Storm Track Intensity Research Articles

Southern Hemisphere Winter Storm Tracks Respond Differently to Low and High CO2 Forcings

Abstract In the Southern Hemisphere, Earth system models project an intensification of winter storm tracks by the end of the twenty-first century. Previous studies using idealized models showed that storm track intensity saturates with increasing temperatures, suggesting that the intensification of the winter storm tracks might not continue further with increasing greenhouse gases. Here, we examine the response of midlatitude winter storm tracks in the Southern Hemisphere to increasing CO2 from two to eight times preindustrial concentrations in more realistic Earth system models. We find that at high CO2 levels (beyond 4×CO2), winter storm tracks no longer exhibit an intensification across the extratropics. Instead, they shift poleward, weakening the storm tracks at lower midlatitudes and strengthening at higher midlatitudes. By analyzing the eddy kinetic energy (EKE) budget, the nonlinear storm-track response to an increase in CO2 levels in the lower midlatitudes is found to stem from a scale-dependent conversion of eddy available potential energy to EKE. Specifically, in the lower midlatitudes, this energy conversion acts to oppositely change the EKE of long and short scales at low CO2 levels, but at high CO2 levels, it mostly reduces the EKE of shorter scales, resulting in a poleward shift of the storms. Furthermore, we identify a “tug of war” between the upper and lower temperature changes as the primary driver of the nonlinear-scale-dependent EKE response in the lower midlatitudes. Our results suggest that in the highest emission scenarios beyond the twenty-first century, the storm tracks’ response may differ in magnitude and latitudinal distribution from projected changes by 2100. Significance Statement The Southern Hemisphere winter storm track is projected to intensify by the end of the century, with the most significant intensification occurring in the higher midlatitudes. However, we show that the intensification is not a linear function of the radiative forcing associated with increasing CO2 levels. In fact, our study shows a poleward shift at very high CO2 levels, with the storm track moving southward. This suggests that the Southern Hemisphere winter storm track may require time-sensitive adaptation strategies, as the impacts of global warming on the storm track may not be a linear function of CO2 concentration in the atmosphere.

A Cloud-Controlling Factor Perspective on the Hemispheric Asymmetry of Extratropical Cloud Albedo

Abstract The Northern and Southern Hemispheres reflect on average almost equal amounts of sunlight due to compensating hemispheric asymmetries in clear-sky and cloud albedo. Recent work indicates that the cloud albedo asymmetry is largely due to clouds in extratropical oceanic regions. Here, we investigate the proximate causes of this extratropical cloud albedo asymmetry using a cloud-controlling factor (CCF) approach. We develop a simple index that measures the skill of CCFs, either individually or in combination, in predicting the asymmetry. The index captures the contribution to the asymmetry due to interhemispheric differences in the probability distribution function of daily CCF values. Cloud albedo is quantified using daily MODIS satellite retrievals, and is related to range of CCFs derived from the ERA5 product. We find that sea surface temperature is the CCF that individually explains the largest fraction of the asymmetry, followed by surface wind. The asymmetry is predominantly due to low clouds, and our results are consistent with prior local-scale modeling work showing that marine boundary layer clouds become thicker and more extensive as surface wind increases and surface temperature cools. The asymmetry is consistent with large-scale control of storm-track intensity and surface winds by meridional temperature gradients: persistently cold and windy conditions in the Southern Hemisphere keep cloud albedo high year-round. Our results have important implications for global-scale cloud feedbacks and contribute to efforts to develop a theory for planetary albedo and its symmetry.

