Forecasts of western North Pacific Winter-time SST anomalies

contributed by Niklas Schneider and Arthur J. Miller

Scripps Institution of Oceanography, La Jolla, California

Winter time anomalies of sea surface temperature in the Kuroshio-Oyashio Extension (KOE) in the western North Pacific are predicted using linear, single baroclinic mode Rossby wave dynamics and observations of Ekman pumping. The underlying hypotheses states that KOE surface temperatures are related to anomalies of thermocline depth, which can be predicted from Rossby waves forced by Ekman pumping anomalies over the North Pacific during the preceding years (Xie et al. 2000, Miller and Schneider 2000, Seager et al. 2001, Schneider et al. 2002). The method and forecast skill are described in Schneider and Miller (2001).

Ekman pumping is estimated from North Pacific wind stress taken from the NCEP/NCAR reanalysis after removal of the average annual cycle and least-square-fit, linear trends from 1958 to 2000. Ekman pumping is averaged between 35N and 40N and integrated along trajectories of Rossby waves with a westward speed of 2.5 cm/s, starting with zero at the eastern boundary at 125W. Anomalies of KOE SST are then estimated from the average Rossby wave predictions west of 170E by linear regression. The regression parameter is determined from NCAR/NCEP reanalysis SST and an hindcast of the Rossby wave model. Forecasts are obtained by using observed wind stress curl until the forecast is initiated. Thereafter, the anomalous wind stress curl is set to zero.

This scheme has skill predicting winter time anomalies of KOE SSTs at leads of two (and possibly three) years, with largest skill in February-April (FMA) season. The skill is larger than that obtained by persistence forecast based on the autoregression of SSTs (Schneider and Miller 2001). As expected from the Rossby wave dynamics, the methods reproduces changes of thermocline depth very well, and is best in predicting the low frequency portion of SST (Fig. 1).

For the coming FMA for years 2002, 2003 the scheme predicts above normal SSTs in KOE region.

Caveat: The forecasts are experimental, the forecast methods are new and subject to future change and improvement.

Acknowledgment:

This work is supported by NOAA (Experimental Climate Prediction Center), the National Science Foundation's Climate Dynamics Division and the Department of Energy.

References:

Miller, A. J., and N. Schneider, 2000: Interdecadal climate regime dynamics in the North Pacific Ocean: Theories, observations and ecosystem impacts. Prog. Oceanogr., 27, 257-260.

Seager, R., Y. Kushnir, N. Naik, M. A. Cane, and J. A. Miller, 2001: Wind-driven shifts in the latitude of the Kuroshio-Oyashio Extension and generation of SST anomalies on decadal timescales. J. Climate, 14, 4249-4265.

Schneider, N. and A. J. Miller, 2001: Predicting western North Pacific ocean climate. J. Climate, 14, 3997-4002.

Schneider, N., A. J. Miller, and D. W. Pierce, 2002: Anatomy of North Pacific decadal variability. J. Climate, in press.

Xie, S. P., T. Kunitani, A. Kubokawa, M. Nonaka and S. Hosoda, 2000: Interdecadal thermocline variability in the North Pacific for 1958-1997: A GCM simulation. J. Phys. Oceanogr., 30, 2798-2813.

Figure captions:

Fig. 1. (Top) Time series of Feb-Mar-Apr (FMA) SST anomalies in the Kuroshio-Oyashio-Extension (KOE) along 40N, 140E-170E. Connected red dots are observations from the NCEP/NCAR reanalysis; thick red line is the three-year average. The smoothed SSTs are repeated as red line in the center and bottom panels. (Center) Solid black line is the hindcast of SST anomalies from linear Rossby wave dynamics forced by NCEP/NCAR reanalysis wind stress. Blue line is observed 100m-400m XBT temperature, a proxy for thermocline depth. (Bottom) Retrospective forecasts (green lines) of KOE SST. Green diamonds are the forecast for FMA of 2002, 2003, 2004 and 2005 obtained from reanalysis winds up to November 2001.