Precipitation Forecasts for the Tropical Pacific Islands Using Canonical Correlation Analysis (CCA)

contributed by Yuxiang He and Tony Barnston

Climate Prediction Center, NOAA, Camp Springs, Maryland

Canonical correlation analysis (CCA) identifies linear relationships between multicomponent predictors and multicomponent predictands, e.g. pattern-to-pattern relationships in space and/or time. Like simpler forms of linear regression, CCA minimizes squared errors in hindcasting the predictands from the predictors. During the last decade, CCA has started being used increasingly in the atmospheric sciences (e.g. Barnett and Preisendorfer, 1987; Graham et al., 1987a; 1987b; Barnston and Ropelewski, 1992; Barnston, 1994; Barnston and He, 1996). Here, CCA is used to predict 3-month precipitation anomalies in the Pacific Islands out to a year in advance, as described in He and Barnston (1996). Because rainfall in the tropical and subtropical Pacific is strongly related to ENSO (Ropelewski and Halpert, 1987; 1996), it is reasonable to expect usable skill in seasonal Pacific rainfall forecasts, and thus worthwhile to establish a real-time prediction system for the benefit of commercial interests in the Pacific Islands. The experimental forecasts shown in this quarterly Bulletin are provided on a monthly basis on the Internet at address: http://nic.fb4.noaa.gov/pacdir/npac.html.

The predictor fields used for the forecasts include quasi-global sea surface temperature (SST), Northern Hemisphere 700 hPa geopotential height, and the predictand precipitation itself (at 61 island stations) at an earlier time. CCA sensitivity experiments indicate that the SST field is the most valuable predictor field, with 700 hPa heights and prior precipitation somewhat helpful. Further details about the skills, the underlying relationships, and the predictors are provided in He and Barnston (1996). The set of predictors is configured as four consecutive 3-month periods prior to the time of the forecast, followed by a variable lead time, and then a single 3-month predictand period. The predictand includes 3-month total rainfall at 61 Pacific Island stations within 25^oN-30^oS, including four Hawaiian stations (see the web site given above for a map of locations) The lead time is defined as the time between the end of the final (fourth) predictor period (i.e., the time of the forecast) and the beginning of the 3-month predictand period. The set of stations predicted has recently been increased from 33 to 61. The rainfall data and climatology for the larger set of stations has been described in a recently published atlas (He et al., 1998).

The expected skill of the forecasts was estimated using a 1-year-out cross-validation (He and Barnston, 1996). These skill estimates indicate that at 1 month lead time the highest correlation skill across the Pacific Islands occurs in Jan-Feb-Mar in the .40s (near .30) averaged over all stations north (south) of the equator, and the lowest occurs from September through December at about 0.15 (near .30) for stations north (south) of the equator. At four months lead, skills are only slightly lower except for the Jan-Feb-Mar average skill north of the equator which drops into the .20s. Skill for only warm or cold ENSO years is higher than the above.

Figure 1 shows forecasts of the standardized precipitation anomaly (X100) for 33 Pacific Island stations using data through November 1998. The top panel shows the forecast for Jan-Feb-Mar 1999 (1 month lead), and the middle and bottom panels for Apr-May-Jun and Jul-Aug-Sep 1999 (four and seven months lead), respectively. The expected skill for these forecasts, based on cross-validation, is shown by the size of the numerals (as opposed to their value, which is the forecast itself): Small numerals indicate low skill (correlation below .3), medium sized numerals usable but modest skill (correlation between .3 and .45), and large numerals moderate or better skill (.45 and higher). A tendency toward enhanced rainfall at many of the off-equator locations is forecast for Jan-Feb-Mar. Along the immediate equator near and east of the date line (e.g. Kiribati), suppressed rainfall is forecast. This pattern is associated with the moderate La Niña condition that developed in May and June this year. Enhanced rainfall is predicted for Hawaii, especially at Oahu and Kauai. Confidence in the above normal rainfall at many of the off-equator stations is fairly good, since we are confident that cold episode conditions will persist through the winter of 1998-99 with the large reservoir of anomalously cold water beneath the surface, coupled with the high SOI and accompanying strong trades. Most statistical and dynamical models continue to indicate a moderate La Niña through boreal spring of 1999. By late boreal spring 1999, climate anomalies associated with the La Niña condition are expected to dissipate.

