49 OCEANIC WINDS AND WEATHER

184. INTRODUCTION – OCEAN METEOROLOGY AND OCEAN CURRENTS

The speed and the comfort, of an ocean passage, will largely depend on the winds encountered and the currents prevailing, so the choice of the best route is of considerable importance. Clearly, a route on which mainly favourable winds of average strength not above force 6 can be expected is far preferable to one where mainly Headwinds, or winds forward of the beam, or gales, prevail. Ocean currents, too, may have a significant effect on the length of a passage. A modest current of only half a knot in the wrong direction means that a yacht has to sail twelve miles each day before she has made any progress towards her destination. This soon mounts up on, say a 4,000-mile passage.

With today’s advanced weather technology and the availability of accurate forecasts, the modern seaman has a great advantage over his forebears, yet there are still numerous craft caught unexpectedly in heavy weather each year in all parts of the World. Exposure to adverse weather remains a major threat to the safety of small craft navigators and their craft. Avoidance or minimization of such exposure in open waters can best be accomplished when mariners learn the basics of ocean climatology and meteorology and avail themselves of official weather forecasts for shipping.

The first of these, the basics of ocean climatology and meteorology, is the subject of the following Chapters, and this knowledge will enable the mariner to decide such important matters as when to leave port when to return when to seek a port of refuge or make a major diversion from a planned route, the best course to avoid heavy weather, the proper route and time of year for a safe passage, when to prepare for a blow and, to a large extent, how to handle a vessel during a storm. For the purpose of these chapters, it is assumed that a student, in his studies of Coastal Navigation, already understands basic elementary Meteorology including the relationship of wind to atmospheric pressure, the frontal theory of the formation of depressions: pressure systems and air masses.

Regarding weather forecasts, broadcasts of weather bulletins for shipping and storm warnings are transmitted by most countries in plain language by R/T and W/T. The form of these forecasts are established by individual State Meteorology services and usually covers specific sea and ocean areas. Details of broadcasts, times, frequencies, sea areas etc., for the World will be found in the Admiralty List of Radio Signals, Volume 111 (Meteorological Services). Storm warnings may be broadcast in two languages, viz., the language of the issuing country and English, and usually contain the following information:

• • • International Call: Securite, Securite, Securite (R/T) or TTT (W/T).
    • Type of Warning (Gale, Storm or Hurricane).
    • Time of Reference (G.M.T.).
    • Type of disturbance causing the warning (e.g. a deep depression) and the pressure at the centre of this disturbance in millibars).
    • Latitude and Longitude of the disturbance at the time in (c).
    • The direction of Movement and Speed of the disturbance at a time in (c).
    • The extent of the area(s) affected.
    • Force and direction of the wind in various parts of the area(s) affected.
    • Further indications, e.g., probable duration of the disturbance.

185. GENERAL PRESSURE AND WIND DISTRIBUTION OF THE OCEANS

The distribution of pressure and the winds which would result over a uniform rotating Earth are shown in fig. 37-1 together with the terms long used by seamen to describe the various regions. This idealised distribution is known as the Planetary System of pressure and wind.

The Earth, however, does not have a uniform surface; there are continental areas and oceanic areas. Furthermore, the amount of heat received from the Sun in any zone varies with the season. The effect of these factors will now be discussed.

The effect of the seasonal change in the Sun’s declination is to cause the belts of pressure and their associated winds to move towards the north in the northern summer, and towards the south in the southern summer. The amount of this oscillation is much smaller than that of the Sun’s movement, and in general amounts only to some four degrees of latitude. In addition, this movement lags some six to eight weeks behind that of the Sun, so that the zones shown in fig. 37-1 reach their most northerly position in July – August and their most southerly position in January – February.

The effect of large landmasses is to modify greatly the planetary distribution of pressure and wind. The belts of high pressure in about Lat. 30°N and S. are split into separate cells of high pressure (anticyclones) situated over the eastern part of each of the oceans; the belt of low pressure in about Lat. 60°N. is likewise modified into separate areas of low pressure situated in the vicinity of Iceland and the Aleutian Islands. In the southern hemisphere, there is little or no land in the zone covered by this low pressure belt, and consequently it extends un-interruptedly around the Earth.

Superimposed upon the above modifications of the general planetary system, temperature over large land masses becomes high in summer and low in winter, whilst over the oceans the seasonal variation is much less. As a result, pressure becomes relatively high over land masses in winter, and low in summer. This seasonal change in pressure distribution causes corresponding modifications to the oceanic winds in the vicinity, and in certain areas results in the establishment of seasonal winds or monsoons.

The features of the pressure distribution described above appear only as a result of averaging large numbers of observations. In tropical and sub-tropical regions variations from the average are slight except on the comparatively infrequent occasions when tropical storms develop, so that the pressure distribution at any time does not usually vary much from the average. Elsewhere this is not true, particularly in temperate latitudes where depressions and anticyclones develop and travel along a variety of tracks (although these are mainly from a westerly point to an easterly point) with great frequency. In these regions barometric pressure is frequently varying and the pressure distribution at any particular time is often completely different from that shown on a chart of mean pressure for that time of year.

