scholarly journals Upper ocean responses in the central western equatorial Indian Ocean During the southwest monsoon season

MAUSAM ◽  
2021 ◽  
Vol 47 (1) ◽  
pp. 21-30
Author(s):  
M, G. JOSEPH ◽  
P.V. HAREESH KUMAR ◽  
P. MADHUSOODANAN
Keyword(s):  
Indian Ocean ◽  
Density Gradient ◽  
Wind Stress ◽  
Mixed Layer ◽  
Time Series Data ◽  
Mixed Layer Depth ◽  
Heat Content ◽  
Equatorial Indian Ocean ◽  
Series Data ◽  
Upper Ocean

 Upper ocean (200 m) response under the pre-onset, and active regimes of southwest (SW) monsoonal forcing at 0°N. 60°E in the Indian Ocean was analysed utilising time series data collection during Indo-Soviet Monsoon Experiment, 1973 (ISMEX- 73). Oceanic response under the pre-onset domination of the wind stress momentum and onset domination of buoyancy flux (B0) was apparent in shoaling/warming and deepening/cooling (12 m/0.50 C in 4 days) of Mixed Layer Depth (MLD). The pre-onset increase was followed by an onset decrease in below layer thermohaline/density gradient and disappearance of Sub-surface Salinity Maximum (SSM). Corespondingly, MLD and its heat content (HCMLD ) were more correlated to (B0) and QN . Upper ocean response during active regime manifested in deepening/colling (20 m/1C in 6 days) of MLD under dominant production of turbulent kinetic energy by wind stress except for the convectively dominant mixing at the beginning and end. With reduction in below-layer thermohaline/density gradient and absence of SSM the correlations between MLD B0 wind stress, QN and HCMLD became insignificant due to increased advective flux during active regime. One dimensional simulation of mixed layer paramerters showed agreement.

Drivers of oceanic exchange through the Indonesian Throughflow since the late 1800s – a synthesis of coral δ18O and high-resolution ocean models

2020 ◽  
Author(s):  
Sujata Murty ◽  
Caroline Ummenhofer ◽  
Markus Scheinert ◽  
Erik Behrens ◽  
Arne Biastoch ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Heat Content ◽  
Phase Changes ◽  
Boreal Winter ◽  
Indonesian Throughflow ◽  
Layer Depth ◽  
Ocean Variability ◽  
The Indian Ocean

<p>The Indonesian Throughflow (ITF) serves as an important oceanic teleconnection for Indo-Pacific climate, altering heat and buoyancy transport from the Pacific to the Indian Ocean. Equatorial Pacific wind forcing transmitted through the ITF impacts interannual to interdecadal Indian Ocean thermocline depth and heat content, with implications for preconditioning Indian Ocean Dipole events. Yet the modulation of Indian Ocean thermal properties at seasonal timescales is still poorly understood. Here we synthesize coral δ<sup>18</sup>O records, instrumental indices (El Niño Southern Oscillation (ENSO), Asian Monsoon), and simulated ocean variability (sea surface salinity (SSS) and temperature (SST), heat content, mixed layer depth) from state-of-the-art NEMO ocean model hindcasts to explore drivers of seasonal to multi-decadal variability. All coral sites are located within main ITF pathways and are influenced by monsoon-driven, buoyant South China Sea (SCS) surface waters during boreal winter that obstruct surface ITF flow and reduce heat transport to the Indian Ocean. Makassar and Lombok Strait coral δ<sup>18</sup>O co-varies with simulated SSS, subsurface heat content anomalies (50-350m) and mixed layer depth at the coral sites and in the eastern Indian Ocean. At decadal timescales, simulated boreal winter ocean variability at the coral sites additionally indicates a potential intensification of the SCS buoyancy plug from the mid- to late-20<sup>th</sup> century. Notably, the variability in these coral and model responses reveals sensitivity to phase changes in the Interdecadal Pacific Oscillation and the East Asian Winter Monsoon. These results collectively suggest that the paleoproxy records are capturing important features of regional hydrography and Indo-Pacific exchange, including responses to regional monsoon variability. Such proxy-model comparison is critical for understanding the drivers of variability related to changes in ITF oceanic teleconnections over the 19<sup>th</sup> and 20<sup>th</sup> centuries.</p>

scholarly journals Subsurface oceanic structure associated with atmospheric convectively coupled equatorial Kelvin waves in the eastern Indian Ocean

