Northern Hemisphere Storm Tracks Research Articles

Impacts of Strong El Niño–Southern Oscillation Events on Wintertime Northern Hemisphere Storm Tracks in Two Pacific Decadal Oscillation Phases during 1950–2010

Northern Hemisphere storm track activities (NHSTs) not only influence the weather and climate along their paths, but they also play a crucial role in climate systems by systematically transporting heat, momentum, and moisture. Distinguish from previous studies focusing on atmospheric circulation anomalies, this study provides further evidence of wintertime NHSTs variation under the influence of strong El Niño–Southern Oscillation (ENSO) events and Pacific Decadal Oscillation (PDO) variation with ERA-20C reanalysis data, from two mathematical aspects of linear superposition and nonlinear modulation. While ENSO warm/cold events lead the entire NHSTs to a general south/north shift, the ENSO impact in two PDO phases exhibits a remarkable difference with the PDO phase. The Pacific storm track (PST) anomalies largely strengthen equatorward and downstream when both ENSO and PDO are in phase, but its anomalies tend to be weakened when ENSO and PDO are out of phase. Generally, the opposite situation occurs with Atlantic storm track (AST) anomalies, which display a strengthening dipole pattern when ENSO and PDO are out of phase. Apparently, the result is roughly a linear superposition of ENSO and PDO-only impacts. Nevertheless, further analyses indicate that the nonlinear modulation of PDO on the ENSO impact on NHSTs exists. With respect to the PST, it exhibits approximately parallel bands of south-north dipole anomalies when ENSO is in the PDO positive phase, but only the south branch remains when ENSO is in the PDO negative phase. Generally, a similar situation occurs to AST anomalies over the Atlantic Ocean. The modulation may be mainly associated with the atmospheric mean flow anomalies and the midlatitude sea surface temperature anomalies to ENSO in different PDO phases. To some extent, the results may be beneficial for understanding the variation of extreme weather events brought by NHSTs.

The Northern Hemisphere Wintertime Storm Track Simulated in the High‐Resolution Community Earth System Model

AbstractWith a pair of high‐resolution (HR, 10 km ocean and 25 km atm) and low‐resolution (LR, 100 km ocean and 100 km atm) Community Earth System Model (CESM) simulations, we investigate and compare the long‐term mean Northern Hemisphere (NH) wintertime storm tracks against reanalysis data. Based on several Eulerian measures, the CESM‐HR shows a poleward shift of the NH storm tracks and a reduced equatorward bias in the North Pacific relative to the CESM‐LR. Those may be associated with the refined topography of the Tibetan Plateau as well as the improved subtropical jet, stationary planetary waves, atmospheric baroclinity, and baroclinic energy conversion over the North Pacific. We also find that sharper oceanic fronts along the Kuroshio Extension accompanied by enhanced local atmospheric baroclinity and strengthened eddy heat flux over the northern North Pacific could be related to those improvements. Nevertheless, the North Atlantic storm track in HR is placed too north, leading to a larger negative bias over the subtropical zone than in LR. The simulated insufficient subtropical sensible and latent heat fluxes and northward zonal winds in HR seemingly coincide with the deficits in the North Atlantic storm track. The stronger NH branch of the Hadley cell and its poleward expansion in HR may be linked to the changes in the tropical precipitation and the poleward shift of the NH storm tracks. Further studies of attributing these biases to a specific aspect of the climate model by sensitivity experiments are warranted for further clarification of the mechanisms.

The role of baroclinic activity in controlling Earth’s albedo in the present and future climates

