Marine heatwaves

Subject: Geography

Section: Physical geography

A marine heatwave is defined as when seawater temperatures exceed a seasonally-varying threshold (usually the 90th percentile) for at least 5 consecutive days. Successive heatwaves with gaps of 2 days or less are considered part of the same event.
Due to increased greenhouse gas emissions, extended periods of extreme warming in seas and oceans have increased in frequency by 50% in the past 10 years and are becoming more severe.

SUMMER MARINE HEATWAVES VS. WINTER WARM SPELLS

Heatwaves can happen in summer and also in winter, where they are known as “winter warm spells”.
These winter events can have important impacts, such as in the southeast of Australia where the spiny sea urchin can only colonise further south when winter temperatures are above 12 °C. A winter warm spell there can help promote colonisation.

WHAT CAUSES MARINE HEAT WAVES?

Marine heat waves can be caused by a whole range of factors, and not all factors are important for each event.
The most common drivers of marine heatwaves include ocean currents which can build up areas of warm water and air-sea heat flux, or warming through the ocean surface from the atmosphere.
Winds can enhance or suppress the warming in a marine heatwave, and climate modes like El Niño can change the likelihood of events occurring in certain regions.

Impact of Marine Heat Waves (MHW):

Higher water temperatures associated with MHWs can cause extreme weather events such as tropical storms and hurricanes, and disrupt the water cycle; making floods, droughts and wildfires on land more likely.
Impact on monsoon:
- The heating of the land in summer creates low pressure over the Indian subcontinent.
- Therefore, the winds blow from the Indian Ocean to the land, carrying the moisture for the monsoon rains.
- When marine heatwaves occur, the winds are pulled to these regions over the ocean (instead of land), reducing the rainfall over land.
- The marine heatwaves in the western Indian Ocean and the Bay of Bengal are found to result in dry conditions over the central Indian subcontinent.
- At the same time, there is a significant increase in rainfall over south peninsular India in response to the heatwaves in the north Bay of Bengal. These changes are in response to the modulation of the monsoon winds by the heat waves.