A drought has begun

A year ago, I showed that Manilla was far from being in a drought. That is not true now. There are severe shortages of rain.

Rainfall status at Manilla, September 2016 and September 2017.

The first graph has rainfall totals up the left margin. They are not expressed in millimetres but as percentile values, Along the bottom margin is the number of months included in calculating each rainfall total.

On the graph, I have compared the rainfall situation today, September 2017, plotted in red with that of September 2016, plotted in grey. Much has changed.

Take, for example, the 12-month (one-year) rainfall total. Rainfall totals for 12 month periods are directly above the value “12” at the bottom of the graph, near the label “Number of Months included”. In data for the month of September 2016 (grey), the 12-month total (actually 802 mm) had been at the 80th percentile, which was very high. In up-to-date data for the month of September 2017 (red), the 12-month total (actually 484 mm) is at the 17th percentile, which is very low.
Although rainfall totals for  periods longer than 12 months have not fallen so much, nearly all of them have fallen. Three that have not are those for 30 months, 36 months and 42 months. They were already low, due to including in them some months of low rainfall several years ago, in 2013 and 2014.

So far, real shortages have occurred mainly within the last 12 months. Beyond that, the two-year rainfall total of 1285 mm, for example, is still near normal, plotting at the 48th percentile.

The second graph shows in detail how shortages that are serious or severe have developed during the last six months. These were the monthly rainfall amounts, with the normal amounts in brackets:

April: 24.0 mm (39.3);
May: 55.6 mm (40.3);
June: 62.8 mm (44.3);
July: 13.2 mm (41.4);
August: 13.8 mm (39.5);
September: 5.5 mm (41.2).

As a result, the current situation is as shown below. There are already severe rainfall shortages, at the 2nd or 3rd percentile, in the two-month and three-month totals to date. There are also serious shortages, at the 8th and 9th percentiles, in the four-month and six-month totals to date.

Drought status at Manilla in September 2017

I will update these graphs each month to show how the situation changes.

[Serious rainfall shortages did not occur in any following months up to January 2018.]


The 2002 rainfall shortages at Manilla

Graph of monthly percentile rainfall in a drought

In 2002, Manilla had a 6-month drought with one of the most extreme rainfall shortages on record. In nearly fifty years since 1966 there have been no other shortages like it.

I have discussed this drought in two posts: “Profile of an Extreme Drought”, and 3-year trends to August 2004 (An extreme 1-year drought).

This post is about the rainfall record only. It compares the percentile values of rainfall totals for groups of months: one month, two months, and so on. The graph shows how the drought began, developed and faded. Other droughts may go through similar stages. I have plotted the pattern of rainfall shortages month by month, showing only even-numbered months. I have plotted them in different colours, with matching “Call-out” labels.

April 2002 (Red): no drought yet.
In April, the monthly rainfall was slightly below average: in the 40th percentile. In this month, nearly all rainfall totals up to the 42-month total were also below average. Only the 6-month total was above average. This set up the conditions for a drought. Notice that rainfall totals for periods longer than 42 months were all well above average. This hardly changed at all in this year. There had been a lot of rain in previous decades.

June 2002 (Orange): 2, 3, and 4-month droughts.
When May rainfall was in the 1st percentile and June rainfall in the 25th percentile, the June 2, 3, and 4-month totals became serious or severe shortages (below the 10th percentile).

August 2002 (Green): 2, 3, 4, 5, 6, and 9-month droughts.
With July rainfall again in the 1st percentile, and August rainfall in the 26th, the drought became extreme. The 4, 5, and 6-month totals were in the 1st percentile: few months had ever had such low figures.

October 2002 (Blue): 3, 4, 5, 6, 9, 12, 15, and 18-month droughts.
September and October both had rainfall in the 18th percentile. That relieved the short-term shortages somewhat, but not those in the medium term. Shortages in the 4, 5, and 9-month totals were in the 1st percentile, but the 6-month total was very much worse. At 76 mm, this 6-month total was the third driest on record, beaten only by August 1888 (43 mm) and September 1888 (69 mm).

December 2002 (Purple): only 9- and 12-month droughts remain.
November rainfall that was near average (40th percentile) and high December rainfall (84th percentile) broke the drought. Only some longer-term effects persisted as severe rainfall shortages in 9- and 12-month totals.

Profile of an Extreme Drought

Rainfall vs Maximum temperature, 2002 droughtAt Manilla, NSW, there was a drought in 2002 that was extreme, but brief. There have been no other extreme droughts at Manilla in the 21st century. The current drought is not as bad (yet).
The first graph shows a profile of the 2002 drought. Low rainfall is at the top, and hot days are on the right. Droughts, with low rainfall and hot days, will be near the top right corner. Normal climate is marked by a rectangle (coloured aqua (aqua)) in the middle.
The climate in these months moved into drought and out of it. January 2001 (Start) had perfectly normal climate with no drought, and so did February 2003 (Finish). Rainfall first became lower than normal after January 2002, and reached a minimum 27 mm below normal in July 2002. Rainfall returned to the normal range by December 2002. Day-time temperature went above the normal range in May 2002, reached a peak 1.3 degrees above normal in September-October 2002, and fell back into the normal range in January 2003. For rainfall lower than normal, the drought lasted ten months: for days hotter than normal, it lasted eight months. In this drought, the time of lowest rainfall came two to three months earlier than the time of hottest days.

