Predicting Near-Term Changes in the Earth System: A Large Ensemble of Initialized Decadal Prediction Simulations Using the Community Earth System Model

Abstract

The objective of near-term climate prediction is to improve our fore-knowledge, from years to a decade or more in advance, of impactful climate changes that can in general be attributed to a combination of internal and externally forced variability. Predictions initialized using observations of past climate states are tested by comparing their ability to reproduce past climate evolution with that of uninitialized simulations in which the same radiative forcings are applied. A new set of decadal prediction (DP) simulations has recently been completed using the Community Earth System Model (CESM) and is now available to the community. This new large-ensemble (LE) set (CESM-DPLE) is composed of historical simulations that are integrated forward for 10 years following initialization on 1 November of each year between 1954 and 2015. CESM-DPLE represents the “initialized” counterpart to the widely studied CESM Large Ensemble (CESM-LE); both simulation sets have 40-member ensembles, and they use identical model code and radiative forcings. Comparing CESM-DPLE to CESM-LE highlights the impacts of initialization on prediction skill and indicates that robust assessment and interpretation of DP skill may require much larger ensembles than current protocols recommend. CESM-DPLE exhibits significant and potentially useful prediction skill for a wide range of fields, regions, and time scales, and it shows widespread improvement over simpler benchmark forecasts as well as over a previous initialized system that was submitted to phase 5 of the Coupled Model Intercomparison Project (CMIP5). The new DP system offers new capabilities that will be of interest to a broad community pursuing Earth system prediction research.

Attributing the U.S. Southwest's Recent Shift Into Drier Conditions

“…State-of-the-art decadal prediction systems struggle to provide skillful forecasts of tropical Pacific SSTs beyond 2-year lead times (Yeager et al, 2018). State-of-the-art decadal prediction systems struggle to provide skillful forecasts of tropical Pacific SSTs beyond 2-year lead times (Yeager et al, 2018).…”

A Review of the Role of the Atlantic Meridional Overturning Circulation in Atlantic Multidecadal Variability and Associated Climate Impacts

“…Decadal predictability and prediction studies are powerful tools for understanding the mechanisms of the observed AMV, and they support the close linkage between AMV and multidecadal AMOC variability. Many recent studies have demonstrated that multidecadal AMOC variability is a significant source for the enhanced decadal predictability and prediction skills of AMV signal (e.g., Hermanson et al, ; Matei et al, ; Msadek et al, ; Robson, Sutton, & Smith, , ; Trenary & DelSole, ; Yan et al, ; Yang et al, ; Yeager et al, , , ; Yeager & Robson, ; Zhang, ; Zhang & Zhang, ). In particular, recent CGCM decadal prediction experiments using observationally based ocean initial conditions exhibit a positive AMOC anomaly at northern high latitudes in the mid‐1990s (Figure a), whereas uninitialized hindcasts including changes in external radiative forcings do not have this positive AMOC anomaly (Figure b).…”

“…The linkage between AMV and multidecadal fluctuations of ITCZ position and Sahel summer monsoon rainfall is also found in AGCM experiments forced by prescribed SST anomalies associated with the observed AMV pattern (i.e., AGCM‐AMV experiments; Mohino et al, ), in the internal variability component of CGCM simulations with varying external forcings (Han et al, ; Ting et al, , ), and in CGCM simulations with the North Atlantic SST restored to the estimated internal component of the observed AMV pattern (i.e., CGCM‐AMV experiments; Figures c and d; Ruprich‐Robert et al, ). The decadal prediction skill of Sahel rainfall depends crucially on the simulated AMV and associated linkage with the Sahel rainfall (Gaetani & Mohino, ; García‐Serrano et al, ; Martin & Thorncroft, ; Mohino et al, ; Monerie et al, ; Mueller et al, ; Sheen et al, ; Yeager et al, ). Initializing a weaker AMOC is crucial for predicting the cooling in the subpolar AMV signal and associated southward shift of the ITCZ in the 1960s (Robson, Sutton, Lohmann, et al, ) and vice versa in the 1990s (Msadek et al, ; Robson et al, ; Robson, Sutton, & Smith, ).…”

“…Observational analyses additionally suggest that AMV is anticorrelated with glacier surface mass balance (Huss et al, ) and spring snowfall (Zampieri et al, ) in the Alps at multidecadal timescales, consistent with the observed warmer spring/summer temperature over Europe during the positive AMV phase (Sutton & Dong, ). Initializing the ocean states properly in climate prediction simulations is critical for predicting the positive shift in AMV and associated European summer climate shifts in the 1990s (Msadek et al, ; Robson et al, ; Robson, Sutton, & Smith, ; Yeager et al, ) and the early twentieth century (Mueller et al, ).…”

The Community Earth System Model Version 2 (CESM2)

“…In addition, CESM1 has also been used in key activities to advance our understanding of the climate system and its variability and predictability, by supplementing CESM1's contributions to the CMIPs with other community driven science efforts. These include the CESM1 Large Ensemble (CESM1(LENS); Kay et al, ), the CESM Decadal Prediction Large Ensemble (CESM‐DPLE; Yeager et al, ), the CESM Last Millennium Ensemble (CESM‐LME; Otto‐Bliesner et al, ), and the CESM Stratospheric Aerosol Geoengineering Large Ensemble (GLENS; Tilmes et al, ).…”