## 4 Discussion  
The SST has a significant impact on air-sea interactions and drives global weather systems and climate patterns. Therefore, changes in the SST, either increase or decrease, can cause a wide variety of impacts on every component of the climate system. The vegetation dynamics across India mainly depend on seasonally occurring rainfall events, especially in the summer monsoon, which is also linked to the regional SST variations in the Indian Ocean [65; 66]. Therefore, it is expected that the vegetation anomalies in India are linked to the SST variability in the Indian Ocean. This is correct on a seasonal scale, and we investigated the details by stratifying the monsoon season into individual months (June, July, August and September). Moreover, there is a significant intraseasonal oscillation in this SIF pattern, which corresponds to the intraseasonal variability of vegetation photosynthesis during different summer monsoon phases. Predominantly, the ACZs in the Indo-Gangetic Plains and the central Indian region displayed notable variability in SIF due to significant intraseasonal variations in the monsoon rainfall at these locations [66; 67]. The idea behind the stratified analysis was to understand the intra-seasonal variability of the SST-SIF relationship rather than establish a general relationship for a season. Indeed, as expected, a significant SST-SIF link was found between the tropical Indian Ocean and Indian vegetation photosynthetic activity in the monsoon onset period. During the initial phase of the summer monsoon, the Indian Ocean acts as the dominant moisture source over the Indian subcontinent, which appears to be the prime reason for observing significant SST-SIF correlations during the summer monsoon onset period. As the monsoon progresses, terrestrial sources become the prominent contributors to atmospheric moisture [68]. Therefore, the rest of this discussion is mostly based on the June SST-SIF relationship. Our findings give another dimension to the impact of climate on the Indian terrestrial  
Figure 9: Regions showing statistically significant composite anomalies of air temperature (\({}^{\circ}\)C) across India during the month of June when SST anomalies of the Western Indian Ocean (**a**,**d**), Northern Indian Ocean (**b**,**e**) and Central Indian Ocean (**c**,**f**) demonstrated positive (upper) and negative (lower) variations. The colour bar shows the scaled density estimate of positive (blue) and negative (orange) air temperature anomalies. The warmer SST in the oceanic regions leads to positive air temperature anomalies, while the colder SST leads to negative air temperature anomalies in the shaded areas. ecosystem by incorporating teleconnections stemming from SST variations in the tropical Indian Ocean. The Possible Physical Mechanisms behind the Significant SST-SIF Relationships during the Onset Phase of the Monsoon  
The inverse nature of the SST-SIF relationship in June indicates that photosynthesis anomalies over the Indian mainland react opposite to SST anomalies in the Indian Ocean regions during the onset phase of the summer monsoon. The persistence of warmer SST anomalies in the tropical Indian Ocean weakens the monsoon circulation, possibly by reducing the land-sea thermal gradient and thus suppresses the rainfall across north and central Indian regions [69]. The weak southwesterly wind is also evident in the composite anomalies of wind speed vectors over the entire study area (Figure S7a-c), and it hinders the moisture transport from the tropical Indian Ocean, as depicted by northeasterly anomalies. The consequent reduction in rainfall and soil moisture and the increase in air temperature create water stress in plants and dampen the photosynthetic activity [70; 71; 72], which led to the negative anomaly of SIF in central and northern India. Similarly, cold SST anomalies in the Indian Ocean help to maintain a strong monsoon circulation.