías, 1987). In terms of frequency, during the warm event there were observed similar percentages than in the third area; meanwhile, during the cold phase, it was possible to differentiate it as the most frequent second area.
The third upwelling area extended from punta Duao (34°53'S) to punta Nugurne (35°57'S). Brandhorst (1971) described Punta Nugurne as an upwelling center. During the cold phase, this area was the most frequent and the most longitudinally extense in upwelled water.
If there is a difference in frequencies of the representative isotherms during cold and warm phases, it doesn't imply that in the warm event upwelled water at the coast was absent. These results are consistent with Peruvian coastal upwelling reports between 1981 and 1984, considering a normal period of time and an El Niño event (Huyer et al. 1987). During El Niño 1982-83 upwelling favorable winds were reported, generating a rise of surface waters characterized by being warmer than those of a normal year.
Most of the intraseasonal low frequency variability (50 days period) in sea surface temperature records, sea level and currents off Peru and northern-central Chile have an equatorial origin (Huyer et al., 1991, Shaffer et al., 1997 and Pizarro, 1999). In the interannual scale this equatorial influence is highly modulated by the El Niño/La Niña cycle.
The influence of the Peru-Chile Undercurrent in the Chilean coasts dynamics has been widely studied, being identified as a poleward flow whose core is located at 150 meter depth, characterized by higher salinity, low dissolved oxygen content and with a variability associated to equatorial originated oscillations (Huyer,1991; Shaffer et al., 1997).
At ENSO 3 cruise, there was a poleward flow of 14 cm s-1, with its core at 180 meter depth approximately. Temperature profiles showed a separation of the isotherms at the coastal zone, in coincidence with the high salinity and low oxygen content core. A more detailed analysis of the intraseasonal oscillations showed the sinking of the 11C isotherm at the coast, associated to a rising in sea level generated by a northerly traveling perturbation. Due to this perturbation it is registered at Callao 14 days before, according to the coastal trapped waves propagation theory.
In general, the geostrophic field of velocities obtained in ENSO cruises showed an alongshore currents and countercurrents system off Valparaiso (Fig. 2). Nevertheless, differences between Autumn and Spring were noticed, where currents varied in intensity and distance offshore between seasons, being the most intense flows observed in autumn. Previous publications have reported the poleward undercurrent located over the continental shelf and slope (Brandhorst, 1971; Huyer et. al., 1991). Results reported in this research, shows that Autumn cruises exhibits poleward flows close to the coast with maximum velocities around 150 meter depth. Also, ENOS 1 and 3 showed a relaxation in the poleward flow and seemed to be far from the coast, associated to the Peru-Chile Countercurrent. This surface poleward current, travels from 8-35°S at 100-300 km offshore (Strub et al., 1995).
Special recognition is presented to the remote Sensing Laboratory at Pontificia Universidad Católica de Valparaíso for having available daily sea surface temperature satellite images and to the Servicio Hidrográfico y Oceanográfico de la Armada de Chile (SHOA) for authorizing the use of sea level information. Also to Dr. Oscar Pizarro from Regional Physical Oceanography and Climate Program at Universidad de Concepción, for the appreciated contributions received in the realization of this research.
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