Fixed-point time series, repeat survey and high-resolution modeling reveal event scale responses of the Northwestern Iberian upwelling

Highlights

• Model and observations define variability on Northwest Iberian Margin.

• Physical characteristics defined to below meso- and event scales through entire year.

• Up/downwelling cycles and buoyant coastal plumes documented in unprecedented detail.

Abstract

The hydrography and circulation of the Northwestern Iberian Margin (NWIM) was studied over an annual cycle from November 2008 to December 2009. The study was focused on the Cape Silleiro border region that separates the rugged northern coastline of the Rías Baixas embayments and the smoother southern coastline, a source of major fresh water outflows. Monthly across-shelf transects off Cape Silleiro monitored hydrography and currents, while an upward-looking ADCP registered currents in 72 m depth on the shelf over the period. To provide greater detail of the variability both spatial and temporal, during the annual cycle, a ROMS-AGRIF configuration of the region was built. Validation of the model against the observations produced robust results that reproduced many aspects of the observed variability. It was therefore possible to examine features of the NWIM, such as the seasonal cycle, wind-induced upwelling and downwelling and their associated horizontal circulations, as well as nearshore countercurrents related to freshwater plumes, in unprecedented detail. Five different scenarios were investigated in the study year through examination of characteristic physical processes: (1) late autumn upwelling, (2) high energy winter conditions, (3) spring upwelling-downwelling cycles, (4) summer upwelling, and (5) autumn transition to high energy conditions. The demonstration of the robust capacity of this model configuration, to reproduce in detail the phenomena observed in this region, suggests that the physical model could be reliably extended to other years in order to provide the basis for multidisciplinary studies in relation to biogeochemical cycles and harmful blooms. Other applications could include transport of sediments, larvae, microplastics and other contaminants as well as operational application for general ocean forecasting.

Introduction

The Northwestern Iberian Margin (NWIM) forms the northernmost part of the North Atlantic Eastern Boundary Upwelling System (EBUS), which like other EBUS is governed by the seasonally varying location of a mid-latitude atmospheric high-pressure system (Relvas et al., 2007). In the Eastern North Atlantic, the anticyclonic Azores High migrates between offshore the Iberian Peninsula during summer and south of the Azores during winter. The intensification of the Iceland Low in winter permits the eastward passage of low pressure systems that enhances variability of atmospheric circulation and causes significant precipitation over the NWIM (Vitorino et al., 2002). On interannual scales the strength of the winter variability is related to the North Atlantic Oscillation (NAO) (Miranda et al., 2002).

As with other EBUS, the large scale oceanic circulation along the roughly meridional coast is dominated by a slow equatorward flow, in this case the Portugal Current (Saunders, 1982), on the eastern limb of the subtropical gyre. The region of the NWIM concentrated on here stretches from Cape Mondego to Cape Finisterre (40°N to 43°N in Fig. 1). North of Cape Silleiro (42°06′N), the coastline is made up of a series of embayments, the Rías Baixas; to the south, it is relatively smooth and oriented approximately NNW as far as Porto (41°06′N). The shelf deepens gently to the shelf edge at the 200-m isobath, and is bounded by a steep slope that plunges to 2000–4000 m. To the south of Porto Canyon (41°20′N) the shelf is around 50–60 km wide, while to the north it narrows. Inshore of the 100-m isobath, the inner shelf generally widens in the southward direction.

The upwelling season starts in late spring in response to the northerly winds resulting from the build-up of the Azores high (Wooster et al., 1976). The continental shelf circulation responds on the scale of an inertial period to the atmospheric forcing and, under constant winds, reaches equilibrium after 3 or more days (McClain et al., 1986). Offshore Ekman transport in the surface layer produces upwelling of subsurface cold, nutrient-rich central waters at the coast (Torres et al., 2003, Rossi et al., 2013). As a consequence, the sea level falls nearshore and an equatorward geostrophic jet develops along the front between coastal cold upwelled waters and the offshore warmer waters (Ambar and Fiuza, 1994). The front typically moves offshore with prevailing upwelling-favorable winds (Haynes et al., 1993) as far as the shelf edge (Rossi et al., 2010, Rollo et al., 2020). As the upwelling season progresses, the front becomes unstable and gives rise to filaments of cold, offshore flowing water at preferred locations, including one south of Cabo Silleiro at 42°N (Haynes et al., 1993, Cordeiro et al., 2015). This filament may be associated with the region of intensified upwelling or colder upwelled waters between Cape Silleiro and Porto (Fig. 1 Relvas et al., 2007, Cordeiro et al., 2018). However, Álvarez-Salgado et al. (2003) found that >70 percent of the variability of the coastal alongshore wind stress that forces the system occurs at time scales shorter than 10 days. The seasonal cycle of the NWIM, therefore, is partly masked by mesoscale phenomena occurring on time scales of days to weeks.

