Changes of reanalysis-derived Northern Hemisphere summer warm extreme indices during 1948–2006 and links with climate variability

Using1948–2006 surface 2 m daily temperature, daily maximum temperature and daily minimum temperature of National Centers for Environmental Prediction (NCEP) reanalysis dataset, summer warm extreme indices, warm days (TG90P), warm-spell days (WSFI), warm day-times (TX90P) and warm nights (TN90P) are calculated for Gaussian grids, a complete Northern Hemisphere (NH) picture of changes of summer warm extremes is presented, and their links with El Niño/La Niña & Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and North Atlantic Oscillation (NAO) are also examined in this paper. The results on the NH summer seasonal basis are as follows. Warm days, warm-spell days, warm day-times and warm nights increase at rates of 2.18, 1.23, 2.31 and 2.00 days/decade respectively during 1948–2006. A broader area is exposed to frequent occurrence of warm extremes in the recent 30 years than in the last 30 years. The 59-year long-term upward trend is characterized by a positive trend reversion in the late 1970s, with a slight downward trend in the last 30 years and a rapid upward trend in the recent 30 years, representing the main form of interdecadal variance of NH warm extremes. Warm days, warm-spell days, warm day-times and warm nights increase at rates of 4.53, 3.36, 4.44 and 4.21 days/decade respectively during 1977–2006. During 1948–2006, the largest increasing rate is at central tropical Atlantic and the largest decreasing rate in Mongolia and north China. Significant (level of 0.05) upward trends cover about half of the NH during 1948–2006 and about a third of the NH during 1977–2006 with the very significant upward trends more focused, while very sparse regions have significant downward trends during these two periods. In the recent 30 years, although NH-land summer warms at a faster rate than NH-water, warm extremes on NH-water increase much faster than those on NH-land, the average warm extreme indices and their increasing trends on NH are most modulated by those on NH-water. TX90P increases more rapidly than TN90P and significant upward (downward) trends in TX90P have more (less) areal coverage than significant upward (downward) trends in TN90P on NH-land during 1948–2006. ENSO mainly modulates interannual variations, especially during PDO warm phase after the 1980s, and it has opposite impacts in Indian Ocean, South China Sea, south and southeast Asia coasts and in the interior Asia. PDO mainly controls the long-term significant upward trend in the central tropical Atlantic and downward trend in Mongolia and north China. NAO indices of different seasons have different impacted locations, the most obvious positive trend reversion in the late 1970s in south Greenland offshore is related to the interdecadal variation of summer NAO. An abrupt shift of NH summer diurnal temperature range in NCEP reanalysis data in 1967 suggests that the derived results related to the very high minimum temperature on NH-land before 1967 be accepted with care.