Influence of wind stress and the Isonzo/Soča River outflow on surface currents in the Gulf of Trieste

Between October and November 2023, the Isonzo/Soča River catchment area experienced exceptionally intense rainfall. The strong precipitations acting on the hydrographic basin at the end of October and beginning of November led to exceptional increases in the Isonzo/Soča runoff into the Gulf of Trieste (GoT) (October 27 and November 3). It was observed that in the days before a precipitation event, a southerly wind with a strength of more than 3 m/s almost always affected the area. Concurrently, rising sea levels and coastal flooding were observed. The event is of particular significance as it was also accompanied by strong coastal storm and waves that caused severe damage to the coast.The aim of this study is to investigate how the exceptional Isonzo outflow, together with the wind patterns associated with the meteorological event, influenced the ocean currents in the surrounding coastal region. To this end, the mechanisms and processes regulating the interaction between river discharges and ocean currents in the Gulf of Trieste were analysed through an integrated analysis combining hydrometric, meteorological and current data.The prevailing winds in the GoT come from the north-eastern (Bora wind) and southern sectors (Sirocco and Libeccio winds). During Bora events, the usual cyclonic circulation is accentuated and the surface currents normally leave the GoT, while during strong southerly wind events the circulation becomes anticyclonic and the surface currents enter the GoT. In the case with a significant river outflow in combination with southerly winds the circulation is anticyclonic in the central part of the gulf and cyclonic in the northern part.HFR sea surface current data confirmed that wind-induced Ekman transport appears to dominate the surface current dynamics in the GoT. Nevertheless, exceptionally intense outflows from the Isonzo, triggered by heavy precipitation and accompanied by southerly winds, can overlay the effects of wind-driven transport, leading to the dominance of river-induced circulation patterns in the GoT.

The increasing amount of data in earth-observing systems allows us to move from considering low-order moments (means and variances) of fluctuating observations to their PDFs (Probability Density Functions). For two years of HFR (High Frequency Radar) sea surface current increments in the Gulf of Trieste (Northern Adriatic Sea) we found the analytical fat-tailed PDF form (a combination of a gaussian and a convolution of two exponentials) using superstatistics and the maximum entropy principle twice: on a short and on a longer time scale. The data observed under different wind regimes (Bora, Sirocco and low wind, from the WRF model local forecasts) follow the same analytical PDF, pointing towards a universal behaviour.We developed an idealised deterministic-stochastic model of the wind-driven sea surface currents in the Gulf of Trieste. The deterministic model consists of a time-dependent Ekman layer system, including the tidal signal, with a quadratic drag. It describes 57% of the variability, missing the fast fluctuations. The stochastic part accounts for the fast fluctuations, reproducing the superstatistical PDFs from the observations. The model, providing a huge amount of data, allows for studying the PDF of the mechanical power-input into the ocean and the associated extreme events.

Supplementary material to "Influence of wind stress and the Isonzo/Soča River outflow on surface currents in the Gulf of Trieste"

Coastal variables monitoring and study are crucial activities for researchers to develop and enhance the knowledge about how climate changes effects are modifying the maritime hydrodynamics.This study focuses on local effects on the hydrodynamic circulation and significant wave height in the Gulf of Trieste (GoT) through coastal high frequency radars (HFRs) measurements and wave buoys. In a semi-enclosed basin, characterized by an average depth equal to 18m and maximum depth of 25m, the variables investigated are surface currents direction (θcurr) and eastward and northward velocity component (ucurr,vcurr), mean wave direction (θw) and spectral significant wave height (Hm0). The HFRs network installed in GoT are composed by four WERA (WEllen RAdar) systems installed in the east and south part of the gulf and operating at a frequency of 24.5 MHz. Two wave buoys are installed in the central and north parts of the GoT at different depths, recording the wave energy spectral distribution characterizing the area. Finally numerical method to estimate power spectral density are implemented in this study with high resolution allowing to increase the knowledge of spatial and temporal hydrodynamic evolution inside the GoT.The HFR measurements can return a spatial information about waves and currents variable, improved by the calibration process. This information is crucial to reproduce the coastal hydrodynamic. Coupling all data is possible to underline the wave spectral evolution inside the gulf, the hydrodynamic peculiarity and have a focus on the extreme events effect on semi-enclosed basins. Joining the HFR and wave buoys data and implementing the numerical model, this research shows innovative methods to deepening knowledge and monitoring the extreme events that characterise the study area.

