Fires are a distinct natural disturbance in combustible vegetation areas and release carbons into the atmosphere in a short time scale1,2. The emission from fires originated from mid and high latitudes, in particular, is comparable to the fuel emission3,4. Hence, to reduce negative social and economic impacts, it is a matter of great importance to investigate dominant factors for generating severe and frequent fires. Fire weather index (FWI)5 has been used as an indicator of fire potential considering that fire ignition, spread, and frequency can be influenced by specific weather conditions such as temperature, relative humidity, precipitation, and wind speed. It has been suggested that those favorable fire weather conditions can be modulated by human-induced global warming and natural modes of climate variability on interannual to interdecadal timescales6-11. The Arctic oscillation and associated jet streams play a role in a fire in the high latitude12-13, while ENSO affects tropical and mid-latitude fires11,14. In a warmer climate, fires are expected to be intensified and occur more frequently with hotter and drier conditions and more lightning occurrences6-8.
The Madden–Julian oscillation (MJO) is the leading mode of subseasonal variability with 20–100-day periods in the tropics. The MJO is represented by the eastward-propagating deep convective system in the Indian Ocean and western Pacific. The MJO teleconnections have a profound impact on rainfall15,16,20, drought19, hurricane17,18, and regional monsoons21,22. The convective heating causes divergent flows in the upper troposphere, generating Rossby wave trains23-26. The MJO teleconnection could be a trigger for extreme weather events through generating a heatwave and drying in high-population mid-latitudes, particularly, East Asia, eastern Europe, and United State23-27. However, impacts of MJO-related weather conditions on fire occurrences on a global scale are not quantified and remain unresolved. Here we present the relationship between daily fire emission and fire weather conditions regulated by MJO convective anomalies over the eastern Indian Ocean and western Pacific. We bring forward that MJO could be a trigger and booster for fire activities in many tropical and mid-latitude regions by generating severe weather conditions favorable for fires.
Modulation of Fire emission by MJO phase
The strongest FWI anomalies (Fig. 1) occur in the southeast US, central Africa, south Asia, eastern Europe, South America, East Asia. When MJO convection occurs in the western Pacific (MJO phase 5 and 6, hereafter WP phases), FWI in those region is positive, while negative when MJO convection is located at Indian Ocean (MJO phase 2 and 3, hereafter IO phase). Note that in the southern US and Australia, FWI is negative during the WP phase, while positive during the IO phase. The significant symmetric changes of the FWI between WP and IO phase could affect fire emission dominantly. The FWI represents favorable weather conditions for fire activities considering temperature, rainfall, humidity, and wind speed. Thus, it is of particular importance to investigate whether the local FWI anomalies driven by MJO are significantly linked to changes in fire emission or not. The horizontal patterns of fire emission anomalies are consistent with corresponding FWI anomalies at both WP and IP phases (Figure 2). During the WP phase, fire emission in central Africa, South Asia, Eastern Europe, South America, and East Asia is over two or three times than during the IO phase, while fire emission in the southern US and Australia during the WP phase is a half-range value compared to during IO phase. Here we found the new results that the fire emissions in the tropics and midlatitudes are modulated by FWI anomalies generated by MJO atmospheric teleconnection. This is surprising because the fire emission in midlatitudes is intensified or weakened significantly by MJO FWO anomalies and fire emission includes extended-range variability (e.g. sub-seasonal variability). The influence of MJO on fire emission is important because a large portion of MJO-related fire emissions occurs near high population midlatitudes.
Heavy fire emission during MJO events
We want to show whether heavy fire emissions are modulated by MJO or not. Figure 3 shows the histogram of the fire emission and how it has changed over the MJO phase. The results show significant changes in mean fire emission during IO and WP MJO phases. Over East Asia, the mean fire emission during the WP phase is four times more likely than the IO phase. In south Asia, eastern Europe, central Africa, and South America, fire emissions during the WP phase are two times more than IO phase. Note that mean fire emission in the southern US and north Australia during WP is much smaller than that during the IO phase.
We now want to show that fire emissions during the MJO period could be larger than during the normal periods. The horizontal map in figure 4 shows extreme fire emissions corresponding percentile of 95% when data are sorted. The results show that for the same rareness events, the heavy fire emission during MJO period is much larger than the normal period due to the generation of heavy fire-favorable weathers (Supplementary Fig. 1), suggesting that MJO could contribute to generating heavy fire emissions in the tropics and midlatitudes. The frequency of February-April daily fire emission during MJO and all days (Supplementary Fig. 2) over major fire emission regions. The light emission frequency during MJO events is similar to that during all days, while heavy (or extreme) emission frequency during MJO is significantly increased when compared to those during all days over most major fire regions, implying that the MJO teleconnection contributes to the generation of big fire emissions in the mid-latitudes.
