Interannual Precipitation Variability Along The Central Mediterranean Regions Of Puglia And Ionian Islands

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.

Figure 1 (a): Topographic map of the Ionian Sea. The ground-based meteorological stations that were used in the study of the precipitation variability are indicated by the small red triangles. The secular linear trends in the observed and the Homer homogenized precipitation series of each station are respectively shown (in mm/decade) within the brackets. However, notice that these trend values are referred to the period 1921-2010 for the Puglia stations (apart from Bari), 1893-2010 for the Ionian Islands, 1956-2010 for Methoni, and to 1938-2010 for Bari. (b): The mean annual precipitation field above land, as implied for the period 1901-2015 from the Climatic Research Unit (University of East Anglia) data set at a resolution 0.5°◊0.5° (color scale at the bottom). Additionally, the mean annual precipitation field above land and sea areas, as implied by the TRMM (Tropical Rainfall Measuring Mission) satellite data set for the period 1998-2015 at a resolution 0.25°◊0.25°, is depicted by the gray lines in mm. (Credit: Anastasios Kalimeris)

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[11]), 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[12] of the monthly precipitation series from ten local coastal stations (Fig.1a) showed[10] 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[13] and Acmant[14] 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[15], 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[16] and a shift in NAO (North Atlantic Oscillation) in the early 1970s, from negative to persistent positive phases.

Figure 2:
(a): The annual precipitation variations in Otranto (blue line). The observed precipitation heights (indicated by the red points) are referred to the sums over each hydrological year, which is considered here as the time interval from July 1st of a year to June 30th of the next year. Estimated annual values for years with missing data are shown by the cyan points. The dashed vertical line indicates the discontinuity in 1964, while the horizontal dashed blue lines show the mean precipitation heights before and after the discontinuity. Finally, the orange curve depicts the strong decadal mode (at 10.4 yrs) in Otranto’s precipitation as detected by Singular Spectrum Analysis (or SSA)[12].
(b): As in the panel (a) but for the southernmost Ionian Island of Zakynthos. The cyan line represents the Homer-corrected version of the annual precipitation in this station, while the horizontal dashed magenta line indicates the mean height before the 1972 discontinuity. The SSA-detected long-scale fluctuating mode is depicted by the orange curve. (Credit: Anastasios Kalimeris)

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[10] to be coherent with ENSO variations (as represented by the Southern Oscillation Index or SOI)[16] 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.

Figure 3:
(a): Amplitude evolution of the precipitation mode at 14 yrs in Zakynthos (blue curve) compared with the corresponding variability component in the SOI (red curve), both implied by wavelet analysis[10].
(b): As in (a) but for the precipitation mode at 21 yrs in Brindisi (green curve) and the corresponding variability component in the SOI (red curve).
(c): As in (a) but for the winter precipitation modes at 7-8 yrs in Corfu (blue curve) and Zakynthos (cyan curve) compared with the corresponding variability component in the NAO (red curve).
(d): As in (c) but for the Corfu, Zakynthos, and NAO variability components at 42 yrs. (Credit: Anastasios Kalimeris)

These findings are more analytically described in the article entitled Variability modes of precipitation along a Central Mediterranean area and their relations with ENSO, NAO, and other climatic patterns, recently published in the journal Atmospheric Research. This work was conducted by Anastasios Kalameris from the Technological Educational Institute of Ionian Islands, Ezio Ranieri from the Polytechnic University of Bari, Dimitra Founda from the National Observatory of Athens, and Caroline Norrant from the University of Lille.


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