Variations of the Global Net Air–Sea Heat Flux during the “Hiatus” Period (2001–10)

Abstract

An assessment is made of the mean and variability of the net air-sea heat flux, Q net , from four products (ECCO, OAFlux-CERES, ERA-Interim, and NCEP1) over the global ice-free ocean from January 2001 to December 2010. For the 10-yr ''hiatus'' period, all products agree on an overall net heat gain over the global ice-free ocean, but the magnitude varies from 1.7 to 9.5 W m 22 . The differences among products are particularly large in the Southern Ocean, where they cannot even agree on whether the region gains or loses heat on the annual mean basis. Decadal trends of Q net differ significantly between products. ECCO and OAFlux-CERES show almost no trend, whereas ERA-Interim suggests a downward trend and NCEP1 shows an upward trend. Therefore, numerical simulations utilizing different surface flux forcing products will likely produce diverged trends of the ocean heat content during this period. The downward trend in ERA-Interim started from 2006, driven by a peculiar pattern change in the tropical regions. ECCO, which used ERAInterim as initial surface forcings and is constrained by ocean dynamics and ocean observations, corrected the pattern. Among the four products, ECCO and OAFlux-CERES show great similarities in the examined spatial and temporal patterns. Given that the two estimates were obtained using different approaches and based on largely independent observations, these similarities are encouraging and instructive. It is more likely that the global net air-sea heat flux does not change much during the so-called hiatus period.

Change of the Global Ocean Vertical Heat Transport over 1993–2010

“…Thus, the ECCO v4r1 estimate of Q net can be considered as an adjusted ERA-Interim estimate that is constrained by ocean dynamics and observations. Intercomparison with other leading flux products (Liang and Yu 2016) shows that ECCO v4r1 corrected a suspicious long-term trend in the ERA-Interim Q net and displayed encouraging agreement with the OAFlux/ CERES product (e.g., Yu and Weller 2007). For a detailed description and validation of Q net , see Liang and Yu (2016).…”

“…Our analyses will therefore focus on the change in the ocean vertical heat transport. This is a follow-up study of Wunsch and Heimbach (2014), in which ocean heat content changes were presented as a function of depth, and of Liang et al (2015), in which only the time mean of ocean vertical heat flux was discussed. This present study is a first step at analyzing decadal changes with ECCO estimates, and motivated by the need to clarify the extent to which the slowdown in surface warming in the 2000s is compensated by an increase in interior ocean heating.…”

“…2c,d) not only in the steady state as suggested in Liang et al (2015) but also in accounting for the temporal variations of vertical heat exchange. Note that dH d is generally smaller than dH a (note the difference in color scale in Fig.…”

“…The contribution of isopycnal mixing to vertical heat transport (rhs1) in the ocean occurs because vertical heat flux is calculated at distinct depth levels, whereas isopycnal mixing acts along tilted isopycnals. This feature can cause heat to diffuse upward in the ocean (e.g., Gregory 2000;Liang et al 2015) and turns out to be particularly important in interpreting the change of the vertical diffusive heat flux. In addition, the global mean of the vertical heat flux that is due to isopycnal mixing shows a subsurface maximum, being close to 1.5 W m 22 , in ECCO v4r1 around 500-m depth.…”

Mechanisms Controlling Global Mean Sea Surface Temperature Determined From a State Estimate

“…Recent comparisons of surface heat flux products (Liang & Yu, 2016) indicate the realism of our  estimates, but we are not aware of any published estimates of ,  for comparison with our results. Although the estimate tracks observed T variability within expected uncertainties, a legitimate question 10.1002/2017GL076821 is how the balances found in Ev4r3 might depend on assumed mixed layer physics and on other model details more generally.…”