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JISAO Winter PDO 1900-2015

Update on the Exceptional +PDO Regime & ENSO

For the 3rd month in a row, according to JISAO, the ongoing +PDO is once again in record territory (@ +2.30), easily beating out February 1941…

Kaplan Global SSTs February 1941, notice the tongue of anomalous cold water within the Kurioshio-Oyashio Extension & the ring of warm water on the west coast of the Americas embedded within the corresponding Peru & California currents.That’s about as classic of a +PDO signature as you’ll ever see.

 

Feb 1941 Global SSTs

 

DJF +PDO North Pacific SST Correlation

 

 

Warm Phase PDO N Pac SST

 

Thus, this past winter’s averaged PDO was also a record, & by quite a bit. The 1939-1942 multi-year El Nino has provided the only consistent & legitimate competition to this exceptional anomaly in the North Pacific, but the PDO preceding that period was arguably way more positive f(rom a multidecadal standpoint) than where we are currently…

JISAO Winter PDO 1900-2015

The top 7 & 8 of these top 10 +PDO winters (all except 1983-84 & 1926-27) were juxtaposed within multi-year El Ninos/+ENSO events.

(huge hint at where ENSO is headed for next winter, starting to appear very likely we’ll have an El Nino once again, for at least one more year, could feasibly go two more years but we’ll save that discussion for later…)

2014-15 2.40

1940-41 2.06

2002-03 1,98

1986-87 1.80

1935-36 1.77

1939-40 1.76

1905-06 1.60

1983-84 1.47

1976-77 1.33

1926-27 1.29

ENSO ONI 1900-1949 El Nino-La Nina

 

This is very likely not just mere coincidence either… I’ve been under the impression until recently that for the most part the (Pacific Decadal Oscillation) PDO is largely an artifact of or a reddened response (longer term consequence, after-effect) to ENSO (El Nino Southern Oscillation). Perhaps there’s more of a two-way relationship than I initially anticipated, particularly with exceptional wintertime +PDO regimes like what we’re experiencing now, which influence the longevity of El Ninos (i.e. extreme + (warm) PDOs favoring longer El Ninos). Of course, last spring’s extraordinary downwelling oceanic Kelvin Wave, lacking an extraordinary +ENSO response like it’s 1997-98 Super El Nino counterpart, contributed appreciably to this current +PDO configuration in the North Pacific…

Here’s the Equatorial Pacific subsurface analysis from the Australian Bureau of Meteorology for December thru March in the winter preceding the 1997-98 Super El Nino, last winter, and of course this year.

Given this information, it’s likely this upcoming Kelvin Wave will be appreciably more efficient at raising the equatorial Pacific SSTs…

DJFM 1997 2014  2015 Eq Pacific Subsurface Anomalies

Our current downwelling KW is appreciably weaker than either last year or 1997-98, however, last year’s large wave has pre-conditioned the tropical Pacific for an El Nino…

Notice vs last year how much warmer the tropical Pacific is & the Atlantic & the AMO has recovered, albeit slightly, to this past winter’s weak El Nino… We’ll need a lot more where that came from to revitalize it. The waters in & around the North Indian Ocean & Maritime Continent are also appreciably warmer, that will play a role in the amount of interference that is exerted onto this El Nino this summer. At least the southern hemisphere is trying to move into the +PDO state…

SSTs Feb 28-March 9 2015 2015 vs 2014

 

Also a considerable proportion of this displaced & anomalously warm water from the West Pacific warm pool was flushed into the extratropical North Pacific via coastally trapped Kelvin Waves & Rossby modes. by last spring’s huge downwelling Oceanic Kelvin Wave. There was also help from persistence & eastward migration of the massive warm pool in the Gulf of Alaska last winter that positively fed back to the record high North Pacific SSTs & the -EPO regime, which once again was very evident this past winter, in concert w/ the very stable Hudson Bay-Greenland Vortex…

The northern hemisphere winter pattern persistence has been anything but short of remarkable over the last 2 winters… (Last year on the left, this past winter on the right). The only true difference lies in the +PDO/ENSO combo, along w/ other forcings, nudging the entire pattern about 15-20 degrees to the east this year, which led to record warmth west of the Rockies.