In this study, two years (2021-2022) of High Frequency Radar (HFR) sea surface current data (30 min time resolution) and modelled near-bottom wind data (1 h time resolution) in the Gulf of Trieste (Northern Adriatic Sea) are analysed through a superstatistical (a superposition of different statistics) approach. Three distinct main wind forcing regimes are present in the Gulf of Trieste: Bora, Sirocco and low wind. Bora and Sirocco are strong winds whose characteristics are different: the Bora is a cold wind that blows in gusts from the East-North-East with a short fetch, the Sirocco is a warm wind that blows from the South with a fetch along the entire Adriatic. The currents in the Gulf of Trieste are forced and highly dependent on such variable wind conditions. It results in a succession of different sea current dynamics on different time scales, asking for a superstatistical analysis of the sea surface current data. From the oceanic signal it is possible to extract two different time scales: a relaxation time τ, the time the system spends to reach the local equilibrium and a larger timescale T, the time for which the signal is locally gaussian. This permits extracting a slowly varying β(t) strictly connected to the original time series’ local variance σ2=β-1. Neither β nor σ2 show well known PDFs and have algebraic tails. Contrary to what one might expect, they show a universal behaviour with respect to the different wind regimes blowing over the Gulf of Trieste.

The Gulf of Trieste is the northernmost part of the Mediterranean and the Adriatic Sea. It is home to lively shipping traffic, tourism, and fishing. The Gulf of Trieste is home to two of the most important ports on the Adriatic coast - Trieste and Koper. We have developed high-resolution operational numerical models for the operational prediction of the distribution of sea surface currents and the dispersion of wastewater from the sewage treatment plant into the sea:(1) Prediction of the distribution of surface currents in the Gulf of Trieste (northern Adriatic Sea) to complement measurements from four HF radars stationed in the Gulf of Trieste. The radar provides current measurements at 30-minute intervals, except during maintenance work and occasional technical faults. In some areas, there are also occasional outages and unreliable current measurements, so high-resolution current models are of additional benefit. The two-day hourly forecast of sea currents provided by our model is displayed on the website along with the HF radar measurements. The most typical user of the HF radar measurements is the Slovenian Maritime Administration, which is responsible for the safety of navigation, management of maritime traffic and maintenance of safety facilities and waterways. The results of our numerical model can provide reliable information on the direction and strength of sea currents, which helps ship operators to plan their routes more efficiently, reduce fuel consumption and emissions, increase shipping safety, and enable better control of oil spills.(2) Predicting the dispersion of wastewater from a wastewater treatment plant into the coastal sea. The Rižana wastewater treatment plant is located in the Gulf of Trieste, just upstream of the mouth of the Rižana river into the port of Koper. The results of the numerical model show whether the wastewater from the treatment plant enters protected areas or bathing waters. The results can help the relevant authorities to predict and prepare for potential pollution events, contribute to a better understanding of the processes in the coastal sea that control the transport, mixing and fate of pollutants, and show which marine areas are particularly vulnerable to pollution.The high-resolution numerical models for predicting surface currents and the dispersion of sewage in the Gulf of Trieste represent a significant advance in maritime safety and environmental management. This underlines the valuable role of advanced modeling in promoting sustainable maritime practices and protecting the fragile ecosystem of the northern Adriatic.

Abstract. Two years (2021–2022) of high-frequency-radar (HFR) sea surface current data in the Gulf of Trieste (northern Adriatic Sea) are analysed. Two different timescales are extracted using a superstatistical formalism: a relaxation time and a larger timescale over which the system is Gaussian. We propose obtaining an ocean current probability density function (PDF) combining (i) a Gaussian PDF for the fast fluctuations and (ii) a convolution of exponential PDFs for the slowly evolving variance of the Gaussian function rather than for the thermodynamic β=1/σ2 in a system with a few degrees of freedom, as the latter has divergent moments. The Gaussian PDF reflects the entropy maximization for real-valued variables with a given variance. On the other hand, if a positive variable, as a variance, has a specified mean, the maximum-entropy solution is an exponential PDF. In our case the system has 2 degrees of freedom, and therefore the PDF of the variance is the convolution of two exponentials. In the Gulf of Trieste there are three distinct main wind forcing regimes: bora, sirocco, and low wind, leading to a succession of different sea current dynamics on different timescales. The universality class PDF successfully fits the observed data over the 2 observation years and also for each wind regime separately with a different variance of the variance PDF, which is the only free parameter in all the fits.

Hydrological disasters are recurrent over the northeast Indian region (NEI) due to heavy downpours within a short span of time. Therefore, the characteristics of rainfall extremes during the southwest monsoon over the NEI, their evolution and dissipation features, are discussed using rainfall data from the Tropical Rainfall Measuring Mission (TRMM) 3B42 V7 and the Climate Prediction Center for 16 years from 2000 to 2015. Circulation features during the lead–lag of extreme events are also examined using the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis products. Thirty‐two extreme events such that rainfall of the day exceeds the 90th percentile were identified. During the extreme events, heavy rainfall is located mainly in the NEI without much spread in the aerial domain. The highest number of extreme rain events is found in July, followed by June and August. Trend analysis of the rainfall indicates a significant decrease in low rainfall events and this decreasing trend is compensated by the significant increasing trend in very heavy (>95 percentile) to extremely heavy (>99 percentile) rainfall events. The average life of the extreme rainfall events is found to be about 3 days during which the NEI receives more than 100 mm/day; however, on individual examination, this ranges from 2 to 5 days. A strengthening of the southerly component of lower level wind from the Bay of Bengal and an updraft due to convergence at 850 hPa over the region favours the rainfall peak. The sea level pressure anomaly gradient and vertical velocity show a sudden development of conditions favourable for a heavy downpour over the northeast region.

