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The Ways of the Sea
Water is an extraordinary molecule, vital for life as we know it. It first appeared on our planet around 4.5 billion years ago, and although science still hasn't found a unifying reason for why Earth has so much water, its arrival began the planet's transformation from spherical rock to a thriving Eden. Water allows chemicals to travel, offers electrons for reactions, dissolves salts and gases, stabilizes temperatures and provides buoyancy, giving relief from the harsh effects of gravity. In this highly active environment, the first life appeared – and evolution did the rest.
Although it is theoretically possible for water from all over the planet to mingle, in reality powerful physical forces and physical rules affect and restrict its movement. So, while some water dynamics are on a colossal scale, traversing the entire globe, others are small and predictable; but all have very specific attributes and associations, reflected in the way we designate distinct bodies of water. The oceans, seas and channels identified by humankind over the centuries are, at root, artificial separations – as man-made as the lines we scrawl over maps to divide countries and empires. But this doesn't mean they have no merit. Although the physical, biological and chemical interactions of our blue planet have set the rules for all aquatic life, this doesn't mean that a water molecule in the Bahamas is interchangeable with one in Bournemouth – you can't just hop from one to the other.
Mapping the seas has not been an easy task, and it is true to say that the final frontier is the great oceans, where there is still much to learn. What we do know is that the oceans have many permutations of the basic physical factors, producing amazing environments to which life has adapted itself and then exploited. The Southern Ocean is a roaring mass of waves and wind, ensuring some of the harshest seafaring conditions on the planet; yet here we also find some of the densest concentrations of aquatic life. By contrast, water molecules further north, around the Equator, sit still and stagnant in the barren expanses of the Pacific: the warmness of these waters does not dictate that life will thrive in them. The picture is far more complicated than that, though, and each ocean has its own set of requirements, sometimes extremely testing, with which its denizens must comply in order to prosper.
All in all, living on our wet planet is complicated and difficult. The rules are strict, but can change in a heartbeat; they are fluid in every sense. Only the very strongest lifeforms survive, and even then you never know when the oceans might change the game all over again.
Once upon a time ...
When the Earth first formed, there were no oceans at all, just a hot ball of rock spinning in space. All the water we now have was in gas form, in what developed into our atmosphere. Only after the Earth had cooled significantly did the water vapour get a chance to condense into clouds and dispense rain into the hollows below – and geologists believe there may have then been a centuries-long downpour (sound like Yorkshire?) as the primal oceans filled.
After this point in the planet's evolution, the oceans' shapes were decided not by water but by land, as continents shifted, and by the changing climate. In fact, 250 million years ago, when all the continents existed as one giant landmass known as Pangaea, there was consequently just the one vast ocean surrounding its coastline. After another 50 million years had passed by and Pangaea had started to break up, individual seas with more distinctive characteristics started to form, most famously the Tethys Sea. Splitting the northerly Laurasia continent from the southern Gondwana, the Tethys provided new ecological niches for life to colonize. Sediments and fossils from the Tethys give us great insight into what this forming world was doing during that geological age. As the continents continued to drift, more seas opened up, and in time the mighty Tethys itself closed, around 65 million years ago.
It is only in relatively recent geological time that today's oceans took shape, when, as the continental drift slowed and the polar ice caps formed, currents started to connect and isolate different bodies of water. Continued movements in landmasses created relatively new seas, such as the Mediterranean, and in the twenty-first century the oceans continue to move, shift and adapt.
A tidal tale
'Time and tide wait for no man ...' – a familiar phrase, but one that I have always enjoyed because of its honest simplicity, especially when it comes to the tide. Long before we invented clocks, nature had its own rhythm, the seasons giving colour to the year. And the tide provides the constant beating heart of the planet.
At a basic level, the tide is widely understood, for it flows in to reach high tide, and ebbs away to make low tide. In doing so, it swallows up the largest beaches, completely exposes harbours, and drives water through the smallest channels. But how does so much water move on a global scale?