Clouds are one of the most influential components of Earth's climate system. Specifically, the midlatitude clouds play a vital role in shaping Earth's albedo. This study investigates the connection between baroclinic activity, which dominates the midlatitude climate, and cloud-albedo and how it relates to Earth's existing hemispheric albedo symmetry. We show that baroclinic activity and cloud-albedo are highly correlated. By using Lagrangian tracking of cyclones and anticyclones and analyzing their individual cloud properties at different vertical levels, we explain why their cloud-albedo increases monotonically with intensity. We find that while for anticyclones, the relation between strength and cloudiness is mostly linear, for cyclones, in which clouds are more prevalent, the relation saturates with strength. Using the cloud-albedo strength relationships and the climatology of baroclinic activity, we demonstrate that the observed hemispheric difference in cloud-albedo is well explained by the difference in the population of cyclones and anticyclones, which counter-balances the difference in clear-sky albedo. Finally, we discuss the robustness of the hemispheric albedo symmetry in the future climate. Seemingly, the symmetry should break, as the northern hemisphere's storm track response differs from that of the southern hemisphere due to Arctic amplification. However, we show that the saturation of the cloud response to storm intensity implies that the increase in the skewness of the southern hemisphere storm distribution toward strong storms will decrease future cloud-albedo in the southern hemisphere. This complex response explains how albedo symmetry might persist even with the predicted asymmetric hemispheric change in baroclinicity under climate change.

The Response of the Northern Hemisphere Storm Tracks and Jet Streams to Climate Change in the CMIP3, CMIP5, and CMIP6 Climate Models

AbstractThe representation of the Northern Hemisphere (NH) storm tracks and jet streams and their response to climate change have been evaluated in climate model simulations from Phases 3, 5, and 6 of the Coupled Model Intercomparison Project (CMIP3, CMIP5, and CMIP6, respectively). The spatial patterns of the multimodel biases in CMIP3, CMIP5, and CMIP6 are similar; however, the magnitudes of the biases in the CMIP6 models are substantially lower. For instance, the multimodel mean RMSE of the North Atlantic storm track for the CMIP6 models (as measured by time‐filtered sea‐level pressure variance) is over 50% smaller than that of the CMIP3 models in both winter and summer, and over 40% smaller for the North Pacific. The magnitude of the jet stream biases is also reduced in CMIP6, but by a lesser extent. Despite this improved representation of the current climate, the spatial patterns of the climate change response of the NH storm tracks and jet streams remain similar in the CMIP3, CMIP5, and CMIP6 models. The SSP2‐4.5 scenario responses in the CMIP6 models are substantially larger than in the RCP4.5 CMIP5 models, which is consistent with the larger climate sensitivities of the CMIP6 models compared to CMIP5.

The fast-acting \u201cpulse\u201d of Heinrich Stadial 3 in a mid-latitude boreal ecosystem

A 3800 year-long radiocarbon-dated and highly-resolved palaeoecological record from Lake Fimon (N-Italy) served to investigate the effects of potential teleconnections between North Atlantic and mid-to-low latitudes at the transition from Marine Isotope Stage (MIS) 3 to 2. Boreal ecosystems documented in the Fimon record reacted in a sensitive way to millennial and sub-millennial scale Northern Hemisphere atmospheric circulation patterns. The high median time-resolution of 58 years allows the identification of five abrupt event-boundaries (i.e., main forest expansion and decline excursions) synchronous with the sharp stadial/interstadial (GS/GI) transitions within dating uncertainties. During Heinrich Stadial 3 (HS 3) we reconstruct more open and dry conditions, compared to the other GS, with a dominant regional scale fire signal. Linkages between local fires and climate-driven fuel changes resulted in high-magnitude fire peaks close to GI/GS boundaries, even exacerbated by local peatland conditions. Finally, palaeoecological data from the HS 3 interval unveiled an internal variability suggesting a peak between 30,425 and 29,772 cal BP (2σ error) which matches more depleted δ18O values in alpine speleothems. We hypothesise that this signal, broadly resembling that of other mid-latitudes proxies, may be attributed to the southward shift of the Northern Hemisphere storm tracks and the associated delayed iceberg discharge events as documented during other HS.