2021 ◽  
Author(s):  
Marina Azaneu ◽  
Adrian Matthews ◽  
Dariusz Baranowski
Keyword(s):  
Indian Ocean ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Phase Speed ◽  
Heat Content ◽  
Life Cycles ◽  
Kelvin Waves ◽  
Eastern Indian Ocean ◽  
Dynamical Response ◽  
Ocean Atmosphere

<p>Atmospheric convectively coupled equatorial Kelvin waves (CCKWs) are a major tropical weather feature strongly influenced by ocean--atmosphere interactions. However, prediction of the development and propagation of CCKWs remains a challenge for models. The physical processes involved in these interactions are assessed by investigating the oceanic response to the passage of CCKWs across the eastern Indian Ocean and MC using the NEMO ocean model analysis with data assimilation. Three-dimensional life cycles are constructed for "solitary" CCKW events. As a CCKW propagates over the eastern Indian Ocean, the immediate thermodynamic ocean response includes cooling of the ocean surface and subsurface, deepening of the mixed layer depth, and an increase in the mixed layer heat content. Additionally, a dynamical downwelling signal is observed two days after the peak in the CCKW westerly wind burst, which propagates eastward along the Equator and then follows the Sumatra and Java coasts, consistent with a downwelling oceanic Kelvin wave with an average phase speed of 2.3 m s<sup>-1</sup>. Meridional and vertical structures of zonal velocity anomalies are consistent with this framework. This dynamical feature is consistent across distinct CCKW populations, indicating the importance of CCKWs as a source of oceanic Kelvin waves in the eastern Indian Ocean. The subsurface dynamical response to the CCKWs is identifiable up to 11 days after the forcing. These ocean feedbacks on time scales longer than the CCKW life cycle help elucidate how locally driven processes can rectify onto longer time-scale processes in the coupled ocean--atmosphere system.</p>

scholarly journals Observed increase in springtime surface partial pressure of CO<sub>2</sub> in the east equatorial Indian Ocean during 1962–2012

2014 ◽  
Vol 11 (1) ◽  
pp. 521-549
Author(s):  
L. Xue ◽  
W. Yu ◽  
H. Wang ◽  
L. Feng ◽  
Q. Wei ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Partial Pressure ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Equatorial Indian Ocean ◽  
Sea Surface ◽  
Atmospheric Co2 ◽  
Global Ocean ◽  
Layer Depth ◽  
Co2 Sink

Abstract. Rapidly rising atmospheric CO2 and global warming may have been impacting the ocean, and, in contrast, the response of surface CO2 partial pressure (pCO2) in the equatorial Indian Ocean is poorly understood. In this study, we attempted to evaluate the variation of springtime sea surface pCO2 in the east equatorial Indian Ocean (5° N to 5° S along 90° E and 95° E, EIO), which is relatively better occupied, using data collected in May 2012, together with the historical data since 1962 (LDEO_Database_V2012). Results showed that sea surface pCO2 in the investigation area increased from ~308 μatm in April 1963, through ~373 μatm in May 1999, to ~387μatm in May 2012, with a mean increase rate of ~1.7μatm yr−1. Given that the EIO during the study period was almost always a CO2 source to the atmosphere, it was obvious that the observed increase of sea surface pCO2 with time in this region was not due to the local uptake of CO2 via air–sea exchange, although quickly increasing atmospheric CO2 had the potential to increase seawater pCO2. Further, we checked the effects of variations in sea surface temperature, salinity, mixed layer depth and chlorophyll a (as a proxy of biological production) on surface pCO2. We found surface ocean warming partially contributed to sea surface pCO2 increase, whereas the effects of salinity, mixed layer depth, and biological activity were not significant. The pCO2 increase in the equatorial waters (CO2 source to the atmosphere) was probably due to the transport of carbon accumulated in the CO2 sink region (to the atmosphere) towards the CO2 source region on a basin scale via ocean circulation. Additionally, our study showed that more and more release of CO2 from the ocean to the atmosphere and big pH reduction (0.07 pH units) in the past 50 yr (from 1963 to 2012) may have occurred in the EIO. It also demonstrated that ocean acidification may have taken place in the global ocean, not just limited to the CO2 sink region.

scholarly journals Intraseasonal Variability of Equatorial Indian Ocean Zonal Currents

