Decadal Climate Predictions in the North Atlantic Region
The Role of Ocean Heat Transport
Leonard Friedrich Borchert|2018
International Max Planck Research School on Earth System Modelling
Ocean Heat Transport, Decadal Climate Prediction, North Atlantic, AMOC, Climate Modelling
The possibility of credible climate forecasts for several years ahead – on the decadal time scale – has received considerable public and economic attention. Scientiﬁc studies quantify the credibility of such forecasts by evaluating the average predictive quality (skill) over the last 50-60 years (in so-called hindcasts). Decadal hindcasts of surface temperatures were shown to be on average particularly skillful in the North Atlantic region. However, the reason for the high skill of these hindcasts is still unclear. Meanwhile, North Atlantic sea surface temperatures (SSTs) are on the decadal time scale strongly inﬂuenced by subpolar ocean heat transport (OHT) variability. I here connect OHT variability and SST predictability and test whether the knowledge of the strength
of subpolar OHT at the beginning of a single SST forecast can improve its credibility. By using initialized global climate simulations of the twentieth century, I conﬁrm previous studies in that OHT variability inﬂuences SST variability for 3–10 years. A characteristic SST pattern of warm anomalies in the northeast Atlantic and cold anomalies in the Gulf Stream region emerges after strong OHT phases and vice versa. This pattern originates from persistently growing upper ocean heat content anomalies that arise from Southward propagating OHT anomalies in the North Atlantic. Extending previous work, I analyze strong and weak OHT phases at 50◦N separately. This reveals an asymmetry between strong and weak phases of ocean heat transport: When subpolar OHT is strong, North Atlantic SSTs show stronger and more persistent decadal anomalies than when subpolar OHT is weak.
For the ﬁrst time, I show that the hindcast skill of northeast Atlantic SSTs 3–10 years ahead is linked to the characteristic SST pattern, and therefore OHT variability in the subpolar North Atlantic. When subpolar ocean heat transport is strong at the initialization of a hindcast, the skill of SST hindcasts in the northeast Atlantic 2 to 9 years into the future is signiﬁcantly higher than when the ocean heat transport is weak at initialization. The asymmetric eﬀect of strong and weak phases of subpolar OHT on SST variability that preconditions asymmetric hindcast skill is robust in non-initialized versions of the same climate model. The skill of decadal SST predictions, therefore, robustly depends on the climate state at the start of a prediction.
I show in this dissertation that hindcast skill changes over time and thus cannot be immediately translated into the credibility of a forecast. Instead, the credibility of a decadal climate forecast depends on the climate state at the start of the forecast. For North Atlantic SST forecasts, the strength of subpolar North Atlantic OHT at the start of the forecast can be used to estimate its credibility. Findings presented in this dissertation suggest that physical mechanisms might be used to improve conventional estimates of the credibility of a climate forecast on the economically and politically relevant decadal time scale.
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