2014-15  2013-14 Winter N hem z500

 

The sudden crash in the AMO over the last few years (potentially brought about by the lack of El Ninos since the 2009-10 moderate-hybrid event & possibly a slow down in the AMOC (Atlantic Meridional Overturning Circulation) as suggested by various pieces of literature, and an extratropical atmospheric bridge between the Aleutian & Icelandic Vortex action centers may have also played a significant role in the recent +PDO spike (The Aleutian & Icelandic vortices tend to manifest in a seesaw-like pattern, with neither vortex usually becoming too intense or fragile without the other negatively responding in the process (i.e. an opposite response, (here’s sort of an over generalization, but you’ll get the picture) strong Aleutian Low=weak Icelandic Low, strong Icelandic vortex=weaker Aleutian Low)

fig02

 

Here’s a nice tidbit of information I posted also at Jeff Masters’ blog which I thought was worth showing, (which of course has been modified to fit the needs of this post & any other ideas I’ve derived since then)

Even though the current SST distribution in the Equatorial Pacific looks like a modoki (Central Pacific El Nino)…

anomnight.3.12.2015

El Nino Modoki & Traditional SSTs EOF

Don’t let the current SST configuration fool you, by & large this has been predominantly a traditional, east Pacific El Nino, w/ considerable interference from the eastern hemisphere (especially in the early stages of the winter, hence a possible reason why the Hudson Bay-Greenland vortex was once again, stable). As you’ll see in the subsequent VP The westward shift in the anomalous +SSTs is more of a function of the climatological behavior of El Ninos as the relentless upwelling via advection of cool water within the Peru Current, coastal upwelling, & the natural easterly trades provide Sverdrup transport & equatorial upwelling that progressively undercuts the displaced warm pool, beginning in the vicinity of the NINO 1-2 regions in the eastern boundary region, to NINO 3-4 in the west across the equatorial Pacific. (In extreme El Ninos where the thermocline in the eastern Pacific may have be equivalent to or even momentarily deeper than the west Pacific like 1997-98 for ex, this initiation for anomalously cold water showing up at the surface after the peak of the EN event may actually appear around 100-120W.)

NESDIS Global SSTs June 2, 1998

sverdrup_balance

Hence, in general, the NINO 1-2 regions are generally more volatile & experience a substantially larger amount of smaller-scale variance, making them naturally more susceptible to transient intraseasonal oceanic/atmospheric stimuli that don’t necessarily nor always accurately reflect the ENSO index…

Top 20 El Ninos OND-MAM (October-December-March-May) Equatorial Pacific SST. The +ENSO base state progressively morphs into the modoki look after peaking in & around NDJ. Hence, taking into account the aforementioned evidence, I suspect this supposed modoki EN in the Pacific is just a temporary feature & may revert back to the basin wide/hybrid look (SST wise) we observed this past winter as we draw closer to next fall & winter…

Top 20 El Ninos OND-MAM SSTs vs 2014-15 Eq Pac SSTs
El Ninos tend to reach their maximum amplitude in/around the boreal winter solstice ~(NDJ) in concert with a favorable equator-pole meridional (north-south) global temperature gradient favorable Hadley Cell, particularly in the eastern equatorial Pacific. Even strong events experience the seasonal westward shift in anomalous +SST, but weaker El Ninos usually don’t have nearly the volume of warm subsurface water, nor the atmospheric-oceanic coupling/bjerknes feedback to overwhelm this incessant & natural cold mixing/upwelling, and thus tend to more rapidly fade into a modoki/Warm Pool El Nino in their decaying stages…

The correlation between El Nino intensity/placement is weak-moderate @ best, & there’s a considerable amount of variability in the behavior among individual events.

ONI & El Nino SST Anomaly Placement

 

OND-MAM El Nino Intensity  Equatorial Pacific SSTs
This past winter was certainly anything but a modoki El Nino, the signals in upper tropospheric VP, total precipitable water, and 850mb zonal winds are more consistent w/ an east-based El Nino…

