Evaluation of synoptic eddy activities and their feedback onto the midlatitude jet in five atmospheric reanalyses with coarse versus fine model resolutions

Interaction between synoptic eddy and mean flow plays a crucial role in maintaining midlatitude westerly jet. In this study, climatologies of synoptic eddy activities and their feedback onto midlatitude jet for 1980–2016 are evaluated and compared through analyzing daily data from five atmospheric reanalyses with different resolutions including one coarse-resolution reanalysis (NCEP2) and four fine-resolution reanalyses (ERA-Interim, JRA-55, MERRA-2, and CFSR). Horizontal resolutions of the atmospheric models generating those reanalyses are approximately equivalent to 210, 79, 60, 50, and 38 km, respectively. Results show that the eddy activities and their feedback onto the midlatitude jet in those fine-resolution reanalyses are consistently and significantly stronger than those in the coarse-resolution reanalysis (NCEP2). The maximal relative increases that are found to occur primarily in the midlatitudes of the Southern Hemisphere are estimated to be up to 55% for the baroclinicity, 53% for the eddy energetics, 59% for the eddy forcing, and even 85% for the eddy feedback onto the mean flow. Those increases are reasonably conjectured to be related to increased model resolutions, since the synoptic eddy genesis is proportional to the low-level atmospheric meridional temperature gradient which is sensitive to the meridional resolution of atmospheric models. Although the coarse-resolution reanalysis resolves synoptic eddies insufficiently and thus underestimates their feedback onto the mean flow, the magnitudes of eddy-driven jets are almost the same among five reanalyses, implying a mismatch between the eddy feedback and the eddy-driven jet in the coarse-resolution reanalysis. Therefore, the results of this study imply the importance of using fine-resolution reanalyses in accurately understanding the midlatitude synoptic eddy–mean flow interaction.

Evaluation of Synoptic Eddy Activities and their Feedback onto the Midlatitude Jet in Five Atmospheric Reanalyses with Coarse Versus Fine Model Resolutions

Interaction between synoptic eddy and mean flow plays a crucial role in maintaining midlatitude westerly jet. In this study, climatologies of synoptic eddy activities and their feedback onto midlatitude jet for 1980–2016 are evaluated and compared through analyzing daily data from five atmospheric reanalyses with different resolutions including one coarse-resolution reanalysis (NCEP2) and four fine-resolution reanalyses (ERA-Interim, JRA-55, MERRA-2, and CFSR). Horizontal resolutions of the atmospheric models generating those reanalyses are approximately equivalent to 210, 79, 60, 50, and 38 km, respectively. Results show that the eddy activities and their feedback onto mean flow in those fine-resolution reanalyses are consistently and significantly stronger than those in the coarse-resolution reanalysis (NCEP2). The maximal relative increases that are found to occur primarily in the midlatitudes of the Southern Hemisphere are estimated to be up to 55% for the baroclinicity, 53% for the eddy energetics, 59% for the eddy forcing, and even 85% for the eddy feedback onto mean flow. Those increases are reasonably conjectured to be related to increased model resolutions, since the synoptic eddy genesis is proportional to the lower-level atmospheric meridional temperature gradient which is sensitive to the meridional resolution of atmospheric models. Although the coarse-resolution reanalysis resolves synoptic eddies insufficiently and thus underestimates their feedback onto the mean flow, the magnitudes of eddy-driven jets are almost the same among five reanalyses, implying a mismatch between the eddy feedback and the eddy-driven jet in the coarse-resolution reanalysis. Therefore, the results of this study imply the importance of using fine-resolution reanalyses in accurately understanding the midlatitude synoptic eddy-mean flow interaction.

Contrasting dynamics of short and long blocks in the Northern Hemisphere

<p>In this study, we aim at identifying dynamical differences between short blocks, which last only five days, and long blocks, which last at least ten days, to better characterise long blocks. We show that long blocks often involve cyclonic Rossby wave breaking, while short blocks are equally associated with cyclonic and anticyclonic wave breaking. This main result is reproduced in several coupled climate models. We propose three mechanisms that might explain the lower number of long anticyclonic blocks: 1/ a downstream reinforcement of the anticyclone during anticyclonic blocks might be associated with a stronger downstream advection of the block; 2/ the mean zonal wind is reinforced by synoptic eddies towards a more northward position during anticyclonic blocks, whereas synoptic eddies force the mean zonal wind to the south of the block during cyclonic blocks, which has been previously shown to be associated with more persistent weather patterns; 3/ strong and/or sustained eddy feedback is needed to maintain long anticyclonic blocks. All these parameters combined might explain why blocks last longer and why anticyclonic blocks are less present at extreme durations.</p>

EXPEREMENT POLYGON-70

In 1970, an experiment named Polygon–70 was carried out by Russian oceanographers in the tropical zone of the North Atlantic. During this experiment, seventeen autonomous buoy moorings were deployed for the period of half-a-year to measure ocean currents in the ocean layer ranging from the surface to the depth of 1500 m. The outcome of this work was the discovery of what later became known as “synoptic eddies of the open ocean”. It revealed the scale, the nature of time variability and energy of these vortices, as well as an interpretation of the observed phenomena from the point of view of the theory of Rossby waves. It was suggested, that the emergence of the vortices was due to baroclinic instability of large-scale flow. These results of Polygon–70 expedition showed its outstanding value and provided basis for further development of studies of ocean vortices.

