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Wei Zhang
,
H.-F. Graf
,
Yee Leung
, and
Michael Herzog

Abstract

This study examines whether there exist significant differences in tropical cyclone (TC) landfall between central Pacific (CP) El Niño, eastern Pacific (EP) El Niño, and La Niña during the peak TC season (June–October) and how and to what extent CP El Niño influences TC landfall over East Asia for the period 1961–2009. The peak TC season is subdivided into summer [June–August (JJA)] and autumn [September–October (SO)]. The results are summarized as follows: (i) during the summer of CP El Niño years, TCs are more likely to make landfall over East Asia because of a strong easterly steering flow anomaly induced by the westward shift of the subtropical high and northward-shifted TC genesis. In particular, TCs have a greater probability of making landfall over Japan and Korea during the summer of CP El Niño years. (ii) In the autumn of CP El Niño years, TC landfall in most areas of East Asia, especially Indochina, the Malay Peninsula, and the Philippines, is likely to be suppressed because the large-scale circulation resembles that of EP El Niño years. (iii) During the whole peak TC season [June–October (JJASO)] of CP El Niño years, TCs are more likely to make landfall over Japan and Korea. TC landfall in East Asia as a whole has an insignificant association with CP El Niño during the peak TC season. In addition, more (less) TCs are likely to make landfall in China, Indochina, the Malay Peninsula, and the Philippines during the peak TC season of La Niña (EP El Niño) years.

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J. M. Castanheira
,
M. L. R. Liberato
,
L. de la Torre
,
H-F. Graf
, and
A. Rocha

Abstract

The annular variability of the northern winter extratropical circulation is reassessed based on reanalysis data that are dynamically filtered by normal modes. One-half of the variability of the monthly averaged barotropic zonally symmetric circulation of the Northern Hemisphere is statistically distinct from the remaining variability and is represented by its leading empirical orthogonal function (EOF) alone. The daily time series of the circulation anomalies projected onto the leading EOF is highly correlated (r ≥ 0.7) with the lower-stratospheric northern annular mode (NAM) indices showing that annular variability extends from the stratosphere deep into the troposphere. However, the geopotential and wind anomalies associated with the leading principal component (PC1) of the barotropic zonally symmetric circulation are displaced northward relative to the zonal mean anomalies associated with the PC1 of the geopotential height variability at single-isobaric tropospheric levels. The regression pattern of the 500-hPa geopotential height (Z500) onto the lower-stratospheric NAM also shows zonally symmetric components displaced northward with respect to those of the leading EOF of the Z500 field.

A principal component analysis (PCA) of the residual variability of the Z500 field remaining after the substraction of the Z500 regressed onto the lower-stratospheric NAM index also reveals a pattern with a zonally symmetric component at midlatitudes. However, this zonally symmetric component appears as the second EOF of the residual variability and is the imprint of two independent dipoles over the Pacific and Atlantic Oceans.

Results show that a zonally symmetric component of the middle- and lower-tropospheric circulation variability exists at high latitudes. At the middle latitudes, the zonally symmetric component, if any exists, is artificially overemphasized by the PCA on single-isobaric tropospheric levels.

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