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Marine heatwaves

The ocean is a complex system and home to countless animal species. Extreme heat also affects the ocean. Climate change plays an important role in this.

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Marine heatwaves are phases during which regions of the ocean are abnormally warm for long periods of time. Such marine heatwaves usually occur when several factors interact:

  • Climate change: If the air is warmer than the underlying water, the atmosphere releases its heat into the ocean. The global ocean absorbs 90% of the additional heat caused by climate change. Climate change therefore increases the risk of marine heatwaves.
  • Solar radiation: If there are no clouds over the ocean for a period ranging from several days to weeks, the uppermost layers of water in the ocean are warmed by the atmosphere and direct incoming solar radiation. The process in the ocean is the reverse of that in the atmosphere: the atmosphere is primarily warmed from the ground upwards, i.e. from below. Warm air rises and mixes with the colder layers of air above it. The ocean, on the other hand, is warmed from above. The warm water does not rise upwards, it is already at the top. This means that heat collects in the upper layers of water and there is little mixing with the colder layers of water below.
  • Wind: In general, ocean water is stably stratified. Colder water is found at the bottom or in lower layers, warmer water at the top. The differences in temperature would only be equalised very slowly if it were not for the wind. Wind at the surface moves the water and thus also mixes it into deeper layers. However, if it is particularly calm, the heat can accumulate in the uppermost layers of water.
  • Aerosols: Aerosols are tiny particles in the atmosphere. The more aerosols there are in the air above the ocean, the less sunlight reaches the surface. As a result, the sea surface cools down. One source of aerosols is, for example, Saharan dust over the North Atlantic or air pollution from cities and ships. These aerosols are often transported across the ocean by the wind. Calm conditions therefore also contribute to the warming of the oceans through the lack of aerosol input.
  • Natural fluctuations in the climate system: Phenomena such as El Niño or the North Atlantic Oscillation (NAO) also regularly contribute to increased ocean temperatures.

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MeteoSwiss is the Federal Office for Meteorology and Climatology. We keep our finger on the pulse of the weather and climate, so it’s all clear as day to you.

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