Elsevier

Progress in Oceanography

Volume 165, July–August 2018, Pages 123-144
Progress in Oceanography

Upper ocean hydrology of the Northern Humboldt Current System at seasonal, interannual and interdecadal scales

https://doi.org/10.1016/j.pocean.2018.05.005Get rights and content

Highlights

  • A seasonal climatology of the NHCS was constructed gathering 50-year of in situ hydrological measurements.

  • Alongshore and cross-shore thermohaline properties and geostrophic circulation show strong variability at a seasonal scale.

  • Abrupt changes in T/S fields are found in the water column during El Nino and La Nina periods.

  • Decadal variability also impacts the Peru upwelling system.

Abstract

Since the 1960s, the Instituto del Mar del Perú (IMARPE) collected tens of thousands of in-situ temperature and salinity profiles in the Northern Humboldt Current System (NHCS). In this study, we blend this unique database with the historical in-situ profiles available from the World Ocean Database for the period 1960–2014 and apply a four-dimensional interpolation scheme to construct a seasonal climatology of temperature and salinity of the NHCS. The resulting interpolated temperature and salinity fields are gridded at a high spatial resolution (0.1° × 0.1° in latitude/longitude) between the surface and 1000 m depth, providing a detailed view of the hydrology and geostrophic circulation of this region. In particular, the mean distribution and characteristics of the main water-masses in the upper ocean of the NHCS are described, as well as their seasonal variations between austral summer and winter. The coastal upwelling region is well documented due to the increased data density along 3 highlighted cross-shore vertical sections off Paita (∼5°S), Chimbote (∼9°S) and San Juan (∼16.5°S). The large and long-term database also allowed us, through a composite analysis, to investigate the impact of the eastern Pacific El Niño and La Niña events on the NHCS hydrology. On average, during these periods, large temperature (±3–4 °C) and salinity (±0.1–0.2) anomalies are observed, impacting the water column of the coastal ocean off Peru down to 100–200 m depth. At 100 km from the coast, these anomalies are associated with a maximum deepening (shoaling, respectively) of the thermocline of 60 m (25 m) during composite El Niño (La Niña) events. At interdecadal scale, a similar approach reveals sea-surface temperature variations of ±0.5°C, associated with a deepening (shoaling) of the thermocline of 5–10 m during warm (cold) periods.

Introduction

The Northern Humboldt Current System (NHCS: 70–90°W; 0–20°S), mainly located off the Peruvian coast, corresponds to the northern part of the Peru-Chile upwelling system and is the region where water exchange between the equatorial and subtropical Eastern South Pacific occurs (Fig. 1). In the NHCS, the circulation is rather complex and transports water-masses (WM) of distinct origins with specific temperature (T) and salinity (S) ranges (Fig. 1). In the first ∼1000 m of the water column the main WM characteristics and large-scale circulation can be summarized as follows (e.g., Wyrtki, 1964, Wyrtki, 1965, Wyrtki, 1966, Wyrtki, 1967, Stevenson, 1970, Kessler, 2006, Fiedler and Talley, 2006; see Fig. 1).

First, the atmospheric South Pacific subtropical anticyclone drives an upper layer subtropical gyre (e.g., Strub et al., 1998, Strub et al., 2013), of which the eastern branch in the NHCS is associated with the equatorward Peru Oceanic Current (POC) (e.g., Gunther, 1936, Wyrtki, 1965, Wyrtki, 1966). The POC transports in the surface layers relatively cold (T < 13 °C) and fresh (S < 34.3) Eastern South Pacific Intermediate Water (ESPIW) from Southern Chile towards the NHCS (Schneider et al., 2003). North of ∼20°S the POC progressively veers westward feeding the South Equatorial Current (SEC) (Chaigneau and Pizarro, 2005), and the ESPIW, which mixes with the warmer (T > 20 °C) and saltier (S > 35.1) Subtropical Surface Water (STSW), spreads northwestward in the lower limit of the thermocline forming a lens of low-salinity water (Schneider et al., 2003).

Second, persistent alongshore winds (Bakun and Nelson, 1991) favor a year-round near-coastal upwelling of cold, nutrient-enriched subsurface waters (T < 19°C and S < 35.0) resulting in high rates of productivity and the most productive eastern boundary upwelling ecosystem in terms of small pelagic fish (e.g., Chavez et al., 2008, Fréon et al., 2009). The upwelled Peruvian coastal water (pcw1) and the offshore STSW form a strong cross-shore density gradient that forces, through geostrophic adjustment, the equatorward Peru Coastal Current (PCC) (e.g., Strub et al., 1998).

