Both of these lead to the development of this pattern, which often becomes more strong/persistent late winter as it interacts w/ the stratospheric polar vortex via wave reflection.

Here's what this -EPO/+TNH or "ABNA" pattern looks like in winter (annotated over their fig 1)

Worth mentioning, this year has the 2 main precursors to a -EPO/+TNH ("polar vortex" pattern) dominated winter in place atm as identified by Zhong & Wu (2023).

Namely, a warmer-than-normal Maritime Continent & West Pac as well as a Eurasian Snow Cover Dipole.

journals.ametsoc.org/view/journal...

Extending the analysis back to 1900 in 20CR (& using the Bronnimann et al (2007) QBO reconstruction) reveals a similar pattern for IPWP strength during eQBO/-ENSO winters:

Imho this SST trend is a "fast" ocean dynamical thermostat response to the increasingly warmer climate

The atmosphere's hydrostatic adjustment to vertical advection of water from depth that warms less quickly in the Eq Pacific is reinforced by Bjerknes feedback, making the trend look "La Nina-like"

This is what the 500mb difference looks like for these east QBO/cool ENSO winters based on IPWP warm pool strength (left)

This is almost exactly what the long-term 500mb trend looks like (right), which isn't surprising given that IPWP is warming at a faster rate than the adjacent tropical oceans

Much like last year, the very warm Tropical Indo-Pacific Warm Pool (IPWP) is going to play a major role in this coming winter.

Compare the current global mean SSTa (left) to the Oct-Nov Warm Pool SST [20S-20N, 90-150E] difference for east QBO/cool ENSO winters only (right)

Of course there are caveats w/ 20CR, but breaking it down by month after Nov-Dec I thought it was neat to see how much more sudden -IOD winters "snap" to a canonical Nina look in Feb

Also, Vimont's SFM pattern that usually precedes El Niño later in the yr is working overdrive in Feb-Mar of -IOD yrs

Finally getting a chance to revisit this thread after Melissa...

I certainly thought it was interesting when I separated weak-moderate La Niña winters *only* by IOD phase you also get the Nov-Dec +PNA tendency & +EPO (much like this year) in -IOD years. Also seems to sniff out the -AAO too.

Multi-panel view of the last few days of Hurricane Melissa:

↖️ GOES-19 infrared brightness temp
↗️ GOES-19 visible satellite
↙️ Hurricane hunter planes & flight paths
↘️ Recon-derived flight level wind swath
⬇️ Estimated minimum pressure from recon dropsondes

Dry air intrusions are suggested to not occur in the right juxtaposition relative to a TC for that to happen in +SRH environments. I think the +SRH in Melissa's environment played at least some role in its overall lack of eyewall replacement cycles.

The "showerhead" effect where stratiform evaporates and/or sublimates, cooling/moistening and preconditioning the mid-levels for later CI in the upshear-right quad is part of the vehicle for this to occur...

This modeling study points out that negative buoyancy-driven low-level subsidence from dry air intrusions are more likely to couple with super gradient flow from the inner core and converge in the low-levels near the inner core and favor secondary eyewall formation (SEF)...

One characteristic of Hurricane #Melissa that'll always stick w/ me is its lack of "classic" eyewall replacement cycles.

I honestly can't help but think the environmental +SRH was a factor for that, as this modeling study from earlier this yr showed:

journals.ametsoc.org/view/journal...

Yep that’s what it looks like 😢

Wow. This is easily the most ridiculous dropsonde I’ve ever seen.

188 knot mean winds in the low-levels with gusts over 250 mph

Absolutely scary and historic hurricane headed into SW Jamaica this morning

Let's play a game of find where local sunset occurred over Hurricane #Melissa

Extreme supergradient flow coupled w/ a deeper inflow layer greatly changes the distribution of winds in the low-levels of a very intense hurricane like #Melissa where you get a strong/shallow low-level jet that more easily mixes to the sfc

We can’t treat all TCs the same in this regard

After the conclusion of this hurricane season, we need to have a real, serious discussion about the utility and validity of sticking with standard reduction techniques for estimating sustained surface winds from flight level in really intense hurricanes.

This latest dropsonde in Hurricane Melissa is mind boggling.

185 knot mean winds in the lowest 150m!!!

This reduces to about 155 knots at the surface, a good bit higher than the current NHC estimate of 145 kt and actually very much in line with satellite consensus (SATCON)

Yep.

Even standard reduction techniques on these sonde profiles or even from flight level winds are probably not as reliable with high-end hurricanes like this due to more efficient BL mixing & a stronger LLJ.

Alex reduces these winds to the surface and accounts for these sort of things. Dropsondes are extremely unlikely to sample the strongest volume of wind in a hurricane, and studies like the one linked here have shown that this leads to an underestimate of the winds by at least 10%, if not more.

Even with all of the structural, intensity, and SST/OHC changes underneath Hurricane #Melissa the last few days, the diurnal cycle still remains undefeated and inevitable

The crazy thing is, even a dropsonde that hits the “perfect” spot in a hurricane is likely to underestimate the winds by at least 10% due to sampling biases

journals.ametsoc.org/view/journal...

Holy mackerel, what a dropsonde….

This explicitly supports Melissa being a 145-150 knot hurricane 😳

Melissa strengthened into a cat 5 hurricane overnight, w/ winds of ~160 mph & central pressure of 913mb.

Even more strengthening is likely later today, which may put this storm in pretty rarified air historically for Atlantic hurricanes.

Awful situation unfolding for much of Jamaica

This is probably the most tone deaf take I've seen from a professional on wxtwitter and social media more broadly speaking during a natural disaster. Not a good look.

Man tropical weenies are getting trolled hard tonight

It is worth mentioning too that Dvorak and other satellite based intensity estimates have been consistently running hot with Melissa/overestimating its intensity

I suspect that’s because of the high tropopause heights we tend to see in the Caribbean in Oct, which ultimately favor colder cloud tops.

Diurnal maximum coupled with exceptional upper-level divergence (to compensate for subsidence warming), & high background tropopause heights, typical of the low-latitude tropical Atlantic in October, means you have a CDO that is *very* efficient at cooling/radiating longwave IR out into space

A renewed bout of intensification is likely ongoing with Melissa as the central dense overcast continues to appreciably cool after sunset.

Also noticing the development of an enveloped eyewall lightning (EEL) signature here, often a signature of a high end category 4 or 5 hurricane