METEOROLOGY Q’s & A’s ICE & CURRENTS

Explain with the help of suitable diagram, the sequential formation of sea
ice.
Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on
the ocean's surface.

Due to the action of winds, currents and temperature fluctuations, sea ice is very
dynamic, leading to a wide variety of ice types and features. Sea ice may be
contrasted with icebergs, which are chunks of ice shelves or glaciers that calve into
the ocean.

For the formation of sea-ice, only the top layer of water needs to cool to the freezing
point. This cooling of the ocean surface by a cold atmosphere makes the surface of
the water dense. This densification of water causes the colder water to sink below
setting up convection currents. This continues to happen till the water at the top
reaches the freezing point which is about -20C for sea water. The convection
phenomenon is restricted only to the surface layer to a level of about 100-150 metres
below the surface.

As the ocean water begins to freeze, the first sea ice to form on the surface is called
Frazil Ice. It is initially in the form of tiny discs, floating flat on the surface and of
diameter less than 0.3 cm. Each disc grows outwards laterally and at a certain point
the disc shape becomes unstable, and the ice grows to have long fragile arms
stretching out over the surface. The arms are very fragile, and soon break off, leaving
a mixture of discs and arm fragments. With any kind of turbulence in the water, these
fragments break up further into random-shaped small crystals floating on the
surface. These ice crystals are also called ice-spicules.

As the soupy layer of Frazil ice thickens; a thin film of ice forms. This film is strong
enough to withstand ripples with the waves passing beneath. When they form in
large quantities, they appear to have thickened sufficiently and this mushy layer so
formed is known as “Grease ice”. The grease ice has a matt appearance.

In quiet conditions, the frazil crystals soon freeze together to form a continuous thin
sheet of young ice. In its early stages, when it is still transparent — this ice is called
Nilas.

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Once nilas has formed, a quite different growth process occurs, in which water
freezes on to the bottom of the existing ice sheet, a process called congelation
growth. This growth process yields first-year ice.

Frazil ice formation may also be started by snowfall, rather than super-cooling.
Waves and wind then act to compress these ice particles into larger plates, of several
meters in diameter, called pancake ice. These float on the ocean surface, and collide
with one another, forming upturned edges. In time, the pancake ice plates may
themselves be rafted over one another or frozen together into a more solid ice cover,
known as consolidated pancake ice. Such ice has a very rough appearance on top and
bottom.

Write Short Note on “Arctic Icebergs”

 An iceberg or ice mountain is a large piece of freshwater ice that has broken off
a glacier or an ice shelf and is floating freely in open water.
 Arctic region contains a basin about 3000 m deep, which is covered by a thin
shell of ice about 4m thick.
 The Arctic Ocean remains almost completely covered by drift ice throughout
the year, whereas the greater part of drift ice surrounding Antarctica melts
each summer.
 The Arctic icebergs are mainly confined to the sea areas off the E and W coast
of Greenland and off the E seaboard of Canada.
 In the Arctic, icebergs originate mainly in the glaciers of the Greenland ice cap
which contains approximately 90% of the land ice of the Northern Hemisphere.
 In the Arctic, the icebergs are of irregular shapes and varying characteristics
such as height, length, submerged-exposed proportions, etc.
 Ice islands may also be found in Arctic, which is a rare form of tubular iceberg,
which is formed by breaking off from the ice shelves.

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METEOROLOGY Q’s & A’s ICE & CURRENTS

Discuss with the aid of suitable sketches the normal season and probable
tracks of North Atlantic icebergs from origin to decay.
Normal season of North Atlantic icebergs: 15th February to 1st July of every year.

Movement or probable track of icebergs in the North Atlantic Region:

1. The icebergs born on the East Coast of Greenland are carried South-westwards
by the East Greenland current.
2. After rounding off at the southern tip of Greenland, they join the icebergs born
on the Greenland’s West Coast. They are then carried Northward by the West
Greenland current.
3. From Baffin Bay, they are brought southward by the Labrador current. By the
time they reach the Grand Banks, they are one to two years old.
4. The volume of an average sized iceberg in this region is about 20,000 cubic
meters. Its rate of drift is erratic, being between 10-70 NM per day.
5. On meeting the Gulf stream, whose temperature is as high as 160C, the average
sized icebergs melt in one or two weeks. The larger ones may take as long as
two months.

Describe the factors which may give rise to ice accretion and methods of
reducing ice accretion.
The main factors which cause ice accretion on ships are:

1. Spray:

 Spray consists of small water droplets formed by mechanical breaking of water

when the waves hit the ship.
 Spray is then transported into air by the prevailing wind and cooled.
 The rate of cooling depends upon the time in the air, size of the droplets and
the air temperature. At low air temperatures (below 00C) the spray freezes into
ice and settles on the windward side of the ship.

