Frédéric Laliberté

Constrained work output of the moist atmospheric heat engine in a warming climate

Because the rain falls and the wind blows Global warming is expected to intensify the hydrological cycle, but it might also make the atmosphere less energetic. Laliberté et al. modeled the atmosphere as a classical heat engine in order to evaluate how much energy it contains and how much work it can do (see the Perspective by Pauluis). They then used a global climate model to project how that might change as climate warms. Although the hydrological cycle may increase in intensity, it does so at the expense of its ability to do work, such as powering large-scale atmospheric circulation or fueling more very intense storms. Science, this issue p. 540; see also p. 475 A more intense hydrological cycle in a warmer world might make atmospheric circulation less energetic. [Also see Perspective by Pauluis] Incoming and outgoing solar radiation couple with heat exchange at Earth’s surface to drive weather patterns that redistribute heat and moisture around the globe, creating an atmospheric heat engine. Here, we investigate the engine’s work output using thermodynamic diagrams computed from reanalyzed observations and from a climate model simulation with anthropogenic forcing. We show that the work output is always less than that of an equivalent Carnot cycle and that it is constrained by the power necessary to maintain the hydrological cycle. In the climate simulation, the hydrological cycle increases more rapidly than the equivalent Carnot cycle. We conclude that the intensification of the hydrological cycle in warmer climates might limit the heat engine’s ability to generate work.

A Statistical Generalization of the Transformed Eulerian-Mean Circulation for an Arbitrary Vertical Coordinate System

AbstractA new method is derived for approximating the mean meridional circulation in an arbitrary vertical coordinate system using only the time-mean and zonally averaged meridional velocity, meridional eddy transport, and eddy variance. The method is called the statistical transformed Eulerian mean (STEM) and can be viewed as a generalization of the transformed Eulerian mean (TEM) formulation. It is shown that the TEM circulation can be obtained from the STEM circulation in the limit of small eddy variance. The main advantage of the STEM formulation is that it can be applied to nonmonotonic coordinate systems such as the equivalent potential temperature. In contrast, the TEM formulation can only be applied to stratified variables. Reanalysis data are used to compare the STEM circulation to an explicit calculation of the mean meridional circulation on dry and moist isentropic surfaces based on daily data. It is shown that the STEM formulation accurately captures all the key features of the circulation. Th...

Moist Recirculation and Water Vapor Transport on Dry Isentropes

AbstractAn analysis of the overturning circulation in dry isentropic coordinates using reanalysis data is presented. The meridional mass fluxes on surfaces of constant dry potential temperature but distinct equivalent potential temperature are separated into southward and northward contributions. The separation identifies thermodynamically distinct mass fluxes moving in opposite directions. The eddy meridional water vapor transport is shown to be associated with large poleward and equatorward mass fluxes occurring at the same value of dry potential temperature but different equivalent potential temperature. These mass fluxes, referred to here as the moist recirculation, are associated with an export of water vapor from the subtropics connecting the Hadley cell to the midlatitude storm tracks.The poleward branch of the moist recirculation occurs at mean equivalent potential temperatures comparable to upper tropospheric dry potential temperature values, indicating that typical poleward-moving air parcels ca...

The representation of ocean circulation and variability in thermodynamic coordinates

The ocean’s circulation is analyzed in Absolute Salinity SA and Conservative Temperature Q coordinates. It is separated into 1) an advective component related to geographical displacements in the direction normal to SA and Q isosurfaces and 2) into a local component, related to local changes in SA–Q values, without a geographical displacement. In this decomposition, the sum of the advective and local components of the circulation is equivalent to the material derivative of SA and Q. The sum is directly related to sources and sinks of salt and heat. The advective component is represented by the advective thermohaline streamfunction C advAQ. After removing a trend, the local component can be represented by the local thermohaline streamfunction C locAQ. Here, C locAQ can be diagnosed using a monthly averaged time series of SA and Q from an observational dataset. In addition, C advAQ and C locAQ are determined from a coupled climate model. The diathermohaline streamfunction C diaAQ is the sum of C advAQ and C locAQ and represents the nondivergent diathermohaline circulation in SA–Q coordinates. The diathermohaline trend, resulting from the trend in the local changes of SA and Q, quantifies the redistribution of the ocean’s volume in SA–Q coordinates over time. It is argued that the diathermohaline streamfunction provides a powerful tool for the analysis of and comparison among ocean models and observation-based gridded climatologies.

