ENSO regulation of far- and mid-infrared contributions to clear-sky OLR

NASA Aqua-derived thermodynamic profiles, calculated spectral clear-sky outgoing longwave radiation (OLR), and vertical velocity fields from meteorological reanalyses are combined to determine the relative proportion of the far-infrared (FIR) and mid-infrared (MIR) spectral contributions to the total clear-sky OLR during different phases of El Niño–Southern Oscillation (ENSO). In the ascending branch of the tropical circulation, the spatial variance of upper tropospheric water vapor is shown to be larger during La Niña than El Niño and is consistent with zonal symmetry changes in the tropical waveguide and associated tropical-extratropical mixing. In the descending branch, upper tropospheric water vapor shows weaker coupling to lower layers that is evidenced by changes in the ratio of FIR to MIR in the clear-sky OLR. Diagnostics from the Geophysical Fluid Dynamics Laboratory AM3 model simulation are generally similar to satellite data, but the ratio of FIR to MIR is 5–10% larger with respect to dynamic regime.