Roles of eddy generation and jet characteristics in setting the annual cycle of Siberian storm track

AbstractThe Siberian storm track is one of the drivers of the East Asian extreme weather events. Using the daily JRA‐55 reanalysis data from 1980 to 2021, this study examines roles of eddy generation and jet characteristics in setting the annual cycle of Siberian storm track. It is found that there are two peaks of Siberian storm track intensity in boreal spring and autumn. The possible reason for such an annual cycle is explored by analyzing the maximum Eady growth rate over the Siberian region and jet characteristics. The stronger Siberian Eady growth rate in boreal spring and autumn, favoring a stronger baroclinic eddy generation, could contribute to the stronger intensity of Siberian storm tracks in these two seasons. Furthermore, the Siberian jet stream cores during boreal spring and autumn are located north of 50° N and resembles more an eddy‐driven jet. While in winter, the subtropical jet stream enhanced and the eddy‐driven jet becomes relatively weaker, which is less efficient to generate midlatitude baroclinic eddies. Besides, the eddy‐driven jet can modulate the horizontal wave propagations from upstream, which also plays a role in amplifying the spring and autumn Siberian storm tracks.

The Midlatitude Response to Polar Sea Ice Loss: Idealized Slab-Ocean Aquaplanet Experiments with Thermodynamic Sea Ice

Abstract Slab-ocean aquaplanet simulations with thermodynamic sea ice are used to study the zonally symmetric mechanisms whereby polar sea ice loss impacts the midlatitude atmosphere. Imposed sea ice loss (difference without and with sea ice with historical CO2 concentration) leads to global warming, polar amplified warming, and a weakening of poleward atmospheric energy transport and the midlatitude storm-track intensity. The simulations confirm an energetic mechanism that predicts a weakening of storm-track intensity in response to sea ice loss, given the change of surface albedo and assuming a passive ocean. Namely, sea ice loss increases the absorption of shortwave radiation by the surface (following the decrease of surface albedo), which increases surface turbulent fluxes into the atmosphere thereby weakening poleward atmospheric energy transport. The storm-track intensity weakens because it dominates poleward energy transport. The quantitative prediction underlying the mechanism captures the weakening but underestimates its amplitude. The weakening is also consistent with weaker mean available potential energy (polar amplified warming) and scales with sea ice extent, which is controlled by the slab-ocean depth. The energetic mechanism also operates in response to sea ice loss due to melting (difference of the response to quadrupled CO2 with and without sea ice). Finally, the midlatitude response to sea ice loss in the aquaplanet agrees qualitatively with the response in more complex climate models. Namely, the storm-track intensity weakens and the energetic mechanism operates, but the method used to impose sea ice loss in coupled models impacts the surface response.

Interdecadal Modulation of the Pacific Decadal Oscillation on the Relationship Between Spring Arctic Oscillation and the Following Winter ENSO

Previous studies indicated that Arctic Oscillation (AO) in boreal spring is an important extratropical trigger for the outbreak of El Niño and Southern Oscillation (ENSO) events in the succedent winter. This study reveals that the Pacific Decadal Oscillation (PDO) has a strong modulation on the linkage between the spring AO and the following winter ENSO. Particularly, impact of the spring AO on the succedent winter ENSO is strong during positive PDO phase (+PDO). By contrast, the spring AO-winter ENSO connection is weak during negative PDO phase (−PDO). During +PDO, positive spring AO induces a marked anomalous cyclone over the subtropical North Pacific via wave-mean flow interaction. The subtropical cyclonic anomaly leads to sea surface temperature (SST) warming and enhanced atmospheric heating there, which could further propagate southward to the tropical central Pacific via wind-evaporation-SST feedback mechanism and, thus, impact the following winter El Niño via the tropical process. During −PDO, the spring AO-generated SST, the atmospheric circulation, and the heating anomalies over the North Pacific are much weaker. As such, spring AO has weak impacts on the winter ENSO. The spring climatological storm track is stronger during +PDO than −PDO years due to an increase in the mean meridional temperature gradient over the North Pacific. Stronger storm track intensity during +PDO leads to stronger synoptic-scale eddy feedback to the mean flow, which results in stronger AO-related SST and atmospheric anomalies over the North Pacific and, thus, the stronger impact of the spring AO on the following winter ENSO.

