Exceptionally extreme drought in Madeira Archipelago in 2012: Vegetation impacts and driving conditions

Liberato M. L. R., Ramos A. M., Gouveia C. M., Sousa P., Russo A., Trigo R.M., Santo F.E.
Agricultural and Forest Meteorology, doi: 10.1016/j.agrformet.2016.08.010

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This work aims at characterizing the exceptional drought that affected Madeira Archipelago (Portugal) during the 2011–2012 hydrological year while including some major impacts but also the main atmospheric circulation mechanism behind the event. Precipitation records from six meteorological stations are used to assess the extreme drought episode by means of a decile classification. The assessment of the drought duration and severity is further corroborated by the Standardized Precipitation Index (SPI) computed for the 3, 6, 12 and 24 months’ time scales which confirmed that the 2012 drought event was one of the events with higher drought intensity at all scales, being classified as extreme at the 6-month time sale (SPI6 < ?1.65) from December 2011 until May 2012 on the majority of the meteorological stations analysed. Long-term precipitation data available since 1865 for the station of Funchal confirm the exceptional dryness of this episode, particularly during the winter season (December to March) corresponding to the driest winter in 150 years. Vegetation activity is assessed through anomalies of NDVI (Normalized Difference Vegetation Index), confirming several large sectors of Madeira under vegetative stress. The southern sector of Madeira Island suffered up to seven months (out of nine) of extremely negative anomalies. From an operational point of view, obtained results reveal the ability of the developed methodology to monitor vegetation stress and droughts in Madeira. The extreme dryness of the Island favoured an unusually intense summer fire season of 2012 (between June and September) in Madeira being the year with highest number of fires in the last decade (with robust data). Furthermore, the main fire hotspots of the 2012 fire season are mostly coincident with the areas affected by drought. The large-scale atmospheric circulation responsible for the setting and intensification of the drought is evaluated using reanalysis data. An extensive high pressure anomaly (maximum above 12 hPa in December) persisted over the North Atlantic during the extended winter months (October to April), centred between the Azores Islands and the UK. This feature is in agreement with a positive NAO index (2.25 in December), a negative EA index (?1.76 in January and ?1.73 in February) and compatible with enhanced north-easterly trade winds over the region. As a consequence of this uncharacteristic dynamics there was a deficit of moisture availability over the region as evaluated by the vertically integrated horizontal water vapour transport with a negative anomaly up to ?120 kg m?1 s?1.