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2. Overflow of Water:

 The overflow of water occurs during rough seas.

 If the scuppers are free of ice, the entire amount of water will drain before it
has sufficient time to freeze on the ship’s deck.
 If for some reason, the water stayed on the deck, a whitish porous slush is
formed on deck which then grows rather rapidly with each over flow of water
on deck.

3. Super Cooled Fog and Rain Drops:

 At times, super cooled fog or rain drops deposit on the ship as snow.
 This kind of icing makes the work area, ladders, passages, etc very slippery
causing unsafe situation onboard the ship. However, it is not of great
significance.

4. Snow Fall:

 Snow falls and deposits on the ship causing ice to accrete on the ship.
 However, as long as the snow is dry it gets blown away by the wind. If the
snow becomes wet by spray, it quickly settles down on the ship and
contributes to very large increase in the weight of the ship.

Methods of reducing ice accretion:

 Icing due to freezing spray can be clearly avoided by keeping away from sea
areas with critical air temperature and wind speed. This can be done with the
help of proper weather routeing techniques. This is however not always
possible.
 Other options available are:
 Seek shelter in the lee of land until the conditions have changed.
 Reduce the speed of the ship or stop the ship entirely.
 Choose a course exactly against the waves and if possible, run with the waves
in order to reduce the amount of icing.
 It might also be possible to avoid extreme waves by navigating near the
leeward shores or in the fairways if possible.

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 Other methods to avoid or reduce ice accretion could be the use of anti-icing
mats, tarpaulins, etc that help in prevention of icing and ice removal very easy.
 Electrical heating of certain vital parts of the ship such as the Radar antenna,
radio masts, etc.

What are the differences between a drift current and a stream current?
Mention a good example of each. Name the prominent currents of the
South Pacific Ocean.
Drift Current: Drift is the direct effect of wind blowing over long stretches of ocean
for long periods. The frictional effect of the wind, on the sea surface, causes the sea
surface to move. It drags the water in the same direction in which the wind is
blowing. This creates wind drift currents. However, Coriolis force deflects the drift
current to the right in the NH and to the left in the SH by about 30 to 45 degrees. The
maximum strength of the drift current is only upto about 2 knots, depending on the
wind speed and its constancy. The drift currents at the surface also transmit the drag
to the successive layers below it which are also influenced by the Coriolis force.

Stream current is due to the tidal stream that is a resultant of the tidal flow and
current in a particular coastal area. The tidal flow is subject to hourly changes and is
caused by gravitational effects of the Moon and Sun. Therefore, the stream currents
are a phenomenon near the coast and change every hour. They are described in the
tidal stream atlases. They are of much higher speed than the drift currents because of
strengthening factors such as gradient, shape of the coast, etc.

Examples of drift currents are:

 North Equatorial currents or South Equatorial currents caused by Trade winds.

 North Atlantic Current, North Pacific current & Southern Ocean current caused
by the Westerlies.

Examples of Stream Currents are:

Gulf Stream - which is the drift current but with increased velocity due to the
strengthening factors such as gradient, shape of the coast, etc. It is one of the fastest
ocean current.

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Prominent Currents of the South Pacific Ocean:

(i) The South Equatorial Current

(ii) East Australian Coast Current
(iii) Southern Ocean Current
(iv) Peru Current (Humboldt current)

Why is surface water current circulation in Arabian Sea varying?

 The surface water current circulation in Arabian Sea is seasonally varying,
which is mainly due to the changes in the wind stress associated with the
Indian monsoon. This is also because the Indian Ocean is blocked by the
continental masses in the north.
 During winter, the flow of the upper ocean is directed westward from near the
Indonesian Archipelago to the Arabian Sea.
 During the summer, the direction reverses, with eastward flow extending from
Somalia into the Bay of Bengal.

A surface current flows in one direction and an undercurrent flows in the

opposite direction through the Gibraltar straits. Explain this fully.
The level of the Mediterranean Sea is lower than that of the Atlantic. This is because
Mediterranean Sea is land-locked which results in a much higher rate of evaporation
and also because the input of water from rains and rivers is very small. A gradient
surface current therefore flows eastward from the Atlantic to the Mediterranean
through the Straits of Gibraltar.

Because of the high rate of evaporation, the water in the Mediterranean Sea is more
saline and hence denser than the water in the Atlantic. Because of this, a sub-surface
current flows westward from Mediterranean to the Atlantic through the Straits of
Gibraltar. (Also, surface current flows from Black Sea to Mediterranean sea and sub-
surface current flows from Mediterranean Sea to Black sea through Dardanells and
Bosporus – because of the same reasons)

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