Impact of spring land temperature anomaly on summer downstream droughts/floods – A new GEWEX/GASS Initiative (ILSTSS2S)
Prof. Yongkang Xue
University of California, USA
SST variability has been shown to have predictive value for land precipitation and significant progress has been achieved in the SST research. Prediction skill for precipitation anomalies in spring and summer months, however, has remained stubbornly low for many years. For instance, during 2015-2016 one of the strongest El Nino events since 1950 was associated with an extraordinary Californian drought, while a 2016-2017 La Nina event has been associated with record rainfall that effectively ended the 5-year Californian drought, contrary to the expected SST - drought/flood relations. While atmospheric internal variability undoubtedly contributes, the sub-seasonal to seasonal (S2S) prediction problem underscores the need to seek explanations beyond SST’s influence alone, to pursue the identification of new mechanisms contributing to droughts/floods and add value for intraseasonal prediction. The GEWEX/GASS Initiative “Impact of initialized land temperature and snowpack on sub-seasonal to seasonal prediction (ILSTSS2S)” puts forward a new approach, which complements SST, snow, and soil moisture research by exploring the possible remote effects of large-scale spring land surface temperature/subsurface temperature (LST/SUBT) anomalies in geographical areas upstream on summer drought or flood- a concept that has been largely ignored by previous extreme hydroclimate events studies.
Preliminary studies have been carried out to explore the relationship between spring LST/SUBT anomalies and summer precipitation anomaly in downstream regions in North America and East Asia (Xue et al., 2016, 2018). Using the worldwide available 2-meter temperature (T2m), which has relatively reliable quality, and maximum covariance analysis method, we have identified that the warm (cold) spring T2m anomaly in western U.S. is associated with the summer wet (dry) conditions in Southern Great Plains and adjacent regions (SGP) and opposite anomaly to the north; it also suggests that warm (cold) spring T2m anomaly in Tibetan Plateau is associated with the summer wet (dry) conditions to the south of the Yangzi River and opposite anomaly to the north. The significant correlation between these two variables is comparable to the well-known SST and precipitation correlations.