DJF Pacific Basin Upper Level VP (Velocity Potential i.e. the irrotational, divergent portion of upper tropospheric flow (upper level divergence). Upper Level Velocity Potential is used as a proxy (approximation) to detect large-scale upward motion/rising air in the tropics, which is most likely induced by conglomerations of tall cumulonimbus clouds that are often associated with the backend of a variety of equatorial wave modes including the MJO… (hence why convection is also a primary determining factor of equatorial wave speed, aside from other conditions like the amplitude of the wave, the background flow, the depth of the fluid, etc) Speaking of convection, ever notice for example, that the 8 octants of the MJO aren’t geographically equivalent in terms of the zonal scale they encompass and that 60 degrees of longitude (~100-160) make up over half of the MJO’s phase space? This answer has to do w/ not only the WISHE, (Wind-Induced Surface Heat Exchange) that is enhanced in the core of the Indo-West Pacific Warmpool (where SSTs that are actually sufficient to adequately sustain latent heating), the relative lack of large-scale continental landmasses to greatly interfere with the MJO’s low-level moisture convergence & sensible heat derived from evaporation (although the topography/diurnal variations associated w/ the sea breeze in the vicinity of the Maritime Continent often causes some disruption), but the higher “convective budget” in the naturally more convectively active eastern hemisphere plays a large part in determining the cycle length of the MJO. Since this deep convection associated w/ the MJO is located within & just in the wake (west of) of the strongest lower level convergence (like most other equatorial waves). This causes major perturbations in the pressure, wind, & moisture, even SST fields, thus forcing the incipient MJO wave to slow down. This increases its life span over regions that are typically more convectively active (like the eastern Indian Ocean/Western Pacific & vis versa over continental landmasses (like Africa/S America) where aforementioned forcing mechanisms to sustain the MJO are largely absent & convectively inactive regions of the tropical oceans (such as the far eastern equatorial Pacific), corresponding to areas w/ lower SSTs mostly induced by cool upwelling & mixing. It’s in these regions where lower level forcings can’t usually maintain the MJO, that it tends to move faster (& behave more like a CCKW), with its remnant upper tropospheric “footprint” remaining largely intact, however (hence why VP is a viable proxy for MJO detection in the western hemisphere, RMM’s utilization of OLR often precludes it from picking up on the “true” ferocity of the MJO beyond the confines of the warm pool.

DJF Pacific VP200 2014-15

 

The VP in thru the end of December resembled a La Nina more so than an El Nino w/ a maximum in the eastern Indian Ocean, it’s no wonder w/ this kind of late fall-early winter tropical forcing (in spite of favorable SAI/SCE) that the vortex was allowed to incubate & couple w/ the lower level Hudson Bay counterpart…

 

ND 2014 Global VP 200

Global .2101 sigma VP Nov-Dec La Ninas

Global VP La Ninas Nov-Dec

What subsequently transpired was a weaker wave #2 signal, w/ anomalous upper level convergence over the central Pacific.

JF 2015 Global VP200

The SSTs were already elevated, and the persistent lack of convection & increase in incoming shortwave radiation brought about by this upper level convergence allowed the waters @ the edge of the warm pool, in & around the international dateline to warm up, and once the climatology became more conducive (esp for more amplified MJO regimes that can “bust” through this convergence zone) & we broke out of the influence of boreal winter ENSO stage,the global pattern was allowed to start the process of reshuffling, beginning w/ a major MJO pulse, & that’s exactly what we’re observing now…DJF Eq Pacific 850mb zonal wind anoms (anomalous westerlies (+ numbers) shown in shades of green, yellow, orange, & red), & vis versa for easterlies in blue)

DJF 850mb zonal wind 2014-15

DJF Eq Pacific Precipitable Water anoms

DJF Precipitable Water 2014-15

You should also notice the westward tilt w/ height between the core of the anomalous upper level Velocity Potential, & the interface of the anomalous 850mb zonal winds (this interface is indicative of the general location of anomalous lower level convergence). This westward tilt with height is a prevalent feature in equatorial waves and can be partially attributed, especially when dealing w/ the MJO, to variation in cloud type & population as a large-scale wave vs the progress of specific & individual mesoscale convective complexes as a function of wave phase (i.e. in relation to the overall MJO wave itself)…

Modoki-Hybrid-Traditional EL Nino VP U850  Precipitable Water vs 2014-15 El Nino

On another note, I have some doubts the NAO is going to flip for an appreciable period into the negative phase as is being depicted by many of the medium range ensembles. Sure, a surge in pacific convection will crank up the Pacific Jet & force a rossby wave train that induces anticyclonic wave breaking in the north Atlantic (which in years like this w/ shorter jet wavelengths, may be more efficient @ disrupting the vortex). This upcoming & primarily, Rossby Wave induced MJO wave…

