Recorded presentation9A.5
Decadal predictability of North Pacific Sea Surface Temperatures
Grant Branstator, NCAR, Boulder, CO; and H. Teng
A unique ensemble of integrations with NCAR's Community Climate System Model is examined for evidence of predictable signals on decadal time-scales in sea surface temperatures (SSTs) of the North Pacific. This ensemble consists of 30 realizations each simulating a prediction of the years 2000-2061. Each realization starts from the identical initial condition except for the atmospheric state, and each is forced by the same external forcing, namely the SRES A1b scenario. Therefore, the ensemble can be treated in the same way as classical atmospheric predictability experiments to quantify the growth of small initial uncertainties and to determine what predictable signals exist and for how long. Extended control runs with the same model exhibit substantial decadal variability in North Pacific SST anomalies suggesting that in this experiment there is the potential for a predictable signal in addition to that produced by the imposed forcing.
Three components of predictability are examined. 1) Gridpoint analysis of intra-ensemble variance indicates that there are two regions of extended predictability in the North Pacific during winter. These include a region centered on the Kuroshio Extension and a swath reaching from the Philipines to the west coast of the United States. In these regions for up to 15 years the spread of ensemble members is less than that expected from natural variability. By contrast, some regions of the North Atlantic and Southern Ocean have predictability beyond 20 years. 2) Analysis of the detrended mean ensemble state shows a smaller contribution to predictability. Initial departures from climatology that are substantially greater than that expected from natural fluctuations are detectable with confidence for about five years in the North Pacific; they last even longer in the North Atlantic. 3) A third predictable component is that produced by growing greenhouse gas concentrations in the experiment. As measured by ensemble average trends, in the North Atlantic these become detectable above the background fluctuations after 10 to 15 years.
Complex EOF analysis indicates that a substantial component of decadal variability in North Pacific SST in this experiment, as well as in the control, is associated with a pattern similar to the Pacific Decadal Oscillation observed in nature. In this model this mode of variability appears to be largely a result of midlatitude atmosphere-ocean coupled processes. Our analysis indicates it is responsible for the enhanced predictability noted in the Kuroshio Extension region. Interestingly, there are suggestions in the ensemble experiment that this mode may be predictable for several decades though we cannot be certain of this. Though there are substantial variations from one ensemble member to another, in the ensemble mean its amplitude reaches peaks near years 2021 and 2043 that are substantially larger than would be expected from natural fluctuations of the model. Even more intriguing is the fact that wavelet analysis shows a very prominent 20 year component to the variability of this mode in many ensemble members, suggesting that its period, if not its phasing, may be especially predictable.
Session 9A, Prediction of climate on seasonal to decadal timescales - II
Wednesday, 14 January 2009, 10:30 AM-12:00 PM, Room 129A
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