ALBERT

Description: The meridional displacement of East Asia jet (EAJ) is characterized by the leading mode of upper tropospheric zonal wind variability over East Asia in boreal summer, and is closely related to the East Asia summer monsoon and downstream climate. Present study reveals that the meridional displacement of EAJ is associated with tropical Indian Ocean (TIO) SST anomalies. When the TIO SST is higher than normal, the overlying tropospheric air warms up through the modulation of the TIO SST on tropical convection. The anomalous convection forces a Kelvin wave wedge penetrating into the equatorial western Pacific, leading to a decrease in precipitation near the Philippines. Combined with the climatological easterly shear over the subtropical western North Pacific, the Pacific-Japan/East Asia-Pacific (PJ/EAP) teleconnection is induced along the East Asia coast. The PJ/EAP-related upper-level anomalous cyclone accelerates westerly in the south flank of EAJ and decelerates westerly in the north flank. Thus, EAJ shifts southward. In contrast, the EAJ shifts northward when the TIO SST is lower than normal. Copyright © 2011 Royal Meteorological Society

Description: Early detection of extreme drought and flood events either over the whole globe or a broad geographical region, and timely dissemination of this information, is indispensable for mitigation and disaster preparedness. Recently, the APEC Climate Center (APCC) has launched a global precipitation variation monitoring product based on the Climate Anomaly Monitoring System-Outgoing Longwave Radiation Precipitation Index (CAMS-OPI) data. Here we quantify the reliability of CAMS-OPI, as well as other gauge-satellite-merged and reanalysis precipitation datasets, for the purpose of monitoring large-scale precipitation variability in East Asia. The ground truth is the newly available gauge-based data from the project titled ‘Asian Precipitation—Highly-Resolved Observational Data Integration Towards Evaluation (APHRODITE) of the Water Resources’. It is found that the seasonal-to-interannual rainfall deficit and surplus given by various reanalysis systems sometimes do not match the spatial patterns seen in the APHRODITE data. Moreover, maps showing the Standardized Precipitation Index (SPI) become less and less reliable as the time scale based on which values are calculated increases. In contrast, the performance of gauge-satellite-based rainfall datasets is satisfactory and the quality of SPI maps does not decay as the time scale increases. Overall, CAMS-OPI is found to be reliable for monitoring large-scale precipitation variations over the East Asian sector. Copyright © 2011 Royal Meteorological Society

Description: The connection between Tropical Pacific and North Pacific variability is investigated in a state-of-the-art coupled ocean-atmosphere model, comparing two 20th century simulations at T30 and T106 atmospheric horizontal resolutions. Despite a better simulation of the frequency and the spatial distribution of the Tropical Pacific anomalies associated with the El Niño Southern Oscillation (ENSO) in the high-resolution experiment, the response in the North Pacific is scarcely different from the low-resolution experiment where the ENSO variability is weaker and at higher than observed frequency. In the North Pacific, the response of surface atmospheric fields to the variability in the Tropical Pacific appears to be affected by local coupling processes significantly different in the two experiments. The coupling between sea level pressure (SLP) and sea surface temperature (SST) in the North Pacific as well as the influence of the Tropical Pacific SST has been measured here by means of the ‘coupled manifold’ technique. In the low-resolution case the SLP variances linked to the fraction of North Pacific SST not influenced by the Tropical Pacific are weak suggesting that the remote influence is strong, consistently with the observations. On the contrary, in the high-resolution experiment the fractions and the patterns of the SLP variances due to the Tropical Pacific SST and those linked to the North Pacific SST are comparable. In the latter case, model systematic errors in the northwestern Pacific influences the local coupling processes thus triggering the remote response. We conclude that an increased atmospheric horizontal resolution does not reduce the coupled model systematic errors in the representation of the teleconnection between the North and the Tropical Pacific and that the validation of coupled models has to consider both remote and local processes. Copyright © 2011 Royal Meteorological Society

