Exploring the active role of water vapor in creating more extreme SSTs and climate variations

Abstract:

While it is well-known that water vapor will play an important role in amplifying the direct warming effects of well-mixed greenhouse gasses like CO₂ and methane, to date relatively little attention has been placed on the spatial variability of water vapor warming effects: increased diabatic forcing from precipitation and long wave radiation. Here, using 1850-2012 atmospheric simulations from the GEOS5 model, 1948-2015 NCEP-NCAR Reanalysis 1 fields, 1979-2015 MERRA atmospheric reanalyses, and 1979-2015 NOAA OLR observations, we explore two potential thermodynamic contributions associated with water vapor. One contribution comes from the diabatic heating of the atmosphere by longwave radiation emissions. Another contribution comes from diabatic heating of the atmosphere by precipitation. This diabatic heating warms the local atmosphere, and over the tropical oceans, typically warms areas that are already warm. This increases local temperature gradients and potentially increases available potential energy both in the vertical (i.e. CAPE) and in the horizontal (i.e. APE). Using MERRA’s detailed thermodynamic budget terms, we examine several recent climate extremes, like the 2011 La Niña and the 2015 El Niño, suggesting that exceptional increases in water vapor radiative warming and precipitation may have helped to make both events more extreme: exceptionally high levels of water vapor in the western Pacific may have helped increase the warm west Pacific – cool Niño 4 SST gradient during the 2011 La Niña. Conversely, in 2015, exceptionally high levels of water vapor in the eastern Pacific may have helped increase the warm Niño 3.4 – cool western Pacific El Niño SST gradient. These water vapor influences can be radiative (warming warm SSTs), as well as dynamic, as enhanced precipitation releases more latent heat. Thus ‘anthropogenic’ water vapor may move around the climate system, helping to exacerbate warming in warm areas of the atmosphere. We examine this hypothesis quantitatively using water vapor-related changes in longwave radiation, diabatic forcing, APE, CAPE, and kinetic energy.