As with the atmosphere, oceans can experience heat waves. The National Weather Service defines an atmospheric heat wave as a period of abnormally hot weather generally lasting more than two days. To be considered a heat wave, the temperatures must be outside the historical averages for a given area.
Marine heat waves are defined as any time the ocean temperatures are warmer than 90% of the previous observations for the region at a given time of year. Marine heat waves can last for weeks, months and even years.
Marine heat wave conditions are monitored and forecast by NOAA. The National Data Buoy Center collects and disseminates quality-controlled marine observations. Global ocean surface temperatures are also monitored using satellite observations. In July 2024, 35% of the global ocean experienced marine heat waves.
The world’s oceans are heating up. Processes like marine heat waves and El Niño/La Niña make it a challenge to untangle the causes of warming anomalies for a specific year or region. However, analysis of the ocean temperature over decades shows the warming trends result from the observed increases in atmospheric carbon dioxide, just like the observed warming trend in atmospheric temperatures.
Warmer waters influence atmospheric weather patterns, such as tropical storms. There are also impacts on marine ecosystems.
An unprecedented marine heat wave dominated the northeastern Pacific from 2013 to 2016. Called “the Blob” because of the large expanse of unusually high temperatures, it upended ecosystems across a huge swath of the Pacific Ocean. The warm temperatures attracted subtropical species rarely seen off the West Coast of the U.S. The krill that humpback whales typically feed on grew scarce, and they switched to feeding on high concentrations of anchovy. The higher temperatures fueled a record bloom of toxic algae that shut down West Coast crabbing from November 2015 through March 2016.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
