Impact of climate variability on coastal dynamics
NAO projection on the Romanian Black Sea coast wind regime
The North Atlantic Oscillation (NAO) variability during the 20th century was characterized by periods of persistent positive-phase with more frequent and stronger winter storms over northern Europe and with less and weaker storms over central and southern Europe. This is a somewhat unusual situation, which occurred in the past only prior to 1650. The NAO index shows a larger variability since the 1960s (between -4.89 and 5.08) in comparison with the previous 100 years of the recorded NAO index time series (between -3.97 and 3.89).
We investigated the influence of North Atlantic Oscillation (NAO) on the Romanian Black Sea coast wind regime; a special attention was paid to NAO control on coastal storm occurrence. Our analysis points out that the storminess on the Romanian Black Sea coast is strongly coupled with the NAO phases (the positive winter wind speed anomalies are associated with NAO negative phases); there is also a high correlation established between the NAO index and the multi-decadal winter storm frequency at Sulina (r = -0.76) and Sfantu Gheorghe (r = -0.77) meteorological stations. Marine storm distribution during the last half of 20th century exhibits a very active interval between 1961 and 1972 that coincides with the strongest negative NAO phase and a relatively quiet period with low variability between 1979 and 2000, which overlaps a strongly positive NAO phase (Fig. 1). During the 1970s a gradual decrease in storm frequency is recorded. In the last few years (2000-2005), when the NAO index was close to zero, the large storm frequency was very low, whereas the medium-intensity storms occurrence increased considerably.
Fig. 1 – Time evolution of storm incidence and Hurrel’s NAO index (A1 – Sulina; B1 – Sf. Gheorghe). Correlation coefficients between winter cases with strong winds (u > 15 m/s for more than 6 hours) and NAO index (A2 – Sulina; B2 – Sf. Gheorghe)
NAO influences on the dynamics of the Danube delta shoreline
Recent studies on macrotidal coasts suggest that the impact of storm surges can be satisfactory assessed only at a short-term scale, when shoreline changes represent an almost immediate response to meteorological and oceanographic forcing. At decadal scale, no direct relationship was observed between storminess and coastline evolution.
The goal of our study was to investigate the role of the climate variability (expressed by North Atlantic Oscillation) upon the multi-decadal Danube delta shoreline mobility. Our results clearly show that shoreline changes at decadal time scales are ultimately driven by the NAO which controls the storminess on the Danube delta coast (shoreline changes and storminess are connected at short-time scale whereas the coastline medium-term (decadal) evolution is controlled by the NAO phases).
We focused on 1961-2006 period when NAO index exhibited the largest negative/positive phases (corresponding to 1961-1972 / 1980-2002 intervals) for the entire period when direct measurements exist for the index to be computed (mid 19th century to present). A comprehensive analysis of data collected over the last five decades on the Danube delta coast (topographic maps, satellite imagery, GPS surveys and beach profiles) revealed two different shoreline dynamics patterns: high mobility during 1960-1979 interval with big retreating and advancing rates and low mobility afterwards (1979-2006) – Fig. 2. The divergence zones in the longshore sediment transport system experienced the highest rates of retreat (~20 m/yr and ~10 m/yr in the first/second time interval), whereas the shoreline advanced fastest along the coast of active lobes (i.e., Chilia and Sf. Gheorghe lobes). The decrease of coastal processes intensity from the second interval was similar for the erosive beaches (with 55-66%) and non-uniform for the accretionary coasts (20-61% for open beaches and 80% for the sheltered secondary deltas).
Time series data for three representative benchmarks were compared with NAO index to assess the continuous shoreline evolution of corresponding dynamic sectors: R 60 and R 28 for the accretionary Sulina and Periteasca beaches and R 33 for the retreating Zatoane beach. A comparison of the shoreline behaviour at these three benchmarks shows an accentuated advance/retreat in the 1960s and 1970s followed by an obvious shift toward less dynamic beaches during 1978–1985 interval (Fig. 3). The shift occurred in 1978 for R 28, in 1982 for R 33, and in 1985 for R 60. The NAO index records a similar shift from dominantly negative to dominantly positive sometime between 1970 and 1980. Thus, the shoreline evolution on both the prograding and retreating sectors appears to be negatively correlated with the NAO index.
Fig. 2 – Shoreline evolution between 1961 – 2006 (A, B: Sulina (0 km) – Sf.Gheorghe (32 km) sector; C: Ciotica – Chituc sector with the southern branch of Sf. Gheorghe secondary delta represented as 0 km)
Fig. 3 – Time evolution of shoreline mobility at R 60, R 33 and R 28 benchmarks and Hurrell’s NAO index
