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ABSTRACT The cross-tropopause mass flux ( CTMF) and long-term trends in stratosphere-troposphere exchange ( STE) over the Tibetan Plateau ( TP) and its surroundings were analyzed using European Centre for Medium-Range Weather Forecasts reanalysis data. The gross CTMF (the sum of upward and downward mass flux) shows an evident wave train structure over the TP, which is mainly related to the horizontal exchange of air masses along the tropopause associated with the sharp meridional gradient in tropopause pressure or vertical discontinuity of the thermal tropopause in winter. The seasonal cycle of the STE over the TP shows that the gross mass flux is downwards in Northern Hemisphere ( NH) winter and upwards in NH summer. The gross CTMF over the TP accounts for 2.96% of the global total CTMF arising from STE processes resolved by Wei method. Both the upward and downward CTMF over the TP exhibit statistically significant positive trends in winter during the period 1979-2009. The strong positive trends of STE in winter over the TP are resulted from the combined effects of the rising tropopause height, enhanced westerlies and decreasing plateau winter monsoon. In summer, both the upward and downward CTMF exhibit statistically significant negative trends over the northern TP, while the trend in upward CTMF is positive over the southern TP, in accordance with the increasing intensity of Asian summer monsoon in recent decades. The sensitivity simulations with a climate model confirm that changes in the Asian monsoon can significantly affect the tropopause and the CTMF over the TP. [ABSTRACT FROM AUTHOR]

This study examines the influence of the Caspian Sea–Hindu Kush Index (CasHKI) on local and synoptic meteorology as well as on dust emissions over southwest (SW) Asia by means of National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP-NCAR) re-analysed mean sea-level pressure (MSLP), geopotential height at 700 hPa and surface meridional wind, along with meteorological data at Zabol, east Iran and Meteosat/Infrared Difference Dust Index (IDDI) retrievals. The analysis focuses on the summer period (June to September) of 2000-2014 and the winter period (November to March) of 1963-2014. The CasHKI values are mostly controlled by the MSLP anomalies over the Caspian Sea (CS) domain, varying from approximately -25 to +35 hPa in winter and from approximately-10 to +14 hPa in summer, but without a clear annual pattern. The CasHKI values are classified into four modes for each month depending on their intensity. In the summer months, the high CasHKI mode is associated with enhanced MSLP over central Asia and deepening of the Indo-Pakistan thermal low associated with the Indian summer monsoon. At 700-hPa level, the high CasHKI mode shows an enhancement of the Arabian ridge, expanding it to the north over Iran and the CS, with a concurrent strengthening of the Indian trough, leading to intensification of northerly winds along east Iran. This results in significant increase in dust activity over SWAsia, which is also apparent in the winter months. Furthermore, the intensification of the northerly flow associated with the high CasHKI modes drops the temperature and increases the relative humidity over Zabol, especially during winter. The SPRINTARS-model simulations also show increased dust emissions and concentrations for the high CasHKI values, confirming that the CasHKI variations modulate the dust activity over SW Asia throughout the year. [ABSTRACT FROM AUTHOR]

Temperature anomalies in the lower stratosphere (namely, at 50- hPa) poleward of 30°N (hereafter, T50) are analysed from reanalysis daily products (i.e. ERA-40 and NCEP/ NCAR) in order to detect those regions with statistically significant trends of T50 during the common 1957-2002 period. We also analyse radiosonde data in order to validate the reanalyses results. The analyses are conducted for the extended polar winter (i.e. from November to May) in order to relate T50 changes to the northern polar vortex variability. In relation to the previous literature, the novelty of this study is double: first, temporal evolution is considered according to regions with similar T50 temporal variability in the Northern Hemisphere; second, trend analyses of stratospheric temperatures are obtained with the use of running windows with variable width of each principal component series computed from monthly mean values. Two main stratospheric regions were identified from both reanalyses, with a statistically significant cooling within the study period: one over the high latitudes and a second one tracing a subtropical ring. The first one was detected in November, December and May during several decades, particularly at the beginning and in the middle of the study period (i.e. 1957-1985). The second region (i.e. the subtropical ring) showed overall cooling for all months along the study period. In addition, the lower stratospheric temperature over northern Europe exhibited an outstanding cooling in May for the whole study period. The results obtained from both reanalyses are practically identical, a fact that provides robustness thereto. Radiosonde data confirm the above-mentioned results, but the magnitude of the trends given by reanalysis products is substantially overestimated in the winter months over high latitudes. [ABSTRACT FROM AUTHOR]

