Summer review

[Glacier Point]

With all the data in, time to wrap up the summer forecast - all the more important as we are likely to learn the most from anomalous seasons such as this.

Overall a pretty average summer with notable rainfall:

http://www.metoffice.gov.uk/weather/season...2007/index.html

So why was it so bad ?

I remember at least one news article sourced from the BBC weather centre pointing the finger at La Nina (Peter Gibbs I think it was). I beg to differ. El Nino Southern Oscillation (ENSO) remained at neutral values all Summer, possibly moving towards a weak Nina during August (the threshold for La Nina is at least -0.5):

http://www.cdc.noaa.gov/Correlation/mei.data

I read also with interest that subsurface values had influenced the overall ocean-atmosphere coupling making the signal from the Pacific seem like a strong Nina. Well fine, but how do you explain how La Nina has intensified and reconcile this with the slight recovery in August and September pressure and temperatures ?

To me, this Summer was absolutely dominated by pressure anomalies over the polar field which were reinforced by low levels of atmospheric inertia and the Sea Surface Temperature Anomalies (SSTA) in the Atlantic. The renalysis supports height rises to the north as the most dominant feature

Overall the Summer was dominated by anomalously high pressure over the North Pole and anomalously low pressure over the UK.

Positive Polar height anomalies rather nicely coincide with the start of the Summer, almost to the day, related to a rapid warming of the western polar region during April and ridging from the Gulf of Alaska during April/May:

This stimulated a negative Arctic Oscillation (AO). For the record, it was the 13th most negative AO Summer, the 10th most negative AO June and 8th most negative AO July. This resulted in the flooding of our quadrant of the hemisphere with lots of polar air, unleased by the delayed break up of the polar vortex in April.

The linear correlation with AO shows why the overall upper height pattern anomaly fits in well with a -AO (remember this is inverted for -AO):

The SSTA may well have played a part. Below is the reanalysis for all Summers with a similar pattern for March to May. This clearly hints at an upper low pressure anomaly for the UK and +ve height rises over Greenland.

So was the shift in the AO predictable ?

Possibly yes. The Quasi Biennial Oscillation (QBO) is known to have some influence over the the amount of blocking over the polar region. This shifted phase during April. Analysis of similar years during a Spring time switch over of the QBO from westerly to easterly phases suggests that out of the four times this has happened, three occassions resulted in a -ve AO (the other occassion associated with a strong El Nino).

Turning to the forecast made here:

http://www.netweather.tv/forum/index.php?s...ic=38327&hl=

Some good and bad points.

Obviously above the outturn, although not as far out as some agencies predicting +2 anomalies. My range was +0.75 - +1.5C. Probably the biggest error was the original rainfall prediction for below average, which was amended during July to above average.

Plus points were the overall methodology (my main driver, the AO, was wrongly assumed but the most infleuntial factor).

Also well highlighted was the July unsettled anomaly and, during the course of the season, the changeover in the pattern at the end of July into August.

The biggest regret was to disregard the SSTA analogue for all years with a similar SSTA March-May in favour of a much smaller sample of more recent years. As it turned out, the all years comparison suggesting an unsettled and pretty summer was on the money. This also places some question mark over us over-estimating the climatic trend. I factored too much into the months preceding the forecast and was wrong - the synoptics actually influenced the result they should have done - a lesson for the winter forecasts possibly.

One important aspect I've learnt from this forecast is the value of the Globally Averaged Angular Momentum (GLAAM) in conjunction with the SSTA. A -GLAAM, such as this year:

GLAAM effectively represents atmospheric inertia. A negative GLAAM is likely to make the SSTA that bit more influential and more directly related to the location in which they ocurred. This meant that the overall mean trough located over the UK was reinfroced by the effects of a mid Atlantic ridge - a response to the trough over the western Atlantic caused by the cold water SSTAs of the Spring there.

So, overall many lessons learned, lessons put to good effect in terms of the Autumn forceast so far and hopefull the all important winter one.

Edited September 22, 2007 by Glacier Point

[Stu_London]

Very interesting read

For us laymen, we could see northern blocking and a southerly tracking jet as the culprit (June/July anyway). Nice to read the scientific analysis and teleconnections behind the charts.

[BrickFielder]

I am not sure I totally agree with your reasoning, but it is an interesting read. I think the root cause of our dismal summer was actually the westerly QBO during the winter which delayed the break up of the polar vortex. This late polar vortex break up mean't the snowfields across Siberia where a little late melting ,which delays warmth building across asia during spring meaning the monsoon winds when they do start up bring much moister air giving a stronger monsoon and forcing the height patterns across the northern Pacific and towards the polar field.

Incidentally we should expect a late drying out of the soil across Asia this autumn pushing the siberian high more towards Russia during the early part of winter. We also have the easterly QBO and the prospect of stratospheric warmings.

This is all enhanced by SSTA's (sea surface temperature anomalies) but I think the driving mechanism was snow cover and soil conditions across northern asia forced by a strong westerly QBO during the winter.

The Gobal averaged angular momentum (GLAAM) certainly looks interesting and has some links to both the asian Monsoon and MJO Activity( strong activity which likely disrupts la nina patterns) but the strongest link is to la nina conditions. One thing to note is that during low GLAMM the jetstream is likely to be fairly south.

[Glacier Point]

Thanks for the alternative thoughts there Brickfielder.

Interesting to note that the QBO featured largely in both our assessments. I'm often surprised how much little attention the QBO receives as a seasonal forecasting tool. It's not the only player, but it can have such a significant impact (possibly at its extremes), both directly and more often indirectly on the global pressure anomaly pattern that it must be considered.

I'm sure Hart won't mind me posting this, but here's the relationship between GLAAM and ENSO (MEI variant):

As we approach the winter, with La Nina looking for a peak value sometime Nov-Jan, this makes it likely that we will see a -QBO -GLAAM combination at play.

Edited September 25, 2007 by Glacier Point