Empirical Study of Atmospheric Responses to the Tropical SST Forcing

Recently GCM studies have indicated that there exist additional atmospheric response patterns beyond the linear ENSO signal and that the Arctic Oscillation (AO) is one of extratropical atmospheric signals that is in part a response to tropical SST forcing. The main purpose of this study is to investigate the additional atmosphere patterns separate from ENSO due to tropical SST forcing that can be found in the observed Northern Hemisphere wintertime variability. Singular value decomposition (SVD) analysis between SST and the NCEP-NCAR reanalyzed 500-mb height (Z500) at both interannual and interdecadal time scales is used to identify these patterns.

At interannual time scales, the leading SVD mode of Z500 pattern exhibits a clear ENSO signature and the corresponding SST pattern resembles the ENSO signature. The 2nd SVD mode of 500-mb height pattern is virtually identical to the AO pattern and the SST pattern reveals the northern tropical Atlantic mode in the Atlantic. The expansion coefficients are highly correlated with AO indices. The 3rd SVD pattern for Z500 is moderately successful at specifying the Cold Ocean- Warm Land (COWL) pattern. An alternative SVD analysis between Z500 and the outgoing longwave radiation indicate that the above SVD modes of SSTs are significantly correlated with the direct thermal forcing in the Tropics.

At decadal timescales, the leading SVD mode is indicative of strong linear coupling between the NH 500-mb height and the Pacific ENSO-like SST field. The SST pattern in the Pacific is seen as Pacific Decadal Oscillation mode and the 500-mb pattern appears most similar to the Pacific-North American (PNA) pattern. The 2nd leading SVD SST pattern has its significant negative loadings in the Pacific south of the equator and in the eastern Atlantic Ocean. The 500-mb height pattern appears most similar to the AO.

Also, we explore the roles of these atmosphere response patterns on connecting the tropical and extratropical SSTs. The composite anomalies indicate that fluctuations in the midlatitude SST are caused by variability in the overlying atmosphere, which in turn originates from oceanic forcing in the Tropics. Both AO and COWL-like atmosphere response patterns function as a link or "bridge" whereby tropical non-ENSO SST anomalies induce the dominant pattern of SST anomalies in the extratropical North Pacific and Atlantic.