Recent droughts and the impact of North Atlantic Oscillation in Iberia; a spatiotemporal analysis based on vegetation temperature condition Index (VTCI)

Dissertation submitted in partial fulfillment of the requirements for the Degree of Master of Science in Geospatial Technologies.

Drought is a complex phenomenon that afflicts huge areas worldwide with high spatial and temporal variability. Being under influence of the Mediterranean climate the Iberian Peninsula is per se prone to droughts. In this study, we applied the Vegetation Condition Temperature Index for a spatiotemporal drought analysis in Iberia. As a compound remote sensing drought index VTCI incorporates information on Normalized Difference Vegetation Index (NDVI) and Land Surface Temperature (LST). Suchlike, it overcomes the disadvantage of NDVI, which faces a lagged vegetation response to droughts and makes it less suitable for real-time drought monitoring. We took advantage of the full range of the VTCI scale and thus describe soil moisture conditions from ‘very dry’ to ‘wet’. In order to study the effect of North Atlantic Oscillation on the establishment of droughts during spring and summer, we correlated VTCI with the NAO winter index of one year. The study was accomplished for the years 2001 until 2005; additionally we adjoined the years 2007 and 2010 for their strong positive (negative) NAO winter index. The results as presented in the VTCI time series illustrate a typical ‘Mediterranean’ pattern with humid springs and persistently dry summers inside the Mediterranean fraction of Iberia. At least two months in spring are regarded to be wet, though with a high temporal and spatial variability: One area can be wet in one year and dry in the next one. March often is drier than middle spring months exhibiting accumulated precipitation deficiency of winter months, whereas a weak start into the growing season can be determining for the crop harvest of one year. The summer months become persistently dry in the Mediterranean region, while the North and Northwest as well as high elevation areas generally obtain sufficient precipitation also during the warm season. Spring months are particularly important for vegetation growth in terms of water availability, though the spatial and temporal pattern changes considerably in the intra-annual perspective. We found a dipolar correlation between VTCI and NDVI particularly in April and June. In April the NW of Iberia shows a high negative correlation between NAO and VTCI, the SE a strong positive correlation. In June the pattern flips showing positive correlation in the NW and negative correlation in the SE. The other summer months exhibit a similar pattern as June though with less significant correlation, while the pattern during the spring months alters, which coincides with the climatic variability during this season. The correlation findings accord with observations from the VTCI time series: We gave the example of the Duero Basin in the NW, where in April (negative correlation) indeed NAO+ years were drier (due to decreased VTCI) than NAOyears (increased VTCI) and the SE exhibited the opposite behaviour. Similar findings can be evidenced for the NW and SE of Iberia in June and July and the Balearic Islands in March. In terms of land cover NAO influences geographical areas rather than certain land cover types, which can be explained by the widely human induced landscape pattern of Iberia. From the land use classes agricultural lands and low vegetation are the classes most vulnerable to droughts; forests are the most resilient land classes. The study of correlation between a certain NAO winter index and drought occurrences is particularly interesting for the development of an outlook system of soil moisture for the period of highest vegetation activity in spring. The study detected come correlations between NAO and VTCI that indeed find spatial response on Iberian landmass.