Therefore, active fire affected pixels were detected for each of the above seven regions/provinces separately, whose areas ranged from 1 x 1 to 6 x 5 degrees latitude-longitude. Then the average proportion of each province covered by clouds was estimated for each scene individually. Cloud detection was performed by employing a single threshold (T₄ < 275K) in the AVHRR channel 4. This single criterion proved sufficient in separating the clouds, since only nighttime data were processed. Assuming that the spatial distribution of fire events is homogenous within each of the seven regions, the number of active fires detected in each GAC image was corrected for each region separately according to the proportion of that region covered by clouds. Then the total all-Borneo fire number in each individual GAC image was derived by aggregating the cloud-corrected fire counts occurring in all of the seven regions.

Figure 2. Active fire counts before and after their adjustment for the different cloud coverage of each GAC image during the 1997-98 El Niño event. Error bars are one standard deviation from the mean.
The difference between the two fire count series was more evident during the months with high fire activity. However, the overall fire pattern remained the same, depicting equivalent variations in fire activity in Borneo during El Niño event (Figure 2). In particular, the intra-seasonal variation was consistent, with the major fire activity to persistently concur in August-October and February-April of the successive (sequential) year. The monthly variation also remained almost constant and no shift in the peak of fire activity occurred. For example, in the 1997-98 event, the peak times (September and March) agreed in both, cloud and non-cloud normalised fire count series. Morover, the rank order of the most affected months was not altered, with respectively August/April second and October/February last.

NOAA satellites operated at different overpass local times over Borneo during the 1982-1998 study period, which is likely to introduce a bias in the derived fire dataset, due to possibly strongly diurnal fire cycle (Eva and Lambin, 1998). Therefore, a further attempt was performed to depict the diurnal cycle of fire activity of Borneo during El Niño periods.

For this reason, active fire as derived from satellite images of the Visible and Infrared Radiometer System (VIRS) onboard the Tropical Rainfall Measuring Mission (TRMM) satellite as well as AVHRR GAC estimates at different overpass times was used. The TRMM-VIRS-derived fire diurnal cycle for Borneo during the 1998 El Niño event was produced and kindly provided by Luis Giglio from Goddard Space Flight Center, NASA (Giglio et al., 2000). The diurnal fire cycle in Borneo during the 1998 El Niño period was found to be strong, for example negligible fire activity was observed between 01:30 and 06:30 hrs local time, with 96% of fire activity occurring within the remaining 19 hours. According to these data, the GAC derived fire counts were normalized for the overpass times of the different NOAA satellites used.

Figure 3. AVHRR GAC derived fire counts for the five 24-months El Niño events, which occurred from 1982 to 1998. These fire counts are adjusted for different cloud coverage and observation time of each GAC image.
The interannual fluctuation of Borneos fire activity for the five studied El Niño events from 1982 to 1998 is shown in figure 3. Obviously, there is a distinct pattern of fire activity. The major fire activity tends to occur between August-October (ASO) of the first year (Year 0), termed as first fire sub-season, and between February-April (FMA) of the following year (Year 1), termed as 2nd fire sub-season. On the contrary, in November-January (NDJ) and March-July (MJJ) of the El Niño period, the fire activity appeared significantly weakened, revealing the domination of local/regional climate conditions driven by the monsoon circulation. As it is depicted in figure 4, during NDJ of Year 0 and MJJ of Year 1, the ENSO index, in most of the studied El Niño events, remained high enough to trigger fire occurrence. However, the Asian monsoon system is active over Borneo during these time periods, particularly during the NDJ of Year 0 when the winter (west) monsoon is substantially stronger than the summer (east) monsoon, resulting strong convection over that region. This is also supported by the fact that although El Niño is being in its mature phase during NDJ of Year 0 (MJJ of Year 1) and the ENSO index is very high, moving towards to (away from) the peak, negligible fire activity occurred. On the contrary, during the MJJ of Year 0, El Niño was still not fully developed, while in NDJ of Year 1, El Niño was already demised, which together with the strong winter monsoon influence, resulted a complete absence of major fire events.

Figure 4. The evolution of the sea surface temperature anomaly in the Niño 3 region of the Pacific Ocean for the five studied El Niño events.
Some interesting observations can be revealed when the fire occurrence is examined according to the total 10-years fire activity. In particular, the majority of fire activity occurred during the 1^st and 2^nd fire sub-seasons in August-October of Year 0 and February-April of Year 1 accounting for the 73.51% of the total El Niño related 10-years time period. The only pronounced exception was the 1993-94 fire event when the major fire activity appeared in the 3^rd fire sub-season in August-October of Year 1, representing the 11.15% of the total 10-years. Some fire activity was also observed in the 3rd fire sub-season of the 1986-87 and 1991-92 El Niño events accounting however only for the 2.37% and 1.03% of the total 10-years. On the other hand, the remaining 11.95% of the total 10-years fire activity were almost evenly distributed to the remaining 72 months of the 5 El Niño events.

During the 1^st fire sub-season, 18.98% of the total 10-years fire activity occurred in 1997-98 El Niño event, 8.24% of the total in 1991-92 event, 3.72% and 2.13% in 1982-83 and 1993-94 respectively, while only 0.70% in 1986-87 El Niño event. During the 2nd fire sub-season, 20.04% occurred in 1982-83, 16.03% in 1997-98, 2.56% in 1991-92, while only 0.87% and 0.23% in 1993-94 and 1986-87 El Niño events respectively. However, the most interesting fact, which is important in the later analysis, is that 80.61% of the total 10-years AVHRR-GAC observed fire activity occurred in the first 16-months from the 24-months studied in each El Niño event.

Figure 5. Cloud and time adjusted AVHRR GAC fire counts (columns) and the Niño 3 anomaly (line) for the five studied El Niño events: (a) 1982-83, (b) 1986-87, (c) 1991-92, (d) 1993-94 and (e) 1997-98. Error bars are one standard deviation from the mean.

The distribution of fires in time was analyzed and compared to the strength of the El Nino-Southern Oscillation (ENSO), as measured by the sea surface temperature anomaly (SSTA) in the Niño 3 region of the Pacific Ocean. The relationship between fire and Niño 3 anomaly variation was explored in three different ways. First, by plotting the cloud and time adjusted AVHRR GAC derived fire counts together with the Niño 3 anomaly in order to observe the general signal of the association (Figure 5). Secondly, a cross-correlation analysis between the monthly fire counts and the Niño 3 anomaly was applied to identify the existence and the best range of possible lag time, in which some ENSO-fire relation occur. And finally, a regression analysis was conducted between different time composites of fire counts and the Niño 3 anomaly to accurately identify and quantitatively measure the strongest possible ENSO-fire relation.