Simulation of ENSO teleconnections to precipitation extremes over the US in the high resolution version of E3SM

We evaluate the simulated teleconnection of El Niño Southern Oscillation (ENSO) to winter season precipitation extremes over the United States in a long (98 years) 1950-control high resolution version (HR, 25 km nominal atmosphere model horizontal resolution) of US Department of Energy’s (DOE) Energy Exascale Earth System Model version 1 (E3SMv1). Model bias and spatial pattern of ENSO teleconnections to mean and extreme precipitation in HR overall are similar to the low-resolution model’s (LR, 110 km) historical simulation (4-member ensemble, 1925-1959). However, over the Southeast US (SE-US), HR produces stronger El Niño associated extremes, reducing upon LR’s model bias. Both LR and HR produce weaker than observed increase in storm track activity during El Niño events there. But, HR improves the ENSO associated variability of moisture transport over SE-US. During El Niño, stronger vertical velocities in HR produce stronger large-scale precipitation causing larger latent heating of the troposphere that pulls in more moisture from the Gulf of Mexico into the SE-US. This positive feedback also contributes to the stronger mean and extreme precipitation response in HR. Over the Pacific Northwest, LR’s bias of stronger than observed La Niña associated extremes is amplified in HR. Both models simulate stronger than observed moisture transport from the Pacific Ocean into the region during La Niña years. The amplified HR bias there is due to stronger orographically driven vertical updrafts that create stronger large scale precipitation, despite weaker La Niña induced storm track activity.

Energetics of transient eddies related to the midwinter minimum of the North Pacific storm-track activity

Storm-track activity over the North Pacific climatologically exhibits a clear minimum in midwinter, when the westerly jet speed sharply maximizes. This counterintuitive phenomenon, referred to as the “midwinter minimum (MWM)”, has been investigated from various perspectives, but the mechanisms are still to be unrevealed. Toward better understanding of this phenomenon, the present study delineates the detailed seasonal evolution of climatological-mean Eulerian statistics and energetics of migratory eddies along the NP storm-track over 60 years. As a comprehensive investigation of the mechanisms for the MWM, this study has revealed that the net eddy conversion/generation rate normalized by the eddy total energy, which is independent of eddy amplitude, is indeed reduced in midwinter. The reduction from early winter occurs mainly due to the decreased effectiveness of the baroclinic energy conversion through seasonally weakened temperature fluctuations and the resultant poleward eddy heat flux. The reduced net normalized conversion/generation rate in midwinter is also found to arise in part from the seasonally enhanced kinetic energy conversion from eddies into the strongly diffluent Pacific jet around its exit. The seasonality of the net energy influx also contributes especially to the spring recovery of the net normalized conversion/generation rate. The midwinter reduction in the normalized rates of both the net energy conversion/generation and baroclinic energy conversion was more pronounced in the period before the late 1980s, during which the MWM of the storm-track activity was climatologically more prominent.

Finding Storm Track Activity Metrics That Are Highly Correlated with Weather Impacts. Part I: Frameworks for Evaluation and Accumulated Track Activity

In the midlatitudes, storm tracks give rise to much of the high-impact weather, including precipitation and strong winds. Numerous metrics have been used to quantify storm track activity, but there has not been any systematic evaluation of how well different metrics relate to weather impacts. In this study, two frameworks have been developed to provide such evaluations. The first framework quantifies the maximum one-point correlation between weather impacts at each grid point and the assessed storm track metric. The second makes use of canonical correlation analysis to find the best correlated patterns and uses these patterns to hindcast weather impacts based on storm track metric anomalies using a leave-N-out cross-validation approach. These two approaches have been applied to assess multiple Eulerian variances and Lagrangian tracking statistics for Europe, using monthly precipitation and a near-surface high-wind index as the assessment criteria. The results indicate that near-surface storm track metrics generally relate more closely to weather impacts than upper-tropospheric metrics. For Eulerian metrics, synoptic time scale eddy kinetic energy at 850 hPa relates strongly to both precipitation and wind impacts. For Lagrangian metrics, a novel metric, the accumulated track activity (ATA), which combines information from both cyclone track frequency and amplitude, is found to be best correlated with weather impacts when spatially filtered 850-hPa vorticity maxima are used to define cyclones. The leading patterns of variability for ATA are presented, demonstrating that this metric exhibits coherent large-scale month-to-month variations that are highly correlated with variations in the mean flow and weather impacts.

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

Global 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.

Impacts of the Madden–Julian Oscillation on Storm-Track Activity, Surface Air Temperature, and Precipitation over North America

In this study, the intraseasonal variations in storm-track activity, surface air temperature, and precipitation over North America associated with the Madden–Julian oscillation (MJO) in boreal winter (November–April) are investigated. A lag composite strategy that considers different MJO phases and different lag days is developed. The results highlight regions over which the MJO has significant impacts on surface weather on intraseasonal time scales. A north–south shift of storm-track activity associated with the MJO is found over North America. The shift is consistent with the MJO-related surface air temperature anomaly over the eastern United States. In many regions over the western, central, and southeastern United States, the MJO-related precipitation signal is also consistent with nearby storm-track activity. An MJO-related north–south shift of precipitation is also found near the west coast of North America, with the precipitation over California being consistent with the MJO-related storm-track activity over the eastern Pacific. MJO-related temperature and storm-track anomalies are also found near Alaska. Further analyses of streamfunction anomalies and wave activity flux show clear signatures of Rossby wave trains excited by convection anomalies related to MJO phases 3 and 8. These wave trains propagate across the Pacific and North America, bringing an anticyclonic (cyclonic) anomaly to the eastern part of North America, shifting the westerly jet to the north (south), thereby modulating the surface air temperature and storm-track activity over the continent. Rossby waves associated with phases 2 and 6 are also found to impact the U.S. West Coast.