Oceanic cloud trends during the satellite era and their radiative signatures

Abstract

The present study analyzes zonal mean cloud and radiation trends over the global oceans for the past 35 years from a suite of satellite datasets covering two periods. In the longer period (1984-2018) cloud properties come from the ISCCP-H, CLARA-A3, and PATMOS-x datasets and radiative properties from the ISCCP-FH dataset, while for the shorter period (2000-2018) cloud data from MODIS and CloudSat/CALIPSO and radiative fluxes from CERES-EBAF are added. Zonal mean Total Cloud Cover (TCC) trend plots show an expansion of the subtropical dry zone, a poleward displacement of the midlatitude storm zone and a narrowing of the tropical Intertropical Convergence Zone (ITCZ) region over the 1984-2018 period. This expansion of the ‘low cloud cover curtain’ and the contraction of the ITCZ rearrange the boundaries and extents of all major climate zones, producing a more poleward and somewhat narrower midlatitude zone and a wider subtropical zone. Zonal mean oceanic cloud cover trends, when examined in terms of distinct latitude zones, two poleward of 50^o and one bounded within 50^oS and 50^oN, show upward or near-zero cloud cover trends in the high latitude zones and consistent downward trends in the low latitude zone. The latter dominate in the global average resulting in TCC decreases that range from 0.72% per decade to 0.17% per decade depending on dataset and period. These contrasting cloud cover increases and decreases between the high and low latitude zones produce contrasting low latitude cloud radiative warming and high latitude cloud radiative cooling effects, present in both the ISCCP-FH and CERES-EBAF datasets. The global ocean mean trend of the Short Wave Cloud Radiative Effect (SWCRE) depends on the balance between these contrasting trends, and in the CERES dataset this balance is a SW cloud radiative warming trend of 0.12 W/m²/decade coming from the dominance of the low-latitude positive SWCRE trends while in the ISCCP-FH dataset it is a 0.3 W/m²/decade SW cloud radiative cooling trend coming from the dominance of the high latitude negative SWCRE trends. The CERES cloud radiative warming trend doubles in magnitude to 0.24 W/m²/decade when the period is extended from 2016 to 2022, implying a strong cloud radiative heating in the past 6 years coming from the low latitude zone.