Internal and Forced Low-Frequency Surface Temperature Variability at Global and Regional Scales

Abstract:

There is evidence for internal models of decadal and multidecadal surface temperature variability that possess relatively narrowband spectral signatures. Among these are the so-called Atlantic Multidecadal Oscillation (“AMO”) and Pacific Decadal Oscillation (“PDO”). Separating these internal variability components from long-term forced temperature changes, however, is a non-trivial task. We apply a semi-empirical approach that combines climate observations and model-simulations to estimate Atlantic- and Pacific-based internal multidecadal variability (termed ‘AMO’ and ‘PMO’, respectively). Using analyses of coupled global climate model simulations, we show that our approach correctly identifies the internal variability components, while several alternative approaches overestimate and misidentify these components and their contribution to hemispheric mean temperatures. Using our method, the AMO and PMO are found to project in nearly equal proportion onto internal multidecadal variability in Northern Hemisphere mean temperature (termed ‘NMO’). A recent NMO cooling trend which contributes to the slowdown or “false pause” in warming of the past decade is seen to reflect a competition between a modest positive peak in the AMO and a substantially negative-trending PMO.