by Judith Curry
So, exactly how are the oceans sequestering heat below 700 m? And how might this heat return to the surface to impact the surface climate?
In a recent thread Has Trenberth found the missing heat?, we discussed the Balmaseda et al paper, which concluded:
In the last decade, about 30% of the warming has occurred below 700 m, contributing significantly to an acceleration of the warming trend. The warming below 700 m remains even when the Argo observing system is withdrawn although the trends are reduced. Sensitivity experiments illustrate that surface wind variability is largely responsible for the changing ocean heat vertical distribution.
The physical mechanisms whereby heating from the atmosphere is sequestered below 700 m are not obvious.
A useful summary of the role of the oceans in climate variability and change is provided by the draft U.S. CLIVAR Science Plan:
Understanding what processes are critical to climate variability and change in the ocean is of importance for several reasons. Primary among these is that the first-order physical processes need to be correctly represented in climate models for improved simulation of ocean climate variability. Various physical processes determine the climate variability of oceans over different space and time scales. Examples of such processes, include ocean mixing; wind driven ocean circulation; heat and freshwater fluxes at the interface of ocean with atmosphere and sea ice that control buoyancy fluctuations and buoyancy driven ocean circulation; penetration of shortwave radiative fluxes and interactions with biological processes in the upper oceans; influence of continental shelves on boundary currents etc.
The fundamental physical processes that influence the ocean, in turn, determine the modes of ocean variability on various space and time scale. Oceanic eddies and waves dominate this variability on time scales of days to months and can be thought of as the oceanic counterpart of weather in the atmosphere. There are various physical mechanisms that lead to generation of ocean eddies – sudden changes in the direction of surface winds and their speed; dynamical instabilities associated with the ocean thermal fronts; interactions between oceanic flows and bottom topography. However such mechanisms are poorly understood.
Due to their ability to exchange energy with the large-scale oceanic state, mesoscale eddies can influence variability in the oceanic circulation and stratification on the variety of time scales. In the Southern Ocean, eddies may also transport heat, salt and biogeochemical tracers (such as carbon) poleward, and therefore, play a key role in the oceanic uptake of heat and carbon. Most climate models lack the ability to resolve ocean eddies and must rely on empirically derived parameterization schemes. Despite advances in the development of such schemes, the inability of models to resolve the mesoscale eddies still counts as a major source of uncertainty in climate simulations. On even smaller scales, sub-mesoscale currents are potentially equally important for the ocean variability, but are even less well understood. Finally, diapycnal mixing associated with breaking of internal gravity waves in the interior and near rough topography likely plays a critical role in the transformation of water properties and also in the dynamics of long space and time scale processes such as Atlantic Meridional Overturning circulation (AMOC).
On seasonal to interannual time scales, modes of ocean variability, such as El Niño – Southern Oscillation (ENSO) and the Atlantic Meridional Mode (AMM), dominate. Ocean variability on this time-scale is governed by dynamical processes in the ocean, and coupled air-sea interaction, including changes in surface winds. Although important advances in understanding the physical mechanisms of these modes have been made, our understanding is not complete and further research is needed.
On longer time scales, variability in the North Pacific and Atlantic has considerable fluctuations in decadal frequencies, and is often referred to as the Pacific Decadal Variability (PDV) and Atlantic Multidecadal Variability (AMV), respectively. Various mechanisms leading to decadal variability in the oceans have been posited, but remain poorly understood. Basic science questions on the role of stochastic atmospheric forcing; role of tropical-extratropical interactions and coupling with the atmosphere; role of interactions between large-scale circulation, mesoscale eddies and slowly propagating Rossby waves in governing ocean variability on decadal time scales remain an area of active research.
On centennial time scales, ocean variability is dominated by the global thermohaline circulation that extends throughout the water column, with one example being the AMOC. On these time scales the buoyancy driven ocean circulation is of fundamental importance. Although the difference in ocean density determined by surface buoyancy exchanges in the high latitudes is the primary factor leading to the thermohaline circulation, oceanic processes such as mixing on much shorter time and space scales are also believed to play a fundamental role.