(There is more detailed analysis of  the 2002 drought in a post dated September 2004.)

The loop on the graph shows this drought as a simple event with a beginning, a middle, and an end. Droughts are not usually seen to be so simple. This graph is made using two “tricks”: anomalies and smoothing. You must judge whether you trust them to describe the drought as it happened.

Continue reading

3-year trends to August 2004

Parametric plots of smoothed climate variables at Manilla

“An extreme 1-year drought”

Trends to August 2004

At Manilla a sudden extreme drought peaked in winter and spring of 2002. Each of seven climate variables went through a large cycle, with a peak at the top right-hand corner of its graph.

(Note added in May 2014: This 2002 event is the most extreme rainfall drought at Manilla so far in the 21st century. Current conditions may perhaps be similar.)

Extreme values of anomalies.

The smoothed anomaly of daily maximum temperature (X-axis on all graphs) exceeded +0.7° in June 2002, and remained above that value until December. It peaked at +1.30° in September-October 2002. (Note added: This stood as a record high value for seven years until October 2009.)
Smoothed monthly rainfall anomaly fell below -14 mm in February 2002, and remained below that value until November. It peaked (negative) at -27.1 mm in July 2002. (Note added: In May 2014 this still stands as a record low value.)
The smoothed anomaly of percent cloudy mornings (more than 4 octas) fell below -7% in July 2002, and remained below that value until December 2002. It peaked (negative) at -11.3% in October 2002. (Note added: In May 2014 this still stands as a record low value. More cloud in recent years has made negative anomalies rare.)
The smoothed anomaly of early morning dew point fell below -0.7° in May 2002, and remained below that value until January 2003. It peaked (negative) at -1.48°  in September-October 2002. (Note added: This stood as a record low value until June 2011. Since then, the air has become much drier, taking anomaly values three times as low.)
The smoothed anomaly of daily temperature range, like that of daily maximum temperature, was positive during the drought. It exceeded +0.7° in April 2002, and remained above that value until November. It peaked at +1.23°  in July-August 2002. (Note added: In May 2014 this still stands as a record high value.)
Smoothed daily minimum temperature anomaly reached a maximum of only +0.38° at the peak of the drought in November 2002. It had risen smoothly from the strongly negative value of -0.91° in November 2001. Through 2002, the rise in daily minimum temperature lagged behind that of daily maximum temperature, resulting in a “continental” type of climate with wide daily temperature range. (Note added: Higher daily minimum temperature anomalies occurred in winter 2007, and much higher ones in spring 2009.)
The smoothed anomaly of subsoil temperature, like that of daily maximum temperature, was positive during the drought. It exceeded +0.7° in August 2002, and remained above that value until February 2003. It peaked at +1.48°  in November 2002. (Note added: This value was exceeded from January to May 2007, and further exceeded in winter 2013.)

Leads and lags.

The climate variables did not all peak at once: some led, and some lagged. In the table below I show three estimates of lead or lag (in months):

  1. I identified dates of peak value on the graphs;
  2. I noted a point mid-way between the date of departure from normal and that of return to normal as the median date;
  3. Where the line on the graph formed a hysteresis loop (shown by a curved arrow), I drew other graphs with a lead or lag imposed to compensate for the actual lead or lag. I estimated the amount of lead or lag that would make the loop into a straight line.

Table of leads and lags of climate anomaly variables

In the table, I take the dates for highest excess of daily maximum temperature as the standard, to show the pattern of leads and lags of the other variables. Rainfall deficit led by 2.3 months and daily temperature range excess led by 1.8 months. Early morning dew point deficit (dry air) did not lead or lag. The deficit in cloud amount lagged by 0.4 months, and the excess of both daily minimum temperature and subsoil temperature lagged by 1.8 months.
(Note added: Leads and lags in more recent climate events at Manilla have a pattern that is similar, but not the same.)

Fully smoothed data – Gaussian smoothing with half-width 6 months – are plotted in red, partly smoothed data uncoloured, and raw data for the last data point in orange. January data points are marked by squares.
Normal values are based on averages for the decade from March 1999. They appear on these graphs as a turquoise (turquoise) circle at the origin (0,0). A range of anomalies called “normal” is shown by a dashed rectangle in aqua (aqua). For values in degrees, the assigned normal range is +/-0.7°; for cloudiness, +/-7%; for monthly rainfall, +/-14 mm.

(Note added in May 2014: A much later post titled “3-year trends to May 2010” is the first of a consecutive series of parametric plots, updated monthly at the time of observation and originally posted elsewhere.
This earlier data was not edited in this form at the time.
To display all existing fully-smoothed data points at least once, I have prepared these back-dated posts in the same format for:
“3-year trends to August 2002” which includes smoothed data September 1999 to February 2002, covering the calendar years 2000 and 2001.
“3-year trends to August 2004” (this post) which includes smoothed data September 2001 to February 2004, covering the calendar years 2002 and 2003.
“3-year trends to August 2006” which includes smoothed data September 2003 to February 2006, covering the calendar years 2004 and 2005.
“3-year trends to August 2008” which includes smoothed data September 2005 to February 2008, covering the calendar years 2006 and 2007.
In these back-dated posts the anomaly values depend on climate normals that are based on the decade ending February 2009, and were thus not available until after that date. I have written the posts as if they were available at the time.
In places I have written some “Notes added in May 2014” (like this) commenting on how values observed at that time relate to more recent events.)