Surface Fluxes Modulate the Seasonality of Zonal-Mean Storm Tracks

Abstract The observed zonal-mean extratropical storm tracks exhibit distinct hemispheric seasonality. Previously, the moist static energy (MSE) framework was used diagnostically to show that shortwave absorption (insolation) dominates seasonality but surface heat fluxes damp seasonality in the Southern Hemisphere (SH) and amplify it in the Northern Hemisphere (NH). Here we establish the causal role of surface fluxes (ocean energy storage) by varying the mixed layer depth d in zonally symmetric 1) slab-ocean aquaplanet simulations with zero ocean energy transport and 2) energy balance model (EBM) simulations. Using a scaling analysis we define a critical mixed layer depth dc and hypothesize 1) large mixed layer depths (d > dc) produce surface heat fluxes that are out of phase with shortwave absorption resulting in small storm track seasonality and 2) small mixed layer depths (d < dc) produce surface heat fluxes that are in phase with shortwave absorption resulting in large storm track seasonality. The aquaplanet simulations confirm the large mixed layer depth hypothesis and yield a useful idealization of the SH storm track. However, the small mixed layer depth hypothesis fails to account for the large contribution of the Ferrel cell and atmospheric storage. The small mixed layer limit does not yield a useful idealization of the NH storm track because the seasonality of the Ferrel cell contribution is opposite to the stationary eddy contribution in the NH. Varying the mixed layer depth in an EBM qualitatively supports the aquaplanet results.

Investigation of boreal storm tracks in historical simulations of INM CM5 and reanalysis data

Ensemble simulations of the 5th version of the INM coupled climate model are employed to analyze the Northern Hemisphere storm track characteristics in the winter season. The results show similar features of the North Atlantic and North Pacific storm tracks that dominate in the Northern Hemisphere in the model simulations and reanalysis data. A composite analysis of the weakening/strengthening of Arctic stratospheric polar vortex events with their influence on the troposphere shows an equatorward/poleward shift in the North Atlantic storm track. As a response to the recent Arctic amplification, a poleward shift in the Pacific storm track and weakening of the North Atlantic storm track have been revealed when comparing two periods: 1998-2014 and 1980-1997.

The Annual Cycle of Northern Hemisphere Storm Tracks. Part II: Regional Detail

AbstractIn Part I of this study, the annual cycle of the Northern Hemisphere storm tracks was investigated using feature tracking and Eulerian variance-based diagnostics applied to both the vorticity and meridional wind fields. Results were presented and discussed for the four seasons at both upper- (250 hPa) and lower- (850 hPa) tropospheric levels. Here, using the meridional wind diagnostics, the annual cycles of the North Pacific and North Atlantic storm tracks are examined in detail. This is done using monthly and 20° longitudinal sector averages. Many sectors have been considered, but the focus is on sectors equally spaced in the two main oceanic storm tracks situated at their western, central, and eastern regions, with the western ones being mainly over the upstream continents. The annual cycles of the upper- and lower-tropospheric storm tracks in the central and eastern Pacific, as well as in the western and central Atlantic sectors, all have rather similar structures. In amplitude, each sector at both levels has a summer minimum and a relatively uniform strength from October to April, despite the strong winter maxima in the westerly jets. However, high-intensity storms occur over a much wider latitudinal band in winter. The storm track in each sector moves poleward from May to August and returns equatorward from October to December, and there is a marked asymmetry between spring and autumn. There are many differences between the North Pacific and North Atlantic storm tracks, and some of these seem to have their origin in the behavior over the upstream East Asian and North American continents, suggesting the importance of seeding from these regions. The East Asian storm track near 48°N has marked spring and autumn maxima and weak amplitude in winter and summer. The 33°N track is strong only in the first half of the year. In contrast, the eastern North American storm track is well organized throughout the year, around the baroclinicity that moves latitudinally with the seasons. The signatures associated with these features are found to gradually decrease downstream in each case. In particular, there is very little latitudinal movement in the storm track in the eastern Atlantic.

