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

Abstract

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 quantify 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). Uncertainty due to internal variability is used as a proxy to quantify the time scales of ocean temperature adjustment at different depths and basins in Community Earth System Model. Time scales of 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.

A New Ocean State After Nuclear War

“…Vertical distributions of physical and biogeochemical tracers take hundreds to thousands of years to achieve steady‐state for a given climate state (DeVries & Primeau, 2011; Sarmiento & Gruber, 2006). Ocean temperature and density perturbation adjustment timescales vary by depth and ocean basin, and are a function of the ocean's internal variability and key circulation components such as MOC, which are in turn affected by climate perturbations (Hogan & Sriver, 2019). The ocean response to such perturbations is asymmetric, with a generally faster response to cooling than to warming due to cooling‐enhanced convection and overturning (Yang & Zhu, 2011), as seen in our simulations.…”

Internal and forced ocean variability in the Mediterranean Sea

“…Moreover, they showed that in several areas the internal ocean variability is more significant than atmospheric variability as a contribution to climate variability for both "the low-frequency variability and the long-term trends of regional ocean heat content". Similarly, in the study by Hogan and Sriver (2019) it was shown that internal variability is fundamental in setting the time scale for the ocean temperature adjustment process, increasing the speed at which the ocean takes up heat from the atmosphere, that is instead highly underestimated by just considering atmospheric variability. Lastly, as regards the scale dependency of the internal variability, it was demonstrated by Tang et al (2019) that by increasing the horizontal spatial resolution of ocean models more intrinsic variability is produced and, similarly, by analyzing the ratio of the externally forced over the internally generated variability (Tang et al (2020)) in the South China Sea, the external forcing is dominant for large scales, while most of the variability is internally generated for small scales.…”