scholarly journals Features of the Equatorial Intermediate Current Associated with Basin Resonance in the Indian Ocean

2018 ◽  
Vol 48 (6) ◽  
pp. 1333-1347 ◽  
Author(s):  
Ke Huang ◽  
Weiqing Han ◽  
Dongxiao Wang ◽  
Weiqiang Wang ◽  
Qiang Xie ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Equatorial Indian Ocean ◽  
Reanalysis Data ◽  
Ocean Model ◽  
Baroclinic Mode ◽  
Energy Propagation ◽  
Equatorial Wave ◽  
The Indian Ocean ◽  
Phase Propagation ◽  
Equatorial Intermediate Current

AbstractThis paper investigates the features of the Equatorial Intermediate Current (EIC) in the Indian Ocean and its relationship with basin resonance at the semiannual time scale by using in situ observations, reanalysis output, and a continuously stratified linear ocean model (LOM). The observational results show that the EIC is characterized by prominent semiannual variations with velocity reversals and westward phase propagation and that it is strongly influenced by the pronounced second baroclinic mode structure but with identifiable vertical phase propagation. Similar behavior is found in the reanalysis data and LOM results. The simulation of wind-driven equatorial wave dynamics in the LOM reveals that the observed variability of the EIC can be largely explained by the equatorial basin resonance at the semiannual period, when the second baroclinic Rossby wave reflected from the eastern boundary intensifies the directly forced equatorial Kelvin and Rossby waves in the basin interior. The sum of the first 10 modes can reproduce the main features of the EIC. Among these modes, the resonant second baroclinic mode makes the largest contribution, which dominates the vertical structure, semiannual cycle, and westward phase propagation of the EIC. The other 9 modes, however, are also important, and the superposition of the first 10 modes produces downward energy propagation in the equatorial Indian Ocean.

scholarly journals Correlation between subsurface salinity anomalies in the Bay of Bengal and the Indian Ocean Dipole and governing mechanisms

2020 ◽  
Author(s):  
Zheen Zhang ◽  
Thomas Pohlmann ◽  
Xueen Chen
Keyword(s):  
Indian Ocean ◽  
Bay Of Bengal ◽  
Indian Ocean Dipole ◽  
Equatorial Indian Ocean ◽  
Ocean Model ◽  
Subsurface Temperature ◽  
Remote Forcing ◽  
Salinity Variability ◽  
Eastern Equatorial Indian Ocean ◽  
The Indian Ocean

Abstract. Lead-lag correlations between the subsurface temperature/salinity anomalies in the Bay of Bengal (BoB) and the Indian Ocean Dipole (IOD) are revealed in model results, ocean synthesis, and observations. Mechanisms for such correlations are further investigated using the Hamburg Shelf Ocean Model (HAMSOM), mainly on the salinity variability. It is found that the subsurface salinity anomaly of the BoB positively correlates to the IOD with a lag of three months on average, while the subsurface temperature anomaly negatively correlates. The model results suggest the remote forcing from the equatorial Indian Ocean dominates the interannual subsurface salinity variability in the BoB. The coastal Kelvin waves carry signals of positive (negative) salinity anomalies from the eastern equatorial Indian Ocean and propagate counterclockwise along the coasts of the BoB during positive (negative) IOD events. Subsequently westward Rossby waves propagate these signals to the basin at a relatively slow speed, which causes a considerable delay of the subsurface salinity anomalies in the correlation. By analyzing the salinity budget of the BoB, it is found that the diffusion dominates the salinity changes near the surface, while the advection dominates the subsurface; the vertical advection of salinity contributes positively to this correlation, while the horizontal advection contributes negatively. These results suggest that the IOD plays a crucial role in the interannual subsurface salinity variability in the BoB.

