Time dependency of the prediction skill for the North Atlantic subpolar gyre in initialized decadal hindcasts

Abstract

We analyze the time dependency of decadal hindcast skill in the North Atlantic subpolar gyre within the time period 1961–2013. We compare anomaly correlation coefficients and temporal interquartile ranges of total upper ocean heat content and sea surface temperature for three differently initialized sets of hindcast simulations with the global coupled model MPI-ESM. All initializations use weakly coupled assimilation with the same full value nudging in the atmospheric component and different assimilation techniques for oceanic temperature and salinity: (1) ensemble Kalman filter assimilating EN4 observations and HadISST data, (2) nudging of anomalies to ORAS4 reanalysis, (3) nudging of full values to ORAS4 reanalysis. We find that hindcast skill depends strongly on the evaluation time period, with higher hindcast skill during strong multiyear trends, especially during the warming in the 1990s and lower hindcast skill in the absence of such trends. Differences between the prediction systems are more pronounced when investigating any 20-year subperiod within the entire hindcast period. In the ensemble Kalman filter initialized hindcasts, we find significant correlation skill for up to 5–8 lead years, albeit along with an overestimation of the temporal interquartile range. In the hindcasts initialized by anomaly nudging, significant correlation skill for lead years greater than two is only found in the 1980s and 1990s. In the hindcasts initialized by full value nudging, correlation skill is consistently lower than in the hindcasts initialized by anomaly nudging in the first lead years with re-emerging skill thereafter. The Atlantic meridional overturning circulation reacts on the density changes introduced by oceanic nudging, this limits the predictability in the subpolar gyre in the first lead years. Overall, we find that a model-consistent assimilation technique can improve hindcast skill. Further, the evaluation of 20 year subperiods within the full hindcast period provides essential insights to judge the success of both the assimilation and the subsequent hindcast quality

Full-field initialized decadal predictions with the MPI earth system model: an initial shock in the North Atlantic

“…These changes are much less pronounced in ORAS4-ANOM leading to a significantly better prediction skill. A similar or better skill for OHC was found by Brune et al (2017) for hindcasts initialized by an oceanic ensemble Kalman filter using full-field temperature and salinity profiles with identical atmospheric nudging as in ORAS4-ANOM. In our full-field systems, the nudginginduced ocean heat flux signals reveal strong fluctuations that drive OHC tendencies in the hindcasts.…”

“…In addition, the initialization from GECCO2 has been found to yield slightly better results as compared to ORAS4, especially over the Northeast Pacific . Also Thoma et al (2015) and Brune et al (2017) have considered baseline1 and a subset of the prototype system in their comparison studies but did not come up with a clear recommendation for either full-field or anomaly initialization. Our study, on the other hand, is based on the completed MiKlip baseline1 and prototype systems, which include 10-year-long multiensemble hindcasts, ten members in case of baseline1 and 30 members in case of prototype, performed every year from 1961 to 2014.…”

Predictability of Multiyear Trends of the Pacific Decadal Oscillation in an MPI‐ESM Hindcast Ensemble

“…The atmospheric component of the model is ECHAM6 (Stevens et al, ; ≈ 2.5° horizontal resolution, 47 levels up to 0.01 hPa), and the oceanic component is MPIOM (Jungclaus et al, ; ≈ 1.5° horizontal resolution, 40 depth levels). Hindcasts are initialized from a simulation, where we assimilate monthly surface and subsurface oceanic temperature and salinity with an ensemble Kalman filter (Brune et al, , ), while atmospheric variables are nudged to monthly European Centre for Medium‐Range Weather Forecast Re‐Analysis data (ERA40: Uppala et al, ; ERA‐Interim: Dee et al, ) and run for 10 years.…”

North Atlantic Sub‐Polar Gyre Climate Index: A New Approach

“…At the same time the deep water formation that takes place in the North Atlantic, renders it a critical region for changes in the Atlantic meridional overturning circulation (AMOC; Hátún et al, 2005). The North Atlantic is a region where skillful predictions of up to a decade can be obtained, namely, in the NASPG as seen in previous studies using various different decadal prediction systems/setups (e.g., Brune et al, 2018;Matei et al, 2012;Polkova et al, 2015;Robson et al, 2018;Smith et al, 2010). However, as indicated by Barrier et al (2015, and references therein) using the Nucleus for European Modelling of the Ocean (NEMO) system, the prediction skill in the NASPG on decadal time scales is high in the western part of the gyre and low in the eastern part and thus the spatial patterns need to be considered.…”