Synchronized tropical Pacific and extratropical variability during the past three decades

Internally generated decadal variability influences global mean surface temperature (GMST), inducing acceleration and slowdown of the warming rate under anthropogenic radiative forcing(1-4). While tropical eastern Pacific variability is important for annual-mean GMST(2,5-8), the cold ocean-warm land (COWL) pattern(9,10) also contributes to continental temperature variability(11-13) in the boreal cold season. Although the two contributors are physically independent(10,12), here we show that, after the mid-1980s, their decadal components vary in phase by chance to strengthen internal GMST trends, contributing to the early 2000s slowdown and early 2010s acceleration. The synchronized tropical Pacific and COWL variability explains the striking seasonality of the recent slowdown and acceleration during which the GMST trend in the boreal cold season is markedly negative and positive, respectively. Climate models cannot simulate the exact timing of the tropical Pacific and COWL correlations because they are physically independent, random-phased modes of internal variability. Natural decadal variability has a role in global mean surface temperature trends. Observational data and modelling show that since the mid-1980s, the tropical eastern Pacific variability and the cold ocean-warm land pattern have covaried to enhance acceleration and deceleration in warming trends.