Two Flavours of Ocean Temperature Change and the Implication for Reconstructing the Historical past of Ocean Warming

Introducing Extra and Redistributed Temperatures. 

By: Quran Wu

Monitoring and understanding ocean warmth content material change is a vital activity of local weather science as a result of the ocean shops over 90% of additional warmth that’s trapped within the Earth system. Ocean warming leads to sea-level rise which is likely one of the most extreme penalties of anthropogenic local weather change.

Ocean warming beneath greenhouse fuel forcing is commonly regarded as further warmth being added to the ocean floor by greenhouse warming after which carried to depths by ocean circulation. This one-way warmth transport diagram assumes that every one subsurface temperature modifications are because of the propagation of floor temperature modifications, and is extensively used to assemble conceptual fashions of ocean warmth uptake (for instance, the two-layer mannequin in Gregory 2000).

Current research, nonetheless, have discovered that ocean temperature change beneath greenhouse warming can be affected by a redistribution of the unique temperature discipline (Gregory et al. 2016). The ocean temperature change because of the redistribution is known as redistributed temperature change, whereas that on account of propagation of floor warming is known as extra temperature change.

A Dye Analogy

To assist clarify the separation of extra and redistributed temperature, allow us to contemplate a dye analogy. Heating the ocean from the floor is like including a drop of dye right into a glass of water that already has a non-uniform distribution of the identical dye. After the dye injection, two issues occur concurrently. First, the newly-added dye progressively spreads into the water within the glass (extra temperature). Second, the dye injection disturbs the water and causes water movement that rearranges the unique dye (redistributed temperature). Each processes contribute to modifications in dye concentrations.

Local weather Mannequin Simulation

Determine 1: Time evolution of global-mean ocean temperature change (in Kelvin) beneath rising greenhouse fuel emission in a local weather mannequin simulation (a). Change in (a) is decomposed into extra temperature change (b) and redistributed temperature change (c).

Extra and redistributed temperatures are each derived from thought experiments; neither of them will be immediately noticed in the actual world. Right here, we show their behaviours utilizing a local weather mannequin simulation beneath rising greenhouse fuel emission. The simulation exhibits that ocean warming begins from the floor, and propagates into depths progressively, reaching 500 m after 50 years (Determine 1a). The ocean warming is generally pushed by extra temperature change (examine Figures 1a with 1b) however strongly disrupted by a downward warmth redistribution close to the floor (cooling on the floor and warming beneath) (Determine 1c). The downward warmth redistribution is attributable to a discount of ocean convection (which pumps warmth upward), as a result of floor warming stabilises water columns.

Implications

Distinguishing extra from redistributed temperature change is necessary as a result of they behave in numerous methods. Whereas one can reconstruct extra temperature at depths by propagating its floor change utilizing ocean transports, the identical can’t be executed with redistributed temperature. It is because temperature redistribution can probably occur anyplace within the ocean, in contrast to further warmth, which might solely enter the ocean from the floor (beneath greenhouse warming). Such a distinction has necessary implications for estimating the historical past of ocean warming from floor observations.

Ocean warming is historically estimated by interpolating in-situ temperature measurements, which have been gathered in discrete areas and occasions, to the worldwide ocean. This in-situ methodology suffers a big uncertainty as a result of the ocean stays poorly sampled till the worldwide deployment of Argo floats (a fleet of robotic devices) in 2005.

A brand new strategy to estimate ocean warming is to propagate its floor signature, that’s sea floor temperature change, downward utilizing info of ocean transports (Zanna et al. 2019). This transport methodology is beneficial as a result of it depends on floor observations, which have an extended historic protection than subsurface observations. Nonetheless, this methodology ignores the truth that a part of floor temperature change is because of temperature redistribution, which doesn’t correspond to subsurface temperature change. In a pc simulation of the historic ocean, we discovered that propagating sea floor temperature change leads to an underestimate of simulated ocean warming on account of redistributive cooling on the floor (as proven in Determine 1c) (Wu and Gregory 2022). This outcome highlights the necessity for isolating extra temperature change from floor observations when making use of the transport methodology to reconstruct ocean warming.

Acknowledgements

Due to Jonathan Gregory for studying an early model of this text and offering helpful feedback and solutions.

References:

Gregory, J. M., 2000: Vertical warmth transports within the ocean and their impact on time-dependent local weather change. Local weather Dynamics, 16, 501–515, https://doi.org/10.1007/s003820000059.

Gregory, J. M., and Coauthors, 2016: The Flux-Anomaly-Compelled Mannequin Intercomparison Undertaking (FAFMIP) contribution to CMIP6: investigation of sea-level and ocean local weather change in response to CO2 forcing. Geoscientific Mannequin Improvement, 9, 3993–4017, https://doi.org/10.5194/gmd-9-3993-2016.

Wu, Q., and J. M. Gregory, 2022: Estimating ocean warmth uptake utilizing boundary Inexperienced’s capabilities: An ideal‐mannequin take a look at of the strategy. Journal of Advances in Modeling Earth Programs, 14, https://doi.org/10.1029/2022MS002999.

Zanna, L., S. Khatiwala, J. M. Gregory, J. Ison, and P. Heimbach, 2019: World reconstruction of historic ocean warmth storage and transport. Proceedings of the Nationwide Academy of Sciences, 116, 1126–1131, https://doi.org/10.1073/pnas.1808838115.