Dynamical Forecasts of Tropical Pacific SST and North American
Climate Using a Two-Tiered Process
contributed by Tony Barnston1, Ming Ji2, Arun Kumar2, Wanqiu Wang2 and Ants Leetmaa1
1Climate Prediction Center, National Centers for Environmental Prediction, NOAA, Camp Springs, Maryland
2Environmental Modeling Center, National Centers for Environmental Prediction, NOAA,
Camp Springs, Maryland
A non-simple coupled ocean-atmosphere model has been developed for use in long-lead climate forecasting in the Coupled Model Branch of the Environmental Modeling Center (EMC) at NOAA's National Centers for Environmental Prediction (NCEP; Ji et al. 1994a,b). The NCEP Medium Range Forecast (MRF) atmospheric model is used with a dynamic Pacific Basin ocean model originated at the Geophysical Fluid Dynamics Laboratory. The MRF has a reduced spatial resolution and is tuned for more realistic tropical circulation. The ocean thermal field, including SST and subsurface temperature, is initialized using an ocean data assimilation system (Ji et al. 1995). As is the case for most AGCMs, this coupled model's atmospheric response to observed tropical Pacific SST is fairly reliable in the tropics but considerably less so in the extratropics. The extratropical response is most realistic during ENSO extremes. The nature of the model's mid-latitude atmospheric response to the ENSO state has been discussed in Livezey et al. (1997). While skillful prediction of the extratropical atmosphere is a major goal, a prerequisite is the ability to predict ENSO itself-the tropical Pacific SST anomaly field. Both SST forecasts and surface climate forecasts for North America are presented here.
SST Forecasts
As has been the case since the original presentation of the NCEP model in this Bulletin in 1993, the expected forecast skill features a horseshoe-shaped spatial pattern of maximum model skill, with highest equatorial skill near the date line and higher skill just north or south of the equator than immediately along it to the east of 165oW. Since 1993 several major improvements have occurred in the model, resulting in higher skill (Ji et al. 1995; Ji et al. 1996). The more recent versions use an ocean data assimilation system (Ji et al. 1995). The basic skill characteristics of the earlier versions of the model remain, such as its geographical distribution and season dependence (e.g. a relative spring skill barrier). The skill of the SST forecasts for SST in the Niño 3.4 region for the first four lead times is shown in Table 1, based on the 1982-97 period. NCEP model forecasts out to 6 months lead are now updated on a weekly basis and are available on Internet site http://nic.fb4.noaa.gov:8000 /research/climate/html.
The NCEP coupled model forecasts for the SST anomaly field averaged over Jun-Jul-Aug, Sep-Oct-Nov, and Dec-Jan-Feb 1999-2000 are shown in Fig. 1, where the systematic model bias for hindcasts over the 1981-95 period has been removed. This forecast is the mean of an ensemble of 7 to 11 individual cases, each based on a different one- to two-week-apart initial ocean condition ranging from early March through early June 1999. The forecasts show the continuation of La Niña conditions through winter 1999-2000, strengthening between now and fall 1999 with the normal seasonally expected increase in the standard deviation. The subsurface equatorial sea temperature (not shown) still indicates a good supply of below-normal water temperature beneath the surface in the eastern part of the basin. Warm water continues to gradually build up beneath the surface in the western Pacific.
North American Surface Climate Forecasts
The predictions of tropical Pacific SST are taken one step farther, and used as a lower boundary condition for integrations of the atmospheric GCM (AGCM) out to comparable lead times. The AGCM forecast is the ensemble mean of 18 individual integrations, each using a different atmospheric initial condition. The nature and skill of the model's mid-latitude atmospheric response to the ENSO state is discussed in Livezey et al. (1997), where skill over the U.S. is found to be moderate during the northern winter during warm and cold ENSO episodes. Earlier work showed that over the Pacific/North American (PNA) region, skills in specifying geopotential heights, temperature and (to a lesser extent) precipitation are usable during the cold half of the year as long as there is an ENSO signal (Livezey et al. 1996). Skill during non-ENSO years is not significantly different from zero. This ENSO-dependency of skill is plausible, given signal-to-noise ratio considerations in the extratropics with respect to ENSO-related anomalous tropical Pacific SST boundary forcing (Kumar and Hoerling 1998).
Here, we present GCM forecasts for 3-month mean North American surface temperature and precipitation out to 8 months of lead time, where lead time refers to the time between the forecast time and the middle month of the 3-month target period. For each location, season and lead time, the expected forecast skill is the product of (1) skill in the forecast of the tropical Pacific SST boundary condition (shown in Table 1 for the first 4 leads out of 8) and (2) skill of the simultaneous simulations (specifications) of the atmosphere, given a perfect SST forecast ("AMIP" skill). The expected forecast skill over the U.S., given a perfect SST forecast, is described in Livezey et al. (1996) and is not shown by geographical region here. These specification skills, averaged over the U.S. for winter and expressed as temporal correlation coefficients, are in the neighborhood of 0.4 for 700 mb height and for surface temperature, and about 0.1 to 0.2 for precipitation, when ENSO is active (i.e., tropical Pacific SST at least 1 standard deviation away from mean). When multiplied by the imperfect correlation skills for predicting tropical Pacific SST, these areal average skills drop accordingly. The final skill is approximately comparable to that of the statistical CCA forecasts, also produced at CPC (Barnston et al. 1994); however, in specific circumstances the skill of the two forecasting approaches may differ substantially. In the regions that experience strong ENSO impacts (e.g. around the Great Lakes and southern Canada, and in the vicinity of the Gulf of Mexico and the Southeast), however, winter skills is moderately high during ENSO episodes at lead times of up to 6 months. The user of the surface forecasts presented here is urged to exercise caution in using the forecasts, considering the lead time and the region of interest. Additionally, spatial displacement biases may cause "centers of action" to appear slightly out of their appropriate location. A model output statistics (MOS) correction scheme (as in Smith and Livezey 1998) would in principle be able to reduce these spatial biases, as well as systematic forecast amplitude errors.
