Spatiotemporal characteristics of water exchange between the Andaman Sea and the Bay of Bengal
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Abstract: A high-resolution customized numerical model is used to analyze the water transport in the three major water passages between the Andaman Sea (AS) and the Bay of Bengal, i.e., the Preparis Channel (PC), the Ten Degree Channel (TDC), and the Great Channel (GC), based on the daily averaged simulation results ranging from 2010 to 2019. Spectral analysis and Empirical Orthogonal Function (EOF) methods are employed to investigate the spatiotemporal variability of the water exchange and controlling mechanisms. The results of model simulation indicate that the net average transports of the PC and GC, as well as their linear trend, are opposite to that of the TDC. This indicates that the PC and the GC are the main inflow channels of the AS, while the TDC is the main outflow channel of the AS. The transport variability is most pronounced at surface levels and between 100 m and 200 m depth, likely affected by monsoons and circulation. A 182.4-d semiannual variability is consistently seen in all three channels, which is also evident in their second principal components. Based on sea level anomalies and EOF analysis results, this is primarily due to equatorial winds during the monsoon transition period, causing eastward movement of Kelvin waves along the AS coast, thereby affecting the spatiotemporal characteristics of the flow in the AS. The first EOF of the PC flow field section shows a split at 100 m deep, likely due to topography. The first EOF of the TDC flow field section is steady but has potent seasonal oscillations in its time series. Meanwhile, the first EOF of the GC flow field section indicates a stable surface inflow, probably influenced by the equatorial Indian Ocean’s eastward current.
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Figure 1. Altimeter of Bay of Bengal (a) and three major passages (b, c, and d). The black solid line represents the 1 800-m isobath, the black dashed lines indicate the three sections used for transport analysis, two red stars mark the mooring sites, the red solid arrows and the blue dashed arrows represent the sea surface circulation patterns during the southwest (SW) monsoon and the northeast (NE) monsoon, respectively.
Figure 2. The climatological monthly variability of temperature and salinity profiles averaged over the RAMA mooring (15°N, 90°E) during 2010–2019, based on RAMA (a and c) and ROMS (b and d). RAMA, Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction; ROMS, Regional Ocean Modeling System model.
Figure 3. The climatological monthly variability of zonal (u) and meridional (v) velocities profiles averaged over the RAMA mooring (0°, 90°E) during 2010–2019, based on RAMA (a and c) and ROMS (b and d). RAMA, Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction; ROMS, Regional Ocean Modeling System model.
Figure 4. The climatological seasonal surface flow (color shaded for current speed (cm/s), arrows for current direction) in winter (a, e), spring (b, f), summer (c, g), and autumn (d, h) in the Andaman Sea during 2010–2019, extracted from the OSCAR (a–d) data and ROMS (e–h), respectively. OSCAR, Ocean Surface Current Analysis-Real Time product; ROMS, Regional Ocean Modeling System model.
Figure 5. Drifter trajectories within the ROMS flow field. The red and blue dots represent two drifter’s trajectory points respectively, while the direction and size of the arrows indicate the drifter’s flow direction and velocity. ROMS, Regional Ocean Modeling System model.
Figure 6. Vertical sections of temperature (a, b, and c), salinity (d, e, and f), and density (g, h, and i) for the Preparis Channel , the Ten Degree Channel, and the Great Channel, respectively.
Figure 7. Vertical sections and transport profiles for the Preparis Channel (a and d), the Ten Degree Channel (b and e) and the Great Channel (c and f). The positive value denotes the inflow from the Bay of Bengal to the Andaman Sea, while the negative value is opposite.
Figure 8. The Empirical Orthogonal Function (EOF) modes for the Preparis Channel (a and d), the Ten Degree Channel (b and e), and the Great Channel (c and f).
Figure 9. The first principal component (PC1), the second principal component (PC2) time series and their power spectrum corresponding to the Preparis Channel (a, d, and g), the Ten Degree Channel (b, e, and h), and the Great Channel (c, f, and i). The red and blue lines indicates the 95% confidence interval.
Figure 10. Schematic of the transports through various channels in the Andaman Sea. The red straight-edged rectangles represent the exchange of seawater through the channels, the red rounded rectangles represent freshwater runoff input, and the black dashed lines represent the cross sections used for calculations. Negative values represent westward transport, and positive values represent eastward transport, with units in 106 m3/s.
Figure 11. Time series and trends of transport in the Preparis Channel (a), the Ten Degree Channel (b), and the Great Channel (c) based on Regional Ocean Modeling Systems model output. The dashed lines represent the average value of the channel transport, the dotted lines represent the linear trend of the channel transport, the thin gray lines and thick black lines indicate the 15-d low-passed filtered time series and 120-d low-passed filtered time series. Positive (negative) transport means that water flows into (out of) the Andaman Sea. The gray-blue shades indicate the inflow into the Andaman Sea, while the orange shades signify the outflow. The upper right of the graph is the average net transport, and the lower right is the regression coefficient of the transport.
Figure 12. Power spectra of full depth water transport in three channels based on unfiltered Regional Ocean Modeling Systems model data. The dashed green, red, and blue lines indicate the 95% confidence interval. PC, Preparis Channel; TDC, Ten Degree Channel; GC, Great Channel.
Figure 13. Mean transport of the Preparis Channel (a), the Ten Degree Channel (b), and the Great Channel (c) during southwest monsoon and northeast monsoon periods during 2010 to 2019, based on ROMS output.
Figure 14. Selected kelvin waves path along the 100-m depth contour of the Bay of Bengal (a), time-longitude plots of sea level anomaly (SLA) from Sea Level Thematic Assembly Centre (b), and Regional Ocean Modeling Systems model output (c). EQ, equator at 90°E; PC, Preparis Channel; GC, Great Channel.
Figure 15. The time series of sea level anomaly (SLA) for the Preparis Channel (PC), Great Channel (GC), and equator at 90°E (EQ) (a), and Pearson correlation of SLA between the EQ, GC, and PC (b, c, and d). The dots in b−d represent the number of days behind when the correlation is the strongest.
Figure 16. Power spectral analysis of the time series of sea level anomaly (SLA). The dashed green, red, and blue lines show the 95% significance interval.
Table 1. The average transport and standard deviation for the three main channels (the Preparis Channel (PC), the Ten Degree Channel (TDC), and the Great Channel (GC)) during winter, spring, summer, autumn, southwest monsoon (SW), northeast monsoon (NE), and annual
Channel Average transport and standard deviation/(106 m3·s–1) Winter Spring Summer Autumn SW NE Annual PC 0.83 ± 0.86 1.49 ± 1.00 1.22 ± 1.11 1.01 ± 1.02 0.78 ± 0.84 0.44 ± 1.08 1.14 ± 0.43 TDC –8.14 ± 2.90 –9.18 ± 3.00 –9.02 ± 4.13 –7.37 ± 4.96 –10.80 ± 1.40 –8.46 ± 2.45 –8.31 ± 1.26 GC 3.24 ± 3.76 3.78 ± 3.54 3.64 ± 3.26 3.52 ± 1.93 1.34 ± 0.34 0.84 ± 0.46 3.63 ± 1.24 
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