The Annual Cycle of Northern Hemisphere Storm Tracks. Part I: Seasons

AbstractIn this paper and Part II a comprehensive picture of the annual cycle of the Northern Hemisphere storm tracks is presented and discussed for the first time. It is based on both feature tracking and Eulerian-based diagnostics, applied to vorticity and meridional wind in the upper and lower troposphere. Here, the storm tracks, as diagnosed using both variables and both diagnostic techniques, are presented for the four seasons for each of the two levels. The oceanic storm tracks retain much of their winter mean intensity in spring with only a small change in their latitude. In the summer they are much weaker, particularly in the Pacific and are generally farther poleward. In autumn the intensities are larger again, comparable with those in spring, but the latitude is still nearer to that of summer. However, in the lower troposphere in the eastern ocean basins the tracking metrics show northern and southern tracks that change little with latitude through the year. The Pacific midwinter minimum is seen in upper-troposphere standard deviation diagnostics, but a richer picture is obtained using tracking. In winter there are high intensities over a wide range of latitudes in the central and eastern Pacific, and the western Pacific has high track density but weak intensity. In the lower troposphere all the diagnostics show that the strength of the Pacific and Atlantic storm tracks are generally quite uniform over the autumn–winter–spring period. There is a close relationship between the upper-tropospheric storm track, particularly that based on vorticity, and tropopause-level winds and temperature gradients. In the lower troposphere, in winter the oceanic storm tracks are in the region of the strong meridional SST gradients, but in summer they are located in regions of small or even reversed SST gradients. However, over North America the lower-tropospheric baroclinicity and the upstream portion of the Atlantic storm track stay together throughout the year.

Seasonal prediction skill and predictability of the Northern Hemisphere storm track variability in Project Minerva

The seasonal prediction skill and predictability of the Northern Hemisphere storm track anomalies in boreal winter (December–January–February, DJF) is examined using seasonal ensemble reforecasts for 1982–2009 from the ECMWF Integrated Forecast System at two different atmospheric resolutions in Project Minerva. It is found that the predictable signals of storm track variations are associated with the two leading EOF modes of ensemble-averaged DJF variances of the high-pass filtered daily meridional winds at 250-hPa level derived from each of the hindcast ensemble members. These two EOF modes are highly correlated both temporarily and spatially between two sets of reforecasts. The first mode (EOF1) mainly shows a latitudinal shift of the storm tracks over the central-eastern North Pacific and the North America continent. The second mode (EOF2) is primarily the pulsing signal exerting on the mean storm track background of the North Pacific. The model predictive skill is verified against observations. The first mode has higher prediction skills and larger skillful regions than the second one. In particular, the first predictable mode is generated by the ENSO-induced wave train, starting from tropical central Pacific and propagating to North America. The skillful region lies in the North Pacific to the west of California, corresponding to the southern lobe of EOF1. The second predictable mode is generated by the North Pacific Mode, which evokes a distinctive wave train, emanating from the tropical western Pacific and propagating northeastward. Its skillful region of the storm track prediction is confined to a small area of Canada western coastlines.

Divergent Responses of Extratropical Atmospheric Circulation to Interhemispheric Dipolar SST Forcing over the Two Hemispheres in Boreal Winter

AbstractGlobal sea surface temperature (SST) evolution exhibits an antiphase variation between the two hemispheres that is referred to as the SST interhemispheric dipole (SSTID) mode. The impacts of the SSTID on extratropical atmospheric circulation in boreal winter are explored by both regression analysis and SST-forced numerical simulations. The responses of extratropical circulation to SSTID thermal forcing bear an equivalent barotropic structure. For the Southern Hemisphere (SH), positive SSTID events lead to a meridional dipolar perturbation in sea level pressure (SLP), similar in pattern to the positive southern annular mode (SAM). Although SSTID-forced SLP anomalies over the Northern Hemisphere (NH) do not exhibit a zonally symmetric pattern as is the case over the SH, they still show signs of a meridional dipole opposite to the SH over the oceans. Divergent circulation responses to SSTID forcing between the two hemispheres are suggested to be associated with contrasting storm-track variations. Positive SSTID events weaken oceanic fronts in both the North Atlantic and North Pacific, and thus lead to the decline of NH storm-track activity by decreasing atmospheric baroclinicity. In the SH, positive SSTID events correspond to the enhancement of SH transients by intensifying the Antarctic polar-frontal zone. Additionally, local baroclinic energy conversions are diagnosed to explain the SSTID-related storm-track variations over both hemispheres. Finally, an investigation of transient eddy feedback indicates that the SSTID mode modulates extratropical atmospheric circulation, primarily by regulating storm tracks and changing the corresponding eddy feedback.

