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). It has been shown that when observed SST fields are prescribed, the coupled model's atmospheric response is fairly reliable in the tropics but considerably less so in the extratropics, as with most AGCMs. 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 treated in some detail in Livezey et al. (1997). While skillful prediction of the extratropical atmosphere is an ultimate 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

In the September and December 1993 issues of this Bulletin, the expected forecast skill of the coupled model version used in 1993 (called CMP6) was shown. A horseshoe-shaped spatial pattern of maximum model skill was noted, 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 165^oW. Since 1993, several major changes have been made to the model, all resulting in incremental improvements in skill. Some information about these model versions is found in the March and June 1998 issues of this Bulletin. These include the CMP9, CMP10 (Ji et al. 1996), and the presently used CMP12. The more recent versions use an ocean data assimilation system (Ji et al. 1995). The basic skill characteristics of the earlier versions of the CMP models still 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.

The CMP12 coupled model forecasts for the SST anomaly field averaged over Sep-Oct-Nov, Dec-Jan-Feb and Mar-Apr-May 1998-99 are show 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 July through early September 1998. The forecasts show the continuation of moderate La Niña conditions with the approach of winter 1998-99. These forecasts show a somewhat weaker cold event than that shown by many of the other models. The subsurface equatorial sea temperature (not shown) indicates an abundance of water at below-normal temperature beneath the surface in the eastern and central part of the basin. Stronger than normal low-level trades are required to allow those negative anomalies to continue to show up in the SST. The implication of the model's forecast is that the trades will, on average, be stronger than normal over the coming 4-6 months.

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 different atmospheric initial condition. The nature of the model's mid-latitude atmospheric response to the ENSO state is discussed in a specification setting in Livezey et at. (1977), where skill over the U.S. is found to be moderate during the northern winter during warm and cold ENSO episodes. Earlier work of the same nature demonstrated that over the Pacific/North American (PNA) region, skill in specifying geopotential heights, temperature and (to a lesser extent) precipitation are usable during the cold

half of the year when a non-neutral ENSO condition exists (Livezey et al. 1996). On the other hand, skill during non-ENSO years is not significantly different from zero. This ENSO-dependency of expected skill is explainable on the basis of 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 in this presentation. This specification skill, 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 the mean). When multiplied by the imperfect correlation skill for predicting tropical Pacific SST, the area averaged skill drops accordingly.

All in all, 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, in the vicinity of the Gulf of Mexico and the Southeast), however, winter skill is locally moderately high during ENSO episodes at lead times of up to 6 months. Thus, 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) might 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. Warmer than normal temperatures are forecasted for much of North America through early boreal winter. In Dec-Jan-Feb 1998-99, negative temperature anomalies appear over Alaska and positive anomalies are positioned in the southern U.S. From Jan-Feb-Mar 1999 through late spring negative anomalies occupy the southeastern U.S., and warm anomalies prevail in the southwestern U.S. beginning in early spring. The expected skill of the forecasts beyond winter 1998-99 is not high enough to regard the forecasts with much practical confidence, and they should be viewed as experimental. Even during Jan-Feb-Mar the appearance of negative anomalies in the southeastern U.S. may indicate a model problem, given the cold ENSO lower boundary condition (Fig. 1) and the expected teleconnection of warm anomalies in the Southeast.

The precipitation forecasts call for dryness in much of the eastern and southern parts of the U.S. through early 1999, while enhanced rainfall along the coast of the Gulf of Mexico is forecast. The dryness through early winter would be expected in view of the cold ENSO state. In a similar fashion to temperature, the precipitation forecasts from mid-spring 1999 onward should be considered cautiously.

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-Lean 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 1980's and 1990's 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.

Figure Captions

Figure. 1. NCEP coupled model SST anomaly forecast fields for Sep-Oct-Nov, Dec-Jan-Feb and Mar-Apr-May 1998-99. The CMP12 version of the model is used. 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.

Figure. 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.

Figure. 3. As in Fig. 2, except for precipitation.