The Southern Ocean (SO) marine atmospheric boundary layer (MABL), regulated by continuous air-sea ex-changes, plays a key role in transporting exchanges between tropics and poles. Among SO sectors, the Indian Ocean sector of SO (ISSO) remained least explored in terms of MABL characterization and is examined in this study. In ISSO, occurrence of sharp oceanic thermohaline fronts regulates the vertical thermodynamic structure of MABL, clouds, and inversions. In this study, these properties of ISSO MABL are investigated over three oceanic domains (Sub-Tropical Indian Ocean (STIO), ISSO, and High-Latitude SO (HLSO)). To achieve this, near-sea surface air-sea exchanges along with profiles of meteorological parameters generated between 25 degrees S and 68 degrees S and 57 degrees E to78 degrees E during three field campaigns conducted in the austral summers of 2017, 2018, and 2020 were utilized. Results showed strong SST-Tair variability across the study region. In STIO, positive SST-Tair indicated Low-Level Cold Air Advection (LLCAA)-induced destabilized and coupled MABLs capped by multiple inversions (>3INV's of strength-0.35 K m-1 above-1200 m) and thin mid/high-altitudes clouds (cloud-base-830 m, cloud-top-2309 m, and cloud-thickness-758 m). Over ISSO and northern HLSO, weak and negative SST-Tair indicated Low-Level Warm Air Advection (LLWAA)-induced stratified and decoupled MABLs. Aided by advective mixing of multiple air-masses, low-level thick multilayered clouds (>2 layers, average cloud-base-604 m, cloud -top-2288 m, and cloud-thickness-1314 m) and multiple strong high-level inversions (>2INV's, strength>0.4 K m-1 ,-1836 m) was observed. In HLSO, weakly positive SST-Tair indicated LLCAA-induced weakly destabilized MABLs capped by mid-altitude inversions (strength-0.22 K m-1,-1632 m) and mid-altitude clouds (cloud -base-979 m, cloud-top-2465 m, and cloud-thickness-1263 m) supported by sublimation leading to virga conditions.

Using ten years of in-situ data from a shallow optically complex coastal ecosystem, this study unravels the coupling between bio-optical variability and seasonally changing hydrography. The data presented is novel, as these are long-term simultaneous measurements of bio-optical properties and constituents covering a wide range of hydrographic conditions at different sampling stations in riverine, estuarine, and coastal shelf. The data represent all three seasons experienced in the region, i.e., Indian Summer Monsoon (ISM), post-ISM, and pre-ISM. It allowed us to conduct a baseline study to characterize the bio-optical variability (absorption or scattering dominated) coupled with stratification for remote sensing applications on ecological health. Four groups of sampling stations, representing varying hydrography, were identified as riverine, maximum stratification, maximum chlorophyll-a (Chl-a) concentration, and estuarine/shelf. The maximum concentrations of total suspended matter (TSM) were always collocated with the strongest stratification site, while maximum Chl-a was always located downstream of maximum stratification. Surprisingly, colored dissolved organic matter (CDOM) values were negligible in the ISM season and increased manifold in other seasons. The source of CDOM (either land-derived or in situ generated) inferred from the spectral slope revealed the dominance of land-derived CDOM in the post-ISM season. Combining the entire data into the same groups showed that groups with maximum TSM and samples from estuarine locations were scattering-dominated. In contrast, the group with maximum Chl-a was absorption dominated. The analysis revealed that the optical behavior of the sampling locations being either absorption or scattering dominated depended on the state of stratification and contribution of Chl-a to the absorption budget.

The study examines the thermodynamic structure of the marine atmospheric boundary layer (MABL) and its effect on the aerosol dynamics in the Indian Ocean sector of Southern Ocean (ISSO) between 30°S-67°S and 57°E-77°E. It includes observations of aerosols and meteorology collected during the Xth Southern Ocean Expedition conducted in December 2017. The results revealed the effect of frontal-region-specific air-sea coupling on the thermodynamic structure of MABL and its role in regulating aerosols in ISSO. The MABL over the subtropical front was unstable and formed a well-evolved mixed layer (≈2400 m) capped by low-level inversions (≈660 m). Convective activities in the Sub-Antarctic Frontal region were associated with the Agulhas Retroflection Current, which supported the formation of a well-developed mixed layer (≈1860 m). The mean estimates of aerosol optical depth (AOD) and black carbon (BC) mass concentrations were 0.095 ± 0.006 and 50 ± 14 ng m-3, respectively, and the resultant clear sky direct shortwave radiative forcing (DARF) and atmospheric heating rate (HR) were 1.32 ± 0.11 W m-2 and 0.022 ± 0.002 K day-1, respectively. In the polar front (PF) region, frequent mid-latitude cyclones led to highly stabilized MABL, supported low-level multi-layered clouds (>3-layers) and multiple high-level inversions (strength > 0.5 K m-1 > 3000 m). The clouds were mixed-phased with temperatures less than -12 °C at 3000 m altitude. Interestingly, there was higher loading of dust and BC aerosols (276 ± 24 ng m-3), maximum AOD (0.109 ± 0.009), clear sky DARF (1.73 ± 0.02 W m-2), and HR (0.029 ± 0.005 K day-1). This showed an accumulation of long-range advected anthropogenic aerosols within baroclinic-boundaries formed over the PF region. Specifically, in the region south of PF, weak convection caused weakly-unstable MABL with a single low-level inversion followed by no clouds/single-layer clouds. Predominant clean maritime air holding a small fraction of dust and BC accounted for lower estimates of AOD (0.071 ± 0.004), BC concentrations (90 ± 55 ng m-3) and associated clear sky DARF and HR were 1.16 ± 0.06 W m-2 and 0.019 ± 0.001 K day-1, respectively.