The resulting general distribution of oceanic winds in January/February/March is shown in fig.37.2 and in July/August/September in fig. 37.3. A description of the climate of the various areas of the world’s ocean now follows.

186. THE DOLDRUMS OR INTER-TROPICAL CONVERGENCE ZONE

On the eastern sides of the Atlantic and Pacific Oceans, the NE and SE trade winds die away as they approach each other, and the two air streams are often separated by a region of calm and light winds which is known as the doldrums. Further west the two trade winds blow more nearly parallel and may finally blow side by side from a nearly easterly direction. The doldrum areas also follow the north and south movement of the Sun in declination but with a much smaller range of latitude. The normal positions of the Doldrums in February and August are shown in Fig. 37.2 and 37.3 respectively, and these are not always coincident with the position of the inter-tropical convergency zone with which the Doldrums are associated.

The belt of separation in the region of the Doldrums between the various air currents whose origins are respectively in the northern and southern hemispheres was formally known to meteorologists as the inter-tropical front, but this term is fallen into disuse, largely because it has been found that there is little resemblance between the so-called fronts of the tropics and those of temperate regions.

The term inter-tropical convergence zone is now used since it is generally agreed that the belt resembles a line of convergence.

The width of the doldrum zones averages about 200 miles, but there are very considerable seasonal and day-to-day variations. The characteristic features of the Doldrums are calms and light variable winds alternating with squalls, heavy rains and thunderstorms. It has been found that doldrum weather is generally worse when the strength of the Trade Winds is greater than normal, and best when the Trades are abnormally light.

187. THE TRADE WINDS

Between the doldrums and the oceanic high-pressure areas in each hemisphere blow the Trade Winds, north-easterly in the northern hemisphere and south-easterly in the southern hemisphere. These winds blow permanently and with great steadiness and persistence throughout the year. The Trade Winds, however, do not blow in the North Indian Ocean or in the western part of the North Pacific, where their place is taken by the Monsoons (see below). In the Atlantic, the mean speed of the Trades is about 13 to 15 knots, with the higher value occurring in the NE Trade.

The highest mean value anywhere (18 knots) is found in the SE Trade of the Indian Ocean. In general, the Trades in each hemisphere blow most strongly at the end of the winter.

The Trade Winds of the Atlantic, eastern Pacific and southern Indian Ocean show up clearly on both Figs: 37-2 and 37-3 as air streams with a high mean rate of travel. In each of these oceans, they blow more or less constantly throughout the year from about Lat. 30° towards the equator. In each case, they extend furthest from the equator on the east side of the relevant high-pressure area and they blow round that area towards the doldrums or equator, changing direction from N to NE (S’ to Se) to ENE (ESE) or even to E except in the South Indian Ocean in the northern summer when the SE Trade Winds cross the equator to become the SW Monsoon.

The Trade Wind areas tend to follow the north and south movement of the Sun’s declination; in the southern hemisphere the movement is small, but in the northern hemisphere in the Atlantic and Pacific Oceans the zone of the highest trade wind speeds moves through 8° to 10°.

Weather in the Trade Wind zones is generally fair or fine, with small detached cumulus clouds, but the generally fair weather of the Trades is liable, at certain seasons and in certain localities, to be interrupted by dangerous tropical revolving storms (which are described in detail in the following chapter). On the eastern sides of the oceans, cloud amounts in the Trades are small and rainfall very scanty while on the western side cloud amounts are larger and rain is comparatively frequent, being a maximum in the summer months. Both clouds and the frequency and intensity of rain increase towards the doldrums. Poor visibility occurs frequently in the eastern parts of the Trade Wind belts, due in part to mist or fog forming at times over the cold currents, and in part to sand and dust being carried out to sea by the prevailing offshore winds. On the western sides of the oceans, visibility is good, except when reduced by heavy rain – and fog is very rare. 

188. THE MONSOONS

In the northern parts of the Indian Ocean and the western Pacific Ocean, the Trade Wind system disappears, to be replaced by a system of seasonal winds known as the monsoons. In winter (northern hemisphere) the great landmass of Asia becomes very cold, and an intense area of high pressure becomes established over Siberia. The anti-cyclonic circulation thus set up results in the establishment in the western N. Pacific, Bay of Bengal and Arabian Sea or north-easterly winds known as the NE Monsoon. In the south Indian Ocean between the Equator and the doldrums, the NE Monsoon is deflected to the left on crossing the equator and is felt here as a north-westerly wind. The limits of this NW wind, or “cross-monsoon” as it is sometimes called, are shown in fig.37-2.

In a similar manner, in the western Pacific south of the equator, the NE winds, being deflected to the left, are felt as far as about Lat. 12.5° to 15.5°. as westerly or north-westerly winds, known as the NW Monsoon.

