2. Area of study
2.1. Bermuda
2.1.2. Climatic characterization
2.1.2.6. Dynamic climatology
The Gulf Stream is an important factor that modifies any air mass passing west of
Bermuda. Frontal systems moving South and East off the North American coast that reach
the islands experience changes in their displacement across the Gulf Stream and Western
Sargasso Sea, giving quite mild winters (Macky 1946). Nevertheless, the Bermuda-
Azores High regulates the surface wind flow. From May to October the High is well
established, where very few migratory highs pressure systems can reach the area
(Vandever & Pearson 1994). In summer, it is located east of Bermuda and when it
becomes elongated extends westward over the north eastern United States, when the
islands are under the influence of persistent troughs; the surface circulation may vary
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The typical synoptic pattern and tropical air mass source regions favours
convective precipitation during the summer. During the autumn season, the Bermuda-
Azores High weakens and moves southward, allowing cold fronts to approach the islands
but hardly ever penetrates into the tropics favouring an easterly flow. Frontal systems are
typically more strongly defined during winter and generally extended into the tropics.
Cold fronts affecting the archipelago can be followed by both cold continental or maritime
polar air masses. Under winter synoptic pattern, overcast and squally conditions can be
frequent. The typical winter synoptic situation is characterized by low pressures moving
eastward from North America and passing south of Nova Scotia favouring a south
westerly surface wind (see Fig.3).
Fig. 3. Sea level pressure (mb) Dec-Jan-Feb composite mean for the winters in the recent climate (1981-2010) representing typical winter synoptic situation over Bermuda (NCEP (National Center for Environmental Prediction) /NCAR (National Center for Atmospheric Research) reanalysis).
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By the end of March, the Bermuda High begins to migrate northward, blocking
cold fronts as they move off the East Coast. This situation favours the development of
intense systems of low pressures from Cuba or the Bahamas moving north-northeast.
Strong winds and short lived rainfall, responsible for sporadic heavy rainfall events, can
be observed over the islands during the spring. Warm air, brought in a moderate south
westerly circulation. In May there is an increasing tendency for Bermuda to come under
the influence of a light easterly circulation in the lower levels as the stronger belt of a
westerly´s move northward. Such situation favouring strong winds and heavy rainfall can
be observed over the archipelago. Severe convective storms and tornadoes are sometimes
associated with these conditions. Warm air, brought in a moderate south westerly
circulation. In May there is an increasing tendency for Bermuda to come under the
influence of a light easterly circulation in the lower levels as the stronger belt of a
westerly´s move northward. Such situation favouring strong winds and heavy rainfall can
be observed over the archipelago. Severe convective storms and tornadoes are sometimes
associated with these conditions.
It is possible to distinguish different synoptic situations responding to different
frontal behaviour:
A. Fronts
A1. Winter fronts
A1.1. Cold fronts followed by a deep cold continental polar air mass.
Thunderstorm activity with gale force winds is commonly observed with them.
A1.2. Fast moving cold fronts followed by a maritime polar air mass.
Asthe front approaches, surface southerly winds rapidly increase. Heavy rains with
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A2. Fall fronts
A2.1. Cold fronts passing the area
A2.2 Cold fronts that dissipate when they approach the islands.
They areassociated with high pressure systems moving into the south-eastern U.S. weak as they
move of the eastern coast of U.S. Under this situation the general flow becomes more
zonal (west-east).
A. 2.3. Stationary fronts located south of Bermuda.
If the cold front is oriented northeast-southwest it usually moves slowly southward. This movement isblocked by the Bermuda-Azores High and the front becomes stationary near 30 degrees
north. A surface easterly flow is generated since the High moves slowly eastward to pass
north of Bermuda.
A2.4. Cold fronts that turn into warm ones.
They are favoured by the formationof a stable wave and/or low in the Gulf of Mexico or off the East Coast of Florida moving
to the northeast.
B. Cyclones
The islands are mainly affected by convective precipitation and frontal systems
(Government of Bermuda 2005 http://www.conservation.bm/publications/projects-
reports/state of the environment 2005.pdf). However, four types of cyclones affect
Bermuda’s weather: the Texas (lows formed west of New Orleans) West Gulf (formed in
the western Gulf of Mexico or east Texas), East Gulf (lows that favour quasi-stationary
front in the eastern Gulf of Mexico) and South Atlantic or Hatteras Low type (storms that
form in the south western region of the North Atlantic with the exception of Tropical
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type is a typical winter related to snowstorms in the south eastern states. When these
storms travel considerably further south of their normal track, they will occasionally pass
south of Bermuda (Vandever & Pearson 1994).
Bermuda is in the path of some of the Atlantic basin hurricanes. Such location,
near the northern limit of the normal Atlantic tropical cyclone re-curvature band, favours
tropical systems approaching from the south (Elsner & Kara 1999) and strong winds or
moderate showers affect the area. These hybrid storms, such as the subtropical cyclones,
are formed in close proximity south of the islands and track northward. The presence of
warm water near Bermuda due to the influence of the Gulf Stream, located more than
1046 km off the coast of the Carolinas, supports the maintenance of hurricanes as they
pass through the region, and the formation of subtropical cyclones.
The tropical season extends from June through November. Nevertheless,
September and October followed by August are the months of higher occurrence of
tropical disturbances in this area (Guishard et al. 2007). Large daily rainfall
accumulations are occasionally observed over the islands resulting from stalled tropical
systems, for example: tropical storm Bertha (WMO 2009; http://www.wmo.int/
pages/prog/www/tcp/Meetings/HC31/documents/Doc.4.2.9_Bermuda.doc). Bermuda
occasionally suffers a direct hurricane hit during the tropical cyclone. The
archipelago experiences longer duration gales from winter storm systems than from
tropical cyclones (Tucker 1972, 1982). Additionally, easterly waves associated with
severe weather are anomalous phenomena over this area.
2.1.3. Oceanographic conditions
The North Equatorial Current located at the southern latitudes of the North
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other one Cuba, which is known as the Antilles Current. The Antilles Current flows
northwest and joins the Florida current, where both of them flow northward as the Gulf
Stream (Vandever & Pearson 1994). Owing to its location southwest of the Gulf Stream
(see Fig.4) lies Bermuda in the warm waters of the Sargasso Sea which are especially
effective at modifying SST since its temporal variations are associated with cyclone
development and lateral water movements (Li et al. 2002). The area does not experience
strong temperature gradients. SST (Surface Sea Temperature) has a seasonal variation of
8-10 ºC (Michaels et al. 1994). The annual average SST varies from 15- 19 degrees
Celsius from January through April compared to 27-29 degrees during August and
September, when it is more likely to observe tropical systems affecting Bermuda. These
warm waters extend west and north to the Gulf Stream and prevent cold air masses from
the American continent (Macky 1948).
Fig.4. Location of the warm and cold currents in the North Atlantic Ocean (https://www.britannica.com/place/Gulf-Stream/images-videos)
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