Seasonal and interannual variations in the nitrogen cycle in the Arabian Sea

Abstract

Abstract. The Arabian Sea plays an important role in the marine nitrogen cycle because of its pronounced mid-water oxygen minimum zone (OMZ) in which bio-available nitrate (NO − 3 ) is reduced to dinitrogen gas (N 2 ). As the nitrogen cycle can respond fast to climate-induced changes in productivity and circulation, the Arabian Sea sediments are an important palaeoclimatic archive. In order to understand seasonal and interannual variations in the nitrogen cycle, nutrient data were obtained from the literature published prior to 1993, evaluated, and compared with data measured during five expeditions carried out in the framework of the Joint Global Ocean Flux Study (JGOFS) in the Arabian Sea in 1995 and during a research cruise of RV Meteor in 2007. The data comparison showed that the area characterized by a pronounced secondary nitrite maximum (SNM) was by 63 % larger in 1995 than a similarly determined estimate based on pre-JGOFS data. This area, referred to as the core of the denitrifying zone, showed strong seasonal and interannual variations driven by the monsoon. During the SW monsoon, the SNM retreated eastward due to the inflow of oxygenenriched Indian Ocean Central Water (ICW). During the NE monsoon, the SNM expanded westward because of the reversal of the current regime. On an interannual timescale, a weaker SW monsoon decreased the inflow of ICW from the equatorial Indian Ocean and increased the accumulation of denitrification tracers by extending the residence time of water in the SNM. This is supported by palaeoclimatic studies showing an enhanced preservation of accumulative denitrification tracers in marine sediments in conjunction with a weakening of the SW monsoon during the late Holocene.

Intensification and deepening of the Arabian Sea oxygen minimum zone in response to increase in Indian monsoon wind intensity

“…However, the simulated OMZ in the Arabian Sea is too weak in this very coarse resolution model, raising concern about its reliability in the region. In the Holocene, large-scale ventilation changes may have played an important role together with the fluctuations in monsoon intensity as suggested by recent studies (Rixen et al, 2014;Gaye et al, submitted;Das et al, 2017). For instance, recent paleo-reconstructions by Das et al (2017) suggest an intensification of the Arabian Sea OMZ from the middle to late Holocene despite a weakening of the SWM winds.…”

Glacial–interglacial changes and Holocene variations in Arabian Sea denitrification

“…Oxygen concentrations in its core and its volume vary in response to the seasonality of ventilation, probably related to the seasonality of isopycnal mixing (Banse et al, 2014;Rixen et al, 2014). Models produce similar patterns with major ventilation from the south during the SW monsoon, while reversing currents, progressive oxygen consumption, and isopycnal mixing reduce oxygen concentrations in the entire basin during the winter monsoon (Resplandy et al, 2012;Rixen et al, 2014).…”

“…Oxygen concentrations in its core and its volume vary in response to the seasonality of ventilation, probably related to the seasonality of isopycnal mixing (Banse et al, 2014;Rixen et al, 2014). Models produce similar patterns with major ventilation from the south during the SW monsoon, while reversing currents, progressive oxygen consumption, and isopycnal mixing reduce oxygen concentrations in the entire basin during the winter monsoon (Resplandy et al, 2012;Rixen et al, 2014). Reoxygenation during the SW monsoon occurs via the invigorated Somali Current in the western Arabian Sea and a northward-flowing undercurrent below 150 m of water depth along the SW coast of India (Resplandy et al, 2012), which was found up to 20 • N (Shetye et al, 1990).…”

Reply to reviewer 2

“…The intensity of the OMZ and denitrification is seasonally variable. Oxygen concentrations in its core and its volume vary in response to the seasonality of ventilation, probably related to the seasonality of isopycnal mixing (Banse et al, 2014;Rixen et al, 2014). Models produce similar patterns with major ventilation from the south during the SW monsoon, while reversing currents, progressive oxygen consumption, and isopycnal mixing reduce oxygen concentrations in the entire basin during the winter monsoon (Resplandy et al, 2012;Rixen et al, 2014).…”