The Karakoram/Western Tibetan vortex: seasonal and year-to-year variability
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Abstract:
The "Karakoram Vortex" (KV), hereafter also referred to as the "Western Tibetan Vortex" (WTV), has recently been recognized as a large-scale atmospheric circulation system related to warmer (cooler) near-surface and mid-lower troposphere temperatures above the Karakoram in the western Tibetan Plateau (TP). It is characterized by a deep, anti-cyclonic (cyclonic) wind anomaly associated with higher (lower) geopotential height in the troposphere, during winter and summer seasons. In this study, we further investigate the seasonality and basic features of the WTV in all four seasons, and explore its year-to-year variability and influence on regional climate. We find the WTV accounts for the majority of year-to-year circulation variability over the WTP as it can explain over 50% ( $${R^2} \geqslant 0.5$$ ) variance of the WTP circulation on multiple levels throughout the troposphere, which declines towards the eastern side of the TP in most seasons. The WTV is not only more (less) active but also has a bigger (smaller) domain area, with a deeper (shallower) structure, in winter and spring (summer and autumn). We find that the WTV is sensitive to both the location and intensity of the Subtropical Westerly Jet (SWJ), but the relationship is highly dependent on the climatological mean location of SWJ axes relative to the TP in different seasons. We also show that the WTV significantly modulates surface and stratospheric air temperatures, north–south precipitation patterns and total column ozone surrounding the western TP. As such, the WTV has important implications for the understanding of atmospheric, hydrological and glaciological variability over the TP.Keywords:
Geopotential height
Atmospheric Circulation
Anomaly (physics)
Subtropical ridge
Abstract Spring Arctic Oscillation (AO) has been revealed to exert a great impact on the Northern Hemispheric climate; however, its seasonal prediction skill has been limited so far. In this paper, we find that the dominant mode of the winter 500‐hPa geopotential height anomalies over the mid‐latitude Eurasia has a close connection with the succeeding spring AO. The analysis on the physical processes indicates that the troposphere–stratosphere interaction plays an important role in the connection between the two. This predominant atmospheric mode in the mid‐latitude Eurasia during winter can change the vertical wave activity and lead to an anomalous stratospheric polar vortex. Thereafter, the signal of the anomalous stratospheric polar vortex propagates downward to troposphere in the subsequent spring, consequently inducing the anomalous AO pattern in the troposphere. The results in this study imply that this previous winter leading atmospheric mode over the mid‐latitude Eurasia may supply a potential source for a spring AO's prediction.
Arctic oscillation
Geopotential height
Middle latitudes
Sudden stratospheric warming
Atmospheric Circulation
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Abstract Central Chile winter (June, July, August (JJA)) rainfall shows positive anomalies during the developing stage of warm events of the Southern Oscillation. Conversely, cold events correspond quite closely to dry conditions. A synoptic characterization of major storms during the most recent warm events is presented. Dry months during cold‐event years are described in terms of average 500‐hPa contour anomaly fields. Significant departures from this general behaviour are also discussed. It is found that major winter storms associated with warm events are related to blocking highs frequently located around the Bellingshausen Sea (90°W) within hemispheric circulation anomaly patterns where zonal wavenumbers 4 and 3 dominate. This phenomenon seems consistent with observed teleconnection wavetrains stemming from the anomalous atmospheric heat source above the equatorial Pacific during ENSO events. Cold years, often immediately preceding or following a warm event, bring dry conditions in the study area owing to a well‐developed south‐east subtropical anticyclone with enhanced zonal westerly flow at middle latitudes. Frequency distributions of 500‐hPa daily blocking indices (BI) at 90°W, derived from 1980 to 1987 European Centre for Medium Range Weather Forecasts hemispheric analyses, show a significant departure towards positive BI values for the available warm‐event winters; the opposite being also true. However, the JJA rainfall variability at Santiago (33.5°S) also seems to be related to the regional strength of the south‐east Pacific anticyclone, as represented by seasonal 500‐hPa geopotential anomalies at Puerto Montt, Chile (41.5°S).
Anticyclone
Anomaly (physics)
Geopotential height
Teleconnection
Zonal flow (plasma)
Subtropical ridge
Atmospheric Circulation
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The leading variability mode of the coupled troposphere‐stratosphere winter circulation in the Northern Hemisphere (NH) describes a close relationship between the strength of the stratospheric cyclonic vortex and the index of a tropospheric wave‐like pattern covering the North Atlantic‐Eurasian region. This mode can be determined by applying singular value decomposition analysis between the time series of winter mean NH 50‐ and 500‐hPa geopotential heights. We compared the features of the leading coupled variability mode between two climate regimes, determined from a 1900‐year integration with the coupled atmosphere‐ocean climate model ECHAM3‐LSG. The two regimes differ on the interdecadal timescale in the strength of the stratospheric polar vortex and therefore in the transmission‐refraction properties of vertically propagating tropospheric waves. The spatial structures of the leading coupled variability mode of observational data better match the corresponding structures of the model's weak polar vortex regime (PVR) than those of the strong one. Because of the more effective tropospheric trapping of stationary wave energy of zonal wave number (ZWN) 2 at midlatitudes, the zonal variability structure of this wave is changed by barotropic effects in the troposphere as well as in the stratosphere. The coupled troposphere‐stratosphere mode and the response of winter circulation were studied in a climate‐change experiment carried out with the same model. We could show that under increased greenhouse gas forcing, both the response and the coupled variability mode between tropospheric and stratospheric circulation itself has a high similarity to the leading coupled mode in the strong PVR.
Geopotential height
Arctic oscillation
Sudden stratospheric warming
Forcing (mathematics)
Atmospheric Circulation
Middle latitudes
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Hindcast
Anomaly (physics)
Sudden stratospheric warming
Geopotential height
Predictability
Quasi-biennial oscillation
Arctic oscillation
Atmospheric Circulation
Forcing (mathematics)
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