Driest April-May for Over 80 Years

6 06 2020

Dale C. S. Destin|

The combined rainfall total for April and May is near record-breaking low levels for Antigua. The total for this Apr-May: 36.8 mm (1.45 in), is the second lowest on record, dating back to 1928. Only Apr-May 1939, 81 years ago, had less rain – 27.9 mm (1.10 in).

Normally, these months would produce a combined rainfall of 189.2 mm (7.45 in). This means that the rainfall for Apr-May is less than 20% of the normal total, a deficit of over 80%.

Such low precipitation for Apr-May only happens once every 67 years, on average. In other words, there is only a 1.5% chance of this happening per year. Most Antiguans alive today have never witnessed such dryness before, for these months, and are very unlikely to witness it again.

Usually, Apr-May would account for 52% of the rainfall for Jan-May but instead it only accounted for less than 14%. Both April and May had similar extreme deficits. There have been only three other occasions when both months registered less than an inch of rain in the same year – 1973, 1939 and 1928.

It was not long ago that we were enjoying ample rainfall. First quarter rainfall, Jan-Mar, was above normal. However, this wonderful start to the year came to a screeching halt.

The horizontal (flat line) from Apr 1 to May 31 is indicative of the rapid downturn in rainfall as compared to the previous three months.

The difference in rainfall between Jan-Mar and Apr-May is the second greatest on record, indicative of the extremely sharp downturn in precipitation. We went from 237.7 mm (9.36 in) for Jan-Mar to 36.8 mm (1.45 in) for Apr-May, a decline of 200.9 mm (7.91 in). Normally, Jan-Mar and Apr-May produce 176.0 mm (6.93 in) and 189.2 mm (7.45 in) respectively.

Few of us alive today have ever seen this kind of change of rainfall, in Antigua. This kind sharp decline in rainfall from Jan-Mar to Apr-May only happens once every 100 years, on average. Only 1967 has had a greater decline, 240 mm (9.45 in), over the similar month periods.

This Apr-May is also the driest two-month period since May-Jun of 2001. It is also the 12th driest combined consecutive two months on record.

This past April and May were clearly extremely dry, and this dryness was magnified by the preceding wetter than normal first quarter.

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Driest April for Antigua in Over Two Generations

1 05 2020

Dale C. S. Destin|

Not since 1944 has an April been drier for Antigua than April 2020. It was the third driest April on record – the driest in 76 years or well over two generations. The last time April was drier, V. C. Bird Snr was president of the Antigua Trades and Labour Union and World War II was still raging.

Potworks Dam, May 1, 2020. Picture courtesy Karen Corbin – Humane Society

The island average for the month is 16.1 mm (0.63 in). Only April 1944, with 5.9 mm (0.23 in) and 1939, with 8.4 mm (0.33 in), have been drier in nearly 100 years.

The total for the month represents only 19% of the normal total of 85.6 mm (3.37 in) – a deficit of 81%, based on the current climatological standard period of 1981-2010.

Such a low rainfall total is very rare for April. How rare? It has only around a 4% chance of occurring or once every 25 years on average, based on record: 1928-2019. This would suggest that we were around 50 years overdue for such a parched April.

The reason for us going so many years overdue for such a near record low total is likely due to “positive” climate change. Rainfall for April has been on a steady wetting increase, rising from an average of 53.1 mm (2.09 in) for the period 1928-1957 to 80.8 mm (3.18 in) for the period 1990-2019.

So, if you were to segment the data, there is around an 11% chance of getting 16.1 mm during the years 1928-1957, as compared to just a 0.6% chance for the period 1990-2019. In other words, our current climate for April is over 18 times less likely to produce such a low rainfall total as compared to the past climate of 1928-1957.

Now, based on the segmented data, the return period for April’s rainfall of 16.1 mm is once every 9 years on average for the past climate. However, the return period for the same value in the current climate is once every 167 years on average. Hence, most persons alive along with their grand and great grand children are unlikely to see a similar or drier April in Antigua.

Compared to other months, this April is the driest since June 2018. And of all the 1108 months on record dating back to 1928, it ranks as the 23rd driest. This puts it in the top 2% of all time driest months.

Notwithstanding the miniscule rainfall for April, the rainfall for the year, thus far, is near normal.  However, if not for a wetter than normal first quarter, the situation would be much more dire.

The reason for the absentee rainfall for April looks to be due to a switch from a positive North Atlantic Oscillation (NAO) to a negative NAO. This is manifest in the swing from above normal pressure over the subtropical North Atlantic to below normal pressure. This switch or swing resulted in a reduction of moist unstable air flowing across the area and depositing rainfall.

The pressure over our islands also went from neutral, over the period January-March 2020, to above normal for April. Such a configuration of the pressure would usually also inhibit rainfall, which was evident.

April 2020 was a remarkably dry month. One that most of us have never seen and will likely never see again – happily.

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The Worst Drought on Record for Antigua

25 03 2016

Dale C. S. Destin |

Antigua is witnessing the worst drought in recent history, dating back to, at least, 1928. The current drought is now over 32 months in length, similar to the drought of 1964-67. However, to date, the record rainfall deficit of 1143 mm (45 in) caused by the current drought, exceeds that of 1964-67 by 254 mm (10 in) or around 29%.

RainfallDeficitGraphic

While we don’t have observed monthly rainfall totals beyond 1928, we do have annual totals going back to 1871. Based on this record, 2015 is now the driest year in the series. This translates to 2015 rainfall total occurring once per 500 years, on average. Thus, it’s perhaps the most intense drought since the arrival of Christopher Columbus.

