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Carlos A. Pires
and
Rui A. P. Perdigão

Abstract

The present work assesses non-Gaussianity and asymmetry within the statistical response of the monthly winter (December–February) precipitation to the North Atlantic Oscillation (NAO) over the North Atlantic–European region (NAE). To evaluate asymmetry, data are split through the median of the NAO index and side correlations are computed for each regime [negative and positive phases of the NAO (NAO− and NAO+, respectively)]. The following statistically significant differences between these correlations are found: (a) near the central North Atlantic, around 40°N, 20°W, and southeast of Iceland, with much stronger correlations in the wet-favorable regime: NAO− in the first location and NAO+ in the second location; (b) around 42°N, 48°W in the west North Atlantic; and (c) south of Greenland and in the west Mediterranean near 36°N, where, in both cases, the correlation is only relevant for the dry-favorable NAO+ regime. Based on the above decomposition, a map of a statistical test of asymmetry, applicable for every bivariate distribution, is shown.

To evaluate redundancy and non-Gaussianity, the mutual information (MI) is computed from information theory. Its positive contributions resulting from the linear correlation, a purely Gaussian term, and non-Gaussianity, which vanishes in pure Gaussian cases, are studied. The MI is estimated through two methods: 1) the truncated Edgeworth expansion of the bivariate probability density function in terms of Hermite polynomials and cumulants, and 2) the maximum entropy method. This method is quite general, while the first one is only applicable for small deviations from Gaussianity. The map of non-Gaussian MI over the NAE domain reveals some coherent regions, where the nonlinear component of the response of monthly winter precipitation to the NAO is more important. The MI is evaluated both for the original pair of variables and for that pair after being subjected to Gaussian anamorphosis in order to prevent the influence of marginal outliers and keep the applicability of the Edgeworth method.

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P. Pires
,
J-L. Redelsperger
, and
J-P. Lafore

Abstract

Equatorial wave systems and their relationships with convective activity are analyzed in the western and central Pacific regions during the Coupled Ocean–Atmosphere Response Experiment (COARE) intensive observation periods. The study uses Geostationary Meteorological Satellite infrared temperature observations and the operational European Centre for Medium-Range Weather Forecasts analyses supplemented with COARE observations.

Spectral and complex principal component analysis are applied to the data. Using the linear theory of equatorially trapped waves as a guideline, the existence of three types of waves is detected. In the 7–10-day period range, n = 1 Rossby waves are found to the east of the date line, in a region of weak convective activity. Over the western equatorial Pacific, where intense convection occurs, the 7–10-day waves do not possess the general characteristics of linear Rossby waves, but they are strongly linked to the active phases of westerly wind bursts and of convection.

Analysis of the meridional wind reveals intense mixed Rossby–gravity waves with a mean 5-day period and westward phase and eastward group velocities. Over the western Pacific, the convection is found to be strongly correlated with the antisymmetric structure of the divergent field, as predicted by the linear theory.

In the 200-hPa divergence field, n = 1 gravity waves are visible, having periods shorter than 2.5 days. They rapidly propagate (about 25 m s−1) both westward and eastward, and have strong correlations with convective clusters.

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