Variability and changes to the mean meridional circulation in isentropic coordinates

We examine the climatology, variability and change in the global mean meridional circulation (MMC) as measured in a dry isentropic coordinate system from 1979–2017 using the ERA-Interim reanalysis. The methodology presents a zonal-mean view of the MMC as a single thermally direct circulation cell in each hemisphere. The circulation is decomposed into 'steady' and 'transient' components which allows us to identify and quantify several MMC features, including the Intertropical Convergence Zone, the descending branches of the Hadley circulation and a 'transient updraft' associated with the extratropical storm track. Large changes were identified in the Southern Hemisphere (SH) in both the Hadley Cell and the extratropical storm track in the late-1990s. These changes intertwine with the Interdecadal Pacific Oscillation that changed from a warm to a cold phase around 2000. Less significant changes were observed in the Northern Hemisphere, although high rates of tropical expansion during boreal summer may have been exacerbated by volcanic eruptions in the 1980s and 1990s. Further to those changes, tropical expansion was observed in autumn, with little change in the extratropical storm track. While potential inhomogeneities in the reanalysis limit the certainty about the magnitude of the identified changes, multiple non-reanalysis-based datasets suggest that large changes did occur in the 1990s in the SH, supporting the presented analysis.

Variability and Changes to the Mean Meridional Circulation in Isentropic Coordinates

We examine the climatology, variability and change in the global mean meridional circulation (MMC) as measured in a dry isentropic coordinate system from 1979–2017 using the ERA-Interim reanalysis. The methodology presents a zonal-mean view of the MMC as a single thermally direct circulation cell in each hemisphere. The circulation is decomposed into 'steady' and 'transient' components which allows us to identify and quantify several MMC features, including the Intertropical Convergence Zone, the descending branches of the Hadley circulation and a 'transient updraft' associated with the extratropical storm track. Large changes were identified in the Southern Hemisphere (SH) in both the Hadley Cell and the extratropical storm track in the late-1990s. These changes intertwine with the Interdecadal Pacific Oscillation that changed from a warm to a cold phase around 2000. Less significant changes were observed in the Northern Hemisphere, although high rates of tropical expansion during boreal summer may have been exacerbated by volcanic eruptions in the 1980s and 1990s. Further to those changes, tropical expansion was observed in autumn, with little change in the extratropical storm track. While potential inhomogeneities in the reanalysis limit the certainty about the magnitude of the identified changes, multiple non-reanalysis-based datasets suggest that large changes did occur in the 1990s in the SH, supporting the presented analysis.

Rossby wave activity associated with Euro-Atlantic weather regimes in the PRIMAVERA historical runs.

<p><span>In this study we </span><span>aim to assess how the upper tropospheric Rossby wave activity is represented in the PRIMAVERA models. </span><span>The low and high resolution historical coupled simulations will be compared with ERA5 reanalysis </span><span>(spanning the 1979-2014 period)</span><span> to enlight</span><span>en</span><span> model deficiencies in representing the spatial distribution </span><span>and temporal evolution</span><span> of Rossby wave activity </span><span>and to emphasize the benefits of </span><span>increased resolution. </span><span>Our analysis focuses </span><span>on </span><span>the wintertime large scale circulation over</span><span> the Euro-</span><span>A</span><span>tlantic </span><span>sector</span><span>. </span></p><p><span>A</span><span> diagnostic based on Local </span><span>W</span><span>ave </span><span>A</span><span>ctivity </span><span>(LWA)</span><span> in isentropic coordinates </span><span>is used </span><span>to </span><span>identify Rossby waves and to </span><span>quantify </span><span>their amplitude</span><span>. </span><span>LWA is partitioned into its stationary and transient components, </span><span>to </span><span>distinguish</span><span> the contribution from </span><span>planetary</span><span> versus </span><span>synoptic scale waves (i.e. wave packets)</span><span>. </span><span>This diagnostic is then combined with another </span><span>one</span><span> to identify persistent and recurrent large scale circulation patterns, the so called weather regimes</span><span>. Weather regimes in the Euro-Atlantic sector are identified with the usual approach </span><span>of EOF decomposition and k-mean clustering applied to daily anomalies of Montgomery streamfunction, </span><span>in order </span><span>to have a consistent framework with LWA </span><span>(</span><span>which is defined in isentropic coordinates</span><span>)</span><span>. </span><span>A</span><span> composite of transient LWA is realised for each weather regime to obtain the spatial distribution of Rossby wave activity associated with each weather regime.</span></p><p><span>Results show a marked intermodel variability in the ability of reproducing the correct (i.e. the one observed in reanalysis data) LWA distribution. Many of the models in fact fails to reproduce the localized (in space) maxima of LWA associated with each weather regime and to distribute LWA over a larger region compared to reanalysis. High resolution helps to correct this bias in the majority of the models, in particular in those where the low-resolution LWA distribution was already close to reanalysis. Finally, the temporal behaviour of the spatially averaged LWA in the examined period is discussed.</span></p>

