Decadal climate predictions in the North Atlantic region: The role of ocean heat transport

• Main Author: Borchert, L.
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Universität Hamburg 2018
• Subjects: North Atlantic; Northeast Atlantic
• Online Access: http://hdl.handle.net/21.11116/0000-0002-0A63-4; http://hdl.handle.net/21.11116/0000-0002-0A69-E

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. Mean- while, 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 pre- vious studies in that OHT variability influences SST variability for 3-10 years. A char- acteristic 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 pat- tern originates from persistently growing upper ocean heat content anomalies that arise from Southward propagating OHT anomalies in the North Atlantic. Extending previ- ous 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 anoma- lies 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 initial- ization of a hindcast, the skill of SST hindcasts in the northeast Atlantic 2 to 9 years into the future is ...