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Tropical terrestrial ecosystems are the critical driver of the land carbon cycle and climate change. Previous studies have reported strong response in the tropical land carbon cycle to precipitation anomalies in post-La Nina years. We show that among the post-La Nina years, 1979 and 1996 stand out strikingly different from other years. 1979 has one of the largest atmospheric CO2 growth rate (CGR) while 1996 has one of the smallest CGR, despite that their annual tropical precipitation anomalies are small. We show that these 2 years correspond to opposite seasonal precipitation anomalies over tropical vegetated lands in the wet-to-dry transition seasons in which negative correlation between precipitation and CGR is the strongest. 1979 has large dry anomaly during the transition season, inhibiting land carbon uptake, while 1996 has large wet anomaly during the transition season, favoring more carbon uptake by the tropical ecosystem. Results here highlight the importance of seasonality of climate anomalies and their timing of occurrences with the rhythm of the carbon-climate coupling.