This study aimed to elucidate the effect of bio-optical complexity on radiant heating rates in coastal waters of the eastern Arabian Sea. The in situ measurements covered a large spatial domain between 9°35'N and 15°43'N and east of 72°58'E and comprised different bio-optical measurements and in-water light field, along the pre-determined nine transects in the vicinity of riverine discharge sites influenced by Indian Summer Monsoon caused precipitation. In addition to the spatial survey, timeseries measurements were also conducted at 15°27'N and 73°42'E at a depth of 20 m. Analyzing the distinctness in surface remote sensing reflectance, data were clustered into four optical water types, representing different bio-optical states. The nearshore waters had the highest concentrations of bio-optical constituents (more bio-optically complex) while the offshore waters had low concentrations of chlorophyll-a and suspended matter (least bio-optically complex). There was a presence of higher colored dissolved organic matter in the offshore waters than in its global estimations. The estimation of radiant heating rates at the surface increased from offshore to nearshore waters. In contrast, the euphotic depth-integrated estimations of radiant heating rate were similar in nearshore and offshore waters. Because the nearshore waters had much shallower bottom and euphotic depths as compared to the offshore, similarity in radiant heating rate estimates seemed to attribute to the higher concentrations of bio-optical constituents in nearshore waters. In conditions with similar surface-reaching irradiance in nearshore and offshore waters, higher attenuation of underwater solar transmission (shallow euphotic depth) occurred when absorption and backscattering by bio-optical constituents increased. The radiant heating rate for the euphotic column in the four bio-optical water types, i.e., O1T (offshore), O2T, O3T, and O4T (nearshore) were 0.225 ± 0.118 °C hr-1, 0.214 ± 0.096 °C hr-1, 0.191 ± 0.097 °C hr-1, and 0.21 ± 0.12 °C hr-1, respectively.

A novel semi-analytical algorithm was developed to retrieve total suspended matter (TSM) in turbid waters. A turbidity index developed from remote sensing reflectance ( $$R_{{{\text{rs}}}}$$ ) of three bands of visible spectrum was employed to estimate TSM. Adopting a partial differentiation approach, wavelength $$\lambda_{1}$$ at which maximum absorption of TSM was identified between 650 nm and 750 nm. The second ( $$\lambda_{2}$$ ) and third ( $$\lambda_{3}$$ ) wavelengths were identified through radiative transfer modelling and parameterization of particulate backscattering to minimize the effect of other optically active substances and account for backscattering effects, respectively. Thus, the wavelengths identified were 679 nm ( $$\lambda_{1}$$ ), 695 nm ( $$\lambda_{2}$$ ), and 704 nm ( $$\lambda_{3}$$ ). Regressing the index with 50 randomly chosen in situ data points (Zuary River estuary, Goa) resulted in a best-fit polynomial form of algorithm. This algorithm was validated with a different set of in situ data points (n = 116), and resulted in a correlation coefficient, r = 0.88. In addition, a comparative analysis of the developed algorithm with forty-one empirical and semi-analytical models of TSM indicated their non-suitability in varying optical conditions. The study further pointed out the significance of 695 nm as an inseparable band of any future optical sensor to retrieve TSM from remotely sensed data.

A new semianalytical algorithm was formulated to retrieve chlorophyll-a (CHL) in optically complex waters using in situ data set of coastal waters of eastern Arabian Sea. The algorithm was derived using CHL index of the form, x = (R-rs((1))(-1)-R-rs((2))(-1)) x R-rs((3)). The first wavelength ((1)) represents the secondary peak of CHL, while the second wavelength ((2)) and third wavelength ((3)) were delineated using a radiative transfer model and partial derivative analysis of hyperspectral remote sensing reflectance, respectively. Further iteration of three wavelengths between 600 and 700 nm resulted in a two-wavelength index, x = (R-rs((1))(-1)-R-rs((2))(-1)) x R-rs((2)). This was further regressed with CHL data initially used for three wavelength index. The final form of algorithm, Goa University Case II (GUC2), c(MCHL)=113.112x(3)-58.408x(2)+8.669x - 0.0384, was validated with in situ CHL ranging between 0.11 and 25.56 g/L, resulted in a strong correlation r(2) = 0.99, RMSE = 0.30, and bias = 0.03. A comparison with NIR-Red two-band, three-band, four-band, synthetic chlorophyll index, and normalized difference chlorophyll index pointed to the nonsuitability of turbid water indices in different water types of the study area. For the first time, a CHL algorithm has been tested successfully in water types outside the region of its formulation. A pixel-to-pixel validation of GUC2-derived MERIS CHL with NASA bio-Optical Marine Algorithm Dataset and Satellite Coastal and Oceanography Research data set resulted in correlation, bias, and RMSE of 0.90, -0.0013, and 1.2499, respectively. Furthermore, GUC2 was successfully tested in Chesapeake Bay for accurate retrieval of CHL from stations with varying turbidity levels. Plain Language Summary Chlorophyll-a (CHL) in coastal and inland water bodies is an index of productivity and has huge implications for identifying fishing zones, eutrophication, hypoxia, and climate change studies. Although, spatiotemporal coverage of Earth via optical remote sensors have long been available, a method to accurately generate CHL in a wide variety of coastal and inland water types (areas prone to human interaction) for a synoptic analysis was desiderated. To address this concern, a new method (an optical algorithm) is developed that when applied to remotely sensed data produced accurate results when compared with widely used indices. The accurate monitoring of CHL is a boost to local fisherman in identifying region of high fishery resources, which enable better catch with less economy and efforts. Thus, this paper depicts the application of space technology to the livelihood of common man.