Contrasting interannual impacts of European and Greenland blockings on the winter North Atlantic storm track

Based on the daily reanalysis, the present study examines the interannual influences of the European blocking (EB) and Greenland blocking (GB) frequencies on the winter North Atlantic storm track (WNAST) intensity. There are contrasting relationships between the two types of atmospheric blockings and the WNAST. The EB and GB frequencies are significantly positively and negatively related to the WNAST intensity, respectively. Composite analysis shows a meridional dipole of the WNAST anomalies associated with both frequent EB and GB. The area of EB-related positive WNAST anomalies is obviously larger than that of the negative anomalies, which directly results in their in-phase relationship. However, the amplitudes of the GB-related negative WNAST anomalies that are larger than those of the positive anomalies directly lead to their out-of-phase relationship. The EB-related intensified WNAST is dependent on the enhanced baroclinic energy conversion and the strengthened atmospheric baroclinicity induced by the anomalous westerly winds on the northern side of the EB-related anticyclone. In contrast, the GB-related weakened WNAST is determined by the attenuated atmospheric baroclinicity and baroclinic energy conversion on the southern flank of the WNAST climatology.

Future Summer Drying in the U.S. Corn Belt and the Role of Midlatitude Storm Tracks

AbstractDuring the summer, the Midwest United States, which covers the main US corn belt, has a net loss of surface water as evapotranspiration exceeds precipitation. The net moisture gain into the atmosphere is transported out of the region to northern high latitudes through transient eddy moisture fluxes. How this process may change in the future is not entirely clear despite the fact that the corn belt region is responsible for a large portion of the global supply of corn and soybeans. We find that increased CO2 and the associated warming increases evapotranspiration. while precipitation reduces in the region leading to further reduction in precipitation minus evaporation (P-E) in the future. At the same time, the poleward transient moisture flux increases leading to enhanced atmospheric moistures export from the corn belt region. However, storm track intensity is generally weakened in the summer due to reduced north-south temperature gradient associated with amplified warming in the midlatitudes. The intensified transient eddy moisture transport as storm track weakens can be reconciled by the stronger mean moisture gradient in the future. This is found to be caused by the climatological low-level jet transporting more moisture into the Great Plains region due to the thermodynamic mechanism under warmer conditions. Our results, for the first time, show that in the future, the US Midwest corn belt will experience more hydrological stress due to intensified transient eddy moisture export leading to drier soils in the region.

Hydrological Cycle Changes Explain Weak Snowball Earth Storm Track Despite Increased Surface Baroclinicity

AbstractSimulations show that storm tracks were weaker during past cold, icy climates relative to the modern climate despite increased surface baroclinicity. Previous work explained the weak North Atlantic storm track during the Last Glacial Maximum using dry zonally asymmetric mechanisms associated with orographic forcing. Here we show that zonally symmetric mechanisms associated with the hydrological cycle explain the weak Snowball Earth storm track. The weak storm track is consistent with the decreased meridional gradient of evaporation and atmospheric shortwave absorption and can be predicted following global mean cooling and the Clausius‐Clapeyron relation. The weak storm track is also consistent with decreased latent heat release aloft in the tropics, which decreases upper tropospheric baroclinicity and mean available potential energy. Overall, both hydrological cycle mechanisms are reflected in the significant correlation between storm track intensity and the meridional surface moist static energy gradient across a range of simulated climates between modern and Snowball Earth.

Alleviation of an Arctic Sea Ice Bias in a Coupled Model Through Modifications in the Subgrid-Scale Orographic Parameterization.

