Emergence time of regional signals in tropical rainfall and sea surface temperature in CMIP5/6 simulations

The concept of emergence time allows to define when anthropogenically-forced signals become larger than the ambient natural climate noise and thus become detectable. While anthropogenic signals in globally-averaged Sea Surface Temperature (SST) usually emerge quite early in the 20th century, regional SST changes are more difficult to detect due to large aliasing by internal climate variability. Yet, changes in relative SST (RSST, SST minus its tropical mean) influence the stability of the atmosphere, and hence rainfall or extreme events like cyclones. Here, we focus on regional SST trends by computing the RSST emergence time in CMIP5/6 simulations and investigate their relationship with rainfall emergence time.

We first propose a new method for estimating the emergence time, based on an actual significance estimate rather than a simple signal to noise ratio, and compare the results with the estimates from traditional methods. By 2100, CMIP projections indicate enhanced warming relative to the tropical mean (positive RSST signal) in the equatorial Pacific, equatorial Atlantic, and the Arabian Sea, and reduced warming in the three subtropical gyres of the southern hemisphere. In broad agreement with observations, the Arabian Sea relative warming and South-Eastern Pacific relative cooling are already detectable in most models (median emergence time < 2020), making those regions suitable for testing a model's ability to predict a regional SST trend. In contrast, the RSST signals in other regions only become detectable after 2050. Patterns of rainfall changes are broadly consistent with the above RSST signals (more/less rain in positive/negative RSST area) but generally emerge one or two decades later. The only region where a rainfall signal emerges before an RSST signal is the central and eastern tropical Pacific, where increasing rainfall signals emerge around 2050 (CMIP median). The absence of currently-detectable regional rainfall trends in CMIP makes it difficult to validate climate models' ability to predict tropical regional rainfall trends.