by Jon Robson
The North Atlantic is a region of the Earth that is characterised by pronounced multi-decadal variability in surface temperatures – a phenomenon that has become known as Atlantic Multi-decadal Variability (AMV, see Sutton et al for a short review). The North Atlantic also appears to be one of the most predictable areas on Earth, with surface temperature and ocean heat content in the subpolar North Atlantic (~50-70°N) apparently predictable up to a decade in advance. This is encouraging given that AMV has also been associated with considerable societal impacts, including modulating the number of Atlantic hurricanes and rainfall over the Sahel (see Figure 1f and 1g).
Figure 1. An overview of Atlantic Multi-decadal Variability (AMV). (a) shows sea surface temperature anomalies for global (blue) and Atlantic (red) and the resulting AMV index (black). (b) and (c) shows proxies of ocean strength in the North Atlantic and sulphate aerosol precursor emissions. (d) shows the spatial extent of the AMV pattern. (e), (f) and (g) shows variability in winter time North Atlantic Oscillation, Hurricane accumulated energy and Sahel Rainfall. Thick lines show 10 year running means.
We recently argued that the high level of predictability of subpolar North Atlantic temperature is consistent with the initialisation of – but not necessarily the prediction of – the thermohaline component of the ocean circulation. Put another way, hindcasts appear skillfully to predict large changes in temperature only when they are started from an anomalous ocean circulation, rather than being able to predict the onset of an anomalous ocean circulation itself. Nevertheless, it is the slow evolution of the anomalous ocean circulation which allows us to predict North Atlantic upper-ocean temperature in advance. The prediction of upper-ocean temperatures can also lead to skillful predictions of other variables; for example, recent advances in 2-5 year predictions of Sahel rainfall and summer surface temperature over China are both related to improved predictions of the North Atlantic.
The particular view of the slow changes in the North Atlantic being governed by slow changes in the thermohaline circulation hasn’t really moved on from the main paradigms of the early 2000s. However, there have been a number of challenges to this view. In particular, multi-decadal changes in regional forcings (particularly Anthropogenic Sulphate Aerosols), or local thermodynamic coupling of the atmospheric variability, have both been proposed to be the main controlling factor of AMV. Long-story-short, we know that many models’ simulation of the Atlantic and AMV is deficient when compared to observations, and the role of the external forcing, in particular, is a major uncertainty. For decadal predictions, the forcings certainly provide skill, particularly in the tropical Atlantic. However, apart from the long-term warming trend, little is known about the role of different forcing factors (e.g. volcanic or anthropogenic aerosols, or solar), nor do we understand how the forcings are leading to skill, or even if the forced responses are realistic.
So where will progress come from over the next few years? Well, interesting changes abound in the North Atlantic at the moment. The subpolar North Atlantic may be transitioning into a cold state similar to that last observed in the 1970s-1980s (see Figure 2), and many people (including yours truly) have published tentative predictions of a further cooling of the North Atlantic. The Atlantic is also now observed at unprecedented levels of detail (for example the RAPID and OSNAP programs), so these changes will be watched closely.
Figure 2. 0-700 m heat content anomalies in the North Atlantic subpolar gyre region computed from the NODC data set. Figure created using KNMI climate explorer.
On the modelling side, recent advances in predicting the North Atlantic Oscillation on multi-year timescales could open the doors to further Atlantic-wide improvements. Near-term Climate Prediction is also now a World Climate Research Program “Grand Challenge”. Finally, international modelling experiments like the Decadal Climate Prediction Project (DCPP, a CMIP6 endorsed MIP) will continue the exploration of “near-term” climate prediction. DCPP will also further co-ordinate the real-time decadal prediction efforts of the community, as well as more process focused sensitivity studies. More generally, the wider CMIP6 activities (from highResMIP to VolMIP) also offer new opportunities.
Finally, the UK ACSIS project is beginning better to co-ordinate and integrate the UK’s scientific expertise in observations and modelling across atmosphere (including composition), ocean and cryosphere, in order to tackle the multi-faceted, multi-disciplinary problem of understanding multi-decadal timescale variability in the North Atlantic.
So, taken altogether, there is a lot of North Atlantic Science to look forward to; I just wish I could find the time to look at all the things I want to!