A Satellite Climatology of Relative Humidity Profiles and Outgoing Thermal Radiation over Earth’s Oceans

Abstract

Satellite observations over Earth’s oceans show two distinct regimes in the relationship between sea surface temperatures (SST) and outgoing longwave radiation (OLR): a temperate regime (OLR increases with increasing SST quasi linearly) and the super greenhouse regime (OLR decrease with increasing SST). Transitions between these regimes occur via nonlinear atmospheric moistening, increasing relative humidity (RH). We perform a clustering analysis of about 450 million satellite-retrieved RH profiles and show that RH profiles over Earth’s oceans can be grouped into six and eight distinct and physically meaningful “primitive” classes for clear-sky and all-sky conditions, respectively. As the different RH-profile classes have distinct effects on OLR, can be associated with large-scale dynamical structures, and their occurrence is particular to different global ocean regions, the “primitive” clustering allows for studying large-scale radiative effects important for characterizing global climates and systematic relationships between SST, OLR, and atmospheric water vapor content. In both clear-sky and all-sky conditions three RH-profile classes correspond to the tropics. In the tropics, increasing SSTs are accompanied by systematic increases in both the occurrence probability and the observed RH magnitudes of the moistest RH-profile classes. Observations of moistest RH-profile classes are usually in a super greenhouse state (data in or likely to transition into the super greenhouse regime), and the formal characterization of them allows us to define an empirical threshold to identify which instantaneous observations are likely in a super greenhouse state. Applying this threshold we are able to identify typical regions in the super greenhouse regime.