Climatology and ENSO variation of Outgoing Longwave Radiation as Depicted By TOVS, AIRS, CERES, and MERRA2

Outgoing Longwave Radiation (OLR) is an essential component of the Earth radiation budget and it changes with natural and anthropogenic climate variations. OLR at a given location is an integrated quantity affected primarily by the Earth’s surface temperature, atmospheric vertical temperature and water vapor profile; and the height and amount of cloud cover. Therefore anomalies and trends of OLR have been useful to study past and present climate processes.

We analyzed OLR acquired from TIROS Operational Vertical Sounder (TOVS), which covers period extending back to 1979, in conjunction with those from AIRS, CERES, and MERRA2. While the comparison of absolute values of OLR shows the difference among different sets of data, large temporal scale variability of OLR track each other very well. The modes of variability of TOVS OLR clearly show the different characteristics of the ENSO related spatial pattern in tropical Pacific during TOVS period prior to 2002. The spatial domains, which are strongly influenced by ENSO activity, are shifted eastward in TOVS compared to those of AIRS and CERES. This is a consequence of the fact that the main region of ENSO activity in the AIRS period is located in the central Pacific, compared to those of the TOVS period. While AIRS OLR has a small positive bias compared to CERES of roughly 3.0 W/m², which is essentially constant in space and time, the comparison of the AIRS and CERES OLR anomaly time series match perfectly on a spatial scale and confirm that both AIRS and CERES derived products are very stable and accurate. The MERRA2 OLR are different from those of AIRS and CERES however, both in terms of intermonth differences, and especially in terms of overall trend. The largest differences is located in convective regions in the topics, where MERRA2 OLR change is for the most part too negative, indicating a possible spurious increase in the amount of high cloud cover over these regions.