Thread by @althecat

#LaNina #ENSO & drought expectations in Africa - a deep dive into latest satellite imagery and model atmospheric precipitable water data.

Thread...

[image: There is remarkably complex water transport activiy underway over North Africa and the Middle East.]

First some background. ENSO, La Nina and El Nino is an oscillation related to sea surface temperatures in the Eastern Pacific. It is an important indicator of drought activity, especially in the US, but also as far away as Africa and NZ. La Nina typically mean less rain.

The image above shows equatorial tropical atmospheric water flows, and helps with understanding of why the Eastern Pacific sea surface temperature is important. Less convective storms in the Eastern Pacific leads to less atmospheric water everywhere.

The image above shows the last 24 hours of satellite imagery of the globe (albeit in a mercator projection). You can see in it that there is a lot of convection in the tropics, which is generating numerous atmospheric rivers in both the Nth and Sth hemispheres.

The image in the initial tweet is repeated here. Whilst there is a persistent La Nina, and has been for 3 years, there is also a unusually high levels of atmospheric water - just one month after Equinox. This will increase and move northwards through summer.

The next set of tweets contains data observations for what we are seeing in the entire tropical belt. Satellite imagery comes from @zoom_earth and PWAT anomaly data from @weathermodels_. And we start in the eastern Pacific where the La Nina oscillation is measured.

Here is a corresponding model forecast for 16 days for the same area. We will look at the ENSO forecast observations next - but in this model forecast we can see robust levels of atmospheric water vapour over the ocean.

This is the La Nina. A massive area of sea surface temperature anomalies that extends all the way to far western Pacific. These temperatures should be suppressing convection, and therefore aerial water flows.

A full technical ENSO forecast is issued very regularly - because it is so important. The latest one - 2 days old - is here. And its conclusion is that La Nina is present and there is 50% probability of it remaining through to summer solstice. cpc.ncep.noaa.gov/products/analy…

If this happens then this La Nina will last longer than last years. These two slides show:
1. That there are convection anomalies present in the Eastern Pacific (i.e. evidence of suppression by La Nina)
2. A broad range of model predictions on what will happen in current months.

And finally a graphic which shows changes in SSTs over a a four week period March-April. Temperatures are rising in the Eastern & South Pacific - the fastest change however is in the North West Pacific and in the Western Indian Ocean.

I am no ENSO expert. Its a complex business. But the question this thread is seeking to answer is whether the La Nina is resulting in a fall in tropical atmospheric water.

My working assumption is that if it isn't then its capacity to cause droughts is diminished.

Here's a satellite image of the Western Pacific over 12 hours for context. This area is one of the largest convective storm sources of atmospheric water in the tropics. It is where the largest and most intense cyclones on the planet develop.

And the PWAT anomaly forecast for the Western Pacific also shows no sign of PWAT (atmospheric water) in the tropical stream being reduced. It does however show what looks like a major typhoon developing.

Further west we have the Indian Ocean which is thought to be the ocean which is being most impacted by Climate Change - and which current data shows has experienced significant SST rises in the West over the past month.

And in PWAT terms the Indian Ocean tropical area is also strong, spectacularly so, partly because the forecast also shows the impact of a very late tropical disturbance/storm in the South Western Indian Ocean.

This can be seen more clearly in this forecast for PWAT anomaly over Africa. The storm is expected to mainly impact Mozambique but outflows from it appear to be contributing to a burst of unusually high levels of moisture over the Arabian Peninsula.

To complete the picture we have the tropical flows over the Atlantic - where there is a lively interaction between the West African Monsoon and the Amazon Monsoon.

For this area we have 2 PWAT forecasts the 1st for the Tropical Atlantic (equatorial water flow is at the bottom) moving west from the West African Monsoon. This is helping to fuel vigorous atmospheric rivers back across the Atlantic in a Nth Easterly direction.

And our final PWAT anomaly plot shows South America. This time the main westerly tropical flows can be seen towards the top.

I now return to the data which prompted this inquiry. For several days we have seen significant storm activity over the Arabian Peninsula.

This image is from today.

The storms are significantly larger than forecast & caused by Indian Ocean, WAM and Amazon water flows.

And they are forecast to intensify over the coming fortnight. This can partly be put down to a possible cyclone in the Indian Ocean, but this activity has started before that storm exists.

The intense Horn of Africa/East African Monsoon - which brings the main rains to the Nile is not due till July - but significant rains are forecast for the Horn of Africa also over the next fortnight.

And we also see a persistent significant PWAT anomaly over the Sahel.

I have been watching to see whether rains in this particular area would follow a similar pattern to last year - in timing and intensity. And specifically whether the strong La Nina would slow them. So far it isn't.

Whilst there are similarities in what we are seeing over Saudi Arabia with last year - the causes of this years rainstorms appear to be different - and stronger.

Here is a raw PWAT forecast for the area. And it is showing levels of PWAT over the Sahel seen last year far later in the year than this.