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A comparison of spring coastal upwelling off central Chile at the extremes of the 1996–1997 ENSO cycle
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Continental Shelf Research 24 (2004) 773–787
A comparison of spring coastal upwelling off central Chile at the extremes of the 1996–1997 ENSO cycle ! c, Sergio A. Vegad Jose! A. Rutllanta,*, Italo Masottib, Janette Calderon a Departamento de Geologia y Geof!ısica, Universidad de Chile, Casilla, Santiago 2777, Chile Laboratoire de Biogeochimie et Chimie Marines (LBCM), Universit!e Pierre et Marie Curie, Case courrier 134, 4, place Jussieu, 75252 Paris Cedex 05, France c ! Meteorologica ! Direccion de Chile, Casilla 63 (Correo Internacional), Santiago, Chile d Laboratoire d’Etudes en Geophysique et Oceanographie Spatiales, 14, Avenue Edouard Belin, Toulouse 31400, Cedex 04, France
b
Received 26 June 2002; received in revised form 22 August 2003; accepted 4 February 2004
Abstract Southerly wind pulses are repeatedly observed in connection with upwelling off Point Curaumilla (33 S), around 20 km south of Valpara!ıso. The remote forcing of coastal upwelling associated with intraseasonal coastal-trapped waves and the local, wind-driven upwelling forcing, are characterised there in terms of sea level, meteorological observations, surface weather charts and satellite-derived SST and chlorophyll a, from observations taken in late austral springs of 1996 (La Nin˜a) and 1997 (El Nin˜o). Warm and cold coastal SST periods lasting from 15 to 20 days are associated with intraseasonal (30–50-day periods) coastal-trapped waves which are detected in time series of adjusted sea level. These waves produce shoaling/deepening cycles in the thermocline depth and hence a modulation of the local wind-driven upwelling efficiency in bringing cold, nutrient-rich waters to the surface. The atmospheric forcing of the wind-driven upwelling pulses is closely related to atmospheric coastal-trapped disturbances (coastal lows) in which the active/relaxed phase of the upwelling occurs ahead of/after the sea-level pressure minimum with clear/overcast skies. Time series of the alongshore pseudo-wind-stress at two coastal locations reveal that the local atmospheric forcing is always active and largely independent of the thermal phase of both the intraseasonal and interannual (ENSO) cycles. Similar differences in satellite derived sea-surface temperatures at the extremes of both the ENSO and intraseasonal cycles of 2–3 C were observed, although upwelling within cold intraseasonal periods was restricted to a narrower coastal strip during El Nin˜o than during La Nin˜a. r 2004 Elsevier Ltd. All rights reserved. Keywords: Humboldt current system; Coastal upwelling; ENSO cycle; Coastal-trapped waves; Valpara!ıso; Chile
1. Introduction During the warm phase of the ENSO cycle (El Nin˜o events) the poleward counter-current along *Corresponding author. Fax: +56-2-6968686. E-mail address: [email protected] (J.A. Rutllant).
western South America strengthens (Kessler and McPhaden, 1995) while the atmospheric subtropical anticyclone, a key constituent of the Southern Oscillation, weakens. These conditions increase the advection and onshore entrainment of warmer waters, producing a rise in the sea level and a corresponding deepening of the nutrient-rich, O2
0278-4343/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2004.02.005
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depleted equatorial sub-surface waters beneath the thermocline. For example, the thermocline off Antofagasta (23 S) deepened from approximately 50 m in January 1997 (pre-El Nin˜o condition) to about 200 m depth during the fully developed El Nin˜o in July 1997 (Gonzalez et al., 1998). At the opposite phase of the ENSO cycle (La Nin˜a) the thermocline shoals and the average sea level decreases, as the Humboldt coastal current and the subtropical gyre speed-up. These interannual changes in the physical environment have a severe impact on fisheries; in fact, anchovy landings of around 1400 thousand tons in 1986 and in 1998 (La Nin˜a years) dropped to about 200 thousand tons in 1987 and 1997 El Nin˜o’s (Yan˜ez et al., 2001). Superimposed on this interannual cycle, intraseasonal (50-day) oscillations in the thermocline depth, associated with equatorially sourced coastal-trapped waves (CTWs), have been reported off the coast of Peru (Enfield, 1987; Kessler and McPhaden, 1995) and Chile (Shaffer et al., 1997). Those CTWs seem to be more prominent in austral summer, particularly at the onset of El Nin˜o events (Shaffer et al., 1997). In the presence of these CTWs, upwelling-favourable winds become more or less effective in bringing nutrientrich cold waters to the surface as a function of the associated thermocline depth fluctuations on intraseasonal to interannual time scales (e.g. Strub et al., 1998). The upwelling regime off central Chile (27 – 37 S) is principally driven by upwelling-favourable (equatorward) wind events lasting 2–3 days with a periodicity of around one event per week (Rutllant, 1993; Rutllant and Montecino, 2002). Farther north upwelling is in general a year-round phenomenon, while seasonality increases as we move into south-central Chile (35–40 S) where upwelling events are concentrated in late spring and summer (e.g. Djurfeldt, 1989; Silva, 1973). Meteorological conditions leading to upwellingfavourable wind events at Punta Lengua de Vaca (30 S) have been identified by Rutllant (1993, 1994) and Rutllant and Montecino (2002). Those southerly (equatorward) wind events are due to an enhanced anticyclonic circulation after the passage of mid-latitude frontal disturbances and the
subsequent southward propagation of coastaltrapped disturbances (coastal lows). After the passage of the minimum in the sea-level pressure, skies usually change from clear to overcast, winds relax and often veer to NW, advecting the nutrient-rich water onshore as it downwells against the shelf. The wind relaxation and onshore advection increase the stability of the water column, favouring phytoplankton growth. This phenomenon seems to be particularly significant in bays adjacent to the main headlands upon which coastal upwelling activity concentrates. This is the case of the Valparaiso (33 S) bay area and Punta Curaumilla (Fig. 1), according to Fonseca and Far!ıas (1987). Oceanographic observations at the Valpara!ıso bay were performed in November 1996 and 1997 to assess the coupled day-to-day variability between the upwelling focus at Punta Curaumilla and the bay area. A strong modulation of the thermocline depth by poleward-propagating intraseasonal sea-level fluctuations was observed there, with associated changes in SST, salinity and dissolved oxygen (Vega et al., 2000). Here, we present an assessment of the meteorological aspects of the local, wind-driven upwelling forcing at the peak of the upwelling season in opposing phases of both intraseasonal and ENSO cycles. Satellite SST and ocean-colour imagery, while illustrating average conditions within oceanic waters associated with the extremes of these cycles, describe the ocean response off Punta Curaumilla to the short-term wind variability. The study area, meteorological and satellite data are described in Section 2. Section 3 presents the results followed by a summary and discussion in Section 4.
2. Study area and methods The study area (Fig. 1) encompasses central Chile including the upwelling centre off Punta Curaumilla (PC) and the Valpara!ıso Bay, located around 25 km northeastward of PC. The remote forcing component associated with intraseasonal CTWs was assessed through sealevel data (Vega et al., 2000). Quality-controlled
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Fig. 1. Study area around central Chile (west coast of South America).
