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vol.68 número2  suppl.TIProcSPATIAL AND TEMPORAL ANALYSIS OF SEAWIFS CHLOROPHYLL IN THE SOUTH TROPICAL PACIFIC OCEANSURFACE CURRENT MAPPING OFF CALIFORNIA WITH RADIOMETRY AND ALTIMETRY índice de autoresíndice de materiabúsqueda de artículos
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Gayana (Concepción)

versión impresa ISSN 0717-652Xversión On-line ISSN 0717-6538

Gayana (Concepc.) v.68 n.2 supl.TIProc Concepción  2004

http://dx.doi.org/10.4067/S0717-65382004000200031 

 

Gayana 68(2) supl. t.I. Proc. : 167-173, 2004 ISSN 0717-652X

SEASONAL AND MESOSCALE VARIABILITY IN THE PERU UPWELLING SYSTEM FROM IN SITU DATA DURING THE YEARS 2000 TO 2004

 

Vincent Echevin1, Ingrid Puillat1, Carmen Grados2 & Boris Dewitte3

1. LODYC/IRD, Paris, France,
2. IMARPE, Lima, Peru,
3. LEGOS/IRD, Toulouse, France.


ABSTRACT

Historical data from hydrological cruises performed off Peru by Instituto del Mar del Peru (IMARPE) during the years 2000-2004 are described and analysed. Hydrographic conditions commonly encountered are briefly described and contrasted with episodes displaying intense surface or subsurface salinity anomalies, in relation to calm wind conditions in summer, intense upwelling and strong mesoscale activity in winter and spring, as evidenced on sea level and wind data. These data suggest a strong variability at different spatial scales, related to a variety of coastal dynamical processes.


 

INTRODUCTION

The Peruvian coastal ocean is a region of permanent and seasonally modulated upwelling, leading to intense primary productivity and enriched fisheries. All year long, the wind is oriented alongshore, hence, upwelling favourable. It reaches its maximum intensity near 14°S-16°S, and weakens towards the north near Punta Falsa (6°S). It is temporally modulated by intense winds events of a few days to a week, which are relatively weak and scarce in spring and summer, and strong and more frequent in winter. The wind-driven surface circulation consists in a shallow, equatorward, coastal jet (Strub et al., 1998), with maximum intensity in winter, known as the Peru coastal Current (PC). This current can be identified by the shoaling of isopycnals in the surface layers nearshore. Offshore, the circulation is dominated by poleward flows, identified as the Peru Chile Counter Current (PCCC) (Huyer et al., 1991, Strub et al., 1995), which advects warm and saline waters from tropical origin (Lukas, 1986). Below the surface coastal current, the subsurface, coastally trapped, Peru-Chile Under Current (PCUC) advects saline (35,0-35,1 PSU) tropical waters poleward. Its signature is characterized by the deepening of isopycnals towards the coast at 50-200 m depth.

The coastal circulation can be strongly modified by variability at time scales from weeks to several years. Intra-seasonal coastal trapped wave activity may displace vertically the nearshore isopycnals and generate alongshore currents. At inter-annual time scales, the ENSO cycle is characterized by the poleward advection of tropical warm waters and the significant deepening of the thermocline (Carr et al., 2002, Halpern et al., 2002) which strongly impact on the primary productivity. However, the recent years, which are the focus of the present study, showed relatively weak inter-annual variability.

For the last decades, Instituto del Mar del Peru (IMARPE) has been conducting several cruises (per year) along its coasts to investigate and follow-up the state of its extremely rich fishery resources and the environment associated. Together with biological measurements (chlorophyll, fish eggs and larvae, juveniles), physical (temperature and salinity), chemical (dissolved oxygen and nutrients), and meteorological measurementes were collected for several cross-shore sections alongshore Peru. We present results for the best-documented sections, in the northern coast (5°-6°S), strongly variable due to intense upwelling and the influence of tropical low and high salinity waters, the widest shelf area between 7°S and 10°S (with a characteristic zonal displacement of high salinity waters towards the coast), and the narrower shelf from 12°S to 16°S, where the strongest upwelling occur (Figure 1).

The measurements were generally collected within 200-300 km from the coast, with a relatively wide sampling distance of ~50 km between CTD stations. The deepest measurements are at ~500 m depth. In the present study, focus is given on recent cruises performed between years 2000 and the beginning of 2004. Altimetric gridded data from Topex/Poseidon and ERS 1-2 and Jason-Envisat (http://www.aviso.fr), as well as Quikscat scatterometer data (http://www.cersat.fr) were used to provide information on the large scale circulation and wind patterns, respectively. Water mass characteristics and spatial structures for the upper 250 m are described, and conditions with striking hydrographic features are contrasted with conditions generally found, and previously described in published literature. In the following sections, we briefly describe the mean spatial structures of the temperature and salinity fields, and focus on a few selected sections which display some peculiar features.

