Publication Abstracts

Chen 2005

Chen, J., 2005: Understanding the Observed Tropical and Midlatitude Radiative Energy Budget in the Context of Long-Term Climate Variations. Ph.D. thesis. Columbia University.

Satellite observations suggest that outgoing longwave radiation flux increased, while reflected solar shortwave flux decreased, by several watts per square meter, at the top of the atmosphere, in the tropics during the 1990s. By applying empirical orthogonal function analysis to the joint frequency distribution of the occurrence of atmospheric parameters, the flux changes are found to be associated with a systematic change of atmosphere circulation, such that the equatorial convective regions have intensified in upward motion and moistened, and both the equatorial and subtropical subsidence regions have intensified in downward motion and become drier and less cloudy, thus indirectly indicating a decadal-time-scale strengthening of the Hadley-Walker circulation.

The dominant interannual El Niño-Southern Oscillation (ENSO) phenomenon and the short length of climate observation records make it difficult to study longer-term climate variations in the spatio-temporal domain. Based on the fact that ENSO signal spreads to remote regions and induces delayed climate variation through atmospheric teleconnections, we develop an ENSO-removal method through which the ENSO signal can be approximately removed at the grid box level from the spatio-temporal field of a climate parameter.

After the ENSO signal is removed, the long-term global warming trend (GW) and the Pacific Pan-Decadal Variability (PPDV) are isolated as the primary long-term climate variations at middle and low latitudes from climate parameter fields with EOF or combined EOF analysis.

The GW trend in the Pacific basin during the last century is only about 0.4K, much weaker than the warming that occurred at the east and west boundary of the Pacific Ocean and outside, including the Atlantic Ocean and the Indian Ocean, where the warming is near 0.8K in the basin-wide mean. Consistent with the zonal surface temperature contrast caused by the weaker warming in the Pacific, the atmosphere circulation change associated with GW is dominated by an anomalous zonal overturning circulation over the Pacific and its surrounding region. The modest warming in the Pacific basin probably is due to its more dynamic nature on the interannual and decadal time scales and/or the leakage of upper ocean water through the Indonesia Throughflow.

The PPDV mode indicates that a climate regime shift that occurred in the Pacific basin during the 1990s, which is comparable to but of opposite-phase to the regime shift that occurred in 1976. Compared to ENSO, the PPDV surface temperature signal is very weak in the Indonesia region, but strong in the North, South and eastern Pacific. The anomalous atmospheric circulation associated with the 1990s PPDV phase change results in a drier upper troposphere in broad areas at low and middle latitudes. Our analysis suggests that the PPDV is a combination of two or three decadal and/or inter-decadal oscillations concerting together through atmospheric teleconnections.

The spatio-temporal structures of the changes of radiation fluxes, cloud amount, and humidity, for the period of 1983-1998, are also addressed with the ENSO-removed combined EOF analysis based on satellite observations. They are consistent with the analysis results obtained from the NCEP/NCAR reanalysis dataset with the same analytical method in the same period. Comparing the results from the above two combined EOF analyses with the spatio-temporal structures of GW and PPDV, we find that the 1990s radiation flux change and the related changes of atmospheric water vapor, cloud and dynamic fields are associated with both GW and PPDV, with PPDV accounting for more variance than GW. That is, the phase change of PPDV is the main mechanism causing the observed earth radiative energy budget change during the 1990s.

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BibTeX Citation

@phdthesis{ch01600h,
  author={Chen, J.},
  title={Understanding the Observed Tropical and Midlatitude Radiative Energy Budget in the Context of Long-Term Climate Variations},
  year={2005},
  school={Columbia University},
  address={New York, N.Y.},
}

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RIS Citation

TY  - THES
ID  - ch01600h
AU  - Chen, J.
PY  - 2005
BT  - Understanding the Observed Tropical and Midlatitude Radiative Energy Budget in the Context of Long-Term Climate Variations
PB  - Columbia University
CY  - New York, N.Y.
ER  -

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