Special Update: El Niño – How We Got Here – Part I

By: Steve Gregory , 2:07AM,GMT on August 16,2015

SATURDAY 15-AUG-15

EL NIÑO’S RAPID RISE

After what turned out to be a feeble attempt to develop into a well-developed El Niño episode last year, a series of strong MJO’s (Madden-Julian Oscillation) combined with an intensifying Positive PDO to bring on what is now, a strong and still strengthening El Niño. And while it’s still way too soon to tell if this event will reach the ‘Super El Niño’ classification (a somewhat qualitative descriptor that has only been applied to the warm ENSO events of 1982/83 and 1997/98) – there’s a reasonable possibility that this year’s event may yet reach the Super El Niño characterization – but an equally realistic possibility it could fall back to a ‘moderate’ intensity event.

Since there’s widespread publicity on the possible impacts on US Weather from a strong or Super El Niño, the focus of this post (and Part II to be posted on Monday) will be on the evolution of this year’s event, comparisons with prior events and some of the mechanisms that brought about the current strong ENSO event. The Monday post will look at some of the forecasts for El Nino.

In the Beginning

In the spring of 2014, a series of MJO’s induced modest Westerly Wind Burst (WWB) which, along with a rapidly evolving growing positive PDO (Pacific Decadal Oscillation) SST pattern, led to the rapid onset of a warm ENSO episode over the equatorial EPAC. However, while the down-welling associated with a moderately strong oceanic Kelvin wave did appear to set the stage for a potentially strong El Niño – there was no ‘follow-thru’ of WWB’s or significant Kelvin waves. So by the start of last summer, the warm episode quickly diminished – along with any chance for a major El Niño event.

However, another set of 2, moderate intensity Kelvin waves during OCT and then again in late NOV led to another rapid warming of SST’s in the equatorial EPAC - and by early DEC - SST anomalies of +.07˚C developed in the ENSO 3.4 region. At the time, it appeared that an El Niño was in fact going to develop. Indeed, increased convection in the tropical EPAC did begin to impact the hemispheric wind pattern – leading to a month of above normal Temps nationwide during December. In addition, the one and only major Precip event of the entire winter brought major rainfall to California – another hallmark of an El Niño event.

But at the same time, the +PDO pattern of SST Temps began to intensify rapidly (reaching record levels in JAN) with an especially strong and deep pool of warm water becoming more pronounced in the Gulf of Alaska (GOA). Finally, with no additional Kelvin waves of significance, the warming of SST’s in the ENSO regions ceased – and as SST anomalies fell below +0.5˚C – and critically, below the threshold needed to sustain above normal convection in the tropical EPAC - the impact of the borderline El Niño ceased. And with a record strong +PDO and especially warm waters in the NE Pacific (including the GOA) – one of the coldest winters in the last 30 years became the hallmark of the 2014/15 winter.

RECORD SETTING MJO EVENT = STRONG EL NIÑO

The 2015 El Niño really began in MAR when a powerful, record strong MJO, accompanied by a very strong WWB in concert with a strong Equatorial Rossby Wave (ERW) propagated across the Pacific during MAR. (The MJO remained quite strong as it moved around the entire globe during a 6 week period.) The combined effects of the MJO & ERW triggered a VERY strong oceanic Kelvin wave which produced an equally impressive down-welling effect from the central Pacific eastward to the South American (SOAMER) coast. SST’s rose rapidly, and the down-welling phase of the Kelvin wave led to much above normal sub-surface Temps as well. Two additional MJO’s and associated Kelvin waves in May and again in JUN led to additional, sustained warming in the central and especially eastern Pacific. Concurrently, SST’s in the equatorial WPAC began to decline, with below normal SST’s (and sub-surface Temps) now in place across much of the WPAC.

El Niño began to impact the hemispheric wind pattern by April, and both the atmospheric flow pattern and oceanic SST pattern now reflect a well-established and still strengthening El Niño with the latest SST anomaly in the ‘benchmark’ Niño 3.4 region over +1.8˚C (values above 1.5˚C are considered indicative of a ‘strong’ event). Some portions of the 3.4 region have readings as high as +3.0˚C.

