Seasonal Forecasts of the Tropical and Extratropical Pacific Ocean

MAM 2001 Forecast

Contributed by Guillermo Auad, Arthur J. Miller, John O. Roads, John N. Ritchie

Experiment Climate Prediction Center, Scripps Institution of Oceanography, UCSD, 0224, La Jolla, CA 92093-0224

Large-scale, seasonal oceanic anomalies in the extratropical regions are generally controlled by anomalous atmospheric forcing (e.g., Miller and Roads, 1990; Cayan, 1992). The accuracy of seasonal forecasts of the extratropical ocean therefore will usually depend on the accuracy of atmospheric seasonal forecasts. Unfortunately, atmospheric forecast skill is generally poor on seasonal timescales (e.g., Roads et al., 2001). Moreover, because extratropical oceanic conditions are very persistent, dynamical forecast skill levels that are superior to persistence forecasts are very difficult to achieve even for short-term forecasting (e.g., Miller et al., 1995). On the other hand, teleconnections from anomalous tropical conditions (e.g., Alexander, 1992) may be able to add some level of skill to forecasts of the extratropical oceanic forcing. Also, some oceanic variations can be controlled by processes that are established by ocean dynamics, such as currents due to intrinsic oceanic instability processes or to Rossby wave propagation, that may have predictability timescales that are longer than those of the atmosphere. Subsurface conditions, such as thermocline depth and geostrophic currents, and their consequent effects on SST may also be easier to predict than surface conditions, such as mixed layer depth or Ekman currents, and the atmospherically controlled part of SST variability.

Presently there several groups around the world that engage in continuous real-time forecasting of oceanic tropical conditions (see Auad et al. 2000 for web sites). ECPC has embarked on a project to develop seasonal forecasts of the large-scale (non-eddy-resolving) extratropical Pacific Ocean conditions. We are exploring the relative importance of oceanic initial conditions, atmospheric forcing forecast skill and dynamical evolution of the ocean on the forecasts. Seasonal forecasts are being displayed in real-time on the web (http://ecpc.ucsd.edu/ocean/) and the latest forecasts are discussed below. Skill levels are being studied and will be reported in the near future.

Unlike forecasting most midlatitude oceanic conditions (Auad et al. 2001), forecasting tropical ocean conditions requires modeling the strong oceanic-atmospheric feedback known to exist in the tropics (e.g., Philander 1990). In our experiments Newtonian damping of SST anomalies and specified wind stress forcing mechanism is used to represent those interactions. The comparison between observed tropical oceanic temperatures and those simulated by an ocean model (http://jedac.ucsd.edu) show a good agreement for both surface and subsurface temperatures.

The ocean model is an isopycnal model (10 layers with nearly constant potential density) fully coupled to a bulk surface mixed layer model (Oberhuber, 1993). The model has been tested in a variety of ocean hindcasting scenarios (e.g., Miller et al., 1994; Auad et al., 2001) and it is here used with 1.5 degree resolution (enhanced to 0.67 degrees north-south resolution near the equator) in the following Pacific Ocean forecast framework. Mean seasonal cycle forcing from momentum, heat, fresh-water and TKE fluxes is specified a priori (see Auad et al., 2001, for a complete discussion). Anomalous forcing of momentum, heat, fresh-water and TKE fluxes are specified from 3-month forecasts of the ECPC GSM (Roads et al., 2001; Roads et al., this issue). Initial ocean conditions are taken from a continuously updated hindcast using atmospheric conditions taken from NCEP analyses up to the time of the launch of the forecast.

Improvements to the Pacific Ocean forecasting system are now being sought. Since no information from observed oceanic conditions is presently being used in the initialization of the ocean model, we are exploring ways of using the NASA and/or NCEP ocean analysis to improve the oceanic initial conditions. Since skill levels of the atmospheric forcing forecast deteriorate with time, we are determining how much skill would have been achievable if accurate atmospheric forcing was specified; these results will then guide improvements in model physics. Since tropical forecasts will necessarily be deficient due to the lack of coupled ocean-atmosphere dynamics in the equatorial region, we are beginning to explore the efficacy of coupling the ECPC GSM to the ocean model. This may improve tropical ocean forecasts and potentially improve extratropical forecasts in regions where feedbacks to the atmosphere may be important such as the Kuroshio-Oyashio Extension region (e.g., Schneider et al., 2001).

Figure 1 shows anomalous SST for March, April and May of 2001. The western tropical Pacific shows a positive anomaly with a maximum value of about 1C on eastern New Guinea. However, the central tropical Pacific, at 150E, warms up by about 0.2-0.3C. The eastern tropical Pacific remains colder than normal throughout this 3-month period. The SST horizontal structure is similar to forecasts issued by other institutions at the time of this writing. Thus, our simplified modeling of the atmospheric-oceanic feedback in the tropics seems adequate. In the extratropical ocean, the main feature is a steady positive anomaly off the US west coast and extending to the central North Pacific at around 30N. The western North Pacific is anomalously cold in March and April and then warms up in May (spring transition). These SST patterns show a mild evolution from those forecast by us earlier (Auad et al 2001).

Figure 2 displays the vertical structure of anomalous temperatures along the equator for March, April and May of 2001. The positive anomaly in the west at about 180-200 m moves eastward and upward throughout this period. As this movement occurs, the amplitude of the positive anomaly slightly decreases. However it also extends in space occupying a larger area in the vertical/longitude plot. The anomalies range from about -3.5șC to +3.5șC.

Figure 3 displays the surface and subsurface anomalous temperatures along the equator for the last week of May 2001. The positive anomaly on the western end at about 180-200 m depth moves eastward and upward throughout this period. Without the effects of monthly averaging, temperature anomalies at 150m and at 220șE are warmer by about 0.5 degree C.

Figure 4 shows anomalous mixed layer depths for March-April-May 2001. The North Pacific shows slow varying changes with the only exception being the Kuroshio Current area and its extension where there is an abrupt shoaling between April and May. We believe that this shoaling is related to the anomalous SST increase seen in Figure 1 for this area which is associated with the anomalous atmospheric warming increase seen between April and May. Off the US west coast there is a moderate deepening of the mixed layer. Along the equator there is a slow deepening of the MLD on its eastern end and a slow shoaling on its western part. This is probably related to the corresponding sinking and shoaling of the thermocline at those locations (not shown but see our web site). This is consonant with the information displayed in Figure 2 in which we noted an eastward and upward displacement of positive anomalies.

This description is in line with those known to happen prior to an El Niño event (e.g., Philander 1990). If this tendency continues through the summer it might result in warm event by late 2001. This is in accordance with current long lead forecasts (e.g., 9 month lead) presented by other institutions. On the central and eastern tropical Pacific the isotherms move to the east (warming). This is related to anomalous model surface currents flowing eastward. We attribute this propagating feature to Kelvin waves traveling along the equatorial waveguide rather than to local wind forcing because the zonal wind stress anomalies are westward (not shown). The mean flow in this area flows westward.

Our hypothesis is that for these short 3-month lead oceanic forecasts the skill heavily depends on the accuracy of the initial conditions and atmospheric forcing rather than coupled ocean-atmosphere interactions, which will dominate for longer, lead times. For example, even with our very simple representation of the oceanic-atmospheric feedback (i.e., Newtonian damping), these forecasts qualitatively and quantitatively agree with others issued using highly sophisticated coupled models.

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