Sea-air CO2 fluxes in the Indian Ocean between 1990 and 2009

The Indian Ocean (44 S-30 N) plays an important role in the global carbon cycle, yet it remains one of the most poorly sampled ocean regions. Several approaches have been used to estimate net sea-air CO2 fluxes in this region: interpolated observations, ocean biogeochemical models, atmospheric and ocean inversions. As part of the RECCAP (REgional Carbon Cycle Assessment and Processes) project, we combine these different approaches to quantify and assess the magnitude and variability in Indian Ocean sea-air CO2 fluxes between 1990 and 2009. Using all of the models and inversions, the median annual mean sea-air CO2 uptake of −0.37 ± 0.06 PgC yr -1 is consistent with the −0.24 ± 0.12 PgC yr -1 calculated from observations. The fluxes from the southern Indian Ocean (18-44 S; -0.43 ± 0.07 PgC yr-1 are similar in magnitude to the annual uptake for the entire Indian Ocean. All models capture the observed pattern of fluxes in the Indian Ocean with the following exceptions: underestimation of upwelling fluxes in the northwestern region (off Oman and Somalia), overestimation in the northeastern region (Bay of Bengal) and underestimation of the CO2 sink in the subtropical convergence zone. These differences were mainly driven by lack of atmospheric CO2 data in atmospheric inversions, and poor simulation of monsoonal currents and freshwater discharge in ocean biogeochemical models. Overall, the models and inversions do capture the phase of the observed seasonality for the entire Indian Ocean but overestimate the magnitude. The predicted sea-air CO 2 fluxes by ocean biogeochemical models (OBGMs) respond to seasonal variability with strong phase lags with reference to climatological CO 2 flux, whereas the atmospheric inversions predicted an order of magnitude higher seasonal flux than OBGMs. The simulated interannual variability by the OBGMs is weaker than that found by atmospheric inversions. Prediction of such weak interannual variability in CO2 fluxes by atmospheric inversions was mainly caused by a lack of atmospheric data in the Indian Ocean. The OBGM models suggest a small strengthening of the sink over the period 1990-2009 of -0.01 PgC decade-1. This is inconsistent with the observations in the southwestern Indian Ocean that shows the growth rate of oceanic pCO 2 was faster than the observed atmospheric CO2 growth, a finding attributed to the trend of the Southern Annular Mode (SAM) during the 1990s

A study on inorganic carbon components in the Andaman Sea during the post monsoon season

Extensive data have been collected on the carbon dioxide system during the post monsoon season in the eastern Bay of Bengal and the Andaman Sea of the northeastern Indian Ocean. The vertical distribution of temperature and salinity in the eastern Bay of Bengal were similar to that in the Andaman Sea down to 700-800 m. Below 1200 m depth the salinity remained constant at 34.90 in the Andaman Sea whereas it decreased to 34.80 in the eastern Bay of Bengal. On the other hand, deep waters (> 1200 m) of the Andaman Sea were warmer than those of the Bay of Bengal by approximately 2 degreesC. Dissolved oxygen concentrations in the subsurface waters were higher in the Andaman Sea than in the central Bay of Bengal and Arabian Sea due to lower rates of regeneration. Total alkalinity, and pCO(2) showed similar distribution patterns both in the eastern Bay of Bengal and the Andaman Sea up to a depth of 1000-1200 m. Below this depth, their concentrations were higher in the latter than compared to former due to warmer waters. Carbonate saturation depth with respect to aragonite was shallow (approximately 300 m) in the Andaman Sea whereas deeper waters were found to be under saturated with respect to calcite.Des données extensives sur le système dioxyde de carbone ont été recueillies en saison post-mousson dans la partie est du golfe du Bengale et la mer d’Andaman (océan Indien NE). La distribution verticale de la température et de la salinité est similaire dans ces deux parties jusqu’à l’horizon 700 à 800 m. En dessous de 1200 m, la salinité demeure constante, autour de 34.90, en mer d’Andaman, alors qu’elle décroît à 34.80 dans le golfe du Bengale. D’un autre côté, l’eau profonde, au-dessous de 1200 m est plus chaude en mer d’Andaman d’environ 2°C comparé au golfe du Bengale. Les teneurs en oxygène dissous des eaux de subsurface sont plus élevées en mer d’Andaman qu’au cœur du golfe du Bengale ou qu’en mer d’Arabie, en raison d’une moindre régénération. L’alcalinité totale et la pCO2 présentent des distributions similaires dans les deux zones jusqu’à une profondeur de 1000 à 1200 m. En dessous, les concentrations sont plus élevées en mer d’Andaman en raison de la présence d’eaux plus chaudes. La profondeur de saturation en carbonates en regard de l’aragonite est moindre (environ 300 m) en mer d’Andaman, alors que les eaux plus profondes sont sous-saturées en calcite

