Tuesday, May 12, 2015

El Niño in the tropical Pacific

Sou | 4:58 PM Go to the first of 5 comments. Add a comment
El Niño has arrived in the tropical Pacific...

From the Australian Bureau of Meteorology:

El Niño in the tropical Pacific

Issued on 12 May 2015

The tropical Pacific is in the early stages of El Niño. Based upon model outlooks and current observations, the Bureau's ENSO Tracker has been raised to El Niño status.

El Niño–Southern Oscillation (ENSO) indicators have shown a steady trend towards El Niño levels since the start of the year. Sea surface temperatures in the tropical Pacific Ocean have exceeded El Niño thresholds for the past month, supported by warmer-than-average waters below the surface. Trade winds have remained consistently weaker than average since the start of the year, cloudiness at the Date Line has increased and the Southern Oscillation Index (SOI) has remained negative for several months. These indicators suggest the tropical Pacific Ocean and atmosphere have started to couple and reinforce each other, indicating El Niño is likely to persist in the coming months.

International climate models surveyed by the Bureau indicate that tropical Pacific Ocean temperatures are likely to remain above El Niño thresholds through the coming southern winter and at least into spring.

Sea Surface Temperature: Here are the changes in the sea surface temperature between 22 December 2014 and 10 May 2015. Click to enlarge as always.

Sea Surface Temperature - 22 December 2014 to 10 May 2015
Source: Bureau of Meteorology

Sub-Surface Temperature: Here are the changes sub-surface between February 2015 and May 2015:

Sub-Surface Temperature - February 2015 to 10 May 2015
Source: Bureau of Meteorology

Southern Oscillation Index (SOI): Here is a chart of the 30 day moving SOI.  The SOI is a measure of the As BoM states: "Sustained positive values of the SOI above +7 may indicate La Niña, while sustained negative values below −7 may indicate El Niño. Values of between about +7 and −7 generally indicate neutral conditions."

30 day moving SOI. Source: Bureau of Meteorology

BoM normally uses 0.8, not 0.7 - is this a climate change? :D

In case you're new to the subject, the SOI is calculated using the pressure differences between Tahiti and Darwin. It's long been used by BoM as an indicator of ENSO status. As stated on the BoM website:
Sustained negative values of the SOI below −8 often indicate El Niño episodes. These negative values are usually accompanied by sustained warming of the central and eastern tropical Pacific Ocean, a decrease in the strength of the Pacific Trade Winds, and a reduction in winter and spring rainfall over much of eastern Australia and the Top End. 

POAMA Projections: Here are the projections for ENSO, from BoM's Predictive Ocean Atmosphere Model for Australia (POAMA). The chart below shows that we've only just crossed into El Niño territory. BoM measures El Niño differently to other weather bureaux:

POAMA model projections for ENSO - Nino 3.4 Source: BoM

Further Reading:


  1. Hi Sou

    The missing link - compared to last year - was the westerly wind burst through the central Pacific. This has continued to drive the SSTanomaly eastward.


    BTW don't be surprised if this turns out to be the Modoki style of El Nino. I'm not yet 100% convinced - but there is historical support for this mode to eventuate


    1. Good reminder George. I picked up on mention of that at the time.


      And yes, who knows what it will do this year.

  2. Interesting to see that an out of season tropical low / tropical cyclone is likely to bring heavy (damaging) rain to the Solomon Islands / Bougainville Island over the coming week.


    The consequential westerly equatorial burst will also further amplify the warming of the central Pacific tropical waters.

    Regards - George.

  3. Media has now picked up on this system.


    On the scale of "unusual" - a southern hemisphere Pacific Ocean TC in July would be right out there - if it were to be formally named. The main impacts however will be very significant out of season heavy rain though the region.


  4. BTW here is the BoM outlook:

    "A tropical low situated to the north of the Solomon Islands is currently moving in a southwestwards direction and could enter the Eastern Region area of responsibility late Tuesday or during Wednesday. The tropical low is expected to develop further and could form into a tropical cyclone near the Solomon Islands on Wednesday or Thursday."


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