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Circulation Changes (circulation + change)
Selected AbstractsWinter 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] The behavior of extreme cold air outbreaks under greenhouse warmingINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 9 2006S. Vavrus Abstract Climate model output is used to analyze the behavior of extreme cold-air outbreaks (CAOs) under recent and future climatic conditions. The study uses daily output from seven GCMs run under late-twentieth century and projected twenty-first century radiative conditions (SRES A1B greenhouse gas emission scenario). We define a CAO as an occurrence of two or more consecutive days during which the local mean daily surface air temperature is at least two standard deviations below the local wintertime mean temperature. In agreement with observations, the models generally simulate modern CAOs most frequently over western North America and Europe and least commonly over the Arctic. These favored regions for CAOs are located downstream from preferred locations of atmospheric blocking. Future projections indicate that CAOs,defined with respect to late-twentieth century climatic conditions,will decline in frequency by 50 to 100% in most of the Northern Hemisphere during the twenty-first century. Certain regions, however, show relatively small changes and others actually experience more CAOs in the future, due to atmospheric circulation changes and internal variability that counter the thermodynamic tendency from greenhouse forcing. These areas generally experience greater near-surface wind flow from the north or the continent during the twenty-first century and/or are especially prone to atmospheric blocking events. Simulated reductions in CAOs are smallest in western North America, the North Atlantic, and in southern regions of Europe and Asia. The Eurasian pattern is driven by a strong tendency for the models to produce sea-level pressure (SLP) increases in the vicinity of the Mediterranean Sea (intermodel mean of 3 hPa), causing greater advection of continental air from northern and central Asia, while the muted change over western North America is due to enhanced ridging along the west coast and the increased frequency of blocking events. The North Atlantic response is consistent with a slowdown of the thermohaline circulation, which either damps the warming regionally or results in a cooler mean climate in the vicinity of Greenland. Copyright © 2006 Royal Meteorological Society. [source] Decadal changes in the link between El Niño and springtime North Atlantic oscillation and European,North African rainfallINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 11 2003Peter Knippertz Abstract The link between El Niño,southern oscillation (ENSO) variability in boreal winter (represented by the NIÑO3 index, i.e. East Pacific sea-surface temperature anomalies) and the large-scale circulation and weather conditions over Europe,northwest Africa in spring is explored, considering station reports of precipitation, sea-level pressure (SLP) anomalies and two North Atlantic oscillation (NAO) indices. It is found that these relations have undergone consistent and simultaneous changes in the 20th century. Three characteristic periods can be identified. During 1900,25 and 1962,87, positive NIÑO3 index values are associated with enhanced precipitation over central Europe and reduced rainfall in southern Europe and northern Africa. The ENSO influence on precipitation over Scotland and Norway is small. The rainfall anomalies can be explained from the advective and dynamical implications of a north,south dipole in SLP correlations (warm ENSO events followed by low pressure in northern Europe and high pressure over the Mediterranean Sea,North Africa). This dipole hardly projects on the commonly used NAO centres (Iceland and Azores/Gibraltar) and thus ENSO,NAO correlations are insignificant. During 1931,56 the NIÑO3 index reveals little influence on precipitation over the Iberian Peninsula and Morocco, but there are large negative correlations with precipitation over Scotland and Norway. This is related to an alteration of the NIÑO3,SLP correlation pattern, which implies high pressure over northern Europe and low pressure over central Europe after warm events, and thus a virtually inverted dipole with respect to the other two periods. The large westward extension of the dipole leads to a significant NAO,NIÑO3 correlation of r = ,0.5. These alterations were accompanied by substantial large-scale circulation changes during the period 1931,56, as revealed by anomalously high pressure and dry conditions over central,western Europe, a change in precipitation-producing SLP patterns for Morocco and an anomalously low number of positive NAO and NIÑO3 index values. It is left for discussion as to whether the decadal variations described are due to a change in the physics of the teleconnection or to stochastic fluctuations. Copyright © 2003 Royal Meteorological Society [source] Trends in New Zealand daily temperature and rainfall extremesINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2001M.J. Salinger Abstract Trends in daily temperature and rainfall indices are described for New Zealand. Two periods were examined: 1951,1998, to describe significant trends in temperature and rainfall parameters; and 1930,1998, to ascertain the effects of two main circulation changes that have occurred in the New Zealand region around 1950 and 1976. Indices examined included frequencies of daily maximum and minimum temperatures, above and below specified percentile levels and at those levels, as well as frequencies of these above and below fixed temperature thresholds. Extreme daily rainfall intensity and frequency above the 95th percentile and the length of consecutive dry day sequences were the rainfall indices selected. There were no significant trends in maximum temperature extremes (,hot days') but a significant increase in minimum temperatures was associated with decreases in the frequency of extreme ,cold nights' over the 48-year period. There was a non-significant tendency for an increase in the frequency of maximum temperature extremes in the north and northeast of New Zealand. A decline occurred in frequency of the minimum temperature 5th percentile (,cold nights') of 10,20 days a year in many locations. Trends in rainfall indices show a zonal pattern of response, with the frequency of 1-day 95th percentile extremes decreasing in the north and east, and increasing in the west over the 1951,1996 period. Changes in the frequency of threshold temperatures above 24.9°C (25°C days) and below 0°C (frost days) are strongly linked to atmospheric circulation changes, coupled with regional warming. From 1930,1950 more south to southwest anomalous flow occurred relative to later years. In this period, 25°C days were less frequent in all areas except the northeast, and there was markedly more frost days in all but inland areas of the South Island compared with the 1951,1975 period. There was more airflow from the east and northeast from 1951 to 1975, the frequency of 25°C days increased and frost days decreased in many areas of New Zealand. In the final period examined (1976,1998), more prevalent airflow from the west and southwest was accompanied by more anticyclonic conditions. Days with a temperature of 25°C increased in the northeast only. Frost day frequencies decreased between 5 and 15 days a year in many localities, with little change in the west of the South Island and at higher elevation locations. Copyright © 2001 Royal Meteorological Society [source] A critical comparison of stratosphere,troposphere coupling indicesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 644 2009Mark P. Baldwin Abstract Assessing stratosphere,troposphere coupling in observational data or model output requires a multi-level index with high time resolution. Ideally, such an index would (1) represent spatial patterns in the troposphere that are most strongly coupled with stratospheric variability and (2) be robust and computationally feasible in both observations and standard model output. Several of the indices used to diagnose extratropical stratosphere,troposphere coupling are based on the Northern and Southern Hemisphere annular modes. The annular mode indices are commonly defined as the leading empirical orthogonal functions (EOFs) of monthly-mean, hemispheric geopotential height. In the lowermost troposphere, the structure of the annular modes is defined as the leading EOF of the near-surface geopotential height field, and these patterns correspond well to the patterns of variability induced by stratospheric circulation changes. At pressure levels above the surface, the structure of the annular modes is typically found by either calculating the local EOF or regressing geopotential height data onto the leading principal component time series of near-surface geopotential height. Here we make a critical comparison of the existing methodologies used to diagnose stratosphere,troposphere coupling, including EOF-based indices as well as measures based on zonal-mean wind at a fixed latitude and geopotential height over the polar cap. We argue in favour of an alternative methodology based on EOFs of daily zonally-averaged geopotential. We find that (1) the daily evolution of stratosphere,troposphere coupling events is seen most clearly with this methodology, and (2) the methodology is robust and requires few subjective choices, making it readily applicable to climate model output available only in zonal-mean form. Copyright © 2009 Royal Meteorological Society [source] | ||||||||||