Brent A. McDaniel

Northern Winter Climate Change: Assessment of Uncertainty in CMIP5 Projections Related to Stratosphere-Troposphere Coupling

Future changes in the stratospheric circulation could have an important impact on northern winter tropospheric climate change, given that sea level pressure (SLP) responds not only to tropospheric circulation variations but also to vertically coherent variations in troposphere-stratosphere circulation. Here we assess northern winter stratospheric change and its potential to influence surface climate change in the Coupled Model Intercomparison Project-Phase 5 (CMIP5) multimodel ensemble. In the stratosphere at high latitudes, an easterly change in zonally averaged zonal wind is found for the majority of the CMIP5 models, under the Representative Concentration Pathway 8.5 scenario. Comparable results are also found in the 1% CO2 increase per year projections, indicating that the stratospheric easterly change is common feature in future climate projections. This stratospheric wind change, however, shows a significant spread among the models. By using linear regression, we quantify the impact of tropical upper troposphere warming, polar amplification, and the stratospheric wind change on SLP. We find that the intermodel spread in stratospheric wind change contributes substantially to the intermodel spread in Arctic SLP change. The role of the stratosphere in determining part of the spread in SLP change is supported by the fact that the SLP change lags the stratospheric zonally averaged wind change. Taken together, these findings provide further support for the importance of simulating the coupling between the stratosphere and the troposphere, to narrow the uncertainty in the future projection of tropospheric circulation changes.

Stratosphere–Troposphere Coupling during Spring Onset

Abstract The authors perform an observational study of the relation between stratospheric final warmings (SFWs) and the boreal extratropical circulation. SFW events are found to provide a strong organizing influence upon the large-scale circulation of the stratosphere and troposphere during the period of spring onset. In contrast to the climatological seasonal cycle, SFW events noticeably sharpen the annual weakening of high-latitude circumpolar westerlies in both the stratosphere and troposphere. A coherent pattern of significant westerly (easterly) zonal wind anomalies is observed to extend from the stratosphere to the earth’s surface at high latitudes prior to (after) SFW events, coinciding with the polar vortex breakdown. This evolution is associated with a bidirectional dynamical coupling of the stratosphere–troposphere system in which tropospheric low-frequency waves induce annular stratospheric circulation anomalies, which in turn, are followed by annular tropospheric circulation anomalies. The reg...

Interannual Variability in the Southern Hemisphere Circulation Organized by Stratospheric Final Warming Events

Abstract A composite observational analysis is presented demonstrating that austral stratospheric final warming (SFW) events provide a substantial organizing influence upon the large-scale atmospheric circulation in the Southern Hemisphere. In particular, the annual weakening of high-latitude westerlies in the upper troposphere and stratosphere is accelerated during SFW onset. This behavior is associated with a coherent annular circulation change with zonal wind decelerations (accelerations) at high (low) latitudes. The high-latitude stratospheric decelerations are induced by the anomalous wave driving of upward-propagating tropospheric waves. Longitudinally asymmetric circulation changes occur in the lower troposphere during SFW onset with regionally localized height increases (decreases) at subpolar (middle) latitudes. Importantly, the tropospheric and stratospheric circulation change patterns identified here are structurally distinct from the Southern Annular Mode. It is concluded that SFW events are l...

The Dynamics of Northern Hemisphere Stratospheric Final Warming Events

Abstract A lag composite analysis is performed of the zonal-mean structure and dynamics of Northern Hemisphere stratospheric final warming (SFW) events. SFW events are linked to distinct zonal wind deceleration signatures in the stratosphere and troposphere. The period of strongest stratospheric decelerations (SD) is marked by a concomitant reduction in the high-latitude tropospheric westerlies. However, a subsequent period of tropospheric decelerations (TD) occurs while the stratospheric circulation relaxes toward climatological conditions. During SFW onset, a wavenumber-1 disturbance at stratospheric altitudes evolves into a circumpolar anticyclonic circulation anomaly. Transformed Eulerian-mean dynamical diagnoses reveal that the SD period is characterized by an anomalous upward Eliassen–Palm (EP) signature at high latitudes extending from the surface to the middle stratosphere. The associated wave-driving pattern consists of zonal decelerations extending from the upper troposphere to the midstratosphe...

Diagnostic Case Studies of the Northern Annular Mode

Abstract Recent observational studies of the northern annular mode (NAM) indicate that significant case-to-case variability exists in the structural evolution of individual events. In particular, certain NAM events remain confined to stratospheric altitudes whereas others readily penetrate downward into the troposphere. We perform observational diagnostic analyses that are targeted at identifying the physical mechanisms behind this distinction. Our results thereby provide a test of the different existing theories regarding stratospheric influences upon tropospheric climate. We contrast robust stratospheric NAM events with differing tropospheric signals in order to identify the underlying dynamical reasons for the observed differences. Piecewise potential vorticity (PV) inversions and Eliassen–Palm flux analyses are performed to study the roles of different forcing mechanisms during NAM onset. Our results indicate that variations in the tropospheric response are readily explained on the basis of piecewise ...

Intraseasonal Dynamical Evolution of the Northern Annular Mode

Abstract The Northern Hemisphere annular mode (NAM) accounts for a significant fraction of the extratropical wintertime atmospheric variability. The dynamics of NAM events have been studied on monthly time scales, but little is known about the physical mechanisms that give rise to NAM variability on shorter time scales. Composite diagnostic analyses based on daily NAM indices are performed with a goal of identifying the dominant processes responsible for the growth and decay of large-amplitude positive and negative NAM events on short intraseasonal time scales. Transformed Eulerian mean, piecewise potential vorticity inversions, and regional Plumb flux diagnoses are employed to deduce the proximate forcings of the zonal-mean wind tendency during maturing and declining NAM stages. A remarkable degree of reverse symmetry is observed between the zonal-mean dynamical evolution of positive and negative NAM events. Anomalous equatorward and downward (poleward and upward) Eliassen–Palm fluxes are observed during...

Submonthly Polar Vortex Variability and Stratosphere–Troposphere Coupling in the Arctic

Abstract A principal component analysis is performed to characterize intraseasonal variability in the boreal stratospheric polar vortex. In contrast to previous studies, the current analysis examines daily zonal-mean variability within a limited spatial domain encompassing the stratospheric polar vortex. The leading EOFs are vertically coherent north–south dipoles in the zonal-mean zonal wind extending through the lower stratosphere. The first mode represents variability in polar vortex strength and is highly correlated with the stratospheric northern annular mode (SNAM). The second mode, the polar annular mode (PAM), represents variability in the latitudinal position of the polar vortex and possesses a poleward-retracted dipole anomaly structure. Composite analyses indicate that large-amplitude PAM events are relatively short lived (1–2 weeks) compared to SNAM events (1 month or longer). Trend analyses further reveal that recent decadal trends in the boreal polar vortex project more strongly onto PAM tha...