Understanding surface currents vs deep ocean currents

Emily Heaslip

Ocean currents play an important role in regulating the climate around the world. There are two main types of ocean currents: surface currents and deep ocean currents. 

Surface currents are driven by global wind systems that are fueled by energy from the sun. These currents bring heat from the tropics to the polar regions; the Gulf Stream, for instance, brings warm water along the eastern coast of the US up to Northern Europe. 

Deep currents, also known as thermohaline circulation, result from differences in water density. These currents occur when cold, dense water at the poles sinks. Surface water flows to replace sinking water, causing a conveyor belt-like effect of water circulating around the globe on a 1000-year journey

Understanding how these different ocean currents work, what impacts ocean currents, and tracking changes in these systems can help us better plan for climate change, map shipping routes, and optimize the placement of offshore renewable energy and aquaculture operations. 

What causes surface currents? 

There are many factors that cause ocean currents. Deep currents are driven by temperature and water density/salinity. Of course, deep currents impact surface currents, which carry warm water to the poles. Surface currents are also driven by global wind systems fueled by energy from the sun. Factors like wind direction and the Coriolis effect play a role. 

The Coriolis effect refers to the way objects are deflected when they travel long distances around the Earth. Our planet’s rotation causes currents to bend to appear to bend to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. 

Coriolis effect causing deflections in expected "straight line" path
Image source

In addition to these regular influences, natural disasters also cause surface currents. Huge storms and underwater earthquakes can trigger ocean currents. Earthquakes that change the landscape of the seafloor can also cause changes in deep sea currents, as the position of underwater landforms changes the positioning of a current. 

While surface currents and deep water currents are related, both are difficult to track. “It is difficult to map an ocean current, as the ocean water is clear and the reflection of light on the water surface does not depend on the velocity or temperature of the floating water. One way to map the ocean currents is by using sea surface temperatures derived from satellite imagery,” wrote ESRI. “Some surface currents carry water with temperature that may significantly vary from nearby water.” 

The difficulty in accurately mapping ocean currents makes it a little more complicated to track how ocean currents are changing with climate change. 

How do ocean currents affect the climate?

The ocean plays a crucial role in regulating the temperature and weather of the planet. Oceans cover 71% of the globe, storing and transferring heat energy around the world. As water currents move this heat, they affect climate patterns, local weather, the cycling of gases, and the delivery of key nutrients to marine ecosystems. 

Unfortunately, the “conveyor belt” of deep ocean and surface currents is slowing. A 2020 IPCC report projected that ocean currents would weaken by as much as a third by 2100 if greenhouse gas emissions continue at their current rate — leading to floods,  higher sea levels, and more extreme weather events. 

[Read more: Tracking Changes in Surface Currents]

Why are currents getting weaker? Put simply, warmer water is less dense. Therefore, warm water circulating to the poles doesn’t sink as well; and, since this is the engine driving the entire conveyor belt, the whole system slows. Plus, the melting of fresh water ice sheets alters the density of water at the poles, further slowing the sinking effect. 

We know that weakened currents and warmer seas mean more stratification, acidification, and lower ability to store carbon dioxide. That’s why it’s critical to keep track of changes in ocean currents to better prepare for large-scale climate change. 

Tracking ocean currents

Beyond measuring temperature, what are some ways that scientists and researchers are currently tracking ocean currents?

Since the 2010 Deepwater Horizon Oil Spill, there’s been increased interest in accurately mapping and tracking ocean currents. Technology like drifter buoys with GPS and acoustic Doppler current profilers (ADCPs) are used at sea level, while planes and drones tackle the task from the sky. 

Taking things to the next level, a team of NASA scientists is using data collected from NASA's Gravity Recovery and Climate Experiment (GRACE) mission to track changes in Atlantic Ocean currents. 

“NASA's GRACE satellites measured Atlantic Ocean bottom pressure as an indicator of deep ocean current speed. This image shows bottom pressure anomaly (mean of November 2009 through March 2010, relative to 2005–2012 mean) over the North Atlantic basin. This pattern of above-average (blue) and below-average (red) seafloor pressure revealed a temporary slowing of the deep currents,” said NASA.

Measuring ocean currents from space may not be the most cost- or time-effective approach, but what’s clear is more data is better. No matter which tool or method is used to track ocean currents, the task of mapping ocean currents is best served by sharing as much data as possible.  

Aggregating data from the shipping industry, marine researchers, climate scientists, and the fishing industry will give us the best chance at understanding and pinpointing changes in ocean currents. Understanding these surface and deep currents will take a coordinated effort from all stakeholders — and Sofar Ocean’s Spotter buoys and ocean data API are tools that can help. 

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