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Clio Michel and Gwendal Rivière

Abstract

The response of the storm track and eddy-driven jet to various steady sea surface temperature (SST) forcings is analyzed using the aquaplanet configuration of the atmospheric general circulation model Action de Recherche Petite Échelle Grande Échelle Climat (ARPEGE-Climat). The SST profiles are zonally homogeneous and piecewise-linear functions of the latitude. It allows for modifying the tropical component that controls the intensity of the subtropical jet without changing the midlatitude SST front and vice versa. Sensitivities to the position, width, and intensity of the midlatitude SST front as well as to the tropical SSTs are investigated. The baroclinicity and the storm-track eddy activity both intensify for a stronger, a wider, or a more equatorward SST front. The eddy-driven jet always lies on the poleward side of the SST front and its relative distance to the front is interpreted in terms of upper-tropospheric Rossby wave breakings.

A focus is then made on the eddy-driven jet variability. The leading EOF of the vertical-average zonal-mean zonal wind is in most cases characterized by latitudinal shifting of the eddy-driven jet. For a more equatorward-shifted front, there is a more efficient positive eddy feedback. The mode has a greater persistence and explains a larger percentage of variance than for a more poleward-shifted front. However, the former case presents smaller latitudinal fluctuations than the latter because of the vicinity of the storm track to the subtropical jet. In the absence of positive eddy feedback, the pulsing of the eddy-driven jet intensity can dominate the variability such as for a high-latitude front having relatively small SSTs in the presence of an intense subtropical jet.

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Clio Michel and Gwendal Rivière

Abstract

The link between Rossby wave breaking (RWB) and the four wintertime weather regimes over the North Atlantic domain is studied in this paper. Using the 40-yr ECMWF Re-Analysis (ERA-40) data, frequencies of occurrence of anticyclonic and cyclonic wave-breaking (AWB and CWB, respectively) events are computed. Each weather regime has its own characteristic pattern of RWB frequencies. CWB events are found to be most frequent for the Greenland anticyclone weather regime whereas AWB events occur more for the Atlantic ridge and the zonal regimes. Time-lagged composites show that the RWB events characterizing each weather regime occur more often during the formation of the regime rather than during its decay. This suggests a reinforcement of the weather regime by RWB. An exception is the blocking weather regime, which is destroyed by an increase of CWB events south of Greenland.

Weather regime transitions are then studied using the low-frequency streamfunction tendency budget. Two types of precursors for the transitions have been identified. One is related to linear propagation of low-frequency transient eddies and the other to nonlinear interactions among the low- and high-frequency transient eddies. The latter has been related to the anomalous frequencies of occurrence of RWB. Two transitions are more precisely analyzed. The transition from blocking to Greenland anticyclone is triggered by a decrease of AWB events over Europe as well as a strong CWB event south of Greenland. The zonal to blocking transition presents evidence of two distinct precursors: one is a low-frequency wave train coming from the subtropical western Atlantic and the other, which occurs later, is characterized by a decrease of AWB and CWB events over western Europe that cannot continue to maintain the westerlies in that region.

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Clio Michel, Annick Terpstra, and Thomas Spengler
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Clio Michel, Annick Terpstra, and Thomas Spengler

Abstract

Polar mesoscale cyclones (PMCs) are automatically detected and tracked over the Nordic seas using the Melbourne University algorithm applied to ERA-Interim. The novelty of this study lies in the length of the dataset (1979–2014), using PMC tracks to infer relationships to large-scale flow patterns, and elucidating the sensitivity to different selection criteria when defining PMCs and polar lows and their genesis environments. The angle between the ambient mean and thermal wind is used to distinguish two different PMC genesis environments. The forward shear environment (thermal and mean wind have the same direction) features typical baroclinic conditions with a temperature gradient at the surface and a strong jet stream at the tropopause. The reverse shear environment (thermal and mean wind have opposite directions) features an occluded cyclone with a barotropic structure throughout the entire troposphere and a low-level jet. In contrast to previous studies, PMC occurrence features neither a significant trend nor a significant link with the North Atlantic Oscillation and the Scandinavian blocking (SB), though the SB negative pattern seems to promote reverse shear PMC genesis. The sea ice extent in the Nordic seas is not associated with overall changes in PMC occurrence but influences the genesis location. Selected cold air outbreak indices and the temperature difference between the sea surface and 500 hPa (SST − T500) show no robust link with PMC occurrence, but the characteristics of forward shear PMCs and their synoptic environments are sensitive to the choice of the SST − T500 threshold.

Open access
Annick Terpstra, Clio Michel, and Thomas Spengler

Abstract

The synoptic and subsynoptic environments associated with polar low genesis are examined. Ambient pre–polar low environments are classified as forward or reverse shear conditions based on the angle between the thermal and mean wind. Forward shear environments are associated with a synoptic-scale ridge over Scandinavia, featuring a zonally oriented baroclinic zone extending throughout the troposphere with a wind speed maximum at the tropopause. Similar to typical midlatitude cyclogenesis, concurrent wavelike development occurs both in the lower and upper troposphere along the baroclinic zone and the mean propagation direction is eastward, parallel to isolines of sea surface temperature. Reverse shear environments exhibit a distinctly different structure and are characterized by a trough over Scandinavia, associated with a synoptic-scale, occluded cyclone. The genesis area exhibits strong cold air advection on its right-hand side and polar low development occurs on the warm side of an intense low-level jet. The environment resembles the characteristics conducive to secondary development associated with frontal instability. Polar lows developing in this configuration propagate mainly southward, perpendicular to isolines of sea surface temperature. The two genesis environments exhibit similar temperature differences between the sea surface and atmosphere near the surface, yet the magnitude of the surface fluxes is approximately double during reverse shear conditions due to stronger low-level winds. The ratio between surface sensible and latent heat fluxes is close to unity for both shear environments.

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Clio Michel, Camille Li, Isla R. Simpson, Ingo Bethke, Martin P. King, and Stefan Sobolowski

Abstract

El Niño–Southern Oscillation (ENSO) is a main driver of climate variability worldwide, but the presence of atmospheric internal variability makes accurate assessments of its atmospheric teleconnections a challenge. Here, we use a multimodel large ensemble of simulations to investigate the ENSO teleconnection response to a low global warming scenario that represents Paris Agreement targets. The ensemble comprises five atmospheric general circulation models with two experiments (present-day and +2°C) in which the same set of ENSO events is prescribed, which allows for quantification of the uncertainty in the ENSO response due to internal variability. In winter, the teleconnection during the positive ENSO phase features a strong negative anomaly in sea level pressure over the northeast Pacific (and vice versa for the negative phase); this anomaly shifts northeastward and strengthens in the warming experiment ensemble. At least 50–75 ENSO events are required to detect a significant shift or strengthening, emphasizing the need to adequately sample the internal variability to isolate the forced response of the ENSO teleconnection under a low warming scenario. Even more events may be needed if one includes other sources of uncertainty not considered in our experimental setup, such as changes in ENSO itself. Over North America, precipitation changes are generally more robust than temperature changes for the regions considered, despite large internal variability, and are shaped primarily by changes in atmospheric circulation. These results suggest that the observational period is likely too short for assessing changes in the ENSO teleconnection under Paris Agreement warming targets.

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