Zoran Pasarić

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.

Trends in precipitation indices in Croatia, 1961–2010

Precipitation data from the period 1961–2010 and from a dense rain-gauge network over Croatia is analysed for spatial characteristics of trends in precipitation amounts and precipitation indices. Besides large spatial variability, the area is characterized by large temporal (seasonal) variability. Thus, analysis is performed on annual and seasonal scales over seven predefined subregions. Ten precipitation indices are selected to assess the intensity and frequency of extreme events as well as their contribution to annual and seasonal precipitation changes. The results reveal that the changes in annual and seasonal amounts are predominantly weak. A significant trend is detected only for annual amounts (negative) in the mountainous region and for summer (negative) in the mountainous littoral, mountainous region and central hinterland. A significant positive trend for autumn appears in eastern mainland. Negative trends in summer are associated with a decrease in frequency of moderate wet days, in maximal 1- and 5-day precipitation and in an increase in light precipitation. A negative annual trend is mainly caused by a decrease in frequency of very wet days and their contribution to the total precipitation. A positive autumn trend is associated with more very wet days and an increase of their contribution to the total precipitation as well as an increase in maximal 1- and 5-day precipitation. This study complements the existing analysis of five Croatian secular data series of extreme precipitation indices by involving the whole precipitation dataset since the mid-twentieth century and fills the gap present in the trend assessment of precipitation trends in Mediterranean and Europe.

A Simple Analytical Model of Periodic Coastal Upwelling

AbstractAn existing reduced-gravity model that reproduces the response of the coastal sea to alongshore wind forcing at subinertial frequencies is extended by allowing for cross-shore wind forcing and by considering superinertial frequencies. The obtained explicit solution shows that the wind-driven currents are predominantly controlled by friction and the Coriolis force at subinertial frequencies and by friction and local acceleration at superinertial frequencies. The effect of the coast is manifested by coastal-trapped variability at subinertial frequencies and baroclinic inertia–gravity waves propagating away from the coast at superinertial frequencies. The pycnocline oscillates at the coast not only at subinertial but also at superinertial frequencies, with the alongshore wind contributing more to the former and the cross-shore wind influencing more the latter. The oscillations are most pronounced when the periodic wind forcing is resonantly coupled to the local inertial oscillations (but only if the ...

Some Pitfalls of the Semiempirical Method Used to Project Sea Level

AbstractThree variants of the semiempirical method for sea level projection are considered. They differ in assuming that the response of sea level to temperature forcing is equilibrium, inertial, or a combination of the two. All variants produce a successful regression of the temperature and sea level data, albeit with controlling parameters that differ among the cases. The related response times vary considerably, with a realistic value (~50 yr) obtained only if both the equilibrium and the inertial dynamics are taken into account. A comparison of sea levels projected by using the three variants shows that the time series are similar through the middle of the twenty-first century but they radically diverge by the end of the twenty-third century. This result is interpreted with the aid of the underlying transfer functions. It suggests that one should be cautious when using the semiempirical method to project sea level beyond the twenty-first century.

Generalised Pareto distribution: impact of rounding on parameter estimation

Problems that occur when common methods (e.g. maximum likelihood and L-moments) for fitting a generalised Pareto (GP) distribution are applied to discrete (rounded) data sets are revealed by analysing the real, dry spell duration series. The analysis is subsequently performed on generalised Pareto time series obtained by systematic Monte Carlo (MC) simulations. The solution depends on the following: (1) the actual amount of rounding, as determined by the actual data range (measured by the scale parameter, σ) vs. the rounding increment (Δx), combined with; (2) applying a certain (sufficiently high) threshold and considering the series of excesses instead of the original series. For a moderate amount of rounding (e.g. σ/Δx ≥ 4), which is commonly met in practice (at least regarding the dry spell data), and where no threshold is applied, the classical methods work reasonably well. If cutting at the threshold is applied to rounded data—which is actually essential when dealing with a GP distribution—then classical methods applied in a standard way can lead to erroneous estimates, even if the rounding itself is moderate. In this case, it is necessary to adjust the theoretical location parameter for the series of excesses. The other solution is to add an appropriate uniform noise to the rounded data (“so-called” jittering). This, in a sense, reverses the process of rounding; and thereafter, it is straightforward to apply the common methods. Finally, if the rounding is too coarse (e.g. σ/Δx~1), then none of the above recipes would work; and thus, specific methods for rounded data should be applied.

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.

On precipitation monitoring with theoretical statistical distributions

A common practice in meteorological drought monitoring is to transform the observed precipitation amounts to the standardised precipitation index (SPI). Though the gamma distribution is usually employed for this purpose, some other distribution may be used, particularly in regions where zero precipitation amounts are recorded frequently. In this study, two distributions are considered alongside with the gamma distribution: the compound Poisson exponential distribution (CPE) and the square root normal distribution (SRN). They are fitted to monthly precipitation amounts measured at 24 stations in Croatia in the 55-year-long period (1961–2015). At five stations, long-term series (1901–2015) are available and they have been used for a more detailed investigation. The accommodation of the theoretical distributions to empirical ones is tested by comparison of the corresponding empirical and theoretical ratios of the skewness and the coefficient of variation. Furthermore, following the common approach to precipitation monitoring (CLIMAT reports), the comparison of the empirical and theoretical quintiles in the two periods (1961–1990 and 1991–2015) is examined. The results from the present study reveal that it would be more appropriate to implement theoretical distributions in such climate reports, since they provide better evaluation for monitoring purposes than the current empirical distribution. Nevertheless, deciding on an optimal theoretical distribution for different climate regimes and for different time periods is not easy to accomplish. With regard to Croatian stations (covering different climate regimes), the CPE or SRN distribution could also be the right choice in the climatological practice, in addition to the gamma distribution.