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Teleconnections: A More Technical Discussion

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On 11/27/2018 at 6:43 PM, Isotherm said:

@Catacol -- keep in mind those two AAM charts you posted have disparate periods for calculation. The 1958-present GLAAM graph depicts monthly averages, whereas, the 90-day chart is obviously intra-monthly, so a transient spike to +3 standard deviations is substantially less than a +3 standard deviation monthly anomaly. We're not even close to the AAM of strong Nino years.

 

Iso - the November Global AAM Anomaly is actually the 2nd highest since 1958, and the Oct-Nov is 4th highest, placing it among the super ninos and other select years. - https://twitter.com/griteater/status/1067482391407673351

 

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On 11/27/2018 at 7:23 PM, Isotherm said:

 

 

Correct, it is still quite impressive and somewhat aberrant given the weak ENSO context. It's multifactorial, and I'll let @Tamara expound further since the inquiry was directed to her (speaking of which, great posts by @Tamara and @Bring Back 1962-63 on the previous page there). However, my brief answer is that the AAM tendency comports quite well w/ past and prognosticated MJO progression. AAM typically reaches a peak as the MJO divergence signal propagates into/through phase 7, and begins to rapidly decline thereafter. We're already seeing the initiation of that diminution reflected in the negative frictional torque which will be represented in the "total" AAM about a week or so from now (approximately). Of course, the direct reasons of the AAM spike include the robust +FT event/WWB with MJO support (and ER component in the signal as well), but more indirectly, in my view at least, the Pacific/west Pacific SST's surrounding the ENSO regions contain more energy than most previous weak ENSO events due to the increased warmth. This seemingly slight increase in SST's in the tropics is significant insofar as convective generation/maintenance/latent heat release/rossby wave train initiation and westerly momentum production. We can debate whether that's due primarily to anthropogenic reasons or not, but the background warmth aids adjunctively in boosting westerly momentum in the Nino event.

 

This seems like a reach imo. You simply don't get AAM anomalies like this without their being a substantial ENSO event in place for subseasonal variability to superimpose itself onto, and while I see the line of rationale here it's way too simplistic to argue that the increased AAM is due to warmer tropical SSTs & actually ignores a lot of other basic processes that are ongoing to offset this. For one thing, evidence for that is highly contentious and at best very limited or non-existent, and not to mention the quasi-moist adiabatic adjustment to surface warming actually increases the static stability in the tropics because the moist adiabatic lapse rate decreases in a warmer world as there's more latent heating to offset adiabatic cooling of ascending air parcels, actually making it harder to generate convection in the tropics despite the warmer BL. This is discussed throughout many pieces of literature and is an even stronger counterpoint to the idea that there's a positive relationship between global temperatures and increased convective activity and this is somehow to blame for the higher AAM lately. 

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30 minutes ago, Isotherm said:

 

 

I disagree and I think it's a plausible hypothesis. If you think you have an alternative mechanism, I'm interested to hear it.

 

See the following papers. The hypothesis is that non-ENSO region SST forcing plays a salient role in modulating zonal wind anomalies, and by extension, induction of either positive or negative atmospheric angular momentum tendency.

 

Warmer tropical and sub-tropical SST's induce an enhanced meridional / poleward SST gradient, which in turn, yield an increased meridional tropospheric temperature gradient. Concordantly, sub-tropical / off-equator zonal wind anomalies are significantly heightened which leads to robust angular momentum increases, as well as a more intense sub-tropical jet. 

 

This first paper ostensible indicates that the most important factor related to the suppressed angular momentum of the 1965-66 El Nino event was the paucity of zonal wind response in the low-latitudes (colloquially, a sub-tropical jet), which is indubitably forced in part by off-equator sea surface temperature anomalies.

 

The following are select quotes from two pertinent papers, in my opinion:

 

https://journals.ametsoc.org/doi/10.1175/1520-0442(2003)016<2526%3AUBIAAM>2.0.CO%3B2

 

The most important factor related to the low AAM value in 1965 is clearly the lack of the typical El Niño response in the zonal wind in low latitudes (i.e., the acceleration of the subtropical jets).

 

Second, the SST anomaly outside the conventional eastern Pacific El Niño region (as quantified by the Niño-3.4 index) may sometimes have a significant impact on the tropical/subtropical zonal wind and global AAM anomalies (Hoerling et al. 2001b). On such occasions, the Niño-3.4 index would not agree well with the global AAM anomaly. 

 

 

If indeed the observed negative response on the equator in 1972/73 is due to SST forcing, then the low correlation between the Niño-3.4 index and the observed AAM anomaly in that event implies that SST anomalies outside the conventional El Niño region are affecting AAM, causing a more negative wind response on the equator.