Description: In this work we present a homogenized high-resolution data set composed of 200 daily precipitation series spanning the last 90 years, located over an area centred on the Trentino—South Tyrol region (central part of the European Alps), in a transition zone between the climates of the southern and northern slopes of the Alps. We analysed the trends of total precipitation (TP), wet days (WD) and average intensity (PI), as well as trends of the number of events and precipitation amounts belonging to 12 different daily intensity categories. For an easier understanding of geographical patterns, we set up a gridded data set in terms of anomalies, with a spatial resolution of 0.1°. All the statistics were analysed for trend over the entire period spanned by the data and on subperiods of variable length. On regional average, we found a weak decrease in TP (about 1%/decade with respect to the 1971–2000 mean) over the entire studied period (1922–2009), which was statistically significant only in spring. Gridded data show that the decrease is related to a reduction in the number of WD in the eastern part of the study area, and a decrement in PI in the western part, with orography playing a clear role in this differentiation. On a daily scale, trends of the strongest events present scarce spatial coherence and are only locally significant, however the results are highly dependent on the period analysed. Comparisons with previous low-resolution studies on the same area underline the importance of a high-resolution data set in characterizing spatial variability of climatic trends in precipitation. Copyright © 2011 Royal Meteorological Society

Description: The relationship between temperature variability inside Scarisoara Ice Cave (NW Romania) and large-scale atmospheric circulation and sea surface temperature (SST) anomalies is investigated. A composite analysis reveals that high (low) temperatures inside the cave are associated with high (low) air temperature over a large area that covers central and Eastern Europe. Coherent large-scale patterns are identified in the field of SST anomalies. The corresponding atmospheric circulation patterns favour relatively warm (cold) air advection towards the cave region during high (low) temperatures inside the cave. A correlation analysis reveals that the atmospheric circulation patterns associated with cave temperature variability is mainly the result of the East Atlantic-Western Russia ( r = − 0.38) teleconnection pattern. Our results have strong implications for the interpretation of climate variability in the region as recorded by the ice block within the Scarisoara Ice Cave, which is more than 3000 years old. Copyright © 2011 Royal Meteorological Society

Description: An examination of synoptic environments conducive to urban thunderstorm development surrounding Atlanta, Georgia, was conducted. Synoptic environmental characteristics were evaluated utilising regional analyses of heights, winds, temperature, and moisture characteristics as well as several thermodynamic variables. A flexible beta cluster analysis was used in order to identify disparate groupings of common synoptic and thermodynamic environments. These groupings were then further analysed through the production of synoptic composites, radar, and lightning distributions. Although the analysis stratified the data by days with weak synoptic flow and maritime tropical airmass characteristics, enough variation existed within the synoptic and thermodynamically grouped parameters to identify four distinct environments present during urban thunderstorms. Environments exhibiting moderate thermodynamic instability were found to produce the highest flash rates and most frequent composite reflectivity days. Copyright © 2011 Royal Meteorological Society

Description: A fully coupled atmospheric-biospheric regional climate model, GEMRAMS, was used to evaluate potential effects of land-use/land-cover changes (LULCC) on near-surface atmosphere over a southern South American domain at seasonal time scales. In GEMRAMS, leaf area index and canopy conductance are computed based on modelled temperature, solar radiation, and the water status of the soil and air, allowing a two-way interaction between canopy and atmosphere. Several austral spring-early summer simulations were conducted using land cover representing current (i.e. agricultural landscape), natural (i.e. before European settlement), and afforestation scenarios for three periods associated with El Niño-Southern Oscillation (ENSO) conditions. The shift to agriculture resulted in a generalized decrease in albedo, reducing the available energy at the near-surface. The energy partitioning between latent and sensible heat fluxes changed, leading to distinct temperature responses. A shift from grass to agriculture led to cooler and wetter near-surface atmospheric conditions. Warmer temperatures resulted from the conversion of wooded grasslands or forest to agriculture. The LULCC-induced signal was spatially heterogeneous and with a seasonal component associated with vegetation phenology. A significant decrease in maximum temperatures in the southern and central Pampas led to a decrease in the diurnal temperature range. Basing on some observational studies in this region our results suggest a potential strong influence of LULCC on the maximum temperatures in central Argentina in summer. Afforestation resulted overall in cooler temperatures. For both LULCC scenarios the direction of the energy fluxes and temperature changes remained in general the same in two extreme ENSO years, although for some vegetation conversions the signal reversed direction. Overall, the impacts were enhanced during a dry year, but the response also depended on the vegetation types involved in the conversion. The effects on precipitation were insignificant in the agriculture-conversion scenario and a general increase was found in the afforested scenario. Copyright © 2011 Royal Meteorological Society

