Downscaled climate projections of future mesopelagic habitat in the California Current Ecosystem

Although the mesopelagic zone occupies a substantial volume of the world's oceans, our results suggest that the livable portion may compress vertically by ~40 m or ~39% by the end of the century. Using an ensemble of three downscaled climate projections from a high emissions scenario, we evaluated the connection between anthropogenic greenhouse gas emissions and changes in light and oxygen at depth, which influence the upper and lower limits of mesopelagic habitat in the central California Current. Although the model projects a small deepening (~ 2 m) of the upper light boundary consistent with increased stratification and reduced upper ocean productivity, the main driver of vertical mesopelagic habitat compression is the significant shoaling (by ~44 m) of the hypoxic boundary over the course of the 21st century. Differences in dissolved oxygen across ensemble members highlight the potential influence of equatorial dynamics and the California Undercurrent in constraining the future availability of mesopelagic habitat along the U.S. West Coast. Mesopelagic ecosystems connect the surface ocean to the deep sea, and a projected decrease in the vertical extent of mesopelagic habitat could have cascading effects on a broader range of marine ecosystem processes and carbon export.

Methods
Downscaled Regional Climate Projections

We use an ensemble of high-resolution regional climate projections representing three different rates of warming under the RCP 8.5 high emissions scenario for the period 2000-2100. The earth system model solutions are first downscaled to 1/10° (~10 km) resolution for the broader California Current (30-48 °N)1,2 and subsequently nested at 1/30° (~3 km) resolution for the central California Current (32-44 °N) to improve the representation of local scale coastal upwelling processes3. In this approach, the 1/10° downscaled projections are forced directly by the CMIP5 earth system models using a time-varying delta method, and the high-resolution nested projections are subsequently forced by the downscaled projections using an offline nesting method (Fig. 1). The three earth system model solutions selected here are GFDL-ESM2M, IPSLCM5, and Hadley-GEM2-E as they include marine biogeochemical fields and represent the spread of the CMIP5 ensemble (GFDL-ESM2M = low rate of warming; IPSLCM5 = rate of warming close to ensemble mean; Hadley-GEM2-E = high rate of warming). While the RCP8.5 scenario may arguably depict an “unrealistically” warm future, we consider it useful here for two reasons: (1) it allowed exploring changes in mesopelagic habitat properties under extreme warming (e.g., identify potential thresholds), and (2) the lowest rate of warming (GFDL-ESM2M) is representative of the high end of the more moderate RCP4.5 scenario.

The physical and biogeochemical fields for both downscaled (1/10°) and nested (1/30°) projections are generated using an implementation of the Regional Ocean Modeling System (ROMS)4,5 for the California Current System coupled to NEMUCSC, a customized version of the North Pacific Ecosystem Model for Understanding Regional Oceanography (NEMURO)6. NEMUCSC includes three limiting macronutrients (nitrate, ammonium, and silicic acid), two phytoplankton functional groups (nanophytoplankton and diatoms), three zooplankton size-classes (microzooplankton, copepods, and euphausiids), three detritus pools (dissolved and particulate organic nitrogen and particulate silica), as well as carbon and oxygen cycling7,8. The three high-resolution nested projections of the coupled ROMS-NEMUCSC model are hereafter referred to as “ROMS-GFDL”, “ROMS-IPSL”, and “ROMS-HADL”.

References

1.         Pozo Buil, M. et al. A Dynamically Downscaled Ensemble of Future Projections for the California Current System. Front. Mar. Sci. 8, 612874 (2021).

2.         Fiechter, J., Pozo Buil, M., Jacox, M. G., Alexander, M. A. & Rose, K. A. Projected Shifts in 21st Century Sardine Distribution and Catch in the California Current. Front. Mar. Sci. 8, 685241 (2021).

3.         Fiechter, J., Edwards, C. A. & Moore, A. M. Wind, Circulation, and Topographic Effects on Alongshore Phytoplankton Variability in the California Current. Geophys. Res. Lett. 45, 3238–3245 (2018).

4.         Haidvogel, D. B. et al. Ocean forecasting in terrain-following coordinates: Formulation and skill assessment of the Regional Ocean Modeling System. J. Comput. Phys. 227, 3595–3624 (2008).

5.         Shchepetkin, A. F. & McWilliams, J. C. The regional oceanic modeling system (ROMS): a split-explicit, free-surface, topography-following-coordinate oceanic model. Ocean Model. 9, 347–404 (2005).

6.         Kishi, M. J. et al. NEMURO—a lower trophic level model for the North Pacific marine ecosystem. Ecol. Model. 202, 12–25 (2007).

7.         Fiechter, J., Santora, J. A., Chavez, F., Northcott, D. & Messié, M. Krill Hotspot Formation and Phenology in the California Current Ecosystem. Geophys. Res. Lett. 47, e2020GL088039 (2020).

8.         Cheresh, J. & Fiechter, J. Physical and Biogeochemical Drivers of Alongshore pH and Oxygen Variability in the California Current System. Geophys. Res. Lett. 47, e2020GL089553 (2020).