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Teleconnection Patterns (teleconnection + pattern)
Selected AbstractsThe eastern Mediterranean teleconnection pattern: identification and definitionINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2007M. Hatzaki Abstract In this study, an attempt is made to investigate possible teleconnection patterns of atmospheric circulation, centered over eastern Mediterranean (EM) with the aid of gridded NCEP/NCAR daily values of geopotential heights for the period 1958,2003. For this purpose, two approaches have been used: correlation analysis and rotated principal component analysis (PCA) on a seasonal and monthly basis. A teleconnection pattern between the EM and northeastern Atlantic was identified at 500 and 300 hPa in winter, which will be referred to as the Eastern Mediterranean Pattern (EMP), appearing as an independent mode of the upper circulation. The pattern also exists in autumn but is substantially weakened with its dipole centers being shifted eastwards. Significant monthly variations were found in the location, strength and structure of the pattern. The employment of a standardized index demonstrated that the negative phase of the EMP prevails throughout the year with the maximum frequency at wintertime. Copyright © 2006 Royal Meteorological Society [source] January northern hemisphere circumpolar vortex variability and its relationship with hemispheric temperature and regional teleconnectionsINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 11 2005Robert V. Rohli Abstract Variability in the hemispheric-scale atmospheric circulation can be directly linked to variations in surface environmental features, such as temperature, precipitation, salinity of water bodies, and pollutant transport. One indicator of the behavior of the hemispheric-scale circulation is the circumpolar vortex (CPV). This research utilizes a geographic information system approach to characterize variability in the Northern Hemispheric (NH) CPV. Specifically, the area, shape, and centroid of the January NHCPV are analyzed for 1959,2001 because it may provide insight about relationships between hemispheric-scale circulation and global temperature change. We also use a new means of characterizing the hemispheric-scale circulation using a ,circularity ratio' (Rc). Results suggest that the January NHCPV has exhibited no long-term trends in area or shape, and that the mean centroid is positioned at approximately 85.3°N, 178.0°W. Regional patterns emerge, which suggest that the area and circularity are associated with variability in surface temperature and moist static energy. Furthermore, the area of the January NHCPV is associated with variability in the Arctic Oscillation, while the shape is tied to variability in the Pacific-North American teleconnection pattern. These results will facilitate understanding of the relationship between hemispheric-scale circulation, regional circulation, and local temperatures. Copyright © 2005 Royal Meteorological Society. [source] The dynamics of NAO teleconnection pattern growth and decayTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 589 2003Steven B. Feldstein Abstract This investigation performs both diagnostic analyses with NCEP/NCAR re-analysis data and forced, barotropic model calculations to examine the dynamical mechanisms associated with the growth and decay of the North Atlantic Oscillation (NAO) teleconnection pattern. The diagnostic calculations include projection and composite analyses of each term in the stream-function-tendency equation. The results of the analyses reveal a complete life cycle of growth and decay within approximately two weeks. The positive NAO phase is found to develop after anomalous wavetrain propagation across the North Pacific to the east coast of North America. This contrasts with the negative NAO phase which appeared to develop in situ. Both high-frequency (period <10 days) and low-frequency (period >10 days) transient eddy fluxes drive the NAO growth. After the NAO anomaly attains its maximum amplitude, the high-frequency transient eddy fluxes continue to drive the NAO anomaly in a manner that is consistent with a positive feedback process. The decay of the NAO occurs through both the divergence term and the low-frequency transient eddy fluxes. The temporal and spatial properties of the divergence term are found to be consistent with Ekman pumping. These results illustrate many important differences between the NAO and Pacific/North American (PNA) teleconnection patterns, perhaps most striking being that the NAO life cycle is dominated by nonlinear processes, whereas the PNA evolution is primarily linear, In addition, the relation between the NAO and the zonal index is discussed. Copyright © 2003 Royal Meteorological Society [source] Fundamental mechanisms of the growth and decay of the PNA teleconnection patternTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 581 2002Steven B. Feldstein Abstract This investigation performs diagnostic analyses on NCEP/NCAR re-analysis data, and also does forced, nonlinear, barotropic model calculations to examine the dynamical mechanisms associated