eNews

#04 2024

Unveiling the hidden depths of marine heatwaves: Insights from the South-West Indian Ocean

By Clea Welch, MSc Student, Egagasini Node, NRF-SAEON

Under the current state of human-induced climate change and global warming, extreme weather events such as floods, droughts and tropical cyclones are becoming more severe, and their devastating impacts are amplifying. However, extreme events are not limited to weather, but also extend into our oceans and can have catastrophic consequences for marine life.

Marine heatwaves (MHWs) are periods of unusually high ocean temperatures that persist for days to months and can cause widespread disruption of marine ecosystems, affecting marine species, coral reefs and even weather patterns. Ocean warming, due to climate change, is causing these events to become more intense and frequent. Yet, our understanding of the true extent and severity of these events remains limited, as traditional MHW studies rely solely on surface temperature data and seldom investigate the vertical structure of these events.

Investigating the hidden depths of MHWs is especially important as the subsurface manifestation of these events is what directly impacts marine life.

South-West Indian Ocean 

Of particular concern is the occurrence of MHWs in the South-West Indian Ocean, a region situated off the southeastern coast of Africa. This area is characterised by intricate oceanic dynamics and hosts a remarkable diversity of marine life, including some of the largest and most varied coral reefs in the world.

“To enhance our ability to predict, understand and mitigate the effects of marine heatwaves, we must emphasise the importance of subsurface investigation of marine heatwaves and prioritise comprehensive ocean monitoring that includes both surface and subsurface data.” ~ Clea Welch, MSc student at the Egagasini Node

The South-West Indian Ocean, a region situated off the southeastern coast of Africa, has some of the largest and most varied coral reefs in the world (Photo: Shutterstock/Jo Dilnot)

The South-West Indian Ocean is characterised by intricate oceanic dynamics and hosts a remarkable diversity of marine life (Photo: Shutterstock/Jo Dilnot)

Here, MHWs are known to reach moderate to severe temperatures that extend and intensify far below the surface. For instance, during studies conducted in January 2020, July 2012 and in October 2007, weak surface temperature anomalies (1–2 °C) were detected that extended down to 800 m deep and intensified up to 4 °C below the surface. While an increase of 3–4 °C in average temperatures may not be much on land, it can be catastrophic for marine life, as ecosystem functioning and species’ physiology are highly temperature sensitive.

Understanding what drives these events 

The first step to predicting future events and mitigating impacts of subsurface-intensified MHWs is understanding what drives these events. In the South-West Indian Ocean, extreme subsurface temperature anomalies persisted where surface MHWs coincided with warm-core eddies – large, rotating bodies of water. These eddies are the likely cause for driving these deep MHWs as they can trap heat below the surface and contribute to more intense subsurface temperature anomalies. This emphasises the need for further subsurface investigation of MHWs in oceanic regions where eddies dominate, as surface MHW identification alone largely underestimates the severity, and subsequently, the impacts of these events.

However, in most oceanic regions, consistent and long-term subsurface data is scarce and our ability to investigate the vertical extent of MHWs is limited. This significantly hinders our ability to develop a thorough understanding of their dynamics and ultimately prevents our ability to implement effective response and mitigation strategies required to effectively protect marine biodiversity from extreme ocean temperature events.

To enhance our ability to predict, understand and mitigate the effects of marine heatwaves, we must emphasise the importance of subsurface investigation of MHWs and prioritise comprehensive ocean monitoring that includes both surface and subsurface data. Staying informed and contributing to these efforts will be essential in preserving the health of our oceans for future generations.

Figure 1: (a) Surface MHW event that occurred on 20 October 2007 and the (b) subsurface temperature anomalies associated with this surface-identified event. The surface MHW was identified from NOAA Optimally Interpolated Sea Surface Temperature (OISST) data and the subsurface temperature anomalies were determined using in situ data from Expendable Bathythermograph (XBT) from the IX21 XBT transect. The black solid line in (a) indicates the XBT transect line used to investigate the subsurface temperature profiles of surface-identified MHWs. The subsurface MHW signal is shown by the large positive anomalies, indicating much warmer than normal temperatures below the surface MHW signal.