2007 ◽  
Vol 20 (13) ◽  
pp. 3036-3055 ◽  
Author(s):  
Debasis Sengupta ◽  
Retish Senan ◽  
B. N. Goswami ◽  
Jérôme Vialard
Keyword(s):  
Indian Ocean ◽  
Wind Stress ◽  
Equatorial Indian Ocean ◽  
Ocean Current ◽  
Upper Ocean ◽  
Thermocline Depth ◽  
Westerly Wind ◽  
Pressure Force ◽  
Easterly Wind ◽  
Wind Bursts

Abstract New satellite and in situ observations show large intraseasonal (10–60 day) variability of surface winds and upper-ocean current in the equatorial Indian Ocean, particularly in the east. An ocean model forced by the Quick Scatterometer (QuikSCAT) wind stress is used to study the dynamics of the intraseasonal zonal current. The model has realistic upper-ocean currents and thermocline depth variabilities on intraseasonal to interannual scales. The quality of the simulation is directly attributed to the accuracy of the wind forcing. At the equator, moderate westerly winds are punctuated by strong 10–40-day westerly wind bursts. The wind bursts force swift, intraseasonal (20–50 day) eastward equatorial jets in spring, summer, and fall. The zonal momentum balance is between local acceleration, stress, and pressure, while nonlinearity deepens and strengthens the eastward current. The westward pressure force associated with the thermocline deepening toward the east rapidly arrests eastward jets and, subsequently, generates (weak) westward flow. Thus, in accord with direct observations in the east, the spring jet is a single intraseasonal event, there are intraseasonal jets in summer, and the fall jet is long lived but strongly modulated on an intraseasonal scale. The zonal pressure force is almost always westward in the upper 120 m, and changes sign twice a year in the 120–200-m layer. Transient eastward equatorial undercurrents in early spring and late summer are associated with semiannual Rossby waves generated at the eastern boundary following thermocline deepening by the spring and fall jets. An easterly wind stress is not necessary to generate the undercurrents. Experiments with a single westerly wind burst forcing show that apart from the intraseasonal response, the zonal pressure force and current in the east have an intrinsic 90-day time scale that arises purely from equatorial adjustment.

scholarly journals MJO-induced Intraseasonal Mixed Layer Depth Variability in the Equatorial Indian Ocean and Impacts on Subsurface Water Obduction

2021 ◽  
Author(s):  
Lingling Liu ◽  
Yuanlong Li ◽  
Fan Wang
Keyword(s):  
Indian Ocean ◽  
Summer Monsoon ◽  
Mixed Layer ◽  
Monsoon Season ◽  
Mixed Layer Depth ◽  
Equatorial Indian Ocean ◽  
Subsurface Water ◽  
Surface Mixed Layer ◽  
Summer Monsoon Season ◽  
Layer Depth

AbstractChange of ocean surface mixed layer depth (MLD) is critical for vertical exchanges between the surface and subsurface oceans and modulates surface temperature variabilities on various timescales. In-situ observations have documented prominent intraseasonal variability (ISV) of MLD with 30-105-day periods in the equatorial Indian Ocean (EIO) where the Madden-Julian oscillation (MJO) initiates. Simulation of Hybrid Coordinate Ocean Model (HYCOM) reveals a regional maximum of intraseasonal MLD variability in the EIO (70°E-95°E, 3°S-3°N) with a standard deviation of ~14 m. Sensitivity experiments of HYCOM demonstrate that among all the MJO-related forcing effects, the wind-driven downwelling and mixing are primary causes for intraseasonal MLD deepening and explain 83.7% of the total ISV. The ISV of MLD gives rise to high-frequency entrainments of subsurface water, leading to an enhancement of annual entrainment rate by 34%. However, only a small fraction of these entrainment events (< 20%) can effectively contribute to the annual obduction rate of 1.36 Sv, a quantification for the amount of resurfacing thermocline water throughout a year that mainly (84.6%) occurs in the summer monsoon season (May-October). The ISV of MLD achieves the maximal intensity in April-May and greatly affects the subsequent obduction. Estimation based on our HYCOM simulations suggests that MJOs overall reduce the obduction rate in the summer monsoon season by as much as 53%. A conceptual schematic is proposed to demonstrate how springtime intraseasonal MLD deepening events caused by MJO winds narrow down the time window for effective entrainment and thereby suppress the obduction of thermocline water.

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