The increased frequency of -EPOs in the 2010s (especially from 2012-13 & beyond) is a question the research community I actively participate in is digging into and I can speak for those who are working to solve this that it's still unknown atm. NAO/AO is believed by many of my colleagues and the community at large to have a significant component that's tied to multidecadal natural variability but there's a large contingency like myself that thinks we're pushing the system to favor +NAOs as the climate warms, in which one part involves the Hadley Cells expanding which pushes the storm tracks and relatively warmer subtropical highs poleward. The mid-latitude jet also strengthens at the height of the tropopause even as the surface meridional temperature gradient decreases as the arctic warms disproportinately faster than the tropics.
More intense Rossby Waves in the northern hemisphere due to the existence of the Himalayas masks any long-term trend attributable to the background climate but the isolation of the southern vortex allows these tendencies to manifest and show up in the data more easily. The two main competing schools of thought on what controls the southern vortex's variability are that it's either the aforementioned influence from the background climate or ozone destruction from CFCs earlier in the 20th century have predisposed the Antarctic vortex to remain intense well into the early-mid 21st century as it tries to steadily recover. Other factors are likely contributing but there's no denying we've been observing a statistically significant positive, upward trend in the antarctic oscillation and intensity of the southern vortex over the course of the reliable record.
The northern vortex has also become more positive in the last few decades (i.e. more +AO/NAO since the satellite era began), but like I said earlier, because the Northern Hemisphere has huge mountain ranges, especially the Himalayas, which significantly amp up the intensity of the longwave troughs and ridges here, it's likely masking any underlying trend that would be due to CFCs, a warming background climate, other lurking variables or all of the above.
I'd definitely be willing to wager we'd see a much bigger trend towards +NAO/+AOs in recent years and decades if it wasn't for the existence of the Himalayas.
Anyway lol didn't mean to rant but thought a few tidbits of information like this would be useful to some here.
Seasonally averaged southern annular mode (SAM) (essentially the equivalent of the AO but in the southern hemisphere)
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