scholarly journals Seasonal and interannual variations of the nitrogen cycle in the Arabian Sea

2013 ◽  
Vol 10 (12) ◽  
pp. 19541-19570 ◽  
Author(s):  
T. Rixen ◽  
A. Baum ◽  
B. Gaye ◽  
B. Nagel
Keyword(s):  
Indian Ocean ◽  
Nitrogen Cycle ◽  
Arabian Sea ◽  
Interannual Variations ◽  
Sea Level Changes ◽  
Carbon Export ◽  
Subsurface Waters ◽  
Glacial Periods ◽  
Seasonal And Interannual Variations ◽  
Sw Monsoon

Abstract. The Arabian Sea is strongly influenced by the Asian monsoon and plays an important role as a climate archive and in the marine nitrogen cycle, because bio-available NO3− is reduced to dinitrogen gas (N2) in its mid-water oxygen minimum layer (OMZ). In order to investigate seasonal and interannual variations of the nitrogen cycle, nutrient data were obtained from the literature prior to 1993, evaluated, and compared with data measured during five expeditions in 1995 as well as a research cruise in 2007. Our results imply that the area characterized by a pronounced secondary nitrite maximum (SNM) was by 63% larger in 1995 than before. This area, referred to as the core of the denitrifying zone, shows strong seasonal and interannual variations driven by the monsoon. During the SW monsoon the SNM retreats eastwards due to the inflow of oxygen-enriched Indian Ocean Central Water (ICW) and it expands westwards during the NE monsoon because of the reversal of the current regime, which allows the propagation of denitrification signals from the Indian shelf into the open Arabian Sea. On an interannual time-scale an enhanced SW monsoon increases NO3− losses by increasing the upwelling-driven carbon export into the subsurface waters. An associate enhanced inflow of ICW increases the transport of denitrification signals from the SNM into the upwelling region and compensates NO3− losses by enhanced NO3− supply from the Indian Ocean. The latter sustains an enhanced productivity, which in turn transfers denitrification signals into the sedimentary records. On glacial interglacial time scales sea level changes affecting the inflow of ICW seem to increase variations in the accumulation of denitrification tracers in the SNM by reducing the residence time during glacial periods.

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

2014 ◽  
Vol 11 (20) ◽  
pp. 5733-5747 ◽  
Author(s):  
T. Rixen ◽  
A. Baum ◽  
B. Gaye ◽  
B. Nagel
Keyword(s):  
Indian Ocean ◽  
Nitrogen Cycle ◽  
Arabian Sea ◽  
Equatorial Indian Ocean ◽  
Global Ocean ◽  
Interannual Variations ◽  
Minimum Zone ◽  
Induced Changes ◽  
Seasonal And Interannual Variations ◽  
Sw Monsoon

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 (NO3−) is reduced to dinitrogen gas (N2). 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 oxygen-enriched 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.

scholarly journals Reviews and syntheses: Present, past, and future of the oxygen minimum zone in the northern Indian Ocean

2020 ◽  
Vol 17 (23) ◽  
pp. 6051-6080
Author(s):  
Tim Rixen ◽  
Greg Cowie ◽  
Birgit Gaye ◽  
Joaquim Goes ◽  
Helga do Rosário Gomes ◽  
...  
Keyword(s):  
Oxygen Consumption ◽  
Indian Ocean ◽  
Nitrogen Cycle ◽  
Bay Of Bengal ◽  
Arabian Sea ◽  
Oxygen Supply ◽  
Northern Indian Ocean ◽  
Minimum Zone ◽  
Oxygen Minimum ◽  
Oxygen Concentrations