sea-level data for both study periods (1996 and 1997) were obtained from tide gauges at Antofagasta (23 390 S) and Valpara!ıso (33 020 S) from the TOGA Sea-Level Centre at the University of Hawaii, USA. The hourly time series were subsequently corrected for atmospheric pressure variations and then low-pass filtered, resulting into an adjusted sea level, as defined in Shaffer et al. (1997). Daily SSTs from AVHRR images (NOAA-14) at 18:00–20:00 UTC with 1.1 km resolution, stored at the Centre for Space Studies of the Universidad de Chile (CEE), were analysed to characterise the space and time variability of the SST field around PC during the 1996–1997 experiments. Images
with more than 30% of the area covered by clouds were discarded. The processed images (see Fig. 8) encompass an area of 200 200 km between latitudes 31.6 and 33.4 S, and longitudes from 71.1 to 73.2 W. For the 1997 experiment chlorophyll a at the ocean surface was assessed through ocean-colour images from SeaWiFS (Seaviewing-Wide Field-of-view Sensor). This sensor operates within the range of visible to near infrared with a resolution of 1.1 km. Images were received and processed at the CEE isolating the same region as in the AVHRR images. From level L1a data, surface chlorophyll a concentrations were calculated through SeaDAS (SeaWiFS Data Analysis System, version 3.2) furnished by the
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Table 1 Selected AVHRR sea-surface temperature (SST) and SeaWiFS ocean colour (OC) satellite images with less than 30% of lowcloud cover collected during the field experiments SST 1996
SST 1997
OC 1997
10 18 22 29 02 06 09
30 09 21 26 30 05 13 27
31 10 14 21 30 02 06 12 28
Nov, 15 Nov Nov, 19 Nov Nov, 23 Nov Nov, 30 Nov Dec, 05 Dec Dec, 07 Dec Dec
Oct, 07 Nov Nov, 13 Nov Nov, 22 Nov Nov, 27 Nov Nov, 03 Dec Dec, 07 Dec Dec, 22 Dec Dec, 31 Dec
Oct, 07 Nov Nov, 13 Nov Nov, 15 Nov Nov, 27 Nov Nov, 01 Dec Dec, 03 Dec Dec, 07 Dec Dec, 27 Dec Dec
National Aeronautics and Space Administration (NASA). Table 1 lists the selected dates of available satellite imagery for both 1996 and 1997 field experiments. The SST fields in degrees Celsius were obtained through thermal channels 4 and 5, following the empirical algorithm developed by Mc. Clain et al. (1982). The geometrical corrections were performed by means of an algorithm developed at the CEE. Space-integrated SSTs, expressed as time series of the percentage of the area between isotherms at 1 intervals, were calculated for November–December 1996 and 1997 (Fig. 4). In order to understand the life cycle of upwelling events and to assess characteristic features of wind-driven upwelling forcing, atmospheric sealevel pressure (SLP), cloudiness, wind speed and direction every 3 h reported at the lighthouse of Punta Angeles (PA), at the southern edge of the Valparaiso Bay, were obtained for November– December 1996 and 1997. Continuous wind-speed records for a few days in November 1996 and for November–December 1997 were also obtained at PC. Additional SLP and wind records from a coastal automatic meteorological station at Punta Lengua de Vaca were also considered. Surface meteorological data were complemented with weather maps from the NCEP–NCAR reanalysis Program (Kalnay et al., 1996). The upwelling-favourable wind intensity has been assessed through a daily upwelling index that considers the afternoon average (12 PM to 12 AM) of the alongshore components of the pseudo-stress
of the wind (APS): APS ¼ Vv
ðm2 s2 Þ;
where the overbar represents the afternoon average of the hourly products of the windspeed V and the meridional (alongshore) component of the wind vector v (e.g. Rutllant, 1993; Rutllant and Montecino, 2002). In this way, APS values reflect not only the afternoon Ekman transport associated with the strengthening of the alongshore equatorward winds but also the Ekman pumping effect at night and morning times when the nearshore winds vanish. In fact, Beardsley et al. (1987), while analysing winds at several distances off the California coast, conclude that the diurnal cycle of the coastal winds vanish approximately within the first 10 miles offshore, while winds farther offshore retain their afternoon strength and direction. The resulting nearshore cyclonic windstress-curl and associated Ekman pumping at night and in the morning maintain upwelling proportional to the mean afternoon alongshore windstress. Given that both PA and PC are located on top of the coastal cliff at 60 and 84 m above sea level, respectively, wind-speed averages were reduced to the 10-m standard level assuming a logarithmic vertical wind profile.
3. Results To set the stage for possible differences in the behaviour of the local and remote forcing of the upwelling during the austral spring campaigns at the extremes of the ENSO cycle, the general meteorological conditions for November–December 1996 (La Nin˜a) and November–December 1997 (El Nin˜o) are analysed through SLP and wind anomalies with respect to the long-term monthly means (NCAR–NCEP reanalysis). Positive SLP anomalies in excess of 4 hPa and stronger trades off the west coast of South America prevailed in December 1996 (Fig. 2a) indicating a poleward-displaced subtropical anticyclone and polar front trough, as expected for La Nin˜a condition. The resulting meridional dipole in the SLP anomaly pattern (+ north/ south) contrasts with the opposite sign one in November
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1997 (Fig. 2b), in which positive SLP anomalies are located to the west of the Antarctic Peninsula. This atmospheric circulation anomaly pattern reflects an enhanced frequency of blocking anticyclones, as usually observed during the developing phase of El Nin˜o events (Rutllant and Fuenzalida, 1991; Renwick, 1998). The negative SLP anomalies farther north result from a weakened subtropical anticyclone (negative phase of the Southern Oscillation) and an equatorward shift of the storm track arising from the high-latitude blocking. In spite of these opposite anomaly patterns in the large-scale atmospheric circulation, the central Chile coast (30–35 S) presented neutral conditions at those times (late spring). Intraseasonal variability along the coastal ocean, as represented by the smoothed time series of adjusted sea level (ASL) at Antofagasta (23 S) and Valpara!ıso (33 S) (Fig. 3) show low-frequency fluctuations progressing polewards (tilted dashed lines) associated with CTWs with periods ranging
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from 30 to 50 days. Associated colder/warmer SST periods, as derived from a combination of in situ (5-m depth temperature at the outer edge of the Valpara!ıso Bay, Vega et al., 2000) and satellite data (Fig. 4), tend to occur when the low-pass filtered sea level is rising/sinking. This 90 phase shift is consistent with SSTs lagging ASLs by about 12 days (Hormaza! bal et al., 2001). A cold intraseasonal period extended from before November 10 to November 26–27 in 1996 as the ASL was rising. The subsequent warm period was at least present until about December 10. In 1997, a similar cold intraseasonal period lasted from around November 23–24 to December 9–10, followed by a distinct warm period until the end of December. The preceding warm period was not as intense, probably due to the smaller amplitude of the ASL intraseasonal wave. Daily alongshore pseudo-windstresses (APSs) at Punta Curaumilla (PC) and from a longer time series of winds recorded at Punta Lengua de Vaca
Fig. 2. Sea-level pressure and wind-vector anomalies with respect to the long-term mean from the NCEP/NCAR Reanalysis Program (Kalnay et al., 1996) for (a) December 1996, and (b) November 1997. Contours every 2 hPa represent SLP anomalies (solid lines: positive anomalies; dashed lines: negative anomalies). Zero-anomaly contours have been enhanced. The scale of wind-vector anomalies is indicated as a 5 m s1 arrow below each map.
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Fig. 3. Adjusted sea-level anomalies at Antofagasta (23 S) and Valparaiso (33 S) for October through December 1996 and 1997. Diagonal lines connect lowest intraseasonal (dashed) values at both locations, indicating a poleward propagation. Observed SSTs (5-m depth) at the outer edge of the Valparaiso Bay are indicated in thick lines near the bottom. Cold intraseasonal SST periods are enclosed in dashed boxes (adapted from Vega et al., 2000).
Fig. 4. Time series of isothermal area percentages (1 intervals) from NOAA-14 for November–December 1996 (left) and 1997 (right).
