The grand circulation currents in the Earth’s atmosphere that create the alternating bands of prevailing winds.
Beyond the 30° latitude of the descending arms of the Hadley cells, at around 60° north or south the surface air, although cooler than at the equator, is still warm enough to rise into the atmosphere and drive another convection loop. And just as with the Hadley cells, the surface winds blowing back towards the equator at the bottom of this loop are smoothly deflected to their right by the Coriolis effect, producing the band of winds called polar easterlies.
The third and final pair of grand circulation currents in the Earth’s atmosphere are the two Ferrel cells, operating in the middle latitudes between 30° and 60°. But unlike the other two, the Ferrel system is passive: it’s not directly driven by its own rising warm air, but by the rolling of the Hadley and Polar cells it nestles between. It’s almost like a freewheeling gear being forced round by two powered cogs turning on either side of it. Where the descending arms of the Ferrel and Hadley cells merge, at around 30° north and south, they form two subtropical ridges of high pressure known as the ‘horse latitudes’. These regions are also characterised by light, variable winds or calm conditions; and so, like the doldrums, sailors learned to be wary of them.
Because the Ferrel cell is driven by the Hadley and Polar cells on either side it turns in the opposite direction. And this fact has been hugely important in the Age of Sail. The surface winds of the Ferrel cells blow not towards the equator but the poles, and so the Coriolis effect instead deflects them in the opposite direction. This is the zone of the westerlies. Two different latitude bands of winds blow to the west – the trade winds of the Hadley cell and the polar easterlies – but if you want to sail east you can only do that within the realms of the two Ferrel cells and the westerly surface winds they produce. This is the route back to Europe from Central and North America, first exploited by Columbus once he realised he needed to sail north into this zone to return home.
The zone of westerly winds has proved just as vital in the Southern Hemisphere. As we noted previously, due to a quirk of the current distribution of the continents through plate tectonics, the Northern Hemisphere is packed with land masses and their mountain ranges which disrupt the flow of winds. The Southern Hemisphere, on the other hand, is dominated by open ocean, free of windbreaks. In particular, below about 40° only the bottom tip of South America and the two islands of New Zealand impede the uninterrupted rush of the westerly winds all the way around the world. The southern westerlies consequently tend to be much stronger than their northern counterparts, and sailors came to call this zone the Roaring Forties. And if they dared to push even further south, risking fierce wind and waves, frigid climate and threat from icebergs, navigators could take advantage of the even stronger Furious Fifties or Shrieking Sixties.
This pattern of alternating bands of winds between the equator and the poles also drives the currents in the world’s oceans which too have been enormously important for knitting together our world into huge trade networks. The neighbouring zones of the easterly trade winds and the westerlies blow the surface water in opposite directions. This, coupled with the fact that the continents block the water from simply circling the world, and that water moving north or south across the globe also feels the Coriolis effect, creates great wheeling surface currents known as ocean gyres. There are five major gyres, in the North and South Atlantic, North and South Pacific, and in the Indian Ocean. These ocean gyres turn clockwise in the northern hemisphere, and anticlockwise in the southern, and like the direction of the wind bands they mirror each other over the equator.
The Canary Current, coursing along the North African coast, was well known to Phoenician and later Iberian sailors, as we saw earlier. This is the eastern arm of the gyre circulating in the North Atlantic; the Gulf Stream, bearing warm waters from the Caribbean up to Northern Europe, forms the western arm. The Gulf Stream was discovered in 1513 when Spanish explorers sailing south along the coast of Florida realised they were being pushed backwards despite sailing with a strong wind. (Because water is so much denser than air, even a gentle ocean current can have a much greater effect on a sailing ship than the wind.) The commercial implications were immediately realised: heavily-laden galleons needed only slip into this wide, fast-flowing river within the ocean to be readily carried north and then round with the westerly winds back home.29 The Brazil Current, running along the east coast of South America, is the mirror-image counterpart of the Gulf Stream and carries ships south into the zone of the westerly winds which they then pick up for rounding Africa into the Indian Ocean.fn11
So overall, in each hemisphere the atmosphere enveloping the planet is divided into three great circulation cells, like giant tubes wrapped around the world, each rolling in place and shifting north and south slightly with the seasons. These produce the major wind zones of the planet – easterly trade winds, westerlies and polar easterlies – which in turn drive the circulating ocean currents. Pretty much the entire wind pattern on Earth can therefore be explained by three simple facts: the equator is hotter than the poles, warm air rises, and the world spins.
