In Depth

ENSO Signal for the Nino 3.4 region

ENSO is currently running at near-neutral levels after a weakly positive ENSO state for much of early 2020. The NMME ensemble for the Nino 3.4 region is pointing towards a likelihood of a weakly negative ENSO state for the latter part of 2020, developing quickly during the summer. However, there is quite a lot of uncertainty over the extent of the change, with some ensemble members staying near-neutral and others going for a marked La Nina episode. The ECMWF and Met Office ensembles for the region are very similar to the NMME ones, but with fewer ensemble members going for a strong La Nina, with the vast majority keeping the ENSO state higher than -1.

Using the MEI v2 ENSO values, the nearest analogues for the current situation are 1981, 1995, 2005 and 2007 - note that the UK had some very contrasting summers in those years!

This mean sea level pressure anomaly chart looks strikingly similar to the one that was forecast by the composites for the spring quarter, but with a weaker signal - the high pressure anomaly centre is about +2.5mb and lies to the west of the British Isles. This anomaly chart suggests slightly above average pressure over the British Isles with the mean westerly flow rather veered from normal. For June there is a strong signal for a high pressure anomaly just to the north-west of the British Isles, indicating that northerly winds may blow frequently during this June. In the composites, the Junes of 1981 and 2007 were both often cyclonic/northerly, while the Junes of 1995 and 2005 were both very northerly in the first half and somewhat more anticyclonic and warmer in the second half.

For July, the composites point to a low pressure anomaly over Scandinavia, indicating a high frequency of cyclonic/north-westerly weather. July 1995 was a mildly cyclonic south-westerly month which came out hot and sunny in most areas due to a strong continental influence, the Julys of 1981 and 2007 were cyclonic, while the July of 2005 was very northerly in the last third of the month.

For August, the dominant signal is for high pressure, with the high pressure anomaly centre of about +5mb centred just to the west of Northern Ireland. The Augusts of 1981, 2005 and 2007 were all anticyclonic and fairly dry and sunny but cool for most, while August 1995 was a record-breaking hot dry sunny month. Thus, there are some fairly consistent signals coming out of the composites despite the very contrasting nature of the summers of 1995 and 2007 in particular. High pressure and northerly winds both looks set to be more frequent than usual in June and August, but with a cyclonic tendency for July.

Stratospheric Conditions

The QBO (Quasi-Biennial Oscillation) has been in a strongly westerly phase for most of the past year, but it moved into a weakly easterly phase during Spring 2020 and is showing signs of moving into a strong easterly phase. This may increase the likelihood of blocking highs developing to the north of Britain during late autumn and winter 2020/21, as well as the likelihood of sudden stratospheric warmings. However, the QBO tends not to have much of an influence over Britain's weather during the summer, and so I have not examined composites for this metric.

MJO

The Madden-Julian Oscillation is less of a strong influence on Britain's weather in the summer than in the winter, but there are some weak associations between the phases and the frequency of particular weather patterns over the British Isles. Into June the forecast is for the MJO to head through phase 8 into phase 1, and to head towards phase 2 towards mid-June. Phases 8 and 1 are typically associated with a higher than average frequency of high pressure, while unsettled/cyclonic weather is more often associated with phases 2 and 3. Phase 3 is associated with a higher frequency of westerly types.

June is often marked by what Philip Eden called the "June Return of the Westerlies" in his book Weatherwise, with a high frequency of blocking early in June but a tendency for a westerly type to set in later in the month with high pressure over the Azores and low pressure moving over the top. This MJO signal suggests that the westerlies are unlikely to set in until late on in the month of June, pointing towards a relatively settled June, perhaps leading into an unsettled and cool July as indicated by the ENSO composites. The phases of the MJO are extremely difficult to forecast more than three weeks out, so little can be said about the MJO's upcoming effects on the weather during July and August at this stage.

Arctic Sea Ice

The pattern of Arctic sea ice anomalies has flipped somewhat during April and May. During the first three months of the year, the Arctic sea ice extent was unusually low around the Bering and Chukchi Sea area, but unusually high for recent years around Svalbard, where temperatures were below the 1981-2010 average in all three months.

In April and May, atmospheric circulation patterns favoured cold air over the Bering/Chukchi region and warm southerlies over the Norwegian and Russian Arctic. The warmth over the Russian and Norwegian Arctic may also owe much to low snow cover over Eurasia, following a record-breaking warm winter in which places such as Helsinki and Moscow recorded remarkably little snow. Thus, now it is the Bering/Chukchi Sea area that has high sea ice extent for recent years, while in the Norwegian and Russian Arctic it is currently low.