Origins and Dynamics of the 90-Day and 30–60-Day Variations in the Equatorial Indian Ocean

2005 ◽  
Vol 35 (5) ◽  
pp. 708-728 ◽  
Author(s):  
Weiqing Han
Keyword(s):  
Indian Ocean ◽  
Sea Level ◽  
Equatorial Indian Ocean ◽  
Ocean Model ◽  
Forced Response ◽  
Western Boundary ◽  
Reflected Waves ◽  
Equatorial Wave ◽  
Baroclinic Modes ◽  
And Sea Level

Abstract Sea level observations in the equatorial Indian Ocean show a dominant spectral peak at 90 days and secondary peaks at 30–60 days over an intraseasonal period (20–90 days). A detailed investigation of the origins and dynamics of these variations is carried out using an ocean general circulation model, namely, the Hybrid Coordinate Ocean Model (HYCOM). Two parallel experiments are performed in the tropical Indian Ocean basin for the period 1988–2001: one is forced by NCEP 3-day mean forcing fields together with the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) pentad precipitation, and the other is forced by monthly mean fields. To help to understand the role played by the wind-driven equatorial wave dynamics, a linear continuously stratified ocean model is also used. Both the observed and modeled 90-day sea level anomaly fields and HYCOM surface current clearly show equatorial Kelvin and first-meridional-mode Rossby wave structures that are forced by the 90-day winds. The wind amplitude at the 90-day period, however, is weaker than that for the 30–60-day period, suggesting that the equatorial Indian Ocean selectively responds to the 90-day winds. This selective response arises mainly from the resonant excitation of the second-baroclinic-mode (n = 2) waves by the 90-day winds. In this case, Rossby waves reflected from the eastern ocean boundary enhance the directly forced response in the ocean interior, strengthening the 90-day peak. In addition, the directly forced response increases monotonically with the increase of forcing period, contributing to the larger variances of currents and sea level at 90 days. Two factors account for this monotonic increase in directly forced response. First, at lower frequency, both Rossby and Kelvin waves associated with the low-order baroclinic modes have longer wavelengths, which are more efficiently excited by the larger-scale winds. Second, responses of the high-order modes directly follow the local winds, and their amplitudes are proportional to both forcing period and wind strength. Although most energy is surface trapped, there is a significant amount that propagates through the pycnocline into the deep ocean. The dominance of the 90-day peak occurs not only at the surface but also in the deeper layers down to 600 m. In the deeper ocean, both the directly forced response and reflected waves associated with the first two baroclinic modes contribute to the 90-day variation. Spectra of the observed sea surface temperature (SST) also show a 90-day peak, likely a result of the selective response of the equatorial Indian Ocean at the 90-day period. Near the surface, the spectral peaks of currents and sea level at the 30–60-day period are directly forced by winds that peak at 30–60 days. In the deeper layers, both directly forced and reflected waves associated with the first two baroclinic modes contribute. Oceanic instabilities can have significant contributions only near the western boundary and near 5°N south of Sri Lanka.

scholarly journals Correlation between subsurface salinity anomalies in the Bay of Bengal and the Indian Ocean Dipole and governing mechanisms

Ocean Science ◽  
2021 ◽  
Vol 17 (1) ◽  
pp. 393-409
Author(s):  
Zheen Zhang ◽  
Thomas Pohlmann ◽  
Xueen Chen
Keyword(s):  
Indian Ocean ◽  
Bay Of Bengal ◽  
Indian Ocean Dipole ◽  
Equatorial Indian Ocean ◽  
Ocean Model ◽  
Subsurface Temperature ◽  
Remote Forcing ◽  
Salinity Variability ◽  
Eastern Equatorial Indian Ocean ◽  
The Indian Ocean

Abstract. Lead–lag correlations between the subsurface temperature and salinity anomalies in the Bay of Bengal (BoB) and the Indian Ocean Dipole (IOD) are revealed in model results, ocean synthesis, and observations. Mechanisms for such correlations are further investigated using the Hamburg Shelf Ocean Model (HAMSOM), mainly relating to the salinity variability. It is found that the subsurface salinity anomaly of the BoB positively correlates to the IOD, with a lag of 3 months on average, while the subsurface temperature anomaly correlates negatively. The model results suggest the remote forcing from the equatorial Indian Ocean dominates the interannual subsurface salinity variability in the BoB. The coastal Kelvin waves carry signals of positive (negative) salinity anomalies from the eastern equatorial Indian Ocean and propagate counterclockwise along the coasts of the BoB during positive (negative) IOD events. Subsequently, westward Rossby waves propagate these signals to the basin at a relatively slow speed, which causes a considerable delay of the subsurface salinity anomalies in the correlation. By analyzing the salinity budget of the BoB, it is found that diffusion dominates the salinity changes near the surface, while advection dominates the subsurface; the vertical advection of salinity contributes positively to this correlation, while the horizontal advection contributes negatively. These results suggest that the IOD plays a crucial role in the interannual subsurface salinity variability in the BoB.

