Future climate evolution in the canary current upwelling system from a regional coupled model

Abstract

The Canary current upwelling system (CCUS) is one of the major eastern boundary coastal upwelling systems in the world, bearing a high productive ecosystem and commercially important fisheries. The CCUS has a large latitudinal extension, and it is divided into upwelling zones with different characteristics. Eddies, filaments and other mesoscale processes characterize upwelling dynamics and are known to have an impact in the upwelling productivity. Thus, for a proper representation of the CCUS, a high horizontal resolution is required. In this study we assess present and future climate of the CCUS using an atmosphere¿ocean regionally coupled model. The regional coupled model consists of a global oceanic component with increased horizontal resolution along the northwestern African coast (reaching the 5 km in Cape Ghir) coupled to a high-resolution regional atmosphere (25 km), which extends its domain to the North Atlantic, including the whole CCUS region. We assess the model¿s present-time performance over the CCUS against relevant reanalysis data sets and compared with an ensemble of global climate models (GCMs) and an ensemble of atmosphere-only regional climate models (RCMs) to evaluate the role of the horizontal resolution. The coupled system reproduces the larger scale pattern of the CCUS and its latitudinal and seasonal variability over the coastal band, improving the GCMs outputs. The model properly reproduces mesoscale structures and is able to simulate the upwelling filaments events off Cape Ghir, which are not well represented in most of GCMs. Our results demonstrate the ability of the regionally coupled model to reproduce both the larger scale and mesoscale processes over the CCUS.

Under RCP8.5 scenario in summer (winter), the upwelling favourable winds increase (decrease) along the Iberian coast and decrease (increase) for the African region. The model simulations suggest that the Azores high is the main driver of these variations in winter, while in summer, the changes are attributed to the intensification of the Iberian thermal low along with an increase in mean sea level pressure over the British islands. This increase may be associated with a weakening of the Atlantic meridional overturning circulation (AMOC). The southernmost region in CCUS, the Mauritania-Senegal upwelling region (MSUR), is expected to experience seasonal changes mainly defined by Cap-Vert (15ºN), where the upwelling favourable winds in the northern subregion will be intensified throughout the year in the future. In the southern MSUR, the upwelling favourable winds are expected to weaken in April-May and strengthen during the winter months. This pattern has been associated with an intensification of the Azores high in winter and spring. Moreover, our study highlights the importance of the high resolution in reproducing coastal upwelling, as we detected changes in upwelling associated with local increases in air temperature over Sahel. During summer, a drastic thermal rise in the African continent will intensify the Saharan thermal low, increasing the upwelling favourable winds in the northern region of MSUR.

However, the efforts of the scientific community to evaluate the effects of climate change on EBUSs have not only focused on the upwelling favourable winds; changes in ocean stratification may also have a significant impact on these upwelling systems. Although both the wind patterns and ocean stratification responses can provide useful information on the future of these vulnerable ecosystems, obtaining a joint response from both mechanisms is essential. In this context, we obtained a joint response from the upwelling source water depth, which is highly dependent on the latitudinal and seasonal variability of the CCUS. We found that ocean stratification plays a primary role in the two northern regions of the CCUS, while in the PUR and MSUR, it plays a secondary role. Nevertheless, both ocean stratification and wind patterns have a significant impact on the future of the CCUS, and our results reveal the importance of studying both mechanisms seasonally and latitudinally.