The current forecasts for North American surface temperature and precipitation are shown in Figs. 2 and 3, respectively. Warmth is forecast in patches of central North America during summer 1999; cold is forecast for Alaska, and part of the eastern U.S. Coolness is forecast for parts of the continent in fall, while Alaska reverses to above normal and stays that way into winter. Although La Niña conditions are expected in winter 1999-2000, the canonical temperature impacts (warm in southern U.S.) do not show up in the model's winter forecast.
The precipitation forecasts feature a tendency toward wetness in the eastern U.S. through late summer 1999, while the U.S. west coast is predicted to be on the dry side straight through to winter. Going into winter 1999-2000 a dry area develops in the eastern and southern U.S. This can be interpreted as being in line with what would be expected with the anticipated continuation of La Niña, although the dryness might not be expected to extend to New England.
References
Barnston, A.G., H.M. van den Dool, S.E. Zebiak, T.P. Barnett, M. Ji, D.R. Rodenhuis, M.A. Cane, A. Leetmaa, N.E. Graham, C.F. Ropelewski, V.E. Kousky, E. A. O'Lenic and R.E. Livezey, 1994: Long-lead seasonal forecasts-Where do we stand? Bull. Amer. Meteor. Soc., 75, 2097-2114.
Ji, M., A. Kumar and A. Leetmaa, 1994a: A multi-season climate forecast system at the National Meteorological Center. Bull. Am. Meteor. Soc., 75, 569-577.
Ji, M., A. Kumar and A. Leetmaa, 1994b: An experimental coupled forecast system at the National Meteorological Center: Some early results. Tellus, 46A, 398-418.
Ji, M., A. Leetmaa and J. Derber, 1995: An ocean analysis system for seasonal to interannual climate studies. Mon. Wea. Rev., 123, 460-481.
Ji, M., A. Leetmaa and V.E. Kousky, 1996: Coupled model forecasts of ENSO during the 1980s and 1990s at the National Meteorological Center. J. Climate, 9, 3105-3120.
Kumar, A. and M.P. Hoerling, 1998: Annual cycle of Pacific/North American seasonal predictability associated with different phases of ENSO. J. Climate, 11, in press.
Livezey, R.E., M. Masutani and M. Ji, 1996: SST-forced seasonal simulation and prediction skill for versions of the NCEP/MRF model. Bull. Am. Meteor. Soc., 77, 507-517.
Livezey, R.E., M. Masutani, A. Leetmaa, H. Rui, M. Ji and A. Kumar, 1997: Teleconnective response of the Pacific-North American region atmosphere to large central equatorial Pacific SST anomalies. J. Climate, 10, 1787-1820.
Smith, T.M., and R. Livezey, 1998: GCM systematic error correction and specification of the seasonal mean Pacific/North American region atmosphere from global SSTs. J. Climate, 11, in press.
Table 1. Expected skill (correlation X100) of the NCEP coupled model in predicting the SST anomaly in the Niño 3.4 region at 4 lead times for 12 overlapping 3-month target seasons. A 1-month lead is, for example, a forecast for JFM made at the end of December.
| LEAD | JFM | FMA | MAM | AMJ | MJJ | JJA | JAS | ASO | SON | OND | NDJ | DJF |
| 1-mon | 90 | 87 | 80 | 76 | 78 | 79 | 80 | 84 | 88 | 91 | 94 | 92 |
| 2-mon | 89 | 84 | 77 | 72 | 73 | 74 | 76 | 81 | 86 | 89 | 92 | 91 |
| 3-mon | 88 | 81 | 74 | 68 | 68 | 70 | 72 | 75 | 83 | 87 | 90 | 90 |
| 4-mon | 88 | 79 | 68 | 64 | 63 | 66 | 68 | 70 | 79 | 84 | 88 | 90 |
Figure Captions
Fig. 1. NCEP coupled model SST anomaly forecast fields for Jun-Jul-Aug, Sep-Oct-Nov, and Dec-Jan-Feb 1999-2000. Each forecast is an average of about 13 individual ensemble members, each based on a different mean of an ensemble of 7 to 11 individual cases based on a different 1- to 2-week-apart initial ocean condition ranging over the last three months (see text). These forecasts were made at the time shown at the bottom of the figure.
Fig. 2. North American surface air temperature anomaly forecasts ( C or K) of the two-tiered NCEP model (coupled model for the SST forecast, then AGCM using the SST forecast as boundary condition) for the coming 8 overlapping 3-month periods. The mean of an 18-member ensemble of GCM integration result is used.
