Based on Soil Moisture and Ocean Salinity (SMOS) data and the Ocean Reanalysis System 5 (ORAS5) dataset, positive salinity anomalies exceeding 2 psu in the northern Bay of Bengal (BoB) and negative salinity anomalies with the peak of the freshening anomalies reaching −2 psu around Sri Lanka were observed in autumn 2010. Here, an analysis of the anomalous salt budget revealed that anomalous horizontal advection contributed most to the variability in salinity in the BoB. With the development of La Niña and negative Indian Ocean dipole (nIOD) in summer and autumn, the strong summer monsoon current and Wyrtki jet combined with the anomalous basin-scale cyclonic circulation led to more high-salinity water entering the northern BoB. In addition, more freshwater was transported southward along the eastern coast of India by east Indian coastal current (EICC) in autumn, resulting in extremely negative salinity anomalies around Sri Lanka and positive salinity anomalies in the northern BoB. Moreover, the freshwater around Sri Lanka was carried farther into the southeastern Arabian Sea by the west Indian coastal current (WICC) in November, which affected the salinity stratification in winter and then influenced the variation of the Arabian Sea Mini Warm Pool (ASMWP) in the following spring. The ASMWP could affect the Indian summer monsoon (ISM) through its influence on the monsoon onset vortex (MOV) over the southeast Arabian Sea (SEAS).

Heterogeneous ice nucleation initiated by atmospheric ice-nucleating particles (INPs) is a key microphysical process for cloud formation. Detecting the ice nucleation ability (INA) and concentration of INPs is essential for improving global climate models. Droplet freezing techniques (DFTs) are among the widely used tools for measuring the immersion freezing of INPs, which is a predominant ice nucleation process in mixed-phase clouds. To enhance the efficiency and accuracy of DFTs, we developed a Freezing Ice Nucleation Detection Analyzer at Westlake University (FINDA-WLU) with an improved hardware setup, user-friendly software, precise droplet freezing detection, and rigorous temperature calibrations. The temperature uncertainty of FINDA-WLU is about ±0.60 °C, considering both vertical heat transfer efficiency and horizontal temperature heterogeneity. The system is tested with Milli-Q ultrapure water and reference materials like Arizona Test Dust and Snomax^®, and the results are consistent with previous studies. We also use the FINDA-WLU to measure INPs in precipitation samples collected in China. Overall, FINDA-WLU proved to be a reliable and precise method for measuring INA and INP concentrations.

In this study, we investigate the connection between planetary equatorial waves, modulated by the Indian Ocean dipole (IOD) and El Niño Southern Oscillation (ENSO), and the interannual variabilities of the salinity distribution in the Bay of Bengal (BoB) in October–December (OND), along with its associated dynamics, using satellite and reanalysis datasets. In OND 2010 and 2016 (1994, 1997, 2006, and 2019), positive (negative) sea surface salinity anomalies (SSSAs) were distributed in the eastern equatorial Indian Ocean (EIO) and Andaman Sea. Moreover, the southward movement of negative (positive) SSSAs along the eastern Indian coast was observed. This phenomenon was caused by large-scale anomalous currents associated with zonal wind over the EIO. During OND 2010 and 2016 (1994, 1997, 2006, and 2019), due to anomalous westerlies (easterlies) over the EIO and anomalous downwelling (upwelling) Kelvin waves, the strengthened (weakened) Wyrtki jet and the basin-scale anomalous cyclonic (anticyclonic) circulation in the BoB gave rise to positive (negative) SSSAs within the eastern EIO and Andaman Sea. In addition, the intensified (weakened) eastern Indian coastal currents led to the southward movement of negative (positive) SSSAs. It is worth noting that downwelling Kelvin waves reached the western coast of India during OND 2010 and 2016, while upwelling Kelvin waves were only confined to the eastern coast of India during OND 1994, 1997, 2006, and 2019. Furthermore, westward salinity signals associated with reflected westward Rossby waves could modulate the spatial pattern of salinity. The distribution of salinity anomalies could potentially influence the formation of the barrier layer, thereby impacting the sea surface temperature variability and local convection.

Marine boundary layer (MBL) clouds exert a strong cooling influence on Earth’s climate, yet their response to aerosol perturbations remains one of the largest uncertainties in climate projections. A key challenge is identifying which aerosol and meteorological factors control cloud droplet number concentration (N_d), especially when boundary-layer structure alters the transport of surface-emitted aerosol to cloud base. Here, we combine aircraft and ground-based measurements from the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) campaign with the aerosol activation parameterization to quantify controlling factors on N_d during different seasons. We found that summer and winter boundary layer decoupling modulates the vertical transport of aerosols within the MBL and thereby influences cloud condensation nuclei number concentration (N_CCN) at cloud base. In both seasons, decoupling occurs for roughly 32 % of the total time, during which ~60 % of accumulation-mode and ~75 % of Aitken-mode aerosols fail to reach the cloud layer. The out-of-cloud accumulation-mode particle number concentration (N_acc) is higher in summer than in winter, whereas the corresponding N_d is comparable between the two seasons. Sensitivity experiments show that N_acc is the dominant control on N_d in summer, whereas cloud droplet formation in winter is additionally sensitive to Aitken-mode aerosol number concentration (N_ait) and size. In contrast, seasonal variations in updraft velocity and aerosol hygroscopicity have weaker effects. These results help clarify how boundary-layer coupling and aerosol size distributions jointly regulate marine cloud droplet formation and provide observational constraints for representing aerosol-marine cloud interactions in climate models.

Marine boundary layer (MBL) clouds exert a strong cooling influence on Earth’s climate, yet their response to aerosol perturbations remains one of the largest uncertainties in climate projections. A key challenge is identifying which aerosol and meteorological factors control cloud droplet number concentration (Nd), especially when boundary-layer structure alters the transport of surface-emitted aerosol to cloud base. Here, we combine aircraft and ground-based measurements from the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) campaign with the aerosol activation parameterization to quantify controlling factors on Nd during different seasons. We found that summer and winter boundary layer decoupling modulates the vertical transport of aerosols within the MBL and thereby influences cloud condensation nuclei number concentration (NCCN) at cloud base. In both seasons, decoupling occurs for roughly 32 % of the total time, during which ~60 % of accumulation-mode and ~75 % of Aitken-mode aerosols fail to reach the cloud layer. The out-of-cloud accumulation-mode particle number concentration (Nacc) is higher in summer than in winter, whereas the corresponding Nd is comparable between the two seasons. Sensitivity experiments show that Nacc is the dominant control on Nd in summer, whereas cloud droplet formation in winter is additionally sensitive to Aitken-mode aerosol number concentration (Nait) and size. In contrast, seasonal variations in updraft velocity and aerosol hygroscopicity have weaker effects. These results help clarify how boundary-layer coupling and aerosol size distributions jointly regulate marine cloud droplet formation and provide observational constraints for representing aerosol-marine cloud interactions in climate models.