A Comparison of Historical versus Recent Precipitation Response in Central Texas to Oceanic Niño-3.4 Index Sea Surface Temperature Anomaly

The El Niño Southern Oscillation (ENSO) and associated Oceanic Niño Index 3.4 (ONI) provide one of the best sources of long-range precipitation forecast skill in Central Texas. Historically, a warm anomaly of the sea surface temperatures in the Pacific equatorial region (El Niño) most often is associated with increased precipitation in Central Texas, particularly in October through June. A cold anomaly of the Pacific sea surface temperatures (La Niña) most often is associated with decreased precipitation in Central Texas. ENSO-neutral periods have shown a variable pattern of precipitation. Forecasters recently have raised questions about whether historical ENSO relationships are still relevant. We have examined the relationships between Pacific sea surface temperatures and precipitation in Central Texas to see if the relationships have continued into the current millennium. Using a 2 degree region of Central Texas from latitude 30° to 32° and longitude -98° to -100°, we analyzed monthly precipitation from 1950 to 1999 and compared it with precipitation from 2000 to 2017 for all months and ENSO conditions. Monthly precipitation data was transformed using a square root function to improve normality prior to parametric analysis and then untransformed for reporting purposes. Based on period averages, precipitation under La Niña and neutral conditions seems unchanged, and under El Niño, conditions have been somewhat wetter. Variations are less than one standard deviation from the prior period for all but July under El Niño conditions. Evaluation of parametric and nonparametric tests for each of the months and conditions reveals precipitation likely is unchanged between the two periods in all but July and October under El Niño conditions. The uncertainty in July and October depends on the chosen confidence level. Due to limited observations of monthly conditions in the recent period, a longer observation period is needed to be more conclusive that the recent period is wetter. Statistically we can say with about a 92 percent confidence, El Niño brings more precipitation than La Niña to Central Texas for the months of February, March, May, June, October, November and December which coincides with the seasonality of ENSO anomalies. We conclude ENSO continues to impact Central Texas precipitation in this recent period similarly to historic periods, and the use of ENSO can continue to provide skill in long-range precipitation forecasting.