Anomalous distribution of distinctive water masses over the Carlsberg Ridge in May 2012

Hailun He,Yejian Wang,Yanzhou Wei,Pengfei Lin,Zhongyan Qiu,Xiqiu Han,Yuan Wang

doi:10.5194/os-16-895-2020

Abstract

Abstract. In May 2012, we conducted a hydrographic survey over the Carlsberg Ridge in the northwest Indian Ocean. In this paper, we use these station data, in combination with some free-floating Argo profiles, to obtain the sectional temperature and salinity fields, and subsequently, the hydrographic characteristics are comprehensively analyzed. Through the basic T–S diagram, three salty water masses, Arabian Sea High-Salinity Water, Persian Gulf Water, and Red Sea Water, are identified. The sectional data show a clear ventilation structure associated with Arabian Sea High-Salinity Water. The 35.8 psu salty water sinks at 6.9∘ N and extends southward to 4.4∘ N at depths around the thermocline, where the thermocline depth is in the range of 100 to 150 m. This salty thermocline extends much further south than climatology indicates. Furthermore, the temperature and salinity data are used to compute the absolute geostrophic current over the specific section, and the results show mesoscale eddy vertical structure different from some widely used oceanic reanalysis data. We also find a west-propagating planetary wave at 6∘ N, and the related features are described in terms of phase speed and horizontal and vertical structures.

Highlights

The northwest Indian Ocean (NWIO) is unique compared with the other two basin-scale oceans (Pacific and Atlantic oceans) because the dominant characteristics are monsoon driven (Schott and McCreary, 2001; Schott et al, 2009)
The wind curl in the NWIO is negative, which is consistent with the annual mean and forces downwelling
This paper reports a onetime hydrographic survey on the Carlsberg Ridge (CR) in the NWIO, where the latitudes cover the equatorial (2.3– 5◦ N) and tropical (5–9.6◦ N) bands

Summary

Introduction

The northwest Indian Ocean (NWIO) is unique compared with the other two basin-scale oceans (Pacific and Atlantic oceans) because the dominant characteristics are monsoon driven (Schott and McCreary, 2001; Schott et al, 2009). The monsoon is strong enough to change the pattern of basin-scale circulation. The monsoon builds up a dominant meridional current in the NWIO, changing the form of the customary zonal current (as in the Pacific and Atlantic oceans) into a meridional current. The NWIO is famous for its role in the so-called Indian Ocean Dipole (Saji et al, 1999; Webster et al, 1999; Han et al, 2014; Chen et al, 2015), which represents the zonal gradient of sea surface temperature in the Indian Ocean. Some studies emphasized the distinct mesoand submesoscale air–sea interactions in the NWIO (Vecchi et al, 2004)

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Discussion

Conclusion