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(LV) for austral springs of 1996 and 1997 (top panels in Figs. 5 and 6) show oscillations causing upwelling pulses. In 1996, the APS variability for the warm and cold periods associated with the ASL intraseasonal oscillations was quite similar (Fig. 5, top panel), with relatively short-lived and high-amplitude cycles in the strength of upwellingfavourable winds. However in 1997, a longer period of strong winds occurred during the cold intraseasonal phase, followed by a long relaxation (Fig. 6, top panel). Table 2 lists upwelling-favourable wind events during both seasons. Because of differences in the APS at PC and LV (see Figs. 5 and 6, top panels), the active phase (upwelling-favourable wind events) for the cold and warm phases of intraseasonal fluctuations was defined as periods in which the APS reaches or exceeds an arbitrary threshold of 40–50 m2 s2. In a similar way, the relaxed phase of the upwelling cycles in Table 2 was defined as periods in which the APS is at or below B20 m2 s2. ‘‘Weak’’ stands for periods in which APSs are slightly below/above the upper/lower threshold. The date of the minimum SLP indicates the passage of a coastal low (CL passage in Table 2). It can be seen that upwelling-favourable winds were active during both seasons, regardless of the thermal phase of the intraseasonal, CTW-related cycles. However, inspection of time series of APS (Figs. 5 and 6, top panels), sea-level pressure (SLP) and fractional low-cloud cover (Figs. 5 and 6, bottom panels) for the 1996 and 1997 campaigns, respectively, reveals that strong upwelling-favourable winds tend to occur in clear skies while the SLP is falling. Conversely, wind relaxations occur in association with increasing SLP and overcast skies. Although this behaviour has been previously established for LV in connection with the passage of coastal lows (Rutllant, 1993; Garreaud et al., 2002), the long active and relaxation periods in 1997 could imply some sort of ocean–atmospheric interaction at intraseasonal time scales. The connection between coastal lows and upwelling-favourable wind events described in Table 2 can be most readily assessed through a composite analysis of daily-mean anomalies of the APSs and SLPs at PA and LV. These anomalies, centred on the day with minimum SLP at PA for
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each event (Fig. 7), are referred to the corresponding average for each of the events (Table 3). Table 3 also includes the average standard deviation for each variable in each of the 13 events. This analysis confirms that upwelling-favourable winds tend to peak/relax while the local SLP is decreasing/ increasing, and that SLP at PA lags SLP at LV according to the direction and typical phase speeds in the propagation of coastal lows (e.g. Garreaud et al., 2002). The offshore extension of the relationship between coastal lows and upwellingfavourable winds can be assessed through a similar composite of reanalysed (NCEP/NCAR Reanalysis) wind and SST fields for all cases listed in Table 2. Fig. 8a documents a maximum equatorward wind anomaly peaking off 32.5 S, centred at 75 W. Similar anomaly fields but with larger equatorward speeds appear when the subset of events during either the cold intraseasonal phase or the 1996 cold interannual phase are considered (not shown). The corresponding composite of mean SST fields, depicted in Fig. 8b, show large offshore SST gradients close to the coast. The SST anomaly composite is meaningless in this case since the offshore SST gradients are smeared out by the large SST interanual and intraseasonal variability off the coast, as documented in the following paragraphs. Satellite imagery shows an active upwelling state around PC on November 19, 1996 (La Nin˜a and cold phase of the intraseasonal wave) that developed in less than 24 h with SSTs below 12.5 C near the upwelling focus at PC and offshore SSTs of about 15 C (Fig. 9a). A secondary upwelling focus appears near 32 S. Two less intense events during this intraseasonal cold phase with minimum SSTs around 13.5 C and similar offshore SSTs occurred on November 10 and 23 (not shown), in connection with the passage of coastal lows (Table 2). The warm-water period that started around November 28, 1996 presented offshore temperatures above 17 C while minimum SSTs never decreased below 15 C in the three upwelling episodes during that period. Fig. 9b depicts the SST field for December 3, corresponding to the 5th event of 1996 (Table 2). SST images in late spring of 1997 (El Nin˜o) also reflect the alternation of intraseasonal warm/cold periods. During the cold
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Fig. 5. Top panel: alongshore pseudo-windstress (APS) in m2 s2 at Punta Lengua de Vaca (LV: white) and Punta Curaumilla (PC: black); bottom panel: sea-level pressure (hPa) and low-cloud cover (tenths) at Punta Angeles (Valparaiso), for November–December 1996. Cold and warm intraseasonal periods are approximately indicated within horizontal arrows. APS values above 40–50 m2 s2 indicate strong upwelling-favourable wind events.
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Fig. 6. Same as in Fig. 5, but for November–December 1997.
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Table 2 Upwelling-favourable wind events within: (a) November 6–December 9, 1996; (b) November 1–December 31, 1997 Event Number
Active phase (dates)
CL Passage (date)
Relaxation phase (dates)
Thermal phase
10 19
10 19–20
— Cold
3 4 5 6
06–09 17–18 (weak) 22–23 29 02 05–06
23–24 30 03 07
24–25 01 03–04 07–08
Cold Warm Warm Warm
(b) 1 2 3 4 5 6 7
13–15 20–23 26–28 30–06 13–14 22–24 27–28
15 23 28 06 15 24 28
16, 18–19 24 29 (weak) 08–12 15–21 25 30
Warm — Cold Cold Warm Warm Warm
(a) 1 2
The active phase is defined as consecutive days in which APSs exceed 40–50 m2 s2. During relaxation APSs stay at or below B20 m2 s2. ‘‘Weak’’ stands for periods in which APSs are slightly below/above the upper/lower threshold. The date of the minimum SLP indicates the passage of a coastal low. Cold and warm phases of the intraseasonal fluctuations are indicated in the last column.
period an almost continuous upwelling event from November 30 to December 6 produced minimum SSTs below 13 C on December 3, while offshore SSTs stayed around 18 C (Fig. 9c). The oceancolour image for that day (Fig. 9e) shows patches of surface chlorophyll a concentrations up to 15 mg m3 in connection with the upwelling plume off PC. The warm period near the end of the experiment brought a more homogeneous thermal field with SSTs above 20 C, as illustrated for December 22 in Fig. 9d, with chlorophyll a concentrations up to 10 mg m3 in coastal embayments to the north of PC on December 23 (Fig. 9f). The overall comparison of the 1996 (La Nin˜a) with the 1997 (El Nin˜o) SST fields yields a B3 C difference in the offshore SSTs, while differences in the SST minima at PC were below 1 C, indicating
that close to the coast the intraseasonal (cold) CTW signal can partially overcome the interannual (warm) ENSO signal, as suggested in Rutllant and Montecino (2002). Similar conclusions can be drawn from the percentage of the total area encompassing isotherms from 12 to 21 C in steps of 1 C (Fig. 4). During 1996 (La Nin˜a), at the peak of the cold intraseasonal period and a few days afterwards, over 50% of the area stayed between 15 C and 16 C, whereas during the 1997 (El Nin˜o) cold-period, half of the area presented SSTs between 17 C and 18 C. During warm intraseasonal periods, 50% of the area was between 17 C and 18 C in 1996 (La Nin˜a) and between 19 C and 20 C in 1997 (El Nin˜o). Therefore, SST amplitudes of the ENSO-related interannual signal and the CTW-related intraseasonal signal are similar (2–3 C) and consistent with coastal SST anomalies during strong El Nin˜o conditions reported elsewhere (e.g. Hormaza! bal et al., 2001; CDB, 1997; Prado and Sievers, 1987). It can also be concluded that upwelling within cold intraseasonal periods is restricted to a narrower coastal strip during El Nin˜o than during La Nin˜a.
4. Summary and discussion Warm and cold SST periods lasting from 15 to 20 days were observed in both late-spring campaigns off central Chile at the extremes of the ENSO cycle. Those SST periods were associated with intraseasonal CTWs producing shoaling/ deepening cycles in the thermocline/nutricline depth that modulated the effectiveness of the local wind-driven upwelling in bringing cold, nutrientrich waters to the euphotic zone. Similar differences in satellite derived sea-surface temperatures at the extremes of both the ENSO and intraseasonal cycles of 2–3 C were observed, although upwelling within cold intraseasonal periods was restricted to a narrower coastal strip during El Nin˜o than during La Nin˜a. No significant differences in the wind-driven upwelling were observed probably due to the neutral condition in the SLP anomaly field in both field experiments. Stronger upwelling-favourable winds tended to occur in clear skies during the local SLP decrease
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Fig. 7. Composites of upwelling-favourable wind events consisting of 5 PM sea-level pressure (SLP: thick lines) and alongshore pseudo-stress (APS: dashed lines) anomalies at P. Angeles (PA), centred on the local SLP minima (day 0), for November–December 1996 (top panel) and November–December 1997 (bottom panel). The corresponding SLP composite at P. Lengua de Vaca (LV: thin lines) is included. Standard deviations for each day and variable of the 6/7 composites in 1996/1997 are indicated either above or below the value of the corresponding anomaly.
