51 OCEAN CURRENTS
195. THE CHARTING OF CURRENT INFORMATION
Practical knowledge of ocean currents is necessary to the ocean navigator, not only for the safety of his vessel, but also for its economical operation. The safety aspect is fairly obvious when one considers the number of shipping casualties caused by encountering an unexpected current, while the economical aspect is witnessed in the considerable time-saving which can be effected on an ocean passage by the judicious use of ocean currents.
An ocean current is a general movement of the water of an ocean owing to a combination of meteorological and oceanographical factors, which may be permanent or semi-permanent. The term, current must not be confused with tidal streams, which are caused by the gravitational effects of the Sun and the Moon, and which are subject to hourly changes. Ocean current circulation, in its widest sense, takes place in three dimensions, but the stronger currents occur in an upper layer which is shallow compared with the ocean depth may flow horizontally, obliquely or vertically, although a surface one, in which a navigator is almost solely interested, only flows horizontally.
Parts of a general surface water circulation were known to the early navigators. Vasco da Gama, who called at Mozambique in 1498 after rounding the Cape of Good Hope, must have experienced the adverse force of the AguL.H.A.s and Mozambique Currents, and so might be regarded as their discoverer. Columbus, in his voyages to America, encountered the N. and S. Equatorial Currents of the Atlantic and stated that he regarded the water movement from E. to W. as proved. Dampier’s Voyages published in 1729 includes an account of the currents experienced in his voyages in the latter part of the seventeenth century. He knew of the Guinea Current and gives relative strengths of the Equatorial Current in different parts of the Caribbean. Benjamin Franklin published the first chart of the Gulf Stream in 1770. By the early nineteenth century seamen had a general working knowledge of the chief trends of current in areas frequented by ships. The first extensive current charts based on ships’ observations were those of Rennell, published in 1832, for the Atlantic and part of the Indian Ocean. In 1845 and succeeding years, Maury published his well known wind and current charts. The early charts and the Admiralty current charts published near the end of the century were based on plotting individual current observations from the log books of naval and merchant ships, the average current being shown by arrows drawn by eye estimation. It was not until 1910 that exact statistical combination of current observations into current roses and vector mean currents was begun.
The current charts so far published by the Meteorological Office are constructed for three-monthly periods, except that monthly charts of the China Sea are now available. Many more current observations are needed before monthly charts for all the oceans can be constructed. Because of seasonal variations to which currents are liable, monthly current charts will give a better picture of the currents at any particular time than the quarterly ones. The following Current Atlases are available from the Meteorological Office: –
Met.0.466. – Quarterly Surface Current Charts – Atlantic Ocean
Met.0.485 – Quarterly Surface Current Charts – Western North Pacific Ocean, with Monthly Chartlets of the China Seas.
Met.0.655 – Quarterly Surface Current Charts – Eastern North Pacific Ocean.
Met.0.435 – South Pacific Ocean Currents.
Met.0.722 – Indian Ocean Currents.
Details of currents in specific areas are contained in the appropriate volumes of the Admiralty Sailing Directions, in Ocean Passages for the World and on the Admiralty Routeing Charts. (The last two publications are described in § 43-47 of this Astro-navigation study). In recent Current Atlases, three types of current chart are given for each quarter (or month) showing:
resultant or mean vector current
The Vector Mean Current Chart (fig. 51.1) shows the overall movement of water over a considerable period of time. For each computing area (approx. 2° of Lat. by 4° of Longitude), the vector mean is computed of all current observations the mid-position of which lie within the area. The vector mean is therefore the resultant rate and direction of current within that area, calculated from the rate and direction of each individual current.
Opposite components cancel each other and the vector mean rate is therefore less than the mean rate of all currents irrespective of direction. In areas of very variable current, the vector mean rate is much less than the average rate and may be extremely small.
