## 1 Introduction  
Anthropogenic global warming is estimated to currently have reached about 1 \({}^{\circ}\)C above preindustrial levels and is estimated to further increase to 1.5 \({}^{\circ}\)C between 2030 and 2052 under current rates of greenhouse gas (GHG) emissions [1]. This increase in global temperature is expectedto lead to an increase in intensity and frequency of extreme events such as heatwaves, drought, and associated wildfires [2; 3; 4]. In 2018, Europe faced such an extreme event characterized by record-breaking temperatures and long-lasting dryness, referred to as \"hotter drought\" [5]. According to the European Drought Observatory of the European Commission, most of Northern and Central Europe was affected by such hotter drought in 2018. On the contrary, much of the Southern Mediterranean Europe experienced relatively cool and moist conditions [6; 7]. For Central Europe, various news and media articles reported severe decrease in crop yields, increase in elderly mortality, and forest fires along with many other adverse impacts. According to a quick attribution study by the World Weather Attribution on the heat in Northern Europe [8], the anthropogenic climate change has more than doubled the probability of occurrence of such a heatwave in the region. Along with severe economic implications and adverse health impact on the public, such record-breaking hotter drought events severely affect ecosystems. A direct comparison of vegetation response to the drought in 2018 and the 2003 European heatwaves using MODIS's vegetation indices concluded that negative impacts in 2018 were even stronger than in 2003, with a strong decline in agricultural land and forests, especially in Central Europe [6]. Such drought events strongly affect the carbon cycle by leading to plant mortality and reductions in ecosystem carbon dioxide (CO\({}_{2}\)) uptake, with the potential to convert ecosystems from carbon sinks into carbon sources [9; 10]. Gross primary productivity (GPP) is a measure of ecosystem CO\({}_{2}\) assimilation. The estimation and modeling of regional or global GPP involves various uncertainties arising from model parameterization [11; 12], thus demanding a more direct and reliable approach to estimate GPP. With recent advances in satellite-based measurements of solar-induced (chlorophyll) fluorescence (SIF), it is now possible to use SIF as a direct proxy for photosynthesis or GPP worldwide [13; 14; 15; 16; 17]. SIF is an electromagnetic signal emitted in the wavelength range of 600 to 800 nm by chlorophyll molecules [18]. Since SIF is directly associated with photosynthesis, it is more physiologically-based compared to the traditionally used vegetation indices (VIs), such as the Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI) [19; 20]. Over the last decade, several studies have demonstrated a strong linear relationship between satellite-based SIF and GPP [13; 15; 21; 22; 23; 24; 25; 26; 27; 28], indicating that SIF might provide a useful approximation of GPP. The SIF retrievals obtained from the Japanese Greenhouse Gases Observing Satellite (GOSAT) and the European Global Ozone Monitoring Experiment-2 (GOME-2) were most widely used in the last decade (2007-2017; see studies in [21; 22; 23; 24; 28]). However, due to coarse spatial resolution of the SIF footprint (10 km diameter for GOSAT and \(40\times 40\) km\({}^{2}\) for GOME-2), these satellites presently impose serious limitations in detecting ecophysiological changes across different vegetation types. The recent NASA's Orbiting Carbon Observatory-2 (OCO-2) satellite (launched in September 2014) provides SIF data at a much higher spatial resolution of \(1.3\times 2.25\) km\({}^{2}\) with increased SIF acquisition frequency, thereby allowing us to resolve SIF responses of various vegetation types under different climatic conditions [15; 29]. SIF acquired from GOSAT and GOME-2 have been previously used to study the effect of drought on vegetation.