This sums up the general pattern of banded winds around the globe. But there is one region of the world with a unique wind system that drove a thriving maritime trading network long before Europeans encountered it.
INTO THE MONSOON SEAS
When you hear the word ‘monsoon’ your mind may well fill with imagery of a verdant and muggy Indian landscape being lashed by torrential rain of fat, heavy drops. The word derives from the Arabic mausim, meaning ‘season’,30 and the monsoons are of course crucial for the wet and dry seasons that shape agriculture across South East Asia. But scientifically speaking, the monsoons are the result of the distinctive atmospheric conditions around South Asia and the pronounced rhythmical reversals in the direction of the prevailing winds. Here was a system of winds completely alien to anything Portuguese sailors had encountered before in the Mediterranean or the Atlantic.
Following in the footsteps (or at least the ship’s wake) of Bartolomeu Dias, another Portuguese explorer, Vasco da Gama, set sail from Lisbon in the summer of 1497 to complete the sea route to India. He took the now customary route along the north-west African coast, re-watered in the Cape Verde islands, and then travelled round the bulge of Africa. But rather than hugging the familiar African coastline into the doldrums in the Gulf of Guinea, he turned his ships southwest into the gaping expanse of the Atlantic, enlarging Dias’ volta do mar into a huge looping course that took him thousands of kilometres away from land.31 Far out at sea he encountered the Brazil Current that carried him steadily south until he picked up the prevailing westerly winds, discovered by Dias a decade earlier, which bore him readily back east to the tip of Africa.
Da Gama and his crew had just spent over three months at sea, travelling around 10,000 kilometres through the Atlantic, making it by far the longest voyage through open ocean undertaken by that time. By comparison, Columbus had spent only 38 days sailing west before his nervous crew became mutinous and demanded to turn back – only to fortuitously sight land two days later.
Da Gama now worked his way round the cape, pushing against the current that sweeps around the south-eastern African coastline. On 16 December 1497 they passed the final stone pillar erected by Dias. By the following March, he reached Mozambique, entering the realm of the Arabic sea traders. In the port of Malindi, in modern-day Kenya, he first encountered Indian merchantmen and here da Gama was able to secure the services of a Gujarati pilot with knowledge in navigating the Indian Ocean.32 Heading off in late April they were blessed with a steady wind to the northeast – da Gama was yet to appreciate the nature of the monsoon winds and the fortuitous timing of his journey – and the fleet headed on a diagonal course cutting right across the Indian Ocean, making for Calicut on the Malabar Coast. On 29 April they noticed the North Star on the horizon: they had re-entered the Northern Hemisphere. Vasco da Gama’s ships arrived
in Calicut on 20 May 1498, after having crossed over 4,000 kilometres of open ocean in only twenty-five days. He had finally achieved the decades-long dream of Portugal’s explorers and traced a sea route from Europe to India and the riches of the Spice Islands.
The Portuguese spent some time exploring the Indian coast before embarking on the voyage home in early October. But now it showed that de Gama’s grasp of the rhythmical mechanics of the monsoon wind patterns was woefully inadequate: no navigator with local knowledge would have attempted to cross southwest to the African coast at this time of year. Da Gama’s ships found themselves battling against a headwind and were forced to beat back and forth, making very slow progress. Worse still, they were frequently becalmed, while their drinking water turned foul and scurvy reared its ugly head amongst the crew.fn12
They eventually reached the East African coast at Mogadishu. Their dismally mistimed return passage had taken 132 days. If only they’d waited two months before attempting the journey they could have sailed before the winter monsoon winds to make the crossing in just a few weeks. By the time the Portuguese finally made it home they had been away for almost exactly two years and travelled around 40,000 kilometres.34 Their feat of courage and endurance had come at the cost of the lives of two-thirds of the crew, many having succumbed to scurvy. The rhythms of the monsoon winds must be heeded.