Basin-Wide Connections of Upper-Ocean Temperature Variability in the Equatorial Indian Ocean

2021 ◽  
pp. 1-50
Author(s):  
Ge Song ◽  
Bohua Huang ◽  
Rongcai Ren ◽  
Zeng-Zhen Hu
Keyword(s):  
Indian Ocean ◽  
Equatorial Indian Ocean ◽  
Reanalysis Data ◽  
Upper Ocean ◽  
Ocean Temperature ◽  
Ocean Reanalysis ◽  
Near Surface ◽  
Surface Warming ◽  
Transition Mechanism ◽  
Wide Range

AbstractIn this paper, the interannual variability of upper-ocean temperature in the equatorial Indian Ocean (IO) and its basin-wide connections are investigated using 58-year (1958-2015) comprehensive monthly mean ocean reanalysis data. Three leading modes of an empirical orthogonal function (EOF) analysis dominate the variability of upper-ocean temperature in the equatorial IO in a wide range of timescales. A coherent interannual band within the first two EOF modes identifies an oscillation between the zonally tilting thermocline across the equatorial IO in its peak phases and basin-wide displacement of the equatorial thermocline in its transitional phases. Consistent with the recharge oscillation paradigm, this oscillation is inherent of the equatorial IO with a quasi-periodicity around 15 months, in which the wind-induced off-equatorial Rossby waves near 5°S-10°S provide the phase-transition mechanism. This intrinsic IO oscillation provides the biennial component in the observed IOD variations. The third leading mode shows a nonlinear long-term trend of the upper-ocean temperature, including the near-surface warming along the equatorial Indian Ocean, accompanied by cooling trend in the lower thermocline originating further south. Such vertical contrary trends may lead to an enhanced stratification in the equatorial IO.

Variations of the Indian Ocean Walker circulation since the Last Glacial Maximum revealed by reconstructed and simulated zonal wind intensity

2021 ◽  
Author(s):  
Xinquan Zhou ◽  
Stéphanie Duchamp-Alphonse ◽  
Masa Kageyama ◽  
Franck Bassinot ◽  
Xiaoxu Shi ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Surface Temperature ◽  
Equatorial Indian Ocean ◽  
Walker Circulation ◽  
Sea Surface ◽  
The Indian Ocean ◽  
Sst Anomaly ◽  
Mean Circulation ◽  
The Last Glacial Maximum ◽  
The Last Glacial

<p>Today, precipitation and wind patterns over the equatorial Indian Ocean and surrounding lands are paced by monsoon and Walker circulations that are controlled by the seasonal land-sea temperature contrast and the inter-annual convection over the Indo-Pacific Warm Pool, respectively. The annual mean surface westerly winds are particularly tied to the Walker circulation, showing interannual variability coupled with the gradient of Sea Surface Temperature (SST) anomaly between the tropical western and southeastern Indian Ocean, namely, the Indian Ocean Dipole (IOD). While the Indian monsoon pattern has been widely studied in the past, few works deal with the evolution of Walker circulation despite its crucial impacts on modern and future tropical climate systems. Here, we reconstruct the long-term westerly (summer) and easterly (winter) wind dynamics of the equatorial Indian Ocean (10°S−10°N), since the Last Glacial Maximum (LGM) based on i) primary productivity (PP) records derived from coccolith analyses of sedimentary cores MD77-191 and BAR94-24, retrieved off the southern tip of India and off the northwestern tip of Sumatra, respectively and ii) the calculation of a sea surface temperature (SST) anomaly gradient off (south) western Sumatra based on published SST data. We compare these reconstructions with atmospheric circulation simulations obtained with the general coupled model AWI-ESM-1-1-LR (Alfred Wegener Institute Earth System Model).</p><p>Our results show that the Indian Ocean Walker circulation was weaker during the LGM and the early/middle Holocene than present. Model simulations suggest that this is due to anomalous easterlies over the eastern Indian Ocean. The LGM mean circulation state may have been comparable to the year 1997 with a positive IOD, when anomalously strong equatorial easterlies prevailed in winter. The early/mid Holocene mean circulation state may have been equivalent to the year 2006 with a positive IOD, when anomalously strong southeasterlies prevailed over Java-Sumatra in summer. The deglaciation can be seen as a transient period between these two positive IOD-like mean states.</p>