The sea ice extent anomaly pattern was quite similar to this year in 1995, 2006, 2007, 2010 and 2016 (note again the recurrence of the contrasting years of 1995 and 2007). The composites for June-August give a much weaker version of the signal from the ENSO composites, with a weak high pressure anomaly to the north-west of Britain. The Junes of those years, on average, were somewhat less westerly than usual over Britain, with the Julys tending to be cyclonic/south-westerly, and the Augusts being more northerly than usual. The cyclonic Augusts of 2006 and 2010 probably offset the anticyclonic ones of 1995 and 2007 here.

Atmospheric circulation, temperature and rainfall forecasts

The Met Office ensemble mean has very slightly above average mean sea level pressure and temperatures for the summer, but overall points to a near average summer. Despite the slightly above average sea level pressure, the ensembles are going for above average rainfall over the British Isles.

The ECMWF ensembles give quite a strong signal for a drier than average summer, with mean sea level pressure above normal particularly across the south, with the biggest pressure anomaly centre over south-western Britain. Temperatures are also forecast to be above the long-term normal, especially in the south. This could point to a north-south split with relatively cool and changeable weather for the north, and warm dry weather in the south.

The Netweather.tv atmospheric circulation forecasts, based on the long-range CFS, go for a less westerly than average June with a large positive pressure anomaly centre to the west of Britain, near average pressure in July and slightly above average pressure (by 0.5-1 hPa) during August.

The long-range CFS based forecasts from NOAA are strongly agreed on a drier than average June, but with more mixed signals for July and August, though the majority of the ensembles are pointing towards a dry July for the south of Britain. For temperature, the signal for July and August is weak, but a warmer than average June is suggested by most of the sets of ensembles.

The JAMSTEC model is suggesting a warmer than average summer, with a signal for below average precipitation for the west of the UK, especially Northern Ireland. The CanSIPS model is going for a more anticyclonic and northerly June than normal, followed by near-average sea level pressure in July and August.

Summer Forecast Conclusions

June looks set to see a continuation of the sort of weather patterns that we've been seeing during May, with some cool northerlies early in the month, but overall the month looks set to be warmer, drier and more anticyclonic than average. Mid to late June is the most likely time for warm sunny weather with high pressure close to the British Isles rather than out to the west, while westerly winds, which have been largely absent since mid-March, are not likely to return until towards the end of June or even into early July.

July is expected to be a more "westerly" month, but still probably drier than average in the south of Britain. For August, some of the indices are suggesting a high chance of anticyclonic and/or northerly types being more frequent than usual. However, while the ENSO and Arctic sea ice composites are suggesting that June and August will be more "northerly" than average, neither the Met Office nor ECMWF seasonal forecast ensembles are pointing towards an above-average frequency of northerlies overall, though both indicate higher than average pressure overall.

1995 and 2007 recur a lot in the analogues, so they are worth mentioning as comparison points. Overall, this summer looks set to be closer to that of 1995 than that of 2007, but while June looks set to be warm overall despite cool spells early on, July and August look likely to be rather cooler than their 1995 counterparts, with more of a westerly (rather than south-westerly) type in July. There is considerable uncertainty by the time we get to August - it could end up being a scorcher like August 1995, but it looks more likely that August 2020 will be fairly dry and sunny but relatively cool for recent years with highest pressure to the west, like the Augusts of 2005 and 2007 were.

Rainfall

Rainfall totals for the summer look set to be below normal, especially for the south of Britain, but with lower confidence in the rainfall anomalies for northern Britain, due to uncertainties over rainfall in July and August, especially July.

June looks set to be drier than normal over much of the UK, with a mix of anticyclonic and northerly types both bringing predominantly dry weather. There is around 70% confidence in a drier than average June over much of the country.

Confidence is lower for July, which is expected to be a more "westerly" month, but with around 50% chance of the south continuing to be on the dry side, while the north will tend to have near or above average rainfall.

The third month of the forecast period is usually associated with the most uncertainty, but with the dominant signals for anticyclonic and/or northerly weather, August will probably be drier than average for the UK as a whole, with the west and south most likely to be somewhat drier than normal.

Temperatures

Temperatures look set to be above the long-term normal during this summer but with a northwest-southeast split, with the south-east about 1.0C warmer than average, and the north-west of the UK much closer to the long-term normal. Due to the uncertainty over August's temperatures, however, there is about 20-30% chance of the summer as a whole turning out somewhat warmer than that, with positive anomalies of over 1.5C in the south.

Despite some cool northerlies early in the month, June looks set to be warmer than normal, typically by between 1 and 2C in most regions (indeed rather like how May has panned out after the cool northerlies in the second week).

For both July and August, temperatures look set to be closer to normal, but confidence in this is lower. With an enhanced north-south split expected during July, July may turn out warmer than normal in the south-east of Britain, contributing to the overall summer anomaly pattern with the highest positive anomalies in the south-east.

August has about 20-30% chance of being much warmer than average, should high pressure set up close to the British Isles, but it is more likely (about 50% chance) to have near average temperatures due to northerlies and high pressure to the west.