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In this study, the Indian Ocean Subtropical Underwater (IOSTUW) was investigated as a subsurface salinity maximum using Argo floats (2000–20) for the first time. It has mean salinity, potential temperature, and potential density values of 35.54 ± 0.29 psu, 17.91° ± 1.66°C, and 25.56 ± 0.35 kg m−3, respectively, and mainly extends between 10° and 30°S along the isopycnal surface in the subtropical south Indian Ocean. The annual subduction rate of the IOSTUW during the period of 2004–19 was investigated based on a gridded Argo dataset. The results revealed a mean value of 4.39 Sv (1 Sv ≡ 106 m3 s−1) with an interannual variability that is closely related to the southern annular mode (SAM). The variation in the annual subduction rate of the IOSTUW is dominated by the lateral induction term, which largely depends on the winter mixed layer depth (MLD) in the sea surface salinity (SSS) maximum region. The anomalies of winter MLD are primarily determined by SAM-related air–sea heat flux and zonal wind anomalies through modulation of the buoyancy. As a result, the annual subduction rate of the IOSTUW generally increased when the SAM index showed negative anomalies and decreased when the SAM index showed positive anomalies. Exceptional cases occurred when the wind anomaly within the SSS maximum region was weak or was dominated by its meridional component. Significance Statement The Indian Ocean Subtropical Underwater (IOSTUW) is characterized as a subsurface salinity maximum in the subtropical south Indian Ocean, and it has potential contribution to regional sea level change and interoceanic salinity exchange. The purpose of this work is to give a more precise description of the IOSTUW and to better understand the formation process of this water mass. Our results shine new light on hydrological characteristics of the IOSTUW based on an observational dataset collected during the past two decades. In addition, the formation of the IOSTUW was found to be closely related to the leading mode of variability in the atmospheric circulation of the Southern Hemisphere though air–sea heat flux and surface wind anomaly.

This study focuses on different evolutions of the low-level atmospheric circulations between eastern Pacific (EP) El Niño and central Pacific-II (CP-II) El Niño. The western North Pacific anomalous anticyclone (WNPAC) originates from the northern South China Sea for EP El Niño, and moves to the western North Pacific (WNP) afterward. Compared with EP El Niño, the origin of the WNPAC is farther west during CP-II El Niño, with the center over the Indochina Peninsula. Moreover, the WNPAC shows a weaker eastward shift. Such discrepancies are attributed to different evolutions of the cyclonic response over the WNP, which can suppress the convection in the western flank of the anomalous cyclone. The eastward retreat of the anomalous cyclone is significant for EP El Niño, but less evident for CP-II El Niño. These discrepancies are related to zonal evolutions of the increased precipitation over the equatorial Pacific. Following the southward migration of the intertropical convergence zone (ITCZ), the deep-convection region extends eastward along the equator, reinforcing the atmospheric response to the eastern Pacific warming in EP El Niño. For CP-II El Niño, the atmospheric response is insignificant over the eastern Pacific without warming. Moreover, the meridional migration of the ITCZ can modulate zonal variations of the easterly trade wind and specific humidity as well. Due to the combined effects of the climatological background and atmospheric anomalies, the specific humidity–induced and wind-induced moist enthalpy advection contribute to different shifts of the precipitation center.

Numerous oceanic mesoscale eddies occur in the South China Sea (SCS). The present study employs eight automatic eddy detection algorithms to identify these mesoscale eddies and compares the results. Eddy probabilities and areas detected by various algorithms differ substantially. Most regions of the SCS with a high discrepancy of eddy probabilities are those with few mesoscale eddies, except for the area west of the Luzon Strait, the area west of Luzon Island between 12° and 17°N, and the southernmost end of the SCS basin. They are primarily caused by strong interference, noncircular eddy shapes, and gentle sea level anomaly (SLA) gradients, respectively. The SLA, winding angle, and hybrid methods can easily detect the mesoscale eddies with wavelike features. The Okubo–Weiss (OW) and the spatially smoothed OW methods better identify grouping phenomena of mesoscale eddies in the SCS. Suggestions are presented on choosing suitable algorithms for studying mesoscale eddies in the SCS. No single algorithm is perfect for all research purposes. For different studies, the most suitable algorithm is different.