Synoptic patterns triggering tornadic storms on the Iberian Peninsula and the Balearic Islands

Abstract

The present article analyses the synoptic framework associated with tornadic events on the Iberian Peninsula and the Balearic Islands. We discussed the spatial and temporal distribution of tornadoes in the study area, subsequently performing a principal component analysis using 500 hPa geopotential height and mean sea level pressure data from ERA5 reanalysis to classify into 12 synoptic patterns the 465 tornadic events (comprising 608 individual tornadoes) reported between 1950 and 2021. Furthermore, we employed 900 hPa specific humidity and temperature, 300 hPa and 900 hPa zonal and meridional wind, CAPE, medium-layer shear (MLS) and the product of MLS and the square root of two times CAPE (WMAXSHEAR03) variables to create the mean composites related to each weather type. We also analysed the influence of several teleconnection patterns on the occurrence of tornadoes. Results showed a wide range of configurations prone to tornadogenesis, although these are usually characterised by a surface low and/or a trough whose axis is oriented north-south close to the Iberian Peninsula. Tornadoes are typically related to warm, moist low-level advections, i.e. maritime 900 hPa wind together with 900 hPa specific humidity relative maxima. The variable WMAXSHEAR03 presents a pattern similar to the tornado distribution for each weather type throughout the study area, whereas CAPE and MLS separately do not (Mediterranean events are mainly associated with high CAPE and moderate MLS environments, whereas Atlantic tornadoes tend to be promoted in small CAPE and strong MLS conditions). Tornadoes in western Iberia are usually formed under the negative phase of the NAO and the AO, whereas in the Mediterranean area they are typically associated with the positive phase of the WeMO and the MO, which contrasts with torrential rainfall events in this subregion.