Baroclinic and barotropic aspects of extratropical wave-mean flow interaction

Baroclinic and barotropic processes are the key components of midlatitude tropospheric dynamics. Baroclinic processes are involved in the growth of extratropical storms, whereas barotropic processes are involved in their decay, suggesting the two processes are closely linked. Their links are conventionally studied through wavemean flow interaction theory and through modes of variability, and both planetary and synoptic scale waves play an important role in interacting with the baroclinic and barotropic mean flow. These processes are studied using multiscale asymptotic methods, which provide a framework for studying wave-mean flow interactions on different spatial and temporal scales. This framework is used to derive the full set of equations for small amplitude planetary and synoptic scale waves and for the zonal mean flow and its interactions with planetary and synoptic waves. In a zonally inhomogeneous framework (planetary-wave amplitudes comparable to synoptic-wave amplitudes) this theory predicts a coupling of baroclinic and barotropic processes through the planetary scale waves, and the interactions between the planetary and synoptic waves only occurring via the zonal mean flow or diabatic and frictional processes. However, in a zonally homogeneous framework (negligible planetary waves) baroclinic and barotropic processes are decoupled, with eddy momentum fluxes only affecting the barotropic flow and eddy heat fluxes only affecting the baroclinic flow, consistent with some recent observational studies. The latter somewhat counterintuitive result is studied in a zonally homogeneous idealized model and in Southern Hemisphere observations, using the baroclinic and barotropic annular modes of variability at different timescales. This shows that the decoupling of the two processes can indeed occur, but is frequency-dependent. The important role of planetary scale waves is explored in a zonally inhomogeneous idealized model and in Northern Hemisphere observations through the variability in the barotropic and baroclinic mean flows in storm track regions, and links with teleconnection patterns are established

A multiscale asymptotic theory of extratropical wave–mean flow interaction

Multiscale asymptotic methods are used to derive wave-activity equations for planetary and synoptic scale eddies and their interactions with a zonal mean flow. The eddies are assumed to be of small amplitude, and the synoptic-scale zonal and meridional length scales are taken to be equal. Under these assumptions, the zonal-mean and planetary-scale dynamics are planetary geostrophic (i.e. dominated by vortex stretching), and the interaction between planetary and synoptic scale eddies occurs only through the zonal mean flow or through diabatic processes. Planetary scale heat fluxes are shown to enter the angular momentum budget through meridional mass redistribution. After averaging over synoptic length and time scales, momentum fluxes disappear from the synoptic-scale wave-activity equation whilst synoptic-scale heat fluxes disappear from the baroclinicity equation, leaving planetary-scale heat fluxes as the only adiabatic term coupling the baroclinic and barotropic components of the zonal mean flow. In the special case of weak planetary waves, the decoupling between the baroclinic and barotropic parts of the flow is complete with momentum fluxes driving the barotropic zonal mean flow, heat fluxes driving the wave activity, and diabatic processes driving baroclinicity. These results help explain the apparent decoupling between the baroclinic and barotropic components of flow variability recently identified in observations, and may provide a means of better understanding the link between thermodynamic and dynamic aspects of climate variability and change

Potential impact of tropopause sharpness on the structure and strength of the general circulation

The wintertime extratropical general circulation may be viewed as being primarily governed by interactions between Rossby waves and the background flow. These Rossby waves propagate vertically and meridionally away from their sources and amplify within the core of the tropopause-level jet, which acts as a waveguide. The strength of this waveguide is in part controlled by tropopause sharpness, which itself is a function of the strength of tropopause inversion layer (TIL), a layer of enhanced static stability just above the tropopause. Here, we report a strong relation between interannual-to-multidecadal variations in the strength of the mid-latitude TIL and features of the general circulation (e.g., jet latitude, strength of the Hadley cell) in a reanalysis and climate models. Similar relationships hold for the variability across climate models. Experiments with a mechanistic model show that a sharper tropopause promotes an intensified general circulation and an equatorward shifted jet.publishedVersio

Multiscale extratropical barotropic variability on the subseasonal-to-seasonal timescale