(…as indicated by the westerly propagation of 850mb zonal winds in the equatorial Pacific, RMM is picking up primarily on a very strong Equatorial Rossby Wave the MJO usually moves east, hence the RMM indices/EOFs may be not be projecting on the MJO at all…)

u.anom.30.5S-5N

…may have been a bit over amplified by the american guidance (the CFS is notorious for producing this kind of over-zealous forecast in the Pacific, these poor forecasts of intraseasonal phenomena like the MJO may also reflect upon it’s dismal performance in over-amplfying El Ninos (particularly during the downwelling phase of the oceanic KW), as was certainly the case last year. Consequently, based on the suspicion for an erroneous MJO forecast, you could also assume the american suite is over embellishing what’s more likely to be (as we observed in late January/early February), largely an east-based -NAO surge, w/ perhaps given seasonal behavior, a bit more success in sustaining/intensifying the regime this time around. There are always exceptions of course, and this ongoing MJO pulse is certainly becoming the “exception”. It’s worth mentioning for future reference, both the European ensemble & CFS have even more issues picking up on MJO initiation in the Indian Ocean, this may be something worth monitoring in the coming weeks as a “true” MJO event attempts to form in the eastern hemisphere….

MJO Amplitude Observed, EPS, & CFSv2

(pic via Kim et al 2014 “Predictability and prediction skill of the MJO in two operational forecasting systems”)

On the other hand, if this upcoming MJO pulse ends up being anything close to the ECMW forecast, w/ >4 sigma  (standard deviation) phase 7 MJO amplitude forecasted, the closest rival to matching the intensity of this potentially record breaking phase 7 event for March (or for the MJO in general) is 1997. The same MJO pulse that helped initiate the 1997-98 super El Nino…

ECMF_phase_MANOM_51m_small

 

JFM MJO 1997 1997-98 Super El Nino

Yikes…

Honestly, the timing of the current downwelling KW & re-activation of the MJO could be anticipated rather far in advance, even well before most model guidance began to hint at the potential of a substantial MJO burst in March a few weeks ago. The MJO tends to reach maximum amplitude in March…

MJO amplitude in the year

& the significantly weaker Pacific zonal SST gradient is closely tied to the seasonal migration of the ITCZ/trade winds, that aside from maintenance of the Indo-West Pac Warm pool influence the equatorward advection of cold water from within the Peru current… (this weaker SST gradient also argues for weaker trade winds, bjerknes/oceanic-atmospheric coupling, that naturally makes the Equatorial Pacific more susceptible to intraseasonal oceanic downwelling Kelvin Wave variance (hence why we often observe dramatic alterations to the ENSO base state & thermocline during this time of the year).

The annual reduction in the near-equatorial SST gradient in the Pacific is very dramatic, a reduction of 4-5C makes a gigantic difference…

Equatorial Pacific SSTs DJF-SON 1948-2014 ENSO

In the heart of the residence period between the Australian & Asian monsoons, the MJO also attains approximate equatorial symmetry, allowing for WWBs (Westerly Wind-Bursts) in the wake of the pulse to be more directly focused into the equatorial counter current that serves as a host to these downwelling Kelvin Waves that displace the warm water out of the West Pac warm pool & allow it to slosh eastward in their wake into the NINO regions further east. This is certainly applicable to first year El Ninos, but the intensification of this El Nino in the beginnings of the spring is actually quite unusual for a multi-year series, and since 1871, I could only find 3 previous examples (1884-86, 1904-06, & 1986-88) that underwent a similar bout of strengthening in their second year. The other multi year events (1913-15, 1939-42, 1951-54, 1957-59, 1968-70, & 1976-78) actually weakened in their second year (likely a rebound upwelling response to the preceding El Nino) until later in the fall, when a resurgent downwelling KW near the autumnal equinox led to warming that secured an El Nino for at least one more year.

Going back to the topic of the ongoing MJO burst, the March MJO Phase 7 bursts (shown in blue) in 2004 & 2012 were nothing to sneeze at either, w/ amplitude nearing or surpassing 3 sigma, and as I’ll explain a bit later in the post, these El Ninos more accurately depict the considerably increasingly unfavorable background state towards El Ninos that has evolved since 1997.