 

The fact that the AAM anomaly in 1972/73 is not well correlated with the Niño-3.4 index, then, suggests that SST anomalies outside the conventional El Niño region may contribute to the low AAM value in 1972/73.

 

 

And the second paper:

 

https://pcmdi.llnl.gov/mips/cmip/cmip_abstracts/huang04.pdf?id=95

 

The robust ∆SST−∆AAM relationship is explained based on a statistical equilibrium argument that links the tropical ∆SST to tropospheric temperature anomaly. During El Nino when ∆SST is concentrated in the tropics, the increase in tropical Pacific SST implies an increase in the meridional gradient of SST, which in our argument produces an increase in the meridional gradient of tropospheric temperature. By thermal wind balance, the latter corresponds to an increase of the vertical shear in the subtropics, or an enhancement of the subtropical jets if the surface wind is assumed unchanged. The enhanced subtropical jets implies a positive ∆AAM, completing the ∆SST−∆AAM connection.

 

 

Concordantly, the sub-tropical jet circum-globally has been very strong this month (Nov. 2018) as per the proxy of z200 zonal wind anomalies:

 

The logic attendant this analysis is reasonable and based in sound, fundamental meteorological tenets in my opinion. A more expansive off-equator + SST anomaly will augment additional zonal wind addition thereby modulating AAM response.

 

70kh80.gif

 

In order to substantiate your hypothesis you have to separate the influence from both ENSO, the QBO, etc. and then have to show that there not only has been an increase in AAM in the observed record, the increase is significant, the increase in AAM has been occurring long enough for a detectable climate signal to be derived, and that the increase is due to climate and not interannual climate variability, quite frankly that's easier said than done and neither of these papers or your statements or composite from NOAA ESRL showed that. 

 

In fact, the second paper notes that increases in equatorial Pacific SSTs favors heightened AAM but they make no further distinction whether this is forced by climate base state changes as you're trying to assert or is simply ENSO being reinforced by subseasonal variability which seems more plausible and easier to substantiate in this case.

 

Before even tackling your idea that simply increases in SST drive increases in convective heating which drive increased AAM, which again completely ignores a lot of other basic processes simultaneously acting to completely dampen this, the idea that there's actually been an increase in meridional SST gradient too is pretty contentious given that tropical Pacific has actually cooled slightly since the mid 1990s, flying in the face of what you're trying to argue plus, the record is simply too short to detect any signal (if there is one) because internal variability is too large.

QsnJOGjoKd.png.e5c875c99475d3ac6d9c51e370629e05.png

 

Increases in both subtropical and tropical SSTs actually prove my earlier point about static stability changes because as temperatures increase, the moist adiabatic lapse rate in the tropics decreases and it's been shown by many (including the linked paper below) because there's more latent heating to offset adiabatic cooling which decreases the ascent rate of parcels embedded within this regime and the strength of the Hadley Circulation which is inherently related to its broadening. This also means that while the mid-latitude jets move poleward in response to basic state climate changes, what we're currently seeing w/ tightening and intensification of the Hadley Cell is inconsistent with the long-term trend and what you'd expect in a warming climate w/ expanded hadley cells.

 

Static stability increases faster than changes in heating related to convection because latent heating is offset by radiational cooling. This also means that your notion that warmer SSTs on the whole lead to more convection is also not true if this is a manifestation of long-term climate changes, because static stability outpaces the contribution from latent heating as temperatures warm.

https://journals.ametsoc.org/doi/pdf/10.1175/2008JCLI2200.1

 

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"Concordantly, the sub-tropical jet circum-globally has been very strong this month (Nov. 2018) as per the proxy of z200 zonal wind anomalies:

 

The logic attendant this analysis is reasonable and based in sound, fundamental meteorological tenets in my opinion. A more expansive off-equator + SST anomaly will augment additional zonal wind addition thereby modulating AAM response."

 

Subtropical warming in concert w/ tropical warming leads to expansion of the Hadley cells, and while total global AAM could increase in that setting, we should see the mid-latitude jet, if anything, intensify and move poleward in concert w/ these changes even on shorter temporal scales. The strong, equatorward displacement of the subtropical jet directly over the latitudes of heightened off-equatorial +SSTAs undermines this idea & suggests other forcing mechanism(s) are probably at work.

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18 minutes ago, Isotherm said:

 

 

So you do see my argument now - as that is exactly what I'm asserting, that total global AAM can be modulated significantly by sub-tropical warming induced zonal wind increases. All I'm asserting is that it's a viable hypothesis -- not saying it's the only mechanism.