An examination of the Northern Hemisphere mid-latitude storm track interannual variability simulated by climate models\u2014sensitivity to model resolution and coupling

The model fidelity in simulating the Northern Hemisphere storm track interannual variability and the connections of this variability to the low frequency atmospheric variations and oceanic variations are examined based on the atmospheric European Centre for Medium-Range Weather Forecasts (ECMWF) model and coupled NCAR Community Climate System Model (CCSM) systems at different horizontal resolutions. The atmospheric general circulation model (AGCM) runs are forced by observed sea surface temperatures (SST) with varying atmospheric resolutions, while the coupled general circulation model (CGCM) runs have a fixed atmospheric resolution but varying oceanic resolutions. The phases, between the North Pacific (NP) and North Atlantic (NA) sectors, of the simulated hemisphere-scale Empirical Orthogonal Function (EOF) modes of the storm track fluctuations change with the model resolution, suggesting the storm track variability in NP and NA basins are largely independent. The models can qualitatively reproduce the basin-scale EOFs of both NP and NA storm track variability. These EOFs are not sensitive to either atmospheric or oceanic model horizontal resolutions, but their magnitudes from the CGCM runs are substantially underestimated. The storm track variations over NP basin are hybrid of internal atmospheric variations and external forcing from the underlying conditions, but the fluctuations over the NA basin are merely atmospheric internal variability. The NP storm track variability from SST forcing accounts for 4.4% of the total variance in observations, while it only has less than 2% of the total in all AGCM simulations. The external forcing to the storm track variations is more realistically reproduced in the higher atmospheric resolution runs. The air–sea coupling makes the SST feedbacks to the atmospheric internal variability, absent in the atmospheric ECMWF model hindcasts, emerge in the coupled CCSM simulations.

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.

The dynamical core of the Aeolus 1.0 statistical–dynamical atmosphere model: validation and parameter optimization

Abstract. We present and validate a set of equations for representing the atmosphere's large-scale general circulation in an Earth system model of intermediate complexity (EMIC). These dynamical equations have been implemented in Aeolus 1.0, which is a statistical–dynamical atmosphere model (SDAM) and includes radiative transfer and cloud modules (Coumou et al., 2011; Eliseev et al., 2013). The statistical dynamical approach is computationally efficient and thus enables us to perform climate simulations at multimillennia timescales, which is a prime aim of our model development. Further, this computational efficiency enables us to scan large and high-dimensional parameter space to tune the model parameters, e.g., for sensitivity studies. Here, we present novel equations for the large-scale zonal-mean wind as well as those for planetary waves. Together with synoptic parameterization (as presented by Coumou et al., 2011), these form the mathematical description of the dynamical core of Aeolus 1.0. We optimize the dynamical core parameter values by tuning all relevant dynamical fields to ERA-Interim reanalysis data (1983–2009) forcing the dynamical core with prescribed surface temperature, surface humidity and cumulus cloud fraction. We test the model's performance in reproducing the seasonal cycle and the influence of the El Niño–Southern Oscillation (ENSO). We use a simulated annealing optimization algorithm, which approximates the global minimum of a high-dimensional function. With non-tuned parameter values, the model performs reasonably in terms of its representation of zonal-mean circulation, planetary waves and storm tracks. The simulated annealing optimization improves in particular the model's representation of the Northern Hemisphere jet stream and storm tracks as well as the Hadley circulation. The regions of high azonal wind velocities (planetary waves) are accurately captured for all validation experiments. The zonal-mean zonal wind and the integrated lower troposphere mass flux show good results in particular in the Northern Hemisphere. In the Southern Hemisphere, the model tends to produce too-weak zonal-mean zonal winds and a too-narrow Hadley circulation. We discuss possible reasons for these model biases as well as planned future model improvements and applications.

