Bidecadal Thermal Changes in the Abyssal Ocean

Wunsch, Carl; Heimbach, Patrick

doi:10.1175/jpo-d-13-096.1

Cited by 52 publications

(53 citation statements)

References 37 publications

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“…Geothermal heating is not the source of abyssal warming in recent decades, with dominant terms being vertical mixing of warmer waters and advection via upwelling (e.g., Purkey and Johnson 2012). However, both mixing and geothermal heating vary spatially, and there are likely regions where the latter (despite acting on centennial time scales) is significant in the global heat budget (Purkey and Johnson 2012;de Lavergne et al 2016;Wunsch and Heimbach 2014).…”

Section: Discussionmentioning

confidence: 99%

“…On a regional scale, geothermal heating can change ocean bottom temperatures by an order of magnitude more than error estimates associated with decadal abyssal temperature trends (e.g., Emile-Geay and Madec 2009;Purkey and Johnson 2010;Kouketsu et al 2011;Wunsch and Heimbach 2014) and increase thermosteric sea level by 0.1-1 mm yr 21 (Piecuch et al 2015). Yet geothermal heating is represented inconsistently by ocean and fully coupled models.…”

Section: Introductionmentioning

confidence: 99%

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The Transient Response of Southern Ocean Circulation to Geothermal Heating in a Global Climate Model

et al. 2016

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Model and observational studies have concluded that geothermal heating significantly alters the global overturning circulation and the properties of the widely distributed Antarctic Bottom Water. Here two distinct geothermal heat flux datasets are tested under different experimental designs in a fully coupled model that mimics the control run of a typical Coupled Model Intercomparison Project (CMIP) climate model. The regional analysis herein reveals that bottom temperature and transport changes, due to the inclusion of geothermal heating, are propagated throughout the water column, most prominently in the Southern Ocean, with the background density structure and major circulation pathways acting as drivers of these changes. While geothermal heating enhances Southern Ocean abyssal overturning circulation by 20%-50%, upwelling of warmer deep waters and cooling of upper ocean waters within the Antarctic Circumpolar Current (ACC) region decrease its transport by 3-5 Sv (1 Sv 5 10 6 m 3 s 21). The transient responses in regional bottom temperature increases exceed 0.18C. The large-scale features that are shown to transport anomalies far from their geothermal source all exist in the Southern Ocean. Such features include steeply sloping isopycnals, weak abyssal stratification, voluminous southward flowing deep waters and exported bottom waters, the ACC, and the polar gyres. Recently the Southern Ocean has been identified as a prime region for deep ocean warming; geothermal heating should be included in climate models to ensure accurate representation of these abyssal temperature changes.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The Transient Response of Southern Ocean Circulation to Geothermal Heating in a Global Climate Model

et al. 2016

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“…Smith et al [54] extended these series back to 1960 and demonstrated an increasing ocean heat uptake, the impact of volcanic eruptions, and that the spike in ocean heat uptake and subsequent decrease in the rate of ocean warming after 2000 in some studies was probably an artifact of errors in XBT bias corrections and/or incomplete ocean coverage. Wunsch and Heimbach [55] used a sophisticated data assimilation technique to infer a deep ocean cooling, in direct contrast to the direct observations of Purkey and Johnson [56]. The reason for this difference in the Wunsch and Heimbach results is unclear but could relate to the omission of the geothermal heat flux from the ocean seafloor (approximately equivalent to the difference between their results to the direct observations) or the dominance of upper ocean observations over the sparse deep observations in their analysis.…”

Section: Sea Level Contributions Steric Sea Level Changementioning

confidence: 98%

Recent Progress in Understanding and Projecting Regional and Global Mean Sea Level Change