The modulation of heavy fire emission by MJO could be verified from recent extreme fire events. August complex fires were extraordinarily large burning in California, 2020. Its size was about 100 km long (north to south) and 50 km wide (east to west). The complex started from 38 separate fires and become one of the largest fires in recorded California history. The complex fire had burned a total of 1million acres, which is about 1% of California's area. Figure 4c shows the temporal evolution of emission and FWI anomalies in the northern California area (39°-40°N, 237°-239°E). During active MJO, two-time series are closely linked to each other. The FWI can capture abrupt peaks of emission anomalies, indicating that the strong emission may have originated from FWI changes induced by the MJO teleconnection. However, during inactive MJO, the changes of both FWI and emission anomalies are relatively small, indicating that MJO teleconnection can contribute to an increase in emission. On the other hand, positive FWI tends to cause the increased emission, while negative FWI is less linked to decreased emission, suggesting that decreased emission may be attributed to other reasons (e.g. human effect). The 2019–20 fire emissions from New South Wales in Australia started early under dry and warm atmospheric conditions. The fires have burned a large portion of the state (55,000 km2). In December 2019, large emission anomalies occurred and moderate emission preceded to late February 2020 (Fig. 4d). Corresponding FWI anomalies reproduce an increase of emission during December 2019 and tend to capture temporal evolution of emission with a correlation of 0.56. These results show that extremely large emissions may be related to the FWI increase induced by MJO teleconnection.
The relationship between fire emission and FWI
This study brings forward that the FWI changes induced by MJO convection should be a key factor in determining the changes in the fire emission, particularly in the mid-latitudes. To further validate this, we compare the relationships between daily FWI and emission anomalies during the active MJO period and total eriods (Fig.5). The results show that the linear relationship between FWI and emission is robust during active MJO periods but very weak during total periods in boreal spring. The relationships between the FWI and emission during total periods are weak with low correlations of 0.17 to 0.34, while they are very strong during active MJO periods with high correlations of 0.58-0.68. The stronger relationship is seen in eastern Europe but weaker in eastern Australia. The strength for the impact of MJO teleconnection (that is, the changes in fire emission by unit change of FWI) depends on the local region and MJO phases. Compared to those during MJO 5 and 6, the strength during MJO 2 and 3 is slightly lower in the southern US, while higher in eastern Australia (Supplementary Fig. 3). On the other hand, there are no significant changes in strength in eastern Asia and eastern Europe.
Contribution of MJO on long-term changes in Fire emission
Previous studies revealed the observed change in MJO residence time28 and convective activities, which may affect MJO-related decadal changes and trends in fire emission. To assess the potential impacts of the observed changes in the MJO on global fire emission, we calculated the composite difference in fire emission between the 2000s and 2010s (Fig. 6a). The changes in the MJO over the western Pacific are linked with increased fire emission over east Asia, India, the Amazon basin in South America, Central Africa, and eastern Europe. Meanwhile, the decadal changes in the IO phase are associated with the increases in fire emissions over the southern US and northern Australia (Fig. 6b). Notably, a trend in fire emission for February-April shows consistent changes over most of these regions (Fig. 6c). The increases in mean fire emission are seen in east Asia, India, the Amazon basin, central Africa, central US, and northern Australia. Increased fire emission in the US, East Asia, Northern Australia, and South America may be attributed to an increasing trend in MJO duration. We confirm that composite differences in fire emission with WP and IO MJO phase show similar results as for the trends.
Challenges for enhanced long-range fire emission prediction using MJO forecast
In this study, we present modulation of fire emission and by MJO and the contribution of MJO on heavy emission in the mid-latitude. The fire emission during the WP phase is two times than IO phase. The fire emission data have a significant subseasonal variability in the tropics and midlatitudes (Supplementary Fig. 4). The fire emission associated with MJO teleconnection can explain 10-20% of total emission from subseasonal to interannual variability in the mid-latitude (Supplementary Fig. 5), particularly in the US and East Asia. A recent study suggested that MJO teleconnection will be expected to be amplified in US regions under a warm climate30, suggesting that the MJO-related fire may be enhanced in the future climate.
The impact of the El Niño event on fire during active MJO periods may be important because the El Nino event changes basic state FWI. In this study, the relatively large decaying El Niño events occurred 2010, 2016, 2019 in boreal spring (Supplementary Fig. 5). During these El Nino years, the MJO-related fire emissions in eastern Europe, East Asia, and eastern Australia are relatively larger than normal years, indicating that the trends in fire emission may be explained at least in part by interaction with El Nino events. The detailed interaction of fire emission with El Nino and MJO will be studied in further study.
The relationship between MJO and fire found in this study could be used for fire prediction. It was known that it takes a few days up to one weeks for Rossby wave excited by MJO convection events to propagate in the mid-latitudes (Supplementary Fig. 6) and a recent state-of-the-art MJO forecast system can predict up to 3-4 weeks in advance34, suggesting that precise MJO prediction may contribute to improving long-range fire forecast in the midlatitudes, reducing social and economic costs of fires.