Hydrography and circulation of the Bay of Bengal during withdrawal phase of the southwest monsoon

Hydrographic data were collected from 3 to 10 September 1996 along two transects; one at 18 degrees N and the other at 90 degrees E. The data were used to examine the thermohaline, circulation and chemical properties of the Bay of Bengal during the withdrawal phase of the southwest monsoon. The surface salinity exhibited wide spatial variability with values as low as 25.78 at 18 degrees N / 87 degrees E and as high as 34.79 at 8 degrees N / 90 degrees E. Two high salinity cells (S > 35.2) were noticed around 100 m depth along the 90 degrees E transect. The wide scatter in T-S values between 100 and 200 m depth was attributed to the presence of the Arabian Sea High Salinity (ASHS) water mass. Though the warm and low salinity conditions at the sea surface were conducive to a rise in the sea surface topography at 18 degrees N / 87 degrees E, the dynamic height showed a reduction of 0.2 dyn.m. This fall was attributed to thermocline upwelling at this location. The geostrophic currents showed alternating flows across both the transects. Relatively stronger and mutually opposite currents were noticed around 25 m depth across the 18 degrees N transect with velocity slightly in excess of 30 cm s(-1). Similar high velocity (> 40 cm s(-1)) pockets were also noticed to extend up to 30 m depths in the southern region of the 90 degrees E transect. However, the currents below 250 m were weak and in general < 5 cm s(-1). The net geostrophic volume transports were found to be of the order of 1.5 x 10(6) m(3) s(-1) towards the north and of 6 x 10(6) m(3) s(-1) towards west across the 18 degrees N and 90 degrees E transects respectively. The surface circulation-patterns were also investigated using the trajectories of drifting buoys deployed in the eastern Indian Ocean around the same observation period. Poleward movement of the drifting buoy with the arrival of the Indian Monsoon Current (IMC) at about 12 degrees N along the eastern rim of the Bay of Bengal has been noticed to occur around the beginning of October. The presence of an eddy off the southeast coast of India and the IMC along the southern periphery of the Bay of Bengal were also evident in the drifting buoy data.La circulation thermohaline et les propriétés chimiques du golfe du Bengale sont étudiées pendant la phase de renversement de la mousson du sud-ouest ; les données hydrologiques ont été collectées du 3 au 10 septembre 1996 sur deux radiales, l'une à 18° N, l'autre suivant 90° E. La salinité de surface présente une grande variabilité spatiale, de 25,78 (par 18° N / 87° E) jusqu'à 34,79 (par 8° N / 90° E), avec deux maxima (plus de 35,2) vers 100 m de profondeur sur la radiale 90° E. La forte dispersion des températures et des salinités, observée entre 100 et 200 m de profondeur, est attribuée à l'eau très salée en provenance de la mer d'Arabie (ASHS). Bien que les eaux superficielles chaudes et peu salées élèvent la topographie de la surface par 18° N / 87° E, la hauteur dynamique présente une baisse de 0,2 m dyn, attribuée ici à la remontée de la thermocline. Les flux géostrophiques sont alternés à travers les deux radiales. Des courants relativement plus forts (dépassant 30 cm s−1) et opposés sont observés vers 25 m de profondeur à travers la radiale 18° N. Des poches similaires de fort courant (plus de 40 cm s−1) dépassent 30 m de profondeur dans la partie sud de la radiale 90° E. Cependant, au-dessous de 250 m de profondeur, les courants sont faibles (moins de 5 cm s−1). Les flux géostrophiques nets sont respectivement de l'ordre de 1,5 × 106 m3 s−1 vers le nord et 6 × 106 m3 s−1 vers l'ouest à travers les radiales 18° N et 90° E. Les schémas de la circulation superficielle sont établis à partir des trajectoires de bouées dérivantes déployées dans l'est de l'océan Indien pendant la même période. Le mouvement est dirigé vers le pôle au début du mois d'octobre, à l'arrivée du Courant Indien de Mousson (IMC) le long du bord oriental du golfe du Bengale, vers 12° N. La dérive des bouées met en évidence la présence d'un tourbillon au large de la côte sud-est de l'Inde et le Courant Indien de Mousson à la périphérie du golfe du Bengal

