Onset, active and break periods of the Australian monsoon

Size: px

Start display at page:

Download "Onset, active and break periods of the Australian monsoon"

Joleen Webster
5 years ago
Views:

IOP Conference Series: Earth and Environmental Science Onset, active and break periods of the Australian monsoon To cite this article: Hakeem A Shaik and Samuel J Cleland 2010

1 IOP Conference Series: Earth and Environmental Science Onset, active and break periods of the Australian monsoon To cite this article: Hakeem A Shaik and Samuel J Cleland 2010 IOP Conf. Ser.: Earth Environ. Sci View the article online for updates and enhancements. This content was downloaded from IP address on 08/10/2018 at 16:28

2 Onset, active and break periods of the Australian monsoon Hakeem A Shaik 1 and Samuel J Cleland 2 1 Northern Territory Regional Office, Australian Bureau of Meteorology, PO Box 40050, NT Regional Office, Bureau of Meteorology, Casuarina NT 0811, Darwin, Australia 2 Bureau of Meteorology, Cape Grim BAPS, Smithton Tasmania TAS 7300, Australia h.shaik@bom.gov.au Abstract. Four operational techniques of monsoon monitoring the Australian monsoon at Darwin have been developed in the Darwin Regional Specialised Meteorological Centre. Two techniques used the rainfall only criteria and look into the onset of wet season rainfall/monsoon rainfall. The other two techniques are based purely on Darwin wind data. The data used for the study ranges from 14 to 21 years. The main purpose of the study is to develop near-real time monitoring tools for the Australian monsoon at Darwin. The average date of onset of the monsoon ranges from 19 December to 30 December. The average date of monsoon onset is 28 December. In eleven out of twenty-one years the onset date remained within three days range between the two rainfall techniques, whereas it is eleven out of fourteen years between the wind techniques. The median number of active monsoon spells in a wet season is 3 for the rainfall techniques and 6 for the wind techniques. The average length of each active monsoon spell is around 4 days for all of the techniques. The date of onset of the monsoon has shown negative correlation with the Southern Oscillation Index (SOI) that is late onset is found to occur in El Niño years while early onset is more likely in La Niña years. 1. Introduction Tropical Australia has a monsoon climate with seasons of wet and dry weather. The Australian monsoon is often weaker than the Indian/Southeast Asian monsoon and the timing with respect to the seasonal cycle of rain and wind is less reliable. The weather over northern parts of Australia has a build-up period prior to widespread monsoonal rainfall. Four techniques have been developed in the Darwin Regional Specialised Meteorological Centre to monitor the north Australian monsoon progress at Darwin. The techniques are based on the previous works such as Nicholls [1], Shaik and Bate [2], Holland [3] and Drosdowsky [4]. c 2010 IOP Publishing Ltd 1

3 2. Data Rainfall from seven locations in the Northern Territory (Australia) for the period 1987 to 2008 was used in the current study. Wind data was obtained from the Darwin Airport observing site for the period 1990 to Monsoon onset monitoring techniques 3.1. Accumulated rainfall Technique 1 (Accum rain) The monsoon onset date is the date on which the accumulated rainfall amount starting from 1 September reached 15 percent of the mean annual station rainfall for all the seven stations used in the study [1] Widespread rainfall Technique 2 (Australian Monsoon Rainfall Index (AMRI) The AMRI for a station is equal to 1 if the 5-day running mean daily rainfall exceeds 150% of the long-term daily mean. If not, the AMRI will be zero. The date of onset or active spell of the monsoon in a region will be the day when the sum of AMRI from the stations within the area is 5 or more. In the present study that will be at least five out of 7 stations report AMRI equal to one [2] and 200 hpa level winds at Darwin Technique 3 (U wind) The date of the onset or active spell of monsoon will be the first occurrence of 5 knots westerly winds at 850 hpa level, 5 knots easterlies at 200 hpa level and Darwin 3-day running mean daily rainfall is 0.2mm. A 3-day running mean was applied to 850-hPa level and 200 hpa level wind data from Darwin Airport starting from 1 November each year Deep-layer mean zonal wind Technique 4 (DLM wind) The date of the onset or active spell was defined as the first day of the first spell of westerly burst in the deep westerly stream that is greater than 8 knots and that lasts two or more consecutive days, overlain by upper level easterlies and Darwin 3-day running mean daily rainfall is 0.2mm. Pressure-weighted deep layer mean wind was calculated as per [4]. Figure 1. Dates of monsoon onset in different years from the four techniques. 4. Date of Onset Figures 1 shows the onset dates for each year and for each technique. The onset date remained same for both wind techniques in 6 years and the onset date remained within three days range in eleven out of fourteen years. In rainfall techniques, out of twenty-one years the onset date was same in 3 years while in 10 years it was within 3 days range. 2

