Spatial shifts in productivity of the coastal ocean over the past two decades induced by migration of the Pacific Anticyclone and Bakun's effect in the Humboldt Upwelling Ecosystem
Graphical abstract
Introduction
Documenting long-term trends in Eastern Boundary Upwelling Systems (EBUSs) is of great importance not only because a large fraction of the human population lives near these shores, but also because these are the most productive marine regions in terms of phytoplankton biomass and fish stocks (Cushing, 1971; Cury et al., 1998; Chavez and Messie, 2009). Indeed, equatorward winds along these shores induce the upwelling of subsurface cold water masses rich in nutrients, which fuel phytoplankton growth and, through the trophic web, provide the necessary energy to sustain some of the largest industrial and artisanal fisheries of the world (Mann and Lazier, 2006; Chenillat et al., 2013; Chavez and Messie, 2009; Salas et al., 2011). Thus, variability of climatic drivers on surface water productivity within EBUS regions impinges directly on our ability to adapt to climate change and on the sustainability of major industrial and local fisheries, with the large direct and indirect socio-economic impacts they generate (Pauli and Christensen, 1995). Consequently, several recent studies have examined long-term trends in wind forcing and physical oceanographic conditions in most EBUSs and, naturally, our understanding of long-term climate effects in these regions of the world's oceans has improved considerably. The effects that these long-term hydrographic changes are having on productivity of the coastal oceans are now a matter of intensive research. However, to understand these temporal trends in primary productivity, studies simultaneously integrating large scale climatic forcing, wind dynamics and sea surface temperature variability are still lacking for the most important EBUSs.
On a global scale, the poleward expansion of the atmospheric Hadley circulation cells, due to the weakening of the thermal differences between the poles and the equator, is known to have already caused the poleward displacement of both the subtropical anticyclones and the subpolar westerlies belts (Previdi and Liepert, 2007; Nguyen et al., 2013). The result has been a poleward expansion and intensification of upwelling favourable winds along most EBUS over the past two decades (McGregor et al., 2007; Lima and Wethey, 2012; Sydeman et al., 2014). The trend is particularly evident in the Southern Pacific, along the Humboldt Upwelling Ecosystem (HUE), where the poleward movement of the Southwestern Pacific Anticyclone (SPA) has displaced the westerlies belt closer to Antarctica (Fan et al., 2014; Ancapichun and Garcés- Vargas, 2015). Recent studies have shown that the seasonal latitudinal migration of the SPA has moved poleward to sit around 36°S during the critical austral spring-summer months that fuel phytoplankton productivity in the coastal section of the south eastern Pacific ocean (Schneider et al., 2017; Aguirre et al., 2018). The displacement has been captured by one of the main modes of climatic variability in the region, the Pacific Annular Mode (SAM), which has shown a clear positive trend in the last decades, thus pointing to low atmospheric pressures over Antarctica compared to those at subtropical latitudes (Marshall, 2003; Wang and Cai, 2013). It has also driven a shift in atmospheric conditions along a broad region of the central coast of Chile, between ca. 30°–35° S, where spring winds have persistently decreased over the past decades (Aguirre et al., 2018), and a poleward region where winds have intensified, rainfall has decreased and surface water cooling has been observed throughout the mixed layer, especially since 2007 (Schneider et al., 2017; Jacob et al., 2018; Narváez et al., 2019). The latitudinal position of the SPA and its regular seasonal migration clearly modulate long-term trends in upwelling phenology and atmospheric climatic conditions (Ancapichun and Garcés- Vargas, 2015; Weller, 2015; Jacob et al., 2018), and impose geographic discontinuities in upwelling regimes, which in the HUE occur around 30° S (Strub et al., 1998; Hormazabal et al., 2001; Navarrete et al., 2005; Tapia et al., 2014). Such discontinuities in upwelling regimes are known to have far-reaching consequences on pelagic and benthic ecosystems, including population dynamics, larval recruitment, adult abundances, the role of species interactions, and genetic and functional structure of benthic communities (Navarrete et al., 2005; Wieters et al., 2009; Tapia et al., 2014; Haye et al., 2014). How climate forcing and the progressive poleward displacement of the SPA will in the long-term affect the biogeographic discontinuity at 30°S remains unclear.
Besides the intensification and poleward migration of subtropical anticyclones, in 1990 Bakun (Bakun, 1990) predicted that the greenhouse effect would warm the oceans slower than adjacent land masses, thus the land-sea pressure differential will become reinforced, strengthening landward breezes which, due to Coriolis, would increase equatorward winds and coastal upwelling at all EBUSs. The phenomenon is known as the Bakun effect (Bakun, 1990; Bakun et al., 2010), and empirical evidence and model results supporting such response to anthropogenic warming have been controversial (Demarcq, 2009; Pardo et al., 2011). Recent meta-analyses of observations and models show that significant intensification of winds, attributable to the Bakun effect, has occurred in all EBUSs but the Canary Current system, where winds have weakened (Sydeman et al., 2014). This result is consistent with several in-situ observations (García-Reyes and Largier, 2010; Weller, 2015) and model predictions for the XXI century (Wang et al., 2015). But other studies, using different databases, have found no evidence for a general intensification of upwelling along EBUSs (Varela et al., 2015). Contradictory results obtained by different studies are due, at least in part, to the fact that long-term responses within EBUSs are not homogeneous, but trends may change across latitudes (Varela et al., 2015). Thus, the regional averages examined to date may provide an incomplete picture of long-term trends in the highly dynamic and spatially variable coastal ocean that characterize all EBUS.