Does anoxia affect mercury cycling at the sediment–water interface in the Gulf of Trieste (northern Adriatic Sea)? Incubation experiments using benthic flux chambers

Climate change, extreme events and increased risk of salmonellosis in Maryland, USA: Evidence for coastal vulnerability

The rapid expansion of urbanization along the world’s coastal areas requires a more comprehensive and accurate understanding of the coastal ocean. Over the past several decades, numerical ocean circulation models have tried to provide such insight, based on our developing understanding of physical ocean processes. The systematic establishment of coastal ocean observation systems adopting cutting-edge technology, such as high frequency (HF) radar, satellite sensing, and gliders, has put such ocean model predictions to the test, by providing comprehensive observational datasets for the validation of numerical model forecasts. The New York Harbor Observing and Prediction System (NYHOPS) is a comprehensive system for understanding coastal ocean processes on the continental shelf waters of New York and New Jersey. To increase confidence in the system’s ocean circulation predictions in that area, a detailed validation exercise was carried out using HF radar and Lagrangian drifter-derived surface currents from three drifters obtained between March and October 2010. During that period, the root mean square (RMS) differences of both the east–west and north–south currents between NYHOPS and HF radar were approximately 15 cm s−1. Harmonic analysis of NYHOPS and HF radar surface currents shows similar tidal ellipse parameters for the dominant M2 tide, with a mean difference of 2.4 cm s−1 in the semi-major axis and 1.4 cm s−1 in the semi-minor axis and 3° in orientation and 10° in phase. Surface currents derived independently from drifters along their trajectories showed that NYHOPS and HF radar yielded similarly accurate results. RMS errors when compared to currents derived along the trajectory of the three drifters were approximately 10 cm s−1. Overall, the analysis suggests that NYHOPS and HF radar had similar skill in estimating the currents over the continental shelf waters of the Middle Atlantic Bight during this time period. An ensemble-based set of particle tracking simulations using one drifter which was tracked for 11 days showed that the ensemble mean separation generally increases with time in a linear fashion. The separation distance is not dominated by high frequency or short spatial scale wavelengths suggesting that both the NYHOPS and HF radar currents are representing tidal and inertial time scales correctly and resolving some of the smaller scale eddies. The growing ensemble mean separation distance is dominated by errors in the mean flow causing the drifters to slowly diverge from their observed positions. The separation distance for both HF radar and NYHOPS stays below 30 km after 5 days, and the two technologies have similar tracking skill at the 95 % level. For comparison, the ensemble mean distance of a drifter from its initial release location (persistence assumption) is estimated to be greater than 70 km in 5 days.

Extreme climate events are expected to occur very frequently and intensively with climate change, and such extreme events can induce irreversible damage to plants and soils, as well as ecosystems. Accordingly, there is a need to understand the effects of extreme climate events on ecosystems. Here, we designed a temperature and precipitation manipulation system to simulate extreme climate events of heat, drought, and heavy rainfall. We constructed three soil surface temperature manipulation levels (control, 3 °C, and 6 °C increases) and three precipitation manipulation levels (control, drought, and heavy rainfall) with six replicates, and operated these from day of year (DOY) 195 to 233 in 2020. Infrared heaters increased the soil surface temperature during the extreme heat treatments. For precipitation manipulation, the automatic rainout shelter excluded ambient rainfall to produce drought conditions and an artificial rainfall simulator with spray nozzles produced heavy rainfall conditions. As a result, the soil surface temperature (°C ± one standard deviation) was higher in the 3 °C and 6 °C heated treatments than in the control by 2.7 ± 0.2 and 5.7 ± 0.5, respectively. The mean soil water content (vol. %) was 12.9 ± 8.6 in the drought treatment, 14.1 ± 7.8 in the control, and 16.1 ± 8.3 in the heavy rainfall treatment during the precipitation manipulation period. The results showed that the system design and operation were as expected. The designed system can be effectively utilized to investigate the responses of plants and soils to extreme climate events.