Air‐Sea fluxes of CO 2 in the Indian Ocean between 1985 and 2018: A synthesis based on Observation‐based surface CO 2 , hindcast and atmospheric inversion models.

International audienceThe Indian Ocean significantly influences the global carbon cycle but it is one of the undersampled regions with reference to surface ocean pCO2. As a part of the Regional Carbon Cycle Assessment and Processes-2 (RECCAP2) project, several approaches, such as interpolated observational climatology, hindcast model, observation-based surface CO2 (empirical models), and atmospheric inversion models have been employed for estimating net sea-to-air CO2 fluxes between 1985 and 2018. The seasonal, spatial and long-term variability in sea-to-air fluxes of CO2 were compared with observational climatology. The mean value of CO2 in the Indian Ocean (north of 37.5oS) for the period of 1985-2018 using all models is estimated to be -0.19±0.1 PgC yr-1 and it is consistent with the observational climatology (-0.07±0.14 PgC yr-1). The Indian Ocean north of 18oS is found to be the mean annual source (0.04±0.05 PgC yr-1) whereas a net sink (-0.23±0.11 PgC yr-1) in the south of 18oS. All models captured observed spatial patterns but underestimated the net source of CO2 in the Oman/Somalia upwelling, the Equatorial Indian Ocean (EIO) and the Bay of Bengal (BoB) whereas CO2 sink is overestimated in the South Indian Ocean (SIO). Overall, all models captured the seasonality in pCO2 levels and CO2 fluxes but overestimated the amplitude of their variability. All models suggested the strengthening of the sink over the period between 1985 and 2018 by 0.02 PgC yr-1 decade-1. A significant increase in the collection of surface ocean pCO2 and atmospheric CO2 measurements improves the model simulations in the Indian Ocea

Coastal vegetation and estuaries are collectively a greenhouse gas sink

Coastal ecosystems release or absorb carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), but the net effects of these ecosystems on the radiative balance remain unknown. We compiled a dataset of observations from 738 sites from studies published between 1975 and 2020 to quantify CO2, CH4 and N2O fluxes in estuaries and coastal vegetation in ten global regions. We show that the CO2-equivalent (CO2e) uptake by coastal vegetation is decreased by 23–27% due to estuarine CO2e outgassing, resulting in a global median net sink of 391 or 444 TgCO2e yr−1 using the 20- or 100-year global warming potentials, respectively. Globally, total coastal CH4 and N2O emissions decrease the coastal CO2 sink by 9–20%. Southeast Asia, North America and Africa are critical regional hotspots of GHG sinks. Understanding these hotspots can guide our efforts to strengthen coastal CO2 uptake while effectively reducing CH4 and N2O emissions