The more constant the current, the closer the vector mean is to the average rate, but the two are never equal because the current has nowhere a constancy of 100%. A Mean vector Current Chart therefore defines the geographical limits of various currents and shows the difference in mean direction and rate between individual current trends. It would, for example, be the chart to consult for calculating the average drifts of boats, derelicts icebergs, etc.
The Current Rose Chart (fig. 51.2) gives all possible information about variability of current. It not only shows the total percentage of current setting in any direction, but also the percentage of current setting in any direction, but also the percentage of currents of various strengths in each direction.
Current roses fall into three main types. Fig. 51-3 (a) shows a typical rose of a region of strong and fairly constant current. Fig. 51-3 (b) shows a typical rose of a monsoon or trade-wind area, where predominance of direction is not so marked as in fig. 51-3 (a), while fig. 51-3 (c) shows a rose of a variable current area, the frequency being nearly equal in all directions. Currents of less than ¼ knot are not included in the rose; the total percentage of these, irrespective of direction, is given in the rose circle, beneath the figure showing the total number of observations.
A Chart of Predominant Current (fig. 51-4) shows the direction of predominant current whenever there is a predominating direction, i.e., where the constancy of current in one direction is 25% or more of all observations. It also gives the average rate of the currents in miles per day actually experienced in the predominant direction. The information given on this chart should be used by the navigator for day-to-day navigation in preference to that given on the vector mean chart, which, as explained above, refers to the resultant flow of water over a 1ong period of time. When using the Predominant Current Chart, the Current Rose Chart should also be consulted to find what degree of variability from the predominant direction and rate may be expected in the region concerned.
The flow-lines on the Predominant Current Chart are of three thicknesses, representing constancies of 25-49%, 50-74% and 75% and over, respectively. Where the constancy exceeds 50% the direction shown on the chart is the most likely one to be experienced at any particular time, this being, of course, more likely when the constancy reaches 75% or more. As the constancy nowhere reaches 100%, no current can be predicted with absolute certainty; hence the advice given above that the Predominant and Rose Charts should be studied together.
196. THE CAUSES AND CHARACTERISTICS OF OCEAN CURRENTS
The primary cause of surface currents in the open ocean is the direct action of the wind on the sea surface; thus there is a close relation between the direction of currents and prevailing winds. A current formed in this way is known as a drift current. A constant wind blowing over great stretches of an ocean will have the greatest effect, and thus the NE and SE Trade winds are the mainspring of the general surface current circulation of the world, which takes the form of several closed systems or eddies in accordance with the winds around the permanent anticyclones centred about 30° N. and S. (see § 38).
The direction of circulation is clockwise in the northern and anticlockwise in the southern hemisphere (fig. 51-6).
When the water in an ocean starts to move, a deflecting force due to the Earth’s rotation comes into play, to the right in the northern and the left in the southern hemisphere. The surface deflection is between 30° and 45° in deep water. For example, in the northern hemisphere in the NE Trades, the surface current will set 45° to the right of SW, i.e., W.
Below the surface, the angle between current and wind increases with depth while the rate of current decreases, owing to internal friction of water until near the bottom of the layer affected by the wind, the water movement is directly opposite to the wind direction.
The total effect of the wind is to give a resultant direction of movement of the whole layer of water at 90° to that of the wind, towards the right in the northern and left in the southern hemisphere. Whenever a layer tends to move from any cause, the resultant direction of movement of the whole layer will be at right angles to that of the original tendency.
The coastal currents, forming the W. and E. sides of the main circulation are due to other causes. When a wind blows parallel to a coastline, or obliquely over it, a slope of the sea surface near the coast occurs and a gradient current results. A gradient current is produced as follows: water tends to run down the slope, but immediately it starts to do so the effect of the Earth’s rotation diverts it to a direction at right angles to the slope. A wind parallel to the coast is the most effective in creating a slope, since the total water movement, being 90° from the wind as explained above, is then directly on to, or away from, the coast. The coastal currents on the E. sides of the oceans are produced in this way, by the removal of water from the coastal regions by the trade winds. The gradient current may flow at the same time as a drift current is being produced by the wind; the surface current experienced being the resultant of the two, the formation of the Benguela Current shown in fig. 51-3 is an example of this.
