5.  Analysis of Wet Season and Dry Season Data for former Badan Meteorologi dan Geofisika (BMG) Seasonal Forecast District (Daerah Prakiraan Musim, DPM)-91.

5.1  Data and definitions

    When East Timor was part of the Republic of Indonesia, the northern part of the former province constituted DPM-91, named "Timor Timur Bagian Utara".  Thus seasonal forecasts were carried out for this district as part of the routine seasonal forecasts published twice yearly by the BMG in their series of annual reports:
*    Prakiraan Musim Hujan 19XX/!9XX+1 (e.g 1995/1996) Di Indonesia (Forecast for the Wet Season 19XX/19XX+1 in Indonesia) , published each September.
*    Prakiraan Musim Kemarau 19XX Di Indonesia (Forecast for the Dry Season 19XX in Indonesia), published each March.

Forecast District 91 is as shown in the following map, scanned and digitised from the above reports.

The BMG carried out forecasts for four parameters, derived from station average rainfall for the following three stations: 476 Serwaru (BMG station No. 476), Tepa (#476a) and Saumlaki (#479c).

PMH: Permulaan Musim Hujan (Wet-Season/Musim-Hujan Onset).  This is defined to have occurred when the 10-day averaged rainfall over a standard Dasarian exceeds 50mm in two successive Dasarian.  In such case the first of the two Dasarians is defined as the time of onset.  In each month there are three standard dasarian: Dasarian I from the 1st to the 10th of the month, Dasarian II from the 11th to 20th, and Dasarian III from the 21st to the end of the month.  Thus each calendar year contains 36 Dasarian.

PMK:  Permulaan Musim Kemarau (Dry-Season/Musim-Kemarau Onset).  This is defined to have occurred when the 3-station average rainfall measures less than 50 mm over two successive dasarian.

CMH:  Curah Musim Hujan (Total Musim Hujan rainfall).  This is defined as the total district average rainfall from the dasarian defined as the Musim Hujan Onset up to the dasarian immediately prior to the Musim kemarau Onset.

CMK:  Curah Musim Kemarau (Total Musim Kemarau rainfall).  This is defined as the total district average rainfall from the dasarian defined as the Musim Hujan Onset up to the dasarian immediately prior to the Musim kemarau Onset.

Data series have been obtained from BMG through cooperative arrangements between BMG and the Bureau of Meteorology.  Time series of each of the four parameters: PMH, PMK, CMH, CMK are available from 1961 to 1999 (though the time series length varies over the four parameters.   From these series two derived parameters can be obtained:

Panjang Musim Hujan (Wet Season Length) defined as the length in dasarian between PMH and the following PMK.
 

Panjang Musim Kemarau (Dry  Season Length) defined as the length in dasarian between PMK and the following PMH.

Normal year:  The median values of each of the six parameters from the 1961-1999 time series are as follows:

Wet-Season Onset (PMH): Dasarian 35 11-20 December
Dry Season Onset  (PMK): Dasarian 10  1-10  April
Wet Season Length (Panjang Musim Hujan): 10 Dasarian (3 months and ten days)
Dry Season Length  (Panjang Musim Kemarau): 25 Dasarian (8 months and ten days) Note: Wet Season Length and Dry Season Length do not add to a year, as different time series lengths are available for the two parameters).

Total Wet Season rainfall (CMH):  2,626 mm
Total Dry Season rainfall (CMK):      279 mm
 
 
 

5.2  Phasing of El Niño events with respect to East Timor Wet Season and Dry Seasons.

Following are the graphs of the Southern Oscillation Index (SOI) over the period of the DPM data extracted from the Bureau of Meteorology National Climate Centre web-page: http://www.bom.gov.au/climate/ahead/

The El Niño events occur when this index (the SOI) goes negative for a prolonged period.  Thus inspection of the figure reveals that El Niño's occurred during the following periods:

Feb 1957  -  May 1958     June 1963 - Jan 1964         April 1965 -  May 1966    Sept 1968 - April 1970
April 1972 - April 1973       Feb 1977 -  April 1978        April 1982 - July 1983    August 1986 -  Feb 1988
Mar 1991 - June 1995 (The so-called 3-year event)    Mar 1997 -  April 1998

All events began in the Feb-Sept period and 7 of the 10 began in the 3 month period Feb-April.  These include the two major El  Niño events 1982-83 and 1997-98.  Averaging the SOI beginning from January of the first year of each of the 7 events yields the following behaviour of the SOI during a "typical El Niño".

