Trends of continental, zonal and seasonal land temperatures in the 20th century

Abstract

We study the evolution of continental, zonal and seasonal land temperature anomalies especially in the early 20th century warming (ETCW) period, using principal component analysis (PCA) and reverse arrangement trend analysis. ETCW is significant in all other continents except for Oceania. Warming in South America is significant from the ETCW onwards, but significant recent warming started in North America and Europe only around 1990. The zonal and seasonal PC2s are both correlated with AMO index, but zonal PC3 is related to Southern oscillation index (SOI) and seasonal PC3 best correlated with wintertime El Nino (NINO34 DJF index). In the southern hemisphere, the recent warming starts first closest to the equator in the 1950s and latest in the southernmost zone in the late 1970s. In the two lowest northern zones (EQ-N24, N24-N44) the warming is significant since the ETCW, and increased warming starts in 1970s, but in two northernmost zones (N44-N64, N64-N90) the cooling after the ETCW delays the start of recent warming until around 1990. All seasons of the northern hemisphere but no season in the southern hemisphere depict a significant ETCW. All the three PCA have almost common PC1 component for the analyzes 1910-2017, i.e., gradual increase of temperature until 1940s, period of declining towards the end of 1950s, a flat phase until the second half of 1970s and steep rise after that. However, the continental PC1 explains only 75.2 % of the variation of the data, while zonal and seasonal PC1s explain 81.7 % and 87.6 % of the corresponding data, respectively.