Implementation and Validating of the Regional Ocean Model System (ROMS) for the Sunda Strait connecting the Java Sea to the Indian Ocean

2021 ◽  
Author(s):  
Subekti Mujiasih ◽  
Jean-Marie Beckers ◽  
Alexander Barth
Keyword(s):  
Time Series ◽  
Indian Ocean ◽  
Vertical Profile ◽  
Ocean Model ◽  
Model System ◽  
Satellite Sst ◽  
Regional Ocean Model System ◽  
Regional Ocean Model ◽  
The Indian Ocean ◽  
Java Sea

<p>Regional Ocean Model System (ROMS) has been simulated for the Sunda Strait, the Java Sea, and the Indian Ocean. The simulation was undertaken for thirteen months of data period (August 2013 – August 2014). However, we only used four months period for validation, namely September – December 2013. The input data involved the HYbrid Coordinate Ocean Model (HYCOM) ocean model output by considering atmospheric forcing from the European Centre for Medium-Range Weather Forecasts (ECMWF), without and with tides forcing from TPXO and rivers. The output included vertical profile temperature and salinity, sea surface temperature (SST), seas surface height (SSH), zonal (u), and meridional (v) velocity. We compared the model SST to satellite SST in time series, SSH to tides gauges data in time series, the model u and v component velocity to High Frequency (HF) radial velocity. The vertical profile temperature and salinity were compared to Argo float data and XBT. Besides, we validated the amplitude and phase of the ROMS seas surface height to amplitude and phase of the tides-gauges, including four constituents (M2, S2, K1, O1).</p>

scholarly journals Impact of the Madden–Julian Oscillation on Summer Rainfall in Southeast China

2009 ◽  
Vol 22 (2) ◽  
pp. 201-216 ◽  
Author(s):  
Lina Zhang ◽  
Bizheng Wang ◽  
Qingcun Zeng
Keyword(s):  
Indian Ocean ◽  
Tropical Indian Ocean ◽  
Summer Rainfall ◽  
Western Pacific ◽  
Madden Julian Oscillation ◽  
Energy Propagation ◽  
Southeast China ◽  
The Indian Ocean ◽  
The Impact ◽  
The Western Pacific

Abstract The impact of the Madden–Julian oscillation (MJO) on summer rainfall in Southeast China is investigated using the Real-time Multivariate MJO (RMM) index and the observational rainfall data. A marked transition of rainfall patterns from being enhanced to being suppressed is found in Southeast China (east of 105°E and south of 35°N) on intraseasonal time scales as the MJO convective center moves from the Indian Ocean to the western Pacific Ocean. The maximum positive and negative anomalies of regional mean rainfall are in excess of 10% relative to the climatological regional mean. Such different rainfall regimes are associated with the corresponding changes in physical fields such as the western Pacific subtropical high (WPSH), moisture, and vertical motions. When the MJO is mainly over the Indian Ocean, the WPSH shifts farther westward, and the moisture and upward motions in Southeast China are increased. In contrast, when the MJO enters the western Pacific, the WPSH retreats eastward, and the moisture and upward motions in Southeast China are decreased. It is suggested that the MJO may influence summer rainfall in Southeast China through remote and local dynamical mechanisms, which correspond to the rainfall enhancement and suppression, respectively. The remote role is the energy propagation of the Rossby wave forced by the MJO-related heating over the Indian Ocean through the low-level westerly waveguide from the tropical Indian Ocean to Southeast China. The local role is the northward shift of the upward branch of the anomalous meridional circulation when the MJO is over the western Pacific, which causes eastward retreat of the WPSH and suppressed moisture transport toward Southeast China.

scholarly journals Dynamics of the seasonal variations in the Indian Ocean from TOPEX/POSEIDON sea surface height and an ocean model

1998 ◽  
Vol 25 (11) ◽  
pp. 1915-1918 ◽  
Author(s):  
Jiayan Yang ◽  
Lisan Yu ◽  
Chester J. Koblinsky ◽  
David Adamec
Keyword(s):  
Indian Ocean ◽  
Seasonal Variations ◽  
Sea Surface Height ◽  
Ocean Model ◽  
Sea Surface ◽  
The Indian Ocean
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