In climate models, the subgrid‐scale orography (SSO) parameterization imposes a blocked flow drag at low levels that is opposed to the local flow. In IPSL‐CM6A‐LR, an SSO lift force is also applied perpendicular to the local flow to account for the effect of locally blocked air in narrow valleys. Using IPSL‐CM6A‐LR sensitivity experiments, it is found that the tuning of both effects strongly impacts the atmospheric circulation. Increasing the blocking and reducing the lift lead to an equatorward shift of the Northern Hemisphere subtropical jet and a reduction of the midlatitude eddy‐driven jet speed. It also improves the simulated synoptic variability, with a reduced storm‐track intensity and increased blocking frequency over Greenland and Scandinavia. Additionally, it cools the polar lower troposphere in boreal winter. Transformed Eulerian Mean diagnostics also show that the low‐level eddy‐driven subsidence over the polar region is reduced consistent with the simulated cooling. The changes are amplified in coupled experiments when compared to atmosphere‐only experiments, as the low‐troposphere polar cooling is further amplified by the temperature and albedo feedbacks resulting from the Arctic sea ice growth. In IPSL‐CM6A‐LR, this corrects the warm winter bias and the lack of sea ice that were present over the Arctic before adjusting the SSO parameters. Our results, therefore, suggest that the adjustment of SSO parameterization alleviates the Arctic sea ice bias in this case. However, the atmospheric changes induced by the parametrized SSO also impact the ocean, with an equatorward shift of the Northern Hemisphere oceanic gyres and a weaker Atlantic meridional overturning circulation.

Impacts of the Atlantic Multidecadal Oscillation on the Relationship of the Spring Arctic Oscillation and the Following East Asian Summer Monsoon

AbstractPrevious studies indicated that spring Arctic Oscillation (AO) can influence the following East Asian summer monsoon (EASM). This study reveals that the Atlantic multidecadal oscillation (AMO) has a pronounced modulation of the spring AO–EASM connection. Spring AO has a close relation with the EASM during the negative AMO (−AMO) phase. However, during the positive AMO (+AMO) phase, the spring AO–EASM connection is weak. During the −AMO phase, a marked dipole atmospheric anomaly pattern (with an anticyclonic anomaly over the midlatitudes and a cyclonic anomaly over the subtropics) and a pronounced tripole sea surface temperature (SST) anomaly pattern is formed in the North Pacific during positive spring AO years. The cyclonic anomaly, SST, and precipitation anomalies over the subtropical western North Pacific (WNP) maintain and propagate southwestward in the following summer via a positive air–sea feedback, which further impacts the EASM variation. During the +AMO phase, the Pacific center of the spring AO (i.e., the anticyclonic anomaly over the midlatitudes) is weak. As such, the cyclonic anomaly cannot be induced over the subtropical WNP by the spring AO via wave–mean flow interaction. Hence, the spring AO–EASM connection disappears during the +AMO phase. The AMO impacts the Pacific center of the spring AO via modulating the Aleutian low intensity and North Pacific storm track intensity. The observed AMO modulation of the spring AO–EASM connection and Pacific center of the spring AO can be captured by the long historical simulation in a coupled global climate model.

On the Efficiency of Baroclinic Eddy Growth and How It Reduces the North Pacific Storm-Track Intensity in Midwinter

Abstract This study investigates the efficiency of baroclinic eddy growth in an effort to better understand the suppression of the North Pacific storm-track intensity in winter. The efficiency of baroclinic eddy growth depends on the magnitude and orientation of the vertical tilt of the eddy geopotential isolines. The eddy efficiency is maximized if the orientation of the vertical tilt creates an eddy heat flux that aligns with the mean baroclinicity (defined as minus the temperature gradient divided by a stratification parameter) and if the magnitude of the vertical tilt is neither too strong nor too weak. The eddy efficiency is, in contrast to most other eddy measures, independent of the eddy amplitude and thus useful for improving our mechanistic understanding of the effective eddy growth. During the midwinter suppression, the eddy efficiency is reduced north of 40°N over a region upstream of the main storm track, and baroclinic growth is reduced despite a maximum in baroclinicity. Eulerian diagnostics and feature tracking suggest that the reduction in eddy efficiency is due to a stronger poleward tilt with height of eddies entering the Pacific through the northern seeding branch, which results in a more eastward-oriented eddy heat flux and a reduced alignment with the baroclinicity. The stronger poleward tilt with height is constrained by the eddy propagation direction, which is more equatorward when the subtropical jet moves equatorward in winter. In addition, the westward tilt with height is too strong. South of 40°N, the eddy efficiency increases during midwinter but in a region far away from the main storm track.

