A5 Suppression of Excessive Mixing and Stratification

Topic

Excessive stratification of the surface ocean occurs when extreme surface warming, or an influx of fresh water from rivers or melting ice, creates a shallow, highly stable mixed layer that inhibits vertical exchange between that layer and the waters beneath it. This suppression of vertical mixing has a direct consequence for ocean biogeochemistry: the surface mixed layer becomes decoupled from the subsurface ocean, and this decoupling deprives photosynthetic organisms living near the surface of access to nutrients that would normally upwell from deeper layers.

An indirect consequence of this increased stratification is deep-ocean deoxygenation, which occurs through two simultaneous processes. On one hand, the already warmer surface ocean retains less dissolved oxygen because oxygen solubility decreases as water temperature rises. On the other hand, the reduced ventilation caused by excessive stratification itself—meaning less surface water sinking to replenish the deep ocean—prevents biological oxygen consumption from being balanced by physical oxygen replenishment; it is this combination that has caused oxygen minimum zones to expand significantly in recent decades amidst climate warming.

Under conditions of particularly extreme stratification, the mixed layer itself shrinks drastically: its thickness can drop from a typical range of 100–150 meters to a mere 20–50 meters during the most severe events, and this reduction in thickness results in significantly less exchange of heat, gases, and nutrients between the mixed layer and the deep ocean compared to normal conditions. This same excessive stratification can produce low-density water layers capable of traveling thousands of kilometers across the ocean—far from their region of origin—and can also create salinity barriers that float atop the denser, saltier water beneath them. When such a salinity barrier becomes sufficiently strong, it completely suppresses winter convection—the seasonal sinking of cold, dense water that normally ventilates the deep ocean each winter.

The Arctic currently offers the clearest example of this entire process: freshwater input from melting ice creates a thin layer of nearly fresh water floating on the saltier water below. This near-fresh layer is stable enough to almost entirely block heat exchange between the two, effectively isolating the deep Arctic saltwater from atmospheric heat year-round. Thus, from the initial decoupling of the mixed layer and the subsurface ocean to deep-ocean deoxygenation, the thinning of the mixed layer, and the formation of salinity barriers, this excessive ocean stratification converges on a single outcome: a surface ocean that is increasingly isolated from the waters beneath it.

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