206
7 Ocean Currents
New Zealand, and the Southern Pacific islands, where the westerlies gather
great strength and persistence, and are known as the 'roaring forties'.
In the arctic and polar zones, the polar easterlies system is established. However, in the high-latitude zones in the Northern Hemisphere, winds blow in
various directions, controlled by local weather disturbances. In the Southern
Hemisphere, the landmass of the Antarctica is surrounded by a vast ocean
where the polar easterlies are a dominant feature of the circulation (Fig. 7.3).
The presence of continents disturbs the wind pattern shown in Fig. 7.3. The
most obvious way in which the presence of continents affects the atmospheric
circulation is through the thermal contrasts which exist between land and sea,
and the land and sea breezes, discussed above, are their simplest examples.
During warm summers, when the air over the land is less dense than that over
the ocean, low pressure areas are centred over continents while high pressure
centres are located over the oceans. In winter the situation is reversed. The
winds associated with seasonally alternating pressure systems are known as
monsoon winds. The most notable regions in which they occur are the Indian
subcontinent and the adjacent Indian Ocean, the West African coast between
latitudes 5° and 15°N, the China Sea and northern Australia (Harvey, 1985).
7.2.4 Storms and Cyclones
A large variety of storms are encountered over the oceans, and they can be
grouped into three main categories: 1. tropical cyclones, 2. mid-latitude cyclones, and 3. hybrid systems. The latent heat released by condensation of
water vapour in clouds and the release of the baroclinic energy stored in horizontal temperature gradients are the main sources of the energy which drives
these systems.
Tropical cyclones are the most powerful and destructive type of storm. They
develop over oceans in the latitudes 8° to 15°N and S. They cannot develop
closer to the equator as the Coriolis effect, required for their formation, is
negligibly small. There are some regions, over tropical and subtropical oceans,
where tropical cyclones occur (Strahler and Strahler, 1992), namely: West
Indies, Gulf of Mexico and Caribbean Sea, western North Pacific, including
the Philippine Islands, China Sea and Japanese Islands, Arabian Sea and Bay
of Bengali, coastal region off Mexico and Central America, south Indian Ocean,
off Madagascar, and western South Pacific, in the region of Samoa and the Fiji
Islands, and the east and north coast of Australia.
High sea-surface temperatures, over 26°C, are required to develop tropical
cyclones which, once formed, move westward through the Trade Wind belt.
However, the movement of tropical cyclones can be complex and difficult to
predict, and there are many exception to the above rule. In the western South
Pacific, cyclone paths are extremely complicated, with sharp speed and direction changes. As a example, in Fig. 7.4 is shown the path of tropical cyclone
'Justin', which for almost 3 weeks during March 1997 disrupted life along the
eastern coast of Australia, in the Coral Sea and over south-east Papua New
7 Ocean Currents
New Zealand, and the Southern Pacific islands, where the westerlies gather
great strength and persistence, and are known as the 'roaring forties'.
In the arctic and polar zones, the polar easterlies system is established. However, in the high-latitude zones in the Northern Hemisphere, winds blow in
various directions, controlled by local weather disturbances. In the Southern
Hemisphere, the landmass of the Antarctica is surrounded by a vast ocean
where the polar easterlies are a dominant feature of the circulation (Fig. 7.3).
The presence of continents disturbs the wind pattern shown in Fig. 7.3. The
most obvious way in which the presence of continents affects the atmospheric
circulation is through the thermal contrasts which exist between land and sea,
and the land and sea breezes, discussed above, are their simplest examples.
During warm summers, when the air over the land is less dense than that over
the ocean, low pressure areas are centred over continents while high pressure
centres are located over the oceans. In winter the situation is reversed. The
winds associated with seasonally alternating pressure systems are known as
monsoon winds. The most notable regions in which they occur are the Indian
subcontinent and the adjacent Indian Ocean, the West African coast between
latitudes 5° and 15°N, the China Sea and northern Australia (Harvey, 1985).
7.2.4 Storms and Cyclones
A large variety of storms are encountered over the oceans, and they can be
grouped into three main categories: 1. tropical cyclones, 2. mid-latitude cyclones, and 3. hybrid systems. The latent heat released by condensation of
water vapour in clouds and the release of the baroclinic energy stored in horizontal temperature gradients are the main sources of the energy which drives
these systems.
Tropical cyclones are the most powerful and destructive type of storm. They
develop over oceans in the latitudes 8° to 15°N and S. They cannot develop
closer to the equator as the Coriolis effect, required for their formation, is
negligibly small. There are some regions, over tropical and subtropical oceans,
where tropical cyclones occur (Strahler and Strahler, 1992), namely: West
Indies, Gulf of Mexico and Caribbean Sea, western North Pacific, including
the Philippine Islands, China Sea and Japanese Islands, Arabian Sea and Bay
of Bengali, coastal region off Mexico and Central America, south Indian Ocean,
off Madagascar, and western South Pacific, in the region of Samoa and the Fiji
Islands, and the east and north coast of Australia.
High sea-surface temperatures, over 26°C, are required to develop tropical
cyclones which, once formed, move westward through the Trade Wind belt.
However, the movement of tropical cyclones can be complex and difficult to
predict, and there are many exception to the above rule. In the western South
Pacific, cyclone paths are extremely complicated, with sharp speed and direction changes. As a example, in Fig. 7.4 is shown the path of tropical cyclone
'Justin', which for almost 3 weeks during March 1997 disrupted life along the
eastern coast of Australia, in the Coral Sea and over south-east Papua New