with the growth and decay of the Pacific/North American teleconnection pattern (PNA). The diagnostic calculations include projection and composite analyses of each term in the stream-function-tendency equation. The results of the diagnostic analyses and model calculations reveal a PNA life cycle that is complete within approximately 2 weeks and is dominated by linear processes. The growth of the two upstream PNA anomaly centres is found to be by barotropic conversion from the zonally asymmetric climatological flow, and the two downstream PNA anomaly centres by linear dispersion. The PNA anomaly growth eventually ceases because of changes in the spatial structure of the anomaly. An analysis of the role of Ekman pumping is performed with a very simple model. The results, although qualitative, suggest that the decay of the PNA may be through Ekman pumping. An examination of the role of transient eddy vorticity fluxes indicates that they play an important role during some stages of the PNA life cycle. Lastly, the model calculations also reveal a crucial role played by the divergence term in maintaining the PNA anomaly in a quasi-fixed position. Copyright © 2002 Royal Meteorological Society. [source] North atlantic oscillatiodannular mode: Two paradigms,one phenomenonTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 564 2000John M. Wallace Abstract The North Atlantic Oscillation (NAO), as defined in the studies of Sir Gilbert Walker ca. 1930, and the zonal-index cycle, as elaborated by investigators at the Massachusetts Institute of Technology some twenty years later, are different interpretations of the same entity, whose time variations are well represented by the leading principal component of the northern hemisphere sea-level pressure field. The NAO paradigm envisions this phenomenon as involving a unique teleconnection pattern in the Atlantic sector that varies on interannual and longer time-scales in association with large-scale atmosphere-ocean interaction. In contrast, the zonal-index-cycle paradigm posits the existence of independent, fundamentally zonally symmetric (or ,annular') modes of variability in the northern and southern hemispheres, both of which fluctuate on intraseasonal as well as interannual time-scales. Spontaneous interactions between the zonally symmetric flow and the eddies are viewed as being largely responsible for the variability of the annular modes at the higher frequencies, and a variety of different mechanisms including, but by no means limited to, atmosphere-ocean interaction are viewed as potentially capable of forcing them at the lower frequencies. The NAO and ,annular mode' paradigms offer contradictory interpretations of the causal linkages that are responsible for the observed correlations between North Atlantic climate variability and variations in a diverse array of zonally averaged quantities. They suggest different research agendas and they evoke quite different images in the popular press. It is argued that the two paradigms cannot be equally valid and that it is in the interests of the community to come to a consensus as to which of them is more appropriate. Rules of evidence are proposed as a basis for making that decision. [source] Can principal component analysis provide atmospheric circulation or teleconnection patterns?INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2008Rosa H. Compagnucci Abstract This investigation examines principal component (PC) methodology and the interpretation of the displays, such as eigenvalue magnitude, loadings and scores, which the methodology provides. The key question posed is, to what extent can S- and T-mode decompositions of a dispersion matrix yield the kinds of interpretations placed on them typically? In particular, a series of experiments are designed based on various amalgamations of three distinct synoptic flow patterns. Since these flow patterns are known, a priori, this allows testing via subtle alterations of the methodology to determine whether there is equivalence between the S- and T-mode decompositions, the degree to which the flow patterns or teleconnections can be recovered by each mode, and the interpretation of each mode. The findings are examined in two contexts: how well they classify the flow patterns, and how well they provide meaningful teleconnections. Both correlation and covariance dispersion matrices are used to determine differences that arise from the standardization. Additionally, unrotated and rotated results are included. By examining a variety of commonly applied methodologies, the results hold for a wider range of studies. Key findings are that eigenvalue degeneracy can influence one mode (but not the other) or both modes for any set of flow patterns resulting in pattern intermixing at times. Similarly, such degeneracy is found in one or both dispersion matrices. Congruence coefficients are used to provide a measure of validity by matching the PC loadings to the parent correlations and covariances. This matching is vital as the loadings exhibit dipoles that have been interpreted historically as physically meaningful, but the present work indicates they may arise purely through