Mirko Orlić

Response of the Adriatic Sea to the bora and sirocco forcing

The response of the Adriatic shelf waters to dominant winter winds is analysed: the bora, which is a cold, katabatic wind blowing from the northeast; and the sirocco, which blows from the southeast and brings warm Mediterranean air. Currents and sea-level changes, caused by momentum exchange at the air-sea interface, are simulated using a three-dimensional numerical model. Numerical predictions are verified through comparison with data collected in the area. It is shown that the bora induces a rather complex response of the Adriatic Sea, with sea levels organized in several lows and highs and the current field dominated by the wind-curl effect. The currents are directed downwind under the bora maxima, upwind under the minima. Simulations of the bora-driven sea levels and currents are supported by results of in situ measurements carried out on the Adriatic shelf. Moreover, they are confirmed by an analysis of the Coastal Zone Color Scanner (CZCS) imagery, showing that during the bora episodes highly-productive Po-influenced waters extend from the river mouth in an offshore direction, due to advection by upwind currents. The sirocco piles up water in the North Adriatic and brings about the occurrence of storm surges along the northern shorelines. This process has been documented amply in the literature. According to the present model, the sirocco-driven currents are controlled by two different mechanisms: bottom slope, supporting the existence of a cyclonic and an anticyclonic gyre in the sea; and wind curl, which is a source of cyclonic vorticity in the basin. The data show that the sirocco may reverse the current usually found along the western coast, when the wind-forced current component becomes stronger than buoyancy-driven residual flow.

Northern Adriatic Response to a Wintertime Bora Wind Event

During winters, the northern Adriatic Sea experiences frequent, intense cold-air outbreaks that drive oceanic heat loss and imprint complex but predictable patterns in the underlying waters. This strong, reliable forcing makes this region an excellent laboratory for observational and numerical investigations of air-sea interaction, sediment and biological transport, and mesoscale wind-driven flow. Narrow sea surface wind jets, commonly known as “bora,” occur when cold, dry air spills through gaps in the Dinaric Alps (the mountain range situated along the Adriatics eastern shore). Horizontal variations in these winds drive a mosaic of oceanic cyclonic and anticyclonic cells that draw coastal waters far into the middle basin. The winds also drive intense cooling and overturning, producing a sharp front between dense, vertically homogenous waters (North Adriatic Dense Water, or NAdDW) in the north and the lighter (colder, fresher), stratified waters of the Po River plume. Once subducted at the front, the NAdDW flows southward in a narrow vein following the isobaths (contours of constant depth) of the Italian coast. In addition to governing the basins general circulation, these processes also influence sediment transport and modulate biological and optical variability

Seasonal and interannual variability of the northern Adriatic surface fluxes

Coastal meteorological data and sea surface temperature collected over a 27-year interval (1966–1992) have been used to evaluate seasonal cycle and interannual variability of surface heat, water and buoyancy fluxes at three locations in the northern Adriatic (Trieste, Rovinj and Mali Losinj). Annual averages of downward heat and water fluxes and upward buoyancy flux ranged between −7 W m−2 and 3 W m−2, 0.3 mm day−1 and 0.8 mm day−1, and −0.4×10−8 m2 s−3 and 0.2×10−8 m2 s−3, respectively. Seasonal cycle of buoyancy flux depended almost entirely on surface heat exchange. Considerable spatial variations of fluxes, controlled by evaporation and to a less extent by sensible heat flux, have been observed at seasonal time scale. The finding was supported by an error analysis, which included estimation of random error (due to uncertainties in basic observations and in method of computation), estimation of the error due to averaging and correction for the use of coastal data while computing open-sea fluxes. Short time series (8 November 1992–21 February 1993) of hourly measurements at two nearby locations (Rovinj and Pula) have been used to show that spatial variability in air–sea forcing was even more pronounced at hourly and daily time scales. In order to discuss long-term variations of surface fluxes, monthly values computed for the 1966–1992 interval were considered. By relating the extreme values of surface fluxes to global meteorological conditions the reliability of our computation was confirmed. In addition, an attempt has been made to relate the interannual variability of the northern Adriatic surface fluxes to the winter dense-water formation processes and to the changes in temperature, salinity and density of water found in the bottom layer of central Adriatic between 1966 and 1980.

