Waxing and Waning of Observed Extreme Annual Tropical Rainfall

TL;DR

This study shows that the probability of extreme annual tropical rainfall events has increased recently, with spatially coherent changes linked to ENSO phases, indicating a fundamental mode of natural variability in tropical rainfall extremes.

Contribution

It reveals a clear association between ENSO phases and the reversal of rainfall extremes, highlighting a natural variability mode affecting tropical rainfall patterns over decades.

Findings

Increased likelihood of extreme rainfall events in recent decades.

Reversal of rainfall extremes between warm and cold ENSO phases.

Spatial coherence of rainfall changes across land and ocean.

Abstract

We begin by providing observational evidence that the probability of encountering very high and very low annual tropical rainfall has increased significantly in the recent decade (1998-present) as compared to the preceding warming era (1979-1997). These changes over land and ocean are spatially coherent and comprise of a rearrangement of very wet regions and a systematic expansion of dry zones. While the increased likelihood of extremes is consistent with a higher average temperature during the pause (as compared to 1979-1997), it is important to note that the periods considered are also characterized by a transition from a relatively warm to cold phase of the El Nino Southern Oscillation (ENSO). To further probe the relation between contrasting phases of ENSO and extremes in accumulation, a similar comparison is performed between 1960-1978 (another extended cold phase of ENSO) and the aforementioned warming era. Though limited by land-only observations, in this cold-to-warm transition, remarkably, a near-exact reversal of extremes is noted both statistically and geographically. This is despite the average temperature being higher in 1979-1997 as compared to 1960-1978. Taken together, we propose that there is a fundamental mode of natural variability, involving the waxing and waning of extremes in accumulation of global tropical rainfall with different phases of ENSO.