Seasonal Loops Between Local Outgoing Longwave Radiation and Surface Temperature.

The relationship between outgoing longwave radiation (OLR) and the surface temperature has a major influence on Earth's climate sensitivity. Studies often assume that this relationship is approximately linear, but it is unclear whether the approximation always holds. Here we show that, on seasonal timescales, clear‐sky OLR is a multivalued function of local surface temperature. In many places, the OLR‐temperature relationship is better approximated by a loop than a line and we quantify the resulting "OLR loopiness", that is, how much clear‐sky OLR varies between different seasons with the same surface temperature. Based on offline radiative calculations, in the tropics OLR loops are mainly caused by seasonal variations in relative humidity that are out of phase with surface temperature; in the extratropics, OLR loops are mainly due to variations in lapse rates. Our work provides a mechanism through which Earth's climate feedback can differ between seasonal and long‐term time scales. Plain Language Summary: When the Earth is warmer, it loses more heat to space. The simplest formula describing this relationship is a line with an upwards slope, where the slope determines how much Earth warms from added greenhouse gases. Here, we report that the relationship between local temperature and rate of heat loss to space can be more complicated on seasonal time scales, often taking the form of loops or other curved shapes. We come up with a way to measure how big these loops are, and explain why they happen. Our results underline that Earth's response to seasonal changes is generally different from Earth's response to global warming. Key Points: Monthly outgoing longwave radiation (OLR) is a multivalued function of local surface temperature, exhibiting loops and other complex shapesOLR loops occur both in clear‐sky and all‐sky data, so the behavior is robust to cloudsOLR loops arise because humidity lags surface temperature in the tropics, and lapse rates lead to surface temperature in the extratropics [ABSTRACT FROM AUTHOR]