Variability of Methane Growth in the Tropics

The El Nino-Southern Oscillation (ENSO) is one of the dominant modes of variability in the atmosphere. Although its main effects are seen in the tropical Pacific Ocean, ENSO has far-ranging impacts and is known to cause variability in precipitation and global average temperatures. ENSO-driven variability in precipitation is especially of interest in the tropics, where the presence of natural wetlands and rice agriculture combined with high temperature result in a significant contribution from these latitudes to global methane emissions. Until recently, conditions have been dominated by relatively weak El Nino conditions. In 2007, however, the phase of ENSO switched coinciding with a period of abnormally high growth in atmospheric methane in both the tropics and polar latitudes as observed by the NOAA/ESRL sampling network, and higher than average precipitation in the tropics. In this study, we evaluate the possible connection between ENSO and methane growth including comparisons between the recent La Nina’s and the strong El Nino of 1997-1998. We consider the ability of a global methane assimilation (CarbonTracker-CH4) as well as bottom-up calculations to capture and properly attribute sources of methane related to ENSO-driven climate variability.