On the vertical structure and propagation of marine heatwaves in the
Marine heatwaves (MHWs) have been recognized as a serious threat to
marine life, yet, most studies so far have focused on the surface only.
Here, we investigate the vertical dimension and propagation of surface
MHWs in the Eastern Pacific using results from a high-resolution
hindcast simulation (1979 to 2019), performed with the Regional Ocean
Modeling System. We detect MHWs using a seasonally varying percentile
threshold on a fixed baseline and track their vertical propagation
across the upper 500 m. We find that nearly a third (∼ 29 %) of the
MHWs extend beyond the surface mixed layer depth (MLD). On average,
these deep-reaching MHWs (dMHWs) extend to 110 m below the MLD and last
five times longer than MHWs that are confined to the mixed layer (184
vs. 36 days). The dMHWs can cause stronger temperature anomalies at
depth than at the surface (maximum intensity of 5.0°C vs. 1.9°C). This
general subsurface MHW intensification even holds when scaling the
temperatures with the respective local variability. A clustering of
dMHWs reveals that 41 % of them are block-like, i.e., continually
remain in contact with the sea surface, 24 % propagate downward, 20 %
propagate upward, while 15 % appear at the surface multiple times.
Although the water column MHW duration, intensity and severity are only
moderately correlated with their corresponding surface-based MHW
characteristics, dMHWs have the potential to be detected from the
surface. Our study can help to augment the remote sensing-based
monitoring of upper ocean exposure to MHWs.