Subsurface Evolution and Persistence of Marine Heatwaves in the Northeast Pacific

Abstract

The reappearance of a northeast Pacific marine heatwave (MHW) sounded alarms in late summer 2019 for a warming event on par with the 2013-2016 MHW known as The Blob. Despite these two events having similar magnitudes in surface warming, differences in seasonality and salinity distinguish their evolutions. We compare and contrast the ocean's role in the evolution and persistence of the 2013-2016 and 2019-2020 MHWs using mapped temperature and salinity data from Argo floats. An unusual near-surface freshwater anomaly in the Gulf of Alaska during 2019 increased the stability of the water column, preventing the MHW from penetrating deep as strongly as the 2013-2016 event. This freshwater anomaly likely contributed to the intensification of the MHW by increasing the near-surface buoyancy. The gradual buildup of subsurface heat content throughout 2020 in the region suggests the potential for persistent ecological impacts. Plain Language Summary Surface marine heatwaves (MHWs) are periods of prolonged and extremely warm regional sea surface temperature that can negatively impact the health and productivity of marine ecosystems. Using surface and subsurface ocean observations, we compare and contrast two recent MHWs to show that salinity variations play an important role in the vertical distribution of temperature anomalies by changing the overall stability of the water column. During the 2019-2020 MHW, the near-surface waters in the Gulf of Alaska were fresher than normal, preventing warm sea surface temperatures from mixing as deeply into the subsurface as in the 2013-2016 MHW. The freshening in 2019 likely enhanced warming in the buoyant surface layer. As warmer temperatures gradually mix downward they can persist long after the surface MHW disappears, suggesting that the ocean can provide memory for long-lived MHWs. The subsurface persistence of MHWs has potential ramifications for long-lasting ecological impacts.

Oceanic Circulation Drives the Deepest and Longest Marine Heatwaves in the East Australian Current System

“…The authors linked the deepest MHWs (800 m) to anticyclonic eddies in the Tasman Sea and found that they were more prominent in winter. Sub-surface warming has also been shown to linger well after the surface signal has disappeared, indicating that the ocean sub-surface maintains long-term temperature memory (Jackson et al, 2018;Scannell et al, 2020). These findings are consistent with the biological impacts observed from the surface down to benthic communities (Marzinelli et al, 2015;Short et al, 2015).…”

“…Although, Adv'-MHWs are not necessarily describing eddy structures, those instances agree with results from Elzahaby and Schaeffer (2019), which showed that the deepest MHWs were associated with ocean advection through anticyclonic eddy structures in a region similar to the Eddy box. While the link between deep MHWs and eddies could be explained by isopycnal heave (Scannell et al, 2020), preliminary analysis showed no clear pattern between Adv'-MHWs and heave. In fact, spice could be an important factor in the region.…”

“…In fact, spice could be an important factor in the region. More analysis is required to investigate the influence of isopycnal heave and water-mass variability on the sub-surface persistence and maximum MHW signals as identified by Scannell et al (2020) in the evolution of the 2008/09 Northeast Pacific MHW.…”

Deep Waters in British Columbia Mainland Fjords Show Rapid Warming and Deoxygenation From 1951 to 2020

“…Upwelling causes deep‐water renewal in all four inlets (Baker & Pond, 1995; Hodal, 2010; Lafond & Pickard, 1975; Wan et al., 2017). Upwelled water travels from the NE Pacific to Rivers Inlet along the 1,026 kgm −3 isopycnal (Hare et al., 2020) and both offshore and deep Rivers Inlet water were anomalously warm following the 2014–2016 marine heatwave (Jackson et al., 2018; Scannell et al., 2020), which accelerated the warming trend after 2016 (Figure 2a and Table 1). Rivers Inlet deep water is significantly denser than waters in Bute or Knight Inlet, yet the fact that some of the warmest deep waters in Bute and Knight Inlet were observed after 2017 suggests that, similar to Rivers Inlet, the source waters for Bute and Knight Inlet also warmed.…”

Forecasting Marine Heatwaves using Machine Learning

“…To classify MHWs, we used the definition presented in (Scannell et al, 2020) which defines MHW as an event when SST exceeds the monthly climatological 90th percentile for at least a month using monthly data from January 1986 to December 2020. To detect MHWs in the OISST dataset we used the Python package Ocetrac (Scannell et al, 2021).…”