During the next few weeks, the SST anomaly in the Niño 3.4 region should exceed the peak values reached during the 2002 and 2009 El Niño events, but current anomalies remain well short of the 1982 and 1997 peaks. However, peak values are normally recorded during the end of the year. But the weak trade winds (i.e. westerly wind anomalies) are likely to contribute to additional warming of the tropical EPAC - with other indicators such as above normal cloudiness and convection near the Date Line, the negative Southern Oscillation Index (SOI), and warm sub-surface Temps in the eastern tropical Pacific Ocean (and cooler than normal readings in the WPAC) all pointing to the strong El Niño continuing into the start of winter – and possibly into the early spring of 2016. To be sure, there still is a CHANCE it can reach ‘Super El Niño’ intensity IF we get another strong MJO and/or WWB (something that cannot be predicted more than a week or two ahead of time.

One of the best metrics for gauging the overall strength of an ENSO event is the MEI (Multi-variate Enso Index) which takes into account SST anomalies and a variety of other atmospheric indicators. With the MEI indicating a continued strong El Niño condition, it is noteworthy that most MEI components now exceed or equal one standard deviation, with every component well within the El Nino state. Overall, the MEI has been around +2 standard deviations for two months running, the highest overall reading since early 1998 (during the 1997/98 Super El Niño event).

Looking at the nearest 6 rankings in this season, and excluding cases with declining June-July values compared to earlier in the year - we are left with five 'analogs' to consider: 1965, 1972, 1982, 1987, and 1997. All five of them maintained strong El Niño status through at least December of their respective first calendar years. Both the 1982-83 and 1997-98 events are ‘unique’ since they are the only events to reach peak values of +3 standard deviations (and hence the term 'Super El Nino').

One other ‘indicator’ of a strong El Nino (but not included in the MEI or any other ENSO related metric) is the Indian Ocean Dipole (IOD). The weekly value of the IOD index at the start of AUG was −0.10 °C. But 3 of five international climate models indicate a positive IOD event will occur during the Fall – and Positive IOD events are more likely to occur during an El Niño with 50% - 60% of all historical El Niño events having seen a positive IOD develop at the same time.

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Fig 1: Here’s hoping El Niños ‘delivers’ rain to California this coming winter...


Fig 2: Graphic depiction of the 4, Niño ‘Regions’ For reference purposes, the “3.4” region is the overlapping portion of the Niño 3 and 4 regions.


Fig 3: Historical Graph for the Niño 3.4 SST anomalies since 1982. With the very latest SST anomaly of +1.85˚C, the still strengthening El Niño may soon exceed the intensity of all but the 1982 and 1997 events of the last 35+ years.


Fig 4: Southern Oscillation Index – SOI Readings below -7 are reflective of an El Niño condition. Short term fluctuations of the SOI, especially during this time of year (winter in the summer hemisphere) – are associated with transient weather phenomena in the South Pacific. But the trend towards a continuously negative reading is now evident.


Fig 5: Outgoing Long-wave Radiation (OLR) Below normal OLR readings are indicative of above normal cloudiness and Precip. The nearly continuous negative OLR readings near the Dateline since the spring is indicative of a well-established El Niño condition. In some ways – this also marks the beginning of the weakening in the El Niño state as SST’s begin to decline very slowly in response to the reduced solar insolation.

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Fig 6: Indian Ocean Dipole While not a particularly useful metric for predicting an El Niño condition – the tendency for a positive IOD during a moderate or strong El Niño (forecast to develop during the next few months) is yet another indicator of the impact a strong El Niño has on the global circulation pattern - and not just NORAMER.


Fig 7: Comparison of SST anomalies during the 1982 and 1997 Super El Niño



Fig 8: Cross section of current sub-0surface Temp anomalies (Top) and animation of last 2 months (bottom). Much above normal sub-surface Temp readings continue across the central and eastern equatorial Pacific as below normal readings persist and expand slightly towards the east – typical of a mature El Niño event.