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Table 3 Average values of SLPs (hPa) and APSs (m2 s2) for each event listed in Table 2, used to define the composite anomalies 1996
1 2 3 4 5 6 7 Ave. s
1997
SLP(PA)
SLP(LV)
APS(PA)
SLP(PA)
SLP(LV)
APS(PA)
— 1014.4 1014.6 1014.6 1015.4 1015.2
— 1012.6 1011.2 1010.6 1011.4 1012.2
72.4 70.8 61.2 52.0 39.1 72.4
1014.8 1.6
1011.6 1.9
59.1 32.7
1018.0 1013.2 1014.6 1011.7 1015.4 1013.2 1013.1 1014.6 1.6
1011.9 1014.1 1009.3 1010.1 1008.5 — — 1010.8 1.7
52.6 58.7 74.7 55.8 17.5 46.2 55.0 51.8 23.6
The overall average for each variable (Ave.) and the average standard deviation s for days 3, 2, 1, 0, 1 and 2 (individually depicted in Fig. 7) are listed in the two bottom rows, respectively.
24S
NOAA-CIRES/Climate Diagnotics Center
24S 2
25S 26S 27S
30S
1.8
1.6
33S
26S 27S
1.4
1.2
19
29S 30S
18
17
31S 1 32S 0.8
34S
33S
16
15
34S 0.6
35S 36S 80W 79W 78W 77W 78W 75W 74W 73W 72W 71W 70W
20
28S
31S 32S
21
25S
28S 29S
NOAA-CIRES/Climate Diagnotics Center
14 35S
0.4
(a)
36S 80W 79W 78W 77W 78W 75W 74W 73W 72W 71W 70W
13
(b)
Fig. 8. Composites (averages) of daily NCEP/NCAR (http://www.cdc.noaa.gov/Composites/Day) reanalysed wind-vector anomalies (a) and mean SSTs (b) for the coastal-low occurrences listed in Table 2 (CL passages). Wind-speed anomalies in m s1 are calculated from 1968–1996 total means. Mean SSTs are in C.
associated with the passage of coastal lows over central Chile. Though NCEP/NCAR Reanalysis includes satellite-derived wind data, the model resolution prevents any possible insight on the
location of the maximum offshore southerly wind speed anomaly (e.g. Garreaud et al., 2001). During El Nin˜o conditions upwelling-favourable winds in northern Chile do not slow down in
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Fig. 9. Sea-surface temperatures (SST) and ocean colour (OC) images during the cold/warm periods within the study area (Fig. 1) (see text for details).
association with a weaker subtropical anticyclone. Coastal alongshore winds, particularly in summer, have a strong regional component arising from
stronger land–sea temperature contrast that enhances the large-scale southerlies in the afternoon. Therefore, the regional alongshore wind strength
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should be strongly dependent on the low-cloud cover anomalies, as suggested by Enfield (1981). In fact, the strong upwelling-favourable winds between November 20 and December 7, 1997 closely follow the cloudless days during that cold-water period. The persistence of these conditions could arise from forced subsidence down the western slope of the Andes induced by higher SLPs to the south of central Chile at the onset of a coastal-low development result in a westward warm advection lowering the SLP and the subsidence inversion base at the coast (Garreaud et al., 2002). Land warming enhances the SLP drop in the afternoon while the colder coastal waters tend to keep the subsidence inversion below the mixing condensation level, inhibiting the low-cloud formation. Then the resulting colder SSTs (strong southerlies and shallow thermocline) and warmer land surface (cloudless skies) produce stronger winds (enhanced land–sea thermal contrast), in a positive feedback loop. During the last warm intraseasonal period in 1997, under El Nin˜o conditions, the relatively high reflectivity in the ocean-colour image on the December 23 at the inner part of bays could be associated with possible anomalous sediment discharge by the Aconcagua river (high runoff because of increased storminess during El Nin˜o years: Rutllant and Fuenzalida, 1991), about 20 km of the Valparaiso Bay. The significant 50-day variability periods in the Valpara!ıso alongshore winds reported in Hormaza! bal et al. (2001) are not discernible in our short 2-month time series. The only apparent relationship in the intraseasonal band is the general correspondence between the cold period, clear skies and strong winds at Punta Curaumilla in 1997. There, the control of the southerly wind strength by enhanced land–sea temperature differences due to clear skies and the associated positive upwelling feedback on SSTs, seem important. Shorter-term (synoptic) variability in the ASL, winds and SST could be produced by differential Ekman transport set up by the opposing winds at the leading and trailing edges of coastal lows, as discussed for the west coast of South Africa in Jury (1986) and Jury and Brundrit (1992).
Acknowledgements The authors wish to thank the Centre for Space Studies (CEE) at the Universidad de Chile for the satellite data processing and analysis and to Prof. Ted Strub for thoughtful comments and careful reading of the manuscript. Sergio Vega and Italo Masotti benefited from Project DIUV 3/96 led by Prof. Helmuth Sievers at the Universidad de Valpara!ıso in the earlier stages of the study. Zaida Salinas and Ricardo Bustos prepared most of the figures. Wind data for Punta Curaumilla were kindly provided by Prof. Carlos Naveas at the Universidad de Playa Ancha, Valpara!ıso. Additional meteorological data were also kindly provided by the Chilean Navy Meteorological Service (SERVIMET) and by the Chilean Weather Service (DMC). The authors are especially indebted to Prof. Mark Jury and an anonymous reviewer for thoughtful comments that led to a very significant improvement of this manuscript. This study was funded by the Program in Atmospheric Dynamics and Climate (PRODAC) and Project CSMAR 02/8-2 at the Universidad de Chile, and by the FONDAP-Humboldt Program.
References Beardsley, R.C., Dorman, C.E., Friehe, C.A., Rosenfeld, L.K., Winant, C.D., 1987. Local atmospheric forcing during the coastal ocean dynamics experiment. 1. A description of the marine boundary layer and atmospheric conditions over a Northern California upwelling region. Journal of Geophysical Research 92 (C2), 1467–1488. CDB, 1997. Climate Diagnostics Bulletin. NOAA. Djurfeldt, L., 1989. Circulation and mixing in a coastal embayment: Gulf of Arauco, Chile. Continental Shelf Research 9 (II), 1003–1016. Enfield, D., 1981. Thermally driven wind variability in the planetary boundary layer above Lima, Peru. Journal of Geophysical Research 86, 2005–2016. Enfield, D.B., 1987. The intraseasonal oscillation in eastern Pacific sea levels: How is it forced? Journal of Physical Oceanography 17, 1860–1876. Fonseca, T., Far!ıas, M., 1987. Estudio del proceso de surgencia en la costa chilena utilizando percepci!on remota. Investigaciones Pesqueras 34, 33–46. Garreaud, R.D., Rutllant, J.A., Quintana, J., Carrasco, J., Minnis, P., 2001. CIMAR-5: A Snapshot of the Lower Troposphere over the Subtropical Southeast Pacific.
ARTICLE IN PRESS J.A. Rutllant et al. / Continental Shelf Research 24 (2004) 773–787 Bulletin of the American Meteorological Society 82, 2193–2207. Garreaud, R.D., Rutllant, J.A., Fuenzalida, H., 2002. Coastal Lows along the Subtropical West Coast of South America: Mean Structure and Evolution. Monthly Weather Review 130, 75–88. Gonzalez, H., Daneri, G., Figueroa, D., Iriarte, J.L., Lefevre, N., Pizarro, G., Quin˜ones, R., Sobarzo, M., Troncoso, A., ! primaria y su destino en la trama 1998. Produccion ! trofica pel!agica y oc!eano profundo e intercambio oc!eano! atmosfera de CO2 en la zona norte de la corriente de humboldt (23 S): posibles efectos del evento El Nin˜o 1997– 98 en Chile. Revista Chilena de Historia Natural 71 (4), 429–458. Hormaz!abal, S., Shaffer, G., Letelier, J., Ulloa, O., 2001. Local and Remote forcing of sea surface temperature in the coastal upwelling system off Chile. Journal of Geophysical Research 106 (C8), 16657–16671. Jury, M.R., 1986. The sudden decay of upwelling off the Cape Peninsula, South Africa: a case study. South African Journal of Marine Science 4, 111–118. Jury, M.R., Brundit, G.B., 1992. Temporal organisation of upwelling in the Southern Benguela Ecosystem by resonant coastal trapped waves in the ocean and atmosphere. South African Journal of Marine Science 12, 219–224. Kalnay, E., Coauthors, 1996. The NCEP/NCAR 40-year reanalysis project. Bulletin of the American Meteorological Society 77, 437–471. Kessler, W.S., McPhaden, M.J., 1995. Forcing in the intraseasonal Kelvin waves in the equatorial Pacific. Journal of Geophysical Research 100, 10613–10632. Mc. Clain, E., Pichel, W., Walton, Ch., 1982. Comparative performance of AVHRR-based multi-channel Sea Surface temperature. Journal of Geophysical Research 90 (C6), 11,587–11,601. Prado, R., Sievers, H.A., 1987. Distribution of physical and chemical water characteristics off peninsula Los Molles, Chile (32 450 S) and its relation to the El Nin˜o 1982/1983 phenomenon. Revista de Biolog!ıa Marina 23 (1), 31–75.
