Author(s): Luis Angel Espinosa; Maria Manuela Portela; Rui Raposo Rodrigues; Martina Zelenakova

Linked Author(s): Luis Angel Espinosa, Maria Manuela Portela

Keywords: North Atlantic Oscillation; Winter extreme rainfall; Madeira Island; Copulas; Teleconnection; Climate variability

Abstract:

This pioneer work provides a bivariate modelling of the outstanding characteristics of (i) the extreme rainfall during winter on the small island of Madeira, Portugal; (ii) associated with the most prominent mode of atmospheric variability in the Northern Hemisphere, i.e., the North Atlantic Oscillation (NAO). The island of Madeira (with an area of 741 km² and quite pronounced relief) spans a wide range of rainfall regimes, or microclimates clearly affected by the island’s location in the North Atlantic, which altogether hamper the analyses of extreme rainfall. Previous studies showed that the influence of the NAO on extreme rainfall is largest in the North Atlantic sector with the likelihood of increased rainfall events from December through February particularly during negative NAO phases. It is worth mentioning that such studies are based on seasonal mathematical definitions of the NAO contrasting to this work using a daily NAO definition. Thus, a copula-based approach was adopted for teleconnection aiming at assigning return periods of daily values of a NAO index (NAOI) coupled with extreme daily rainfalls (for the 50-year period from 1st of December 1967 to February 2017) at six representative rain gauges located in the northern, southern, eastern and western coastal zones, and in the central highlands of Madeira. In other words, with the hypothesis that the NAO can explain extreme rainfall, and assuming that the phenomenon has a multivariate nature, the return periods of the bivariate problem were computed as joint return periods using selected copulas. Results show that: (i) the constructed copulas can characterise the dependence of the underlying joint distributions, which in turn provide useful analytical expressions of the return periods of the bivariate problem (i.e., of previous NAOI and extreme rainfall); and that (ii) in recent years there is strong evidence of an increasing climate variability with more anomalous and persistent daily negative NAOI accompanied also with higher extreme rainfall events. Note that small island environments are highly vulnerable to natural disasters such as cyclones and storm surges. They are also susceptible to flash floods, landslides and debris flows as a result of extreme rainfall. It is therefore, essential to study variability of extreme rainfall at shorter time scales along with atmospheric observations from a multivariate perspective. This in turn may help to understand the atmospheric physics and the mechanisms that can shift a heavy rainfall event into one producing floods and alluviums on the island. Even though the teleconnection was not well established for all the analysed rain gauges, the use of copulas still provided relevant information towards the understanding of the vulnerability of Madeira Island to large-scale climatic variations.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221318

Year: 2022