The Critical Role of Non-Normality in Partitioning Tropical and Extratropical Contributions to PNA Growth

The Pacific–North American (PNA) teleconnection pattern has been linked both to tropical phenomena, including the Madden–Julian oscillation (MJO) and El Niño–Southern Oscillation (ENSO), and to internal extratropical processes, including interactions with the zonally varying basic state and synoptic eddies. Many questions remain, however, concerning how these various relationships act, both separately and together, to yield observed PNA variability. Using linear inverse modeling (LIM), this study finds that the development and amplification of PNA anomalies largely results from the interference of modes strongly coupled to sea surface temperatures (SST), such as ENSO, and modes internal to the atmosphere, including the MJO. These SST-coupled and “internal atmospheric” modes form subspaces that are not orthogonal, and PNA growth is shown to occur via non-normal interactions. An internal atmospheric space LIM is developed to examine growth beyond this interference by removing the SST-coupled modes, effectively removing ENSO and retaining MJO variability. Optimal PNA growth in the internal atmospheric space LIM is driven by MJO heating, particularly over the Indian Ocean, and a retrograding northeast Pacific streamfunction anomaly. Additionally, the individual contributions of tropical heating and the extratropical circulation on PNA growth are investigated. The non-normal PNA growth is an important result, demonstrating the difficulty in partitioning PNA variance into contributions from different phenomena. This cautionary result is likely applicable to many geophysical phenomena and should be considered in attribution studies.

North Atlantic Circulation Regimes and Heat Transport by Synoptic Eddies

Meridional transport of heat by transient atmospheric eddies is a key component of the energy budget of the middle- and high-latitude regions. The heat flux at relevant frequencies is also part of a dynamical interaction between eddies and mean flow. In this study we investigate how the poleward heat flux by high-frequency atmospheric transient eddies is modulated by North Atlantic weather regimes in reanalysis data. Circulation regimes are estimated via a clustering method, a jet-latitude index, and a blocking index. Heat transport is defined as advection of moist static energy. The focus of the analysis is on synoptic frequencies but results for slightly longer time scales are reported. Results show that the synoptic eddy heat flux is substantially modulated by midlatitude weather regimes on a regional scale in midlatitude and polar regions. In a zonal-mean sense, the phases of the North Atlantic Oscillation do not significantly change the high-latitude synoptic heat flux, whereas Scandinavian blocking and the Atlantic ridge are associated with an intensification. A close relationship between high-latitude (midlatitude) heat flux and Atlantic jet speed (latitude) is found. The relationship between extreme events of synoptic heat flux and circulation regimes is also assessed and reveals contrasting behaviors in the polar regions. The perspective that emerges complements the traditional view of the interaction between synoptic eddies and the extratropical flow and reveals relationships with the high-latitude climate.

Coherent changes in large-scale thermal structure and baroclinic life cycle of synoptic eddies in the Northern Hemisphere under global warming

<p><strong>There is a growing concern that human-induced climate change has been affecting weather systems. However, robust observational evidences that confirm the links between global warming and synoptic phenomena at the global scale are lacking. Here we reveal robust covarying signals between poleward temperature gradient and baroclinic life cycle of synoptic (1-10 days) eddies under global warming. We note that the changes in temperature structure in Northern Hemisphere winter and summer in the past decades are different. In boreal winter, the tropospheric warming has been larger in tropical upper troposphere and around 30°N than for the midlatitude (30-60°N). This inhomogeneous warming resulted in the enhancement of poleward temperature gradient in the subtropical upper troposphere and in the lower midlatitude (30-45°N). We observed correlated increasing trends in the entire baroclinic life cycle of synoptic eddies</strong><strong> — </strong><strong>including eddy fluxes of heat and momentum, and zonal mean jet</strong><strong> — </strong><strong>associated with steepened poleward temperature gradients in these regions in the winter Northern Hemisphere over the past four decades. By contrast, in the summer Northern Hemisphere, the overall tropospheric warming over the mid- to high-latitude land areas has been accompanied by weakly reduced synoptic eddy activities and zonal mean flow. Our findings suggest that if greenhouse gas–induced warming continue to change the atmospheric thermal structure as projected in a warming climate, extratropical synoptic disturbances and large-scale circulations may change accordingly. </strong></p>

ENERGY INTERACTION OF THE JETS AND EDDIES OF ANTARCTIC CIRCUMPOLAR CURRENT IN THE NEAR-SURFACE LAYER OF THE SOUTHERN OCEAN

Kinetic energy six jets of the Antarctic Circumpolar Current (ACC), and of synoptic eddies generated by these jets is studied in application to the near-surface layer of the Antarctic Circle on the base of the satellite altimeter data during 1993–2015. The main results of the study were as follows: a) prevalence of the energy of middle jet of the Subantarctic Current over energy of the rest ACC jets in the whole of the Antarctic Circle; b) five times excess of the mean energy of jets proper over the mean summary (cyclones plus anticyclones) energy of eddies; c) two times excess of mean energy of cyclonic eddies over energy of anticyclones.