Abstract
In the 1960s North Atlantic sea surface temperatures (SST) cooled rapidly. The magnitude of the cooling was largest in the North Atlantic subpolar gyre (SPG), and was coincident with a rapid freshening of the SPG. Here we analyze hindcasts of the 1960s North Atlantic cooling made with the UK Met Office’s decadal prediction system (DePreSys), which is initialised using observations. It is shown that DePreSys captures—with a lead time of several years—the observed cooling and freshening of the North Atlantic SPG. DePreSys also captures changes in SST over the wider North Atlantic and surface climate impacts over the wider region, such as changes in atmospheric circulation in winter and sea ice extent. We show that initialisation of an anomalously weak Atlantic Meridional Overturning Circulation (AMOC), and hence weak northward heat transport, is crucial for DePreSys to predict the magnitude of the observed cooling. Such an anomalously weak AMOC is not captured when ocean observations are not assimilated (i.e. it is not a forced response in this model). The freshening of the SPG is also dominated by ocean salt transport changes in DePreSys; in particular, the simulation of advective freshwater anomalies analogous to the Great Salinity Anomaly were key. Therefore, DePreSys suggests that ocean dynamics played an important role in the cooling of the North Atlantic in the 1960s, and that this event was predictable.
Highlights
Decadal prediction, wherein climate models are initialised from the observed climate state to improve predictions of near-term climate, has recently gained much attention (Smith et al 2007; Keenlyside et al 2008; Meehl et al 2009)
This study has explored predictions of the 1960s North Atlantic cooling using the perturbed physics version of the UK Met Office Decadal Prediction System (DePreSys)
By comparing DePreSys predictions started in the mid-1960s just before the cooling event ( 1963-1968) to predictions that do not assimilate the observed information (NoAssim), it is shown that:
Summary
Wherein climate models are initialised from the observed climate state to improve predictions of near-term climate, has recently gained much attention (Smith et al 2007; Keenlyside et al 2008; Meehl et al 2009). To build confidence in decadal predictions it is important to understand the reasons why the SST predictions are improved by initialisation. An important approach for building confidence in decadal predictions is the use of specific case studies of large decadal change events to test the prediction systems. North Atlantic has been important for understanding that dynamical ocean changes, and a subsequent increase in the northward ocean heat transport, were key for this event to be predicted (Robson et al 2012; Yeager et al 2012). A notable event that has yet to be studied in the context of initialised decadal predictions is the cooling of the North Atlantic ocean in the 1960s (see Fig. 1). In this study we will assess the predictability of the 1960s cooling in the UK Met Office’s Decadal Prediction System (DePreSys; Smith et al 2010), and assess the important mechanisms.
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