Well, the first answer usually given is that the tides are controlled by the gravitational pull of the Moon. However, this is only half right. The tides are also controlled by the gravitational pull of the Sun, with both Sun and Moon exerting a similar control. As beach-goers know, the timing of tides differs each day, usually by around 50 minutes, depending on location. More than that, some shorelines can experience two high and two low tides daily, while others can have just one, and yet other shores vary depending on the time of the month. And then, twice a month respectively, there are spring tides and neap tides. Spring tides occur when the Sun and Moon are in alignment, either on opposite sides of the Earth or complementing each other on the same side. The combined gravitational pull of the celestial bodies brings higher high tides and lower low tides, covering and then exposing more shoreline than normal. But when the Sun and the Moon are 90 degrees apart when viewed from Earth, the opposite occurs. The gravitational pull is compromised and the result is a neap tide, in which little water moves in any direction.
Over thousands of years, humankind has studied the tides, acquiring a pretty good grasp of the whole phenomenon. More remarkable, though, is the way that marine animals are wonderfully in tune with the rhythm of the seas and in synchronicity with the tidal ebb and flow.
Around the world, there are some extraordinary daily tidal events. The Bay of Fundy in Canada has the highest tide on the planet, exposing more than 26 kilometres of shoreline during the ebb of a high spring tide. Put another way, that's an estimated 160 billion tonnes of seawater flowing in and out of the bay twice a day. Scientists attribute the phenomenon to the topography of the bay itself, which complements the wavelengths of the incoming tides to create an effect called tidal resonance.
Massive movements of water like this do not go unnoticed by man or beast, and areas of large tidal flow often exhibit great biodiversity. For us, the chance to harness the power of the tides is just too tempting, and across the globe tidal barriers are constructed as a method of producing clean, renewable energy.
By contrast, there are points around the planet where the arrangement of the continental shelves, landmasses and local topography result in there being very little tidal movement, and in some places almost nothing at all. These are termed amphidromic points. This is not to say that there are no low or high tides, but these points are like the fulcrum of a seesaw, never changing amid the contrasts on either side.
The current picture
Looking at a map of the ocean currents reveals a really quite incredible and intricate system, like some wonderful clockwork mechanism. You can see exactly where the water goes as it moves around Earth's landmasses.
The major currents work in a reasonably uniform way. This means that in the northern hemisphere the currents move in clockwise whirls – gyres – while in the southern hemisphere they travel anticlockwise. This is due to the 'coriolis effect', a phenomenon produced by the Earth's spinning rotation, pulling at both the water and the driving winds. As the currents move across the oceans, they are heated at the Equator, and then at higher latitudes their heat dissipates – and creates the weather we experience. Think of the Gulf Stream. This current begins in the tropical Gulf of Mexico and then moves across the Atlantic towards the UK; it typically brings with it the mild winters and warm summers a country at our latitude would not otherwise have, thanks to the heat it picked up at its Caribbean origin.
As warm water moves away from the Equator, it pulls in nutrient-rich cold water from the dark depths and/or the polar regions that is often directly responsible for an explosion of life. This phenomenon is evident in South America, where the bloom of plankton and the subsequent population explosion of anchoveta provide the basis for an entire ocean food chain, right up to humans, all of it originating in those cold waters.
There is only one current that seems to break all the rules – and this is known as the Antarctic circumpolar current. Driven by the turning of the Earth and the roaring winds, and without any landmass to stop it circulating, this current rips around the bottom of the planet in one huge loop. It can easily be ranked as the roughest sea on the planet.
Turn, turn, turn: the coriolis effect
Currents around the planet are driven by many factors – the wind, temperature, landmasses – but they're also influenced by the constant spinning of the Earth. It's hard to explain, but it's really important, and it's called the coriolis effect.
In a nutshell, as the Earth spins on its axis, objects near the Equator have to travel faster than objects nearer the poles to complete a single rotation. This is because they have further to travel to complete a single rotation than objects further away from the Equator, where the rotational distance is shorter. Water molecules are being subjected to this force all the time, and those molecules fractionally closer to the Equator move slightly faster than those further away. The effect of these fractional differences is to cause a spiral to form. While this effect is very weak on the Equator, where the large majority of water molecules are all subject to the same forces, as you move further towards the poles the effect increases.