Interactions across modes of variability can also occur over different time and spatial scales that modulate the full range of ocean climate variability. In the Indian Ocean variability associated with ENSO and the Indian Ocean Dipole influences the interannual variability of the MJO. ENSO variability in the tropical Pacific also has low-frequency modulation, with some epochs having larger variability compared to other epochs, and models disagree on the dominant mechanisms. Higher frequency variations associated with tropical instability waves in the equatorial eastern Pacific has been hypothesized to affect ENSO variability and prediction on the seasonal time scale, and have been proposed as a potential mechanism for asymmetry in the amplitude of warm and cold ENSO events. Interactions between modes of oceanic variability across different time and spatial scales, along with the mechanisms that govern such interactions, are currently not well understood.
For background on some of these physical processes, I recommend Chapter 11 of my text Thermodynamics of Atmospheres and Oceans, which is posted [Ch 11] [chapter11 figs]. The table of contents for this chapter are:
Chapter 11 Thermohaline Processes in the Ocean
- 11.1 Radiative Transfer in the Ocean
- 11.2 Ocean Surface Layer
- 11.3 Surface Density Changes and the Ocean Mixed Layer
- 11.4 Instability and Mixing in the Ocean Interior
- 11.5 Deep Water Formation
- 11.6 Global Thermohaline Circulations
It is rather easy to imagine the heat transfer processes in the upper 700 m, see this recent paper published in Nature Climate Change:
Retrospective prediction of the global warming slowdown in the past decade
VIrginie Guemas, Francisco Doblas-Reyes, Isabel Andreu-Burillo
Abstract. Despite a sustained production of anthropogenic greenhouse gases, the Earth’s mean near-surface temperature paused its rise during the 2000–2010 period1. To explain such a pause, an increase in ocean heat uptake below the superficial ocean layer2, 3 has been proposed to overcompensate for the Earth’s heat storage. Contributions have also been suggested from the deep prolonged solar minimum4, the stratospheric water vapour5, the stratospheric6 and tropospheric aerosols7. However, a robust attribution of this warming slowdown has not been achievable up to now. Here we show successful retrospective predictions of this warming slowdown up to 5 years ahead, the analysis of which allows us to attribute the onset of this slowdown to an increase in ocean heat uptake. Sensitivity experiments accounting only for the external radiative forcings do not reproduce the slowdown. The top-of-atmosphere net energy input remained in the [0.5–1] W m−2 interval during the past decade, which is successfully captured by our predictions. Most of this excess energy was absorbed in the top 700 m of the ocean at the onset of the warming pause, 65% of it in the tropical Pacific and Atlantic oceans. Our results hence point at the key role of the ocean heat uptake in the recent warming slowdown. The ability to predict retrospectively this slowdown not only strengthens our confidence in the robustness of our climate models, but also enhances the socio-economic relevance of operational decadal climate predictions.
However, how to sequester heat below 700 m is not obvious and ocean circulations seem to be required, since turbulent mixing doesn’t occur much below 700 m and mixing in the ocean interior is a slow process. On such short time scales as evident in figure 1 of the Balmasda et al paper, it seems that mesoscale eddies rather than the large-scale organized circulations would need to be the mechanism for the heat transfer to deep layers. I haven’t looked at the ocean reanalysis data in any detail; there may be regional clues as to how this heat sequestration is occurring.
And then assuming that the heat storage in the deep ocean occurs as described by Balmaseda et al., here is what Kevin Trenberth thinks will happen.
From a National Science Foundation article on April 15th, 2010:
“The heat will come back to haunt us sooner or later,” says NCAR scientist Kevin Trenberth, the lead author. “The reprieve we’ve had from warming temperatures in the last few years will not continue. It is critical to track the build-up of energy in our climate system so we can understand what is happening and predict our future climate.”
If the heat is well mixed in the deep ocean below 700 m, exactly how could that heat return to the surface? The second law of thermodynamics suggests that a well mixed heat reservoir in the deep ocean would actually be very inefficient at returning heat to the surface.
We need to understand how the ocean exchanges heat vertically, between the upper ocean and deep ocean, and whether mixing in the deep ocean is more efficient than currently thought. Until we understand this, we won’t know to what extent this heat will remain sequestered in the deep ocean.