Quasi-tropical cyclone caused anomalous autumn coccolithophore bloom in the Black Sea

Abstract

A quasi-tropical cyclone (QTC) observed over the Black Sea on 25-29 September 2005 caused an exceptionally strong anomalous autumn coccolithophore bloom that lasted for more than 1.5 months. The QTC induced intense upwelling, causing a decrease in sea surface temperature of 15 span classCombining double low line"inline-formula"g/spanC and an acceleration of the cyclonic Rim Current up to extreme values of 0.75 m sspan classCombining double low line"inline-formula"-1/span. The Rim Current transported nutrient-rich Danube plume waters from the northwestern shelf to the zone of the cyclone action. Baroclinic instabilities of the plume boundary caused intense submesoscale processes, accompanied by mixing of the shelf and upwelling of the waters. These processes triggered the initial growth of remote sensing reflectance (span classCombining double low line"inline-formula"iR/irs/span) on the offshore front of the plume, indicating the beginning of the coccolithophore bloom. Furthermore, the bloom shifted to the zone of the strongest upwelling in the western cyclonic gyre. Intense vertical entrainment of nutrients in this area caused the increase in chlorophyll span classCombining double low line"inline-formula"ia/i/span concentration (Chl), which was then followed by a strong bloom of coccolithophores. Advection by the Rim Current spread the bloom over the entire southern part of the Black Sea, more than 1000 km from its initial source. A month after the QTC action, span classCombining double low line"inline-formula"iR/irs/span in these areas reached a value of 0.018 srspan classCombining double low line"inline-formula"-1/span, corresponding to an estimate of a coccolithophore concentration of 10span classCombining double low line"inline-formula"7/span cells per liter.