Vertical Reach and Life-Cycle Process Exposures of Marine Heatwaves and Cold Spells in the East Sea

Marine heatwaves (MHWs) and cold spells (MCSs) are catalogued at the sea surface, yet their vertical extent shapes ecological exposure and the processes they experience. We present a surface-anchored vertical-reach census of detrended MHWs and MCSs in the East Sea (Japan Sea, 34–45°N) from daily GLORYS12 reanalysis (1993–2022, 29 levels to 318 m). Objects are tracked independently at every level, and each surface (0.49 m) object is characterised by the depth to which the same-sign extreme is continuous from the surface. With the 90th percentile of this depth, 42% (49%) of the 3,169 (3,306) surface MHW (MCS) objects are shallow-linked (< 50 m), 17% (21%) transitional and 39% (28%) deep-reaching (> 100 m). Deep-reaching objects peak in winter, last 1.6 times longer and cover 2.3 times the area of shallow-linked objects, and at 50–110 m the water was extreme on the surface-run start day at 42–67% of their run starts (37–42% before it), against 14–16% (12–14%) for shallow-linked ones. Below 100 m, 59–78% of subsurface objects have no continuous column to the surface and 43–56% no surface extreme above them. Reference-invariant advective diagnostics over the concurrent mixed layer show that deep-reaching objects grow under weaker amplifying surface-flux anomalies than shallowlinked ones (−0.037 and −0.040 K day−1 for MHWs and MCSs; 6 versus 26 and 13 versus 34 W m−2), under eight reach rules and after duration stratification and covariate adjustment. The mean-flow advection contrast was consistently positive for MHWs and positive only after covariate adjustment for MCSs.