Abstract. Decreasing concentrations of dissolved oxygen in the ocean are considered one of the main threats to marine ecosystems as they jeopardize the growth of higher organisms. They also alter the marine nitrogen cycle, which is strongly bound to the carbon cycle and climate. While higher organisms in general start to suffer from oxygen concentrations < ∼ 63 µM (hypoxia), the marine nitrogen cycle responds to oxygen concentration below a threshold of about 20 µM (microbial hypoxia), whereas anoxic processes dominate the nitrogen cycle at oxygen concentrations of < ∼ 0.05 µM (functional anoxia). The Arabian Sea and the Bay of Bengal are home to approximately 21 % of the total volume of ocean waters revealing microbial hypoxia. While in the Arabian Sea this oxygen minimum zone (OMZ) is also functionally anoxic, the Bay of Bengal OMZ seems to be on the verge of becoming so. Even though there are a few isolated reports on the occurrence of anoxia prior to 1960, anoxic events have so far not been reported from the open northern Indian Ocean (i.e., other than on shelves) during the last 60 years. Maintenance of functional anoxia in the Arabian Sea OMZ with oxygen concentrations ranging between > 0 and ∼ 0.05 µM is highly extraordinary considering that the monsoon reverses the surface ocean circulation twice a year and turns vast areas of the Arabian Sea from an oligotrophic oceanic desert into one of the most productive regions of the oceans within a few weeks. Thus, the comparably low variability of oxygen concentration in the OMZ implies stable balances between the physical oxygen supply and the biological oxygen consumption, which includes negative feedback mechanisms such as reducing oxygen consumption at decreasing oxygen concentrations (e.g., reduced respiration). Lower biological oxygen consumption is also assumed to be responsible for a less intense OMZ in the Bay of Bengal. According to numerical model results, a decreasing physical oxygen supply via the inflow of water masses from the south intensified the Arabian Sea OMZ during the last 6000 years, whereas a reduced oxygen supply via the inflow of Persian Gulf Water from the north intensifies the OMZ today in response to global warming. The first is supported by data derived from the sedimentary records, and the latter concurs with observations of decreasing oxygen concentrations and a spreading of functional anoxia during the last decades in the Arabian Sea. In the Arabian Sea decreasing oxygen concentrations seem to have initiated a regime shift within the pelagic ecosystem structure, and this trend is also seen in benthic ecosystems. Consequences for biogeochemical cycles are as yet unknown, which, in addition to the poor representation of mesoscale features in global Earth system models, reduces the reliability of estimates of the future OMZ development in the northern Indian Ocean.

scholarly journals Nitrous oxide and methane during the 1994 SW monsoon in the Arabian Sea/northwestern Indian Ocean

1999 ◽  
Vol 104 (C12) ◽  
pp. 30067-30084 ◽  
Author(s):  
Robert. C. Upstill-Goddard ◽  
Jonathan Barnes ◽  
Nicholas J. P. Owens
Keyword(s):  
Nitrous Oxide ◽  
Indian Ocean ◽  
Arabian Sea ◽  
Sw Monsoon

Variability of Sea Surface Salinity in the Southeastern Arabian Sea: Driving mechanisms and influence on the Arabian Sea mini Warm Pool

2020 ◽  
Author(s):  
Akhil Valiya Parambil ◽  
Matthieu Lengaigne ◽  
Jerome Vialard ◽  
Krishnamohan Krishnapillai Sukumarapillai ◽  
Keerthi Madhavan Girijakumari
Keyword(s):  
Indian Ocean ◽  
Seasonal Cycle ◽  
Arabian Sea ◽  
Vertical Mixing ◽  
Warm Pool ◽  
Sea Surface ◽  
Interannual Variations ◽  
Salinity Stratification ◽  
Driving Mechanisms ◽  
Southeastern Arabian Sea

&lt;p&gt;With sea surface temperatures (SST) exceeding 30&amp;#730;C in May, the southeastern Arabian Sea (SEAS) hosts one of the warmest open ocean region globally, which appears to play an important role in the summer monsoon onset. Freshwater input from the Bay of Bengal precede the SEAS warm pool build-up by a few months, and are believed to influence its temperature through its impact on oceanic stability and vertical mixing of heat. SSS interannual variations in the SEAS region have not been extensively described before, and their potential feedback on the warm pool build-up and the monsoon are still debated. In the present study, we describe the SEAS SSS seasonal and interannual variability, its driving mechanisms and potential impact on the monsoon. To that end, we analyse experiments performed with a regional 25-km ocean model, both forced and coupled to a regional atmospheric model. The forced and coupled simulations both reproduce the main oceanic features in the SEAS region, including the salinity seasonal cycle and interannual variability. Winter salinity stratification inhibits the vertical mixing of heat, thereby warming the mixed layer by ~0.5&amp;#176;C.month&lt;sup&gt;-1&lt;/sup&gt;. This salinity-induced warming is however compensated by a salinity-induced cooling by air-sea fluxes. Salinity stratification indeed yields a thinner mixed layer which is more efficiently cooled by negative surface heat fluxes at this season. Overall, salinity has thus a negligible impact on the SST seasonal cycle. SEAS SSS interannual variations are largely remotely driven by the Indian Ocean Dipole (IOD), an indigenous interannual climate mode in the equatorial Indian Ocean. The IOD remotely impacts coastal currents along the Indian coastline, and hence modulates freshwater transport from the Bay of Bengal into the SEAS. This yields positive SSS anomalies in the SEAS during the boreal winter that follows positive IOD events. Those SSS anomalies however do not appear to significantly alter the interannual surface layer heat budget. Coupled model sensitivity experiments, in which the influence of haline stratification on vertical mixing is neglected, further confirm that the SEAS winter freshening does not significantly influence the SEAS warm-pool build-up nor the monsoon onset&lt;/p&gt;

