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Zachary D. Lawrence and Gloria L. Manney

Abstract

Characteristics of the Arctic stratospheric polar vortex are examined using reanalysis data with dynamic time warping (DTW) and a clustering technique to determine whether the polar vortex exhibits canonical signs of preconditioning prior to sudden stratospheric warmings (SSWs). The DTW and clustering technique is used to locate time series motifs in vortex area, vortex edge-averaged PV gradients, and vortex edge-averaged wind speeds. Composites of the motifs reveal that prior to roughly 75% of SSWs, in the middle to upper stratosphere, PV gradients and wind speeds in the vortex edge region increase, and vortex area decreases. These signs agree with prior studies that discuss potential signals of preconditioning of the vortex. However, similar motifs are also found in a majority of years without SSWs. While such non-SSW motifs are strongly associated with minor warming signals apparent only in the middle and upper stratosphere, only roughly half of these can be associated with later “significant disturbances” (SDs) that do not quite meet the threshold for major SSWs. The median lead time for sharpening vortex edge PV gradients represented in the motifs prior to SSWs and SDs is ~25 days, while the median lead time for the vortex area and edge wind speeds is ~10 days. Overall, canonical signs of preconditioning do appear to exist prior to SSWs, but their existence in years without SSWs implies that preconditioning of the vortex may be an insufficient condition for the occurrence of SSWs.

Open access
Gloria L. Manney, Michaela I. Hegglin, and Zachary D. Lawrence

Abstract

The relationship of upper tropospheric jet variability to El Niño–Southern Oscillation (ENSO) in reanalysis datasets is analyzed for 1979–2018, revealing robust regional and seasonal variability. Tropical jets associated with monsoons and the Walker circulation are weaker and the zonal mean subtropical jet shifts equatorward in both hemispheres during El Niño, consistent with previous findings. Regional and seasonal variations are analyzed separately for subtropical and polar jets. The subtropical jet shifts poleward during El Niño over the Northern Hemisphere (NH) eastern Pacific Ocean in December–February (DJF) and in some Southern Hemisphere (SH) regions in March–May and September–November (SON). Subtropical jet altitudes increase during El Niño, with significant changes in the zonal mean in the NH and during summer/autumn in the SH. Although zonal mean polar jet correlations with ENSO are rarely significant, robust regional/seasonal changes occur: The SH polar jet shifts equatorward during El Niño over Asia and the western Pacific in DJF and significantly poleward over the eastern Pacific in June–August and SON. During El Niño, polar jets are weaker in the Western Hemisphere and stronger in the Eastern Hemisphere, especially in the SH; conversely, subtropical jets are stronger in the Western Hemisphere and weaker in the Eastern Hemisphere during El Niño in winter and spring. These opposing changes, along with an anticorrelation between subtropical and polar jet wind speeds, reinforce subtropical–polar jet strength differences during El Niño and suggest ENSO-related covariability of the jets. ENSO-related jet latitude, altitude, and wind speed changes can reach 4°, 0.6 km, and 6 m s−1, respectively, for the subtropical jets and 3°, 0.3 km, and 3 m s−1, respectively, for the polar jets.

Open access
Gloria L. Manney, Michelle L. Santee, Zachary D. Lawrence, Krzysztof Wargan, and Michael J. Schwartz

Abstract

A comprehensive investigation of the climatology of and interannual variability and trends in the Asian summer monsoon anticyclone (ASMA) is presented, based on a novel area and moments analysis. Moments include centroid location, aspect ratio, angle, and “excess kurtosis” (measuring how far the shape is from elliptical) for an equivalent ellipse with the same area as the ASMA. Key results are robust among the three modern reanalyses studied. The climatological ASMA is nearly elliptical, with its major axis aligned along its centroid latitude and a typical aspect ratio of ~5–8. The ASMA centroid shifts northward with height, northward and westward during development, and in the opposite direction as it weakens. New evidence finding no obvious climatological bimodality in the ASMA reinforces similar suggestions from previous studies using modern reanalyses. Most trends in ASMA moments are not statistically significant. ASMA area and duration, however, increased significantly during 1979–2018; the 1958–2018 record analyzed for one reanalysis suggests that these trends may have accelerated in recent decades. ASMA centroid latitude is significantly positively (negatively) correlated with subtropical jet-core latitude (altitude), and significantly negatively correlated with concurrent ENSO; these results are consistent with and extend previous work relating monsoon intensity, ENSO, and jet shifts. ASMA area is significantly positively correlated with the multivariate ENSO index 2 months previously. These results improve our understanding of the ASMA using consistently defined diagnostics of its size, geometry, interannual variability, and trends that have not previously been analyzed.

