Marine heatwaves (MHWs) are rapidly intensifying phenomena threatening marine ecosystems worldwide, and new research sheds light on their hidden aspects beneath the surface. For the first time, scientists have characterized the vertical structure of subsurface MHWs within shallow nearshore environments, using nearly two decades of data from the California Current upwelling system.
The study, led by researchers at California Polytechnic State University, focused on San Luis Obispo Bay, California, and highlights significant differences between surface and subsurface MHW occurrences. The researchers employed high-resolution temperature data collected between 2005 and 2023 using automated profiling instruments stationed at about 10 meters depth.
Marine heatwaves are defined as extended periods of unusually warm water, typically linked to detrimental ecological consequences, such as the loss of seagrasses and corals. Yet, previous studies mainly examined surface MHWs via satellite data, often overlooking the potential impacts of subsurface phenomena.
With global sea temperatures rising due to climate change, marine heatwaves are projected to increase both in frequency and intensity, disrupting marine ecosystems and threatening coastal economies. What sets this research apart is its focus on the less understood subsurface MHWs, which may exert distinct ecological pressures on marine life.
The researchers discovered concerning trends: approximately one-third of bottom MHW events occurred independently from surface counterparts, underscoring the inadequacy of satellites which predominantly monitor surface temperatures. Lead researcher Gavin Plume noted, "Despite being in a shallow depth of only 10 m, surface and subsurface extreme temperature anomalies (MHWs) were disconnected and occurred independently from one another."
This disconnect poses challenges for monitoring and managing marine ecosystems, especially as most assessments have relied on less accurate data derived from satellite technology. The automation of the profiling system allowed for near-continuous measurements, yielding new insights about the characteristics of subsurface MHWs across various seasons.
One of the key findings revealed the distinct seasonal patterns of MHWs: during the fall and winter, MHWs showed increased frequency and intensity due to weak stratification of the water column, allowing them to penetrate greater depths. Conversely, during the summer, strong stratification limited the vertical range of MHWs, resulting in more occurrences of bottom- or surface-trapped events with shorter durations and reduced intensities. "During the summer months, strong stratification limited the vertical extent of MHWs, leading to surface- and bottom-trapped events with shorter durations and intensities," explained the researchers.
Coastal upwelling—when cold, deep waters are pushed to the surface—was identified as a significant factor affecting MHW dynamics. The findings confirmed previous assertions linking variations in coastal upwelling to the initiation and termination of MHWs. Notably, instances of weak upwelling were seen to coincide with the start of MHWs.
This research equips marine managers and ecologists with indispensable information for monitoring marine heatwaves and their ecological consequences. It emphasizes the necessity for expanded monitoring frameworks of subsurface MHWs globally as oceans worldwide warm. Coupled with more reliable data collection techniques, enhanced local and regional management practices could be informed to mitigate adverse impacts encountered during marine heatwaves.
Importantly, the researchers stress the need for finer-resolution modeling and long-term observational data, asserting, "This study highlights the urgent need for expansion of subsurface monitoring of MHWs globally amid a warming planet." This imperative suggests the significant role subsurface marine heatwaves play within our oceans, underlining the increasing importance of incorporating vertical structures of MHWs within future marine ecological studies.
With these insights, efforts to strengthen global responses to marine heatwaves can proceed thoughtfully, aligning with the pressing realities of climate change and its pervasive effects on marine environments.