But their ships had returned with holds full of cinnamon, cloves, ginger, nutmeg, pepper, and rubies, whereas Columbus’ first expedition had found little of any worth. So although it was Columbus’ eight-month expedition of 1492 that is most remembered today, in many ways da Gama’s 1497 voyage was far more impressive. He had discovered what Columbus set out, but failed, to find: the sea route to the riches of the East.
THE MONSOON METRONOME
The monsoon winds are driven by exactly the same process you’ll be familiar with from the changing breezes on a trip to the seaside. During the day, the land warms up more quickly, and to a higher maximum temperature, than the surface of the sea alongside it. This causes the air over the land to rise, and the cooler air over the sea is sucked into the low pressure region left behind, driving a convection current with a steady wind that blows from the sea to the land – an onshore breeze. Conversely, the ground cools much more quickly after sunset, and so the warmer, rising sea air pulls in behind it air from the land to drive an offshore breeze. If you sit on the beach over sunset you can often feel the distinct reversal in wind direction. The only difference is that monsoons occur on a much grander scale, and seasonally rather than daily. In the summer the land mass of continents heats up more quickly than the surrounding sea surface, driving a monsoon wind that pulls in moist air from over the ocean. Through the winter, the ocean retains more of its warmth so that the convection cell reverses, the monsoon winds flip direction, and dry air from higher in the atmosphere descends onto the continent.
Seasonal monsoon winds are created by the temperature differences between several continental land masses and their surrounding ocean. West Africa as well as North and South America also experience weak monsoons but the monsoon winds over India and South East Asia are by far the strongest on Earth, and this comes down to geography. The Tibetan Plateau is the world’s largest and highest, measuring roughly 2,500 by 1,000 kilometres, and rising on average more than 5 kilometres above sea level. When the ground of the Tibetan plateau warms in the summer sunshine it also heats up the air in the upper atmosphere. This gives a big boost to the rising air currents at the beginning and end of the summer monsoon season. Even more important in driving the strong monsoon winds is the Himalayan mountain range along the southern edge of the plateau. This acts like a high wall, a barrier that blocks cold, dry air from the north being sucked down over India and mixing with the warm, moist air from the ocean, which would subdue the atmospheric circulation. The Himalayas essentially insulate India and provide the conditions for a powerful monsoon effect.35 So the strong monsoon winds of south Asia are another consequence of plate tectonics – the result of India crashing into Eurasia about 25 million years ago.
India sits in its enveloping ocean like the central spike in a huge ‘M’, and as it warms up with the beginning of summer, the rising air currents suck in moist air from the ocean that encircles it, which then itself ascends, cools and condenses into clouds that release huge amounts of monsoon rain. The Intertropical Convergence Zone, as we saw earlier, snakes around the waistband of the Earth where the trade winds blowing from the north and south meet each other. During the summer, the heating of India and the effects of the Tibetan Plateau and the Himalayas are so pronounced that the ITCZ is pulled over 3,000 kilometres north of the equator, and then swings far to the south again in winter. Thus the ITCZ band sweeps through the region, the trade winds from the Southern Hemisphere push right over the equator in summer, and in winter the northerly winds extend into the Indian Ocean and islands of the East Indies.
The seasonal pattern of reversing winds in the Monsoon Seas.