 

But these subtropical increases in SSTAs would not contribute to enhanced subtropical AAM because it overlies where the momentum increases are occurring which violates thermal wind relationships. Increase in thermal wind near 45-50N would be a completely different story however because this would be consistent w/ both Hadley Cell expansion and the observed SST distribution but you're arguing that it's contributing to subtropical AAM increases ~30N. It also does not contribute to subtropical AAM due to aforementioned increases in static stability that take place under such a regime to suppress convectively-induced westerly momentum accelerations. Static stability outpaces latent heating contributions because it's not offset by radiational cooling which is discussed in the paper I linked to your response. 

 

Global AAM shows no discernible trend since 1976 that would even argue there's a climate-induced signal modulating it which substantially weakens your hypothesis. If anything, we've actually seen more negative AAM in the subtropics during the last 20 years or so, which serves to only further derail your hypothesis.

 

totaam_sig.58-curr.reanal.gif.ef13a3149084cd8207e337838f5b6ab6.gif

 

 

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28 minutes ago, Isotherm said:

 

 

 

Regarding your first comment: In order to substantiate your hypothesis you have to separate the influence from both ENSO, the QBO, etc. and then have to show that there not only has been an increase in AAM in the observed record, the increase is significant, the increase in AAM has been occurring long enough for a detectable climate signal to be derived, and that the increase is due to climate and not interannual climate variability,

 

I think you realize that this would be very difficult to virtually impossible to prove, at least at this stage, in light of Earth being a very poor, "lab rat" for isolating variables; analogously, it's precisely the same reason why those who assert that climate change is mostly/entirely due to anthropogenic reasons are walking on tenuous scientific grounds, because in order to substantiate that hypothesis, one needs to extract out other contaminating variables. 

 

Further, to me, your SST change map aids in bolstering my thought process re my hypothesis. While equatorial Pacific SST's have decreased slightly, equatorial west Pacific SST's have increased [where SST's are always warmer in the absolutes]; sub-tropical west Pacific SST's have increased; and, high-latitude Pacific SST's have actually cooled. This, in my view, corroborates the notion that zonal wind anomalies, off-equator, would be enhanced via strengthening meridional thermal gradients, and by extension, provide additional momentum injection into the atmosphere. 

 

I'm not sure if you're entirely understanding what I'm arguing, but hopefully this clarifies. The question is: has static stability truly outpaced/countervailed effects of SST induced heating in the Pacific domain? To me, the SST change map actually argues otherwise, and favors significantly increased zonal winds in the sub-tropics. 

 

This interpretation is just flat-out wrong, zonal winds have not increased in the subtropics, in fact, they've been decreasing in the same period as this SST composite is valid for & they completely oppose the ongoing AAM behavior w/ strong +AAM in the subtropics.

 

MG3h5cx0Vh.png.178c746c820386460ad10d29c1fd4bb2.png

compday.fLLiZX174i.gif.338d3fd2395e3349324e49fa5defc6cd.gif

 

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38 minutes ago, Isotherm said:

 

 

So you do see my argument now - as that is exactly what I'm asserting, that total global AAM can be modulated significantly by sub-tropical warming induced zonal wind increases.

 

All I'm asserting is that it's a viable hypothesis -- not saying it's the only mechanism.

 

Furthermore, you also need to account for where this subtropical warming is occurring in relation to the waveguide because it doesn't always lead to AAM increases in the subtropics. The harsh reality is that where the subtropical SSTAs are increasing the most, in the subtropical western North Pacific, actually leads to poleward anticyclonic wave breaking at the longitude of the exit region of the Pacific jet which corresponds to -AAM in the subtropics and that's exactly what the difference composite above shows.

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Some really interesting reading - thank you. I'm still scratching my head a bit in terms of the size of the AAM surge over several months, and the consequent compensation reaction to higher levels of momentum is underway and more punchy than might have been the case had Glaam remained in moderate bounds. glcalctend_sig.90day.gif.2ca65cbb08a62af0b061a968c8e68866.gif

 

Will the spiral of upwards momentum be sustained and pushed to equally high levels in December as the strong MJO signal reconnects with phase 5/6? The ventrice site continues to show as strong an MJO progression out to week 2 that I can remember seeing in recent winters

 

twc_globe_mjo_vp200x.png.80ef1de7d4a73ac5fc1219d9e94c5700.png

 

and with potent early season wave 1 activity ongoing and set to hit a good and sustained peak

ecmwfzm_ha1_f120.gif.d9e21e1ee90ae5e839d79206dae51513.gif

as Tamara stated - there are interesting times ahead for the vortex as we head towards the heart of winter.

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Reading that last page was awesome. A respectful discussion between 2 well respected wx experts then Tam with the bomb of a post is why 33andrain is the supreme forum for weather 

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