Understanding the SAM influence on the South Pacific ENSO teleconnection

Abstract

The relationship between the El Niño Southern Oscillation (ENSO) and the Southern Hemisphere Annular Mode (SAM) is examined, with the goal of understanding how various strong SAM events modulate the ENSO teleconnection to the South Pacific (45°-70°S, 150°-70°W). The focus is on multi-month, multi-event variations during the last 50 years. A significant (p \ 0.10) relationship is observed, most marked during the austral summer and in the 1970s and 1990s. In most cases, the significant relationship is brought about by La Niña (El Niño) events occurring with positive (negative) phases of the SAM more often than expected by chance. The South Pacific teleconnection magnitude is found to be strongly dependent on the SAM phase. Only when ENSO events occur with a weak SAM or when a La Niña (El Niño) occurs with a positive (negative) SAM phase are significant South Pacific teleconnections found. This modulation in the South Pacific ENSO teleconnection is directly tied to the interaction of the anomalous ENSO and SAM transient eddy momentum fluxes. During La Niña/SAM? and El Niño/ SAM-combinations, the anomalous transient momentum fluxes in the Pacific act to reinforce the circulation anomalies in the midlatitudes, altering the circulation in such a way to maintain the ENSO teleconnections. In La Niña/SAM-and El Niño/SAM? cases, the anomalous transient eddies oppose each other in the midlatitudes, overall acting to reduce the magnitude of the high latitude ENSO teleconnection.

Winter and spring controls on the summer food web of the coastal West Antarctic Peninsula

“…Wind speeds and the number of windy days are significantly reduced during a À SAM in spring/summer (October-February, Pearson's R, Po0.005). The intensity of the local atmospheric response to SAM is also related to ENSO-SAM interactions 22,[29][30][31] . Previous studies have shown an intensified atmospheric response (and thus sea ice as well) when a þ SAM/La Niña or À SAM/El Niño are coincident (for example, a À SAM coincident with El Niño can amplify blocking high conditions favourable for high winter sea ice and/or late spring retreat), relative to the response when þ SAM or À SAM occurs when ENSO is otherwise neutral, and vice versa 22,30,31 .…”

The impacts of El Niño on the observed sea ice budget of West Antarctica

“…This is associated with a weakening of the ASL [Marshall and Thompson, 2016;Turner, 2004], resulting in increased sea ice in the Bellingshausen Sea and decreased sea ice in the Ross Sea [Kwok and Comiso, 2002;Dash et al, 2013;Kwok et al, 2016]. This tropical teleconnection and its influence on the Antarctic have been found to be enhanced when El Niño coincides with the negative phase of the Southern Annular Mode (SAM) [Fogt et al, 2011[Fogt et al, , 2012 • Supporting Information S1…”

Evolution of the Southern Annular Mode during the past millennium

“…We restrict the mid-latitude proxy data to the South American continent to produce a reconstruction that is specifically related to SAM variability in the Drake Passage sector. We do this because although the SAM is classically described as a zonally symmetric climate feature, its variability and tropical climate interactions have seasonal asymmetries that are particularly strong in the South Pacific and Antarctic Peninsula regions [13][14][15] . Analysis with European Centre for Medium-Range Weather Forecasts Reanalysis data (ERA-Interim) 16 since AD 1979 and climate simulations spanning the past millennium demonstrates, however, that on annual average and longer timescales SAM variability in the Drake Passage sector is highly representative of the circumpolar mean state of the SAM (Supplementary Figs 5 and 6).…”