Synoptic-intraseasonal variability control on high chlorophyll-a events in the Puyuhuapi Fjord, Chilean Patagonia

Abstract. Intraseasonal climate variability as, the Madden-Julian Oscillation (MJO), and synoptic-scale systems modify the normal conditions of the atmosphere and ocean, causing anomalies in sea surface temperature (SST) and salinity (S) which could create an environment conducive to algal bloom events in fjord systems, which in some cases can be toxic (HABs). In this work, an analysis of the atmospheric forcings on the synoptic-to-intraseasonal scale (SY-IS), that precede and proceeds to extreme high chlorophyll-a (chl-a) events was made in the Puyuhuapi fjord (44.7º S 72.8º W), during the summer season (December–February, DJF) between the years 2010–2018. Extreme events of high chl-a are defined when chl-a anomalies exceed the 90th percentile, and day 0 was defined as the maximum anomalous value. Six extreme events, corresponding to 83 % of the total, were detected in the year 2016, a year with strong El Niño southern Oscillation (ENSO). From the analysis of the SY-IS patterns of persistent atmospheric anomalies during these 2016 events and their similarities, we detected that 4 events presented the characteristic of the passage of a low-pressure system, starting at least 7 days before the extreme chl-a event, with negative anomalies of sea level pressure and surface temperature, a change in wind direction and an increase in salinity at surface waters. we propose an atmospheric-oceanographic mechanism that induces favourable conditions for high phytoplanktonic activity in summertime: the passage of a low-pressure system, that weakens stratification and induces upwelling of deeper, colder and nutrient-rich waters favouring an increase in phytoplankton activity and the occurrence of extreme events of high chl-a in Puyuhuapi fjord. Furthermore, this work suggests that active phases 6 and 7 of the MJO might reinforce, on the SY-IS time scale, in DJF 2016. In the case of microalgae blooms, in addition to the well-known seasonal and interannual behaviors, it is important to superimpose the high-frequency variability. To improve the predictive ability of algal blooms and their relationship with climate conditions is essential for managing and mitigating their negative impacts on aquatic ecosystems, human health, and the economy.