SLA–chlorophyll-a variability and covariability in the Algero-Provençal Basin (1997–2007) through combined use of EOF and wavelet analysis of satellite data

Abstract

Data reduction and signal decomposition techniques have been applied to a large bio-physical remotely sensed dataset covering the decade 1997-2007. The aim was the estimation of the spatial (basin and sub-basin scales) and temporal (seasonal and interannual) variability of sea level anomalies and chlorophylla concentration in the Algero-Provençal Basin, as well as the study of their covariability. Empirical orthogonal functions, wavelet analysis, singular value decomposition and correlation maps have been successfully used to individuate the patterns of (co)variability of the investigated fields. The seasonal variability of the phytoplanktonic biomass is divided in two distinct modes, timewise and spacewise separated. Positive interannual events are related to anomalies in 1999 and 2005, while the main (negative) anomaly is that of summer 2003, associated to the European 2003 heatwave. The analysis of the sea level anomalies shows a minimum in the formation of anticyclonic Algerian eddies during that period. The largest anticorrelation between sea level anomalies and phytoplanktonic biomass is found in the central zone of the basin, suggesting a clear biological response to the shoaling/deepening of the isopycnae and so to the nutrient injection into the euphotic layer. The analysis suggests that the driver of the variability Responsible Editor: of the nutricline depth in this central area is the displacement (seasonal) of the North Balearic Front and the formation/action of the frontal eddies.

Pelagic primary production in the Algero-Provençal Basin by means of multisensor satellite data: focus on interannual variability and its drivers

“…Nevertheless, this part is also characterized by the strongest coupling between chlorophyll a (Chla; a good and prompt proxy for phytoplanktonic biomass) and the displacement of isopycnae. In other words, effects on the phytoplanktonic abundance of nutrient injection in euphotic layer, generated by isopycnae displacements, is much more evident here than in northern and southern areas (Olita et al 2011).…”

“…Excluding the shallowest shelf areas, it could be roughly divided in three main parts, considering both circulation and bio-physical characteristics accordingly to our previous study (Olita et al 2011). The southern part, namely the Algerian Basin (AB), is characterized at the surface by the Atlantic Water (AW) flowing into the Mediterranean Sea through the Gibraltar Strait (Millot et al 1999).…”

“…Biological response of this basin to physical forcing (to which we often refer in this paper with the expression bio-physical relationships) has also been analysed in several studies (e.g. Lohrenz et al 1988;Raimbault et al 1993;Levy et al 1998Levy et al , 2000Garcon et al 2001;Moran et al 2001;Taupier-Letage et al 2003;Olita et al 2011). In our previous paper (Olita et al 2011), we focused the attention to the relationships between sea level anomalies (due to mesoscale activity and indicators of isopycnae displacements), and concentration of chlorophyll a in the APB for the decade 1997-2007.…”

Recent improvements in mesoscale characterization of the western Mediterranean Sea: synergy between satellite altimetry and other observational approaches

“…At its start, near 0° longitude, the Algerian Current is fairly narrow (30-50 km), flowing close to the coast. As the current progresses eastward, its width and separation from the coast vary and, due to baroclinic instabilities in its flow, meanders are regularly formed and can eventually detach from the current and become both cyclonic and anticyclonic coastal eddies (Olita et al 2011). Some of the stronger eddies, which are predominantly anticyclonic, may become "open sea eddies" and reach the southern Balearic Islands (Millot 1999).…”

Mesoscale Eddies in the Algerian Basin: do they differ as a function of their formation site?

“…Hereafter, for the purposes of the present study, we will consider only the Algerian anticyclonic Eddies, indicated with the acronym AEs. The general description of their behaviour, as reported in the bibliography, is that they move along the African coast at a few Km per day and are blocked at the level of the Sardinia Channel (Millot, 1985(Millot, , 1999Font et al, 1998Font et al, , 2004Olita et al, 2011), where they are usually forced by the local topography to detach from the coast and follow a cyclonic pathway (Millot, 1987;Vignudelli, 1997;Cotroneo et al, 2016;Aulicino et al, 2018).…”