A3 Thermal Anomalies and Variability

Topic

Oceanic thermal anomalies are persistent deviations in sea surface temperature from a region's typical climatology; these deviations have consequences extending far beyond the ocean itself, altering atmospheric circulation, modifying precipitation patterns, and affecting marine biodiversity. It is precisely this persistence over time that causes an oceanic thermal anomaly to impact systems as diverse as the climate and marine life.

Among all oceanic thermal anomalies, El Niño and La Niña are the most extensively studied: during an El Niño event, the sea surface temperature in the eastern equatorial Pacific warms to between two and four degrees above normal, whereas during a La Niña event, the opposite occurs, and the same region cools. This warming or cooling of the eastern equatorial Pacific triggers atmospheric teleconnections, causing El Niño and La Niña to affect the global climate system rather than just the region of their origin.

Marine heatwaves are a distinct type of oceanic thermal anomaly; they are defined as periods when sea surface temperatures remain above the 90th percentile of their climatological norm for at least five consecutive days, typically lasting anywhere from weeks to months. Two mechanisms drive these marine heatwaves: reduced winds—which diminish both evaporation and the upwelling of deep, cold water that normally cools the surface—and the deepening of the pycnocline (the layer separating surface waters from deep, cold waters), which further isolates that deep, cold water from the surface.

Another oceanic thermal anomaly—though opposite in nature to those previously mentioned—is the persistent cold patch appearing south of Greenland in the North Atlantic, characterized by temperatures one to two degrees below normal. This cold patch is attributed to the weakening of the Atlantic Meridional Overturning Circulation: when this circulation transports less warm water northward, the very area where that water sinks cools down; it is this cooling that leads to the intensification of the North Atlantic cold patch being viewed as a possible early warning sign that the Atlantic Meridional Overturning Circulation continues to weaken.

Underlying these more localized anomalies is a broader trend affecting the entire ocean: since 1955, the ocean has absorbed 93 percent of all the excess heat gained by the climate system, warming by an average of one-tenth of a degree per decade within the top 700 meters. This background warming increases ocean stratification, which in turn hinders vertical mixing, reduces the supply of nutrients to the sunlit zone where plankton thrive, and ultimately alters the distribution of marine species.

Consequently, as the climate warms, marine heatwaves are expected to become even more frequent, intense, and widespread than they are today, causing severe ecological impacts on coral reefs, seagrass meadows, and the fisheries upon which so many coastal communities depend. From El Niño and La Niña—which disrupt the global climate—to the North Atlantic cold patch that could signal a shift in ocean circulation, and the background warming that makes marine heatwaves increasingly likely, all these oceanic thermal anomalies converge in a single ocean that is growing steadily warmer and more stratified.

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