CH06 The Sea

Question 1

In general, where do the strongest currents occur?

Answer 1

Upper layer of Oceans, although they occur at all depths.

Question 2

What are the drivers of currents and tides?

Answer 2

Primarily the direct action of the wind on sea surface - mainly the trade and monsoon winds - causes surface currents.
Coriolis effect has a greater impact with depth.
Slow moving TRS.
BA charts.

Question 3

What is a gyre?

Answer 3

The large continuous circulations of water existing in all major oceans.
Centred around 30° N/S with peripheries in 10° and 50° N/S.

Question 4

What publications contain details of currents?

Answer 4

Ocean passages of the world.
Appropriate Admiralty Sailing Directions.
Admiralty Routing Charts.

Question 5

List the warm currents.

Answer 5

Gulf stream 
Mozambique current
Japan current
Agulhas current
Brazil current
East Australian Coast current

Question 6

List the cold currents.

Answer 6

Labrador current
Kamchatka current
East Greenland current
Falkland current
Peru current
California current 
Benguela current

Question 7

Why might we find the tide is not what was predicted?

Answer 7

Meteorological conditions such as:
Wind piles up seas in the direction it blows.
Barometric pressure greater/lesser than expected.
Extended period of high pressure may delay time of HW.

Question 8

What is a positive surge?

Answer 8

Where the sea is piled up by the wind to increase sea level.
Greatest effect in shallower waters such as gulfs or bights.

Question 9

What is a negative surge?

Answer 9

Where sea level is reduced to as much as 1m by forces which are poorly understood.
Of particular importance to ships with small UKC.
Commonly found in estuaries and shallow waters.
Warnings are propagated via VHF and other warning systems.
ALRS3.

Question 10

What is a storm surge?

Answer 10

Positive or negative surge associated with depressions.
Positive occurs in the vicinity of the depression and is intensified in shallow water.
Negative occurs away from the depression and is essentially the waters which ‘feed’ the positive.

Question 11

What is a seiche?

Answer 11

A temporary disturbance in water level (wave originating from the disturbance) caused by atmospheric pressure.

Question 12

What is a squall?

Answer 12

A sudden violent gust of wind or localised storm, especially one bringing rain, snow or sleet.

Question 13

What determines wave height?

Answer 13

How long the wind has been blowing.
The wind strength.
The currents.

Question 14

What is swell?

Answer 14

Swell is the wave motion caused by a meteorological disturbance.

Question 15

Describe Beaufort force 0?

Answer 15

Calm

Sea like a mirror

Question 16

Describe Beaufort force 1?

Answer 16

Light air
Ripples with the appearance of scales are formed but without foam crests
1-3kts
0.1m
Sea state 1

Question 17

Describe Beaufort force 2?

Answer 17

Light breeze
Small wavelets, still short but more pronounced; crest have a glassy appearance and do not break
4-6kts
0.2m
Sea state 2

Question 18

Describe Beaufort force 3?

Answer 18

Gentle breeze
Large wavelets; crests begin to break; foam of glassy appearance' perhaps scattered white horses
7-10kts
0.6m
Sea state 3

Question 19

Describe Beaufort force 4?

Answer 19

Moderate breeze
Small waves, becoming longer' fairly frequent white horses
11-16kts
1.0m
Sea state 3-4

Question 20

Describe Beaufort force 5?

Answer 20

Fresh breeze
Moderate waves. taking a more pronounced long form' many white horses are formed (chance of some spray)
17-21kts
2.0m
Sea state 4

Question 21

Describe Beaufort force 6?

Answer 21

Strong breeze
Large waves begin to form; the white foam crests are extensive everywhere (probably some spray)
22-27kts
3.0m
Sea state 5

Question 22

Describe Beaufort force 7?

Answer 22

Near gale
Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind
28-33kts
4.0m
Sea state 5-6

Question 23

Describe Beaufort force 8?

Answer 23

Gale
Moderately high waves of greater length' edges of crests begin to break into spindrift' foam is blown in well-marked streaks along the direction of the wind
34-40kts
5.5m
Sea state 6-7

Question 24

Describe Beaufort force 9?

Answer 24

Severe or strong gale
High waves; dense streaks of foam along the direction of the wind; crests of waves begin to topple, tumble and roll over; spray may affect visibility
41-47kts
7.0m
Sea state 7

Question 25

Describe Beaufort force 10?

Answer 25

``` Storm Very high waves with long overhanging crests; the resultant foam, in great patches, is blown in dense white streaks along the direction of the wind; on the whole, the surface of the sea takes a white appearance' the tumbling of the sea becomes heavy and shock-like; visibility affected 48-55kts 9.0m Sea state 8 ```

Question 26

Describe Beaufort force 11?

Answer 26

``` Violent storm Exceptionally high waves (small and medium sized ships might be for a time lost to view behind the waves); the sea is completely covered with long white patches of foam lying along the direction of the wind; everywhere the edges of the wave crests are blown into froth; visibility affected 56-63kts 11.5m Sea state 8 ```

Question 27

Describe Beaufort force 12?

Answer 27

``` Hurricane The air is filled with foam and spray; sea completely white with driving spray; visibility severely affected +64kts 14m Sea state 9 ```

Question 28

What is a Tsunamis?

Answer 28

Waves caused by submarine volcanic eruptions. Length of waves over 100m. Height less than 1m. Speed 300-500kts When these long waves reach shallow waters they can build up to heights of 20m.

Question 29

Where would you find info on regions prone to abnormal waves?

Answer 29

Admiralty Sailing Directions and Ocean Passages of the World.

Question 30

What are sandwaves?

Answer 30

Analogous with sand dunes. Moved/built up by action of tides. Mariners should be mindful of potential changes to charted depths where this phenomena occurs. Dover and Malacca Straits both have them.