Third, in the Northern region of the NHCS the upwelled pcw is advected northwestward by the PCC and SEC, and mixes with the water upwelled from the Equatorial Undercurrent (EUC) along the equator. It results in a cold tongue extending from Northern Peru to the west of Galapagos Islands associated with Equatorial Surface Water (ESW) and characterized by relatively low temperature (T < 21°C) and high salinity (S > 34.5) (Wyrtki, 1966, Fiedler and Talley, 2006).

Fourth, North of ∼2–3°S and nearshore is found the relatively warm (T > 23°C) and fresh (S < 33.5) Tropical Surface Water (TSW) that results from an excess of precipitation beneath the Inter Tropical Convergence Zone and the Gulf of Panama, and freshwater input from large Colombian and Ecuadorian rivers (Enfield, 1976, Strub et al., 1998, Fiedler and Talley, 2006, Poveda and Mesa, 2000). This TSW can be transported poleward along the north Peruvian coast by the surface Ecuador-Peru Coastal Current (Chaigneau et al., 2013), or northwestward by the surface Ecuador Coastal Current close to the Ecuadorian coast (not shown in Fig. 1) (Collins et al., 2013). The strong temperature and salinity gradients separating TSW from ESW are observed in the first ∼100 m of the water column form the Equatorial Front (Pak and Zaneveld, 1974, Fiedler and Talley, 2006).

In the subsurface layers (∼100–1000 m depth), the NHCS encompasses two main water masses, namely the Equatorial Subsurface Water (ESSW) and the Antarctic Intermediate Water (AAIW). ESSW (T < 17 °C and S > 35), which is transported poleward along the continental slope by the Peru Chile Undercurrent (PCUC) (Silva and Neshyba, 1979, Chaigneau et al., 2013) is the main source of the upwelled pcw driving the high productivity of this ecosystem (Brockmann et al., 1980, Huyer et al., 1987, Toggweiler et al., 1991, Albert et al., 2010). Furthermore, the offshore spreading of the ESSW between ∼100 m and ∼600 m depth forms the core of the most intense and shallower subsurface oxygen minimum zone of the World Ocean (Karstensen et al., 2008, Paulmier and Ruiz-Pino, 2008, Chavez and Messié, 2009, Llanillo et al., 2013). Below the ESSW, the relatively cold (T ∼ 5 °C) and fresh (34.3 < S < 34.5) AAIW spreads northward at intermediate depths (e.g., Tsuchiya and Talley, 1998).

The mean WM characteristics and large-scale circulation patterns vary seasonally according to atmospheric heat flux variations (e.g., Takahashi, 2005) and wind forcing variations associated with the position and strength of the large-scale atmospheric anticyclone (e.g., Strub et al., 1998, Strub et al., 2013). However, one of the key features of the NHCS is its connectivity to the basin-wide equatorial dynamics that strongly impacts the hydrography, coastal currents and productivity of the eastern South Pacific at intraseasonal (e.g., Belmadani et al., 2012, Illig et al., 2014, Echevin et al., 2014), seasonal (e.g., Echevin et al., 2011) and interannual scales (e.g., Barber and Chavez, 1983, Chavez et al., 2003, Colas et al., 2008, Montes et al., 2011, Espinoza-Morriberón et al., 2017). Among these different timescales, the most important phenomenon is probably the well-known El Niño Southern Oscillation (ENSO), which is the strongest mode of natural climate fluctuation at interannual scales (Philander, 1990, Wallace et al., 1998, Timmermann et al., 1999). Although ENSO can manifest as central Pacific or eastern Pacific events, the NHCS is mainly affected by “canonical” eastern Pacific events (Ashok et al., 2007, Kao and Yu, 2009, Takahashi et al., 2011, Vimont et al., 2014). Indeed, during the warm (El Niño) and cold (La Niña) phases of eastern Pacific ENSO events, the physical and biogeochemical characteristics of the NHCS as well as the large-scale circulation (Colas et al., 2008, Montes et al., 2011) and the whole ecosystem have been shown to be strongly impacted (Barber and Chavez, 1983, Barber and Chavez, 1986, Chavez et al., 2003, Jahncke et al., 2004). At interdecadal scales, the Pacific Decadal Oscillation (PDO) (Mantua et al., 1997), often described as a long-lived El Niño-like pattern of Pacific climate variability (Zhang et al., 1997, Mantua and Hare, 2002), seems to also impact the NHCS as shown from multidecadal variations in the productivity and main ecosystem components (Chavez et al., 2003, Thomas et al., 2009, Bertrand et al., 2011, Gosselin et al., 2013, Guinez et al., 2014).

