Climate Model Used to Explore Possible Future Changes in the North Pacific Ocean

PIFSC scientists Jeffrey Polovina, Phoebe Woodworth, and Evan Howell, and their colleague John Dunne (NOAA Geophysical Fluid Dynamics Laboratory (GFDL) have used a complex computer model to explore possible future changes in the North Pacific ecosystem. They employed the NOAA GFDL Earth System Model (ESM2.1), which incorporates a biogeochemical model and an ocean phytoplankton model run under the A2 greenhouse gases forcing scenario. Output from the ESM2.1 model included a suite of physical, chemical, and phytoplankton variables, aggregated monthly from 1998-2100, with a spatial resolution ranging from 1 degree at high latitudes to 1/3 degree at the equator. The collaborators used model phytoplankton density to define subarctic, subtropical, and eastern tropical Pacific biomes. Then they examined how the spatial extent, phytoplankton density, phytoplankton species composition, and primary production of these biomes changed over time.

Sea surface temperature (SST) isotherms for 10°C, 20°C, and 30°C. Black lines represent projected mean position of isotherms during 1998-2017, while red lines represent their projected mean positions during 2080-2099. As surface waters warm, the 10°C and 20°C isotherms will shift northward, and the area of waters exceeding 30°C (between the 30°C isotherms) will expand to account for about half of the subtropics. Results were derived using the NOAA Geophysical Fluid Dynamics Laboratory Earth System Model ESM2.1.

A key model projection was that by the end of the 21st century, the subtropical biome will expand northward and southeastward, increasing in area by 29% at the expense of the subarctic and eastern tropical Pacific biomes, which would shrink by 29% and 27%, respectively, from their initial sizes in 1998. Further, with warming occurring within each biome, cooler thermal habitats would be predicted to shrink or vanish as isotherms move northward and warmer habitats expand or are created. For example, currently the subtropical SST ranges largely between 20°C and 30°C. By the end of the century, the area covered by sea surface waters 30°C or warmer is expected to account for about half of the subtropics. Since there currently is so little habitat with SST exceeding 30°C, it is not clear how this large increase in habitat with surface water warmer than 30°C will affect the subtropical gyre ecosystem.