In a state of ENSO-limbo

While models are hell-bent on a big El Nino, the start of June carries over with climate drivers in a 'neutral' phase.

Today the Bureau of Meteorology (BoM) releases its June climate driver update. Many are expecting the declaration of the key ENSO battleground of NINO3.4 to be reaching the +0.8°C sea surface temperature. The +0.8°C anomaly is the threshold required to meet BoM’s ‘El Nino Alert’, however, it must be sustained for 3 months in order to meet the Active El Nino phase requirements.

This indicator is also known as the Oceanic Niño Index (ONI), keeping in mind that American climatologists at NOAA recognise the threshold as less-conservative +0.5°C.

Another ENSO phase indicator is the Southern Oscillation Index (sea level pressure divergence between Tahiti & Darwin). Currently, this 30-day index exceeds the El Nino threshold value of -7, averaging -15 on the index last month. The index must be sustained for 2 months for BoM to declare the ‘El Nino Alert’ phase. It looks like we will require June’s average to allow BoM to raise the ENSO phase to the ‘Alert‘ level.

NOAA’s most recent Sea Surface Temperature Anomaly (4th June).

The latest 3-month ONI depicts the deep swing from the La Nina phase last year to ENSO ‘neutral’ by Southern Autumn, this year.

So, whilst ALL climate models are indicating a swift transition to El Nino by July we should anticipate there to be a lag as to how the atmosphere responds - meaning we may not see direct El Nino effects for weeks after reaching the +0.8°C threshold.

In an El Nino phase, this lag is often characterised by persistent low-level (850mb) westerly winds that drive the phase and atmospheric cycle.

In NOAA’s most recent update, we can observe the prevalence of westerly winds in the Western Equatorial Pacific Ocean, as a result of convective storms in Papua New Guinea over the last month. However, this flow is not considered overly strong or persistent.

The lack of ‘persistent‘ Westerly is best depicted in the below time series plot of observed 850mb equatorial winds, highlighting the frequent swing between westerly winds (amber/orange shading) and easterly (blue shading) wind phases since February. Therefore a neutral ENSO pattern has been established in the atmosphere since around March.

With the above in mind, I am struggling to see why BoM wouldn’t elevate the ENSO phase to ‘Alert‘ levels. At least 2 of the 4 criteria are likely to be met.

There are observations of trending positive SSTA’s that meet the ‘alert‘ threshold.

This said we are yet to see those sustained SSTs (NINO3.4 +0.8°C) and evidence of atmospheric responses such as those Westerlies driving convection away from Australia and the Maritime continent.

The Indian Ocean Dipole (IOD) is the other climate driver vying for influence over the Australian continent. Like ENSO, the IOD is currently identified as being in its ‘neutral’ phase. In recent weeks we have started to see temperatures cool in the Eastern region (near equatorial Indonesia) with a warming phase underway in the West, near Northern Africa.

That said, we have seen waters warm around equatorial Indonesia over the last 3 months.

Furthermore, the depth of this cooling/heating is a little less studied (compared to ENSO) and as such there is more uncertainty in the outlook, particularly if ENSO doesn’t verify to the extent the models suggest.