Clark

Church

Gregory

et al. 2015

Curr Clim Change Rep

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Considerable progress has been made in understanding the present and future regional and global sea level in the 2 years since the publication of the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change. Here, we evaluate how the new results affect the AR5's assessment of (i) historical sea level rise, including attribution of that rise and implications for the sea level budget, (ii) projections of the components and of total global mean sea level (GMSL), and (iii) projections of regional variability and emergence of the anthropogenic signal. In each of these cases, new work largely provides additional evidence in support of the AR5 assessment, providing greater confidence in those findings. Recent analyses confirm the twentieth century sea level rise, with some analyses showing a slightly smaller rate before 1990 and some a slightly larger value than reported in the AR5. There is now more evidence of an acceleration in the rate of rise. Ongoing ocean heat uptake and associated thermal expansion have continued since 2000, and are consistent with ocean thermal expansion reported in the AR5. A significant amount of heat is being stored deeper in the water column, with a larger rate of heat uptake since 2000 compared to the previous decades and with the largest storage in the Southern Ocean. The first formal detection studies for ocean thermal expansion and glacier mass loss since the AR5 have confirmed the AR5 finding of a significant anthropogenic contribution to sea level rise over the last 50 years. New projections of glacier loss from two regions suggest smaller contributions to GMSL rise from these regions than in studies assessed by the AR5; additional regional studies are required to further assess whether there are broader implications of these results. Mass loss from the Greenland Ice Sheet, primarily as a result of increased surface melting, and from the Antarctic Ice Sheet, primarily as a result of increased ice discharge, has accelerated. The largest estimates of acceleration in mass loss from the two ice sheets for 2003-2013 equal or exceed the acceleration of GMSL rise calculated from the satellite altimeter sea level record over the longer period of 1993-2014. However, when increased mass gain in land water storage and parts of East Antarctica, and decreased mass loss from glaciers in Alaska and some other regions are taken into account, the net acceleration in the ocean mass gain is consistent with the satellite altimeter record. New studies suggest that a marine ice sheet instability (MISI) may have been initiated in parts of the West Antarctic Ice Sheet (WAIS), but that it will affect only a limited number of ice streams in the twenty-first century. New projections of mass loss from the Greenland and Antarctic Ice Sheets by 2100, including a contribution from parts of WAIS undergoing unstable retreat, suggest a contribution that falls largely within the likely range (i.e., two thirds probability) of the AR5. These new results increase confidence in the AR5 likel...

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“…As a specific example of the sampling problems, consider Figure 4a, which is an estimate of the heat content, H (λ, φ), of the oceanic water column as a 20-year average (Wunsch & Heimbach 2014); λ and φ are the longitude and latitude, respectively. For present purposes, this field is taken as the truth, as it is known to computer-word accuracy on a complete global grid, permitting the near-perfect calculation of mean temperatures,…”

Section: Inhomogeneities: Signal and Samplingmentioning

confidence: 99%

Global Ocean Integrals and Means, with Trend Implications

Wunsch

2016

Annu. Rev. Mar. Sci.

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Understanding the ocean requires determining and explaining global integrals and equivalent average values of temperature (heat), salinity (freshwater and salt content), sea level, energy, and other properties. Attempts to determine means, integrals, and climatologies have been hindered by thinly and poorly distributed historical observations in a system in which both signals and background noise are spatially very inhomogeneous, leading to potentially large temporal bias errors that must be corrected at the 1% level or better. With the exception of the upper ocean in the current altimetric-Argo era, no clear documentation exists on the best methods for estimating means and their changes for quantities such as heat and freshwater at the levels required for anthropogenic signals. Underestimates of trends are as likely as overestimates; for example, recent inferences that multidecadal oceanic heat uptake has been greatly underestimated are plausible. For new or augmented observing systems, calculating the accuracies and precisions of global, multidecadal sampling densities for the full water column is necessary to avoid the irrecoverable loss of scientifically essential information.

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Bidecadal Thermal Changes in the Abyssal Ocean

Cited by 52 publications

References 37 publications

The Transient Response of Southern Ocean Circulation to Geothermal Heating in a Global Climate Model

The Transient Response of Southern Ocean Circulation to Geothermal Heating in a Global Climate Model

Recent Progress in Understanding and Projecting Regional and Global Mean Sea Level Change

Global Ocean Integrals and Means, with Trend Implications

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