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Renwick, J.A., 1998. ENSO-related variability in the frequency of South Pacific blocking. Monthly Weather Review 126, 3117–3123. Rutllant, J., 1993. Coastal Lows and Associated Southerly Wind Events in North-Central Chile. Preprints IV International Conference on Meteorology and Oceanography of the Southern Hemisphere. Hobart, Australia, 29 March–2 April 1993. American Meteorological Society, 268–269. Rutllant, J., 1994. Modelling the day to day wind variability offshore Central Chile at about 30 deg. South. International Centre for Theoretical Physics (Trieste-Italy). Internal Report IC/94/166, 22 pp. (available from author). Rutllant, J., Fuenzalida, H., 1991. Synoptic aspects of the Central Chile rainfall variability associated with the southern oscillation. International Journal of Climatology 11, 63–76. Rutllant, J., Montecino, V., 2002. Multiscale upwelling forcing cycles and biological response off north-central Chile. Revista Chilena de Historia Natural 75, 217–231. Shaffer, G., Pizarro, O., Djurfeldt, L., Salinas, S., Rutllant, J., 1997. Circulation and low frequency variability near the Chile Coast: remotely forced fluctuations during the 1991–1992 El Nin˜o. Journal of Physical Oceanography 27, 217–235. Silva, N., 1973. Variaciones estacionales de temperatura, salinidad y contenido de ox!ıgeno en la zona costera. Investigaciones Marinas (Valpara!ıso) 4 (3), 89–112. Strub, P., Mes!ıas, J.M., Montecino, V., Rutllant, J., Salinas, S., 1998. Coastal Ocean Circulation off Western South America. In: Robinson, A., Brink, K. (Eds.), The Sea, Vol. 11, pp. 273–313 (Chapter 10). Vega, S., Rutllant, J., Masotti, I., 2000. Local and Remote Forcing of Coastal Upwelling near Valpara!ıso, Chile (33 S) in late spring of 1996 (La Nin˜a) and 1997 (El Nin˜o). Preprints VI International Conference on Southern Hemisphere Meteorology and Oceanography. Santiago, Chile, 3–7 April 2000. American Meteorological Society, 39–40. Yan˜ez, E., Barbieri, M.A., Silva, C., Nieto, K., Espindola, F., 2001. Climate variability and pelagic fisheries in northern Chile. Progress in Oceanography 49, 581–596.
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A comparison of spring coastal upwelling off central Chile at the extremes of the 1996–1997 ENSO cycle ! c, Sergio A. Vegad Jose! A. Rutllanta,*, Italo Masottib, Janette Calderon a Departamento de Geologia y Geof!ısica, Universidad de Chile, Casilla, Santiago 2777, Chile Laboratoire de Biogeochimie et Chimie Marines (LBCM), Universit!e Pierre et Marie Curie, Case courrier 134, 4, place Jussieu, 75252 Paris Cedex 05, France c ! Meteorologica ! Direccion de Chile, Casilla 63 (Correo Internacional), Santiago, Chile d Laboratoire d’Etudes en Geophysique et Oceanographie Spatiales, 14, Avenue Edouard Belin, Toulouse 31400, Cedex 04, France
b
Received 26 June 2002; received in revised form 22 August 2003; accepted 4 February 2004
Abstract Southerly wind pulses are repeatedly observed in connection with upwelling off Point Curaumilla (33 S), around 20 km south of Valpara!ıso. The remote forcing of coastal upwelling associated with intraseasonal coastal-trapped waves and the local, wind-driven upwelling forcing, are characterised there in terms of sea level, meteorological observations, surface weather charts and satellite-derived SST and chlorophyll a, from observations taken in late austral springs of 1996 (La Nin˜a) and 1997 (El Nin˜o). Warm and cold coastal SST periods lasting from 15 to 20 days are associated with intraseasonal (30–50-day periods) coastal-trapped waves which are detected in time series of adjusted sea level. These waves produce shoaling/deepening cycles in the thermocline depth and hence a modulation of the local wind-driven upwelling efficiency in bringing cold, nutrient-rich waters to the surface. The atmospheric forcing of the wind-driven upwelling pulses is closely related to atmospheric coastal-trapped disturbances (coastal lows) in which the active/relaxed phase of the upwelling occurs ahead of/after the sea-level pressure minimum with clear/overcast skies. Time series of the alongshore pseudo-wind-stress at two coastal locations reveal that the local atmospheric forcing is always active and largely independent of the thermal phase of both the intraseasonal and interannual (ENSO) cycles. Similar differences in satellite derived sea-surface temperatures at the extremes of both the ENSO and intraseasonal cycles of 2–3 C were observed, although upwelling within cold intraseasonal periods was restricted to a narrower coastal strip during El Nin˜o than during La Nin˜a. r 2004 Elsevier Ltd. All rights reserved. Keywords: Humboldt current system; Coastal upwelling; ENSO cycle; Coastal-trapped waves; Valpara!ıso; Chile
1. Introduction During the warm phase of the ENSO cycle (El Nin˜o events) the poleward counter-current along *Corresponding author. Fax: +56-2-6968686. E-mail address: [email protected] (J.A. Rutllant).
western South America strengthens (Kessler and McPhaden, 1995) while the atmospheric subtropical anticyclone, a key constituent of the Southern Oscillation, weakens. These conditions increase the advection and onshore entrainment of warmer waters, producing a rise in the sea level and a corresponding deepening of the nutrient-rich, O2
0278-4343/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2004.02.005
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depleted equatorial sub-surface waters beneath the thermocline. For example, the thermocline off Antofagasta (23 S) deepened from approximately 50 m in January 1997 (pre-El Nin˜o condition) to about 200 m depth during the fully developed El Nin˜o in July 1997 (Gonzalez et al., 1998). At the opposite phase of the ENSO cycle (La Nin˜a) the thermocline shoals and the average sea level decreases, as the Humboldt coastal current and the subtropical gyre speed-up. These interannual changes in the physical environment have a severe impact on fisheries; in fact, anchovy landings of around 1400 thousand tons in 1986 and in 1998 (La Nin˜a years) dropped to about 200 thousand tons in 1987 and 1997 El Nin˜o’s (Yan˜ez et al., 2001). Superimposed on this interannual cycle, intraseasonal (50-day) oscillations in the thermocline depth, associated with equatorially sourced coastal-trapped waves (CTWs), have been reported off the coast of Peru (Enfield, 1987; Kessler and McPhaden, 1995) and Chile (Shaffer et al., 1997). Those CTWs seem to be more prominent in austral summer, particularly at the onset of El Nin˜o events (Shaffer et al., 1997). In the presence of these CTWs, upwelling-favourable winds become more or less effective in bringing nutrientrich cold waters to the surface as a function of the associated thermocline depth fluctuations on intraseasonal to interannual time scales (e.g. Strub et al., 1998). The upwelling regime off central Chile (27 – 37 S) is principally driven by upwelling-favourable (equatorward) wind events lasting 2–3 days with a periodicity of around one event per week (Rutllant, 1993; Rutllant and Montecino, 2002). Farther north upwelling is in general a year-round phenomenon, while seasonality increases as we move into south-central Chile (35–40 S) where upwelling events are concentrated in late spring and summer (e.g. Djurfeldt, 1989; Silva, 1973). Meteorological conditions leading to upwellingfavourable wind events at Punta Lengua de Vaca (30 S) have been identified by Rutllant (1993, 1994) and Rutllant and Montecino (2002). Those southerly (equatorward) wind events are due to an enhanced anticyclonic circulation after the passage of mid-latitude frontal disturbances and the
subsequent southward propagation of coastaltrapped disturbances (coastal lows). After the passage of the minimum in the sea-level pressure, skies usually change from clear to overcast, winds relax and often veer to NW, advecting the nutrient-rich water onshore as it downwells against the shelf. The wind relaxation and onshore advection increase the stability of the water column, favouring phytoplankton growth. This phenomenon seems to be particularly significant in bays adjacent to the main headlands upon which coastal upwelling activity concentrates. This is the case of the Valparaiso (33 S) bay area and Punta Curaumilla (Fig. 1), according to Fonseca and Far!ıas (1987). Oceanographic observations at the Valpara!ıso bay were performed in November 1996 and 1997 to assess the coupled day-to-day variability between the upwelling focus at Punta Curaumilla and the bay area. A strong modulation of the thermocline depth by poleward-propagating intraseasonal sea-level fluctuations was observed there, with associated changes in SST, salinity and dissolved oxygen (Vega et al., 2000). Here, we present an assessment of the meteorological aspects of the local, wind-driven upwelling forcing at the peak of the upwelling season in opposing phases of both intraseasonal and ENSO cycles. Satellite SST and ocean-colour imagery, while illustrating average conditions within oceanic waters associated with the extremes of these cycles, describe the ocean response off Punta Curaumilla to the short-term wind variability. The study area, meteorological and satellite data are described in Section 2. Section 3 presents the results followed by a summary and discussion in Section 4.
