Record Balkan floods of 2014 linked to planetary wave resonance

Abstract

In May 2014, the Balkans were hit by a Vb-type cyclone that brought disastrous flooding and severe damage to Bosnia and Herzegovina, Serbia, and Croatia. Vb cyclones migrate from the Mediterranean, where they absorb warm and moist air, to the north, often causing flooding in central/eastern Europe. Extreme rainfall events are increasing on a global scale, and both thermodynamic and dynamical mechanisms play a role. Where thermodynamic aspects are generally well understood, there is large uncertainty associated with current and future changes in dynamics. We study the climatic and meteorological factors that influenced the catastrophic flooding in the Balkans, where we focus on large-scale circulation. We show that the Vb cyclone was unusually stationary, bringing extreme rainfall for several consecutive days, and that this situation was likely linked to a quasi-stationary circumglobal Rossby wave train. We provide evidence that this quasi-stationary wave was amplified by wave resonance. Statistical analysis of daily spring rainfall over the Balkan region reveals significant upward trends over 1950-2014, especially in the high quantiles relevant for flooding events. These changes cannot be explained by simple thermodynamic arguments, and we thus argue that dynamical processes likely played a role in increasing flood risks over the Balkans.

Keywords: Flooding; Rossby Waves; Vb-cyclone; Wave-resonance; climate change; extreme weather.

Fig. 1. Daily precipitation averaged over the BHSC region during the first half of 2014, showing clusters of heavy rainfall in April and around 3 May (dark blue), before the record-breaking rainfall events associated with cyclone Yvette (14 May; dark blue).

Horizontal lines indicate monthly means for April and May 2014 (dashed) and their climatology (dotted).

Fig. 2. Accumulated rainfall (blue colors) over southeastern Europe from 13 to 16 May 2014 (in millimeters) and the position of cyclone Yvette tracked by its lowest core pressures (red line).

Large red dots represent the position at 12:00 a.m. with the date labeled, and small red dots indicate positions at 6:00 a.m., 12:00 p.m., and 6:00 p.m.

Fig. 3. Increased heavy rainfall days over the BHSC region during spring.

(A) Daily rainfall distribution for days in MAMJ (dots) plus linear regressions of the mean, median, and the 95th and 99th quantile for 1950–2014. (B) Linear trend of different quantiles in daily rainfall during spring showing strong and significant trends in the upper tail. (C) Number of days exceeding the 95th and 99th rainfall percentile during spring in the BHSC region.

Fig. 4. Upper-level hemispheric circulation in May 2014.

(A) Total wind field (in meters per second) at 300 mb over the Northern Hemisphere mid-latitudes on 13 May (daily mean). Color displays the magnitude of the flow, and arrows indicate direction. (B) Meridional component of the wind speed (in meters per second) averaged from 6 to 20 May 2014, revealing a quasi-stationary wave 6 pattern. (C) Spectral contributions of different wave numbers to the meridional wind field averaged from 35°N to 60°N and from 6 to 20 May 2014. (D) Latitudinal distribution of the square of the stationary wave number given by (l²a² + k²), which gives the location of possible turning points for any given k indicated on the right vertical axis. It shows that a waveguide for wave number 6 forms from 34°N to 40°N (dashed lines). The vertical dashed black lines indicate the latitudinal positions of the turning points for wave number 6 as given by the crossing of the horizontal dashed black lines with the right y axis.