Revisit to atmospheric oscillations over global oceans: a combined climatology/modality approach

Low frequency atmospheric oscillations are among the most important climatic events related to global change. Using 144 years (1854–1997) of extended reconstruction of monthly mean sea level pressure (SLP) data, a reinvestigation is carried out to further reveal and characterize the spatiotemporal patterns of major atmospheric oscillations over global oceans with a combined climatology/modality approach. In the space domain, 12 centres of SLP climatology have been identified, on the basis of which five significant atmospheric oscillations are observed, namely, the North Atlantic Oscillation (NAO), the North Pacific Oscillation (NPO), the Southern Oscillation (SO), the South Atlantic Oscillation (SAO), and the South Indian Oscillation (SIO). The first three of them confirm existing oscillations with additional findings while the last two are so named for the first time. Our analysis demonstrates that the validity of the so-called annular mode associated with the Arctic Oscillation (AO, or the northern annular mode, NAM) and the Antarctic Oscillation (AAO, or the southern annular mode, SAM) cannot be held as a whole. Instead, what can be found, poleward from 20°N, is actually the coexistence of a strong NAO and a weak NPO which are partially and conditionally correlated. Poleward from 20°S, the SIO and SAO show a certain degree of consistency and might therefore be treated as a semi-annular mode at most. In the time domain, it is revealed that even the two best defined atmospheric oscillations, i.e. the NAO and the SIO, exhibit considerable abnormal departures from their regular pattern on a decadal time scale, reflecting the unstable nature of the atmospheric oscillations. In the frequency domain, the intrinsic periodicities along with modal structures of leading SLP modes at major centres of high/low are derived for the first time. Most of the atmospheric oscillations are found to be frequency selective. The persistent NAO is the only wide-band oscillation which remains valid for periods ranging from semiannual to interdecadal. The SO, SAO, SIO, and NPO are all band sensitive in terms of dipole shape, location, and intensity. Therefore, the effect of ‘frequency dispersion’ is at least partially responsible for the observed instability in prominent atmospheric oscillations.