Seasonal and interannual variations of mixed layer salinity in the southeast tropical Indian Ocean

2016 ◽  
Vol 121 (7) ◽  
pp. 4716-4731 ◽  
Author(s):  
Ningning Zhang ◽  
Ming Feng ◽  
Yan Du ◽  
Jian Lan ◽  
Susan E. Wijffels
Keyword(s):  
Indian Ocean ◽  
Mixed Layer ◽  
Tropical Indian Ocean ◽  
Interannual Variations ◽  
Seasonal And Interannual Variations

Contribution of Tropical Cyclones to the Global Precipitation from Eight Seasons of TRMM Data: Regional, Seasonal, and Interannual Variations

2010 ◽  
Vol 23 (6) ◽  
pp. 1526-1543 ◽  
Author(s):  
Haiyan Jiang ◽  
Edward J. Zipser
Keyword(s):  
Tropical Cyclone ◽  
Indian Ocean ◽  
North Indian Ocean ◽  
Northwest Pacific ◽  
Intensity Level ◽  
Interannual Variations ◽  
Central Pacific ◽  
South Indian ◽  
The Impact ◽  
Seasonal And Interannual Variations

Abstract Based on the University of Utah Tropical Rainfall Measuring Mission (TRMM) precipitation feature (PF) database, tropical cyclone PFs (TCPFs) are identified for over 600 storms that reached tropical storm intensity level or above around the globe during eight TC seasons from the period of 1998–2006. Each TC season includes 6 months yr−1. Six basins are considered: Atlantic (ATL), east-central Pacific (EPA), northwest Pacific (NWP), north Indian Ocean (NIO), south Indian Ocean (SIO), and South Pacific (SPA). TRMM 2A25- (precipitation radar) and 3B42- (multisatellite) derived rainfall amounts are used to assess the impact of tropical cyclone (TC) rainfall in altering the regional, seasonal, and interannual distribution of the global total rainfall during the TC seasons in the six basins. The global, seasonal, and interannual variations of the monthly rainfall inside TCPFs are presented. The fractional rainfall contributions by TCPFs are compared in different basins. The TRMM 2A25 and 3B42 retrievals are compared in terms of the rainfall contribution by TCs. After constraining TC rainfall for being within 500 km from the TC center, 2A25 and 3B42 show similar results: 1) TCs contribute, respectively, 8%–9%, 7%, 11%, 5%, 7%–8%, and 3%–4% of the seasonal rainfall to the entire domain of the ATL, EPA, NWP, NIO, SIO, and SPA basins; 2) both algorithms show that, regionally, the maximum percentage of TC rainfall contribution is located in EPA basin near the Mexico Baja California coast (about 55%), SIO close to the Australia coast (about 55%), and NWP near Taiwan (about 35%–40%); 3) the maximum monthly percentage of TC rainfall contribution is in September for the ATL basin, August and September for EPA, August for NWP, May for NIO, March for SIO, and January and February for SPA; 4) the percentage of rainfall contributed by TCs is higher during El Niño years than La Niña years for EPA and NWP basins. The trend is the reverse for ATL and NIO, and nearly neutral for SIO and SPA. However, this study does not include enough years of data to expect the findings to be representative of long-term statistics of the interannual variations.

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