The season of the NE Monsoon is from about October to March, and that of the NW Monsoon is from about November to March. Winds are light in the North Indian Ocean but fresh and sometimes strong in the China Sea and in the North Pacific east of Formosa and the Philippines. Elsewhere they are generally moderate or light. Weather in the Arabian Sea and Bay of Bengal is generally fine, cloud amounts are small, and rain is rare except in the extreme southwest of the Bay of Bengal, where it is cloudy with considerable rain to windward of the coasts of Southern India and Sri Lanka (Ceylon). In the open ocean of the Western Pacific weather is generally fair, with an increase of clouds and showers by night and in the early morning. In the China Sea North of Lat. 20° N overcast skies are the rule and light rain or drizzle is common. Among the islands to the south and southeast of the Philippines, weather is generally cloudy and unsettled, with considerable rainfall.

In the summer (northern hemisphere) the Asiatic High disappears and is replaced by a deep and extensive area of low pressure centred approximately over north-west India and as a result of the vast landmass of southern Asia becoming greatly heated. The SE Trade Winds of the South Indian Ocean and western South Pacific Ocean are drawn across the equator, are deflected to the right by the effect of the Earth’s rotation, and join in the cyclonic wind circulation set up around the Asiatic Low. The resulting south-westerly wind which is felt in the Arabian Sea, Bay of Bengal, China Sea and the western North Pacific is known as the SW Monsoon (see fig 37-3).

The season of the S.W. Monsoon is from about May to September. This is only a light wind in the western Pacific and China Sea and is moderate to fresh elsewhere, except in the western part of the Arabian Sea where it blows fresh or strong and frequently attains gale force at the height of the season.

Weather in the Arabian Sea and the Bay of Bengal is generally cloudy and unsettled during the SW Monsoon, being worse when the Monsoon is blowing strongest. Both cloud and rainfall increase greatly to windward of high ground, and the SW Monsoon bring copious rainfall to the windward coasts of India and SE Asia, sometimes associated with poor visibility, which is not as a rule persistent. In the open ocean of the western Pacific north of the equator, conditions are generally fair, but with an increase of clouds and showers during the night and early morning. Visibility is good in the open ocean except when reduced by rain, but fog is prevalent on the China coast in the Spring and Summer, reaching its maximum frequency in April off Hong Kong, in June of the Yangtze, and in July off Shantung.

In general, the monsoon winds when fully established tend to be somewhat stronger than the Trade Winds, In no oceanic areas are the adjacent land masses of sufficient size to cause so radical a change in the oceanic winds as are experienced in the North Indian Ocean and western Pacific. Nevertheless, fairly well-marked seasonal changes in the prevailing winds occur over the oceans adjacent to other land masses.

The most important areas thus affected are as follows:

WEST AFRICA In the northern summer pressure is low over northern Africa and the doldrums belt is distorted towards the North. The SE Trades on crossing the equator are deflected to the right, and are experienced as south-westerly winds between the equator and Lat. 10° N. and east of about Longitude 30° W. the season of these south-westerly winds is at its height in August (see fig. 37-3), and they bring cloudy and very wet weather to these coasts, in marked contrast to the fine hot weather experienced in the winter months.

SE UNITED STATES OF AMERICA Here again the alternate heating and cooling of the North American continent cause a prevalence of northwesterly winds over the adjacent oceans in the winter and of southerly winds in the summer. 

EASTERN PACIFIC OCEAN The establishment of relatively low pressure over the North American continent in the summer, and the consequent bending of the doldrums belt towards the North, resulting in the SE Trades being deflected to the right on crossing the equator and in the prevalence of southwesterly winds at this season between the equator and about Lat. 10° – 15° N. and east of about Longitude 120° W. (see fig. 37-3). These south-westerly winds replace the normal North East Trade Wind which prevails in this area during the northern winter and bring wet, cloudy weather to the west coast of Central America.

WESTERN SOUTH ATLANTIC OCEAN During the southern summer, an area of low pressure is formed over Brazil. This causes north easterly winds to prevail over the adjacent ocean between about Lat. 12° S. and 30° S. (see fig. 37-2).

189. THE VARIABLES AND THE WINDS OF THE TEMPERATE ZONES

In the regions where the centres of the subtropical anticyclones are most often located, that is over the middle of the oceans between latitude 30° – 35° N. and S., between the Trade Winds and the Westerlies further poleward, there are generally calm and light winds and fine, clear weather; in fact, typical anti-cyclonic weather. These areas are generally termed Variables, but in the North Atlantic, this region is known as the Horse Latitudes from the unfortunate necessity, in sailing-ship days, of throwing overboard the horses being carried to America and the West Indies, because of the shortage of water and fodder, when the ship’s passage was unduly prolonged owing to light winds.

On the poleward side of the Variables are those disturbed regions where the mobile depressions and anticyclones of the temperate zone are found.

As these pressure systems move, generally from a westerly point to an easterly point, they cause considerable variations in wind direction and force at any given place. On the whole, there is a predominance of westerly winds and a relatively high mean rate of travel of the surface air masses in some easterly direction, thus accounting for the winds in these areas being designated the Westerlies. Gales are frequent, especially in winter; the weather is subject to rapid changes, and fine weather is seldom prolonged.

Figs. 37-2 and 37-3 show that in the southern hemisphere, the Westerlies form a continuous belt around the globe, so often reaching gale force that they are known as the roaring forties, after the latitudes where they are generally located.