How Heat and Freshwater Move Through the Ocean
The ocean warms up most at the equator. Ocean circulation then acts like a giant conveyor belt, moving this heat to higher latitudes where it's eventually released into the atmosphere.
Socratis Loucaides / Elaine McDonagh
Freshwater moves between the atmosphere and the ocean through evaporation, rain, river runoff, and ice melting. These patterns are different from how heat moves. A key result is that the Atlantic Ocean, which loses a lot of water to evaporation, ends up being saltier at the surface than other oceans. Changes in ocean circulation directly change the regional distribution of heat and freshwater.
The Ocean's Response to Climate Change
In our warming climate, the ocean has absorbed a huge amount of extra warmth, often called "excess heat." This is incredibly important because the ocean takes up more than 90% of the excess heat in the entire climate system, which moderates the warming we experience and its impacts on other parts of the Earth.
A warming climate speeds up the "hydrological cycle," meaning freshwater exchanges become more intense. This leads to a simple, dramatic pattern: the salty parts of the ocean get saltier, and the fresh parts get fresher.
The ocean's role is absolutely central to the climate system, controlling how heat and freshwater are absorbed and moved around the planet.
The regional distribution of heat and freshwater is what drives regional sea level and its variability. Warmer water also holds less dissolved oxygen, so a warming ocean is also a deoxygenating ocean. On a regional scale, these changing conditions (temperature, salinity, and oxygen) directly impact ocean ecosystems, with major consequences for ocean health and our global food systems.
Warming at the surface makes the top layer of the ocean less dense, which increases the density difference between the shallow and deep ocean. This gradient is called "stratification." Increased stratification makes it harder for the ocean to mix or exchange water between the layers. This reduction in vertical mixing can, in turn, impact the formation of deep water.
Our Research and Impacts
Our researchers have studied how global heat transport and Atlantic freshwater transports will change in the future (Mecking and Drijfhout, 2023; Mecking et al in preparation).
Recent work has been published on how heat transport convergence modifies ocean heat content in the North Atlantic (Moat et al 2024). We also have ongoing work examining how a combination of heat and freshwater transport causes feedback to the AMOC (Sinha et al 2025).