JFM MJO 2004

JFM MJO 2012

 

 

Also, uncertainty remains as the southern hemisphere stubbornly hangs onto the cold multidecadal PDO state w/ the horseshoe of anomalous cool water within the Peru current, likely being induced by the persistent +Southern Annular Mode/stronger than normal Antarctic Vortex & re-emergence of what may still be an underlying multidecadal -PDO regime, it’s just that the North Pacific is busy contending w/ a host of other more pressing issues, particularly trying to deal w/ the exhaust of last spring’s downwelling KW…

Southern Hem z700 December 1-March 2 2015

S hem z700 anomalies Dec 1-March 10 2015

 

+AAO z700 regression

Loading Pattern of AAO Index

 

+AAO Global SST correlation, note how this corresponds well to only the southern hemisphere in the current setup, with an opposing cold PDO/warm AMO signature. FMA Global SSTs AAO correlation

 

Consequently, the persistent +NAM/AO is also not conducive to a returning El Nino. In fact, the difference in SLP in the North Pacific between returning El Ninos since 1900 & the strongest “bounce-back” La Ninas in that same time period has striking semblance to the correlated NAM/AO pattern, arguing that it may in fact be a primary determinant in deciding the fate of ENSO following a first-year El Nino…

SLP Returning El Ninos vs Top 10 Bounce Back La Ninas  JFM AO Pacific SLP Correlation

 

The North & South Pacific are highly polarized w/ opposing multidecadal PDO states. This kind of opposing configuration will result in interference with this El Nino & will likely keep it from achieving the “Super” status of incredible events like 1877-78, 1982-83, & 1997-98

 

+PDO-PDO Pacific FMA SST correlation  Mar 2 2015 Global SSTs

 

Similar to last year, even though we’ll likely see this El Nino once again make another surge near the autumnal equinox (Sept-Oct), the most important question rests on the survival of this El Nino through the summer & how it handles the southeast Asia/Indian Monsoons, and coincident eastern hemisphere interference. This interference with El Ninos, likely connected to the long-term, punctuated warming of the Indian Ocean, over the last several decades has steadily increased, and has shoved the entire NINO circulation further east into the Pacific…

Global VP Pre 1976-78 El Ninos. Very distinct wavenumber 1 signal, centered directly over the central Pacific…

Global 200VP Pre 1976-78 El Ninos

 

 

Post 1976-78 El Ninos Global VP, notice the evolution towards a weaker, wavenumber 2 look, w/ increasing interference from the Indian Ocean

Global 200VP Post 1976-78 El Ninos

This increasing interference & eastward progression/weakening of the attendant anomalous Walker circulation associated w/ an El Ninos becomes even more evident in the NDJFM upper level VP in the post 1998 era, where in fact, the Indian Ocean actually becomes the dominant center of action in terms of Upper Level forcing. Amazing…Post 1998 NDJFM 200VP

 

This issue w/ multidecadal warming leading to a stronger upper level circulation in the eastern hemisphere is being exacerbated by the -IOD, that tends to (but not always) evolve in conjunction w/ La Nina events as the ascending portion of the Walker Cell causes anomalous eastward flow of air in the vicinity of the Maritime Continent on it’s western flank, favoring downwelling in the eastern boundary region of the Indian Ocean, & hence a -IOD configuration w/ anomalously cooler SSTs towards eastern Africa & warmer SSTs near Sumatra. (7 of 11 +IOD events (identified by the Australian Bureau of Meteorology) were associated w/ El Ninos, & 4 of 9 La Ninas w/ -IODs. Of course the Wyrtki jets can dramatically change all of this, but I remain doubtful given the intensity of this -IOD. Since 1958, 1958-59 is the only full-blown El Nino to develop in concert w/ a -IOD. 1960, 1992, & 1993 were +ENSO events that evolved in-step w/ a -IOD.

iod1

 

