Miroslava Pasarić

Long-term meteorological preconditioning of the North Adriatic coastal floods

Abstract Flooding of the North Adriatic coast is examined through 14 years of hourly sea-level data, recorded at Bakar. The threshold-exceeding sea levels are studied in respect to four major components: (i) tides, (ii) elevations generated by synoptic and smaller-scale meteorological disturbances (storm surges and seiches of the Adriatic), (iii) low-frequency oscillations (0.01

Response of the Adriatic sea level to the air pressure and wind forcing at low frequencies (0.01-0.1 cpd)

Low-frequency (0.01–0.1 cpd) variability of air pressure, wind, and sea level is examined through 6- to 8-year records of data collected at three locations along the east coast of the Adriatic and one on the west coast. Seasonal energy spectra show that processes at these timescales are more energetic in winter than in summer. There is substantial wind energy at timescales corresponding to planetary atmospheric waves. In order to explain the stronger-than-isostatic adjustment of sea level at low frequencies to the air pressure forcing, recorded in different parts of the Mediterranean, the present empirical analysis is based on a physically more tractable model, relating sea level slope to the air pressure gradient and wind stress integral. The multiple input regression and the cross-spectral analysis yield a spatially variable response: over the deeper sea region sea level slope is fully explained by isostatic adjustment to the air pressure gradient alone; over the shelf a much stronger-than-isostatic response (−1.7 cm/mbar) is greatly reduced (−1.3 cm/mbar), but not fully accounted for, by the action of wind. Next the multiple linear regression method is carefully reexamined; a simple statistical model is developed to show that in multiple-input linear models with mutually correlated inputs, small errors in one of the inputs produce biased estimates of all the response parameters. The apparent discrepancy between the theoretically predicted and the estimated response is attributed to the bias.

A note on local and non-local properties of turbulence in the bora flow

On the basis of two-month measurements of the bora wind at Senj, Croatia, with a 1 s temporal resolution, properties of the bora turbulence are inspected using the records of three bora episodes. The spectrum is divided into two parts: high-frequency turbulence (periods less than 1 min) and the low-frequency part (periods between 1 and 10 min) where pulsations appear. We have found that the high-frequency turbulence is generated locally by surface roughness and local wind shear. On the other hand, the low-frequency turbulence, i.e. the pulsations, seems to be independent of the local properties and can therefore be treated as an organized non-local effect. This is in accordance with the studies of the pulsations in the Boulder downslope windstorm.

Severe flooding along the eastern Adriatic coast: the case of 1 December 2008

This paper addresses an extraordinary storm surge in the Northern Adriatic that was more pronounced on the eastern than on the western shore. On 1 December 2008, Adriatic monitoring stations detected exceptionally high sea levels; the oldest Croatian tide gauge station recorded the highest water level in its operating history at the time. Apart from the Northern Adriatic, large portion of the Dalmatian Coast was also exposed to high water levels, while Venice experienced a less-dramatic event. This marine storm was different from the capital storm of 4 November 1966 during which the surge had the highest impact ahead of Venice and along the north-western coastline. The 2008 event is studied here in detail, and the mechanisms that resulted in the different flooding of the two shores are identified. The study is based on hourly sea level, air pressure and wind data measured along both basin sides together with ECMWF reanalysis fields. Four components of sea-level evolution are identified: the storm surge, tide, Adriatic seiche and, low-frequency variability. The 2008 event was the outcome of a fine interplay between the first three components, which were all superimposed on the raised sea level due to low-frequency variability. The marine storm differed from the 1966 storm in the atmospheric forcing and relative timing of all contributing processes. The 2008 flooding of the eastern coast was mainly due to the Sirocco-wind shear, whereas the 1966 flooding of the western coast was due to the combined effect of almost uniform Sirocco and bottom slope.

Summer breakout of trapped bottom dense water from the northern Adriatic

[1] The paper deals with an intriguing dense-water breakout episode in mid-August 2004 which has been observed in the bottom layers of the oil rig located in the middle of the northern Adriatic. Various data (temperature series and vertical T-S profiles, currents, meteorological measurements, and satellite images) have been analyzed in order to understand conditions which preceded, were active, and followed the breakout episode. A stationary bottom pool of dense water, generated during the previous winter, has been suspected to be a source of the dense water observed during the breakout, with a permanent position established by a stationary northern Adriatic cyclonic-anticyclonic gyre system. The breakout lasted for 3 days, advecting the bottom waters more than 2°C colder than residing waters at the oil rig site. The main result of modeling experiments concerns the generative force for the observed breakout which was found to be a mesoscale storm that occurred over the open north Adriatic on 8 August 2004. The storm has been reproduced by the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®) atmospheric model which was then used to force the Princeton Ocean Model (POM) at the surface. Results of simulations reveal the capability of the storm to break the thermohaline fronts through the wind-induced baroclinic transport and downwelling at the exposed shorelines. This is the first study in the Adriatic which evaluates the impact of mesoscale summer storm to the sea, driving bottom layer circulation through the convergence/divergence dynamics in addition to the direct impact on the sea surface through the wind stress forcing.

Mediterranean Sea-Level Variability in the Second Half of the Twentieth Century: A Bayesian Approach to Closing the Budget

Regional sea levels in the Mediterranean sub-basins, the Black Sea and the Atlantic close to Gibraltar between 1930 and 2015, are constructed, based on high-quality tide-gauge data in the wider Mediterranean area, to identify long-term trends against decadal and multidecadal changes. Regional sea-level variability induced by direct atmospheric forcing and steric changes is determined, respectively, from air pressure and temperature and salinity data. Vertical land movements due to glacial isostatic adjustment are also taken into account. Focusing on linear trend in the period 1950–1990, the individual contributions to the trend are calculated and sea-level budget is examined within each region, according to proposed physical model. The trends with their uncertainty intervals are determined using Bayesian statistics. In the Atlantic off Gibraltar and in the Black Sea, the regional sea-level trends were close to the global values; in the Mediterranean, they were close to zero. Sea-level rise in the Atlantic was supported by regional atmospheric loading and thermohaline changes, while the trend underlying the residual part of sea-level variability was comparable to the global mass contribution. Throughout the Mediterranean and in the Black Sea, atmospheric forcing and steric effects induced lowering of sea level. In the Mediterranean, and partly in the Black Sea, these regional effects compensated the effect of global mass increase. It is concluded that over the 1950–1990 interval, the sea-level budget is closed within the, rather wide, credible limits, which are obtained when autocorrelation of the linear-fit residuals is taken into account.