Barotropic variability plays an important role in a variety of extratropical atmospheric processes, such as annular modes, teleconnections, and baroclinic life cycles, which occur on a wide range of timescales. Extratropical dynamics is dominated by high-frequency (periods shorter than 10 days) transient waves, which drive barotropic variability through baroclinic life cycle events. However, other types of waves (e.g. low-frequency, with periods longer than 10 days, and stationary waves) also play an important role in shaping extratropical dynamics on various timescales. This study uses reanalysis data in the context of the zonal momentum budget to address the relative importance of stationary, low-frequency and high-frequency waves in driving barotropic variability at high (synoptic) and low (subseasonal-to-seasonal) frequencies both locally in storm track regions and in the zonal mean in both hemispheres. The analysis reveals that the eddy forcing of barotropic variability on synoptic timescales is dominated by the interaction between low-frequency (and stationary) and high-frequency waves, and not by high-frequency self-interactions. On longer (subseasonal-to-seasonal) timescales the picture is more complex, with increased importance of low-frequency self-interactions but still largely negligible high-frequency self-interactions. A better understanding of the mechanisms driving barotropic variability on subseasonal-to-seasonal timescales may help advancing predictability on these timescales

Recent Hadley circulation strengthening: a trend or multidecadal variability?

Under embargo until: 2022-12-03This study explores the possible drivers of the recent Hadley circulation strengthening in the modern reanalyses. Predominantly, two recent generations of reanalyses provided by the European Centre for Medium-Range Weather Forecasts are used: the fifth-generation atmospheric reanalysis (ERA5) and the interim reanalysis (ERA-Interim). Some results are also evaluated against other long-term reanalyses. To assess the origins of the Hadley cell (HC) strength variability we employ the Kuo-Eliassen (KE) equation. ERA5 shows that both HCs were strengthening prior to 2000s, but they have been weakening or remained steady afterwards. Most of the long-term variability in the strength of the HCs is explained by the meridional gradient of diabatic (latent) heating, which is related to precipitation gradients. However, the strengthening of both HCs in ERA5 is larger than the strengthening expected from the observed zonal-mean precipitation gradient (via Global Precipitation Climatology Project, GPCP). This suggests that the HC strength trends in the recent decades in ERA5 can be explained partly as an artifact of the misrepresentation of latent heating and partly through (physical) long-term variability. To show that the latter is true, we analyze ERA5 preliminary data for the 1950-1978 period, other long-term (e.g. 20th century) reanalyses, and sea surface temperature observational data. This reveals that the changes in the HC strength can be a consequence of the Atlantic multidecadal oscillation (AMO) and related diabatic and frictional processes, which in turn drive the global HC variability. This work has implications for further understanding of the long-term variability of the Hadley circulation.publishedVersio

On the coupling between barotropic and baroclinic modes of extratropical atmospheric variability

The baroclinic and barotropic components of atmospheric dynamics are usually viewed as interlinked through the baroclinic life cycle, with baroclinic growth of eddies connected to heat fluxes, barotropic decay connected to momentum fluxes, and the two eddy fluxes connected through the Eliassen-Palm wave activity. However, recent observational studies have suggested that these two components of the dynamics are largely decoupled in their variability, with variations in the zonal mean flow associated mainly with the momentum fluxes, variations in the baroclinic wave activity associated mainly with the heat fluxes, and essentially no correlation between the two. These relationships are examined in a dry dynamical core model under different configurations and in Southern Hemisphere observations, considering different frequency bands to account for the different timescales of atmospheric variability. It is shown that at intermediate periods longer than 10 days the decoupling of the baroclinic and barotropic modes of variability can indeed occur as the eddy kinetic energy at those time scales is only affected by the heat fluxes and not the momentum fluxes. The baroclinic variability includes the oscillator model with periods of 20-30 days. At both the synoptic timescale and the quasi-steady limit the baroclinic and barotropic modes of variability are linked, consistent with baroclinic life cycles and the positive baroclinic feedback mechanism, respectively. In the quasi-steady limit the pulsating modes of variability and their correlations depend sensitively on the model climatology