JJASO Global SSTs +vs -IOD

The long-term warming of the Indian Ocean has also had a significant impact on the Asian/Indian monsoons & the upper tropospheric Tropical Easterly Jet. The tropical easterly jet is created in large part as a quasi-geostrophic response to the thermal/pressure differences created by the relatively cooler adjacent northern Indian Ocean to the south & Tibetan Anticyclone to the north, which over the course of the summer, progresses northward across the Indian subcontinent. The more vigorous latent/sensible heating underneath this area of upper level high pressure causes the air to expand like a balloon (also creating a coincident lower-level thermal low above the planetary boundary layer), forcing the heights to rise over the relatively warmer landmass, while the cooler adjoining ocean, w/ higher retention for heat, experiences a relative lowering in the height field. The pressure gradient force dictates that air should move towards the lower height field over the Indian Ocean, however, the Coriolis force deflects this equatorward moving air towards the west, which along w/ tight thermal gradient & consequent equatorward sensible heat flux, (i.e. also moving southward from the warmer subcontinent to the cooler ocean) lead to the formation of the Tropical Easterly Jet near the tropopause.However, the punctuated warming of the Indian Ocean, most noteworthy following El Nino events, is induced by the refraction of off-equatorial rossby waves that carry leftover warm water from the ENSO regions in the preceding El Nino westward back into the Indo-West Pacific warm pool. This restored warm water seeps through crevices in the Maritime Continent archipelago, in a process more widely known as Indonesian Throughflow (ITF),

Indonesian Throughflow Diagram

…and along w/ the increase in incident solar radiation imparted by the preceding & weakened Walker Cell, causes the SSTs in the Indian Ocean to rise.

You can see these off-equatorial Rossby Waves, (especially in the northern hemisphere) via +SSH (sea surface height) anomalies that propagate initially move poleward along the coast of South America during the height of the El Nino, and then progress westward w/ increasing time, eventually reaching the western Pacific in the following spring & summer in & around 5-10 degrees N, with a portion of the once-displaced water being diverted into the Indian Ocean, and another piece going into the warm Kurioshio current east of Japan, and eventually, into the Kurioshio-Oyashio Extension (KOE). Thus, this leads to longer term implications on the PDO, aside from the North Pacific atmospheric bridge that couples with El Ninos via an extensive Pacific Jet & powerful Aleutian Low.

 

Atmospheric Bridge ENSO-PDO

 

The evolution of the SSH anomalies & off-equatorial Rossby Wave are most noteworthy & most adequately modeled in the Super El Ninos of 1982-83 & 1997-98…

December 1982 & 1997 SSH anomalies during the height of these extreme El Ninos. Note the anomalously high sea surface height in the far eastern Pacific, (in yellow, orange, & red) & lower heights further west.

Global Sea Surface Height Anomaly Dec 1982 El Nino

Global Sea Surface Height Anomaly Dec 1997 El Nino

 

By mid spring, the higher sea surface heights have started to develop & translate westward just north of the equator

Global Sea Surface Height Anomaly Apr 1983 El Nino

Global Sea Surface Height Anomaly Apr 1998 El Nino

 

In the middle of the following summer, the +SSHs are fully confined within the north Equatorial Current via a westward propagating Rossby wave, and a tongue of lower SSHs are starting to show up in the counter-current. The beginning of a La Nina event…

Global Sea Surface Height Anomaly Jul 1983 El Nino

Global Sea Surface Height Anomaly Jul 1998 El Nino

 

This rise in Indian Ocean SST, though is not accompanied by an equivalent negative response to La Ninas, and the SST change is typically far more gradual. This behavior leads to major upward step-changes in the Indian Ocean SST following major El Ninos, creating a progressive, multidecadal warming, overall as noted by the OISSTv2 dataset (in this case plotted by Bob Tisdale) & Kaplan SSTs.

Notice the punctuated SST spikes following the end of the 1982-83, 1986-87, 1991-92, 1997-98, 2006-07, & 2009-10 El Ninos (the Mt. Pinatubo & El Chichon volcanic eruptions masked the after-effects of the 1982-83 & 1991-92 events), with a relative plateau/minimal decline between these successive events. Precisely why the response isn’t as great during La Ninas (which in many cases last longer than the accompanying El Ninos that induce the dramatic uptick in SSTs) is still a major question I’m trying to resolve…

Kaplan SSTv2 Indian Ocean Warming 1948-2015

 

Indian Ocean OISSTv2 SSTs 1981-Feb 2015

 