Figure 1. Bottom topography and sections 1 to 8 off Peru.

HYDROGRAPHIC CONDITIONS OFF THE COAST OF PERU

a) Mean and seasonal variability of the temperature field

The mean state of water masses in the Peru upwelling, as observed along the IMARPE sections, consists in water temperatures in the range of 15°C-16°C at 50 m and 12°C-13°C at 250 m depth. These waters are capped by a seasonal thermocline in austral summer, where temperature can exceed 25°C near 5°S and 22°C near 12°S, unless the nearshore thermocline is perturbed by upwelling events (section 4, summer 2003, not shown). In the latter case, the summer nearshore surface temperature can decrease to 17°C. In spring and winter, a mixed layer of ~30-50 m depth develops offshore (section 4, spring 2002, not shown). It is generally shallower nearshore due to increased stratification because of fresher coastal waters. During the most intense upwelling events, the nearshore temperature can reach 15°C.

b) Mean salinity field

The salinity fields display much more spatial and temporal variability than the temperature fields. From 5°S to 12°S, offshore saline surface waters (35,0-35,1 PSU) generally form a salinity front with fresher (34.9-35.0 PSU) coastal waters. This front generates a strong cross-shore gradient of the mixed layer depth, which strongly decreases nearshore. However, the surface salinity can be high (>35.0-35.1 PSU) close to the coast (section 2, spring 2001; section 3, spring 2003, not shown) associated to zonal displacements of oceanic more saline water masses to the coast, mainly in spring and summer seasons.

Near the coast, a subsurface layer with maximal salinity of ~35.0-35.1 PSU can be identified at depths between the surface and 100 m as for instance on section 2 in spring 2003 (Figure 2a) , spring 2002, summer 2004, and on section 4 in fall 2000 and winter 2000 (not shown). This salinity maximum corresponds to the signature of the PCUC, which can also be traced by the deepening isopycnals (s= 26.0-26.2, Figure 2b) between 0-100 km towards the coast (section 2, spring 2002, spring 2003, section 3, spring 2003, not shown) indicating poleward geostrophic flow. In fall and winter, saline waters found in the core of the undercurrent can be traced to the surface near the coast (section 4, spring 2003, summer 2003, section 6, spring 2002, not shown, and Figure 2a). This high surface salinity could be the signature of recently upwelled waters.

Figure 2. Salinity (a) and density (b) along section 2, spring 2003.

c) Seasonal variability and mesoscale structures in the salinity field

The mean structure of the salinity field can vary significantly during the seasons:

1) Spring and summer:
- To the north of Peru (~5°S), sporadically during spring, and more frequently and persistently during summer, surface fresher waters (S< 34.5 PSU) of tropical origin displace to latitudes not higher than 7°S and form a layer of warm, relatively fresh, stratified waters over the whole section (section 1, summer 2000, 2001, 2002 (Figure 3) ). This fresher layer can be disrupted by moderate upwelling (section 2, summer 2004) and possibly advected or maintained offshore (section 1, spring 2002, not shown). This suggest that they are diverted offshore but also northward by some dynamical processes yet to be identified.

- Offshore, subsurface fresh anomalies (~34.7-34.8 PSU) can be encountered (section 1 and 2, spring 2003, section 2, summer 2001, not shown). Due to the short range of the sections, it is unclear whether they are large scale, thin, layered structures, or upducted/subducted lenses of upwelled waters of coastal origin, as suggested by the presence of surface anomalies of similar characteristics nearshore (section 2, spring 2001, not shown).

- The situation displayed on section 6 in spring 2001 is quite striking (Figure 4). There are four distinct salinity anomalies ranging from 34.8 to 34.9 located along the s=26.0-26.2 isopycnal. The source of these anomalies seems to be the water mass at S=34.8, q= 12°C below 150 m depth, at 300 km from the coast. This water mass has been upwelled from greater depths to 150 m, salinity lenses detach from the water mass, and are upwelled towards the coast along the shoaling isopycnals. During their upward migration, they mix with the more saline surrounding waters, and their salinity increase. This situation suggests that the localized, low salinity surface anomalies encountered near shore could be upwelled from offshore deeper waters. The source water could have been upwelled nearshore during a previous upwelling event in the end of winter. However, the presence of a strong cyclonic eddy evidenced on altimetry, which propagates alongshore at ~200-300 km distance from the coast, could be involved in the mechanism of the upwelling event of section 6. A similar type of salinity anomaly is also encountered more to the south, and the amplitude of the anomalies are even more intense (S<34.65, section 7 and 8, spring 2001, section 8, spring 2003, Figure 5). These fresh cores are localized at 100 m depth and may upwell off San Juan (17°S). Their origin is associated with the Subantarctic Water Masses (Grados et al, 2001) but a dynamical process, which could be trapping by eddies, has prevented them from mixing with surrounding waters. The proximity of the Nazca ridge (near sections 7 and 8) could influence the strong local mesoscale activity by trapping eddies and therefore generate fresh water lenses.