More detailed forecasts for 9 U.S.-affiliated and 18 non-U.S.-affiliated Pacific Island stations (first 9 stations, second 9 stations) are shown in Fig. 2, in the form of long-lead rainfall forecasts from 1 to 13 seasons lead (solid bars) along with their expected skills (lines). The horizontal axis reflects the lead time, whose corresponding actual target period for this forecast is indicated in the legend along the top of the figure (e.g. 1=Jan-Feb-Mar 1999). The same ordinate scale is used for both forecasts and skills (standardized anomaly and temporal correlation coefficient, respectively). Sometimes skill may increase as the lead is increased because a more forecastable target season has been reached. The forecasts and their skills differ not only due to their differing ENSO-responsiveness caused by general location differences in the Pacific basin, but also due to differences in orientation with respect to the local orography (if any).

Hawaii and the other U.S. affiliated stations are forecast to have near to above normal rainfall this boreal winter. South of the equator at the non-U.S.-affiliated islands, the same forecast generally applies but with exceptions in the eastern part of the basin. Locations close to the equator from 170 E and eastward (e.g. Kiribati) are forecast to be dry through boreal spring, and should already be making drought mitigation efforts as needed.

The CCA modes (not shown; He and Barnston, 1996) clearly show ENSO as the dominant (nearly exclusive) influence on tropical Pacific climate, especially during Nov-Dec-Jan-Feb-Mar-Apr-May (and even earlier than Nov. along the immediate equator near and somewhat east of the dateline). The current forecasts show a moderate (not severe) influence from the La Niña conditions which are stronger than those of 1995-96 and 1983-84, but not as strong as 1988-89 or the two strong cold events of the mid-1970s.

References

Barnett, T.P. and R. Preisendorfer, 1987: Origins and levels of monthly and seasonal forecast skill for United States surface air temperatures determined by canonical correlation analysis. Mon. Wea. Rev., 115, 1825-1850.

Barnston, A.G., 1994: Linear statistical short-term climate predictive skill in the Northern Hemisphere. J. Climate, 7, 1513-1564.

Barnston, A.G. and C.F. Ropelewski, 1992: Prediction of ENSO episodes using canonical correlation analysis. J. Climate, 5, 1316-1345.

Barnston, A.G. and Y. He, 1996: Skill of CCA forecasts of 3-month mean surface climate in Hawaii and Alaska. J. Climate, 9, 2579-2605.

Graham, N.E., J. Michaelsen and T. Barnett, 1987a: An investigation of the El Niqo-Southern Oscillation cycle with statistical models. 1. Predictor field characteristics. J. Geophys. Res., 92, 14251- 14270.

Graham, N.E., J. Machaelsen and T. Barnett, 1987b: An investigation of the El Niqo-Southern Oscillation cycle with statistical models. 2. Model results. J. Geophys. Res., 92, 14271-14289.

He, Y. and A.G. Barnston, 1996: Long-lead forecasts of seasonal precipitation in the tropical Pacific islands Using CCA. J. Climate, 9, 2020-2035.

He, Y. A.G. Barnston and A.C. Hilton, 1998: NCEP/Climate Prediction Center Atlas No. 5: A precipitation climatology for stations in the tropical Pacific basin; effects of ENSO. U.S. Dept. of Commerce, NOAA, 280pp.

Ropelewski, C.F. and M.S. Halpert, 1987: Global and regional scale precipitation patterns associated with the El Niqo/Southern Oscillation. Mon. Wea. Rev., 115, 1606-1626.

Ropelewski, C.F., and M.S. Halpert, 1996: Quantifying Southern Oscillation-precipitation relationships. J. Climate, 9, 1043-1059.

Acknowledgments: This work was supported under NOAA grant NA26-GP0149 and NA46-GP0217 and NSF grant ATM-93-21354.

Fig. 1. CCA-derived precipitation standardized anomaly forecast (X100) for 61 Pacific Islands stations for Jan-Feb-Mar, Apr-May-Jun and Jul-Aug-Sep 1999. Latest data for these forecasts is November 1998. The cross-validated skill expected for the forecasts is indicated by the size of the numerals (not their value, which shows the forecast itself). Small numeral size indicates correlation skill of less than 0.30, considered unusable; medium size is used for 0.30<skill<0.45 which is modest but usable; large size indicates skill>0.45, considered a relatively more reliable forecast.

Fig. 2. Time series of CCA-based long-lead precipitation anomaly forecasts, and their expected skills, out to one year into the future for 9 U.S.-affiliated Pacific Island stations (first page) and 18 non-U.S.-affiliated stations (first 9 stations, second 9 stations). The bars indicate the forecast values (as standardized anomalies) and the lines indicate the associated skills (as temporal correlation coefficients). Both forecasts and skills use the same ordinate scale. The target season is indicated on the abscissa, ranging from 1 (Jan-Feb-Mar 1999) through 13 Jan-Feb-Mar 2000); see the legend at top.