How Northern High-Latitude Volcanic Eruptions in Different Seasons Affect ENSO

The impact of northern high-latitude volcanic (NHV) eruptions on El Nino-Southern Oscillation (ENSO) is investigated based on ensemble simulations with the Community Earth System Model. The seasonality of the atmospheric circulation influences the NHV aerosol dispersion, causing stronger (weaker) Northern Hemisphere cooling after the January and April (July and October) eruptions. ENSO's response is found to be more dependent on NHV eruption seasons than that on tropical eruption seasons. The January eruption causes an El Nino in an eruption year [year (0) hereafter] while an El Nino occurs in year (1) after the October eruption. No significant El Nino occurs after the April (July) eruption. A diagnostic analysis reveals that these El Ninos' developments are attributed to the positive zonal, meridional advective, and thermocline feedbacks, triggered by the western Pacific westerly anomalies. The anomalous North Pacific cyclone (NPC) and Asian monsoon are key systems to excite anomalous westerlies, which are caused by the NHV-induced midlatitude cooling and Eurasian continent-North Pacific thermal contrast. After the January eruption, the anomalous NPC develops in early summer and connects with a weakened Asian summer monsoon, which excites anomalous westerlies over the Indo-western Pacific, activating the Bjerknes feedback. For the October eruption, the anomalous NPC and enhanced East Asian winter monsoon bring cold air to the Maritime Continent and warm the subtropical central North Pacific through surface heat flux exchange, exciting the westerly anomalies. These results suggest that the strong dependence on the seasonal timing of NHV should be a critical element of data-model comparisons.