Qualitative assessment of synoptic patterns linked to different types of high-frequency sea level extremes in Mediterranean

Qualitative analysis of synoptic conditions associated with extreme high-frequency sea level oscillations recorded at selected Mediterranean tide gauge stations is presented. Two types of extreme events are considered: (1) events in which high-frequency component is dominant component of the residual sea level height; and  (2) events in which the contributions of both high-frequency and low-frequency component to residual signal are nearly equal. We show that, on average, events of type (1) are accompanied by westerly winds at 500 hPa height, north-to-south temperature gradient at 850 hPa, and weak gradients of mean sea level pressure field. Events of type (2) are, on average, characterized by south-westerly winds at 500 hPa height, inflow of warmer, southern air from Africa towards the affected regions, detectable at 850 hPa height, and more enhanced gradients of mean sea level pressure. Further sub-classification of both types of events, based on the wind direction at 500 hPa height is proposed. Three subtypes of events are considered for each of the two groups, events characterized by (a) north-westerly; , (b) south-westerly, and (c) westerly winds. For events of type (1) we find that subtype (a) is characterized by the advection of colder air of northern latitudes over affected areas at 850 hPa height and strong gradients in mean sea level pressure field, caused by high-pressure fields found to the west and low-pressure fields to the east from the affected areas. In contrast, for subtype (b) we observe the inflow of warmer, southern air towards the affected areas at 850 hPa height and mean sea level pressure lows at and around all affected locations. Subtype (c) is characterized by mostly homogenous mean sea level pressure fields, with typical north-to-south temperature gradients at 850 hPa. For events of type (2), we observe that all three subtypes of events qualitatively resemble the subtypes discussed in context of events of type (1). However, air pressure lows observed in mean sea level pressure field in case of type (2) events are noticeably deeper. These new findings are expected to contribute to overall understanding of synoptic conditions driving different types of sea level extremes, potentially leading to further development in forecasting of these events.