4 Figure 2. Number of active monsoon spells in each season. The average and median date of onset with technique 1 was 19 December and 20th December. The average and median date of onset for technique 2 was 25 December. The dates of the onset analysed by technique 3 has the mean as well as the median date of onset was 30 December. The average and the median date of onset remained as 28 and 27 December respectively with the DLM wind technique. The difference in monsoon onset dates between the techniques is mostly due to the synoptic patterns of individual years. Figure 3. Length of each monsoon spell in each season for AMRI technique. 5. Active spells of monsoon Figure 2 shows the number of active monsoon spells in each season for the three techniques. The number of active monsoon spells in a season ranged from 1 to 6 in AMRI (rainfall) technique whereas the wind techniques had 3 to 10 active spells each year. In some years the rainfall had only one active spell but the wind technique had 10 spells ( season). These years are dominated by dry monsoon periods where the wind pattern persists without much rainfall. Some of these years are prominent with short period wind bursts or not enough deep layer monsoon wind structure. The median number of active spells was 3 with the AMRI technique and for the wind techniques it was 6 spells. 3

Figure 4. Length of each monsoon spell in each season for Uwind technique. 6. Length of active monsoon spell Figures 3 to 5 show the length of each wet spell (in days) using different techniques.

With the U wind technique the length of the maximum monsoon spell was 19 days though the average length of the spell was 4 same as of AMRI technique.

5 Figure 4. Length of each monsoon spell in each season for Uwind technique. 6. Length of active monsoon spell Figures 3 to 5 show the length of each wet spell (in days) using different techniques. With AMRI the wet spell length ranged from one day to 12 days and the average spell length was 4 days. With the U wind technique the length of the maximum monsoon spell was 19 days though the average length of the spell was 4 same as of AMRI technique. With the DLM wind technique the average spell length 3.3 days was highly modulated by several one-day spells. Several short duration spells in the DLM wind technique indicate that the deep layer monsoon structure was not prevailing for a longer duration or the strength of the deep layer was not enough to sustain for longer periods. Figure 5. Length of each monsoon spell in each season for DLM wind technique. 7. Date of Monsoon onset and Southern Oscillation Index (SOI) Previous studies indicate a strong relationship between the Australian monsoon and the ENSO (El Niño - Southern Oscillation (Meehl and Arblaster [5]). The relationship between monsoon onset and various types of SOI values is studied. The types of SOI include October SOI, November SOI, mean of September to November SOI, past month SOI, past 3 months mean and past 6 months mean to the onset month. For example, past 3 months SOI for an onset date in 4

6 January will be the mean of the October, November and December SOI. Figure 6 presents the relationship between the November SOI and the date of onset. Figure 7 presents the relationship between the September-November (3 month) mean SOI and the date of onset. The lines on the graph indicate the linear best fit (BF) for each technique. The correlation coefficients obtained are presented in figure 8. Figure 6. Relationship between November SOI and the monsoon onset. Figure 7. Relationship between September to November (3 month mean) SOI and the monsoon onset. All of the three techniques have shown a general trend of late onset in El Niño years and early onset during La Niña events. The AMRI technique has shown a better correlation with the past three months SOI and also with the September to November SOI. 5

7 Figure 8. Correlation between SOI and onset dates. 8. Conclusions The average date of onset of the monsoon ranges from 19 December to 30 December. The average onset date using only AMRI, U wind and DLM wind techniques is 28 December. In eleven out of twenty-one years the onset date remained within three days range between the two rainfall techniques. Eleven out of fourteen years the onset date remained within three days range in the wind techniques. Out of the four techniques, the DLM wind and the AMRI techniques represent the monsoon onset better than the other techniques. The median number of active monsoon spells in a wet season is 3 for the rainfall techniques and 6 for the wind techniques. The average length of each active monsoon spell is around 4 days for all of the techniques. The date of onset of the monsoon has shown negative correlation with the Southern Oscillation Index (SOI), that is late onset is found to occur in El Niño years while early onset is more likely in La Niña years. 9. References: [1] Nicholls N 1984 A system for Predicting the onset of the north Australian wet season J. Climatol [2] Shaik A H and Bate P 2000 On the estimation of the date of the monsoon onset and withdrawal over Northern Australia Australia New Zealand Climate forum, Hobart April 2000 [3] Holland G J 1986 Interannual Variability of the Australian Summer Monsoon at Darwin: Mon.Wea.Rev [4] Drosdowsky W 1996 Variability of the Australian Summer Monsoon at Darwin: J. Climate [5] Meehl G A and Arblaster J M 1998 The Asian-Australian monsoon and El Niño- Southern Oscillation in the NCAR Climate System Model J. Climate

Appendix E Mangaone Stream at Ratanui Hydrological Gauging Station Influence of IPO on Stream Flow

NZ Transport Agency Peka Peka to North Ōtaki Expressway Hydraulic Investigations for Expressway Crossing of Mangaone Stream and Floodplain Appendix E Mangaone Stream at Ratanui Hydrological Gauging Station