Upper-ocean response to the passage of tropical cyclones in the Azores region

Abstract

Tropical cyclones (TCs) are extreme climate events that are known to strongly interact with the ocean through two mechanisms: dynamically through the associated intense wind stress and thermodynamically through moist enthalpy exchanges at the ocean surface. These interactions contribute to relevant oceanic responses during and after the passage of a TC, namely the induction of a cold wake and the production of chlorophyll (Chl a) blooms. This study aimed to understand these interactions in the Azores region, an area with relatively low cyclonic activity for the North Atlantic basin, since the area experiences much less intense events than the rest of the basin. Results for the 1998–2020 period showed that the averaged induced anomalies were on the order of +0.050 mg m−3 for Chl a and −1.615 ∘C for SST (sea surface temperature). Furthermore, looking at the role played by several TCs characteristics we found that the intensity of the TCs was the most important condition for the development of upper-ocean responses. Additionally, it was found that bigger TCs caused greater induced anomalies in both variables, while faster ones created greater Chl a responses, and TCs that occurred later in the season had greater TC-related anomalies. Two case studies (Ophelia in 2017 and Nadine in 2012) were conducted to better understand each upper-ocean response. Ophelia was shown to affect the SST at an earlier stage, while the biggest Chl a induced anomalies were registered at a later stage, allowing the conclusion that thermodynamic exchanges conditioned the SST more while dynamical mixing might have played a more important role in the later stage. Nadine showed the importance of the TC track geometry, revealing that the TC track observed in each event can impact a specific region for longer and therefore result in greater induced anomalies.