the methodology. Overall, we observe that S-mode results can be interpreted as teleconnection patterns and T-mode as flow patterns for well-designed analyses that are meticulously scrutinized for methodological problems. Copyright © 2007 Royal Meteorological Society [source] Winter snow depth variability over northern Eurasia in relation to recent atmospheric circulation changesINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 13 2007V. Popova Abstract Mean snow depth time-series for February (1936,2001) over northern Eurasia (incl. Norway, Finland and the former USSR), interpolated into 5 × 5° grid points, are studied using empirical orthogonal function (EOF) analysis. First, five statistically significant rotated PCs are correlated to Northern Hemisphere (NH) teleconnection patterns at the 700 hPa height: North Atlantic Oscillation (NAO), Polar-Eurasia (Pol), Pacific-North American (PNA), West Pacific (WP), and Scandinavia (Scand). The impact of the NH circulation modes on snow depth variations is evaluated using the multiple stepwise backward regression (MSBR). Analyses of the snow depth PCs indicate that within the northern Eurasia territory, there are several regions with snow accumulation, respondent to certain circulation modes. PC1 describes low-frequency snow depth variation to the north from 55 to 60°N between the White Sea and the Lena river basin, and is positively correlated with NAO and negatively,with Scand. MSBR shows that in 1951,1974 the leading role in snow depth variability belongs to Scand. After 1975, Scand has passed over the leading role to NAO. Scand and NAO are also responsible for the surface air temperature changes over the northern Eurasia. Snow depth PC1 and wintertime temperature are closely related to each other. PC2 describes quasi-decadal snow depth variability over eastern Europe and is negatively correlated with NAO. For the Baltic and White Sea coasts, Fennoscandia, and the center of the East European plain, decrease of snow accumulation, related to a positive NAO phase, seems to be caused by mild winters. For the southwestern and central regions of eastern Europe, negative snow depth anomalies could also be caused by decrease of precipitation associated with the eastward shift of cyclone tracks related to the positive NAO phase. Two regions, where snow depth variations are described by PC1 and PC2, respectively, reveal the border between the opposite recent tendencies of snow accumulation. Copyright © 2007 Royal Meteorological Society [source] Seasonality of the northern hemisphere circumpolar vortexINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2007Kalyn M. Wrona Abstract In previous research, Rohli et al. (2005) identified long-term features of the northern hemispheric circumpolar vortex (NHCPV) in January. This research provides a seasonal analysis using December and February to augment the previously analyzed January data in representing winter, along with April, July, and October data to represent spring, summer, and autumn, respectively. A representative 500 hPa geopotential height contour was selected to delineate the NHCPV in each of the five months. The area, shape, and centroid of the monthly December, February, April, July, and October NHCPV are computed for 1959,2001 to supplement the previously identified January properties. These geometrical features of the NHCPV reveal relationships between hemispheric-scale circulation and temperature anomalies throughout the year. A circularity ratio (Rohli et al., 2005) is used to characterize the shape of the hemispheric-scale circulation. Results suggest that only October exhibit long-term trends in either area or circularity, with July being the most variable month in area and October being the most variable month in circularity. Centroids tend to be skewed toward the Pacific basin, except in spring, but few systematic temporal shifts in centroid position were noted for any month. The NHCPV is correlated with atmospheric teleconnection patterns in several months. For example, as was the case for January (Rohli et al., 2005), the Arctic Oscillation (AO) is associated with the area of the December, February, and April NHCPV, while in December the circularity is positively correlated to the AO Index. Also, the Pacific-North American index is correlated with the area of the December and February NHCPV and with the shape of the December and October NHCPV. Copyright © 2006 Royal Meteorological Society [source] The eastern Mediterranean teleconnection pattern: identification and definitionINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2007M. Hatzaki Abstract In this study, an attempt is made to investigate possible teleconnection patterns of atmospheric circulation, centered over eastern Mediterranean (EM) with the aid of gridded NCEP/NCAR daily values of geopotential heights for the period 1958,2003. For this purpose, two approaches have been used: correlation analysis and rotated principal component analysis (PCA) on a seasonal and monthly basis. A teleconnection pattern between the EM and northeastern Atlantic was identified at 500 and 300 hPa in winter, which will be referred to as the Eastern