Variations of Northern Hemisphere Storm Track and Extratropical Cyclone Activity Associated with the Madden–Julian Oscillation

This study investigates the intraseasonal variations of the Northern Hemispheric storm track associated with the Madden–Julian oscillation (MJO) during the extended boreal winter (November–April) using 36 yr (1979–2014) of reanalysis data from ERA-Interim. Two methods have been used to diagnose storm-track variations. In the first method, the storm track is quantified by the temporal-filtered variance of 250-hPa meridional wind (vv250) or mean sea level pressure (pp). The intraseasonal anomalies of vv250 composited for eight MJO phases are characterized by a zonal band of strong positive (or negative) anomalies meandering from the Pacific all the way across North America and the Atlantic into northern Europe, with weaker anomalies of opposite sign at one or both flanks. The results based on pp are consistent with those based on vv250 except for larger zonal variations, which may be induced by surface topography. In the second method, an objective cyclone-tracking scheme has been used to track the extratropical cyclones that compose the storm track. The MJO-composite anomalies of the “accumulated” cyclone activity, a quantity that includes contributions from both the cyclone frequency and cyclone mean intensity, are very similar to those based on pp. Further analysis demonstrates that major contribution comes from variations in the cyclone frequency. Further analysis suggests that the intraseasonal variations of the storm track can be primarily attributed to the variations of the mean flow that responds to the anomalous MJO convections in the tropics, with possible contribution also from the moisture variations.

Past changes in the North Atlantic storm track driven by insolation and sea-ice forcing

Abstract Changes in the location of Northern Hemisphere storm tracks may cause significant societal and economic impacts under future climate change, but projections of future changes are highly uncertain and drivers of long-term changes are poorly understood. Here we develop a late Holocene storminess reconstruction from northwest Spain and combine this with an equivalent record from the Outer Hebrides, Scotland, to measure changes in the dominant latitudinal position of the storm track. The north-south index shows that storm tracks moved from a southern position to higher latitudes over the past 4000 yr, likely driven by a change from meridional to zonal atmospheric circulation, associated with a negative to positive North Atlantic Oscillation shift. We suggest that gradual polar cooling (caused by decreasing solar insolation in summer and amplified by sea-ice feedbacks) and mid-latitude warming (caused by increasing winter insolation) drove a steepening of the winter latitudinal temperature gradient through the late Holocene, resulting in the observed change to a more northern winter storm track. Our findings provide paleoclimate support for observational and modeling studies that link changes in the latitudinal temperature gradient and sea-ice extent to the strength and shape of the circumpolar vortex. Together this evidence now suggests that North Atlantic winter storm tracks may shift southward under future warming as sea-ice extent decreases and the mid- to high-latitude temperature gradient decreases, with storms increasingly affecting southern Europe.

위성 자료가 재분석자료의 대규모 대기 순환장에 미치는 영향: JRA-55와 JRA-55C 비교 연구

The effects of satellite observations on large-scale atmospheric circulations in the reanalysis data are investigated by comparing the latest Japanese Meteorological Association's reanalysis data (JRA-55) and its family data, JRA-55 Conventional (JRA-55C). The latter is identical to the former except that satellite observations are excluded during the data assimilation process. Only conventional datasets are assimilated in JRA-55C. A simple comparison revealed a considerable difference in temperature and zonal wind fields in both the stratosphere and troposphere. Such differences are particularly large in the Southern Hemisphere and whole stratosphere where conventional ground-based measurements are limited. The effects of satellite observations on the zonal-mean tropospheric circulations are further examined in terms of the Hadley cell, eddy-driven jet, and mid-latitude storm tracks. In both hemispheres, JRA-55C exhibits slightly weaker and narrower Hadley cell than JRA-55. This is consistent with a weaker diabatic heating in JRA-55C. The eddy-driven jet shows a small difference in its latitudinal location only in the Southern Hemisphere. Likewise, while the Northern-Hemisphere storm tracks are quantitatively similar in the two datasets, Southern-Hemisphere storm tracks are relatively weaker in JRA-55C than in JRA-55. Their difference is comparable to the uncertainty between reanalysis datasets, indicating that satellite data assimilation could yield significant corrections in the zonal-mean circulation in the Southern Hemisphere.

