We intuitively know that the ocean is constantly moving: the tides ebb and flow, waves swell and crash, plane wreckage moves mysteriously across latitudes to be beached on the other side of the world. But what makes the ocean move? The steady heartbeat of the ocean is its global-scale currents.
Ocean circulation is the global-scale movement of water, driven by separate processes on two scales: surface currents in shallow water, and deep currents along the bottom layers of the ocean. Surface currents are driven by energy exchange between shallow ocean water and the wind. The shear stress of air blowing along the surface of the water imparts momentum from the wind to the water, initiating a drift current in the shallow surface layer. This wind also deforms the shape of the surface, creating choppy waves, which in turn changes the grip the wind has on that sea surface. Over time, circulating winds and the rotation of the Earth can combine to create gyres, swirling vortices of water thousands of kilometers in diameter. And sometimes, water isn’t the only thing swept along: the Great Pacific Garbage Patch is an example of a man-made gyre, a massive wasteland of floating garbage kept together by the swirling of surface currents.
Deep water beyond the reach of air-sea interactions is pumped by thermohaline circulation, driven by differences in the heat and salinity of units of water. The ocean is stratified into layers with slightly different compositions, temperatures, and densities. Warm, fresh water rises slowly towards the surface, and cool, salty water sinks towards the ocean floor, buoyed or sunk by its relative density to the surrounding water. This sinking displaces water at the bottom of the ocean, instigating a creeping conveyor belt of water transport. This process transports 1015 watts of heat from the tropics to the polar latitudes, which accounts for about one-quarter of the total heat transport of the atmosphere and oceans combined. Reaching every ocean basin across the globe, this process merges the Earth’s oceans into a connected global system, facilitating the global transport of energy, nutrients, and living creatures. This circulation progresses at a snail’s pace, typically moving at only 0.4 inches per second, and cycles the deep, dense water at the bottom of the world’s oceans approximately every 600 years.
This process transports 1015 watts of heat from the tropics to the polar latitudes, which accounts for about one-quarter of the total heat transport of the atmosphere and oceans combined.
This ocean heartbeat allows ocean life to flourish as well. As deep water is pulled to the surface by upwelling events, nutrient-rich water is flushed upwards through the water column. Phytoplankton, primary producers and the foundation of the oceanic food web, bloom around these nutrient flows and kickstart a zone of extremely rich biological activity. These upwelling events become biotic magnets for all levels of the food chain, and humans are no exception: approximately one-quarter of the global marine fish catch comes from upwelling zones, which account for only 5 percent of the total ocean.
As we have increasingly discovered in the 21st century, human actions can have cascading effects on delicate, crucial natural processes. Rising atmospheric CO2 and the resulting warming of Earth’s oceans have wreaked havoc on marine systems and could potentially interrupt ocean circulation. A weakening — or even total arrest — of global ocean circulation could spell disaster for huge swaths of the Earth’s ecosystems. As deep water ceases to be cycled to the surface, it will become depleted of oxygen, choking all marine life — which is the projected cause of many mass extinction events throughout geologic time. Ceasing upwelling events would cause a collapse in the plankton population, which would in turn kneecap the rest of the marine food chain. The weather in northern and western Europe would turn cold and unpredictable, as the stabilizing effect of Atlantic thermohaline circulation dissipates, and storms and floods in many regions of the world will increase in frequency and intensity. Already, the Atlantic meridional overturning circulation, the largest thermohaline circulation pattern, has shown evidence of 15 – 20 percent weakening over the past 200 years, with scientists speculating that human impact is accelerating this process, causing a devastating 13 centimeters of sea level rise.
As deep water ceases to be cycled to the surface, it will become depleted of oxygen, choking all marine life — which is the projected cause of many mass extinction events throughout geologic time.
The movement of Earth systems can seem immoveable, and humans have been wont to take them for granted. But we’ve begun to learn that our choices create untold environmental impact and can disrupt cycles which have continued for millennia. The effects of changing circulation patterns will not be kind to us, and we will soon find out what happens to us when the ocean’s pulse quiets.
Geophysical Research Letters (2013). DOI: 10.1002/2013GL057992
Encyclopedia of Ocean Sciences (2001). DOI: 10.1006/rwos.2001.0111