Investigating the Influence of Cloud Radiative Effects on the Extratropical Storm Tracks

AbstractRecent studies have focused on the role of cloud radiative effects (CRE) in governing the mean atmospheric circulation and its response to climate change. This study instead examines the role of CRE in climate variability in the extratropics. Cloud locking experiments are performed using the Community Earth System Model. In these experiments, CRE are scrambled, such that they maintain the same climatology but no longer match the model's dynamical fields. The results of these experiments indicate that high‐frequency interactions between CRE and dynamics have a small (≤5–10%) but statistically significant damping effect on the intensity of the extratropical storm tracks, particularly in the Southern Hemisphere. Individual midlatitude cyclones have decreased intensity and shorter lifetime. These effects arise largely from clouds' radiative modification of static stability below 700 hPa. The coupling among clouds, radiation, and dynamics thus has a modest but potentially important influence on the extratropical storm tracks.

Quasi-stationary extratropical wave trains associated with distinct tropical Pacific seasonal mean convection patterns: observational and AMIP model results

In this paper, mechanisms for the formation of quasi-stationary extratropical wave trains associated with four distinct winter patterns of seasonal mean tropical Pacific convection anomalies, including the western CP (W-CP) pattern, La Nina (LN) pattern, CP El Nino (CPEN) pattern and EP El Nino (EPEN) pattern, are investigated by utilizing observational and six Atmospheric Model Intercomparison Project Phase 5 (AMIP5) high-skilled models (ACCESS1.0, CanAM4, CCSM4, CMCC-CM, HadGEM2-A, MPI-ESM-LR) datasets. The observational results show that tropical Pacific forcing induces upper-level anomalous Rossby wave source that force vorticity over the central North Pacific through modulating the local Hadley circulation, then excite eastward propagation of quasi-stationary planetary waves penetrating into the North Atlantic, and finally establish the associated teleconnections in mid- and high-latitudes by local synoptic eddy–mean flow interaction. Different structures of wave trains may be explained by evident differences in intensity and scale of the extratropical vorticity forcing originating from distinct convection anomalies over the tropical Pacific. The MME results further confirm the possible physical processes obtained from observations, but there still exist some significant differences. These cover the North Pacific and North America and can possibly be attributed to the models’ simulation ability of extratropical vorticity forcing and local storm track intensity. In addition, the stratosphere–troposphere interaction plays an important role for extratropical atmospheric circulation anomalies. The stratospheric polar vortex not only modulates the underlying wave train, especially for the North Atlantic/Europe sector, but is also affected by the upper-tropospheric disturbance in high-latitudes through the upward wave activity flux.

On the Inverse Relationship between the Boreal Wintertime Pacific Jet Strength and Storm-Track Intensity

AbstractPrevious studies show that in boreal winters when the Pacific jet is extremely strong, the Pacific storm track is, however, unexpectedly weak. Using a recently developed technique, namely, the multiscale window transform (MWT), and the MWT-based localized multiscale energetics analysis, we investigate in this study the underlying mechanism of this counterintuitive phenomenon, based on ERA-40 data. It is found that most of the synoptic storms are generated at latitudes far north of the jet core, which lowers the relevance of the jet strength to the storm-track intensity, and the inverse relationship between the Pacific jet strength and storm-track intensity is mainly attributed to the internal dynamics. In the strong jet state, on one hand, the jet is narrow, and thus the jet winds at high latitudes are weak, resulting in weak baroclinic instabilities and hence reduced eddy growth rate; on the other hand, although baroclinic instabilities are strong at the jet core, inverse kinetic energy (KE) cascades are even stronger (by 43%). The resultant effect is that more eddy energy is transferred back to the background flow, leaving an overall weak storm track in a strong Pacific jet. In addition, diabatic processes are found to account for the inverse relationship: it is greatly weakened (by 25%) in the strong-core jet state. Apart from these, we also find that the role that barotropic canonical transfer plays in the inverse relationship is opposite to that in the formation of the midwinter minimum (MWM), another counterintuitive phenomenon in the Pacific storm track.

