O papel do oceano na modulação de eventos extremos no Sul/Sudeste do Brasil

Abstract

Understanding the processes responsible for extreme events is crucial for public safety and managing coastal and offshore economic activities. The current scientific comprehension has demonstrated that ocean physics may have a great impact on these phenomena and that examining the ocean input can lead to significant insights into how extreme events evolves. With regard to extreme events in south/southeastern Brazil, there is a high frequency of extratropical cyclones and these frontal systems have been pointed out as the main driver for the development of large wave events (LWEs). Furthermore, the first-ever documented South Atlantic Hurricane (Catarina), which made landfall in southern Brazil, began as an extratropical cyclone. In this study, we apply different methods to give a diagnostic of the contribution of the ocean on both Catarina’s modulation and LWEs. The former by applying the genesis potential index (GPI) to give a quantitative diagnostic of the contribution of the environmental factors known to be linked to tropical cyclogenesis. The latter by coupling in a one-way mode the hydrodynamic model Regional Ocean Modeling System (ROMS) and the wave model Simulating WAves Nearshore (SWAN) to study the effect of currents on LWEs produced under the influence of cyclones. For the study of Hurricane Catarina, results indicate two regions of large positive GPI anomalies: where Catarina (1) developed subtropical cyclone structure and underwent tropical transition, and (2) acquired hurricane status. We also examine the ocean’s potential to sustain tropical cyclone intensification through the ocean heat content (OHC). Although some areas of positive OHC anomalies may have indicated conditions favorable to cyclogenesis, it was not sufficient to explain the main stages of Catarina’s development. On the other hand, the GPI in March 2004 was the highest among March of all years (1990–2019), suggesting that this may be a valid index for obtaining a threshold for tropical cyclogenesis in south/southeastern Brazil. Regarding the five LWEs studied, chosen due to the distinct importance that current-to-wave effects had for the period, the results indicate that the addition of currents on the wave model improved the accuracy of the peaks of significant wave height, generally increasing them (by as much as 27%). The increase in wave height occurred mainly near the shelf edge and slope, where the southwestward-flowing Brazil Current opposes the dominant direction of cyclone-generated waves (usually from S/SE to S/SW). Modulation of wave height by currents in the sub-inertial band explained up to 95% of the total variance of the difference between results with and without ocean currents, and this band was largely related to the Brazil Current. Its near-inertial counterpart explained up to 22% of the total variance and was primarily due to near-inertial currents in the region. This study provides, for the first time, conclusive evidence that wave-current interaction can impact LWEs in south/southeastern Brazil.