Reliability of GCM (general circulation model) simulations is a significant predictor selection criteria and seems to be essential for outcomes of the statistical downscaling. This article investigates the reliability of a wide ensemble of CMIP5 climate models in reproducing seasonal atmospheric circulation patterns over Europe and the North Atlantic, namely sea level pressure (SLP) and 500 hPa geopotential height (G500). GCMs were evaluated with respect to 1971-2000 NCEP/NCAR reanalysis data by means of MAE, correlation coefficient and standard deviation. An attempt was made to determine the range of biases introduced into the statistical models by biases in the GCM simulations. For this purpose, canonical correlation analysis (CCA) models linking large-scale atmospheric circulation and air temperature in Poland were driven by historical GCM simulations expressed as anomalies with reference to the reanalysis data. It was shown that reliability of GCMs varies considerably, both seasonally and on an inter-model basis, thus becoming unfavourable in the context of statistical downscaling application. Overall, GCM ability to reproduce G500 values is better than for SLP. The most important finding from the statistical downscaling perspective is that SLP and G500 biases may introduce considerable air temperature biases (T-GCMbiases) into the downscaled reconstructions or projections of climate change. Seasonally, the highest T-GCMbiases were found in DJF, when a significant overestimation of air temperature results from a GCM tendency to overestimate the meridional pressure gradient. A similar situation appears to be a significant factor in MAM and SON. Multiyear average GCM biases do not, however, explain a significant part of the inter-model variability of T-GCMbias, thus highlighting the need for a more in-depth evaluation. Nevertheless, good GCM performance seems to ensure that T-GCMbias is not of a considerable value. Overall, the results of the article emphasise the necessity for undertaking an application-oriented assessment of GCM reliability prior to any statistical downscaling approach. [ABSTRACT FROM AUTHOR]

ABSTRACT This paper contributes to knowledge of the impact of global warming on viticulture, using major vineyard regions of New Zealand as a case study to illustrate regional disparities in climate change impacts resulting from downscale effects of larger scale atmospheric circulation. Recent trends in air temperature in New Zealand vineyard areas are investigated. Trends since 1941 in Marlborough show an increase in temperature range, with both rising maximum temperatures and declining minimum temperatures, but no change in the annual mean. More hot days and frosts are of concern to viticulturalists, and this trend not only differs from other major vineyard areas, but also occurs at other sites in eastern parts of the country. The relationship between changes in atmospheric circulation over New Zealand and observed trends in temperature and frost occurrence at Blenheim is investigated using the Kidson weather type classification for the period 1958-2010 and the Trenberth M1 index. The increased temperature range is expected to be associated with more frequent clear skies along the east coast of the country, resulting from changes in weather patterns (more frequent anticyclones, fewer low-pressure systems, increased zonal and southerly flow). These changes in weather patterns are shown to be closely linked to larger scale changes in atmospheric circulation via the Southern Annular Mode (SAM) and Southern Oscillation. The results show that significant regional variations in the impact of global warming can occur over areas of complex terrain such as New Zealand. Observed differences in local temperature and frost trends can be caused by the interaction of changing weather systems with mountainous terrain. These changing weather systems themselves are seen to be the result of major shifts in the larger scale atmospheric circulation. These results are important for assessing possible impacts on viticulture and in developing adaptation strategies for agriculture in response to predicted future climates. [ABSTRACT FROM AUTHOR]

Changes of the upper-tropospheric temperature teleconnection and the associated atmospheric circulations in spring (Mar-April-May, MAM) during the mid-Holocene are preliminarily examined by using the CCSM3 simulation data archived in the Paleoclimate Modeling Intercomparison Project (PMIP) database. The simulation result shows that in the mid-Holocene the atmospheric circulation over the Asian-North Pacific region showed the variability of the teleconnection pattern resembling the present Asian-Pacific oscillation (APO), which is characterized by a warming (cooling) upper troposphere over Asia corresponding to a cooling (warming) upper troposphere over the North Pacific and indicates a thermal contrast between Asia and the North Pacific. In comparison with the present climate, the mid-Holocene upper-tropospheric temperature in spring was modelled to be lower over Asia and higher over the North Pacific, indicative of a weaker spring APO. Associated with such a condition, both the upper-tropospheric high (low) and the lower-tropospheric low (high) over Asia (central-eastern Pacific) were weakened in the mid-Holocene. Accordingly, anomalous descending (ascending) motion appeared over East Asia (central-eastern Pacific). This circulation background is unfavourable (favourable) for the occurrence of precipitation, and thus the mid-Holocene precipitation decreases (increases) in this region as compared with the present. Hence, the change of the spring thermal contrast between Asia and the North Pacific played a crucial role in the anomalies of the atmospheric circulations and precipitation over the Asian-Pacific region during the mid-Holocene. Copyright © 2012 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

Low ozone level episodes (also known as mini-holes) have been analysed over Valencia, Spain, during the period from 1 January 2000 to 31 December 2007. In this study, 24 episodes, lasting 1 or 2 days each, were detected. Most of them can be explained dynamically by the existence of an anticyclone over the Azores, which sends air masses coming from the Caribbean over the Iberian Peninsula. A positive anomaly in the vertical temperature profile in the high troposphere and one negative anomaly in the medium and low stratosphere are also observed in most of the analysed episodes. The total ozone column decrease is accompanied by a rise in the UV erythemal radiation levels. This increase is quantified at 1.29 times the decrease in the total ozone column. This is known as the radiation amplification factor (RAF) of UV erythemal radiation. Copyright © 2010 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

The article presents a study on linear and nonlinear trends in the frequency of stratospheric and tropospheric potential vorticity (PV) streamers, specifically the breaking synoptic-scale Rossby waves in the tropopause region covering the period from 1958 to 2002. It is observed that several factors affect the streamer and wind trends such as changes in the wintertime stratospheric circulation, greenhouse gas concentration, as well as changes in subtropical and extra-tropical sea surface temperature (SST) patterns. Several studies related to the research are highlighted. The methods and results of the study are also discussed.