The coriolis effect's greatest impact is on the planet's winds, as seen in weather patterns where large spirals of cloud can cover whole oceans. The winds then drive the oceans into the planet-spanning gyres and currents we rely on for our weather, for sea travel and for food. Without the coriolis effect and the spinning of the Earth, life would be dull indeed: our only major water movement would be the cycle of hot water to cold water in a very boring convection current.
Rip currents: a real drag
Knowing about rip currents could save your life. They are a powerful force to be reckoned with, and while they are found off some beaches all year round, they can appear anywhere under the right conditions.
Strong rip currents form when large waves and strong winds drive in towards the beach. Once a wave of water has broken, it wants to roll back into the sea; but behind it are more and more waves pushing in, trying to force it back towards the beach. So, the mass of water finds the path of least resistance back into the ocean. On long sandy beaches, this path can begin as a very slight depression in the sand and very quickly develop into a trench. Gulleys that form between rocks in the sand allow rip currents to establish themselves.
Although a rip current eventually merges back into the ocean to reappear as more waves, the problem is that their speed and strength can drag a swimmer out behind the breakers and into deep water very quickly indeed. Some rip currents will move at 8 kilometres an hour – faster than anyone trying to swim against it. So, if you do get caught in a rip, you have two options: either let it take you all the way out and then signal for help, or swim parallel to the beach until you get out of the current, before swimming back to shore.
Rip currents can be hard to spot, so local knowledge and looking out for warning signs are important. But if you see a thin strip of darker, calmer water at right-angles to the beach, cutting through white breaking waves, that's a rip current.
Into the vortex: whirlpools
Whirlpools are awesome, be they the little ones in the bath when you pull the plug or the wilder ones in the natural world. I have been lucky enough to see the strongest whirlpools on the planet. In the Saltstraumen strait in Norway, currents of up to 40 kilometres an hour smash 400 million cubic litres of water through a gap only 150 metres across, producing the world's strongest tide and a watery chaos truly living up to its name: the Maelstrom.
The formation of whirlpools is actually quite simple. They are just the meeting of two opposing currents, which, as they pass each other, interact and spiral downwards. The strength of the vortex, or downward pull, will depend on the power of the original currents. Most whirlpools struggle to pull down perhaps a metre or more; but the one at Saltstraumen can be 5 metres deep in just its cone. Getting pulled down into it would mean certain death for any swimmer, but fish don't seem to be bothered by large whirlpools for the most part: any small fish or plankton sucked downwards are likely to resurface unharmed, while larger adult fish can simply skirt around the base of the vortex without concern.
More ingeniously, fish exploit smaller whirlpools for food: they use the eddying currents to minimize the effort needed to swim, while positioning themselves ready to feed rapidly on anything edible that the vortex brings down to them.
The stuff of disaster movies, tsunamis are one of the most destructive and terrifying natural events that can be witnessed on Earth. Unpredictable and unstoppable, they are a constant reminder that we are guests on this planet and not its masters.
Tsunamis mainly occur through sudden movements in the Earth's tectonic plates. These underwater earthquakes cause a massive shift in the water column above as one plate jerks suddenly beneath another, forcing the other plate to pitch upwards. This sends an enormous jolt of energy into the sea, and water is displaced either upwards or downwards. At the surface, directly above the epicentre of the quake, a single wave forms and radiates out in all directions. At first, this wave may not be very high at all, and the water column may have been displaced by less than a metre. But all the water down to the ocean floor will have been affected, and all that incredible energy means that the wave can travel at 800 kilometres an hour or faster. As the tsunami starts to reach land, its wave height drastically increases as all those energized water molecules start to bunch up on top of each other, reaching extraordinary heights. At the same time, as the wave height grows and the wavelength shortens, speed is lost – the wave might now be travelling at 50 kilometres an hour.(Continues…)
Excerpted from "Blowfish's Oceanopedia"
Copyright © 2017 Tom Hird.
Excerpted by permission of Atlantic Books Ltd.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Table of Contents
1 The Ways of the Sea,
3 Coastal Seas,
4 Coral Reef,
5 Open Ocean,
6 Deep Ocean,
7 Frozen Seas,
8 Threats to the Oceans,