How Ocean Currents Work

The ocean is never still. Even in the absence of wind, even at depths where sunlight never reaches, water is moving. It moves slowly in some places and quickly in others, across distances measured in thousands of kilometres and on timescales measured in centuries. Those movements, collectively called ocean currents, are one of the primary mechanisms by which the planet redistributes heat, nutrients, and carbon. Understanding them is part of understanding why the ocean is changing and what that change means for the systems built around it.

Surface currents are driven primarily by wind. Steady winds blowing across the ocean push water in consistent directions, and as that water moves it is deflected by the rotation of the earth into large rotating systems called gyres. The North Atlantic gyre and the North Pacific gyre are the largest of these, each circulating slowly clockwise in the northern hemisphere. Around the edges of these gyres, currents intensify. The Gulf Stream on the western edge of the North Atlantic gyre is one of the most powerful currents on the planet, carrying warm tropical water northward along the eastern coast of North America before turning east across the Atlantic toward Europe. It moves close to a hundred times the combined flow of all the world’s rivers. Its presence is why London has a milder winter climate than Labrador, which sits at a similar latitude.

On Canada’s Atlantic coast, the more immediately relevant current runs in the opposite direction. The Labrador Current carries cold, nutrient-rich water south from the Arctic along the coasts of Labrador and Newfoundland, meeting the warmer Gulf Stream water in the Northwest Atlantic. That meeting of cold and warm water creates one of the most biologically productive zones in the ocean. The Grand Banks historically supported some of the richest fisheries on earth. The conditions that made them productive depended on that collision of currents, and as those currents shift, the productivity and species composition of the region shifts with them.

On Canada’s Pacific coast, the dominant feature is the North Pacific Current, which flows eastward across the Pacific before splitting near the coast of British Columbia. One branch turns south as the California Current, which drives the upwelling that sustains Pacific fisheries productivity. Another turns north as the Alaska Current. The seasonal pattern of these currents, combined with wind-driven upwelling that brings cold, nutrient-rich water to the surface along the BC coast, underlies the productivity that supports salmon, halibut, and herring. In the Arctic, surface currents are driven by fresh water from melting ice and river runoff, creating conditions distinct from both Atlantic and Pacific systems.

Beneath the surface, a slower and more consequential circulation operates. Differences in water density, driven by temperature and salinity, cause water to sink in certain regions and rise in others. Cold water is denser than warm water, and saltier water is denser than fresh. In regions where surface water cools dramatically or becomes saltier, as happens near the poles when sea ice forms, that water becomes heavy enough to sink toward the ocean floor. As it descends it drives circulation through the deep ocean, eventually rising again elsewhere in a cycle that moves heat, carbon, and nutrients across the planet over periods of decades to centuries. This system, called the thermohaline circulation, carries heat from the tropics toward the poles and returns cold water from depth. The Atlantic component of this circulation, the Atlantic Meridional Overturning Circulation, carries warm water north and returns cold water at depth, and it is what maintains the mild climate of northwestern Europe and the productive conditions of the Northwest Atlantic.

That system is showing signs of change. As the Arctic warms and ice melts, large volumes of fresh water are entering the North Atlantic. Fresh water is less dense than salt water, and its addition to the surface reduces the density-driven sinking that powers the overturning circulation. Multiple research groups have found evidence that the Atlantic Meridional Overturning Circulation has weakened over recent decades, though the pace and ultimate extent of that weakening remain areas of active scientific investigation. A significant slowdown would alter heat distribution across the North Atlantic, shift the position of storm tracks, affect precipitation patterns in Western Europe, and change the temperature and productivity conditions in Atlantic Canada’s fisheries. Those are not distant scenarios. They are plausible near-term changes to conditions that Canadian fisheries, coastal infrastructure, and insurance markets are already operating within.

In the Arctic, currents are reorganizing in response to the loss of sea ice and the addition of fresh water. The Beaufort Gyre, a large clockwise circulation in the western Arctic Ocean, has been accumulating fresh water at an accelerating rate. When and if that fresh water is released into the North Atlantic, it would add to the freshening pressure on the overturning circulation already underway. The timing and magnitude are uncertain, but the direction is not.

For financial decisions connected to the ocean, currents are not background information. They are the delivery mechanism for the conditions that fisheries, aquaculture, coastal infrastructure, and conservation finance depend on. When currents shift, the underlying system shifts with them.