Tropical cyclones (TCs) significantly alter the upper oceanic environment through physical and biogeochemical processes. This study investigates the impact of category 3 TC Tej (20–25 October 2023) on oceanic conditions in the Arabian Peninsula, using satellite remote sensing and model-based ocean reanalysis products. Ocean changes were assessed during the cyclone active period (20–25 October 2023) to capture its immediate impact on the oceanic environment. The cyclone induced Sea Surface Temperature (SST) cooling of 2.5°C to 4.0°C, particularly within cyclonic eddies and along the right side of the storm track, with cooling effects penetrating the upper 100 meters due to wind-induced vertical mixing (mixed layer depth to 40-50) Ekman upwelling. Concurrently, Chlorophyll-a (Chl-a) concentrations increased up to 6 mg/m³, reflecting a fourfold rise driven by nutrient entrainment. Enhanced subsurface Chl-a was observed down to 50 meters. Sea Surface Height (SSH) dropped below 0.05 m in cyclonic eddies (12°N, 56°E) and rose above 0.8 m in anticyclonic eddies (15°N, 55°E), revealing contrasting vertical motions. Surface pH declined by 0.05-0.1 units, while dissolved inorganic carbon (DIC) concentrations increased to 2.1-2.2 mol/m³ in the upper 100 meters, attributed to enhanced CO₂ uptake and the upwelling of CO₂-rich waters. Cyclonic eddies were linked to lower SSH, higher Chl-a, and elevated DIC levels. This study provides the first high-resolution vertical analysis of post-Tej biogeochemical responses in the Arabian Peninsula, highlighting eddy-modulated nutrient redistribution as a key driver of phytoplankton bloom. These findings enhance understanding of the coupled physical-biogeochemical impacts of TCs in the region.

Phytoplankton, the primary producers of marine ecosystems, play a crucial role in sustaining primary production. This study investigates spatiotemporal variations in phytoplankton functional groups (PFGs) and their environmental drivers across the Bohai, Yellow, and East China Seas (BYECS) from 2003 to 2022. Sea surface temperature exhibited a significant warming trend (sea surface temperature [SST]; τ > 0.48, p < 0.01), increasing from 16.39°C in 2011 to 17.98°C in 2021, mean at 17.24 ± 0.39°C and a breakpoint around 2009. Smaller phytoplankton, including prokaryotes (PROKAR, 27.86%; mean = 1.59 ± 0.96 mg m−3) and pico-phytoplankton (PICO, 28.99%; mean = 1.70 ± 0.95 mg m−3) showed strong negative correlations with SST (r = −0.64 and −0.69) and pronounced declines after 2015. Haptophytes and nanoplankton also decreased significantly (τ < −0.52, p < 0.001) with a breakpoint around 2011. In contrast, larger groups; diatoms (5.28%; mean = 0.41 ± 0.15 mg m−3) and micro-phytoplankton (MICRO, 8.44%; mean = 0.60 ± 0.19 mg m−3) were positively associated with SST (r = 0.38–0.49), indicating enhanced resilience under warming conditions. Total chlorophyll-a (TChl-a; 3.38 ± 3.74 mg m−3) declined significantly (τ < −0.57, p < 0.001) and showed a strong negative correlation with SST (r = −0.64), indicating reduced biomass after 2011. PICO and MICRO contributed 30–45% and 20–35% of TChl-a, respectively. Nutrients showed no significant long-term trends. Overall, findings reveal a warming-driven shift from small to larger phytoplankton groups, with decrease in TChl-a reflecting structural and productivity changes in the BYECS ecosystem. These findings are BYECS-specific and should be interpreted with caution due to uncertainties in remote-sensing PFG products and the potential influence of riverine inputs, stratification, and mixing.