Fig 9: Graphic showing the evolution of the record setting MJO last MAR The MJO set in motion the rapid development of El Niño when it triggered a powerful oceanic Kelvin wave which in turn, produced very strong down-welling in the central and eastern Pacific, resulting in strong warming of SST’s.

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Fig 10: Graphical representation of the MJO location and relative strength. The current MJO signal is very weak and incoherent – despite some apparent strengthening last week. Forecasts from all of the major global models call for a continuation of a weak and incoherent MJO. From a 'cyclic' standpoint, we are a bit more likely to see another MJO sometime in September.


Fig 11: Earlier graphic showing the correlation between Kelvin waves and the associated up- and down- welling effect on total ocean heat content.

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Fig 12: Follow-up graphic to Fig 11 above. The correlation of lower level wind anomalies (~850mb or ~5,000’) with the associated Kelvin waves and their impact on Upper-Ocean Heat anomalies. Remarkably, the trailing ‘up-welling’ phase (short dashed lines) had virtually no impact on sensible SST anomalies, but are reflected in changes to sub-surface Temps and the total ocean heat content OHC). But in terms of the large scale impact, the down-welling (warming) phase on SST’s and total heat content has over-whelmed any follow-up cooling during the trailing, up-welling phase of the Kelvin waves.


Fig 13: Sub-surface Temp anomalies at the 105m depth (the typical level of the ‘thermocline’ As mentioned above, the up-welling phase of the passing Kelvin wave usually produces cooling of the upper ocean. Some of the cooling can be seen in the above time-based graphic at the 105 meter depth (approx. location, on average, of the Thermocline).


Fig 14: Comparison of SST anomaly evolution during the 1982 and 1997 Super El Niño events (Threw this in for fun.,.)


Fig 15: Evolution of Sub-Surface Temp anomalies since JAN Clearly seen is the increased warming of sub-surface Temps in the central and eastern Pacific while cooling developed in the western Pacific.


Fig 16: Same as Fig 15 – but comparing current conditions (bottom) with those of just 1 month ago. Warming in the eastern Pacific continues and has actually expanded westward - while sub-surface readings in the WPAC lowered even more.


Fig 17: ENSO 3-D Simulation Shows the relationship of warming ocean Temps and increasing height of the ocean height as warmer water ‘expands’ compared to lower ocean heights when the column of water cools.


Fig 18: Latest JASON Satellite derived image for ocean height anomalies. Strong warming of the ocean in the equatorial Pacific results in rising sea surface heights (‘red to white’ colors) – but much below normal ocean heights in the WPAC ass shown by the cool ‘blue to deep purple’ color tones. Note the still deep (but diminishing) depth of warm water in the GOA. This will be a ‘key player’ in the atmospheric pattern over North America during the coming winter. (To be discussed at a later time.)

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Fig 19: Animation comparing Ocean Heights during the 1997/98 ‘Super El Niño and’ the current Strong El Niño event


Fig 20: Latest available SST Anomalies across the globe As mentioned in a prior post – there appears to be a ‘teleconnection’ of sorts between oceanic SST anomalies in the WPAC and those in the GOA. Cooling in WPAC anomalies tends to signal an impending lowering of SSTA’s in the GOA with a 3-6 month ‘lag’. While not yet ‘proven’ – it will be interesting to see if this relationship unfolds during the rest of the year – especially since SST’s in the NE Pacific can play an important role in NORAMER winter temperatures, and Precip pattern in the West (especially California!).


Fig 21: SSTA Changes during the past month. As alluded to in Fig. 20 above – cooling of anomalies has occurred in the far western Pacific – AND in the GOA - during the past month, as SST’s continued to increase in the Niño 3.4 region. SST have also increased over the tropical Atlantic – although so far, it has meant nothing in terms of ATL cyclone formations!


Fig 22: MEI Values for the ‘Top 7’ strongest El Nino’s of the last 65 years. Using the MEI metric – the current event is already the 3rd strongest El Niño on record.


SPECIAL UPDATE ON EL NIÑO PART II WILL BE POSTED MONDAY, AUG 15 ALONG WITH
THE REGULAR WEATHER UPDATE


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Steve