Weak El Niño and winter climate in the mid- to high latitudes of eurasia

Abstract

According to the sea surface temperature anomaly (SSTA) intensity in the Niño-3.4 region and the east- west gradient across the Pacific, three types of El Niño are identified in this work. An event with larger than average intensity is defined as a strong El Niño, all others are considered to be weak events. Almost all strong El Niños are concurrent with a large gradient, which is featured by negative SSTAs in the western Pacific and positive SSTAs in the equatorial eastern Pacific (EP) and Indian Ocean (IO). According to the east-west gradient, the weak events can be subdivided into gradient-weak (GW) El Niño and equatorial-weak (EW) El Niño. The GW El Niño characterizes a great east-west gradient without a significant IO SSTA. In contrast, theEWevent features a positive SSTA over the tropical IO and EP. The impact ofGWEl Niño on the North Atlantic-Eurasia continent (NA-Eurasia) displays a negative North Atlantic Oscillation (NAO)-like atmospheric anomaly, resulting in a drier and cooler-than-normal winter over Eurasia. Observational and numerical evidence indicate that the prolonged subtropical jet from the North Pacific to NA acts as a waveguide that captures the planetary Rossby waves generated by the GW El Niño. This waveguide favors the propagation of the perturbations into the downstream regions, which would affect the NA-Eurasian climate. However, theEWEl Niño is accompanied by a relatively weak subtropical jet that cannot impact the NA-Eurasian climate significantly. For the strong El Niño, the absence of theNAOsignal can be attributed to the counteracting of the teleconnections triggered by the Pacific and the tropical IO.