Description: The variability of seasonal precipitation anomalies over Europe displays significant spectral peaks at quasi-quadrennial periods, appearing in power spectra of the two leading principal components of the field. Band-pass filtering of these time series reveals the presence of two different signals within the quadrennial band, each associated to distinct precipitation, sea level pressure (SLP) and sea surface temperature (SST) patterns, obtained here by means of linear regression and composite analysis. One of the quadrennial components seems to be connected with the impact of El Niño-Southern Oscillation (ENSO) over Europe, that leads to anomalous SLP over the North Sea, southwestern Scandinavia and central Europe, along with precipitation anomalies of the same sign in a wide band extending from the British Isles into eastern Europe, fringed by anomalies of the opposite sign along the Norwegian coast and in the western Mediterranean area. The other quadrennial component bears no correlation with the Tropical Pacific SST anomalies, and seems connected with SST anomalies in the equatorial Atlantic, and in the Mediterranean, North and Norwegian Seas. The precipitation anomalies feature a contrast between the northern and southern regions of Europe akin to that associated with the North Atlantic Oscillation (NAO). The SLP pattern resembles the NAO structure, but with a marked eastward shift of its two centres of action towards central Europe and the Barents Sea, respectively. Copyright © 2011 Royal Meteorological Society

Description: The relationships between large-scale atmospheric circulation types and seasonal regimes of daily precipitation over Iran are assessed using daily precipitation from a high-resolution gridded dataset provided by the Asian Precipitation-Highly Resolved Observational Data Integration Towards the Evaluation of Water Resources (APHRODITE) Project. Regional spatial modes of daily precipitation variability were identified by S-mode Principal Component Analysis (PCA) with Varimax rotation, applied to the subset of days when at least 10% of all grid-points over Iran received precipitation ≥ 5 mm. The study refers to the period 1961–2004 and is carried out for each season (excluding summer) separately. To characterize the dynamical features associated with each regional precipitation regime (PR), composites of daily atmospheric fields are computed by only averaging days with rotated PCA scores ≥ 1.5 (strong positive phase). In autumn and winter, Iran is divided into five PRs, while four PRs are identified in spring. Results suggest that the spatial distribution of precipitation over Iran is largely governed by the geographical position of both the mid-tropospheric trough over the Middle East and the Arabian anticyclone. In fact, in almost all PRs, the trough, as a pre-conditioning factor, leads to regional-scale ascending motions, whereas the Arabian anticyclone induces low-tropospheric moisture transports from southern water bodies into the cyclonic systems near Iran, triggering rain-generating conditions. Copyright © 2011 Royal Meteorological Society

Description: Synoptic weather typing depends on the method and its implementation. This paper demonstrates the utility of a 2-phase batch self-organising map procedure (CP2) in comparison with a procedure (CP1) consisting of T-mode principal component analysis followed by convergent K-means clustering, for classifying the synoptic weather types over east Australia. Four classifications were obtained for the 52-year NCEP/NCAR dataset, one from CP1 and three from CP2. These classifications were examined in terms of grouping quality, mean type maps, type frequencies, lifetime and transitions, and in relation to the Southern Oscillation Index (SOI). The results showed that the classifications derived from different procedures are inter-confirmative and capture a similar set of major synoptic situations influencing east Australia, each having counterparts in previous studies and conforming well to local synoptic experience. In particular, this study demonstrates that CP2 is a promising tool for the purpose of weather typing with a two-fold utility: CP2 can perform data projection (neighbourhood width 〉 = 1) and provide a flexible means for visualising the broad distribution of the daily weather patterns in the dataset; and the procedure can also conduct cluster analysis (neighbourhood width = 0) and produce results equivalent to those from CP1. It was found that performing data clustering or data projection may affect the inference associated with type frequencies. While focusing on methodology, the analysis has also shown that the frequency of synoptic weather types fluctuates on the seasonal, interannual, and decadal scales. From the 1970s to 1995 there was a decline in the occurrence of a few weather types associated with an east-coast trough extending over New South Wales from the north, and a slight increase in the activity of some anticyclonic types. Significant correlations were also identified between SOI and frequencies of synoptic weather types on the annual and seasonal timescales. Copyright © 2011 Royal Meteorological Society