Assessment of the impact of ocean observations on the predictability of the THC

Experiments have been performed to explore the predictability of the Atlantic branch of the thermohaline overturning circulation, here referred to as the Meridional Overturning Circulation (MOC). We have utilised the idealised model world framework which is useful for examining the predictability of variables like the overturning circulation for which a long historical record of direct observations are unavailable. Using the HadCM3 model we show: -monthly temperature and salinity observations taken globally in the top 2000m of the ocean are potentially capable of initialising the MOC, -observations below 2000m improve skill slightly, especially in the Southern Ocean -the additional assimilation of atmospheric variables improves skill in the first year but has little impact beyond that (at least in HadCM3), -assimilation of sea surface temperature (SST) alone does not sufficiently constrain the ocean and can lead to spurious MOC strengths, -the high latitude North Atlantic (essentially the sub-polar gyre (SPG) region) is key to initialise for successful MOC predictions, even if the tropical Atlantic is still initialised. Multi-model observing system simulation experiments (OSSEs) were performed using pseudo observations typical of the real world both before and after Argo became available: - a range of different techniques for using the pseudo observations to initialise coupled models were used. To a lesser or greater extent all models showed predictability when either the 2008 (with Argo) or the 1990 (without Argo) type observations were used. Both sets of observations provided forecasts that were clearly better than a persistence forecast, -the current observing system (2008), that includes the Argo array of profiling floats, gives better forecasts of the MOC than previous (1990) observing networks - although we only find this to be statistically significant for the first five years.