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Michael Riemer and Frédéric Laliberté

Abstract

This 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 potential temperature () air into the eyewall through the frictional inflow layer. In contrast to previous ideas, vertical shear decreases midlevel ventilation in these experiments. Consequently, the difference in eyewall between the no-shear and shear experiments is largest at low levels. Vertical shear, however, significantly increases detrainment from the eyewall and modifies the thermodynamic signature of the outflow layer. Finally, vertical shear promotes the occurrence of a novel class of trajectories that has not been described previously. These trajectories lose entropy at cold temperatures by detraining from the outflow layer and subsequently warm by 10–15 K. Further work is needed to investigate in more detail the relative importance of the different pathways for TC intensity change and to extend this study to real atmospheric TCs.

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Frédéric Laliberté and Paul J. Kushner

Abstract

The dynamics of late summer Arctic tropospheric heat content variability is studied using reanalyses. In both trends and interannual variability, much of the August heat content variability in the Arctic midtroposphere can be explained by the total—sensible plus latent—heat content variability at the midlatitude near surface in July. Climate models suggest that this connection is part of the global warming signal in September–November, but in reanalyses the connection is most strongly present in July–August variability and trends. It is argued that heat content signals are propagated from the midlatitude near surface to the Arctic midtroposphere approximately along climatological moist isentropes. High-frequency data reveal that the propagating signal is primarily driven by a few strong meridional heat flux events each summer season. Composite analysis on these events shows that August meridional heat fluxes into the Arctic midtroposphere are succeeded by positive heat content anomalies in the lower troposphere a few days later. This second connection between the Arctic midtroposphere and the Arctic lower troposphere could be sufficient to explain some of the recent Arctic 850-hPa temperature variability north of 75°N.

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Frédéric Laliberté, Tiffany Shaw, and Olivier Pauluis

Abstract

A theoretical model describing the structure of the dry and moist isentropic circulations in the lower troposphere is derived. It decomposes the meridional flow in the troposphere into three contributions: a dry equatorward flow, a cold moist equatorward flow, and a warm moist poleward flow in the mixed layer. The model is based on observations of the meridional mass fluxes joint distribution in potential temperature and equivalent potential temperature. It updates an existing model of the dry circulation by emphasizing the role of moisture in the mixed layer. The model is used to derive an expression for the ratio of moist to dry circulation strengths and this expression is used to assess the influence of surface thermodynamics on the circulations. It predicts that the moist circulation should be between 1.5 and 2 times as strong as the dry circulation and that this relative strength should not increase indefinitely with increasing surface temperature variability. The model also yields an expression for the ratio of total meridional heat fluxes to meridional sensible heat fluxes. This expression indicates that while an increase in the total heat fluxes occurs when surface temperature variability increases (via an increase in latent heat flux), it cannot increase indefinitely. The results suggest that changes in surface thermodynamic conditions must be constrained to constrain changes in the meridional overturning circulation associated with a warming climate.

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Robert Fajber, Paul J. Kushner, and Frédéric Laliberté

Abstract

Evidence from models and observations suggests that the vertical distribution of entropy in the extratropical troposphere reflects the horizontal distribution of entropy at the surface. This isentropic linkage, which is accomplished through moist isentropic mass transport driven by extratropical waves, becomes more apparent when the effect of latent heat release by condensing moist parcels is accounted for. This study focuses on the stratification of the Arctic troposphere, which is connected by moist isentropes to the midlatitude surface. A relatively simple moist general circulation model without radiative feedbacks involving water vapor or clouds is used to study the linkage between the midlatitude surface and the Arctic midtroposphere. Zonally symmetric midlatitude thermal perturbations switched on at the surface drive a moist potential temperature response in the Arctic midtroposphere with a lag of about 2 weeks. This response increases the gross moist vertical stability in the Arctic while generally decreasing it, or increasing it only weakly, in the midlatitudes. The moist isentropic streamfunction is shifted poleward owing to the poleward entropy flux response and is shifted upward (i.e., to higher entropy) owing to the zonal-mean entropy response. The results suggest a potential novel mechanism by which the midlatitudes might influence polar lapse rates and their associated radiative feedbacks.

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Olivier Pauluis, Tiffany Shaw, and Frédéric Laliberté

Abstract

A 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. The error in the streamfunction is less than 10%.

The STEM formulation is subsequently used to analyze the circulation induced by latent heat transport and to understand the processes responsible for setting the effective stratification in the troposphere. The eddy sensible heat transport dominates in the midlatitudes and in the winter hemisphere, while the eddy latent heat transport dominates in the subtropical regions and in the summer hemisphere. For the dry isentropic circulation, the approximate effective stratification is dominated by the vertical stratification, whereas for the moist isentropic circulation it is dominated by the eddy variance contribution. The importance of the eddy variance in setting the stratification is in agreement with previous work.