Not only the last year has been the driest on record, but so to have the last two years (24 months). Further, the last 32 months – July 2013 to February 2016, is the driest such period on record. We are missing about a year’s worth of rainfall.

Surface water contributes to around 30% of our potable water mix. However, since the drought started, the country has been completely out of surface water twice with an aggregate duration of around 14 months. We were out of surface water April to September 2014 and again from August 2015 to present.

The drought was caused by a number of climate actors not necessarily all acting at the same time. These include mainly an abundance of the dry and dusty Saharan air layer (SAL) from Africa, positive North Atlantic Oscillation (NAO), negative Tropical North Atlantic (TNA) Index and El Nino. It is fairly well established that these phenomena, in the mentioned phases, cause less than normal rainfall across our area with the converse being true.

The drought got kicked off by the SAL along with unpredictably strong vertical wind shear, sinking air and the weakening of the Atlantic Ocean overturning circulation. Happily, this resulted in a failed hurricane season in 2013, but unfortunately it also plunged us into what has become our worst rainfall deficit on record.

Contributing to the persistent drought, the NAO has been predominantly positive over the duration of the drought with only nine of the last 32 months having negative (rain-favoured) values. Meanwhile, the TNA was negative (unfavourable rainfall values) for most of January 2014 to June 2015. In 2015, El Nino developed and reach super (record) strength during the latter half of the year.

Droughts are expensive, and severe droughts are severely expensive. It’s believed that the drought has cost the country hundreds of millions of dollars, directly and indirectly. I will address this matter more in a subsequent blog.

The current drought is anticipated to become the longest on record – a further very unwelcome new record. Initial predictions had the drought easing significantly or ending around mid-year. However, our last set of forecasts has it continuing into the second half of the year.





Once in a Generation Dry March for Antigua!*

30 03 2015

Dale C. S. Destin |

We are about to exit a severely dry March for most of Antigua. So far for the month, the Met Office, located at the V. C. Bird International Airport, has measured only 8.4 mm of rainfall. This severe dryness only happens once in a generation, on average. This total is not expected to change much between now tomorrow, when the month ends. Hence, this is likely to be among the top five driest Marches and the top 10 driest month of all times.

Top 4 Driest Marches for Coolidge, Antigua
March 2015 in perspective

Barring unexpected appreciable rainfall over the next 24 hours, this will be the fourth driest March on record dating back to 1928. Only March 1999, 1944 and 1930 have been drier.

Relative to other months, this March ranks 10th of all 1047 months of recorded rainfall, at the Airport, for the period.

The probability of March having such little rainfall is quite low; it’s around 3%. This means that this severe dryness we have experienced only happens once in every 33 Marches or once in a generation, on average.

Looking at the year on a whole, this is a 17 year event. The probability of a (any) month of a year, at the Airport, yielding 8.4 mm or less is around 6%. This means that this happens once every 17 years, on average.

March is the second driest month at the Airport with an average rainfall of 46 mm (1.81 in). The minimum and maximum rainfall totals on record are zero and 179.1 mm (7.05 in) respectively.

Drought

Currently, we are in our worse drought, in terms of intensity, since 2002/2003 and it is likely to get worse. We have been in a drought since September 2013.

It reached serious levels in 2014 but was reduced to slight levels in October 2014. At the end of January 2015 it had reintensified to moderate levels and with this month being brutally dry, it is set to drop to severe levels.

Projections

However, seasonal forecasters are not optimistic about rainfall for much of the rest of 2015. Indications are that El Nino, which tends to restrict our rainfall, mainly in the rainy season, is expected to continue deep into the year.

Additionally, the tropical North Atlantic sea surface temperatures are projected to remain near normal for much of the rest of the year. This is generally not helpful for rainfall in our nook of the world.

If El Nino strengthens to moderate levels or worse, our rainfall deficits are likely to be worse that those of last year’s. Recall that last year that we had a water crisis which are yet to get over.

Further, strategies and tools are required to sustainably deal with our water insecurity issues. Two such required tools are an integrated water resource management programme and an integrated drought management programme. Meanwhile, be conservative and efficient with your water usage.

[*It turned out that unexpected heavy downpours stopped March from having “once in a generation” dryness. The heavy downpours that stopped this March from being the fourth driest occurred after midnight on March 31. However, In meteorology, the last day of a month ends at 8 am on the first day of the next month. .

So, instead, the eventual total rainfall for March, at the Airport, was 11.9 mm, the driest since 2001, the seventh driest March and the 18th driest month on record. This kind of dryness, instead, happens once in every 16 years, on average]





How dry has this September been?

19 09 2012

By Dale C. S. Destin

Antigua is currently experiencing a meteorological drought, which started in February of this year. Notwithstanding the active hurricane season, so far, September has been especially dry. The first half of the month only yielded 1.3 mm / 0.05 inch of rainfall at the V. C. Bird International Airport. This tied with September of 1986 for the lowest total at the Airport on record (1971-2012). Below normal rainfall was anticipated for the month so this does not come as a surprise. This sort of rainfall is consistent with a warm Pacific Ocean (El Nino) and a lukewarm/cold tropical north Atlantic (TNA) Ocean. The lowest total rainfall for the month of September on record at the Airport is 27.2 mm or 1.07 inches (1978, at the start of a strong El Nino and cold TNA), while the highest is 410.2mm or 16.15 inches (1995, during a moderate La Nina Episode and warm TNA). Thus, based on record, there has never been a sub-inch total for the month; only two other month has never experienced sub-inch rainfall – August and December. However, at the current rate, and based of the outlook, sub-inch rainfall is quite possible and would obviously make it a record dry September. Further, it would make the drought become severe.








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