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Forecasts of U.S. Coastal SST
Using Canonical Correlation Analysis
contributed by Anthony Barnston and Yuxiang He
Climate Prediction Center, NOAA, Camp Springs, Maryland
Canonical correlation analysis (CCA) is used here to predict the SST adjacent to the U.S. coast
around Hawaii, the mainland West Coast, the Gulf of Mexico coast and the Southeast Coast.
Coastal SST forecasts are provided for commercial fishing interests as well as tourists (e.g.
bathers, surfers). As described in several other articles in this Bulletin, CCA identifies linear
relationships between multicomponent predictors and multicomponent predictands. In the present
application, fields of near-global SST, Northern Hemisphere 700 mb height and the coastal SST
predictand itself are used as predictors, covering four consecutive 3-month periods ending at the
time of the forecast. To prevent overfitting in the weaker predictor fields, the SST field is
weighted double on entry to the analysis, following standardization. The predictand is the coastal
SST at various 3-month periods following the time of the forecast, ranging from zero lead
(beginning at forecast time) out to a year ahead. The predictand SST is presented in 2o x 2 oboxes,
using COADS data from 1955 to 1979 and blended (in situ plus satellite) or optimum
interpolation (OI) data thereafter. The predictor and predictand data are filtered separately using
standard EOF analyses prior to the CCA, as done originally by Barnett and Preisendorfer (1987)
and in subsequent applications such as Graham et al. (1987a,b), Barnston and Ropelewski (1992),
Barnston (1994), He and Barnston (1996), Shabbar and Barnston (1996) and Johansson et. al
(1997).
The expected skill of the forecasts was estimated using cross-validation, in which each year in
turn was held out of the model development sample and used as the forecast target. Figure 1 in
the December 1996 issue of this Bulletin shows the seasonal cycle of areal average skill of the
SST forecasts (temporal correlation between forecasts and observations), for several lead times
for (a) the West Coast of the mainland U.S., (b) the Gulf of Mexico and Southeastern coasts, and
© the Hawaiian coasts. Moderate skills are found for the SST along the West Coast for the cold
half of the year for leads of up to 4 months. At 6 to 13 months lead, skills are highest in spring.
Skill is moderate for the Gulf/ Southeast coasts for all seasons except winter at short leads, and
for late summer/early fall at both short and long leads. In the Hawaiian region skills are relatively
lower. CCA skills are higher than the skill of persistence forecasts (with respect to the fourth of
the four predictor periods), with the exception of Hawaiian skills for 1-month lead forecasts in the
fall in which case CCA skill is at its highest but persistence skill is even higher.
The geographical distribution of skill for short lead winter forecasts was discussed and shown in
Fig. 2 of the December 1996 issue of this Bulletin, as well as the associated CCA predictand
loading pattern for the leading mode. The coastal SST anomalies in winter are shown to be
closely related to the ENSO state during the several months preceding the time of the forecast. An
El Niño is associated with positive SST anomalies along the West Coast, negative anomalies in
the Gulf of Mexico and Southeast Coast, and positive (negative) anomalies in southern (northern)
Hawaii. The fact that Hawaii lies in a south-north transition zone with respect to ENSO-related
anomalies implies lower predictive skill there, since it cannot benefit greatly from knowledge
about the likely ENSO state as can most of the other coastal regions examined here.
Figure 1 shows the coastal SST forecasts for Sep-Oct-Nov 1997 (top) and Jan-Feb-Mar 1998
(bottom), made in early June 1997. The numbers indicate standardized SST anomalies (X100),
and their font size depict their expected skill. Forecasts with expected skill of less than 0.3
correlation are omitted. For Sep-Oct-Nov (a 2.5-month lead forecast), positive anomalies are
predicted with modest skill off the Pacific Northwest coast and in southern Hawaii, while
modest-skill negative anomalies are forecast in the Gulf of Mexico and the Southeast coast.
Anomalies of these same signs presently exist in these areas. The 6.5-month lead forecast for
Jan-Feb-Mar 1998 indicates warmth along the U.S. west coast with modest to moderate skill at
most locations. The anomaly magnitudes tend to be a little lower along the immediate coast than
slightly farther out into the ocean basin. The loading patterns associated with this long lead skill
(not shown) reflect a decadal trend toward warm-ENSO-like conditions particularly in the
northern late winter and spring seasons, of which a warm west coast SST pattern is one part.
Another factor is that an increase in the strength of the present warm ENSO condition is expected
by the CCA during 1997, peaking during winter 1997-98.
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 Niño-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 Niño-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.
Johansson, Å., A.G. Barnston, S. Saha and H.M. van den Dool, 1997: The level and origin of
seasonal forecast skill in northern Europe. J. Atmos. Sci., 54, in press.
Shabbar, A. and A.G. Barnston, 1996: Skill of seasonal climate forecasts in Canada using
canonical correlation analysis. Mon. Wea. Rev., 124, 2370-2385.
Fig. 1a, b (two separate links). Forecast for coastal SST for Sep-Oct-Nov 1997 (top) and
Jan-Feb-Mar 1998 (bottom) made at 1.5 and 6.5 months lead, respectively (using data through
mid-June 1997). The value of the numbers shows uninflated standardized SST anomaly (X100)
with respect to the 1961-1990 mean, and the physical size of numerals indicates estimated skill
level (correlation) in four categories: blank for skill < .30, small for .30 < skill < .45, medium for
.45 < skill < .60, and large for skill > .60.
