Relationships between wintertime modes of atmospheric variability on intermediate and long timescales

Abstract

Variability in daily wintertime (DJF) 500 hPa geopotential heights on short (< 6 days), intermediate (6-30 days), and long (> 30 days) timescales is examined. Sectoral EOF analysis shows the leading patterns of variability on short timescales to correspond to the climatological storm tracks, and on long timescales to resemble the North Atlantic Oscillation (NAO) and the Pacific-North America (PNA) pattern. The intermediate timescale variability is best described by large-scale, zonally-oriented patterns: In the Atlantic sector the leading pattern is a dipole with centers of action over Greenland and Scandinavia; The leading Pacific pattern is dominated by a center of action over the Gulf of Alaska, flanked by a pair of lower amplitude centers of opposing sign.Non-linear relationships between fluctuations on long timescales and shorter timescale variance are determined using maximum covariance analysis (MCA). In the Atlantic sector, the negative polarity of a pattern resembling the NAO is associated with enhanced intermediate-timescale variability in a region extending from Baffin Bay to Scandinavia. In the Pacific sector, the negative polarity of a PNA-like pattern is associated with an enhancement of intermediate-timescale variance for a region centered over Alaska and extending over the North Pacific. Similar relationships are demonstrated by regression of the variance fields onto monthly indices of the NAO and PNA, and by the compositing of intermediate timescale variance by polarity of these modes. Hence, the leading patterns of intermediate timescale variability are shown to be enhanced during low-index months of the NAO and PNA.The skewness of the 500 hPa height field, an indicator of blocking activity polewards of the midlatitude jet, is attributable to contributions from intermediate and monthly timescale skewness, as well as a cross-frequency term involving monthly anomalies and intermediate timescale variance. This cross-frequency term is shown to be the dominant source of skewness in the total field, and a simple conceptual model for how large-scale modes of variability generate skewness is discussed.Composites of extreme positive and negative intermediate timescale events over Baffin Bay and Anchorage demonstrate a high degree of symmetry, aside from a reversal of sign. This symmetry is shown to extend throughout the lifetimes of these events over these locations, and they are shown to exhibit the westward phase propagation and downstream development characteristics of Rossby waves. The spatial patterns of these events resemble the two polarities of the leading sectoral EOFs on this timescale, and their symmetry is consistent with the normal distribution of their principal component timeseries. Therefore, it is suggested that the intermediate timescale is governed primarily by linear, Rossby wave dynamics. Rossby wave generation is favored for these regions during periods of low-index NAO and PNA, and the non-linear appearance of the positive skewness at these locations may be explained as a superposition of the intermediate timescale Rossby waves on a background state set by the NAO or PNA.