The mesoscale variability manifests as intermittent occurrence of upwelling, jets and counter-flows, meanders, eddy development, and upwelling filaments, superimposed on the seasonal changes (Peliz et al., 2002, Peliz et al., 2005, Serra et al., 2002). While extended periods of equatorward wind appear to result in exclusively equatorward currents over outer shelf and upper slope (Rollo et al., 2020), even brief periods of calm or northward wind can allow coastal downwelling and counterflows to develop (Torres et al., 2003) and to extend into the Rias Baixas (Barton et al., 2016).

In winter, wind has a persistent northward component and the Iberian Poleward Current (IPC) advects warm waters from the south along the continental slope and shelf edge (Frouin et al., 1990, Haynes and Barton, 1990, Peliz et al., 2005). The relatively warm, and also saline, surface water of the IPC can be tracked into the Bay of Biscay and further north (Pingree and Le Cann, 1990). This flow has been attributed to the interaction of the meridional density gradient and intermittent southerly winds with the continental slope and shelf (Huthnance, 1984, Peliz et al., 2003). The IPC is thought to be present all year, although it weakens and spreads offshore in spring to become confined to the subsurface during summer, below the upwelling jet (Huthnance et al., 2002). At the start of the upwelling season in May and June, the offshore IPC has been observed to coexist with shelf and slope equatorward flows (Torres and Barton, 2007). Following the end of the upwelling season, from September through January, as southerly winds are increasingly prevalent, the IPC becomes more surface intensified and jet-like (Teles-Machado et al., 2016). The poleward flow is frequently associated with eddies and smaller scale instabilities spun up either side of the current core (Peliz et al., 2003). The year-round variability of forcing results in occurrence of sporadic winter upwelling events (Vitorino et al., 2002), which can have significant biological repercussions (Santos et al., 2004, Ribeiro et al., 2005).

The passage of atmospheric low pressure frontal systems is responsible for precipitation events and increased river outflow on the Iberian coast. The sporadic injections of buoyant river outflow modify the typical annual cycle of stratification of the nearshore surface layers associated with local heat and momentum fluxes between atmosphere and ocean. Winter mixing generally homogenizes temperature and salinity down to 100 m, occasionally 200 m (Reboreda et al., 2014) but shallow buoyant plumes form because of continental runoff of fresh waters and local precipitation. Summer stratified conditions are modified over the shelf by upwelling and also by river outflow. In the absence of other forcing, waters discharged from western Iberian rivers form a plume deflected adjacent to the coast in the direction of coastal trapped waves, as expected (Chao and Boicourt, 1986, Horner-Devine et al., 2015). Peliz et al. (2002) reported that the individual river plumes can merge into a single Western Iberian Buoyant Plume (WIBP). Under northerly winds, westward Ekman transport spreads the plume seawards across the shelf to disappear by mixing with oceanic waters, while under southerly winds, the plume is constrained against the coast and the halocline below it deepens nearshore, even reaching the bottom and becoming vertical with strong winds (Otero et al., 2008, Mendes et al., 2016). During periods of high continental runoff and persistent downwelling, the plume waters from the rivers to the south may reach and enter the Rías Baixas (Sordo et al., 2001). There the surface fresh waters accumulate and warm through shortwave radiation (Barton et al., 2015) so that in subsequent upwelling events, these waters are exported to the shelf, with lower salinity and higher temperature than the locally upwelled waters (Álvarez-Salgado et al., 2000, Torres and Barton, 2007).

Basic understanding of the shelf and slope conditions has been obtained from extensive observational efforts over the years, although these have often been sporadic, with different goals and restricted to sub-regions. However, understanding is limited because slow, often low-resolution ship surveys made in different locations during different years under rapidly changing conditions complicate the interpretation because the NWIM is driven by processes on scales from years to days acting together in a complex manner.

The mean circulation and its inter-annual and seasonal changes in the NWIM have been studied through numerical modeling (Nolasco et al., 2013, Teles-Machado et al., 2016). Idealized studies have helped comprehend the development of the coastal upwelling and the formation of filaments (Røed and Shi, 1999, Meunier et al., 2010, Rossi et al., 2010), while the poleward slope currents have been modeled by Peliz et al. (2003). Other model configurations, with enough spatial resolution to resolve processes on scales down to kilometers, have addressed the winter dynamics of plumes and their interaction with the Rías Baixas (Otero et al., 2008, Otero et al., 2013, Sousa et al., 2014, Mendes et al., 2016).

For the first time, we integrate high-resolution modeling with systematic spatial and Eulerian observations on different scales over an annual cycle to explore details of the shelf/slope hydrography and circulation of the NWIM and its relation to the open ocean (Section 2). A year of cross-shelf transects and moored observations, complemented by a model able to resolve the transient processes, has produced new insights into details of shelf conditions (Section 3). The results are placed in the context of experience in this and other upwelling regions (Section 4) before remaining gaps and recommendations for further study are discussed.