Vertical Redistribution of Oceanic Heat Content

Abstract

Estimated values of recent oceanic heat uptake are on the order of a few tenths of a W m 22 , and are a very small residual of air-sea exchanges, with annual average regional magnitudes of hundreds of W m 22 . Using a dynamically consistent state estimate, the redistribution of heat within the ocean is calculated over a 20-yr period. The 20-yr mean vertical heat flux shows strong variations in both the lateral and vertical directions, consistent with the ocean being a dynamically active and spatially complex heat exchanger. Between mixing and advection, the two processes determining the vertical heat transport in the deep ocean, advection plays a more important role in setting the spatial patterns of vertical heat exchange and its temporal variations. The global integral of vertical heat flux shows an upward heat transport in the deep ocean, suggesting a cooling trend in the deep ocean. These results support an inference that the near-surface thermal properties of the ocean are a consequence, at least in part, of internal redistributions of heat, some of which must reflect water that has undergone long trajectories since last exposure to the atmosphere. The small residual heat exchange with the atmosphere today is unlikely to represent the interaction with an ocean that was in thermal equilibrium at the start of global warming. An analogy is drawn with carbon-14 ''reservoir ages,'' which range from over hundreds to a thousand years.

Nonlinearities in patterns of long‐term ocean warming

“…On century time scales, for very low forced scenarios, even negative deep ocean temperature trends are possible (Figure e, blue lines). This is consistent with slightly decreasing trends in recent decadal observations [ Wunsch and Heimbach , ; Llovel et al , ; Liang et al , ]. Locally, slightly negative and positive trends can occur after the upper 2000 m are fully equilibrated.…”

“…Thermal expansion of ocean waters due to heat uptake from the atmosphere is a large contributor to recent and near‐future sea level rise [ Church et al , , ; Levermann et al , ]. General circulation models (GCM) differ in the amplitude of simulated thermal expansion due to different base states, the total amount and vertical extent of the heat uptake, heat redistribution, and differences in the representation of vertical heat transport processes, advection, isopycnal, and diapycnal mixing [ Gregory , ; Kuhlbrodt and Gregory , ; Hallberg et al , ; Church et al , ; Exarchou et al , ; Melet and Meyssignac , ; Liang et al , ]. Figures a–c show zonal averaged ocean temperature anomaly patterns at the end of the century for three different scenarios simulated by the Coupled Model Intercomparison Project Phase 5 (CMIP5) models.…”

Dependence of global radiative feedbacks on evolving patterns of surface heat fluxes

“…We use a 80 year long segment of a quasi-equilibrated coupled control simulation with preindustrial forcing to generate a climatological reference state Q-flux ( Figure 1d), which globally and annually averages to zero. The annual mean climatological Q-flux is dominated by tropical upwelling regions taking up heat and Western boundary currents, the Nordic seas, and the Southern Ocean releasing heat, and compares well with observations [Liang et al, 2015]. The reference state for all our experiments is this climatological Q-flux in an equilibrated climate with CO 2 levels of 4 times its preindustrial value, leading to 6.4 K warming compared to the control simulation.…”

“…Resulting follow-up questions include the following: Which local features and physical mechanisms of the heat uptake in which geographical combination are most efficient in changing the magnitude of the global feedback parameter and are we sure they will occur in reality and are not modeling artifacts [e.g., Zhang et al, 2010;Exarchou et al, 2014;Liang et al, 2015]? What is the critical size of a region with a certain ocean heat uptake to influence the magnitude of the global feedbacks [L'Hévéder et al, 2015;Kang and Xie, 2014, and Figures S9 and S10]?…”

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

“…Their variability on smaller spatial and shorter time scales is much larger. For instance, the 20-yr mean of the ECCO Q net estimates shows basin-scale variability that is above 200 W m 22 ; also, temporal variability in many regions, such as the major western boundary currents, is up to 200 W m 22 as well (e.g., Liang et al 2015). This implies that the Q net response to climate change and climate variability could be strongly localized.…”