## 1 Introduction  
Droughts impact not only ecosystem functions but also the well-being of affected human populations [1; 2]. The temporal and geospatial responses to drought are varied, and they threaten regions that are less well-adapted to water stress. A better understanding of the relationship between photosynthesis and water stress has broad reaching implications for our understanding of the global carbon and hydrological cycles. Roughly half of the global variability in terrestrial carbon cycling can be attributed to carbon dioxide fluxes in tropical Africa, but the impact of drought on these fluxes is still modeled with significant uncertainty [3]. Further research on continental carbon fluxes is limited by data availability and characterization of regional productivity responses to drought [4; 5]. The future of ecosystem productivity in the tropics is limited by how well plants cope with water stress but the influence of seasonal water stress on productivity, particularly in Africa's tropical regions, has rarely been characterized [6; 7]. African climate is characterized by summer rainfall in the northern and southern tropics with an equatorial bimodal regime in-between. Regional variations notwithstanding (Figure 1), rainfall in tropical East Africa is delivered during the boreal spring and fall (Figure 2), accompanying local solar insolation maxima. The rainy season from March to May, known as the \"long rains\" in Kenya, \"Belg\" in Ethiopia, and \"Gu\" in Somalia, includes the majority of annual regional rainfall. A second rainy season occurs during the boreal fall and is referred to as the \"short rains\" in Kenya, \"Keremt\" in Ethiopia, and \"Deyr\" in Somalia. For simplicity, Kenyan terminology is employed in this study along with March, April, May (MAM) and October, November, December (OND) to refer to respective rainy months. In the area of our interest (black boxes in Figure 1), annual mean precipitation is 439 mm/year; this is a dry region where the two growing seasons (Figure 3) are susceptible to even small decreases in precipitation. Climate extremes on the African continent are well studied due to their potential to produce far-reaching economic and social consequences. East Africa has faced at least one major drought per decade over the last half century [8]. That said, regional droughts are typically isolated to only one of the two rainy seasons; consecutive failed rains are uncommon phenomena given the differing influences on precipitation for each rainy season. The successive failure of 2010 short rains and 2011  
Figure 1: Average annual mean precipitation from Global Precipitation Climatology Project (GPCP; left) and Tropical Rainfall Measuring Mission Multisatellite (TRMM; right) climatology data, demonstrating the variability in rainfall regimes in the area. For zonally averaged values, the drought region is defined as the land area between \(1.5^{\circ}\) S and \(4^{\circ}\) N and \(38.5^{\circ}\) E to \(46.5^{\circ}\) E (black box). Figure 2: The climatology of rainfall over the drought region as presented by the two precipitation products used in this study, GPCP and TRMM. long rains produced the worst East African drought in the last 60 years [9], causing humanitarian crises in East Africa. This study seeks to characterize the productivity response of tropical East Africa to water stress while identifying the unique spatial and temporal characteristics of the 2010-2011 drought. Related research on gross primary production (GPP) change in other tropical ecosystems and drought impacts relies on satellite-derived measurements because in situ measurements there remain scarce, largely due to operational constraints [10]. Here, we use relatively new satellite measurements of solar induced chlorophyll fluorescence (SIF) [11] to study the change in East African productivity during the drought, in conjunction with other atmospheric and terrestrial datasets. In Section 2, we describe the data and methods that we used. The results and discussion are presented in Sections 3 and 4.