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Frédéric Laliberté, Tiffany Shaw, and Olivier Pauluis

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An 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 can ascend to the tropopause. The analysis suggests that these air parcels ascend on the equatorward side of storm tracks by following moist isentropes reminiscent of upright deep convection, while on the poleward side their moist isentropes are indicative of large-scale slantwise convection. In the equatorward branch, the analysis describes typical air parcels that follow their dry isentropes until they get injected into the boundary layer where they are subsequently moistened.

The moist recirculation along with the mean equivalent potential temperature of its poleward and equatorward components are used to recover an approximate overturning circulation on moist isentropes from which it is shown that the moist recirculation accounts for the difference between the meridional circulation averaged on dry and on moist isentropes.

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Joakim Kjellsson, Kristofer Döös, Frédéric B. Laliberté, and Jan D. Zika

Abstract

The zonal and meridional components of the atmospheric general circulation are used to define a global thermodynamic streamfunction in dry static energy versus latent heat coordinates. Diabatic motions in the tropical circulations and fluxes driven by midlatitude eddies are found to form a single, global thermodynamic cycle. Calculations based on the Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) dataset indicate that the cycle has a peak transport of 428 Sv (Sv ≡ 109 kg s−1). The thermodynamic cycle encapsulates a globally interconnected heat and water cycle comprising ascent of moist air where latent heat is converted into dry static energy, radiative cooling where dry air loses dry static energy, and a moistening branch where air is warmed and moistened. It approximately follows a tropical moist adiabat and is bounded by the Clausius–Clapeyron relationship for near-surface air. The variability of the atmospheric general circulation is related to ENSO events using reanalysis data from recent years (1979–2009) and historical simulations from the EC-Earth Consortium (EC-Earth) coupled climate model (1850–2005). The thermodynamic cycle in both EC-Earth and ERA-Interim widens and weakens with positive ENSO phases and narrows and strengthens during negative ENSO phases with a high correlation coefficient. Weakening in amplitude suggests a weakening of the large-scale circulation, while widening suggests an increase in mean tropical near-surface moist static energy.

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Sjoerd Groeskamp, Jan D. Zika, Trevor J. McDougall, Bernadette M. Sloyan, and Frédéric Laliberté

Abstract

The ocean’s circulation is analyzed in Absolute Salinity S A and Conservative Temperature Θ coordinates. It is separated into 1) an advective component related to geographical displacements in the direction normal to S A and Θ isosurfaces and 2) into a local component, related to local changes in S A–Θ 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 S A and Θ. The sum is directly related to sources and sinks of salt and heat. The advective component is represented by the advective thermohaline streamfunction . After removing a trend, the local component can be represented by the local thermohaline streamfunction . Here, can be diagnosed using a monthly averaged time series of S A and Θ from an observational dataset. In addition, and are determined from a coupled climate model. The diathermohaline streamfunction is the sum of and and represents the nondivergent diathermohaline circulation in S A–Θ coordinates. The diathermohaline trend, resulting from the trend in the local changes of S A and Θ, quantifies the redistribution of the ocean’s volume in S A–Θ 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.

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Kristofer Döös, Joakim Kjellsson, Jan Zika, Frédéric Laliberté, Laurent Brodeau, and Aitor Aldama Campino

Abstract

The thermohaline circulation of the ocean is compared to the hydrothermal circulation of the atmosphere. The oceanic thermohaline circulation is expressed in potential temperature–absolute salinity space and comprises a tropical cell, a conveyor belt cell, and a polar cell, whereas the atmospheric hydrothermal circulation is expressed in potential temperature–specific humidity space and unifies the tropical Hadley and Walker cells as well as the midlatitude eddies into a single, global circulation. The oceanic thermohaline streamfunction makes it possible to analyze and quantify the entire World Ocean conversion rate between cold–warm and fresh–saline waters in one single representation. Its atmospheric analog, the hydrothermal streamfunction, instead captures the conversion rate between cold–warm and dry–humid air in one single representation. It is shown that the ocean thermohaline and the atmospheric hydrothermal cells are connected by the exchange of heat and freshwater through the sea surface. The two circulations are compared on the same diagram by scaling the axes such that the latent heat energy required to move an air parcel on the moisture axis is equivalent to that needed to move a water parcel on the salinity axis. Such a comparison leads the authors to propose that the Clausius–Clapeyron relationship guides both the moist branch of the atmospheric hydrothermal circulation and the warming branches of the tropical and conveyor belt cells of the oceanic thermohaline circulation.

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Jan D. Zika, Nikolaos Skliris, A. J. George Nurser, Simon A. Josey, Lawrence Mudryk, Frédéric Laliberté, and Robert Marsh

Abstract

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.

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