Summertime increases in upper-ocean stratification and mixed-layer depth

Area of Science:

• Oceanography
• Climate Science
• Marine Ecosystems

Background:

• The ocean's surface mixed layer is crucial for regulating global climate through heat and carbon exchange.
• It also supports marine ecosystems and oxygenates deep ocean layers.
• Changes in the mixed layer due to climate change are not well understood.

Purpose of the Study:

• To investigate changes in the ocean's mixed layer depth and stability.
• To analyze trends from 1970 to 2018 using oceanographic observations.
• To understand the implications for marine primary production and ocean dynamics.

Main Methods:

• Utilized oceanographic observations from 1970 to 2018.
• Applied a physically based definition of upper-ocean stability.
• Analyzed trends in density contrast and mixed layer depth across global ocean regimes.

Main Results:

• The density contrast across the mixed layer base significantly increased (8.9 ± 2.7% per decade).
• Contrary to expectations, the mixed layer deepened (2.9 ± 0.5% per decade, or 5-10m/decade).
• Concurrent stratification and deepening are linked to surface warming, freshening, and intensified turbulence.

Conclusions:

• The study reveals substantial, previously underestimated changes in the world's upper ocean.
• Findings challenge prior assumptions about mixed layer thinning with increased stratification.
• Results necessitate a re-evaluation of drivers impacting marine primary production and ocean ecosystems.