A decadal variance decomposition method is applied to the Northern Hemisphere (NH)
500-hPa geopotential height (GPH) and the sea level pressure (SLP) taken from the
Last Millennium (850-1850 AD) experiment with the coupled climate model CCSM4,
to estimate the contribution of the intra-decadal variability to the inter-decadal variability.
By removing the intra-decadal variability from the total inter-decadal variability, the residual
variability is more likely to be associated with slowly varying external forcings and slow-
decadal climate processes,and therefore is referred to as slow-decadal variability. The
results show that the (multi-)decadal changes of the NH 500-hPa GPH are primarily
dominated by slow-decadal variability, whereas the NH SLP field is primarily dominated
by the intra-decadal variability. At both pressure levels, the leading intra-decadal modes
each have features related to the El Niño-Southern Oscillation (ENSO), the intra-decadal
variability of the Pacific Decadal Oscillation (PDO) and the Arctic Oscillation (AO); while
the leading slow-decadal modes are associated with external radiative forcing (mostly with
volcanic aerosol loadings),the Atlantic Multi-decadal Oscillation (AMO) and the slow-decadal
variability of AO and PDO. Moreover, the radiative forcing has much weaker effect to the
SLP than that to the 500-hPa GPH.
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