The South Asian Monsoon Circulation in Moist Isentropic Coordinates

The atmospheric circulation during the South Asian summer monsoon season is analyzed in moist isentropic coordinates. The horizontal mass transport is sorted in terms of its equivalent potential temperature and is separated into the upper- and lower-tropospheric contributions. This technique makes it possible to trace the transport of air parcels over long distances, identify regions of convective motion in the tropics, and assess the impacts of diabatic processes. The goal here is to assess the thermodynamic characteristics of the atmospheric overturning associated with the South Asian monsoon and to connect this thermodynamic structure to horizontal transport. The monsoon is associated with a low-level inflow of warm and moist air, compensated by an upper-tropospheric outflow at high potential temperature. The South Asian monsoon differs, however, from other monsoonal systems in two important ways. First, the ascending air exhibits an unusually high equivalent potential temperature, which results in global lifting of the tropopause during the boreal summer. Second, on a seasonal basis the main monsoon regions appear to be shielded from dry air intrusion from the extratropical regions.

Axisymmetric Potential Vorticity Evolution of Hurricane Patricia (2015)

Operational numerical models failed to predict the record-setting rapid intensification and rapid overwater weakening of Hurricane Patricia (2015) in the eastern North Pacific basin, resulting in large intensity forecast errors. In an effort to better understand the mesoscale processes contributing to Patricia’s rapid intensity changes, we analyze high-resolution aircraft observations collected on 22–23 October. Spline-based variational analyses are created from observations collected via in situ measurements, Doppler radar, and full-tropospheric dropsonde profiles as part of the Office of Naval Research Tropical Cyclone Intensity (TCI) experiment and the National Oceanic and Atmospheric Administration Intensity Forecasting Experiment (IFEX). We present the first full-tropospheric calculation of the dry, axisymmetric Ertel’s potential vorticity (PV) in a tropical cyclone without relying on balance assumptions. Detailed analyses reveal the formation of a “hollow tower” PV structure as Patricia rapidly approached its maximum intensity, and a subsequent breakdown of this structure during Patricia’s rapid overwater weakening phase. Transforming the axisymmetric PV analyses from radius–height to potential radius–isentropic coordinates reveals that Patricia’s rapid intensification was closely related to the distribution of diabatic heating and eddy mixing. During Patricia’s rapid overwater weakening phase, eddy mixing processes are hypothesized to be the primary factor rearranging the PV distribution near the eye–eyewall region, diluting the PV previously confined to the hollow tower while approximately conserving the absolute circulation.

The Thermodynamic Cycles and Associated Energetics of Hurricane Edouard (2014) during Its Intensification

This study expands on a previous analysis of the intensification of Hurricane Edouard (2014) in the isentropic coordinates to further examine the thermodynamic processes that lead to the strengthening of the storm. Thermodynamic cycles are extracted using the methodology known as the Mean Airflow as Lagrangian Dynamics Approximation. The most intense thermodynamic cycle here is associated with the air rising within the hurricane eyewall. Its structure remains mostly steady during the early development of Edouard but evolves rapidly as the storm intensifies. Through intensification, the ascent shifts toward high values of entropy under the effect of enhanced surface heat fluxes and stronger surface winds, while reaching higher altitudes and lower temperatures. The near–rapid intensification onset of Edouard corresponds to an increase in the energy input into the cycle and an increase in the amount of kinetic energy generated. The external heating fluctuates considerably in the two low-level legs with a period of about 16–24 h, indicative of diurnal variation in the thermodynamic cycle. During the intensification of Edouard, the mechanical work production and the Carnot efficiency both increase dramatically, which can be attributed to the increase in energy transport and deepening of the thermodynamic cycle. In addition, there is a substantial increase of the mechanical work done during the horizontal expansion of air parcels near Earth’s surface, and a larger fraction of the kinetic energy generated is used to sustain and intensify the horizontal flow rather than to provide a vertical acceleration in the updrafts.

Local Finite-Amplitude Wave Activity as a Diagnostic for Rossby Wave Packets

Upper-tropospheric Rossby wave packets (RWPs) are important dynamical features, because they are often associated with weather systems and sometimes act as precursors to high-impact weather. The present work introduces a novel diagnostic to identify RWPs and to quantify their amplitude. It is based on the local finite-amplitude wave activity (LWA) of Huang and Nakamura, which is generalized to the primitive equations in isentropic coordinates. The new diagnostic is applied to a specific episode containing large-amplitude RWPs and compared with a more traditional diagnostic based on the envelope of the meridional wind. In this case, LWA provides a more coherent picture of the RWPs and their zonal propagation. This difference in performance is demonstrated more explicitly in the framework of an idealized barotropic model simulation, where LWA is able to follow an RWP into its fully nonlinear stage, including cutoff formation and wave breaking, while the envelope diagnostic yields reduced amplitudes in such situations.

