General Circulation Model Diagnostic and Predictability Studies

Accomplishments FY96

Model Sensitivity Studies

A detailed investigation has been conducted of the sensitivity of the NCEP atmospheric seasonal forecast model to tropical Indo-Pacific SST anomalies, with a view of identifying those tropical areas that have the greatest impact on the seasonal mean circulation over North America. A related but separate investigation has assessed the relative impact of SST anomalies in the Nino-3 and Nino-4 regions. Particular attention was paid to the nonlinear aspects of the response to the SST anomalies in these regions. It was found that the response is progressively more nonlinear as one moves farther downstream of the PNA region into the Atlantic sector. In particular, the GCM simulates the same sign of an North Atlantic Oscillation-like pattern for both positive and negative SST anomalies in Nino-4.

A study has been conducted on the extent to which the different patterns of tropical Pacific sea surface warming, which conventionally are combined into a single "El Niño" pattern, drive different climate signals. In other words, is the atmosphere sensitive to the departures of an individual El Niño event's SST forcing from the composite El Niño structure? If so, then there is more boundary-forced seasonal predictability than implied by simple ENSO composite climate signals, and prediction of the detailed SST distribution becomes especially important. This problem has been addressed by analyzing the NCEP GCM's response to the tropical Pacific SST forcing for various observed El Niños in the 1950-95 period. The GCM response consists of a 13-member ensemble of integrations, thereby ensuring detection of the climate signal for each event. The results reveal that the spatial pattern of the extratropical circulation response consists largely of a single deterministic structure. Thus, although the observed seasonally-averaged circulation anomalies differ appreciably from one El Niño event to another, these circulation differences are apparently unrelated to differences in tropical SST forcing, and thus appear unpredictable from knowledge of that forcing alone. The mechanisms for this apparent insensitivity are being explored, and the robustness of the NCEP model results are also being explored through analyses of similar experiments using other GCMs, including the GFDL and NCAR climate models.

GFDL Consortium Project

In support of the GFDL-University Consortium project, CDC scientists have developed both a print and Web-based atlas of atmospheric fields using a recent GFDL GCM simulation. Both atlases include standard fields, such as the mean and standard deviation of precipitation, and more unique analyses, such as the model's representation of the Madden-Julian Oscillation and the mid-winter minimum in storm activity over the North Pacific Ocean. The web-based atlas, which is linked from <URL:http://www.cdc.noaa.gov/gfdl/>, consists of a series of menus that allow the user to focus on a given region, and/or specify the space and time over which the model field will be averaged. A similar web-based atlas of NCEP Reanalysis data is also available from CDC, enabling direct on-line comparisons between the GFDL GCM simulations and the reanalysis climate data.

A GFDL GCM simulation has been used to study the surface flux variability over the North Pacific and Atlantic Oceans during winter. The surface flux anomalies are organized by the low-level atmospheric circulation, in agreement with previous observational studies. Surface flux variability in the 3-10 day time band is clearly associated with mid-latitude storms. The SLP and surface flux anomalies are also strong in the 10-30 day band but are located further north, are broader in scale, and propagate ~3-4 times more slowly eastward than the synoptic disturbances. More than half of the variability occurs on sub- monthly timescales. The anomalous wind speed has the greatest influence on surface flux anomalies in the subtropics and western Pacific, while the flux anomalies are more closely associated with air temperature and moisture anomalies in the northeast Pacific and over the Atlantic north of 40N.

General Circulation Model Studies of Air-Sea Interactions

The role of air-sea interactions on climate variability has been examined by comparing a simulation in which an atmospheric GCM is coupled to a mixed layer ocean model in the North Atlantic to a control simulation where climatological North Atlnatic SSTs are specified. The dominant pattern of the anomalous air temperature, which is similar in the two runs, decays much more slowly in the coupled run. The enhanced persistence of this pattern is tied to two processes: reduced thermal damping and the re-emergence of SST anomalies. There is a reduction in the damping of air temperature anomalies in the coupled run, as the ocean can adjust to the overlying atmosphere rather than constrain it. The re-emergence mechanism occurs when thermal anomalies in the deep ocean mixed layer in winter are stored within the summer seasonal thermocline and return to the surface when the mixed layer deepens again in the following fall. This happens in the ocean model in the northwest Atlantic, in agreement with observations. These results have implications for the persistence of climate regimes over the North Atlantic and Europe.