Upwelling occurs in the coastal currents on the E. sides of the oceans since cold water rises from below to replace that drawn away from the coast by the wind. The balance between the replacement of water by upwelling and its removal by the wind is such that the slope of the surface and therefore the strength of the gradient current remains the same, so long as the wind direction and strength do not change. The slope is less than 1″ in a distance of 10 miles. Upwellings on the E. sides of the oceans are responsible for the Benguela Current, the Canary Current, the Peru Current and the California Current.
A slope of the sea surface, and hence a gradient current, can also occur if masses of water of different densities lie adjacent to one another. Such density differences arise from differences of temperature, or salinity, or more usually, of both; the level of the warmer or less saline water will be a little higher than that of colder or saltier water when two such masses are adjacent on the surface. An example of a thermal gradient current is found in the Arabian Sea and Bay of Benguela during February to April when the NE Monsoon is still blowing but the current on the western coasts sets against the wind. This is due to the cooling of the water at the head of the Arabian Sea and Bay of Bengal between Nov and Jan, where the temperature difference sets up a slope downhill towards the cooler water to the northward so producing the current contrary to the NE Monsoon wind.
Currents, whether drift of gradient, may be classified as warm or cold currents as follows:- they may be warm or cold currents corresponding to the Lat. in which they flow, such as the warm Equatorial Currents of all oceans and the cold Southern Ocean current encircling the globe; they may be warm or cold currents the temperature of which does not correspond to the Lat. in which they flow, such as the warm Gulf Stream or warm Kuro Shio, which transport the warm water of the Equatorial currents to higher latitudes, and the Labrador Current or Oya Shio Current which transport the cold water from the Arctic basin to lower latitudes. Cold currents from high latitudes have a special significance for navigators because they transport ice to lower Latitudes and are responsible for the high frequency of fog and poor visibility in some areas,
197. THE CURRENT CIRCULATION OF THE OCEANS
Fig. 51-6 shows the general surface-current circulation of the world and represents the trend of water movement in the long run, emerging from the more or less variable movement of individual currents. Apart from major changes in direction of some currents in the two Monsoon seasons, there are other minor seasonal changes in the position of currents which cannot be shown on a single chart.
In the North Atlantic, THE N. EQUATORIAL CURRENT and the N. SUB-TROPICAL CURRENT sweep westwards between Lat. 10° N. & 30° N. The part of the N. Equatorial Current which runs to the N.W. consists of relatively warm water which, owing to the shape of the land, is forced through the West Indies into the Caribbean at a rate sometimes as high as 3 kts. The water piles up in the Gulf of Mexico; there it is further heated before it escapes through the only exit open, to it – the narrow Florida Strait. It reaches the open sea as a belt of excessively warm water called the GULF STREAM, moving at 4-5 kts. The coral banks of the Bahamas deflect it northward along the coast of S. Carolina where the stream becomes broader, and further N. it meets the LABRADOR CURRENT and is deflected to the right. After passing the Longitude of the easternmost part of North America the Gulf Stream, as such, ceases to exist, but the prevailing westerly winds continue the set as the N. ATLANTIC CURRENT. On approaching Europe, this current divides, one branch continuing N. eastwards to the Arctic, and the other branch, the PORTUGAL CURRENT runs S. and E. towards the African coast. The southern end of the Portugal Current comes under the influence of the NE Trade Wind and extends as the CANARY CURRENT through the Canary Isles to Cape Verde, where it turns westerly again into the North Equatorial Current.