Role of vertical and horizontal mixing in the tape recorder signal near the tropical tropopause

Nearly all air enters the stratosphere through the tropical tropopause layer (TTL). The TTL therefore exerts a control on stratospheric chemistry and climate. The hemispheric meridional overturning (Brewer–Dobson) circulation spreads this TTL influence upward and poleward. Stratospheric water vapor concentrations are set near the tropical tropopause and are nearly conserved in the lowermost stratosphere. The resulting upward propagating tracer transport signal of seasonally varying entry concentrations is known as the tape recorder signal. Here, we study the roles of vertical and horizontal mixing in shaping the tape recorder signal in the tropical lowermost stratosphere, focusing on the 80 hPa level. We analyze the tape recorder signal using data from satellite observations, a reanalysis, and a chemistry–climate model (CCM). By modifying past methods, we are able to capture the seasonal cycle of effective vertical transport velocity in the tropical lowermost stratosphere. Effective vertical transport velocities are found to be multiple times stronger than residual vertical velocities for the reanalysis and the CCM. We also study the tape recorder signal in an idealized 1-D transport model. By performing a parameter sweep, we test a range of different strengths of transport contributions by vertical advection, vertical mixing, and horizontal mixing. By introducing seasonality into the transport strengths, we find that the most successful simulation of the observed tape recorder signal requires vertical mixing at 80 hPa that is multiple times stronger compared to previous estimates in the literature. Vertical mixing is especially important during boreal summer when vertical advection is weak. Simulating the reanalysis tape recorder requires excessive amounts of vertical mixing compared to observations but also to the CCM, which hints at the role of spurious dispersion due to data assimilation. Contrasting the results between pressure and isentropic coordinates allows for further insights into quasi-adiabatic vertical mixing, e.g., associated with overshooting convection or breaking gravity waves. Horizontal mixing, which takes place primarily along isentropes due to Rossby wave breaking, is captured more consistently in isentropic coordinates. Overall, our study emphasizes the role of vertical mixing in lowermost tropical stratospheric transport, which appears to be as important as vertical advection by the residual mass circulation. This questions the perception of the tape recorder as a manifestation of slow upward transport as opposed to a phenomenon influenced by quick and intense transport through mixing, at least near the tape head. However, due to the limitations of the observational dataset used and the simplicity of the applied transport model, further work is required to more clearly specify the role of vertical mixing in lowermost stratospheric transport in the tropics.

Role of vertical and horizontal mixing in the tape recorder signal near the tropical tropopause

Nearly all air enters the stratosphere through the tropical tropopause layer (TTL). The TTL therefore exerts a control on stratospheric chemistry and climate. The hemispheric meridional overturning (Brewer-Dobson) circulation spreads this TTL influence upward and poleward. Stratospheric water vapor concentrations are set near the tropical tropopause and are nearly conserved in the lowermost stratosphere. The resulting upward propagating tracer transport signal of seasonally varying entry concentrations is known as the tape recorder signal. Here, we study the roles of vertical and horizontal mixing in shaping the tape recorder signal in the tropical lowermost stratosphere. We analyze the tape recorder signal using data from satellite observations, a reanalysis, and a chemistry-climate model (CCM). Modifying past methods, we are able to capture the seasonal cycle of effective vertical transport velocity in the tropical lowermost stratosphere, which is found to be multiple times stronger than residual vertical velocities for the reanalysis and the CCM. We also study the tape recorder signal in an idealized one-dimensional transport model. By performing a parameter-sweep we test a range of different strengths of transport contributions by vertical advection, vertical mixing, and horizontal mixing. Introducing seasonality in the transport strengths we find that the most successful simulation of the observed tape recorder signal requires quadrupled vertical mixing in the lowermost tropical stratosphere compared to previous estimates in the literature. Vertical mixing is especially important during boreal summer when vertical advection is weak. The reanalysis requires excessive amounts of vertical mixing compared to observations but also to the CCM, which hints at the role of spurious dispersion due to data assimilation. Contrasting the results between pressure and isentropic coordinates allows further insights into quasi-adiabatic vertical mixing, e.g. associated with breaking gravity waves. Horizontal mixing, which takes place primarily along isentropes due to Rossby wave breaking, is captured more consistently in isentropic coordinates. Overall our study emphasizes the role of vertical mixing in lowermost tropical stratospheric transport, which appears to be as important as vertical advection by the residual mass circulation. This questions the perception of the "tape recorder" as a manifestation of slow upward transport as opposed to a phenomenon influenced by quick and intense transport through mixing, at least near the tape head.