Increase in ocean stratification reduces the aggregation power of fine-scale physical structure

The near-surface ocean dynamics includes a variety of processes spanning characteristic horizontal scales from internal waves (IW) to submesoscale and mesoscale features. Recent works based on acoustic data showed that the vertical displacements of the oxycline depth, which separates the well-mixed oxygenated surface layer from the less oxygenated deeper ocean, provide a robust proxy of isopycnals displacements over a wide range of horizontal scales. Using a high-resolution acoustic data set in the Northern Humboldt Current System (NHCS) off Peru, the impact of fine-scale to mesoscale upper ocean dynamics over zooplankton and fish patchiness is investigated. The analysis reveals the existence of distinct features for the fine-scale range below ~2-3 km, and clearly points out the existence of intense IW and submesoscale activity over the entire NCHS region. We also address the potential impact of climate variability on fine-scale oases for life that are created by upper ocean turbulence. Indeed stratification changes induced by climate variability could impact both the number and intensity of physical structures and, consequently, distribution patterns of marine life. Using 12 scientific surveys conducted between 2002 and 2011 off Peru, we show that fine-scale structures aggregate fewer organisms when the stratification increases. Climate variability might thus reduces the number and intensity of physical structures, and consequently, size and density of organisms patches with a potential negative impact on resultant trophic interactions.