In the South Atlantic, the S. EQUATORIAL CURRENT and the S. SUB-TROPICAL CURRENT set westwards between 2°.N. and 20°.S. About 300 miles W. of Recife it divides, one branch running S. to become the BRAZIL CURRENT and the other running NW along the coast of S. America to join the N. Equatorial Current. At about the Lat. of the River Plate the Brazil Current turns eastward and merges with the SOUTHERN OCEAN CURRENT. The FALKLANDS CURRENT runs northwards up the E. coast of South America as far as the River Plate. On the eastern side of the S. Atlantic, an offshoot of the Southern Ocean Current forms the BENGUELA CURRENT which sets to the N. along the W. coast of Africa to the equator, where it joins the S. Equatorial Current. The area of the doldrums, where there is insufficient wind to set up a drift current, affords a suitable area for the return of water displaced by the N. & S. Equatorial Currents and this is called the EQUATORIAL COUNTER CURRENT.
It will be seen from fig, 39-4 that the currents of the Pacific Ocean differ very little from those of the Atlantic, the principal difference being the periodic change of drift current in the China Sea resulting from the opposing Monsoons.
In the Pacific, the EQUATORIAL CURRENT flowing westwards to the Philippines, is deflected to the NE and becomes the warm, dark HURO SHIO CURRENT running up the E. coast of Japan before curving E. and merging into the N. PACIFIC CURRENT. The KAMCHATKA CURRENT corresponds to the Labrador Current of the North Atlantic, and flows southwards from the Bering Sea to the Kuril Islands where it becomes the OYA SHIO as far as the northern islands of Japan. Some of the Oya Shio sinks below the warmer and lighter water of the Kuro Shio and the rest is deflected eastwards across the Pacific as the ALEUTIAN CURRENT. The CALIFORNIAN CURRENT is a cold current that sets southward along the coast of North America and Mexico before turning W. into the North Equatorial Current. The DAVIDSON CURRENT sets northwards alongside the W. coast of the United States inside the California Current during the winter as a result of the prevailing southerly winds along this coast during that season.
In the S. Pacific Ocean the S. EQUATORIAL CURRENT and the S. SUB-TROPICAL CURRENT flow westwards to the numerous islands between 160°. E. & 170°.E. before swinging southwards as the E. AUSTRALIAN COAST CURRENT which sets at a rate of 2–3 kts until it meets the SOUTHERN OCEAN CURRENT, when it is deflected eastwards towards New Zealand. The PERU CURRENT, also known as the HUMBOLDT CURRENT, sets to the N. along the W. coast of South America before finally merging with the S. Equatorial Current.
The currents of the Indian Ocean are greatly dependent on the Monsoons. The EQUATORIAL CURRENT divides on reaching Mauritius, one part flowing N. and one S. of Madagascar. The branch which goes N. again divides when it reaches the African coast, one part continuing northwards as the N. GOING – E. AFRICA COAST CURRENT during the SW Monsoon, when it often exceeds 3 kts. In the NE Monsoon, this N.-going current is not experienced N. of about Lat. 5° S., but a S.-GOING E. AFRICA COAST CURRENT is set up by the Monsoon, which turns left on crossing the equator and merges with the Equatorial Counter Current. The part of the Equatorial Current which passes N. of Madagascar and is then deflected to the S. by the African coast is known as the MOZAMBIQUE CURRENT. The branch of the Equatorial Current which passes S. of Madagascar merges with the Mozambique Current and runs southward along the E. Coast of Africa as the AGULHAS CURRENT, one of the strongest currents in the world, frequently running at a rate of between 3-5 kts. On the other side of the S. Indian Ocean the W. AUSTRALIAN CURRENT runs northward along the W. coast of Australia, eventually turning W. and merging with the Equatorial Current.
In the Mediterranean Sea the rate of evaporation is high and the inflow of water from the rivers entering it is not sufficient to maintain the level of the sea. Water therefore flows in from the Atlantic through the Strait of Gibraltar and is deflected to the right by the effect of the Earth’s rotation, so that the inflow current is forced to run along the whole length of the N. African coast, and a counter-clockwise circulation is maintained. The east’ly current in the western basin of the Mediterranean Sea may attain a speed of between 2-3 kts, but elsewhere the currents are generally weak and variable.