In effect, the geography of India disrupts the ‘normal’ wind patterns that we encounter around the rest of the globe. From one season to the next, the winds over the whole of South East Asia periodically flip direction, like the great breathing in and out of huge planetary lungs. Through the eleventh to fifteenth centuries, long before the arrival of Portuguese mariners, ships making use of these winds to sail across the Indian Ocean and among the myriad islands of the East Indies created a dynamic and diverse trading network, with bustling ports along the routes.36
The reversal of the monsoon winds is as regular and predictable as a metronome, and simply by timing your voyages right you can sail to where you need to go on favourable winds, load up on goods and re-provision your ship, and then simply wait for the winds to shift direction and carry you all the way home again. Navigating in the Indian Ocean or around the East Indies is therefore different from sailing the Atlantic or the Pacific. There the trick is to move north or south between neighbouring atmospheric circulation cells with either the tropical easterly trade winds or the mid-latitude westerlies – you pick the wind direction you need by a change in space. But the trick for navigating the monsoon seas is to wait for the seasonal reversal and sail back pretty much the way you came – you pick the right wind direction by a change in time. And this is something that Vasco da Gama completely failed to appreciate when he entered the Indian Ocean in 1498.
EMPIRE OF WATER
From the year after da Gama’s return, the Portuguese began sending annual expeditions to India along his new route.fn13 These mariners also learned their lesson from da Gama’s punishing return voyage and quickly acquired knowledge of the rhythms of the monsoon winds that dictate the sailing schedules through the Indian Ocean and islands of South East Asia. Now in possession of this key navigational understanding, and with their large, cannon-wielding ships and experience of building strong fortifications born of centuries of incessant warfare in Europe, the Portuguese rapidly asserted their dominance in the region and continued further east in their quest for the source of the spices. In 1510 they conquered Goa, turning it into their main base of operations around the Indian Ocean,fn14 and the following year they took Malacca so as to control the maritime traffic through these straits. Once they had ascertained where the Spice Islands were located, they sent an expedition to occupy the Moluccas in 1512. The Portuguese also gained permission to establish trading centres in Macau, on the south Chinese coast, in 1557, and in Nagasaki, Japan, in 1570.
By 1520, the income from Portugal’s spice trade across the Indian Ocean provided nearly 40 per cent of the Crown’s total revenue. Portugal had created a new kind of empire, made powerful and wealthy not through possession of large areas of territory but by the strategic control of sprawling oceanic trade networks on the other side of the world – an empire of water.39
Where the Spanish and Portuguese had led the way, the Dutch, British and French followed. The rivalry between these marine trading powers triggered colonial wars a
round the world as they attempted to eject each other from strategic ports and forts, and control chokepoints to dominate the critical sea passages. Through exploration and maritime trade Europe’s centre of gravity shifted decisively from east to west. Europe was no longer the world’s western extremity, the distant terminus of the network of the Silk Road threading its way across Asia. And the Mediterranean – the inland sea that had witnessed millennia of city states, kingdoms, and empires competing with each other for dominance – became almost parochial, fading from its previous centrality into relative insignificance.
The New World, and the new maritime routes to India and the Orient, offered Europeans access to a seemingly inexhaustible trove of territory and resources, wealth and power. As European navigators decoded the secrets of the planet’s wind patterns and ocean currents they reached across the great expanses of the world’s oceans, linked formerly unconnected regions of the planet, and began the process of globalisation.40 The Age of Exploration was therefore not just a process of filling in the world map with strange new lands, but also of discovering invisible geographies. European sailors learned how to use the alternating bands of planetary winds and wheeling ocean currents like a great interlinked system of conveyor belts, to carry them where they wanted to go.
The early exploration ships were slender-hulled and rigged for the greatest manoeuvrability around unknown coastlines, and in particular for beating into the wind. But these small caravels with triangular ‘lateen’ sails required large numbers of expert crew and had little stowage space for cargo alongside the necessary provisions. The ideal design for transoceanic trade is a broad ship rigged with large square sails, which is much simpler to handle and so minimises the crew size whilst maximising the hold space for supplies and profit-making cargo. These square-rigged ships, exemplified by the Spanish galleons, catch a great deal of motive force but can only ever ride with the wind: beating against the breeze is virtually impossible.41 This meant that in contrast to the early years of exploration, the trade routes that came to establish the European imperial presences overseas were strongly dictated by the direction of the prevailing winds, and this had profound implications for patterns of colonisation and the subsequent history of our world. The three most important of these were the Manila Galleon Route, the Brouwer Route and the Atlantic Trade Triangle.
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