26 June 2019

The Effect of Internal Variability on Ocean Temperature Adjustment in a Low-Resolution CESM Initial Condition Ensemble

Ocean temperature adjustment is analyzed in large CESM ensembles with different representations of internal climate variability.

Science

Due to its large heat capacity and circulation, the ocean contributes significantly to global heat uptake, global heat transport, spatial temperature patterns, and variability. Quantifying ocean heat uptake across different temporal and spatial scales is important to understand Earth's climate response to anthropogenic warming.

Here we evaluate ocean adjustment time scales from two different fully coupled climate model ensembles using the Community Earth System Model. Both ensembles use the same model version, anthropogenic and natural forcings, and coupling configurations, but we initialize the ensembles in two different ways: (1) sampling joint internal variability of the ocean–atmosphere system (unique atmosphere and ocean conditions) and (2) sampling the internal variability of the atmosphere only (unique atmosphere, identical ocean conditions).

Impact

We find that internal variability within the ocean plays a significant role in how fast the global ocean takes up heat from the atmosphere, and global ocean temperature adjustment can take hundreds of years. The variability and model responses are represented differently in CESM depending on how the model is initialized.  The ensemble sampling only the atmospheric initial conditions under-represents the internal ocean variability, which in turn slows down the temperature evolution of the global ocean. Results have important implications for quantifying model drift, biases, and uncertainties in climate model ensemble projections.

Summary

This study quantifies the effect of internal variability on ocean temperature adjustment under anthropogenic global warming in 2 different CESM ensembles with different initialization techniques. Time scales of temperature equilibration are longer in the deep ocean than the upper ocean, highlighting the vertical structure of dynamic adjustment. The Atlantic equilibrates on shorter time scales (82 years above 1,000 m, 140 years below 1,000 m) relative to the Pacific (106 years above 1,000 m, 444 years below 1,000 m) in Community Earth System Model due to the large North Atlantic Deep Water formation and strong overturning circulation in the Atlantic. These results have broad implications for analyzing internal climate variability, ocean adjustment, and drift in global coupled model experiments and intercomparisons.

Contact
John Weyant
Stanford University
Publications
Hogan, E, and RL Sriver.  2019.  "The Effect of Internal Variability on Ocean Temperature Adjustment in a Low‐Resolution CESM Initial Condition Ensemble."  Journal of Geophysical Research: Oceans 124(2): 1063-1073.  https://doi.org/10.1029/2018jc014535.