The Surprising Skill of Sea Surface Salinity in Seasonal Prediction of Precipitation on Land
[12-Dec-2018] Schmitt, R.W.
Presented at the 2018 AGU Fall Meeting
Most water on Earth is in the ocean and it is the ultimate source of terrestrial rain. The latent heat flux due to evaporation also dominates the transfer of energy from ocean to the atmosphere and land. While about 50 % of surface evaporation from the ocean falls back as local precipitation, the rest is exported from the evaporation-dominated subtropical high pressure systems. This generates high sea surface salinity (SSS) in the subtropical gyres and low SSS in the high and low latitude oceans and coastal regions that receive runoff. About one third of the water exported from the subtropical oceans rains out on land. Anomalously large water export leads to higher SSS, guaranteeing that some part of the climate system will experience more rain; lower SSS indicates less export and less rain elsewhere. We have found that seasonal anomalies in SSS in particular areas of the ocean have remarkable skill for predicting future rainfall in certain regions on land. That is, springtime SSS anomalies in different regions of the North Atlantic are good predictors of summer rain in the Sahel of Africa and the US Midwest, with significantly higher skill than any of the usual climate indices (ENSO, NAO, etc). The delay mechanism involves soil moisture and feedbacks on the atmospheric circulation. Using a global analysis of fall SSS anomalies, we find even stronger predictability of winter time precipitation in the US Southwest. SSS can be superior to SST for rainfall predictions in different regions, seasons and lead times, with the best predictability generally in the summer and winter seasons. SST tends to be a better predictor for short time leads (weeks) and SSS better for long time leads (months). Rainfall predictability is analyzed using multiple linear regression and artificial intelligence techniques and seasonally quantified in the US, Europe and Africa. The use of such ocean based prediction schemes to win a substantial cash prize in a rainfall forecasting contest for the US West is described. These discoveries promise to improve sub-seasonal to seasonal prediction of rainfall on land and emphasize the importance of sustained ocean observations in our rapidly changing climate.