The Ionian Sea (Fig.1a) is a highly dynamic area where many geodynamic[1,2,3], oceanographic[4,5], and climatic processes evolve[6,7,8,9,10]. Precipitation monitoring along this area is aimed at the study of the Mediterranean climatic variability and is entangled with a broad-band of human activities.
The estimated mean annual precipitation field over the period 1901-2015 is illustrated in Fig.1b.
The region of Puglia, Italy, along with the Ionian Islands complex, represents a marine environment which is meridionally extended along the central Mediterranean. Precipitation observations from ground-based stations go back to the 19th century (starting as early as 1809 in Corfu), but nevertheless, they are available in a scientifically exploitable form after the 1890s in the Ionian Islands and after the 1920s in Puglia.
Multi-spectral decomposition analysis of the monthly precipitation series from ten local coastal stations (Fig.1a) showed the prevalence of mixed secular trends (+0.86 to −0.65 mm/yr) in Puglia and significant negative trends (−0.42 to −2.70 mm/yr) in the Greek Ionian region, actually composed of: (a) multidecadal fluctuations at 35−60 yrs particularly met in the Adriatic coast and the Ionian Islands, and (b) two major climatic discontinuities (abrupt decreases) in the mean annual height, jointly detected by the Homer and Acmant homogeneity algorithms in 1962-64 at the Otranto-Corfu area (Fig.2a) and in 1971-72 at the southern Greek Ionian region and Taranto (Fig.2b). The 1962-64 event seems to be associated with the global climate shift of the 1960s that was triggered by oceanic processes in the North Atlantic, while the 1971-72 event was probably caused by the onset of exceptional activity in the El Niño Southern Oscillation (ENSO) during the late 1960s and a shift in NAO (North Atlantic Oscillation) in the early 1970s, from negative to persistent positive phases.Furthermore, significant variability modes at the quasi-decadal and shorter-scales (2.5−15 yrs) dominate the interannual precipitation along the Taranto Gulf and the Ionian Islands, which account for 20-60% of the total variance. Such modes are found to be coherent with ENSO variations (as represented by the Southern Oscillation Index or SOI) from the 1890s to the mid-1920s and from the early 1970s to the 2000s (Fig.3a). The winter precipitation variability at 6−9 yrs of the entire area is strongly affected by the NAO, particularly along the Ionian Islands, where the local multi-decadal modes further reveal significant anti-phase coherence with the NAO (Fig.3c,d).
The strong sub-decadal variability that mainly prevails at the Ionian seaside stations (which are freely exposed to the southern sirocco-type air flow) reveal significant in-phase coupling with the Scandinavian (SCAND) pattern, particularly in the Taranto Gulf, where a remarkably strong mode was detected for 2.3 yrs in Leuca, accounting for 40% of the local precipitation variance. Along the Adriatic coast of Puglia, the main precipitation variations are spotted at the bi-decadal scales. These modes seem to be correlated with ENSO (Fig.3b), the East Atlantic pattern and possibly with the intrinsic variability of the Atlantic Ocean. The strong mode at 10−11 yrs in Otranto (Fig.2a) is probably associated with the transition from the Taranto Gulf to the Adriatic coast climatic regime.
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