The influence of declining sea ice on shipping activity in the Canadian Arctic

Significant attention has focused on the potential for increased shipping activity driven by recent observed declines in Arctic sea ice cover. In this study, we describe the first coupled spatial analysis between shipping activity and sea ice using observations in the Canadian Arctic over the 1990-2015 period. Shipping activity is measured using known ship locations enhanced with a least-cost path algorithm to generate ship tracks, and quantified by computing total distance travelled in km. Statistically significant increases in shipping activity are observed in the Hudson Strait (150-500 km travelled year-1), the Beaufort Sea (40-450 km travelled year-1), Baffin Bay (50-350 km travelled year-1), and regions in the southern route of the Northwest Passage (50-250 km travelled year-1). Increases in shipping activity are significantly correlated with reductions in sea ice concentration (Kendalls tau up to -0.6) in regions of the Beaufort Sea, Western Parry Channel, Western Baffin Bay, and Foxe Basin. Changes in multi-year ice dominant regions in the Canadian Arctic were found to be more influential on changes to shipping activity compared to seasonal sea ice regions.

The Atmospheric General Circulation in Thermodynamical Coordinates

The zonal and meridional components of the atmospheric general circulation are used to define a global thermodynamic stream function in dry static energy versus latent heat coordinates. Diabatic mo ...

Winter intensification of the moist branch of the circulation in simulations of 21st century climate

[1] In this paper, changes in isentropic circulations associated with global warming in the AlB model outputs for the 20th and 21st centuries are analyzed. The changes in the circulations on dry and moist isentropes are quantified through the use of three bulk measures of the circulations: mass transport, entropy transport and effective stratification. The circulation on dry isentropes is expected to weaken due to a reduction of the meridional heat transport and to an increase in stratification. In contrast, the moist branch of the circulation, measured in terms of the difference between the circulations on moist and dry isentropes, strengthens during the winter months. This intensification is characterized not only by an increase in the eddy latent heat transport but also by an increase in the mass transport. This indicate a larger poleward mass flow of warm moist subtropical air into the stormtracks leading to enhanced moist ascent within baroclinic eddies.

Maintenance and Broadening of the Ocean’s Salinity Distribution by the Water Cycle

The global water cycle leaves an imprint on ocean salinity through evaporation and precipitation. It has been proposed that observed changes in salinity can be used to infer changes in the water cycle. Here salinity is characterized by the distribution of water masses in salinity coordinates. Only mixing and sources and sinks of freshwater and salt can modify this distribution. Mixing acts to collapse the distribution, making saline waters fresher and fresh waters more saline. Hence, in steady state, there must be net precipitation over fresh waters and net evaporation over saline waters. A simple model is developed to describe the relationship between the breadth of the distribution, the water cycle, and mixing—the latter being characterized by an e-folding time scale. In both observations and a state-of-the-art ocean model, the water cycle maintains a salinity distribution in steady state with a mixing time scale of the order of 50 yr. The same simple model predicts the response of the salinity distribution to a change in the water cycle. This study suggests that observations of changes in ocean salinity could be used to infer changes in the hydrological cycle.

Secondary Circulation of Tropical Cyclones in Vertical Wind Shear: Lagrangian Diagnostic and Pathways of Environmental Interaction

AbstractThis study introduces a Lagrangian diagnostic of the secondary circulation of tropical cyclones (TCs), here defined by those trajectories that contribute to latent heat release in the region of high inertial stability of the TC core. This definition accounts for prominent asymmetries and transient flow features. Trajectories are mapped from the three-dimensional physical space to the (two dimensional) entropy–temperature space. The mass flux vector in this space subsumes the thermodynamic characteristics of the secondary circulation. The Lagrangian diagnostic is then employed to further analyze the impact of vertical wind shear on TCs in previously published idealized numerical experiments. One focus of this analysis is the classification and quantitative depiction of different pathways of environmental interaction based on thermodynamic properties of trajectories at initial and end times. Confirming results from previous work, vertical shear significantly increases the intrusion of low–equivalent...

Changes in ocean vertical heat transport with global warming

Heat transport between the surface and deep ocean strongly influences transient climate change. Mechanisms setting this transport are investigated using coupled climate models and by projecting ocean circulation into the temperature-depth diagram. In this diagram, a “cold cell” cools the deep ocean through the downwelling of Antarctic waters and upwelling of warmer waters and is balanced by warming due to a “warm cell,” coincident with the interhemispheric overturning and previously linked to wind and haline forcing. With anthropogenic warming, the cold cell collapses while the warm cell continues to warm the deep ocean. Simulations with increasingly strong warm cells, set by their mean Southern Hemisphere winds, exhibit increasing deep-ocean warming in response to the same anthropogenic forcing. It is argued that the partition between components of the circulation which cool and warm the deep ocean in the preindustrial climate is a key determinant of ocean vertical heat transport with global warming.