What Are the Main Causes of Positive Subtropical Low Cloud Feedbacks in Climate Models?

Climate feedbacks over the historical period (here defined as 1850–2014) have been investigated in large ensembles of historical and single forcing experiments (hist‐ghg, hist‐aer, and hist‐nat), with 47 members for each experiment. Across the historical ensemble with all forcings, a range in estimated Effective Climate Sensitivity (EffCS) between approximately 3–6 K is found, a considerable spread stemming solely from initial condition uncertainty. The spread in EffCS is associated with varying Sea Surface Temperature (SST) patterns seen across the ensemble due to their influence on different feedback processes. For example, the level of polar amplification is strongly correlated with the amount of sea ice melt per degree of global warming. This mechanism is related to the large spread in shortwave clear‐sky feedbacks and is the main contributor to the different forcing efficacies seen across the different forcing agents, although in HadGEM3‐GC3.1‐LL these differences in forcing efficacy are shown to be small. The spread in other feedbacks is also investigated, with the level of tropical SST warming strongly correlated with the longwave clear‐sky feedbacks, and the local surface‐air‐temperatures well correlated with the spread in cloud radiative effect feedbacks. The metrics used to understand the spread in feedbacks can also help to explain the disparity between feedbacks seen in the historical experiment simulations and modeled estimate of the feedbacks seen in the real world derived from an atmosphere‐only experiment prescribed with observed SSTs (termed amip‐piForcing).

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Oceanic cloud trends during the satellite era and their radiative signatures

Tselioudis,

Rossow,

Bender

et al. 2024

Preprint

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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 50o and one bounded within 50oS and 50oN, 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.

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What Are the Main Causes of Positive Subtropical Low Cloud Feedbacks in Climate Models?

Cited by 3 publications

References 64 publications

Oceanic cloud trends during the satellite era and their radiative signatures

Oceanic cloud trends during the satellite era and their radiative signatures

Feedbacks, Pattern Effects, and Efficacies in a Large Ensemble of HadGEM3‐GC3.1‐LL Historical Simulations

Oceanic cloud trends during the satellite era and their radiative signatures

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