Tropical cyclones commonly occur over the (sub)tropical areas of the world’s oceans and are often associated with catastrophic weather events. They are usually smaller than their midlatitude extratropical counterparts but can exhibit very strong winds, sometimes well over 250 km/h for the strongest ones classified as category 5 on the Saffir-Simpson scale.
Even though these violent weather systems only develop over the open ocean with SST's of at least 26 degrees centigrade, a smaller and less well-known type of cyclone can develop over smaller (and much cooler) basins when conditions are right: tropical-like cyclones (TLC’s). While these hybrid storms are infrequent, their development can heavily disrupt maritime operations. In this article we specifically target the Black Sea where the weather can also take a turn for the worst.
Even though the Black Sea spans well over 400.000 km² and SST’s can locally reach more than 25°C during summer, the basin is just simply too small and cool for the development of tropical cyclones. Under specific conditions though, with high SST placed underneath cool mid- and upper levels of the atmosphere, weaker tropical-like weather systems can occur over the region; frequently bearing a striking resemblance to ‘real’ tropical cyclones.
On satellite imagery most fully grown tropical cyclones (Hurricanes and Typhoons) show up as vast pearly white rotating disks with a distinct cloud free eye. They exhibit a warm symmetric core, enclosed by a highly active eyewall and clearly visible upper-level outflow ventilation. Their weaker tropical-like cousins tend to have similar characteristics, but they are usually less strongly/deeply developed and can have (a)symmetrical non-frontal cloud bands surrounding the core. Also, these hybrid systems occur on a much smaller scale (total diameter well under 300 km) with strongest sustained winds (sometimes well over 30 m/s) limited to a radius of 50 to 100 km from the system’s center.
The first rounds of thunderstorms will not impact the background flow that much though. Only once a process called CISK can take place this all changes. CISK (Convective Instability of the Second Kind) describes the self-strengthening process of large scale latent heat release in an increasingly moist environment through convection, leading to a lowering of surface pressure. In response to the changing pressure gradient, ambient air moves in towards the newly formed low causing stronger low level convergence which leads to renewed convection, even more latent heat release (as well as upper level outflow) and another lowering of surface pressure. This then triggers more vigorous convection, etc. This cycle continuously repeats itself, intensifying the cyclone as long as the developing circulation remains present over warm enough water (generally in the range of 18 to 26°C) without interactions of too much wind shear and/or land masses.
Fig. 3 Schematic representation of the CISK cycle, where positive feedback loops between widespread convection and low level convergence cause cyclogenesis.
As the Black Sea is limited size-wise, sooner or later the storm will encounter land and decay quickly as its main source of fuel is cut off and the circulation is distorted due to frictional forces. This also limits the frequency; on average a mature tropical-like cyclone, with sustained winds of at least 32 m/s, occurs only once every decade. More common are tropical depressions (<17 m/s) or tropical storms with winds >17 m/s. But still, on average they occur only once every year somewhere over the basin, with highest likelihood of occurrence between September and January.
Tropical-like cyclones undergo a three staged development if the synoptics remain right long enough. The pre-eyed stage (phase I) is characterized by increasing surface winds and clustering thunderstorms, usually most strongly developed along the western flank of the developing cyclone. Upper level support is the main driver at first, but CISK soon takes over.
In phase II (stationary or mature phase) CISK is predominant. An increasingly distinct eye-like structure develops as the axisymmetric cloud band wraps around the deepening warm core. Thunderstorm activity is frequent at first, but slowly moves away from the center as massive amounts of released latent heat stabilize the airmass. Surface winds peak and overall precipitation activity is at its highest in this phase. If the (non-frontal) cloud band completely surrounds the core a distinct eye can be observed.
Once the system reaches land, or the synoptics won’t allow any further development, phase III dictates a quick decay with less overall activity. Winds weaken rapidly at this point as the surface low fills. The storm’s cyclonic structure becomes messy and eventually unrecognizable as such.
As most tropical-like systems over the Black Sea only have a limited life span of a few days, and the bands with strongest winds remain rather close to the cyclone’s core, the impact on wave height is usually significant only on a regional scale. With sustained winds ranging from 15 to well over 30 m/s, locally 3 to 5m can be added to Hs, with highest waves usually occurring in the southeastern quadrant of the storm where storm motion aligns with prevailing winds.
Atmospheric pressure in the storm’s center usually isn’t low enough to cause a significant storm surge, as is common with strong tropical cyclones. However, some observations in the Mediterranean (where TLC’s are called ‘Medicanes’) have occasionally shown minor bulges up to 30cm near the low’s center. Causing additional flood risks upon landfall.
Also, SST can drop quite dramatically in the wake of a mature cyclone. Sudden drops of 10 to 15 degrees centigrade have been observed over the Black Sea (storm Falchion, 25-29 Sept 2005), as deep mixing and upwelling allowed cold water to surface in the storm’s wake.
Even though tropical-like cyclones are a rare occurrence over the Black Sea, they can wreak havoc and cause serious damage to equipment and personnel if you are unlucky enough to get caught in one. In the weather room of Infoplaza our meteorologists are always on the lookout for systems like this and will give out early warnings if necessary. Interested in our services? Feel free to contact us at any time.