To what extent the positive or negative trends in equatorward winds, the driving force of upwelling, have led to changes in primary productivity of coastal waters is now being intensively investigated (Bakun et al., 2015; Gómez-Letona et al., 2017). The effect of altered upwelling winds on surface primary productivity can have contrasting effects over coastal and offshore domains and analyses must therefore take this into account. For instance, stronger coastal upwelling generates turbulence and mixing alongshore which, together with enhanced seaward transport by the Ekman drift, may lead to higher offshore phytoplankton concentrations at the expense of a decrease in nearshore primary productivity (Lachkar and Gruber, 2012; Anabalón et al., 2016). In contrast, a scenario with higher phytoplankton biomass nearshore induced by enhanced upwelling-driven nutrient inputs is also possible (Bakun et al., 2010). Moreover, the influence of upwelling intensification or weakening on phytoplankton biomass may depend on the eco-physiological requirements of different phytoplankton groups (Smayda, 2000) or changes in other nutrient sources, such as riverine inputs (Masotti et al., 2018; Jacob et al., 2018).
In this study, we aim to quantify and disentangle the role of SPA dynamics and Bakun's effect on upwelling spatiotemporal fluctuations at the Humboldt Upwelling System. To this end, time series of sea-land thermal differentials, sea level atmospheric pressure, meridional wind stress, satellite sea surface temperature (SST) and chlorophyll-a concentration (Chl-a) with the highest possible spatiotemporal resolution for the region, together with in situ measurements of nearshore Chl-a, were inspected along central Chile between 26°–36° S in search of interannual and seasonal trends. With this approach we were able to evaluate the strength of such trends, their spatial distribution across the region and their correspondence with the displacement of the SPA and Bakun's effect. The direction and spatial structure of the temporal trends measured in all these variables will allow us further comparisons with other models and observations developed both within the Humboldt Upwelling System and in other EBUSs.
Section snippets
Physical conditions: winds, SPA migration and Bakun's effect
Processed, science-quality satellite data for SST and Chl-a were retrieved from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua spacecraft (https://coastwatch.pfeg.noaa.gov/erddap/griddap/erdMH1sstd8day.html;https://coastwatch.pfeg.noaa.gov/erddap/griddap/erdMH1chla8day.html). In particular, 8-day composites with 4 km of pixel resolution were retrieved at different distances from the shore (20, 40, 60, 80, 100, 200, 300, 400 and 500 km) between 26°–36° S off the
Results
Time series of monthly SPA latitudinal position revealed strong seasonality, with the core of the anticyclone located at 29° S in austral winter (July and August) and at 35° S at the end of austral summer (Fig. 1B). The GAMM analyses revealed a slight southward trend along the entire time series, and within each season, linear regressions showed sharp and significant poleward displacement in spring (Fig. 1A), as the SPA shifted from 31° S in 2003 to 35° S in 2015. Given this sharp spring trend,
Discussion
Here we documented the existence of contrasting long-term trends in wind forcing and SST along different portions of the Humboldt Upwelling Ecosystem, and show how these climate-driven changes have translated into dissimilar domains of surface water productivity in the costal ocean. At the equatorward domain of the region, north of 30° S, phytoplankton productivity (Chl-a biomass) has increased in nearshore habitats since the early 2000's. At the poleward section, between 31°–35° S and over the
Conclusions
In summary, two main effects of anthropogenic global warming on upwelling systems seem to be responsible for two contrasting domains of long-term dynamics in primary productivity of the coast, with limits between these domains around 30° S to 31° S. To the north, Bakun's increased sea-land thermal contrast would enhance upwelling-favourable winds and coastal primary productivity. To the south of the latitudinal break, there is an ample region of decreasing upwelling winds that extends down to
Declaration of Competing Interest
None.
Acknowledgements
We thank research assistants and students, especially Ivan Albornoz, for helping in the collection of water samples at ECIM and Mirtala Parragué for managing the in-situ Chlorophyll-a dataset. The National Fund for Scientific and Technological Development, FONDECYT (Chile), supported with post-doctoral grants to NW [3150072], AO [3150425] and JB [3160294]. BRB was supported by FONDECYT 1181300 and the Millenium Nucleus Center for the Study of Multiple Drivers on Marine Socio-Ecological Systems
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