This multidecadal warming of the Indian Ocean brought about by punctuated, upward step-changes in the SSTs via Indonesian Throughflow, at the culmination of El Nino events has weakened the thermal gradient between the Tibet Anticyclone & the northern Indian Ocean, thus causing the TEJ to follow suit.  You can see by this NCEP/NCAR reanalysis of the 150mb zonal (U) wind, the jet core within the Tropical Easterly Jet has weakened by about 10 m/s since the 1950s & about or a little over 5 m/s since the beginning of the satellite era when more reliable upper level observations became available. The TEJ underwent appreciable zonal contraction, especially through the 1990s, although there is some recent rejuvenation in the African sector, in concert w/ the slightly wetter/more active Sahel.

1950-2009 TEJ 150mb U Wind

 

The weakening trend of the TEJ has only continued, and over the last 5 years, U wind 25-27.5 m/s contour, which once encompassed a longitudinal band ranging from equatorial Africa to the Maritime Continent, currently only makes up the jet core itself…

JJAS 2010-2014 150mb U wind TEJ

 

Perhaps, this general weakening of the TEJ slowed the African Sahel’s response to the +AMO in 1995 and prolonged the multidecadal drought there which got underway at the end of the last +AMO regime in 1970. Note, the sahel rainfall in the antecedent period was locked in phase w/ the AMO, that isn’t necessarily the case now however. I should also thrown in the fact that this Easterly Jet is indeed unique in the sense it is only possible through this sort of land-sea configuration, with tropical ocean juxtaposed equatorward to a neighboring, large, conglomerate, continental landmass.

1900-2013 Sahel Rainfall

 

Thinking about the broad picture and what this aforementioned information means, ENSO is the global weather/cliamte system’s response to amend internal disequilibrium that’s imparted by a wide variety of external forcing mechanisms. The system can only efficiently respond to this disequilibrium at a specific harmonic/resonance, hence why it tends to peak near & just before the boreal winter solstice. Decreasing the natural equator-pole thermal gradient in the boreal winter (which has been proxied by the robust warming that has taken place in the arctic since the beginning of the satellite era, particularly during the winter), causes the Hadley Cell & attendant Walker Circulation to expand & weaken, w/ consequently less robust tropical convective forcing within the associated ascending portion in the core of the Hadley Cell, thus leading to a general & pronounced weakening of ENSO phenomena itself (or as Anthony Masiello more adequately termed, “ENSO wave weakening”). We saw a taste of this in the 1920s-1940s, and something of at least similar magnitude could conceivably happen once again in the “relatively” near future, although the consequences may be more significant this time around with solar activity fixing to really go into the tank as we progress onto the backside of solar cycle 24, with what’s likely to be deepest solar minimum since the Dalton Minimum in 1790-1830…

 

Comparison of Solar Cycle 5 (within the Dalton Minimum) & the ongoing cycle 24 via Geoff Sharp. Very similar…

Solar Cycle 5 & 24 comparison

 

This Hadley Cell broadening & attendant Indian Ocean warming, increasing the amount of interference imparted by the eastern hemisphere onto the Walker Circulation, among other things, will certainly make life difficult for any developing El Nino, and even though we are experiencing an MJO pulse w/ similar tenacity to the one preceding 1997-98 Super El Nino, the background state, even w/ a record +PDO state, is still not very welcoming…

On another somewhat related note, considering how the Hudson Bay-Greenland Vortex is fully coupled w/ its stratosphere counterpart & has been utterly dominating the mid/high-latitude mass transports, the guidance of mid-lat rossby waves, and thwarted previous attempts to destroy it, without the proverbial destruction of this vortex, it’s current intensity & preceding ferocity/persistence may allow it to become effectively (& @ least somewhat) resistant to external tropical forcing, including the MJO… It’s only when this exceedingly stable Hudson Bay-Greenland Vortex is dismantled that we’ll have a chance @ eradicating the +ENSO tendency & finally have a legitimate shot at a La Nina again. The most likely way to do this, IMO, will probably involve a major, mid-winter sudden stratospheric warming event, lowering the tropical tropopause & reversing the gradient (as a function of a transfer of mass), favoring the precipitous growth & initiation of a strong MJO wave (~85-90% of major SSWEs have been subsequently followed by massive MJO pulses), which can then interact w/ ENSO to attempt to kickstart a “true”, major global pattern change & reverse the ENSO base state (credit to WeatherPhil at the Weatherforums for pointing this out to me)…
Until then I hope everyone enjoys the multi-yr El Nino :)