Prediction of Tropical Sea Surface Temperatures

Investigations have been conducted on the predictability of tropical Atlantic and tropical Indo-Pacific sea surface temperature anomalies. Related to this work, real-time forecasts of tropical Indo-Pacific SST anomalies are now being published monthly in the Climate Diagnostics Bulletin, and quarterly in the Experimental Long-Lead Forecast Bulletin. Preliminary investigations with tropical Atlantic SST anomalies indicate that skillful forecasts may be made for the northern tropical Atlantic (6N-18N), with SST in the Nino-3 region providing a 6- to 8-month predictor. Forecasts in the equatorial (6N-6S) and southern (6S-18S) tropical Atlantic so far have been less skillful, but continue to be explored.

Plans FY 97

Model Sensitivity Studies

The following activities will continue to be performed in collaboration with scientists at NCEP's Climate Modelling Branch, the NOAA/ERL Geophysical Fluid Dynamics Laboratory (GFDL), and the Universities of Colorado and Illinois:

• Determine the sensitivity of the North American climate to details of tropical Pacific forcing. The sensitivity to both SST forcing and to diabatic forcing associated with deep convection will be examined. The GCM component of this study will continue to use the NCEP climate forecast model so as to further understand the predictability characteristics of that system, and, more generally, to estimate the upper-limits of dynamical seasonal predictability.

• Analyze the seasonal dependence of the climate response to ENSO. In particular, determine the importance of the phase relation between the climatological seasonal cycle and the seasonal cycle of anomalous SST forcing associated with El Niño.

• Diagnose a 13-member ensemble of 45-year runs for 1950-95 with prescribed global SSTs made with the same NCEP GCM. This model will also be used in the development of a two-tiered seasonal forecasting system which prescribes SST anomalies predicted by Penland's statistical ENSO model.

• Continue development of a three-pronged approach to the extratropical seasonal prediction problem, given Penland's seasonal tropical SST forecasts. This work involves the following parts:
  1. Develop an empirical regression model for predicting extratropical 500 mb heights, surface temperature and precipitation, given anomalous seasonal-mean tropical SSTs;
  2. Develop an intermediate regression-dynamical model in which a tropical diabatic heating field is predicted from empirical heating-SST relationships. This heating is then specified as a diabatic forcing in an adiabatic atmospheric GCM to produce a seasonal extratropical forecast, and
  3. Develop a prediction system in which Penland's SST forecasts are fed directly into the NCEP atmospheric GCM as anomalous boundary conditions, and the GCM is run to produce a seasonal extratropical forecast.
  An ensemble of model runs, with different initial conditions, is planned for approaches (2) and (3). Sufficient runs will be performed to estimate the altered probability distribution functions of extratropical circulation indices for the predicted seasonal SST anomaly pattern.

GFDL Consortium Project

• Document the variability of surface fluxes in the GFDL GCM and NCEP reanalyses, and investigate the relationship of the fluxes to SST anomalies and the overlying atmospheric circulation.

General Circulation Model Studies of Air-Sea Interactions

• Examine the extent to which the winter-to-winter re- emergence of SST anomalies occurs within the North Atlantic and North Pacific Ocean using new basin-wide ocean datasets.

Prediction of Tropical Sea Surface Temperatures

• Experimental real-time forecasts of the northern tropical Pacific will be provided, starting early 1997.

• Further diagnostic analyses will be performed of the stochastic forcing of Indo-Pacific SST anomalies, concentrating on the interplay of heat fluxes, wind stresses, and mixed-layer depth.