Natural variability contributes to model-satellite differences in tropical tropospheric warming.

Area of Science:

• Climate Science
• Atmospheric Science
• Oceanography

Background:

• General circulation models (GCMs) exhibit a persistent discrepancy with satellite observations regarding tropical midtroposphere temperature (TMT) trends.
• GCMs simulate TMT warming at roughly double the rate observed from 1979-2018.

Purpose of the Study:

• To investigate the causes of the discrepancy between GCMs and satellite-observed tropical TMT trends.
• To assess the role of natural climate variability and Coupled Model Intercomparison Project Phase 6 (CMIP6) models in explaining this difference.

Main Methods:

• Analysis of a large ensemble of climate model simulations from a single GCM.
• Comparison of simulated tropical TMT trends (1979-2018) with satellite observations.
• Assessment of CMIP6 model simulations and their relationship with sea-surface temperature patterns.

Main Results:

• A subset of GCM simulations showed tropical TMT trends within the range of satellite observations.
• Subdued sea-surface warming in the tropical central and eastern Pacific correlated with reduced tropical TMT trends, mirroring observed patterns.
• 13% of CMIP6 model realizations and 25% of global average simulations aligned with observed TMT trends, irrespective of climate sensitivity.

Conclusions:

• Multidecadal natural climate variability significantly influences tropical TMT trends, reconciling model- GCM and satellite data discrepancies.
• The magnitude of tropical tropospheric warming is not solely determined by climate sensitivity.
• Natural variability is crucial for understanding differences between observed and simulated tropospheric warming on 40-year timescales.