Honestly, it’s about time, I was wondering when we’d see another one of these. Since 1871, we’ve only went >10 years or more between the ending & commencement of multi-yr +ENSO events once (& that was very early on in the record, where the validity of extended MEI is questionable), the last one ended in 2005, so this is coming just in time…

Here’s the list of multi year +ENSO events since 1871

1876-1878

1884-1886

1899-1901

1904-1906

1913-1915

1918-1920

1929-1931

1935-1938

1939-1942

1951-1954

1957-1959

1968-1970

1976-1978

1986-1988

1990-1995

2002-2005

2014-?

 

Possibly there’s some influence from the TPQDO (Tropical Pacific Quasi-Decadal Oscillation) The Tropical Pacific Quasi Decadal Oscillation can be thought of as a lower frequency addition to the ENSO index, and ss described by Stergios Mismos, is a naturally-induced oscillation of the tropical pacific similar to El Ninos, and lends observational evidence for a warmer tropical Pacific coinciding with increased solar forcing. Sverdrup transport persistently expels heat into the subtropics via surface divergence, and raises the thermocline in response to the easterly trade winds, forcing a -TPQDO. Stronger easterlies thus, drive the Pacific into a state that favors heat accumulation in the west Pacific & the warm pool region that is later discharged through El Ninos when external forcings (among other things) warrant…

TPQDO Index  2014-16 Possible Multi Yr Warm ENSO-El Nino

 

I’ve also taken note of a quasi-regular pattern of increasing La Nina intensity & longevity on a interdecadal scale of about 15-20 years… This behavior may just be a manifestation of the IPO (Interdecadal Pacific Oscillation) or perhaps a lower frequency extension of the recharge-discharge oscillator ENSO theory consisting of multiple flips between the + and – ENSO base states. Regardless, what’s worth looking at here is that this observed phenomena operates on time scales that are generally inconsistent w/ & fall in between that of ENSO & PDO (although there may be some overlap and these cycles take up a decent proportion of the ENSO/PDO variability). Over a period of about a decade and a half to 20 years or so we undergo a period of progressively increasing La Nina intensity, which then, after the last & largest La Nina event of the series, is followed by a multi year warm ENSO neutral or El Nino event, and then cycle repeats…

15-20 yr ENSO-La Nina Intensity Variability

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In my analysis of the return frequency/period of La Ninas over the last 150 years, there is a legitimate spectral peak around 15-20 years, which lays generally in between higher frequency ENSO persistence (4-7 years) & the appreciably lower frequency multidecadal PDO signal (25-30+ years). There’s also a minimum corresponding to the length of the solar cycle, interesting…

La Nina Return period 1871-2014 MEI ENSO PDO IPO

 

El Niño’s return frequency doesn’t show nearly the spectral maximum in this range, however, comparing the pre 1968 data (which precludes a high reporting bias in lower years) & all of the data, a well defined anomaly shows up once again in & around the solar/schwabe cycle length, (in this case a spectral max) highlighting the sun’s influence in regulating ENSO, including both La Ninas & El Ninos… You can probably also pick out relatively smaller secondary maximum & rather anomalous minimum in around the 25-30 year lead, which could possibly correspond to the PDO, but I personally wasn’t expecting that to show up in the data (hence, why we do research in the first place)

El Nino return Period (1871-2014)

 

It may seem crazy, (as I thought I certainly was for buying into the TPQDO about this time last year), based on the aforementioned information regarding this interdecadal cycle of La Nina ferocity/longevity & the ongoing multi year El Nino/+ENSO event, I would suspect ENSO to most likely flip back into an extended weak La Nina or cold neutral state at the end of this multi year El Nino (that may happen either this year or possibly the following year if we continue to closely follow in the footsteps of 1939-1942). Of course, whatever transpires, will have significant implications on the upcoming winters & hurricane seasons, which I will talk about a bit more (especially the latter) in my next post, that will likely come sometime later in May.

If you have any comments, questions, or concerns about anything in this post, or would just like to say hi, feel free to comment below.

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