2. Study area and methods The study area (Fig. 1) encompasses central Chile including the upwelling centre off Punta Curaumilla (PC) and the Valpara!ıso Bay, located around 25 km northeastward of PC. The remote forcing component associated with intraseasonal CTWs was assessed through sealevel data (Vega et al., 2000). Quality-controlled
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Fig. 1. Study area around central Chile (west coast of South America).
sea-level data for both study periods (1996 and 1997) were obtained from tide gauges at Antofagasta (23 390 S) and Valpara!ıso (33 020 S) from the TOGA Sea-Level Centre at the University of Hawaii, USA. The hourly time series were subsequently corrected for atmospheric pressure variations and then low-pass filtered, resulting into an adjusted sea level, as defined in Shaffer et al. (1997). Daily SSTs from AVHRR images (NOAA-14) at 18:00–20:00 UTC with 1.1 km resolution, stored at the Centre for Space Studies of the Universidad de Chile (CEE), were analysed to characterise the space and time variability of the SST field around PC during the 1996–1997 experiments. Images
with more than 30% of the area covered by clouds were discarded. The processed images (see Fig. 8) encompass an area of 200 200 km between latitudes 31.6 and 33.4 S, and longitudes from 71.1 to 73.2 W. For the 1997 experiment chlorophyll a at the ocean surface was assessed through ocean-colour images from SeaWiFS (Seaviewing-Wide Field-of-view Sensor). This sensor operates within the range of visible to near infrared with a resolution of 1.1 km. Images were received and processed at the CEE isolating the same region as in the AVHRR images. From level L1a data, surface chlorophyll a concentrations were calculated through SeaDAS (SeaWiFS Data Analysis System, version 3.2) furnished by the
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Table 1 Selected AVHRR sea-surface temperature (SST) and SeaWiFS ocean colour (OC) satellite images with less than 30% of lowcloud cover collected during the field experiments SST 1996
SST 1997
OC 1997
10 18 22 29 02 06 09
30 09 21 26 30 05 13 27
31 10 14 21 30 02 06 12 28
Nov, 15 Nov Nov, 19 Nov Nov, 23 Nov Nov, 30 Nov Dec, 05 Dec Dec, 07 Dec Dec
Oct, 07 Nov Nov, 13 Nov Nov, 22 Nov Nov, 27 Nov Nov, 03 Dec Dec, 07 Dec Dec, 22 Dec Dec, 31 Dec
Oct, 07 Nov Nov, 13 Nov Nov, 15 Nov Nov, 27 Nov Nov, 01 Dec Dec, 03 Dec Dec, 07 Dec Dec, 27 Dec Dec
National Aeronautics and Space Administration (NASA). Table 1 lists the selected dates of available satellite imagery for both 1996 and 1997 field experiments. The SST fields in degrees Celsius were obtained through thermal channels 4 and 5, following the empirical algorithm developed by Mc. Clain et al. (1982). The geometrical corrections were performed by means of an algorithm developed at the CEE. Space-integrated SSTs, expressed as time series of the percentage of the area between isotherms at 1 intervals, were calculated for November–December 1996 and 1997 (Fig. 4). In order to understand the life cycle of upwelling events and to assess characteristic features of wind-driven upwelling forcing, atmospheric sealevel pressure (SLP), cloudiness, wind speed and direction every 3 h reported at the lighthouse of Punta Angeles (PA), at the southern edge of the Valparaiso Bay, were obtained for November– December 1996 and 1997. Continuous wind-speed records for a few days in November 1996 and for November–December 1997 were also obtained at PC. Additional SLP and wind records from a coastal automatic meteorological station at Punta Lengua de Vaca were also considered. Surface meteorological data were complemented with weather maps from the NCEP–NCAR reanalysis Program (Kalnay et al., 1996). The upwelling-favourable wind intensity has been assessed through a daily upwelling index that considers the afternoon average (12 PM to 12 AM) of the alongshore components of the pseudo-stress
of the wind (APS): APS ¼ Vv
ðm2 s2 Þ;
where the overbar represents the afternoon average of the hourly products of the windspeed V and the meridional (alongshore) component of the wind vector v (e.g. Rutllant, 1993; Rutllant and Montecino, 2002). In this way, APS values reflect not only the afternoon Ekman transport associated with the strengthening of the alongshore equatorward winds but also the Ekman pumping effect at night and morning times when the nearshore winds vanish. In fact, Beardsley et al. (1987), while analysing winds at several distances off the California coast, conclude that the diurnal cycle of the coastal winds vanish approximately within the first 10 miles offshore, while winds farther offshore retain their afternoon strength and direction. The resulting nearshore cyclonic windstress-curl and associated Ekman pumping at night and in the morning maintain upwelling proportional to the mean afternoon alongshore windstress. Given that both PA and PC are located on top of the coastal cliff at 60 and 84 m above sea level, respectively, wind-speed averages were reduced to the 10-m standard level assuming a logarithmic vertical wind profile.
3. Results To set the stage for possible differences in the behaviour of the local and remote forcing of the upwelling during the austral spring campaigns at the extremes of the ENSO cycle, the general meteorological conditions for November–December 1996 (La Nin˜a) and November–December 1997 (El Nin˜o) are analysed through SLP and wind anomalies with respect to the long-term monthly means (NCAR–NCEP reanalysis). Positive SLP anomalies in excess of 4 hPa and stronger trades off the west coast of South America prevailed in December 1996 (Fig. 2a) indicating a poleward-displaced subtropical anticyclone and polar front trough, as expected for La Nin˜a condition. The resulting meridional dipole in the SLP anomaly pattern (+ north/ south) contrasts with the opposite sign one in November
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1997 (Fig. 2b), in which positive SLP anomalies are located to the west of the Antarctic Peninsula. This atmospheric circulation anomaly pattern reflects an enhanced frequency of blocking anticyclones, as usually observed during the developing phase of El Nin˜o events (Rutllant and Fuenzalida, 1991; Renwick, 1998). The negative SLP anomalies farther north result from a weakened subtropical anticyclone (negative phase of the Southern Oscillation) and an equatorward shift of the storm track arising from the high-latitude blocking. In spite of these opposite anomaly patterns in the large-scale atmospheric circulation, the central Chile coast (30–35 S) presented neutral conditions at those times (late spring). Intraseasonal variability along the coastal ocean, as represented by the smoothed time series of adjusted sea level (ASL) at Antofagasta (23 S) and Valpara!ıso (33 S) (Fig. 3) show low-frequency fluctuations progressing polewards (tilted dashed lines) associated with CTWs with periods ranging
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from 30 to 50 days. Associated colder/warmer SST periods, as derived from a combination of in situ (5-m depth temperature at the outer edge of the Valpara!ıso Bay, Vega et al., 2000) and satellite data (Fig. 4), tend to occur when the low-pass filtered sea level is rising/sinking. This 90 phase shift is consistent with SSTs lagging ASLs by about 12 days (Hormaza! bal et al., 2001). A cold intraseasonal period extended from before November 10 to November 26–27 in 1996 as the ASL was rising. The subsequent warm period was at least present until about December 10. In 1997, a similar cold intraseasonal period lasted from around November 23–24 to December 9–10, followed by a distinct warm period until the end of December. The preceding warm period was not as intense, probably due to the smaller amplitude of the ASL intraseasonal wave. Daily alongshore pseudo-windstresses (APSs) at Punta Curaumilla (PC) and from a longer time series of winds recorded at Punta Lengua de Vaca
Fig. 2. Sea-level pressure and wind-vector anomalies with respect to the long-term mean from the NCEP/NCAR Reanalysis Program (Kalnay et al., 1996) for (a) December 1996, and (b) November 1997. Contours every 2 hPa represent SLP anomalies (solid lines: positive anomalies; dashed lines: negative anomalies). Zero-anomaly contours have been enhanced. The scale of wind-vector anomalies is indicated as a 5 m s1 arrow below each map.
