Hot Days and ENSO

Graphical log of max temps and hot days

More frequent hot days do not come in a three year cycle, but in a 1.5 year cycle related to ENSO.

The Hot Day data set

The graph of number of hot days per year

Log of annual hot days in 16 yearsThe graph on the left is one I posted earlier. The height of each data point represents the number of hot days in a year, plotted near January. The pattern of points led me to join them by a smooth curve. This curve swings up and down rather regularly, with five peaks and five dips in the fifteen years. That is, more frequent hot days seem to come in a three-year cycle.
Is this cycle “real”? Should we look for a cause? Will the cycle continue?
Probably not! The points of measurement are one year apart. Cycles that are only three years long may be “aliases” of different and shorter undetectable cycles. (See Note below on Nyquist frequency.)

More detailed hot day data

Other graphs already shown include further data: the number of hot days in each month, and the 13-year average number of hot days in each calendar month. From these I have calculated a relative frequency. That is, the ratio of the actual number to the average number for that month.
Only the months of November, December, January and February have enough hot days to calculate a relative frequency, but these can show changes within the hotter months of each year.

The daily maximum temperature data set

A graph that I posted in “El Niño and my climate” shows a curve of smoothed monthly means of daily maximum temperature anomalies. The yearly cycle of summer-to winter temperature has been removed. I have also applied a smoothing function, which makes the monthly points of measurement effectively two or three months apart. As a result, cycles longer than about six months can be detected.
There are about 10 peaks and 10 dips in the 15.5 year curve. They define a cycle of about 1.5 years wavelength. That cycle is so much longer than the minimum-detectable six month cycle that “aliasing” is not likely.
The reality of this temperature curve is supported by its close similarity to the recognised curve of the El Niño – Southern Oscillation (ENSO), as read from NINO3.4 Pacific Ocean sea surface temperature anomalies.

A combined graph of hot day and temperature data

The graph at the top of the page presents the monthly smoothed maximum temperature anomaly again, using the scale at the left. To this I have added data on the number and frequency of hot days.
The annual number of hot days is shown in blue, in blue boxes. The boxes are placed higher or lower according to the number, but the height is adjusted to match other data better.
A “Hot Day Index” is shown by blue diamonds. This index is based on the relative frequency of hot days in each month that has data.  I have re-scaled the values to improve the match. (See Note on Re-scaling below.)

Matching hot days with temperature

Continue reading

Manilla’s Hot Days

I have used my 13-year weather record to find the number of hot days in each year and in each month. Earlier I did the same for frosty mornings. Because the summer, which has the most hot days, crosses from one calendar year to the next, I have begun each year at July.
I have called days warmer than 35° “hot days”, and days warmer than 40° “very hot days”.

Total hot days

The first graph shows the number of hot and very hot days in each year. The most hot days were in the year ’09-’10, which had 44; the fewest were in the year ’11-’12, which had only 4. The 13-year average is 26, but the number of hot days is quite different from year to year.

Counting only the very hot days, ’03-’04 had the most (6), and four years had none at all. On the average, two days exceeded 40° in a year.
Hot days each year, and seasonal distribution.

Months with hot days

The second graph shows how the number of hot and very hot days peaks strongly in January, with very few earlier than November or later than March. On average, Manilla’s summer has about 22 days warmer than 35°, while spring has 3, and autumn only 1.

The other graphs show how each year had a different pattern of hot days. The highest monthly peaks, each 19 hot days, came in January 2003 (following drought) and January 2007. Annual peaks also came in January in 2008 and 2012, but these peaks were extremely low: only 4 and 3 hot days. Continue reading

Spring 2009 warm and dry

Weather log spring 2009

This spring was warm and dry, partly due to the extreme heat wave in the final two weeks. In that period, records were broken for the highest temperature in any month: highest daily maximum (42.6°), highest daily maximum temperature anomaly (+13.5°), highest daily minimum (27.8°), and highest weekly average (31.3°).

Compared to decade averages, the mean maximum (28.4°) was up 2.1° and the mean minimum (11.7°) up 1.3°. For dryness, the morning Dew Point (6.0°) was down 1.5°, and cloudiness (29%) down 1%. Although this spring was very warm and dry, spring 2002 had been much warmer and drier. Values then were: max 29.2°, min 12.1°, Dew Point 5.4° and cloudiness 19%.
In spring 2002 there had been only 66 mm of rain: the fifth lowest on record. Rainfall this spring was 121 mm. Similar spring totals were seen in 2007 (122 mm), 2006 (139 mm), and 2003 (125 mm). These values are on the 30th percentile (which is not very low), but seem low compared to recent extremely wet springs in 2008 (295 mm), 2005 (260 mm) and 1999 (262 mm). The long-term spring median is 156 mm.

Climate spring 2009

Data. Rainfall data is from Manilla Post Office, courtesy of Phil Pinch. Temperatures, including subsoil at 750 mm, and other data are from 3 Monash Street, Manilla.