Although small in size, the Gulf of Trieste (GoT), a marginal coastal basin in the northern Adriatic Sea, is characterized by very complex dynamics and strong variability of its oceanographic conditions. In April–May 2012, a persistent, large-scale anticyclonic eddy was observed in the GoT. This event was captured by both High Frequency Radar (HFR) and Lagrangian drifter observations collected within the European MED TOSCA (Tracking Oil Spill and Coastal Awareness) project. The complexity of the system and the variety of forcing factors constitute major challenges from a numerical modeling perspective when it comes to simulating the observed features. In this study, we implemented a high-resolution hydrodynamic model in an attempt to reproduce and analyze the observed basin-wide eddy structure and determine its drivers. We adopted the Massachusetts Institute of Technology General Circulation Model (MITgcm), tailored for the GoT, nested into a large-scale simulation of the Adriatic Sea and driven by a tidal model, measured river freshwater discharge data and surface atmospheric forcing. Numerical results were qualitatively and quantitatively evaluated against HFR surface current maps, Lagrangian drifter trajectories and thermohaline data, showing good skills in reproducing the general circulation, but failing in accurately tracking the drifters. Model sensitivity to different forcing factors (wind, river and tides) was also assessed.

The 1998 extremely heavy rainfall events over East Asia are investigated through partial lateral forcing (PLF) experiments with the Weather Research and Forecasting model to determine the impacts of the synoptic (SY), intra-seasonal (IS), and inter-annual (IA) forcing across the lateral boundary on the extreme climate anomalies. The large-scale lateral boundary forcing was derived from an ensemble reanalysis dataset and decomposed into climatological, SY, IS, and IA components. The PLF experiments show that the IS forcing not only triggers the monsoon onset and drives two northward propagation events of the subtropical front but also has dominant contributions to the two heaviest rainfall events over the Yangtze River Basin (YRB) and South China, suggesting the critical role of the intra-seasonal variability in the devastating 1998 floods. Previous studies perceived that the northward propagating IS oscillation from the tropics regulates the extreme heavy rainfall of East Asia summer monsoon in 1998. However, we find that the IS forcing from the mid-latitude plays a more important role than the forcing from the tropics in generating the two extreme rainfall events in 1998. During the first extreme event in June, the IS forcing across the western boundary is the major cause of the northward advance of the subtropical front and the heavy rainfall over the YRB and South China, with the IS forcing across the northern boundary providing the second largest contribution. During the second extreme event (July 15–August 5), the IS forcing from the eastern boundary plays a dominant role in driving the southward retreat and northward advance of the subtropical front, causing another heavy rainfall over the YRB and South China. The western and northern IS forcing also has large contributions to the second extreme event. We have estimated the contributions to the seasonal anomalous rainfall by the three types of forcing. The SY forcing tends to have a moderate effect on the YRB rainfall but significant reduction of the rainfall in South China. The IS forcing has dominant contributions to the seasonal-mean rainfall anomalies over all three sub-regions of China (North China, the YRB, and South China). The IA forcing mainly enhances the rainfall in South China but reduces the precipitation in the YRB slightly. This study portends a promising application of regional climate models to identify key factors causing extreme climate events. The PLF methodology can be used to study a broad range of climate phenomena and to understand the effects of variety of dynamic and physical processes in climate variability and predictability.

PurposeAs the result of historical mining at Idrija (Slovenia), mercury (Hg) contamination in the Gulf of Trieste (northern Adriatic Sea) is still an issue of environmental concern. The element has been conveyed into the coastal area by the Isonzo/Soča River inputs of freshwater and suspended particles for centuries. This research aims to investigate the occurrence of Hg bound to the settling sediment particles (SSP) in the coastal water and to assess the sedimentary Hg fluxes.MethodsSettling sediment particles were collected at four sites located in the innermost sector of the Gulf, a shallow and sheltered embayment where the accumulation of fine sediments is promoted. Six sampling campaigns were performed under different environmental conditions in terms of discharge from the Isonzo River and 12 sediment traps were installed in the upper and bottom water column for SSP collection. Settling sediment particles (SSP) were collected approximately every 2 weeks and analysed for grain size and total Hg.ResultsSettling sediment particles (SSP) consisted predominantly of silt (77.7 ± 10.1%), showing a concentration of Hg ranging overall between 0.61 and 6.87 µg g−1. Regarding the daily SSP fluxes, the minimum (7.05 ± 3.26 g m−2 day−1) and the maximum (92.4 ± 69.0 g m−2 day−1) values were observed under conditions of low and high river discharge, respectively. The daily Hg fluxes displayed a notable variability, up to an order of magnitude, both in the surface water layer (3.07–94.6 µg m−2 day−1) and at the bottom (11.3–245 µg m−2 day−1), reaching the maximum values following periods of high river flow.ConclusionsThe Isonzo River inputs of suspended particulate matter continue to convey Hg into the Gulf of Trieste, especially following river flood events, which represent one of the most relevant natural factors affecting the variations of the Hg flux in the investigated area.