# Analysis of Four Years of Global XCO\({}_{2}\) Anomalies as
## 4 Results
### Seasonal XCO\({}_{2}\) Anomalies  
Figure 4 illustrates the mean seasonal XCO\({}_{2}\) anomalies calculated from the period September 2014 to August 2018. The different seasons are defined as September-October-November (SON), December-January-February (DJF), March-April-May (MAM), and June-July-August (JJA). The large-scale patterns on SON, DJF, MAM look very similar to each other and to the annual mean, while in JJA the anomaly distribution looks quite different, with strong negative anomalies over the Northern Hemispheric mid-latitudes. We note that no OCO-2 observations are available during winter months (DJF) over the northern high- and mid-latitudes, due to persistent cloudiness and low signal levels. Figure S4 in the supplementary material shows the number of data points available for each season from September 2014 to August 2018. The standard error of the mean is illustrated in Figure S5. In order to better understand this seasonal variability, we analyze the seasonal distribution of the SIF (Figure 5), also measured by OCO-2, as well as the fluxes from NOAA's CarbonTracker model (Figure 6). Both SIF and CarbonTracker flux spatial distributions show how the negative anomalies observed during JJA are related to the biospheric sink (i.e., high SIF and negative fluxes). This is particularly visible in the eastern part of North America, where the highest SIF values are observed. At the same time, low SIF values and high XCO\({}_{2}\) anomalies are observed in the western part of North America. The high XCO\({}_{2}\) anomalies observed in part of the northern hemisphere during JJA are partially related to the seasonal minimum of the background (Figures 2 and S1). These patterns,  
Figure 4: Seasonal XCO\({}_{2}\) anomalies 2014-2018 for September-October-November, December-January-February, March-April-May, and June-July-August (top to bottom panels, respectively). however, do not contradict the patterns in the CarbonTracker flux map and are similar to the seasonal FLEXPART modeling results described in Section 4.3. The XCO\({}_{2}\) anomaly patterns in Africa show positive values in the northern biomass burning area during winter months (DJF) and relatively smaller anomalies during SON and MAM. During summer months, we find mainly negative XCO\({}_{2}\) anomalies over the same area. These features correspond directly to those observed in the CarbonTracker flux maps, i.e., strong emissions from biomass burning during winter and uptake during summer (Figure 6, right column). SIF is also higher during summer months, when the XCO\({}_{2}\) anomalies are negative. In the southern biomass burning area in Africa, we observe the largest XCO\({}_{2}\) anomalies during SON and JJA, when the biomass burning emissions are the strongest (Figure 6, right column) and SIF values relatively small. The largest negative XCO\({}_{2}\) anomalies are seen in DJF and MAM, when there are reduced emissions from biomass burning. From Figure 4, we find that the XCO\({}_{2}\) anomalies over the Highveld area in South Africa are the largest during JJA and SON, when the CO\({}_{2}\) uptake by photosynthesis is minimum. The limited number of data points available over Amazonia makes the quantification of the XCO\({}_{2}\) anomalies in South America more challenging (Figure S4). This is also illustrated in Figure S5 with high values of the standard error of the mean. The XCO\({}_{2}\) anomaly values are positive throughout the year, although, smaller anomalies are observed in JJA when the biospheric fluxes are negative. Signatures from biomass burning and positive biospheric fluxes are visible as larger positive anomalies during SON. In the Southern Cone, the XCO\({}_{2}\) anomalies are most negative during SON and DJF, which is in line with the SIF and biospheric flux maps, indicating that these anomalies are primarily biogenic. In Indochina, negative XCO\({}_{2}\) anomalies correspond to the largest SIF values observed during JJA, although from the biospheric flux maps, we would expect positive fluxes during JJA.