Sources and characteristics of seasonal-interannual variability of subsurface undercurrents in the Indonesian Throughflow outflow region

doi:10.3969/j.issn.1001-909X.2024.04.001

Abstract

Abstract:

Using eddy-resolving numerical simulation data and historical hydrological observation data, this study investigates the sources, seasonal and interannual variability of two subsurface undercurrents under the Indonesian Throughflow—the Ombai Undercurrent located in the Ombai Strait and the Timor Undercurrent located in the Timor Channel. The results indicate that these two undercurrents exist at depths of approximately 200-800 m, which are a quasi-permanent undercurrent system. The formation of the Ombai Undercurrent is mainly related to the eastward extension of the South Java Undercurrent, while the water source of the Timor Undercurrent is more complex, mostly a mixture of the South Java Undercurrent and the Leeuwin Undercurrent. Both subsurface undercurrents exhibit significant seasonal and interannual variations, with a significant semiannual period at the seasonal scale, typically peaking during the Indian Ocean monsoon transition period (April, May, and October). Combining historical wind, satellite altimeters, and temperature and salinity observation data, it is found that the meridional pressure gradient in the subsurface layer related to local wind and their upwelling is the dominant factor leading to their seasonal changes. At the interannual scale, there is a period of 2-4 years for subsurface undercurrents, which is significantly correlated with the Indian Ocean dipole.

Key words: Indonesian Seas, Southeast Indian Ocean, subsurface undercurrent, Ombai Undercurrent, Timor Undercurrent, seasonal variation, interannual variation, Indonesian Throughflow

CLC Number:

P731.2

SHI Wanli, HU Shijian. Sources and characteristics of seasonal-interannual variability of subsurface undercurrents in the Indonesian Throughflow outflow region[J]. Journal of Marine Sciences, 2024, 42(4): 1-11.

Figures/Tables 10

Fig.1 The average velocity above 750 m was observed by INSTANT and simulated by OFES in the Ombai Strait (a) and the Timor Channel (b)and ITF transport anomaly observed by INSTANT and simulated by OFES (c) (In fig.a and b, positive flow velocity values indicate eastward flow, while negative values indicate westward flow, the same applies to the following text. Sizes of blue dots denote the absolute values of the difference between observation and OFES, and the red line shows the least square fitting.)

Fig.2 Topography (a) and currents, temperature and salinity at different depths (b-d)in the ITF outflow region (In fig.a,red arrows represent the direction of surface currents. In fig.b-d, arrows represent currents, data from OFES; contour line represents temperature, data from WOA18; color filling represents salinity, data from WOA18.)

Fig.3 Zonal velocities of OFES in the Ombai Strait(a-d) and the Timor Channel(e-h)across the 124°E section (The black contour lines represent the velocity, where solid lines represent positive values and dashed lines represent negative values; the blue contour line represents the 26.7σθ potential density line.)

Fig.4 Climatological ocean currents on the 26.7σθ potential density surface based on OFES in various months (Arrow represents ocean current, color filling represents zonal current velocity.)

Fig.5 Topography and four key regions in the ITF outflow region (a); T-S diagram of A、B、C and D regions (b); the lead-lag relationship of South Java Undercurrent (SJUC) and Ombai Undercurrent (c); the lead-lag relationship of Northwest Shelf (NWS) and Timor Undercurrent (d) (Temperature, salinity, and currents data are from the OFES.)

Fig.6 Anomalies of meridional pressure gradient and undercurrent transports in the subsurface layer of the Ombai Strait (a) and the Timor Channel (b),the composite diagrams of absolute dynamic topography of AVISO and wind anomaly of ERA5 from months with positive (c) and negative (d) transport anomalies (The pressure gradient and undercurrent transports were calculated from WOA18 temperature, salinity and OFES velocity, respectively.)

Fig.7 Wavelet spectrum of subsurface undercurrent transport

Fig.8 The relationship between Ni?o Index, DMI Index and the subsurface undercurrent transport anomaly

Fig.9 Correlation coefficient between subsurface undercurrent transport and DMI Index in each month

Tab.1 Sources of undercurrent in different months

潜流名称	月份
潜流名称	1月	4月	7月	10月
翁拜潜流		SJUC	SJUC	SJUC
帝汶潜流		萨武海和卢温潜流	SJUC和卢温潜流	SJUC

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