ALBERT

Description: The effects of the 2003 European heatwave on the sea surface layer of the Central Mediterranean were studied using a regional 3-D ocean model. The model was used to simulate the period 2000 to 2004 and its performance was validated using remotely-sensed and in situ data. Analysis of the results focused on changes in the Sea Surface Temperature (SST) and on changes to the surface and sub-surface current field. This permitted us to identify and quantify the anomalies of atmospheric and sea surface parameters that accompanied the heatwave. The dominant annual cycle in each variable was first removed and a wavelet analysis then used to locate anomalies in the time-frequency domain. We found that the excess heating affecting the sea surface in the summer of 2003 was related to a significant increase in air temperature, a decrease in wind stress and reduction of all components of the upward heat flux. The monthly averages of the model SST were found to be in good agreement with remotely-sensed data during the period studied, although the ocean model tended to underestimate extreme events. The spatial distribution of SST anomalies as well as their time-frequency location was similar for both the remotely-sensed and model temperatures. We also found, on the basis of the period of the observed anomaly, that the event was not limited to the few summer months of 2003 but was part of a longer phenomenon. Both the model results and experimental data suggest the anomalous heating mainly affected the top 15 m of ocean and was associated with strong surface stratification and low mixing. The skill of the model to reproduce the sub-surface hydrographic features during the heatwave was checked by comparison with temperature and salinity measurements. This showed that the model was generally in good agreement with observations. The model and observations showed that the anomalous warming also modified the currents in the region, most noticeably the Atlantic Ionian Stream (AIS) and the Atlantic Tunisian Current (ATC). The AIS was reduced in intensity and showed less meandering, mainly due to the reduced density gradient and low winds, while the ATC was enhanced in strength, the two currents appearing to modulate each other in order to conserve the total transport of Modified Atlantic Water.

Description: The effects of anomalous weather conditions on the sea surface layer over the Central Mediterranean were studied with an eddy resolving regional ocean model by performing a 5-year long simulation from 2000 to 2004. The focus was on surface heat fluxes, temperature and dynamics. The analysis of the time series of the selected variables permitted us to identify and quantify the anomalies of the analysed parameters. In order to separate the part of variability not related to the annual cycle and to locate the anomalies in the time-frequency domain, we performed a wavelet analysis of anomalies time series. We found the strongest anomalous event was the overheating affecting the sea surface in the summer of 2003. This anomaly was strictly related to a strong increase of air temperature, a decrease of both wind stress and upward heat fluxes in all their components. The simulated monthly averages of the sea surface temperature were in a good agreement with the remotely-sensed data, although the ocean regional model tended to underestimate the extreme events. We also found, on the basis of the long-wave period of the observed anomaly, this event was not limited to the few summer months, but it was probably part of a longer signal, which also includes negative perturbations of the involved variables. The atmospheric parameters responsible for the overheating of the sea surface also influenced the regional surface and sub-surface dynamics, especially in the Atlantic Ionian Stream and the African Modified Atlantic Water current, in which flows seem to be deeply modified in that period.

Description: The Sicily Channel Regional Model forecasting system was tested using an optimization package for the initial and lateral boundary conditions. Spurious high frequency oscillations during the spin-up time were successfully reduced both in duration and magnitude by optimizing the time tendency of the free surface elevation using the Variational Initialization and Forcing Platform method developed in the framework of the Mediterranean Forecasting System Towards the Environmental Prediction project. The effect of optimization was most profound for the free surface elevation, where all oscillations with periods shorter than 4 h were suppressed. The overall forecast skill was assessed on a 5 day case study starting on 6 April 2005, characterized by a fast passage of a deepening atmospheric low-pressure field with strong winds and marked wind direction change. We compared the predicted variables with in-situ and remotely sensed data. The forecasts of temperature, including the sea surface temperature, and salinity were quite successful, while the forecasted currents, especially within the surface layer, were not in good agreement with the measurements.

Description: Ocean forecasts over the Central Mediterranean, produced by a near real time regional scale system, have been evaluated in order to assess their predictability. The ocean circulation model has been forced at the surface by a medium, high or very high resolution atmospheric forcing. The simulated ocean parameters have been compared with satellite data and they were found to be generally in good agreement. High and very high resolution atmospheric forcings have been able to form noticeable, although short-lived, surface current structures, due to their ability to detect transient atmospheric disturbances. The existence of the current structures has not been directly assessed due to lack of measurements. The ocean model in the slave mode was not able to develop dynamics different from the driving coarse resolution model which provides the boundary conditions.

Description: The Sicily Channel Regional Model forecasting system was tested using an optimization package for the initial and lateral boundary conditions. Spurious high frequency oscillations during the spin-up time were successfully reduced both in duration and magnitude by optimizing the time tendency of the free surface elevation using the Variational Initialization and Forcing Platform method developed in the framework of the Mediterranean Forecasting System Towards the Environmental Prediction project. The effect of optimization was most profound for the free surface elevation, where all oscillations with periods shorter than 4 h were suppressed. The overall forecast skill was assessed on a 5 day case study starting on 6 April 2005, characterized by a fast passage of a deepening atmospheric low–pressure field with strong winds and marked wind direction change. We compared the predicted variables with in–situ and remotely sensed data. The forecasts of temperature, including the sea surface temperature, and salinity were quite successful, while the forecasted currents, especially within the surface layer, were not in good agreement with the measurements.