Abstract
Abstract This study uncovers an early 1990s change in the relationships between El Niño–Southern Oscillation (ENSO) and two leading modes of the Southern Hemisphere (SH) atmospheric variability: the southern annular mode (SAM) and the Pacific–South American (PSA) pattern. During austral spring, while the PSA maintained a strong correlation with ENSO throughout the period 1948–2014, the SAM–ENSO correlation changed from being weak before the early 1990s to being strong afterward. Through the ENSO connection, PSA and SAM became more in-phase correlated after the early 1990s. The early 1990s is also the time when ENSO changed from being dominated by the eastern Pacific (EP) type to being dominated by the central Pacific (CP) type. Analyses show that, while the EP ENSO can excite only the PSA, the CP ENSO can excite both the SAM and PSA through tropospheric and stratospheric pathway mechanisms. The more in-phase relationship between SAM and PSA impacted the post-1990s Antarctic climate in at least two aspects: 1) a stronger Antarctic sea ice dipole structure around the Amundsen–Bellingshausen Seas due to intensified geopotential height anomalies over the region and 2) a shift in the phase relationships of surface air temperature anomalies among East Antarctica, West Antarctica, and the Antarctic Peninsula. These findings imply that ENSO–Antarctic climate relations depend on the dominant ENSO type and that ENSO forcing has become more important to the Antarctic sea ice and surface air temperature variability in the past two decades and will in the coming decades if the dominance of CP ENSO persists.
Highlights
The linkages between the Southern Hemisphere (SH) climate and the tropical Pacific sea surface temperatures (SSTs), those associated with El Niño– Southern Oscillation (ENSO), have been extensively studied (e.g., Karoly 1989; Mo 2000; Yuan 2004; Fogt and Bromwich 2006; L’Heureux and Thompson 2006; Lee et al 2010)
The EP ENSO is the conventional type of ENSO similar to that portrayed by Rasmusson and Carpenter (1982), which typically onsets and develops near the South American coast and has its SST anomalies centered in the equatorial eastern Pacific
We examined the decadal variations in the relationships among ENSO, the southern annular mode (SAM), and the Pacific–South American (PSA) to understand how the ENSO influence on the high-latitude Southern Hemisphere has varied during the past seven decades
Summary
The linkages between the Southern Hemisphere (SH) climate and the tropical Pacific sea surface temperatures (SSTs), those associated with El Niño– Southern Oscillation (ENSO), have been extensively studied (e.g., Karoly 1989; Mo 2000; Yuan 2004; Fogt and Bromwich 2006; L’Heureux and Thompson 2006; Lee et al 2010). We use observational and reanalysis datasets to examine if and how the early 1990s change of ENSO type affects the relationships among ENSO, PSA, and SAM and how these changing relationships impact SH climate variability. ENSO forcing to high-latitude Southern Hemisphere: 1) an eddy–mean flow interaction mechanism in which an El Niño–induced intensification of the Hadley circulation and equatorward shift in the subtropical jet influence the propagation of midlatitude transient eddies and their associated eddy–mean flow interactions to give rise to a negative phase of the SAM (e.g., Seager et al 2003) and 2) a stratospheric pathway mechanism in which El Niño events affect the propagation of planetary waves into the stratosphere and induce polar temperature and vortex anomalies that subsequently descend into the troposphere to excite the SAM (e.g., Mechoso et al 1985; Hurwitz et al 2011; Son et al 2013; Evtushevsky et al 2015).
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