Focuses on the relationship between sea-surface temperature (SST) inter-annual variability at the subtropical and midlatitudes of the southern Atlantic and Indian Oceans and its links with the atmospheric circulation in the Southern Hemisphere are investigated over the 1950?1999 period. How exploratory analysis using singular value decomposition and further investigations based on simple indices show that a large part of regional SST variability is common between the southwestern parts of both basins at subtropical and midlatitudes during the austral summer.

Investigates the temporal and spatial variability of the seasonal mean of maximum air temperature in Romania and its links with the large?scale atmospheric circulation. Significant increasing trends were found for winter and summer mean maximum temperature in Romania, with upward shifts around 1947 and 1985 respectively. During autumn, a decreasing trend with a downward shift around 1969 was detected. These changes seem to be real, since they are connected to similar changes in the large-scale circulation. The intensification of the southwesterly circulation over Europe since 1933, which overlapped with the enhancement of westerly circulation after the 1940s, could be the reason for the change in winter mean maximum temperature.

Analyzes the decadal variability of the Danube river flow in the lower basin and its connection with the North Atlantic Oscillation (NAO) for the period 1931?1995. Associated linkages with precipitation (PP) in the European sector, global sea surface temperature (SST) and atmospheric circulation for the period 1931?81, and the 500 hPa geopotential heights (G500) over the Northern Hemisphere for the period 1948?95 are also investigated. The results show that there is an out-of-phase relationship between the time series of the Danube river flow anomalies and the NAO index. The time series of a PP index presents a time evolution similar to that of the river flow anomalies. Correlation maps between the river flow anomalies and global SST show coherent large-scale patterns. Physically consistent sea level pressure and 500 hPa geopotential height are obtained.

Monthly lower stratosphere temperature anomalies in a layer centred about 70 hPa, from the MSU data set are analysed for the period 1979–1997. T-mode approach principal component analysis (PCA) is used in order to obtain the leading spatial anomaly patterns and their sequences of occurrence throughout the period under study. Five principal components (PCs) are significantly different from the spatial distribution of noisy data. The patterns given by the PC scores represent ten typical spatial anomaly patterns: five correspond to the direct mode, that is to say anomaly fields with the same sign as the PC score patterns, and five have the opposite sign. The first three PCs represent simple spatial temperature anomaly distributions, with zonal wave 0 to wave 2 wave structures. The following significant PCs, orders four and five, display a more complex spatial behaviour, with wave 3 wave structures. The first two PC's frequency distribution in time, given by the PC loadings time series, do not show noticeable changes throughout the period analysed. The remaining three PCs show changes in their frequency of occurrence that might be associated with the negative trends in the lower stratosphere temperature, as well as to the other different features observed in the real temperature anomaly time series for the grid points in the Southern Hemisphere. The latter are studied with the PCA in the S-mode approach in the companion paper (Compagnucci et al., 2001. International Journal of Climatology 21: 419–437). Copyright © 2001 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

The lower stratosphere monthly temperature anomalies over the Southern Hemisphere derived from soundings made by the Microwave Sounding Unit (MSU) between 1979 and 1997 are analysed. Specifically MSU channel 4 temperature retrievals are considered. Principal component (PC) analysis with the S-mode approach is used in order to isolate grid points that covary in a similar manner and to determine the main features of their temporal behaviour. The first six PCs explain 81.3% of the variance and represent the different time variability patterns observed over the Southern Hemisphere for the ten area clusters determined by the method. The most important feature is common to all the PC score pattern–time series and corresponds to a negative linear trend present in almost all the Southern Hemisphere except over New Zealand and surrounding areas. The negative trend is largest over Antarctica. The remaining features of the temporal variability are different for each PC score and therefore for each cluster region over the Southern Hemisphere. The first PC score pattern shows the impact of the Chichón and Mt Pinatubo eruptions that each produced a 2-year warming over the tropical and sub-tropical lower stratosphere. This variability is orthogonal with the behaviour present over Antarctica. There are different anomalies between 1987 (El Niño) and 1988 (La Niña). This second PC does not show any evidence whatsoever of the volcanic eruptions. The semi-annual wave is present in the anomaly recurrence at mid to high latitudes. Over very low latitudes, close to the Equator, the Quasi-Biennial Oscillation (QBO) band of frequency is also present. Copyright © 2001 Royal Meteorological Society [ABSTRACT FROM AUTHOR]