Abstract
Our decadal climate prediction system, which is based on the Max-Planck-Institute Earth System Model, is initialized from a coupled assimilation run that utilizes nudging to selected state parameters from reanalyses. We apply full-field nudging in the atmosphere and either full-field or anomaly nudging in the ocean. Full fields from two different ocean reanalyses are considered. This comparison of initialization strategies focuses on the North Atlantic Subpolar Gyre (SPG) region, where the transition from anomaly to full-field nudging reveals large differences in prediction skill for sea surface temperature and ocean heat content (OHC). We show that nudging of temperature and salinity in the ocean modifies OHC and also induces changes in mass and heat transports associated with the ocean flow. In the SPG region, the assimilated OHC signal resembles well OHC from observations, regardless of using full fields or anomalies. The resulting ocean transport, on the other hand, reveals considerable differences between full-field and anomaly nudging. In all assimilation runs, ocean heat transport together with net heat exchange at the surface does not correspond to OHC tendencies, the SPG heat budget is not closed. Discrepancies in the budget in the cases of full-field nudging exceed those in the case of anomaly nudging by a factor of 2–3. The nudging-induced changes in ocean transport continue to be present in the free running hindcasts for up to 5 years, a clear expression of memory in our coupled system. In hindcast mode, on annual to inter-annual scales, ocean heat transport is the dominant driver of SPG OHC. Thus, we ascribe a significant reduction in OHC prediction skill when using full-field instead of anomaly initialization to an initialization shock resulting from the poor initialization of the ocean flow.
Highlights
In decadal or near-term climate prediction, proper initialization with observations is important for improving forecast quality (e.g., Cox and Stephenson 2007; Hawkins and Sutton 2009)
We provide skill estimates for the two investigated generations of the MiKlip decadal prediction system based on skill scores of hindcasts of sea surface temperature (SST) and North Atlantic ocean heat content (OHC)
Based on an older version of the MPI climate prediction system, we previously identified a strong sensitivity of prediction skill for upper layer OHC in the North Atlantic to the choice of the ocean reanalysis product applied in the initialization (Kröger et al 2012)
Summary
In decadal or near-term climate prediction, proper initialization with observations is important for improving forecast quality (e.g., Cox and Stephenson 2007; Hawkins and Sutton 2009). In this assimilation step, for a long time, nudging to anomalies instead of full fields of the ocean estimates was favored to suppress drift and initial shocks in the forecasts (e.g., Smith et al 2007; Keenlyside et al 2008; Pohlmann et al 2009). A variety of reasons such as application of independent, uncoupled atmosphere and ocean reanalyses or application of a reanalysis product of poor quality in the assimilation procedure can lead to an imbalanced initial state in the coupled prediction system that triggers initial shocks in the forecasts (e.g., Balmaseda and Anderson 2009; Mulholland et al 2015; Pohlmann et al 2017). We investigate the impact on prediction skill when applying full-field nudging with a different reanalysis data set since a strong sensitivity of skill to the choice of the ocean reanalysis product was
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