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 is true for 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 discussed 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. CMP12 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 CMP12 coupled model forecasts for the SST anomaly field averaged over Dec-Jan-Feb, Mar-Apr-May and Jun-Jul-Aug 1999 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 September through early December 1998. The forecasts show the continuation of moderate La Niña conditions for the winter 1998-99, weakening in spring and largely dissipating by summer. These forecasts show a somewhat weaker cold event than that shown by some 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 through boreal spring 1999.

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 of the model's mid-latitude atmospheric response to the ENSO state is discussed in a specification setting 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 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. 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 skill for predicting tropical Pacific SST, the areal average skill drops accordingly. The final skill is presently approximately comparable to that of the statistical CCA forecasts, also produced at CPC (Barnston et al. 1994); however, in specific circumstances the skills 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. Warmth is forecast for parts of northern North America during winter 1998-99, but no significant anomalies appear in the mainland U.S. In spring, marked anomalous warmth is predicted over the central and southwestern U.S, with colder than normal temperature in Alaska and northwestern Canada. The expected skills of the forecasts beyond spring 1999 are not high enough to regard them with much practical confidence, and they should be viewed as experimental. Even during winter the lack of the expected warmth along the southern tier of the U.S. may indicate a model problem, given the cold ENSO lower boundary condition (Fig. 1) and the expected teleconnection. The hint toward cold in the Pacific Northwest in Jan-Feb-Mar is consistent with expectations, while the slight warmth near the Great Lakes runs counter to expectations.

The precipitation forecasts are more in line with what would be expected in North America with a La Niña, calling for dryness in much of the mid-Atlantic and southern parts of the U.S. through spring 1999. Wetness in the Pacific Northwest and southwestern Canada is consistent with expectation; this wetness may be underestimated along the southern tier of Canada to the Great Lakes, consistent with a suspected possible dry bias in this model. In similar fashion to temperature, the precipitation forecasts beyond spring 1999 onward should be considered very 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-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.

Fig. 1. NCEP coupled model SST anomaly forecast fields for Dec-Jan-Feb, Mar-Apr-May and Jun-Jul-Aug 1999. 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.

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.

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