Why does El Niño enhance the jet stream?

Question

As quoted over here. This is especially interesting to me because El Niño tends to warm up the climate, and warmer climates are generally associated with weaker jet streams.

El Niño has other effects further into North America. It tends to enhance the jet stream, creating a wall that prevents Arctic air (and the Polar Vortex) from dipping down to mid-latitudes. East Coast winters are generally drier and warmer during El Niño years, which is probably good news to those still smarting from this recent frigid season. The mild winter has interesting downstream effects, like a boost for the U.S. economy during the Christmas season.

Answer 1

I'm not sure if it's correct to say that El Niño enhances the jet stream, reading NWS they seem to state that the affect is more of a shift in the position of the jet stream over the Pacific and North America. I guess any enhancement would be due to a greater temperature contrast between the poles and equator.

Answer 2

Just to expand on the answer by @Siv:

Previous studies have indeed suggested teleconnections between the jet stream and ENSO (for example: Horel 1981). Recall that, at a fundamental level, the jet stream (a westerly geostrophic wind pattern) is driven by horizontal North-South temperature gradients across cold and warm air masses (thermal wind). Thus, it can be enhanced by increasing the temperature gradient of the two air masses.

Because winter frontal zones have characteristically larger temperature gradients than summer fronts, the jet stream is strongest during the boreal winter. It also moves equatorward to $30^o$ from its typical position in the summer of $50-60^o$.

And as discussed in Chunzai Wang, 2002:

During El Niño–La Niña events, the atmospheric westerly jet stream tends to move meridionally [...] El Niño (La Niña) corresponds to westerly (easterly) wind anomalies in the upper troposphere of the midlatitude Pacific, associated with the equatorward (poleward) displacement of the jet stream.

So, the enhancement/reduction is due to the displacement of the jetstream and associated increase/decrease in the temperature gradient across the cold/warm air masses that are generating the jet stream in the first place.

---

Horel, J. D., and J. M. Wallace, 1981: Planetary-scale atmospheric phenomena associated with the Southern Oscillation. Mon. Wea. Rev, 109, 813–829.

Chunzai Wang, 2002: Atmospheric Circulation Cells Associated with the El Niño–Southern Oscillation. J. Climate, 15, 399–419