Seasonal Atmospheric Responses to Reduced Arctic Sea Ice in an Ensemble of Coupled Model Simulations

Abstract Arctic sea ice decline is expected to continue throughout the twenty-first century as a result of increased greenhouse gas concentrations. Here we investigate the impact of a strong Arctic sea ice decline on the atmospheric circulation and low pressure systems in the Northern Hemisphere through numerical experimentation with a coupled climate model. More specifically, a large ensemble of 1-yr-long integrations, initialized on 1 June with Arctic sea ice thickness artificially reduced by 80%, is compared to corresponding unperturbed control experiments. The sensitivity experiment shows an ice-free Arctic from July to October; during autumn the largest near-surface temperature increase of about 15 K is found in the central Arctic, which goes along with a reduced meridional temperature gradient, a decreased jet stream, and a southward shifted Northern Hemisphere storm track; and the near-surface temperature response in winter and spring reduces substantially due to relatively fast sea ice growth during the freezing season. Changes in the maximum Eady growth rate are generally below 5% and hardly significant, with reduced vertical wind shear and reduced vertical stability counteracting each other. The reduced vertical wind shear manifests itself in a decrease of synoptic activity by up to 10% and shallower cyclones while the reduced vertical stability along with stronger diabatic heating due to more available moisture may be responsible for the stronger deepening rates and thus faster cyclone development once a cyclone starts to form. Furthermore, precipitation minus evaporation decreases over the Arctic because the increase in evaporation outweighs that for precipitation, with implications for the ocean stratification and hence ocean circulation.

Stationary Eddies and the Zonal Asymmetry of Net Precipitation and Ocean Freshwater Forcing

Abstract Transport of water vapor in the atmosphere generates substantial spatial variability of net precipitation (precipitation minus evaporation). Over half of the total spatial variability in annual-mean net precipitation is accounted for by deviations from the zonal mean. Over land, these regional differences determine differences in surface water availability. Over oceans, they account, for example, for the Pacific–Atlantic difference in sea surface salinity, with implications for the deep overturning circulation. This study analyzes the atmospheric water budget in reanalyses from ERA-Interim and MERRA, to investigate which physical balances lead to zonal variation in net precipitation. It is found that the leading-order contribution is zonal variation in stationary-eddy vertical motion. Transient eddies modify the pattern of zonally anomalous net precipitation by moving moisture from the subtropical and tropical oceans onto land and poleward across the Northern Hemisphere storm tracks. Zonal variation in specific humidity and stationary-eddy horizontal advection play a secondary role. The dynamics leading to net precipitation via vertical motion in stationary eddies can be understood from a lower-tropospheric vorticity budget. The large-scale variations of vertical motion are primarily described by Sverdrup balance and Ekman pumping, with some modification by transient eddies. These results suggest that it is important to understand changes in stationary eddies and their influence on the zonal variation of transient eddy fluxes, in order to understand regional changes in net precipitation. They highlight the relative importance of different atmospheric mechanisms for the freshwater forcing of the North Pacific and North Atlantic.

The Ability of CMIP5 Models to Simulate North Atlantic Extratropical Cyclones*

Abstract The ability of the climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) to simulate North Atlantic extratropical cyclones in winter [December–February (DJF)] and summer [June–August (JJA)] is investigated in detail. Cyclones are identified as maxima in T42 vorticity at 850 hPa and their propagation is tracked using an objective feature-tracking algorithm. By comparing the historical CMIP5 simulations (1976–2005) and the ECMWF Interim Re-Analysis (ERA-Interim; 1979–2008), the authors find that systematic biases affect the number and intensity of North Atlantic cyclones in CMIP5 models. In DJF, the North Atlantic storm track tends to be either too zonal or displaced southward, thus leading to too few and weak cyclones over the Norwegian Sea and too many cyclones in central Europe. In JJA, the position of the North Atlantic storm track is generally well captured but some CMIP5 models underestimate the total number of cyclones. The dynamical intensity of cyclones, as measured by either T42 vorticity at 850 hPa or mean sea level pressure, is too weak in both DJF and JJA. The intensity bias has a hemispheric character, and it cannot be simply attributed to the representation of the North Atlantic large-scale atmospheric state. Despite these biases, the representation of Northern Hemisphere (NH) storm tracks has improved since CMIP3 and some CMIP5 models are able of representing well both the number and the intensity of North Atlantic cyclones. In particular, some of the higher-atmospheric-resolution models tend to have a better representation of the tilt of the North Atlantic storm track and of the intensity of cyclones in DJF.