Pedro Vélez-Belchí, Spanish Oceanographic Institute
During 2018, continued the cooling and freshening that begin in 2015; 2014 was the saltier and warmest year in the record for the NACW waters. The average values in the 200-800dbar layer is similar to those found at the beginning of the 2000s. The upwelling of the CCLME continues to strengthen, although 2015 was still the coolest and fresher year in the record for the upwelling influenced surface waters.
The Canary Basin sits at the boundary between the oceanic waters of the subtropical Atlantic gyre and the upwelling waters from the Canary Current Large Marine Ecosystem (CCLME) off the coast of Northwest Africa. Since the early 2000’s the Canary Islands archipelago region has been monitored by the Spanish Institute of Oceanography [Tel et al, 2016]; the oceanic waters west of Lanzarote (stations 11-23, Figure 1) and the Coastal Transition Zone (CTZ) of the upwelling region of the Canary Current Large Marine Ecosystem (stations 1-10, Figure 1) At the upper levels, the area is under the influence of the southward flowing Canary current and the Canary Upwelling current, associated to the upwelling front (Figure 1). At intermediate levels, it is under the influence of the tongue of slow propagating Mediterranean waters and the slope current known as the Canary Intermediate Poleward Current [Hérnandez-Guerra et al, 2017; Vélez-Belchí et al, 2018].
The waters above the seasonal thermocline, are characterized on the θ/S diagram by scattered temperature and salinity values due to seasonal heating and evaporation. These waters occupy the upper 300 m in the oceanic region, and the upper 100 m in the stations under the effect of the coastal upwelling which are considered the surface waters. Below the seasonal thermocline and through the permanent thermocline is the North Atlantic Central Water roughly between 300 m and 700 m depth. These waters are characterized on the θ/S diagram by an approximately straight-line relationship between potential temperature (11.4ºC < θ < 14.9ºC) and salinity (35.6 <S < 36.1). At intermediate levels, roughly between 700 m and 1200 m, two distinct water masses are found in the Canary Islands region, the fresher (S<35.3) and slightly lighter Antarctic Intermediate Waters (AAIW), and the saltier (S>35.4) and heavier Mediterranean Waters (MW).
Between the 1990s and the early 2000s there was a decrease in the temperature and salinity of all upper-layer waters. Thi was followed in the mid-2000s by a marked increase in both temperature and salinity, which peaked in 2014 in the hottest and saltier year in the record. Since 2015, both temperature and salinity have decreased, and at the end of 2017, the mean temperature and salinity was similar to that observed in the late 1990s [Vélez-Belchí et al, 2015].
In the Oceanin waters at the surface, although the time series do not resolve propely the seasonal cycle, the overal ocean warming observed 0.19±1.35°C decade-1 coincided with the Sea surface temperature observations from satellite.
In the depth stratum that characterize the NACW waters (200-800 dbar), there is an overall statistically significant warming of 0.08±0.07°C decade-1 and increase of salinity of 0.008±0.012°C per decade-1 (Figure 2). The overall increase in temperature and salinity almost compensate in density, corroborating that the observed trends are due to deepening of the isoneutral surfaces rather than changes along the isoneutral surfaces. This overall increase in salinity and temperature for the NACW waters was also observed in the CTZ, although with slightly smaller values for the trend due to the influence of the upwelling. The variability in the CTZ is higher due to the proximity of the upwelling region, and the frequent intrusions of upwelling filaments. For the same reason, the uncertain is higher in the trend estimations.
The surface waters in the CTZ shows a non-statistically significant cooling of -0.33±0.49°C decade-1, and a non-statistically significant decrease in salinity of -0.058±0.069 decade-1, both coherent with an increase in the upwelling in the Canary Current Large Marine Ecosystem (CCLME). The upwelling of the CCLME continues to strengthen, and 2015 was the coolest and fresher year in the record for the upwelling influenced surface waters. Sea surface temperature observations from satellite observations corroborate changes in the upwelling regime inferred from the in situ observations, with different areas showing increases in upwelling. However, the magnitude of the observed trend in the satellite SST is different, probably due to the thin layer of ocean that the satellite observes.
Figure 4. (a) Sea surface trends (°C decade‐1) computed from NOAA high‐resolution (1/4°) blended analysis of Daily SST for the 1982–2017 period. The dashed grey line denote the three upwelling areas as defined by Cropper et al. (2014): the weak annual upwelling zone (26°N‐35°N), the permanent annual upwelling zone (21°N‐26°N) and the Mauritania‐Senegalese upwelling zone (12°N ‐20°N). The thin solid grey lines correspond to the isotherms (18°C, 20°C, 22°C, 24°C and 26°C) of the mean field for each season. (b) Yearly values of the SST for the four locations represented in (a): UP1, west of Cape Beddouza in the weak permanent annual upwelling zone, UP2, south of Cape Bojador in the permanent annual upwelling zone, OC1, in the oceanic waters north of the Canary Islands, and DW1, west of Cape Timiris, in the Mauritania‐Senegalese upwelling zone. Trend values (dashed line in °C decade‐1), and statistical significant range at the 95% confidence level are indicated in the legend for each one of the locations.
In the stratum corresponding to the intermediate waters (800–1400 m), weak cooling and decreasing salinity has been observed since the 1990s, however the changes are not statistically significantly different from zero, in the oceanic region or in the Coastal Transition Zone (CTZ). Both time series show high variability due to the two very different intermediate water masses present in the region, i.e the Mediterranean waters (MW) and the Antarctic Intermediate Waters (AAIW)
In the layer corresponding to the upper NADW (1700-2600 dbar), there was a weak warming and increase in salinity that is not statistically significantly different from zero. However, in stratums corresponding to the NADW (2600-3600 dbar), a marginally statistical significant freshening (-0.002±0.002°C decade-1) is observed, although no trend could be confirmed int temperature (-0.009±0.01°C decade-1)
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Vélez-Belchí,P., González-Carballo, M., Pérez-Hernández, M. D. and Hernández-Guerra, A. (2015). Open ocean temperature and salinity trends in the Canary Current Large Marine Ecosystem. In: Valdés, L. and Déniz-González, I. (eds). Oceanographic and biological features in the Canary Current Large Marine Ecosystem. IOC-UNESCO, Paris. IOC Technical Series, No. 115, pp. 299-308.
Vélez‐Belchí, P., M. D. Pérez‐Hernández, M. Casanova‐Masjoan, L. Cana, and A. Hernández‐Guerra (2017), On the seasonal variability of the Canary Current and the Atlantic Meridional Overturning Circulation, J. Geophys. Res. Oceans, 122, 4518–4538, doi:10.1002/2017JC012774.