Stratospheric Effects of ENSO-Related Tropospheric Circulation Anomalies

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  • 1 Northwest Research Associates, Inc., Bellevue, Washington
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Abstract

Using 30 years of Northern Hemisphere geopotential data, from 1000 to 10 hPa, the link between three fundamental modes of tropospheric variability [the Pacific/North America (PNA), Western Pacific Oscillation (WPO), and Tropical/Northern Hemisphere (TNH) patterns] and the extratropical wintertime northern stratospheric circulation is explored. These modes of variability are known to be influenced by El Niño/Southern Oscillation (ENSO) and may provide a mechanism for ENSO to influence interannual variability of the strato-spheric flow. Aside from any link to ENSO, these modes may be important in providing tropospheric wave forcing to the stratosphere.

These modes may be characterized by three of the leading rotated empirical orthogonal functions of the wintertime 5OO-hPa height field. An index of the amplitude of each of the modes is given by time series of principal components of that mode. By examining composites of years with high-index values minus years with low-index values it is shown that the zonal-mean stratospheric wind is not significantly influenced by any of the three tropospheric modes. However, the variability of these tropospheric modes appears to be associated with variations of stratospheric amplitudes of geopotential waves 1 and 2.

A comparison is made between the stratospheric influence of the equatorial quasi-biennial oscillation (QBO) and the three tropospheric modes during the December-February period. The influence from the QBO is weak in the troposphere and strongest in the stratosphere. The tropospheric modes’ influence is strongest in the troposphere. At 50 hPa, the QBO's influence is a strong modulation of the strength of the polar night jet and, in terms of geopotential height, is about twice that of the PNA, WPO, or TNH modes. The effect of the QBO on the stratospheric climate appears to be considerably larger than that from the three tropospheric modes.

Abstract

Using 30 years of Northern Hemisphere geopotential data, from 1000 to 10 hPa, the link between three fundamental modes of tropospheric variability [the Pacific/North America (PNA), Western Pacific Oscillation (WPO), and Tropical/Northern Hemisphere (TNH) patterns] and the extratropical wintertime northern stratospheric circulation is explored. These modes of variability are known to be influenced by El Niño/Southern Oscillation (ENSO) and may provide a mechanism for ENSO to influence interannual variability of the strato-spheric flow. Aside from any link to ENSO, these modes may be important in providing tropospheric wave forcing to the stratosphere.

These modes may be characterized by three of the leading rotated empirical orthogonal functions of the wintertime 5OO-hPa height field. An index of the amplitude of each of the modes is given by time series of principal components of that mode. By examining composites of years with high-index values minus years with low-index values it is shown that the zonal-mean stratospheric wind is not significantly influenced by any of the three tropospheric modes. However, the variability of these tropospheric modes appears to be associated with variations of stratospheric amplitudes of geopotential waves 1 and 2.

A comparison is made between the stratospheric influence of the equatorial quasi-biennial oscillation (QBO) and the three tropospheric modes during the December-February period. The influence from the QBO is weak in the troposphere and strongest in the stratosphere. The tropospheric modes’ influence is strongest in the troposphere. At 50 hPa, the QBO's influence is a strong modulation of the strength of the polar night jet and, in terms of geopotential height, is about twice that of the PNA, WPO, or TNH modes. The effect of the QBO on the stratospheric climate appears to be considerably larger than that from the three tropospheric modes.

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