Figure 2 Difference in the first annual mean ensemble mean 500m ocean

1. GCEP Case Studies in Decadal Climate Predictability Leon Hermanson and Rowan Sutton, Department of Meteorology, University of Reading, UK 1. Introduction Is climate predictable beyond seasonal (El Niño) timescales? To what extent does knowledge of initial conditions constrain long-term climate forecasts? Previous studies show potential predictability on annual time scales is greatest for ocean variables and is weak for climate variables. This study investigates whether there is predictability beyond the limit of seasonal time scales for climate variables. Recognising that predictability is likely to be dependent on initial conditions we consider model-based case studies rather than average predictability. Two case studies examined here show potential predictability in tropical precipitation and West European surface temperatures two years ahead, but differ substantially in the details. Initial phase: Pre-industrial to modern day Generation of initial conditions Two initial conditions chosen for case study 1860 1950 ~1980 Figure 2 Difference in the first annual mean ensemble mean 500m ocean heat content for each of the four case studies. Units are 10-19J. 2. Generation of Initial Conditions Figure 1 shows the methodology. An ensemble of 20th century simulations with HadCM3 are used to generate initial conditions for the case studies. Members of this ensemble which show large, persistent, regional differences in ocean heat content are used to initialise the case study ensembles. Figure 2 gives an indication of the differences in the initial ocean heat content for the four case studies shown here. Note that large anomalies exist in all ocean basins. 3. Predictability Plumes Figure 3 shows examples from all four case studies of plumes for three different ocean variables: global mean ocean heat content (OHC), a Pacific sea surface temperature (SST) index and the Atlantic meridional overturning circulation (MOC) at 30°N. As expected from previous studies the OHC (first column) is predictable between 2—7 years ahead. This is despite the ocean heat content being strongly influenced by external forcing. The second column shows a sea surface temperature (SST) index of the Interdecadal Pacific Oscillation (IPO). The IPO is a basin-wide multi-decadal pattern in the Pacific that can modulate El Niño Southern Oscillation (ENSO) teleconnections. Predictability of the index is generally low, but interestingly in plot (e) it looks like predictability returns in year 8. Further work is needed to determine whether cases of “returning predictability” such as this one, are really predictable. The third column shows the Atlantic MOC, which is linked to ocean meridional heat transport and important for western European climate. It is potentially predictable between 3─6 years ahead here. Figure 3 Predictability plumes from the four case studies (one per row). The first column is global 500m ocean heat content, the second an SST index of the Interdecadal Pacific Oscillation and the third the Atlantic meridional overturning circulation. The dark line shows the ensemble mean and the shading one standard error for that mean. The predictability as determined by a t-test is printed on each plot. 4. Predictability of Climate in the Second Year Figure 4 shows differences between annual mean ensemble mean maps of surface temperature and precipitation in the second year for experiments 2 and 4. It is clear that in some regions predictability exists for both these variables beyond ENSO time scales. Comparing the two experiments it is clear that the initial conditions strongly influence the predictability. The other case studies also show evidence of climate predictability in the second year, but not always in the same areas. Figure 5 shows predictability plumes for some seasonal and regional mean quantities for all four experiments. It shows that two experiments (1&4) show 3 year predictability for western European temperatures. Two experiments (1&2) also have two year predictability in tropical South Atlantic precipitation. The mechanisms which give rise to these predictabilities are currently being investigated, but the most common mechanism is persistence of ocean heat content anomalies. Figure 1 Schematic of experiment set-up. An ensemble is started in 1950 from a single run (black). From this ensemble case studies are chosen. Each case study consists of two ensembles (red and purple) of ten members each. All integrations are forced by observed changes in radiation. Figure 5 Same as figure 3, but for various seasonal mean quantities (season indicated on plot). Figure 4 Differences in second annual mean ensemble mean 1.5m surface temperature (°C) and precipitation (mm/day) for experiments 2 and 4. Line contours show significance levels (dashed for negative anomalies). • 5. Conclusions • Perfect model experiments have been carried out with HadCM3 on a case-by-case basis to investigate decadal predictability of transient climate. • Predictability in the ocean varies from 0—2 years for SST to 2—7 years for OHC and 3—6 years for the MOC, this is broadly in line with previous studies. • Though predictability of atmospheric climate variables is usually less than for ocean variables, for some initial conditions there can be predictability up to 3 years for regional variables. • The length of predictability and the region and season in which it occurs is very state dependent and differed in all four experiments. • The mechanism that gives rise to predictability is commonly persistence of OHC anomalies. • Cases of returning predictability will have a more complex mechanism.