Using Observations and Climate Models to Investigate Recent Variability in Upper Ocean Warming

Abstract:

The global ocean is responsible for storing more than 90% of the heat associated with observed greenhouse-gas-attributed warming. Our previous work used multiple in-situ estimates as well as altimetry in conjunction with a large suite of climate models to quantify how estimates of global upper-ocean warming since 1970 are likely biased low. This apparent underestimation, consistent with earlier studies, was attributed to poor observational coverage of the Southern Hemisphere.

Recent 2006-2013 Argo ocean heat content (OHC) change estimates have placed 67-98% of increased heat in the Southern Hemisphere. The new estimates contrast markedly to long-term evaluations that place only 35-49% of the warming since 1970 in the Southern Hemisphere – even though the Southern Hemisphere contains more than 60% of the upper ocean volume. Although unforced variability is likely to play a larger role on resolved trends over this shorter time period, results from climate models provide one mechanism for investigating the role of variability across time scales.

Extending our previous work, we investigate the hemispheric distribution of OHC using a number of observed change estimates along with a large suite of CMIP5 model simulations. We consider changes over decadal (Argo) to multi-decadal (multi-platform) time scales and contrast the observation-based hemispheric partitioning of OHC changes to a distribution of results from individual model simulations.