Metrics of the Hadley circulation strength and associated circulation trends

This study compares trends in the Hadley cell (HC) strength using different metrics applied to the ECMWF ERA5 and ERA-Interim reanalyses for the period 1979–2018. The HC strength is commonly evaluated by metrics derived from the mass-weighted zonal-mean stream function in isobaric coordinates. Other metrics include the upper tropospheric velocity potential, the vertical velocity in the mid-troposphere, and the water vapour transport in the lower troposphere. Seven known metrics of HC strength are complemented here by a metric of the spatially averaged HC strength, obtained by averaging the stream function in the latitude–pressure (φ–p) plane, and by the total energy of zonal-mean unbalanced circulation in the normal-mode function decomposition. It is shown that metrics, which rely on single-point values in the φ–p plane, produce unreliable 40-year trends in both the northern and southern HCs, especially in ERA-Interim; magnitudes and even the signs of the trends depend on the choice of the HC strength metric. The two new metrics alleviate the vertical and meridional inhomogeneities of the trends in HC strength. The unbalanced energy metric suggests a positive HC trend in both reanalyses, whereas the metric based on averaging the stream function finds a significant positive trend only in ERA5.</p

Metrics of the Hadley circulation strength and associated circulation trends

This study compares trends in the Hadley cell (HC) strength using different metrics applied to the ECMWF ERA5 and ERA-Interim reanalyses for the period 1979–2018. The HC strength is commonly evaluated by metrics derived from the mass-weighted zonal-mean stream function in isobaric coordinates. Other metrics include the upper tropospheric velocity potential, the vertical velocity in the mid-troposphere, and the water vapour transport in the lower troposphere. Seven known metrics of HC strength are complemented here by a metric of the spatially averaged HC strength, obtained by averaging the stream function in the latitude–pressure (φ–p) plane, and by the total energy of zonal-mean unbalanced circulation in the normal-mode function decomposition. It is shown that metrics, which rely on single-point values in the φ–p plane, produce unreliable 40-year trends in both the northern and southern HCs, especially in ERA-Interim; magnitudes and even the signs of the trends depend on the choice of the HC strength metric. The two new metrics alleviate the vertical and meridional inhomogeneities of the trends in HC strength. The unbalanced energy metric suggests a positive HC trend in both reanalyses, whereas the metric based on averaging the stream function finds a significant positive trend only in ERA5.publishedVersio

Downstream suppression of baroclinic waves

Baroclinic waves drive both regional variations in weather and large-scale variability in the extratropical general circulation. They generally do not exist in isolation, but rather often form into coherent wave packets that propagate to the east via a mechanism called downstream development. Downstream development has been widely documented and explored. Here we document a novel but also key aspect of baroclinic waves: the downstream suppression of baroclinic activity that occurs in the wake of eastward propagating disturbances. Downstream suppression is apparent not only in the Southern Hemisphere storm track as shown in previous work, but also in the North Pacific and North Atlantic storm tracks. It plays an essential role in driving subseasonal periodicity in extratropical eddy activity in both hemispheres, and gives rise to the observed quiescence of the North Atlantic storm track 1–2 weeks following pronounced eddy activity in the North Pacific sector. It is argued that downstream suppression results from the anomalously low baroclinicity that arises as eastward propagating wave packets convert potential to kinetic energy. In contrast to baroclinic wave packets, which propagate to the east at roughly the group velocity in the upper troposphere, the suppression of baroclinic activity propagates eastward at a slower rate that is comparable to that of the lower to midtropospheric flow. The results have implications for understanding subseasonal variability in the extratropical troposphere of both hemispheres

Coupled stratosphere-troposphere-Atlantic multidecadal oscillation and its importance for near-future climate projection

Northern Hemisphere (NH) climate has experienced various coherent wintertime multidecadal climate trends in stratosphere, troposphere, ocean, and cryosphere. However, the overall mechanistic framework linking these trends is not well established. Here we show, using long-term transient forced coupled climate simulation, that large parts of the coherent NH-multidecadal changes can be understood within a damped coupled stratosphere/troposphere/ocean-oscillation framework. Wave-induced downward propagating positive stratosphere/troposphere-coupled Northern Annular Mode (NAM) and associated stratospheric cooling initiate delayed thermohaline strengthening of Atlantic overturning circulation and extratropical Atlantic-gyres. These increase the poleward oceanic heat transport leading to Arctic sea-ice melting, Arctic warming amplification, and large-scale Atlantic warming, which in turn initiates wave-induced downward propagating negative NAM and stratospheric warming and therefore reverse the oscillation phase. This coupled variability improves the performance of statistical models, which project further weakening of North Atlantic Oscillation, North Atlantic cooling and hiatus in wintertime North Atlantic-Arctic sea-ice and global surface temperature just like the 1950s-1970s