Central Idea

MHWs have engulfed regions like the north-east Pacific, southern Indian Ocean, the Atlantic, and the Mediterranean, as reported by Mercator Ocean International.
In April, the ocean’s surface temperature peaked at 21.1 degrees Celsius, breaking the previous record set in 2016.

What are Marine Heat Waves (MHWs)?

While we often associate heatwaves with the atmosphere, they can also occur in the ocean, known as Marine Heatwaves (MHWs).
These prolonged periods of excessively warm ocean temperatures can have significant consequences for marine ecosystems and industries.
MHWs can happen in both summer and winter, with “winter warm-spells” affecting specific regions and species.

Threshold Criteria: A marine heatwave is characterized by seawater temperatures exceeding a seasonally-varying threshold (often the 90th percentile) for at least 5 consecutive days.
Continuity of Events: Successive heatwaves with gaps of 2 days or less are considered part of the same MHW event.

heat wave

Air-sea heat flux: Ocean currents and air-sea heat flux are common drivers of MHWs, leading to the build-up of warm water in specific areas.
Influences of Wind and Climate Modes: Wind patterns can enhance or suppress MHWs. They influence the likelihood of events occurring in certain regions.
Large-Scale Climate Drivers: Events like the El Nino Southern Oscillation (ENSO) can also contribute to the formation of MHWs.
Intensification with Global Warming: Rising global temperatures have resulted in longer-lasting, more frequent, and intense MHWs in recent decades.
Human Influence: 87% of MHWs can be attributed to human-induced warming, with the oceans absorbing significant amounts of heat due to greenhouse gas emissions.
Oceans as Heat Sink: Oceans have absorbed 90% of the additional heat caused by greenhouse gas emissions, increasing global mean sea surface temperature by nearly 0.9 degrees Celsius since 1850.

Ecosystem Structure: MHWs can disrupt ecosystem structure, supporting certain species while suppressing others.
Kelp Forest Destruction: MHWs along the Western Australian coast in 2010-2011 devastated kelp forests and fundamentally altered the ecosystem of the coast.
Economic Losses: MHWs can cause economic losses, particularly in fisheries and aquaculture industries.
Vulnerability of Temperature-Sensitive Species: Species such as corals are highly vulnerable to MHWs. The 2016 marine heatwaves in northern Australia caused severe bleaching of the Great Barrier Reef.

Catastrophic Effects: MHWs have led to the death of numerous marine species, altered migration patterns, and caused coral bleaching, endangering coral reefs.
Coral Bleaching: High ocean temperatures in the tropical Atlantic and Caribbean in 2005 led to a massive coral bleaching event, impacting over 80% of surveyed corals, with severe consequences for marine life.
Invasive Species and Ecological Imbalance: MHWs fuel the growth of invasive alien species, disrupting marine food webs and posing threats to wildlife. Ex. Whale entanglements in fishing gear.

Amplifying Storms: Higher ocean temperatures associated with MHWs make storms like hurricanes and cyclones stronger, leading to severe weather events and flooding.
Threat to Coral Reefs: Half a billion people depend on coral reefs for food, income, and protection, but MHWs pose a grave threat to these ecosystems, impacting human livelihoods.
Socio-Economic Impact: Coastal communities are particularly vulnerable to the socio-economic impacts of MHWs, affecting fisheries and tourism.