On the link between the North Atlantic storm track and precipitation deuterium excess in Reykjavik

This study establishes a robust process‐based dynamical link between the variability of the deuterium excess (d) in monthly winter precipitation in Reykjavik and the frequency of strong large‐scale ocean evaporation (SLOE) events in the subpolar North Atlantic. SLOE events are induced by cold advection in the rear of extratropical cyclones. The link between SLOE and d emerges from the fact that a substantial fraction of moisture that precipitates in Reykjavik in winter originates from SLOE events. It is shown that positive anomalies of monthly d co‐occur with positive anomalies in the frequency of SLOE events at the moisture source of Reykjavik precipitation. An intensification and a northeastwards shift of the North Atlantic storm track acts as a driver for the enhanced SLOE frequency in months with positive anomalies in d. This link also implies that in the subpolar North Atlantic, variations of d in monthly (or seasonal) precipitation reveal information about the storm track dynamics in the vicinity of Iceland. Potential implications of this finding for the interpretation of d as a proxy for the location and intensity of storm tracks are discussed.

Application of Intelligent Dynamic Bayesian Network with Wavelet Analysis for Probabilistic Prediction of Storm Track Intensity Index

The effective prediction of storm track (ST) is greatly beneficial for analyzing the development and anomalies of mid-latitude weather systems. For the non-stationarity, nonlinearity, and uncertainty of ST intensity index (STII), a new probabilistic prediction model was proposed based on dynamic Bayesian network (DBN) and wavelet analysis (WA). We introduced probability theory and graph theory for the first time to quantitatively describe the nonlinear relationship and uncertain interaction of the ST system. Then a casual prediction network (i.e., DBN) was constructed through wavelet decomposition, structural learning, parameter learning, and probabilistic inference, which was used for expression of relation among predictors and probabilistic prediction of STII. The intensity prediction of the North Pacific ST with data from 1961–2010 showed that the new model was able to give more comprehensive prediction information and higher prediction accuracy and had strong generalization ability and good stability.

A Moist Static Energy Framework for Zonal-Mean Storm-Track Intensity

Abstract A moist static energy (MSE) framework for zonal-mean storm-track intensity, defined as the extremum of zonal-mean transient eddy MSE flux, is derived and applied across a range of time scales. According to the framework, storm-track intensity can be decomposed into contributions from net energy input [sum of shortwave absorption and surface heat fluxes into the atmosphere minus outgoing longwave radiation (OLR) and atmospheric storage] integrated poleward of the storm-track position and MSE flux by the mean meridional circulation or stationary eddies at the storm-track position. The framework predicts storm-track decay in spring and amplification in fall in response to seasonal insolation. When applied diagnostically the framework shows shortwave absorption and land turbulent surface heat fluxes account for the seasonal evolution of Northern Hemisphere (NH) intensity; however, they are partially compensated by OLR (Planck feedback) and stationary eddy MSE flux. The negligible amplitude of Southern Hemisphere (SH) seasonal intensity is consistent with the compensation of shortwave absorption by OLR and oceanic turbulent surface heat fluxes (ocean energy storage). On interannual time scales, El Niño minus La Niña conditions amplify the NH storm track, consistent with decreased subtropical stationary eddy MSE flux. Finally, on centennial time scales, the CO2 indirect effect (sea surface temperature warming) amplifies the NH summertime storm track whereas the direct effect (increased CO2 over land) weakens it, consistent with opposing turbulent surface heat flux responses over land and ocean.