Figure 3. Salinity field along section 1, summer 2002.


Figure 4. Salinity along section 6, spring 2001.


Figure 5. Salinity along section 8, spring 2001.

DURING WINTER

- In winter, quite different salinity conditions occur due to intense upwelling events. Mesoscale, localized fresh salinity anomalies (S<34.5 PSU, Figure 6) are evidenced over a great range of depths during or after major upwelling events. In winter 2001, two intense upwelling events took place in mid-august a few days before the cruises. The salinity value is unexpectedly low, and complete checking of the CDT cast is under way. However, a water mass of similar characteristics (S<34.5 PSU) was evidenced more to the south along two sections (sections 3 and 6) at depths between 300 m and 500 m (not shown). These salinity anomalies were not encountered along all sections for a given cruise, which suggests two comments: first, this confirms the well known discontinuity of the upwelling, which is strongest at some specific locations along the coast. Second, the IMARPE measurements, which are not synoptic, may have been performed too late after the relaxation of the upwelling event to display the signature of this event along some of the sections. Furthermore, the salinity anomalies are not located close to the coast but rather in the open ocean (section 2, winter 2000, winter 2001). This raises the question of their origin: do they result from coastal upwelling of deep waters or from a mesoscale eddy generated offshore during or after the strong wind event?

Combined TP/ERS altimetric data suggest a coastal process. It shows a relatively weak (2-4 cm relative to the local average sea level) negative sea level anomaly near section 2, consistent with an upwelling signal, which appears nearshore at the end of august 2000. In winter 2001, high negative anomalies (8-10 cm relative to the local average sea level) were evidenced on August 22nd, 2001 near section 2, following two intense wind events between mid-august and beginning of september 2001 (not shown). However nearshore altimetric data quality is rather questionable, and the short scales of the upwelling can not be resolved on gridded data at 1/3° resolution. This can only indicate that the saline anomalies generate cyclonic anomalies as a response to recent upwelling events. The anticyclonic nature of the anomalies is confirmed by the density structure along the sections resulting from the salinity anomalies, which generate deep geostrophic equatorward flow nearshore and poleward flow offshore (section 2, winter 2000, winter 2001, not shown).

Figure 6. Salinity along section 2, spring 2001.

On all figures, vertical red lines are CTD casts.

CONCLUSIONS

The sections analysed in this study confirm the presence of several spatial patterns previously described in the literature (Strub et al., 1998 and references herein), such as the shoaling isotherms and isopycnals, typical of upwelling regimes, and the relatively high salinity subsurface signature of the PCUC. However, the seasonal and mesoscale variability of salinity structures display various local features. Near Punta Falsa (6°S), fresh surface waters, of tropical nature, may propagate to the south during the spring and mainly summer season. During the other seasons, these events were not encountered, likely because of surface mixing due to the stronger winds, and to the stronger equatorward Peru coastal current, which advects tropical waters offshore, contributing to the equatorial cold tongue (Strub et al., 1998). In spring, several lower salinity lenses are evidenced at ~100 m depth. These salinity lenses related to the core of Subantarctic Water Masses seem to be relatively isolated and poorly mixed with surrounding waters, especially near the Nazca ridge, a topographic feature which could influence the vertical structure of eddies and the trapping of salinity cores. Some of the anomalies are clearly upwelled along isopycnals and reach the surface nearshore, freshening the relatively saline coastal waters which originate from the PCUC in normal conditions. A most striking event is the intense upwelling events that occurred in winter 2000 and 2001 (Figure 6), and brought to the surface low salinity waters from depths greater than 500 m, at various places from 5°S to at least 12°S along the Peru coast. The various forcings (wind events, influence of the tropical circulation in the northern and central Peru, the role of Nazca ridge), and the subtropical circulation which may trigger these events, remain to be investigated through model studies.

ACKNOWLEDGEMENTS

Luis Vásquez and Walter García Diaz are acknowledged for collecting and processing part of the in situ data. The Institut de la Recherche pour le Développement (IRD) is acknowledged for funding Ingrid Puillat through a post-doc scholarship, and W. Garcia Diaz through the IRD Action Thématique Inter-disciplinaire "Humboldt" supervised by P. Soler.

BIBLIOGRAPHY

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