Hazardous Conditions: Winter

• Northwesterly winds and waves. Prevailing winds at Bizerte during the winter are from the west to northwest, and a major threat to the port is created by strong northwesterly winds and waves. The winds can reach 40 to 50 kts, and be accompanied by swell waves exceeding 10 ft in the anchorage. Local authorities state that they occur primarily from October through May, and can be anticipated two days in advance of onset because they can be associated with Mistral events in the Gulf of Lion.

• The Mistral is a cold, strong northwesterly to north-northeasterly offshore wind along part or all of the coast of the Gulf of Lion. Its influence occasionally extends beyond the Gulf of Lion to affect the weather of the entire Mediterranean Basin. The Mistral is the result of a combination of the following factors:
  • The basic circulation that creates a pressure gradient from west to east along the coast of southern France. This pressure gradient is normally associated with Genoa cyclogenesis.

  • A foehn wind effect caused by cold air associated with the Mistral moving downslope as it approaches the southern coast of France and thus increasing the wind speed.

  • A jet-effect wind increase caused by the orographic configuration of the coastline. This phenomenon is observed at the entrance to major mountain gaps such as the Carcassone Gap, Rhone Valley, and Durance Valley. It is also observed in the Strait of Bonifacio between Corsica and Sardinia.

  • A wind increase over the open water resulting from the reduction in the braking effect of surface friction. Mistral wind speeds often exceed 40 kt and occasionally have reached 100 kt in gusts along the coastal region from Marseille to Toulon. Over the open water in the Gulf of Lion, Mistral wind speeds locally greater than 40 kt occurred in nearly 8% of total observations. The strongest winds associated with a Mistral generally occur over the Gulf of Lion, decreasing south-eastward. However, synoptic situations producing severe Mistrals will often produce associated strong wind regimes extending as far as North Africa, Sicily and Malta. Although the Mistral is prevalent during all seasons, severe cases are most common during winter and spring.

• Weather associated with Mistral events is generally good near the Mistral source, but as the cold air moves out over the warmer water, convective cloudiness increases. Very poor atmospheric visibility also has been reported up to a height of 98 ft during cases of extremely strong Mistrals because of a layer of spray that extends above the water surface.

• North African Low. Another potential cause of strong north to northwesterly winds at Bizerte is a North African low which moves northeastward across the coast of Tunisia toward Sicily. Strong winds are likely to the west of the northeastward tracking low, especially when the low is accompanied by a tongue of cold air aloft (evident at 500 mb).

• East and Northeast Winds. East and northeast winds have a less severe impact at Bizerte than northwest winds, but affect port operations by causing steering difficulties for ships entering the port. The winds can have several causes, including the development and passage of North African lows south of Bizerte.

• North African lows develop over the desert region south of the Atlas Mountains. The synoptic situation favoring development is the presence of an upper trough lying over Spain with its axis lying northeast to southwest, producing a deep southwesterly flow over northwest Africa. The presence of a cold front is apparently immaterial for the development of a low, but when one is present, development usually occurs before the front reaches the mountain range.

• The lows which have the potential to produce east and northeasterly winds at Bizerte follow an eastward track south of the Atlas Mountains before moving over the Mediterranean Sea across the coast of Tunisia at or near the Gulf of Gabes.

• When North African lows occur south of the Atlas Mountains, strong easterly to southeasterly winds are likely over the southern Mediterranean and will result in high seas in the Strait of Sicily. A North African low is most likely to form over Tunisia when the long-wave trough is oriented northeast-southwest across the Tyrrhenian Sea. Cold continental polar air will be advected in from eastern Europe and a pocket of cold air (-25degC at 500 mb) will form between Sardinia, Sicily and Tunisia. The subtropical jet also will be evident over North Africa. Wind speeds at 500 mb over Tunisia and Libya will be 55 kt or more. The speed of movement with these systems is related to the time of year in which they develop. During late autumn and early winter, lows moving out of this area are noted for their extremely slow movement due to their association with a cut-off low aloft.

• During late winter and early spring, as the number of North African cyclones increases, North Africa becomes the primary cyclogenesis area for the region. Unlike lows developing early in the winter, these lows are generally associated with open, short wave troughs. They produce little precipitation, but frequently produce high winds in close proximity to their centers. Their increased speed of movement compared with the early winter systems also make them unique. Some lows have been noted to move eastward out of North Africa at 40 to 50 kt.