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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 and 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 predictors, 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 optimum interpolation data from 1990 to the present. 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
(c) 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.
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 is discussed and shown in Fig.
2 of the December issue of this Bulletin, as well as the associated CCA predictand loading
pattern. 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 many of the other coastal regions examined here.
Figure 1 shows the coastal SST forecasts for Jun-Jul-Aug 1997 (top) and Mar-Apr-May 1998
(bottom), made in early March 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 Jun-Jul-Aug (a 3-month lead forecast) positive anomalies are
predicted with moderate skill in much of the Gulf of Mexico and the south coast of Florida, with
lower skill weak negative anomalies along the northeast coast of Florida. Positive anomalies exist
presently along the eastern Gulf and southeast U.S. coast, perhaps in association with the
prolonged cool ENSO episode. Positive anomalies are forecast in two locations near Hawaii, but
with low skill. The 12-month lead forecast for Mar-Apr-May 1998 indicates warmth along the
U.S. west coast with moderately high skill at many locations. Skill tends to be 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 spring season, of which a warm west coast SST pattern is one part. A
secondary factor is that a switch to a moderate warm ENSO condition is expected by the CCA
during 1997, continuing into early 1998.
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, accepted.
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. 1. Forecast for coastal SST for Jun-Jul-Aug 1997 (top) and Mar-Apr-May (bottom) 1998
made at 3 and 12 months lead, respectively. 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.
