Seasonality in the biplot of Northern Hemisphere temperature anomalies

Abstract

Northern Hemisphere mean monthly temperature anomalies for the 1890–2010 period are examined for seasonality. The statistical method of biplotting visually synthesizes the major features of the time series for the 12 calendar months into a single plot. The common upward trend in all months and the winter–summer temperature contrast capture more than 80% of the total data variance. A temperature seasonalization is established: winter (January–March), summer (May–October) and transitional months (November, December and April). Two uncorrelated factors underlie this seasonality. The first is the North Atlantic thermohaline circulation, which is indicated by the Atlantic Multidecadal Oscillation and which statistically determines the summer temperature anomalies. The second is the cold ocean–warm land pattern, which is indicated by the hemispheric land–ocean temperature contrast and which statistically determines the winter anomalies. The interannual effect of the El Niño–Southern Oscillation, volcanism and also the Arctic Oscillation/North Atlantic Oscillation mode compete with the background trend to produce a few extreme and outlier years.