Adriatic seiche decay and energy loss to the Mediterranean

Abstract A salient feature of sea level records from the Adriatic Sea is the frequent occurrence of energetic seiches of period about 21 h. Once excited by a sudden wind event, such seiches often persist for days. They lose energy either to friction within the Adriatic, or by radiation through Otranto Strait into the Mediterranean. The free decay time of the dominant (lowest mode) seiche was determined from envelopes of handpassed sea level residuals from three locations (Bakar, Split and Dubrovnik) along the Croatian coast during twelve seiche episodes between 1963 and 1986 by taking into consideration only time intervals when the envelopes decreased exponentially in time, when the modelled effects of along-basin winds were smaller than the error of estimation of decay time from the envelopes and when across-basin winds were small. The free decay time thus obtained was 3.2±0.5 d. This value is consonant with the observed width of the spectral peak. The decay caused by both bottom friction and radiation was included in a one dimensional variable cross section shallow water model of the Adriatic. Bottom friction is parameterized by the coefficient k appearing in the linearized bottom stress term ρ0u (where u is the along-basin velocity and ρ0 the fluid density). The coefficient k is constrained by values obtained from linearization of the quadratic bottom stress law using estimates of near bottom currents associated with the seiche, with wind driven currents, with tides and with wind waves. Radiation is parameterized by the coefficient f appearing in the open strait boundary condition ζ =auh/c (where ζ is sea level, h is depth and c is phase speed). This parameterization of radiation provides results comparable to allowing the Adriatic to radiate into an unbounded half plane ocean. Repeated runs of the model delineate the dependence of model free seiche decay time on k and a, and these plus the estimates of k allow estimation of a. The principle conclusions of this work are as follows. 1. (1) Exponential decay of seiche amplitude with time does not necessarily guarantee that the observed decay is free of wind influence. 2. (2) Winds blowing across the Adriatic may be of comparable importance to winds blowing along the Adriatic in influencing apparent decay of seiches; across-basin winds are probably coupled to the longitudinal seiche on account of the strong along-basin variability of across-basin winds forced by Croatian coastal orography. 3. (3) The free decay time of the 21.2 h Adriatic seiche is 3.2±0.5 d. 4. (4) A one dimensional shallow water model of the seiche damped by bottom stress represented by Godins (1988) approximation to the quadratic bottom friction law ρ0CDu|u| using the commonly accepted drag coefficient CD = 0.0015 and quantitative estimates of bottom currents associated with wind driven currents, tides and wind waves, as well as with the seiche itself with no radiation gives a damping time of 9.46 d; radiation sufficient to give the observed damping time must then account for 66% of the energy loss per period. But independent estimates of bottom friction for Adriatic wind driven currents and inertial oscillations, as well as comparisons between quadratic law bottom stress and directly measured bottom stress, all suggest that the quadratic law with CD=0.0015 substantially underestimates the bottom stress. Based on these studies, a more appropriate value of the drag coefficient is at least CD=0. In this case, bottom friction with no radiation leads to a damping time of 4.73 d, radiation sufficient to give the observed damping time then accounts for 32% of the energy loss per period.

Oscillations of the inertia period on the Adriatic Sea shelf

Abstract Wind, current and hydrographic data, taken during three summer seasons (1979, 1980 and 1983) on the Adriatic Sea shelf, have been analysed for evidence of the inertia-period oscillations. The data originated from four stations: one close to the lateral boundary, one at a typical mid-basin location, and two close to the longitudinal boundary of the Adriatic Sea. The inertia-period oscillations occurred in episodes lasting for a few days. Vertically, the oscillations displayed a simple structure: the clockwise current-vector rotations were opposed in phase across the thermocline. The partition of energy between two layers depended on the thermocline depth. Horizontally, the inertia-period currents accounted for about 10% of the total current variance at stations close to the longitudinal boundary, and for 20–30% at the stations farther offshore. The oscillations in the current field were accompanied by temperature variations. The complexity of the phenomenon could well be explained by the internal mode and a few horizontal modes of the two-layer sea contained in a rotating rectangular channel. The two-layer fluid model was also found to be capable of introducing the non-adiabatic problem in an illuminative way. The typical Adriatic Sea wind stress (0.25 N m −2 ) caused in the model inertia-period currents of 5–10 cm s 1 , and pycnocline displacements of ∼1 m—in fair agreement with the observations. The linearized bottom friction damped the oscillations with the realistic decay time (1–2 days).

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

Seasonal variability of inertial oscillations in the Northern Adriatic

Abstract The inertial content of the current time series taken at two stations in the Northern Adriatic between May 1988 and November 1990, and hydrographic data measured at the same stations in the period 1966–1991, were analyzed. The object of the investigation was to document the seasonal variability of the inertial signal and its decay time. It was shown that amplitudes of the inertial oscillations are maximal in summer, having an average surface value of about 10 cm s−1, and that they are within the noise level throughout winter. Such variation is due to seasonal changes in the static stability of the water column, and is also influenced by seasonal variability of decay time of the inertial oscillations. The ratio of the amplitudes observed above and below pycnocline is controlled by the pycnocline depth, as predicted by a two-layer model. Decay times of the inertial oscillations were found to range between 20 and 80 h, with a maximum recorded in summer. This is consistent with damping imposed on the two-layer sea by the interface and bottom friction, which in turn is heavily dependent on stratification. The contribution of the interface friction to damping varies from less than 5% in summer to about 30% in spring and autumn.

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.

Introduction to special section: Recent Advances in Oceanography and Marine Meteorology of the Adriatic Sea

[1] Owing to its strategic location, rich fisheries and natural resources, the Adriatic Sea possesses a long history of scientific exploration (Cushman-Roisin et al. [2001] provides a comprehensive review). The two volumes of this Adriatic Sea special section contribute to this rich literature by collecting results from a recent period of intense research activity. During 2002–2003, the combined efforts of several large, multi-disciplinary international programs brought an exceptional array of observational and numerical resources to bear on contemporary questions in Adriatic research. Supported by the U.S. Office of Naval Research, NATO, the Croatian Ministry of Science and Technology and the Italian Ministry of the Environment and Ministry of Universities and Research, scientists from several countries pursued interrelated collaborative research programs. Broad-ranging studies included investigations of circulation at kilometer to basin scales, physical and biological response to bora wind forcing, sediment transport and mucilage formation. Extensive numerical efforts contributed to the success of many of these programs, providing interpretation and understanding and, in some cases, extensions into short-term forecasting.