Open access
Zachary M. Subin, Charles D. Koven, William J. Riley, Margaret S. Torn, David M. Lawrence, and Sean C. Swenson

Abstract

At high latitudes, changes in soil moisture could alter soil temperatures independently of air temperature changes by interacting with the snow thermal rectifier. The authors investigated this mechanism with model experiments in the Community Land Model 4 (CLM4) with prescribed atmospheric forcing and vegetation state. Under equilibrium historical conditions, increasing CO2 concentrations experienced by plants from 285 to 857 ppm caused local increases in soil water-filled pore space of 0.1–0.2 in some regions throughout the globe. In permafrost regions that experienced this moistening, vertical- and annual- mean soil temperatures increased by up to 3°C (0.27°C averaged over all permafrost areas). A similar pattern of moistening and consequent warming occurred in simulations with prescribed June–September (JJAS) rainfall increases of 25% over historical values, a level of increase commensurate with projected future rainfall increases. There was a strong sensitivity of the moistening responses to the baseline hydrological state. Experiments with perturbed physics confirmed that the simulated warming in permafrost soils was caused by increases in the soil latent heat of fusion per unit volume and in the soil thermal conductivity due to the increased moisture. In transient Representative Concentration Pathway 8.5 (RCP8.5) scenario experiments, soil warming due to increased CO2 or JJAS rainfall was smaller in magnitude and spatial extent than in the equilibrium experiments. Active-layer deepening associated with soil moisture changes occurred over less than 8% of the current permafrost area because increased heat of fusion and soil thermal conductivity had compensating effects on active-layer depth. Ongoing modeling challenges make these results tentative.

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Eun-Pa Lim, Harry H. Hendon, Amy H. Butler, David W. J. Thompson, Zachary D. Lawrence, Adam A. Scaife, Theodore G. Shepherd, Inna Polichtchouk, Hisashi Nakamura, Chiaki Kobayashi, Ruth Comer, Lawrence Coy, Andrew Dowdy, Rene D. Garreaud, Paul A. Newman, and Guomin Wang

Abstract

This study offers an overview of the low-frequency (i.e., monthly to seasonal) evolution, dynamics, predictability, and surface impacts of a rare Southern Hemisphere (SH) stratospheric warming that occurred in austral spring 2019. Between late August and mid-September 2019, the stratospheric circumpolar westerly jet weakened rapidly, and Antarctic stratospheric temperatures rose dramatically. The deceleration of the vortex at 10 hPa was as drastic as that of the first-ever-observed major sudden stratospheric warming in the SH during 2002, while the mean Antarctic warming over the course of spring 2019 broke the previous record of 2002 by ∼50% in the midstratosphere. This event was preceded by a poleward shift of the SH polar night jet in the uppermost stratosphere in early winter, which was then followed by record-strong planetary wave-1 activity propagating upward from the troposphere in August that acted to dramatically weaken the polar vortex throughout the depth of the stratosphere. The weakened vortex winds and elevated temperatures moved downward to the surface from mid-October to December, promoting a record strong swing of the southern annular mode (SAM) to its negative phase. This record-negative SAM appeared to be a primary driver of the extreme hot and dry conditions over subtropical eastern Australia that accompanied the severe wildfires that occurred in late spring 2019. State-of-the-art dynamical seasonal forecast systems skillfully predicted the significant vortex weakening of spring 2019 and subsequent development of negative SAM from as early as late July.

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