Mediterranean Pattern (EMP), appearing as an independent mode of the upper circulation. The pattern also exists in autumn but is substantially weakened with its dipole centers being shifted eastwards. Significant monthly variations were found in the location, strength and structure of the pattern. The employment of a standardized index demonstrated that the negative phase of the EMP prevails throughout the year with the maximum frequency at wintertime. Copyright © 2006 Royal Meteorological Society [source] North American weather-type frequency and teleconnection indicesINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 1 2003Scott C. Sheridan Abstract The impact of teleconnections upon the surface climate has largely been examined via a response in monthly mean temperature or total precipitation. In this paper, a different approach is undertaken, by examining the response of synoptic weather-type frequencies to different teleconnection phases. For over 330 stations in the USA and Canada, the Spatial Synoptic Classification scheme has classified each day in each station's period of record into one of seven weather-type categories, based on thermal, moisture, and other characteristics. The differences in how frequently these different weather types occur in different phases of the North Atlantic Oscillation (NAO) and Pacific,North American (PNA) teleconnection patterns is assessed, for Canadian stations from 1953 to 1993, and for US stations from 1950 to 1999. For PNA, a significant shift in the transitional frequency is observed, suggesting changes in storm track. Concomitantly, a large shift in Dry Polar and Moist Tropical frequencies is observed across the continent. Across the West, in +PNA wintertime months far fewer Dry Polar days are observed. Across the eastern USA, these polar intrusions are more common, and Moist Tropical is diminished significantly. The frequency of the transitional situation is also correlated with NAO phase, with differences as large as a factor of two across much of Canada and the northern USA. In northeastern Canada, there is a large replacement of Moist Polar conditions with Dry Polar conditions during +NAO. Farther south, however, across the eastern USA, both polar weather types occur much less often with +NAO. Although previous research has discovered eastern North American connections to the NAO, this research has shown that the connections often extend into the interior West during much of the year. Particularly strong in the spring, Dry Tropical conditions are much more common with +NAO throughout much of the continent, as far west as the Great Basin. Copyright © 2003 Royal Meteorological Society. [source] Evaluating Antarctic sea ice variability and its teleconnections in global climate modelsINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 8 2002Jiping Liu Abstract This study evaluates simulated Antarctic sea ice edge (SIE) variability and its teleconnections in three global coupled climate models (GISS, NCAR and GFDL) against the observations. All models do a reasonable job in simulating the seasonal advance and retreat of the Antarctic sea ice fields. The simulated GISS and NCAR SIE distributions are in agreement with observations in summer and autumn, whereas the GFDL model does best in spring and winter. A common problem is the poor simulation of the observed SIE in the Weddell Sea. All models are not particularly good at simulating the observed regionally varying SIE trends. A comparison of dominant empirical orthogonal function modes of surface air temperature (SAT) variability in each model associated with observed modes show that the models generally capture features of the more prominent covarying spatial patterns such as an El Niño,southern oscillation (ENSO)-like pattern in the tropical Pacific. The simulated teleconnection patterns between detrended Antarctic SIE anomalies and detrended global SAT anomalies in each model are evaluated for comparison with observed teleconnection patterns. All models capture the ENSO-like phenomenon to some degree. Also, the GISS and NCAR models capture the Antarctic dipole pattern and meridional banding structure through the Pacific. The Antarctic SIE regions showing the strongest extrapolar teleconnections differ among the models and between the models and observations. Almost all models miss the observed polar,extrapolar teleconnections in the central Indian, western extreme of the tropical and southern Pacific, and over the tropical continents. Copyright © 2002 Royal Meteorological Society. [source] Observed and SST-forced seasonal rainfall variability across tropical AmericaINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2001Vincent Moron Abstract Three experiments starting from different initial conditions have been made with the ECHAM-4 atmospheric General Circulation Model (GCM) integrated at T30 resolution forced with the observed sea-surface temperature (SST) over the period 1960,1994. The tropical America modes of seasonal rainfall anomalies whose time variation is most accurately simulated by the GCM have been searched for using Singular Value Decomposition Analyses (SVDA) and Canonical