Due to these peculiar characteristics, the NHCS has spurred intensive multidisciplinary studies through national and international cooperative programs, mainly initiated in the 1960s with the beginning of a long-term oceanographic monitoring program conducted by the IMARPE. It consists of repeated ship surveys (4–5 per year) that occupy several hydrographic cross-shore sections between the Peruvian coast and as far as 200 nm offshore, covering the main upwelling and fishery areas of the NHCS. This program, which has been improved regularly throughout the years, aims to provide to the Peruvian government the scientific basis for the rational exploitation and sustainability of the Peruvian fishery resources. Although the in-situ temperature and salinity data acquired during these surveys have been used to depict some physical characteristics of the near-coastal WM in the NHCS, the results were mainly published in national journals (e.g., IMARPE, 1965, Zuta and Guillén, 1970, Zuta and Urquizo, 1972, Urquizo et al., 1987, Morón, 2011) and an integrated and comprehensive study based on all available information is still lacking. Besides, existing global climatologies, such as the World Ocean Atlas (Locarnini et al., 2010, Antonov et al., 2010) and the CSIRO Atlas of Regional Seas (CARS2009) (Ridgway et al., 2002, Dunn and Ridgway, 2002) have not incorporated the nearshore in-situ data collected during IMARPE cruises after the 1980s; in addition, the coarser resolution (e.g., 0.5° × 0.5°) of these products does not fully resolve coastal features, particularly in upwelling regions as the NHCS.

Thus, the main goal of this study is to obtain a high-resolution (0.1° × 0.1°) seasonal climatology of the hydrology and WM characteristics in the NHCS by merging the IMARPE original dataset with other in-situ observations from available international cruises or autonomous floats (see Section 2). The resulting NHCS regional climatology provides a detailed description of the mean and seasonal variations of the physical properties (temperature, salinity and density) and geostrophic circulation along the Peruvian coast. This product may serve as a reference for multidisciplinary studies or to validate regional model simulations. Furthermore, given the good spatio-temporal coverage of the blended database, the mean impact of the interannual ENSO events and interdecadal warm and cold phases on the WM characteristics of the NHCS are also investigated. While the obtained results are of interest for the oceanographic community they can also be relevant for regional climate studies, since the surface WM encountered in the near-coastal regions of the NHCS tend to stabilize the marine boundary layer leading to the largest and most persistent stratocumulus deck in the world (Klein and Hartmann, 1993, Wood et al., 2010, Zheng et al., 2011).

Section snippets

Data sources

The vertical T and S profiles used to compute the climatology were gathered from (i) the World Ocean Database (WOD, http://www.nodc.noaa.gov/OC5/WOD/pr_wod.html) of the National Oceanographic Data Center (NODC) and (ii) the IMARPE. T and S profiles collected between January of 1960 and December of 2014 in the region extending from 30°S to 10°N and from 70°W to 100°W were used (Fig. 2). Vertical profiles from the WOD were acquired by several types of instruments (Johnson et al., 2006):

Surface temperature, salinity and density fields

General hydrographic features of the NHCS are illustrated by the mean seasonal patterns of SST, SSS and sea-surface density (SSρ) in austral summer and winter (Fig. 4). Close to the coast, SST displays a well-marked nearshore front, stronger in summer (∼4 °C over ∼100–200 km, Fig. 4a) than in winter (∼2 °C over ∼100–200 km, Fig. 4d). This near-coastal thermal front separates the cold upwelled pcw, that have a quite homogeneous salinity of 34.9–35.1, from the warmer and saltier offshore STSW,

Conclusion and discussion

The main objectives of this study were (i) to provide an updated description of the hydrography and geostrophic circulation in the NHCS, (ii) to investigate the mean impact of the interannual ENSO events on the temperature and salinity structure and (iii) to determine the interdecadal changes since the 1960s. Based on the combined analysis of in-situ temperature and salinity profiles from the World Ocean Database, complemented by more than 370 cruises realized by IMARPE between 1960 and 2014,

Acknowledgements

We express our gratitude to crewmembers, oceanographers and supporting staff of IMARPE, especially L. Vásquez and the physical oceanography team, and international institutions that collected and made freely available the data used in this work. Argo data (http://doi.org/10.17882/42182) were collected and made freely available by the International Argo Program and the national programs that contribute to it. (http://www.argo.ucsd.edu, http://argo.jcommops.org). The Argo Program is part of the

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