Simon Noone, Conor Murphy and Peter Thorne
Drought has begun to fade from the national conversation following a week of rain, but observations show that Dublin remains in the grip of the one of the most intense flash droughts, probably the most, since at least the middle of the nineteenth Century.
There are numerous definitions of drought. None are perfect. In Ireland we have tended to use an index called the Standardised Precipitation Index (SPI) (McKee et al. 1993). This makes sense in that most of the time it is a lack of rainfall that leads to drought in our relatively moderate climate. But there are other indexes such as the Standardised Precipitation and Evapotranspiration Index (SPEI) (Vicente-Serrano et al.2010) which, in addition, account for removal of water via evaporation and plant transpiration. This index is arguably better for characterising flash droughts that tend to occur in the warm season. It uses a combination of monthly rainfall and temperature records.
The long-term context
The Noone et al. (2015) assessment of variability and change in precipitation over the period 1850–2010 indicates positive trends in winter and negative trends in summer precipitation. Noone et al. (2017) produced a 250 year drought catalogue for the Island of Ireland by applying SPI-12 ( a 12 month drought index) to identify long hydrological drought rich periods. The results show that Ireland is drought prone but recent decades are unrepresentative of the longer-term drought climatology. Ireland has experienced seven long drought rich periods over the period 1850-2015 impacting the whole of the island of Ireland; (1854–1860, 1884–1896,1904–1912, 1921–1923, 1932–1935, 1952–1954 and 1969–1977). But it is also possible to have ‘flash’ droughts, such as that in 2018.
Phoenix Park is one of the longest continuous meteorological stations in Ireland. Records started in the early 1800s and continue through present. This long record is thanks to the significant efforts of Met Eireann, Ordnance Survey Ireland and others. Although instrumentation has changed over time, the site has always stood in the grounds of Ordnance Survey Ireland and the regional environment has remained largely unchanged. The precipitation series was homogenised by ICARUS as part of Noone et al. (2015). The temperature record has not been homogenised to date.* Met Eireann make monthly and daily summaries available. To perform the present assessment we have appended the July daily values aggregated to monthly averages** to the long monthly series and calculated SPEI-3 (a 3 month drought index) for the entire series.
Phoenix Park site photo sourced from Met Eireann. 53°21‘50” N, 06°20’00’’ W, 48 m above mean sea level
SPEI-3 at or very close to record levels
The provisional value of SPEI-3 for May to July is -2.70 which is the most extreme value on record, beating October 1995 by a whisker***. The next most extreme was August 1995. Before that the index values are sufficiently different to conclude that either 2018 or 1995 represent the worst flash drought since modern meteorological records began in Ireland, with 2018 marginally more likely.
SPEI-3 values calculated at Phoenix Park January 1850 to July 2018. Red lines show 3 month accumulative deficits and blue lines show 3 month accumulative surplus.
One important thing to note is that the two most extreme prior months occurred in the same year. This shows that drought can appear to diminish only to quickly return. We may well have had a week of rain (although even that in Phoenix Park amounted to only 47% of the long term monthly average rainfall for July), but that does not even begin to undo months of deficit.
Table of the ten most extreme values of SPEI-3 in the Phoenix Park record. Negative values denote deficits. The more negative the index the more severe the drought conditions.
Perhaps more worrying still is the seasonal forecasts for the coming two months which show a real possibility of the meteorological set-up that led to the current drought returning and persisting. Don’t be fooled by a week of recent rains. We are potentially far from done yet.
Current seasonal forecast output from the US National Oceanic and Atmospheric Administration CFSv2 system. Daily deficits of >-0.2mm/day are predicted. Note large uncertainties exist in seasonal prediction systems which should only be used as indicative guidance.
*A reason to say probably rather than definitively
** Another reason to say probably rather than definitively in that monthly summaries have quality control applied by Met Eireann which may lead to a mismatch between the aggregated to monthly daily reports and eventual monthly summary of record.
*** Yet another reason to say probably rather than definitively.
McKee TB, Doesken NJ, Kleist J. (1993). The relationship of drought frequency and duration of time scales. In Eighth Conference on Applied Climatology. Am. Meteorol. Soc. January 17–23, 1993, Anaheim CA, 179–186
Noone S, Murphy C, Coll J, Matthews T, Mullan D, Wilby RL, Walsh S. (2015). Homogenization and analysis of an expanded long-term monthly rainfall network for the Island of Ireland (1850–2010). Int. J. Climatol. 36: 2837–2853, doi: 10.1002/joc.4522.
Noone S, Broderick C, Duffy C, Matthews T, Wilby R. and Murphy C. (2017), A 250‐year drought catalogue for the island of Ireland (1765–2015). Int. J. Climatol, 37: 239-254. doi:10.1002/joc.4999