Correlation Analysis (CCA) between observed and model fields. The leading modes revealed by SVDA and CCA are highly similar, even though the ordering of the modes showed some fluctuation. A first skilful rainfall anomaly mode has weights of the same sign almost everywhere in tropical America, except along the western coast and the sub-tropical margins. This mode appears in all of the four seasons assessed. A second major skilful mode is usually a bipolar north,south (N,S) rainfall anomaly pattern (clear in December,March, DJFM; March,May, MAM; and June,September, JJAS). A large portion of the skill of the first rainfall anomaly mode (same sign anomalies across tropical America except small patches along the western coast) is through variance that is in common with the Southern Oscillation Index (SOI). In addition to forcing from the central/eastern tropical Pacific SST, there also appears a contribution from contrasting SST anomalies in the tropical Atlantic. This rainfall mode is usually a regional portion of a more large-scale mode encompassing at least the whole tropical zone (especially in DJFM, MAM and September,November, SON). Analysis of the relationship of this mode with GCM circulation features reveals that a rainfall deficit (respectively excedent) over the main rainbelt of the tropical America region is associated with strengthening (respectively weakening) of the sub-tropical westerly jet streams, a global warming (respectively cooling) of the tropical atmosphere, an anomalous divergence (respectively convergence) in the lower levels and an anomalous convergence (respectively divergence) in the upper levels over tropical America and in the region of the Atlantic Inter-tropical Convergence Zone (ITCZ). Such global features are not so apparent for the dominant mode of JJAS, even though the correlations with El Niño,Southern Oscillation (ENSO) indicators (as SOI or NINO3 SST index) are as high as for the other seasons. The bipolar N,S rainfall anomaly mode in tropical America is mostly related to anomalous N,S gradient of SST anomalies in the tropical Atlantic. The atmospheric circulation anomalies emphasize changes in 850 hPa meridional winds in the tropical Atlantic. However, there is also interannual variance of this rainfall mode in both the model and observations that is unexplained by tropical Atlantic SSTs, but which is explained by central/eastern tropical Pacific SSTs and, potentially, SSTs from other tropical and extratropical areas. This is especially true in MAM. Some differences in the details of the model and observed teleconnection patterns are noted. Such differences can be used to statistically adjust the model simulations using the CCA or SVDA modes as basis patterns. Both statistical approaches have been applied and the results are consistent between the two. The increase of skill is stronger when temporal correlation (the pattern correlation) between the model and observed pattern is high (low) as for JJAS. The skill is moderate to high around the whole Amazon basin, but remains relatively low inside the Amazon basin, though reliability of the observations themselves may influence this result. Averaged over all the seasons, about 15,35% (35,55%) of the interannual grid-box (regional) seasonal rainfall variance is skilfully simulated from the observed SST forcing. Copyright © 2001 Royal Meteorological Society [source] The dynamics of NAO teleconnection pattern growth and decayTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 589 2003Steven B. Feldstein Abstract This investigation performs both diagnostic analyses with NCEP/NCAR re-analysis data and forced, barotropic model calculations to examine the dynamical mechanisms associated with the growth and decay of the North Atlantic Oscillation (NAO) teleconnection pattern. The diagnostic calculations include projection and composite analyses of each term in the stream-function-tendency equation. The results of the analyses reveal a complete life cycle of growth and decay within approximately two weeks. The positive NAO phase is found to develop after anomalous wavetrain propagation across the North Pacific to the east coast of North America. This contrasts with the negative NAO phase which appeared to develop in situ. Both high-frequency (period <10 days) and low-frequency (period >10 days) transient eddy fluxes drive the NAO growth. After the NAO anomaly attains its maximum amplitude, the high-frequency transient eddy fluxes continue to drive the NAO anomaly in a manner that is consistent with a positive feedback process. The decay of the NAO occurs through both the divergence term and the low-frequency transient eddy fluxes. The temporal and spatial properties of the divergence term are found to be consistent with Ekman pumping. These results illustrate many important differences between the NAO and Pacific/North American (PNA) teleconnection patterns, perhaps most striking being that the NAO life cycle is dominated by nonlinear processes, whereas the PNA evolution is primarily linear, In addition, the relation between the NAO and the zonal index is discussed. Copyright © 2003 Royal Meteorological Society [source] | ||||||||||