The 2019 marine heatwave at Ocean Station Papa

(Adapted from Kohlman et al., 2024)

Marine Heat Waves (MHW; Hobday et al., 2016) are prolonged periods of extreme ocean sea surface temperature (SST) anomalies. MHWs are typically identified in satellite SST and/or ocean color records; subsurface data and interdisciplinary variables are often lacking. Ocean Station Papa (OSP) provided long-term, interdisciplinary, subsurface data to examine the physical and biochemical characteristics of a MHW in the Northeast Pacific.

MHWs may have multi-faceted causes, as well as impacts on primary production and higher trophic levels. A recent paper by Kohlman et al. (2024) examines the 2019 NE Pacific MHW using gridded satellite SST data and in-situ observations from multiple OSP platforms including the NOAA Pacific Marine Environmental Lab (PMEL) surface mooring, two OOI Flanking Moorings, the Applied Physics Lab (U. Washington) waverider mooring and shipboard samples from OOI and the Department of Fisheries and Oceans, Canada.

The 2019 MHW was identified in satellite SST data, but the Kohlman et al. study also assessed vertical stratification and the subsurface extent of the temperature signal. The PMEL surface mooring provided temperature and salinity (T/S) down to 300 m. The OOI Flanking Moorings extended the T/S data to 1500 m. The resulting composite time series from 2013-2020 is shown in Fig. 1. Both the extended 2013-2015 MHW and the 2019 MHW are identifiable. Subsurface temperature anomalies during 2013-2014 were strongest above the mixed layer depth (MLD). In the winter and spring of 2017, deeper waters (120–300 m) remained anomalously warm. This anomaly persisted into 2018 due to strong stratification from a fresher surface layer. During the 2019 MHW, anomalously warm waters extended down to 1000 m, whereas the 2013-2015 MHW extended only to about 150 m.

The authors used interdisciplinary data available from Station Papa platforms to assess the drivers and impacts of the 2019 MHW. They found that weaker winds and smaller significant wave height prior to the summer of 2019 created favorable pre-conditioning in the form of an unusually shallow winter MLD. During the MHW, they found that dissolved inorganic carbon and pCO2 decreased, while pH increased. Shipboard samples indicated a decrease in nutrients and an increase in primary productivity. Finally, they speculated that the increased productivity may have had an impact on higher trophic levels – more blue whale calls were recorded in 2019 at Station Papa than normal for Aug-Sep.

This project shows that the characteristics of MHWs are complex. Sustained, multi-disciplinary, subsurface observations are needed to unravel the drivers, pre-conditioning, characteristics, and impacts of these events. Station Papa, among the longest sustained ocean time series sites, is uniquely suited due to the task due to the collaborative observing effort at the site.

Figure 1. Subsurface temperature anomalies at Staton Papa during 2013-2020. Data from the surface to 300 m are from the PMEL surface mooring. Data below 300 m are from the OOI Flanking Moorings. Anomalies are relative to the 1999-2020 Argo climatology. The density-based mixed layer depth (black) and isothermal depth (purple) are overlaid. From Kohlman et al., 2024.

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References:

Hobday, A.J., Alexander, L.V., Perkins, S.E., Smale, D.A., Straub, S.C., Oliver, E.C.J., et al. (2016). A hierarchical approach to defining marine heatwaves. Prog. Oceanog., 141(0079–6611), 227–238. https://doi.org/10.1016/j.pocean.2015.12.014.

Kohlman, C., Cronin, M.F., Dziak, R., Mellinger, D.K., Sutton, A., Galbraith, M., et al. (2024). The 2019 marine heatwave at Ocean Station Papa: A multi‐disciplinary assessment of ocean conditions and impacts on marine ecosystems. J. Geophys. Res., 129, e2023JC020167. https://doi.org/10.1029/2023JC020167.