Variability of ocean heat uptake: Reconciling observations and models

Abstract

[1] This study examines the temporal variability of ocean heat uptake in observations and in climate models. Previous work suggests that coupled Atmosphere-Ocean General Circulation Models (A-OGCMs) may have underestimated the observed natural variability of ocean heat content, particularly on decadal and longer timescales. To address this issue, we rely on observed estimates of heat content from the 2004 World Ocean Atlas (available at http://www.nodc.noaa.gov/OC5/indprod.html, hereinafter referred to as WOA-2004) compiled by Levitus et al., 2005. Given information about the distribution of observations in WOA-2004, we evaluate the effects of sparse observational coverage and the infilling that Levitus et al. use to produce the spatially complete temperature fields required to compute heat content variations. We first show that in ocean basins with limited observational coverage, there are important differences between ocean temperature variability estimated from observed and infilled portions of the basin. We then employ data from control simulations performed with eight different A-OGCMs as a test bed for studying the effects of sparse, space-varying and time-varying observational coverage. Subsampling model data with actual observational coverage has a large impact on the inferred temperature variability in the top 300 and 3000 m of the ocean. This arises from changes in both sampling depth and in the geographical areas sampled. Our results illustrate that subsampling model data at the locations of available observations increases the variability, reducing the discrepancy between models and observations.

Complementary observational constraints on climate sensitivity

“…In our analysis, cutting off the heavy tail required (i) neglecting model structural uncertainty, (ii) a reduction of the aerosol forcing uncertainty to negligible levels, and (iii) reduced uncertainty in oceanic heat uptake. Our analysis suggests that one promising avenue to decide whether the true climate sensitivity is indeed located in the heavy upper tail of current estimates is through improving the skill of the existing ocean observation system to estimate the anthropogenic heat uptake [AchutaRao et al, 2006;Levitus et al, 2005]. The skewness of the climate sensitivity estimate in our analysis hinges critically on the observation error of the oceanic heat content observations: reducing the uncertainty about the oceanic heat uptake shortens the upper tail of the resulting climate sensitivity estimates (Figure 2e).…”

Decadal Trends in Sea Level Patterns: 1993–2004

“…and AchutaRao et al (2006) instead sampled models to mimic the Levitus et al (2001) calculation and found large discrepancies between global and hemispheric averages, including, in the former, sign reversals [somewhat in conflict with the results of Barnett et al (2001)]. Harrison and Carson (2007) discuss the great difficulties in inferring global upper ocean averages from the data after 1950 and call attention to the need for far better understanding of the accuracies of quoted subglobal averages.…”

Patrones históricos y escenarios térmicos futuros en mares mexicanos

“…Según AchutaRao et al 2006, los cambios en la capacidad del océano para absorber calor son una fuente de incertidumbre en las predicciones de modelos acoplados (atmósfera-océano), que se vuelve particularmente relevante en escalas decadales o mayores. En general, la importancia de la variabilidad interna se incrementa conforme la escala espacial y temporal se reduce (Hawkins & Sutton 2009).…”