Thus the "normal El Niño behaviour is for the event to begin around February-March, for it to undergo approximately 14 months of negative values of the SOI; for the SOI to return to a value close to zero around the end of April of the following year.  After that the SOI can go in any direction ranging from La Niña (positive SOI) through to a repeat of the El Niño sequence.   Interpreted in this context, the three-year event becomes three separate El Niño events as follows:
Feb 1991-Jun 1992;    Sept 92 - Oct 93;   Mar 94 -  Jun 95.... Thus two of these events (1991-92, 1994-95) fit the classical pattern as depicted in the above graph.

Taking the above median dates of wet season and dry season onset and plotting them on the above canonical El Niño graph, we obtain the following:

Thus the beginning of an El Niño event occurs around the time of the onset of the dry season for the northern part of East Timor, and continues through to the end of the following wet season.

5.3  Impact of an El Niño event on dry season and wet season onset and total rainfall for DPM-91 (Northern Part of East Timor)

Correlation analysis.

Dry Season Onset:  Since the dry onset occurs around the time the SOI is changing sign, there is little relationship between this parameter and the value of the SOI.  This is confirmed by correlation analysis:
correlation (Dry onset, preceding Jan-Mar SOI) = -0.1 (50 yrs data).
correlation (Dry onset, simultaneous Apr-Jun SOI) = -0.0 (50 yrs data).

Total Dry Season rainfall:  This is strongly influenced by the value of the SOI
Correlation (Dry season rainfall, simultaneous Apr-Jun SOI) = 0.6  (50 yrs data)
Correlation (Dry season rainfall, simultaneous July - Sep SOI) = 0.6

    Thus 35% of the variance in dry season rainfall in northern East timor is accounted for by the value of  the SOI.  There is also a predictive capacity here as the relationship holds for SOI values averaged over the first three months of the season.  The sign of the relationship is such that when the SOI is negative (El Niño phase) the dry season rainfall is below normal.

Wet Season Onset.  This also is strongly influenced by ENSO (El Niño - Southern Oscillation) such that in the negative phase (El Niño) the wet onset is later than normal, the percentage of variance accounted for being 25 %.
correlation (Wet onset, preceding Jul-Sep SOI) = -0.5 (40 yrs data).
correlation (Wet onset, simultaneous Oct-Dec SOI) = -0.5 (40 yrs data)
 

Wet Season total rainfall.  Despite the fact that that an El Niño event typically continues through to the end of the wet season, the statistical relationship between wet season rainfall and the SOI is small, accounting for less than 5% of the variance.  The sign of the relationship is such that there is less total rain during an El Niño.
Correlation (Wet season rainfall, simultaneous Oct-Dec SOI) = 0.1  (40 yrs data)
Correlation (Wet season rainfall, simultaneous Jan-Mar SOI) = 0.2
 

b)  Case studies of the seven recent canonical El Niño events:

    The following bar-charts show the long term median values of the various measures of wet and dry season activity (upper bar in each chart) accompanied by the value obtained in the seven recent canonical El Niño events, as described above.

Considering first total dry season rainfall.  This was below the normal median for all seven events; and in four of these events was as little as 40% or less of normal.  This represents a significant impact of an El Niño on northern East Timor.
 

The wet season onset was close to normal for several of the events, including the major 1997-98 event.  However in the 1982-83 and in the 1977-78 El Niño events the onset was more than 5 dasarian (50 days) late...which again represents a significant potential impact.

Wet Season rainfall.  Despite the fact  the correlation analysis showed a very small impact on wet season rainfall, looking at the individual El Niño events, on a case by case basis, the effect of an El Niño is quite large.  For all seven events the wet season rainfall was below normal and for three of the events (1982-83, 1994-95, 1997-98) it was less than 50% of normal.   This seems to contradict the statistical/correlation result above showing a very weak relationship between wet-season rainfall and the SOI.  Hence I show (below) a scatter plot of the values making up the small (0.2) correlation.

Scatter plot between wet season rainfall and simultaneous SOI

The correlation between these two series is only 0.2. However this is influenced by a small number of years of low rainfall with close to zero SOI values (92/93, 94/95, 95/96, 96/97).  These are all recent years; so I am a little suspicious of the data.. and will go back to the relevant sections in BMG to check it.  In the meantime, by inspection for most years there is clearly a strong negative relationship between total wet season rainfall and the SOI: such that when there is an El Niño (SOI negative) the rainfall is below normal.