Tuesday, 11 January 2005: 11:30 AM
Climate response over North America to a simultaneous El Niņo and volcanic eruption
Both El Niņo/Southern Oscillation (ENSO) and volcanic eruptions affect the North American climate. Between the years of 1982 and 1998, the two strongest El Niņos on record and two major tropical volcanic eruptions occurred. Using the extended empirical orthogonal function (EEOF) technique, we identified El Niņo and La Niņa years between 1982-1998. We analyzed monthly precipitation observations from the CMAP data set and NCEP/NCAR reanalysis of monthly temperature and 200 mb zonal wind using relative extrema of the principal component time series from the EEOF analysis to allow us to obtain a composite average of the remote response over North America during a winter El Niņo. We also conducted simulations with various versions of the GFDL's AM2/LM2 atmospheric general circulation model (AGCM), driven by observed sea surface temperatures (SSTs), for the period 1982-1998. The simulations also included an ensemble of 50-year long control runs, all initialized in January of 1950 and an ensemble of 17-year long control runs using newer ozone parameterization, initialized January 1, 1982. We also conducted 17-year perturbed experiments forced by observed volcanic aerosols. For the 50-year runs, we conducted ten ensemble members and analyzed the mean. For the 17-year runs, we conducted four ensemble members for control and volcanic forcing, and analyzed the mean. The first EEOF of SST anomalies in the equatorial Pacific is the El Niņo signal, which explains 67% of the variance during the winter months (November through March). The precipitation field in both the observations and the atmospheric model displays an ENSO-related signal during the winter months in the southeast region of the United States, the Pacific Northwest, and the California coast. The 200 mb zonal wind also displays an ENSO-related signal in the atmospheric model and observations. During the warm phase of ENSO, the subtropical jet strengthens and the polar jet weakens. These two mid-latitude jet streams dictate the path that cyclones follow, an explanation for the increased precipitation in certain areas throughout the continent. Significance tests performed on spatial correlations between the observations and the AGCM data confirm that the ENSO signal in the model is significant at the 99.9% confidence level. The AGCM ENSO-related surface temperature signal is poorly correlated with the observations, suggesting that the volcanic signal alters the ENSO signal between 1982-1998. However, forcing from the June 1991 Mt. Pinatubo eruption does not significantly change the remote response of temperature, precipitation and 200 mb zonal wind during ENSO winters in North America in the atmospheric model. Reasons for this counterintuitive result are discussed.