Variations in Near-Global Sea Level Pressure

Analysis of a near-global sea level pressure (SLP) data set for the period 1951-80 was carried out in terms of complex empirical orthogonal function A large-scale, propagating SLP signal was discovered that seems to include variability previously associated with the Southern Oscillation, monsoons and tropical midlatitude teleconnection patterns In this sense, the new feature offers an empirical framework for many of the well-known features of the global climate system. Extensive comparisons show the feature to be observable in the raw data field, and thus not a figment of the analysis technique. The large SLP signal may have its origins in a SLP/snow feedback loop that acts over the region from Siberia to India. It is suggested that the maintenance of the SLP signal involves SLP/precipation feedbacks and, more importantly, the excitation of a major natural mode of climate variation. The SLP signal is only excited at time scales larger than about 18 mouths, i.e., time scales characteristic of oceanic processes. The GCM results of Lau also suggest the importance of ocean-atmosphere interactions in maintaining the SLP signal. However, forcing solely by SST does not appear capable of explaining the entire SLP signal. The North Atlantic 0scillation (NAO) was found to be another natural mode of global climate variation. The spatial response of this mode. which closely resembled a simple standing wave, may be excited over a wide range of frequencies. The appearance of this mode in both a Geophysics Fluid Dynamic Laboratory (GFDL) model simulation and NCAR Community Climate Model (CCM) simulation with limited interannual forcing suggests the NAO to be a singular expression of internal atmospheric dynamics The NAO unrelated in both space and time to the large SLP signal discussed above. Both the NAO and the large-scale SLP modes exhibit dichotomous behavior in time. On one hand they can appear as quasi-regular, continuous elements of the global climate system. Alternatively, they exhibit behavior characteristic of a multiequilibrium system.