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Fig. 3. Adjusted sea-level anomalies at Antofagasta (23 S) and Valparaiso (33 S) for October through December 1996 and 1997. Diagonal lines connect lowest intraseasonal (dashed) values at both locations, indicating a poleward propagation. Observed SSTs (5-m depth) at the outer edge of the Valparaiso Bay are indicated in thick lines near the bottom. Cold intraseasonal SST periods are enclosed in dashed boxes (adapted from Vega et al., 2000).
Fig. 4. Time series of isothermal area percentages (1 intervals) from NOAA-14 for November–December 1996 (left) and 1997 (right).
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(LV) for austral springs of 1996 and 1997 (top panels in Figs. 5 and 6) show oscillations causing upwelling pulses. In 1996, the APS variability for the warm and cold periods associated with the ASL intraseasonal oscillations was quite similar (Fig. 5, top panel), with relatively short-lived and high-amplitude cycles in the strength of upwellingfavourable winds. However in 1997, a longer period of strong winds occurred during the cold intraseasonal phase, followed by a long relaxation (Fig. 6, top panel). Table 2 lists upwelling-favourable wind events during both seasons. Because of differences in the APS at PC and LV (see Figs. 5 and 6, top panels), the active phase (upwelling-favourable wind events) for the cold and warm phases of intraseasonal fluctuations was defined as periods in which the APS reaches or exceeds an arbitrary threshold of 40–50 m2 s2. In a similar way, the relaxed phase of the upwelling cycles in Table 2 was defined as periods in which the APS is at or below B20 m2 s2. ‘‘Weak’’ stands for periods in which APSs are slightly below/above the upper/lower threshold. The date of the minimum SLP indicates the passage of a coastal low (CL passage in Table 2). It can be seen that upwelling-favourable winds were active during both seasons, regardless of the thermal phase of the intraseasonal, CTW-related cycles. However, inspection of time series of APS (Figs. 5 and 6, top panels), sea-level pressure (SLP) and fractional low-cloud cover (Figs. 5 and 6, bottom panels) for the 1996 and 1997 campaigns, respectively, reveals that strong upwelling-favourable winds tend to occur in clear skies while the SLP is falling. Conversely, wind relaxations occur in association with increasing SLP and overcast skies. Although this behaviour has been previously established for LV in connection with the passage of coastal lows (Rutllant, 1993; Garreaud et al., 2002), the long active and relaxation periods in 1997 could imply some sort of ocean–atmospheric interaction at intraseasonal time scales. The connection between coastal lows and upwelling-favourable wind events described in Table 2 can be most readily assessed through a composite analysis of daily-mean anomalies of the APSs and SLPs at PA and LV. These anomalies, centred on the day with minimum SLP at PA for
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each event (Fig. 7), are referred to the corresponding average for each of the events (Table 3). Table 3 also includes the average standard deviation for each variable in each of the 13 events. This analysis confirms that upwelling-favourable winds tend to peak/relax while the local SLP is decreasing/ increasing, and that SLP at PA lags SLP at LV according to the direction and typical phase speeds in the propagation of coastal lows (e.g. Garreaud et al., 2002). The offshore extension of the relationship between coastal lows and upwellingfavourable winds can be assessed through a similar composite of reanalysed (NCEP/NCAR Reanalysis) wind and SST fields for all cases listed in Table 2. Fig. 8a documents a maximum equatorward wind anomaly peaking off 32.5 S, centred at 75 W. Similar anomaly fields but with larger equatorward speeds appear when the subset of events during either the cold intraseasonal phase or the 1996 cold interannual phase are considered (not shown). The corresponding composite of mean SST fields, depicted in Fig. 8b, show large offshore SST gradients close to the coast. The SST anomaly composite is meaningless in this case since the offshore SST gradients are smeared out by the large SST interanual and intraseasonal variability off the coast, as documented in the following paragraphs. Satellite imagery shows an active upwelling state around PC on November 19, 1996 (La Nin˜a and cold phase of the intraseasonal wave) that developed in less than 24 h with SSTs below 12.5 C near the upwelling focus at PC and offshore SSTs of about 15 C (Fig. 9a). A secondary upwelling focus appears near 32 S. Two less intense events during this intraseasonal cold phase with minimum SSTs around 13.5 C and similar offshore SSTs occurred on November 10 and 23 (not shown), in connection with the passage of coastal lows (Table 2). The warm-water period that started around November 28, 1996 presented offshore temperatures above 17 C while minimum SSTs never decreased below 15 C in the three upwelling episodes during that period. Fig. 9b depicts the SST field for December 3, corresponding to the 5th event of 1996 (Table 2). SST images in late spring of 1997 (El Nin˜o) also reflect the alternation of intraseasonal warm/cold periods. During the cold
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Fig. 5. Top panel: alongshore pseudo-windstress (APS) in m2 s2 at Punta Lengua de Vaca (LV: white) and Punta Curaumilla (PC: black); bottom panel: sea-level pressure (hPa) and low-cloud cover (tenths) at Punta Angeles (Valparaiso), for November–December 1996. Cold and warm intraseasonal periods are approximately indicated within horizontal arrows. APS values above 40–50 m2 s2 indicate strong upwelling-favourable wind events.
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Fig. 6. Same as in Fig. 5, but for November–December 1997.
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Table 2 Upwelling-favourable wind events within: (a) November 6–December 9, 1996; (b) November 1–December 31, 1997 Event Number
Active phase (dates)
CL Passage (date)
Relaxation phase (dates)
Thermal phase
10 19
10 19–20
— Cold
3 4 5 6
06–09 17–18 (weak) 22–23 29 02 05–06
23–24 30 03 07
24–25 01 03–04 07–08
Cold Warm Warm Warm
(b) 1 2 3 4 5 6 7
13–15 20–23 26–28 30–06 13–14 22–24 27–28
15 23 28 06 15 24 28
16, 18–19 24 29 (weak) 08–12 15–21 25 30
Warm — Cold Cold Warm Warm Warm
(a) 1 2
The active phase is defined as consecutive days in which APSs exceed 40–50 m2 s2. During relaxation APSs stay at or below B20 m2 s2. ‘‘Weak’’ stands for periods in which APSs are slightly below/above the upper/lower threshold. The date of the minimum SLP indicates the passage of a coastal low. Cold and warm phases of the intraseasonal fluctuations are indicated in the last column.
period an almost continuous upwelling event from November 30 to December 6 produced minimum SSTs below 13 C on December 3, while offshore SSTs stayed around 18 C (Fig. 9c). The oceancolour image for that day (Fig. 9e) shows patches of surface chlorophyll a concentrations up to 15 mg m3 in connection with the upwelling plume off PC. The warm period near the end of the experiment brought a more homogeneous thermal field with SSTs above 20 C, as illustrated for December 22 in Fig. 9d, with chlorophyll a concentrations up to 10 mg m3 in coastal embayments to the north of PC on December 23 (Fig. 9f). The overall comparison of the 1996 (La Nin˜a) with the 1997 (El Nin˜o) SST fields yields a B3 C difference in the offshore SSTs, while differences in the SST minima at PC were below 1 C, indicating
that close to the coast the intraseasonal (cold) CTW signal can partially overcome the interannual (warm) ENSO signal, as suggested in Rutllant and Montecino (2002). Similar conclusions can be drawn from the percentage of the total area encompassing isotherms from 12 to 21 C in steps of 1 C (Fig. 4). During 1996 (La Nin˜a), at the peak of the cold intraseasonal period and a few days afterwards, over 50% of the area stayed between 15 C and 16 C, whereas during the 1997 (El Nin˜o) cold-period, half of the area presented SSTs between 17 C and 18 C. During warm intraseasonal periods, 50% of the area was between 17 C and 18 C in 1996 (La Nin˜a) and between 19 C and 20 C in 1997 (El Nin˜o). Therefore, SST amplitudes of the ENSO-related interannual signal and the CTW-related intraseasonal signal are similar (2–3 C) and consistent with coastal SST anomalies during strong El Nin˜o conditions reported elsewhere (e.g. Hormaza! bal et al., 2001; CDB, 1997; Prado and Sievers, 1987). It can also be concluded that upwelling within cold intraseasonal periods is restricted to a narrower coastal strip during El Nin˜o than during La Nin˜a.
