## 3 Results and Discussion
### Discussion  
El Nino and La Nina are opposite phases of a natural climate pattern across the tropical Pacific Ocean that swings back and forth every 3-7 years on average. Over the 13-year study period of this research, a strong El Nino and a strong La Nina both occurred. Following a brief and weak 2009-2010 El Nino event (which our EOF analysis did not sufficiently resolve due to the monthly temporal resolution of the SIF dataset), the 2010-2012 La Nina event was one of the strongest La Nina on record. It caused Australia to experience its wettest September on record in 2010, the 2010 Pakistan floods, the 2010-2011 Queensland  
Figure 2: **(left)** Global rotated EOF spatial patterns of the first three SIF modes calculated for the 2007–2019 period. **(right)** Monthly time series (color-shaded, 2007–2019) of the principal components (PCs) associated with the first three global rotated EOF modes. Monthly time series (dashed lines) of correlated climate indices (ENSO for PC1, NAO for PC2, and PDO for PC3) are normalized and superimposed. No time-lag or running mean are performed on these time series. The Pearson correlation coefficient and p-value are given for each time series pair. (Australia) floods, and the 2011 East Africa drought. This La Nina event also led to above-average tropical cyclone activity in the North Atlantic Ocean during 2010-2012. The 2015-2016 El Nino phenomenon, according to the World Meteorological Organization, was one of the three strongest El Ninos since 1950. It produced higher SST and moisture, and contributed to a record-breaking tropical cyclone season in the central and eastern Pacific. The winter of 2015-2016 brought above normal rainfall and mild temperatures to the south central United States and Europe. This El Nino also contributed to the Earth's warming trend, with 2014 and 2015 being two of the warmest years on record to date. The combination of heat and low rainfall brought a very early start to the 2015-2016 Australian bushfire season and widespread regional droughts in South Africa and eastern South America. The impact of ENSO on SIF (GPP) can be visualized by comparing SIF distributions in 2011, which had a strong La Nina, to 2015, which had a strong El Nino. One way to quantify their difference is to compute the annual mean SIF (AMS) change: (AMS\({}_{2015}\)-AMS\({}_{2011}\)) (Figure 3). Relative to SIF in 2011, SIF in 2015 was drastically decreased (by \(>\)50%) in Australia, southern Africa, and eastern South America. A significant SIF (GPP) decrease (10-20%) also occurred in central North America, most of Europe, and southern Asia. In contrast, other regions, including northwestern and southwestern North America, northern Africa, and the Middle East had a SIF (GPP) increase in 2015. The net global mean SIF difference between AMS\({}_{2015}\) and AMS\({}_{2011}\) is \(-\)0.02 mW m\({}^{-2}\) sr\({}^{-1}\) nm\({}^{-1}\), showing that the 2011 La Nina\(-\)2015 El Nino ENSO cycle reduced global SIF (GPP). This net change between 2011 and 2015 is equivalent to 12.5% of the 2007-2019 global mean SIF (0.16 mW m\({}^{-2}\) sr\({}^{-1}\) nm\({}^{-1}\)). SIF is largely dependent on solar radiation, air temperature, and precipitation (e.g., [41]). The latter two are strongly modulated by teleconnections and compound the effects of climate drivers (e.g., ENSO, AMO, PDO, NAO). Discussion of such effects is beyond the scope of the present study, but they were investigated by other researchers in the past. For example, [PERSON] et al. ([43]) suggested that the PDO, along with the AMO (which transitioned from its cold to warm phase in 1994), strongly influences decadal drought patterns in North America. If a positive AMO (like we are in now) continues, drought frequency will be enhanced over much of the northern United States during a positive PDO phase and over  
Figure 3: Difference of annual mean SIF (AMS) between 2011 and 2015, calculated as (AMS\({}_{2015}\)-AMS\({}_{2011}\)), highlighting the dramatic changes in SIF between the 2015 El Niño and 2011 La Nina.