Southeast Greenland Winter Precipitation Strongly Linked to the Icelandic Low Position

Abstract Greenland’s largest precipitation flux occurs in its southeast (SE) region during the winter, controlled primarily by easterly winds and frequent cyclogenesis in the North Atlantic. Several studies have attempted to link SE Greenland precipitation to the North Atlantic Oscillation (NAO) but results are inconsistent. This work uses reanalysis, automatic weather station data, and regional climate model output to show that the east–west position of the Icelandic low is a better predictor of SE Greenland precipitation (average correlation of r = −0.48 in DJF) than climate indices such as the NAO (r = −0.06 in DJF). In years when the Icelandic low is positioned extremely west, moisture transport increases up to ~40% (or up to 40 kg m−1 s−1) off the SE Greenland coast compared to when the low is in an extreme east position. Furthermore, in years when the Icelandic low is positioned extremely west, storm track density and intensity increase just off the SE coast of Greenland. Thus, the Icelandic low’s longitudinal position dominates SE Greenland ice sheet’s wintertime precipitation, a positive term in the ice sheet mass balance. Given SE Greenland’s importance in the overall ice sheet mass balance, the position of the Icelandic low is therefore important for making projections of future sea level.

Interannual variability of the North Pacific winter storm track and its relationship with extratropical atmospheric circulation

Interannual variability of the North Pacific storm track and the three-dimensional atmosphere circulation during winter are investigated using NCEP/NCAR reanalysis data during 1950–2015. Results show that year-to-year variations of the storm track exhibit two principal modes, i.e. the monopole intensity change and the meridional shift of the storm track, respectively. The intensity change mode is linked to weakening of the Siberian high, northward shift of the western Pacific jet stream and Aleutian Low, and well corresponding to the Western Pacific teleconnection. The meridional shift mode is related to intensification and south-eastward extension of western Pacific jet stream and Aleutian Low, and linked to the Pacific-North America teleconnection. The internal atmospheric dynamics responsible for the storm track variability is further investigated from the perspective of wave-flow energy conversion. For the intensity change mode, accompanied by the enhanced baroclinity over the entrance region of the storm track, more energy is converted from mean available potential energy to eddy available potential energy and then transferred to eddy kinetic energy, which is favorable for the overall enhancement of the storm track intensity. For the meridional shift mode, more energy is transformed from mean available potential energy to eddy available potential energy and further transferred to eddy kinetic energy over the southern (northern) areas of the storm track, contributing to the southward (northward) shift of the storm track. Additionally, the increased (decreased) conversion from mean-flow kinetic energy to eddy kinetic energy over the north-eastern Pacific region is also in favor of the southward (northward) shift of the storm track.

A Midwinter Minimum in North Atlantic Storm Track Intensity in Years of a Strong Jet

AbstractThis study investigates the occurrence of a midwinter suppression in synoptic eddy activity within the North Atlantic storm track. It is found that eddy kinetic energy over the Atlantic is reduced during winter relative to fall and spring, despite the stronger wintertime jet and enhanced baroclinicity. This behavior is similar to the well‐known Pacific midwinter minimum, yet the reduction over the Atlantic is smaller and persists for a shorter period. To examine the conditions favorable for this phenomenon, we present an analysis of years with stronger jet intensity versus years of weaker jets over the Atlantic and Pacific basins. When the wintertime jet is stronger, the midwinter suppression of eddy activity is more pronounced, and the jet is more equatorward. Since the climatological Atlantic jet is weaker relative to the Pacific jet, the conditions for a midwinter suppression in the Atlantic are generally less favorable, yet a midwinter suppression often occurs in years of a strong jet.