4. Summary and discussion Warm and cold SST periods lasting from 15 to 20 days were observed in both late-spring campaigns off central Chile at the extremes of the ENSO cycle. Those SST periods were associated with intraseasonal CTWs producing shoaling/ deepening cycles in the thermocline/nutricline depth that modulated the effectiveness of the local wind-driven upwelling in bringing cold, nutrientrich waters to the euphotic zone. Similar differences in satellite derived sea-surface temperatures at the extremes of both the ENSO and intraseasonal cycles of 2–3 C were observed, although upwelling within cold intraseasonal periods was restricted to a narrower coastal strip during El Nin˜o than during La Nin˜a. No significant differences in the wind-driven upwelling were observed probably due to the neutral condition in the SLP anomaly field in both field experiments. Stronger upwelling-favourable winds tended to occur in clear skies during the local SLP decrease
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Fig. 7. Composites of upwelling-favourable wind events consisting of 5 PM sea-level pressure (SLP: thick lines) and alongshore pseudo-stress (APS: dashed lines) anomalies at P. Angeles (PA), centred on the local SLP minima (day 0), for November–December 1996 (top panel) and November–December 1997 (bottom panel). The corresponding SLP composite at P. Lengua de Vaca (LV: thin lines) is included. Standard deviations for each day and variable of the 6/7 composites in 1996/1997 are indicated either above or below the value of the corresponding anomaly.
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Table 3 Average values of SLPs (hPa) and APSs (m2 s2) for each event listed in Table 2, used to define the composite anomalies 1996
1 2 3 4 5 6 7 Ave. s
1997
SLP(PA)
SLP(LV)
APS(PA)
SLP(PA)
SLP(LV)
APS(PA)
— 1014.4 1014.6 1014.6 1015.4 1015.2
— 1012.6 1011.2 1010.6 1011.4 1012.2
72.4 70.8 61.2 52.0 39.1 72.4
1014.8 1.6
1011.6 1.9
59.1 32.7
1018.0 1013.2 1014.6 1011.7 1015.4 1013.2 1013.1 1014.6 1.6
1011.9 1014.1 1009.3 1010.1 1008.5 — — 1010.8 1.7
52.6 58.7 74.7 55.8 17.5 46.2 55.0 51.8 23.6
The overall average for each variable (Ave.) and the average standard deviation s for days 3, 2, 1, 0, 1 and 2 (individually depicted in Fig. 7) are listed in the two bottom rows, respectively.
24S
NOAA-CIRES/Climate Diagnotics Center
24S 2
25S 26S 27S
30S
1.8
1.6
33S
26S 27S
1.4
1.2
19
29S 30S
18
17
31S 1 32S 0.8
34S
33S
16
15
34S 0.6
35S 36S 80W 79W 78W 77W 78W 75W 74W 73W 72W 71W 70W
20
28S
31S 32S
21
25S
28S 29S
NOAA-CIRES/Climate Diagnotics Center
14 35S
0.4
(a)
36S 80W 79W 78W 77W 78W 75W 74W 73W 72W 71W 70W
13
(b)
Fig. 8. Composites (averages) of daily NCEP/NCAR (http://www.cdc.noaa.gov/Composites/Day) reanalysed wind-vector anomalies (a) and mean SSTs (b) for the coastal-low occurrences listed in Table 2 (CL passages). Wind-speed anomalies in m s1 are calculated from 1968–1996 total means. Mean SSTs are in C.
associated with the passage of coastal lows over central Chile. Though NCEP/NCAR Reanalysis includes satellite-derived wind data, the model resolution prevents any possible insight on the
location of the maximum offshore southerly wind speed anomaly (e.g. Garreaud et al., 2001). During El Nin˜o conditions upwelling-favourable winds in northern Chile do not slow down in
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Fig. 9. Sea-surface temperatures (SST) and ocean colour (OC) images during the cold/warm periods within the study area (Fig. 1) (see text for details).
association with a weaker subtropical anticyclone. Coastal alongshore winds, particularly in summer, have a strong regional component arising from
stronger land–sea temperature contrast that enhances the large-scale southerlies in the afternoon. Therefore, the regional alongshore wind strength
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should be strongly dependent on the low-cloud cover anomalies, as suggested by Enfield (1981). In fact, the strong upwelling-favourable winds between November 20 and December 7, 1997 closely follow the cloudless days during that cold-water period. The persistence of these conditions could arise from forced subsidence down the western slope of the Andes induced by higher SLPs to the south of central Chile at the onset of a coastal-low development result in a westward warm advection lowering the SLP and the subsidence inversion base at the coast (Garreaud et al., 2002). Land warming enhances the SLP drop in the afternoon while the colder coastal waters tend to keep the subsidence inversion below the mixing condensation level, inhibiting the low-cloud formation. Then the resulting colder SSTs (strong southerlies and shallow thermocline) and warmer land surface (cloudless skies) produce stronger winds (enhanced land–sea thermal contrast), in a positive feedback loop. During the last warm intraseasonal period in 1997, under El Nin˜o conditions, the relatively high reflectivity in the ocean-colour image on the December 23 at the inner part of bays could be associated with possible anomalous sediment discharge by the Aconcagua river (high runoff because of increased storminess during El Nin˜o years: Rutllant and Fuenzalida, 1991), about 20 km of the Valparaiso Bay. The significant 50-day variability periods in the Valpara!ıso alongshore winds reported in Hormaza! bal et al. (2001) are not discernible in our short 2-month time series. The only apparent relationship in the intraseasonal band is the general correspondence between the cold period, clear skies and strong winds at Punta Curaumilla in 1997. There, the control of the southerly wind strength by enhanced land–sea temperature differences due to clear skies and the associated positive upwelling feedback on SSTs, seem important. Shorter-term (synoptic) variability in the ASL, winds and SST could be produced by differential Ekman transport set up by the opposing winds at the leading and trailing edges of coastal lows, as discussed for the west coast of South Africa in Jury (1986) and Jury and Brundrit (1992).
Acknowledgements The authors wish to thank the Centre for Space Studies (CEE) at the Universidad de Chile for the satellite data processing and analysis and to Prof. Ted Strub for thoughtful comments and careful reading of the manuscript. Sergio Vega and Italo Masotti benefited from Project DIUV 3/96 led by Prof. Helmuth Sievers at the Universidad de Valpara!ıso in the earlier stages of the study. Zaida Salinas and Ricardo Bustos prepared most of the figures. Wind data for Punta Curaumilla were kindly provided by Prof. Carlos Naveas at the Universidad de Playa Ancha, Valpara!ıso. Additional meteorological data were also kindly provided by the Chilean Navy Meteorological Service (SERVIMET) and by the Chilean Weather Service (DMC). The authors are especially indebted to Prof. Mark Jury and an anonymous reviewer for thoughtful comments that led to a very significant improvement of this manuscript. This study was funded by the Program in Atmospheric Dynamics and Climate (PRODAC) and Project CSMAR 02/8-2 at the Universidad de Chile, and by the FONDAP-Humboldt Program.
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