SEASONAL AND INTERANNUAL VARIATIONS OF ATMOSPHERIC CO2 AND CLIMATE

Interannual variations of atmospheric CO2 concentrations at Mauna Loa are almost masked by the seasonal cycle and a strong trend; at the Sou th Pole, the seasonal cycle is small and is almost lost in the trend a nd interannual variations. Singular-spectrum analysis (SSA) is used he re to isolate and reconstruct interannual signals at both sites and to visualize recent decadal changes in the amplitude and phase of the se asonal cycle. Analysis of the Mauna Loa CO2 series illustrates a haste ning of the CO2 seasonal cycle, a close temporal relation between Nort hern Hemisphere (NH) mean temperature trends and the amplitude of the seasonal CO2 cycle, and tentative ties between the latter and seasonal ity changes in temperature over the NH continents. Variations of the s easonal CO2 cycle at the South Pole differ from those at Mauna Loa: it is phase changes of the seasonal cycle at the South Pole, rather than amplitude changes, that parallel hemispheric and global temperature t rends. The seasonal CO2 cycles exhibit earlier occurrences of the seas ons by 7 days at Mauna Loa and 18 days at the South Pole. Interannual CO2 variations are shared at the two locations, appear to respond to t ropical processes, and can be decomposed mostly into two periodicities , around (3 years)(-1) and (4 years)(-1), respectively. Joint SSA anal yses of CO2 concentrations and tropical climate indices isolate a shar ed mode with a quasi-triennial (QT) period in which the CO2 and sea-su rface temperature (SST) participation are in phase opposition. The oth er shared mode has a quasi-quadrennial (QQ) period and CO2 variations are in phase with the corresponding tropical SST variations throughout the tropics. Together these interannual modes exhibit a mean lag betw een tropical SSTs and CO2 variations of about 6-8 months, with SST lea ding. Analysis of the QT and QQ signals in global gridded SSTs, joint SSA of CO2 and delta(13)C isotopic ratios, and SSA of CO2 and NH-land temperatures indicate that the QT variations in CO2 mostly reflect upw elling variations in the eastern tropical Pacific. QQ variations are d ominated by the CO2 signature of terrestrial-ecosystem response to glo bal QQ climate variations. Climate variations associated with these tw o interannual components of tropical variability have very different e ffects on global climate and, especially, on terrestrial ecosystems an d the carbon cycle.