Birth of the Biscane

On 15 September 2016 storm “Stephanie” exhibited unprecedented tropical characteristics over the Bay of Biscay. This contribution describes the synoptic-dynamic development of the storm, and evaluates the skill of the European Centre of Medium-Range Weather Forecasts (ECMWF) deterministic forecasts in predicting its warm core. During the second week of September, a blocking pattern prevailed over Europe and caused the upper-level trough upstream to elongate meridionally over the eastern North Atlantic. The trough later developed into a cut-off over the northern Iberian Peninsula, where it remained for the next two days. In the meantime, a low-pressure system formed and moved cyclonically around the quasi-stationary cut-off trough, thereby distorting the associated cold front and eventually undergoing occlusion. After this, on the day before landfall at the Spanish coast, the storm acquired tropical-like characteristics. In the Mediterranean Sea, extra-tropical storms occasionally follow the TT development pathway despite cooler SSTs than in the Tropics, usually due to the presence of an upper-level cut-off trough, which enhances bulk atmospheric instability. As a result, Mediterranean tropical-like cyclones, also referred to as “Medicanes” (from Mediterranean hurricanes), form. By analogy with them, we name this storm a “Biscane” (Biscay hurricane).

Despite that fact that the storm occurred entirely over the sea, various types of in-situ and remote sensing observations are available to document the most interesting part of the evolution from the end of the occlusion until dissipation. A cloud-free central area was observed in satellite imagery and matched a ring-shaped pattern of rain in ground-based radar reflectivity. Reaching an intensity equivalent to a tropical storm, an axially symmetric wind field was observed from a space-borne scatterometer. The time series of pressure and wind values recorded by the buoy “Gascogne”, over which the storm center passed, is reminiscent of a tropical cyclone with a distinct, small-scale inner-core structure. Cyclone phase space diagrams (Hart, 2003), adapted to the specific characteristics of this storm, clearly corroborate the transition from a frontal cold-core to a symmetric warm-core system, often referred to as “tropical transition” (TT). To the best of our knowledge, this is the first-ever documented case of this kind over the Bay of Biscay. The ECMWF deterministic forecast predicted well the track, intensity, and TT of the storm four days ahead.

We speculated that the tropical transition of Stephanie was fostered by anomalously high sea surface temperatures (SSTs) present in September 2016 over the Bay of Biscay. While these high SSTs are likely related to the prevalence of anticyclonic weather in the weeks preceding the storm, average SSTs over the Gulf of Biscay increased by about 1°C in the last 35 years and weather systems of this kind may become more frequent in a future warmer climate.