Thread by @RobertWLee

New paper from myself, Steve Woolnough, @CharltonPerez and Frederic Vitart published online today in #AGUpubs Geophysical Research Letters.
@AtmosScience @UniRdg_Met @ECMWF
#VotePlanet #MJO #ENSO #teleconnections #NAO
doi.org/10.1029/2019GL…

Summary:
Using a reanalysis to incorporate observations from the past 39 winters, we find a robust dependence of teleconnections from the Madden-Julian Oscillation (MJO) to the North Atlantic/European region weather regimes on the state of the El Niño–Southern Oscillation (ENSO)

More details:
Our study provides the first evidence to support this: in the form of contingency tables, using the method and format from @cassouman40, Nature (2008); geopotential height composites, to show the flow pattern; and analysis of the physical mechanisms

Winter weather regimes in the North Atlantic & European region are useful when considering the ‘subseasonal’ forecasting period, approx. 10–20 days ahead, indicating the average weather covering many countries for many days

The weather regimes are named NAO– (aka Greenland Blocking), NAO+ (aka Zonal), Atlantic Ridge, and Scandinavian Blocking. An NAO+ regime, for example, often brings mild, windy, and wet weather for central latitudes – the British Isles through Denmark

Subseasonal forecasts of weather regimes include can help authorities and businesses to plan ahead in sectors such as agriculture, energy, health (see linked example), aviation and transportation, water, and retail
doi.org/10.1016/j.wace…

ENSO characterises the periodic variation of sea surface temperatures and accompanying atmospheric circulation over the tropical Pacific Ocean, with the warm phase known as ‘El Niño’ and the cool phase as ‘La Niña’, reaching peak intensity during boreal winter

The MJO is an eastward moving ‘pulse’ of suppressed and enhanced cloud and rainfall near the equator which typically recurs every 30 to 60 days during the boreal winter. However, the MJO is not active about one third of the time
metoffice.gov.uk/weather/learn-…

We find that ENSO modifies the MJO, making these regions of wet and dry weather narrower or broader in longitude. This then has a consequence on the type of teleconnection that is triggered

Our study shows that during El Niño winters the teleconnection from MJO phases 1–3 makes the NAO+ regime occur twice as often, with the signal travelling the jet streams. Whereas during La Niña this teleconnection is absent and there is no increase in NAO+ regime occurrence

During La Niña years we find the teleconnection from the MJO phases 6–8 makes the NAO– regime occur up to 2.5x as often – this signal travels via the stratosphere, warming it and slowing the polar vortex, with the total pathway taking around 20 days

Our findings on the mechanisms for the dependence of the teleconnections on the ENSO state are summarized in the paper in this scientific schematic for the key combinations of MJO and ENSO

This statistical method, using the ENSO and MJO conditions, indicates which weather regimes we might expect 10-20 days in advance. Combining this with dynamical models, which can forecast the MJO up to 10 days ahead, may improve the weather outlook for up to 30 days ahead

We end by discussing implications, and suggest that this dependence of the teleconnections on the ENSO state should be well represented in weather and climate models, and has significant implications for the achievable skill in predicting weather regimes on subseasonal timescales

This work is part of the @InterDecProject, looking at inter-regional linkages on subseasonal-to-decadal timescales, and has been funded by @NERCscience via the 2015 joint call from the @Belmont_Forum and @JPIClimate. Thank you to all & reviewers!