Relevance of relative sea surface temperature for tropical rainfall interannual variability

The coupling between sea surface temperature (SST) anomalies and rainfall is an important driver of tropical climate variability. Observations however reveal inconsistencies, such as decreased convective rainfall in regions with positive SST anomalies in the tropical Indian and Atlantic Oceans during and after El Nino-Southern Oscillation (ENSO) events. The upper troposphere warms during El Nino events, stabilizing the atmosphere. SST anomalies only account for the influence of the surface. Using theoretical arguments, we show that relative SST (RSST; defined as SST minus its tropical mean) anomalies, by also accounting for the influence of upper tropospheric temperature anomalies on gross moist stability, explain tropical convection interannual variations better than SST anomalies in observations and climate models. This relation further improves when RSST anomalies are weighted by the precipitation climatology to account for low and high-precipitation regimes. Using RSST is thus essential to understand El NinoSouthern Oscillation teleconnections and interactions between modes of tropical climate variability. Plain Language Summary Rainfall is an important driver of coupled air-sea climate variability in tropical regions, through its influence on atmospheric motions. Sea surface temperature anomalies influence tropical rainfall, as a warm anomaly is expected to destabilize the atmosphere, yielding a rising motion and increased rainfall. Decreased rainfall however sometimes occurs over positive sea surface temperature anomalies, notably during El Nino events, because these events tend to warm the upper atmosphere in the entire tropics, hence stabilizing the atmosphere and inhibiting rainfall. We show that the relative sea surface temperature, defined as the sea surface temperature minus its average over the tropical oceans, explains precipitation anomalies better than sea surface temperature. Relative sea surface temperature is thus a simple and powerful indicator to better understand year-to-year climate variability, notably El Nino-Southern Oscillation influence on rainfall in the Atlantic and Indian Oceans and its interactions with other tropical ocean-atmosphere coupled modes.