Intraseasonal Variability of the Equatorial Undercurrent in the Indian Ocean

Abstract

By analyzing in situ observations and conducting a series of ocean general circulation model experiments, this study investigates the physical processes controlling intraseasonal variability (ISV) of the Equatorial Undercurrent (EUC) of the Indian Ocean. ISV of the EUC leads to time-varying water exchanges between the western and eastern equatorial Indian Ocean. For the 2001–14 period, standard deviations of the EUC transport variability are 1.92 and 1.77 Sv (1 Sv ≡ 106 m3 s−1) in the eastern and western basins, respectively. The ISV of the EUC is predominantly caused by the wind forcing effect of atmospheric intraseasonal oscillations (ISOs) but through dramatically different ocean dynamical processes in the eastern and western basins. The stronger ISV in the eastern basin is dominated by the reflected Rossby waves associated with intraseasonal equatorial zonal wind forcing. It takes 20–30 days to set up an intraseasonal EUC anomaly through the Kelvin and Rossby waves associated with the first and second baroclinic modes. In the western basin, the peak intraseasonal EUC anomaly is generated by the zonal pressure gradient force, which is set up by radiating equatorial Kelvin and Rossby waves induced by the equatorial wind stress. Directly forced and reflected Rossby waves from the eastern basin propagate westward, contributing to intraseasonal zonal current near the surface but having weak impact on the peak ISV of the EUC.

MJO-Induced Intraseasonal Mixed Layer Depth Variability in the Equatorial Indian Ocean and Impacts on Subsurface Water Obduction

“…The ocean model used in this study is the Hybrid Coordinate Ocean Model (HYCOM; e.g., Wallcraft et al 2009) version 2.2.18, which has been successfully utilized to investigate MJOrelated oceanic variations (e.g., Li et al 2014Li et al , 2015Li et al , 2017Chen et al 2015Chen et al , 2019. Multisatellite Precipitation Analysis (TMPA) level 3B42 product (Kummerow et al 1998;Huffman et al 2007), and 0.75°×0.75° 2-m air temperature and humidity from ECMWF interim reanalysis (Dee et al 2011).…”

“…The equatorial Indian Ocean (EIO) is home to most strong MJO events (Zhang 2005) and exhibits strong intraseasonal variability (ISV) in SST (e.g., Duvel et al 2004;Han et al 2007;Duncan and Han 2009;Jayakumar et al 2011), ocean salinity (e.g., Matthews et al 2010;Grunseich et al 2011;Li et al 2015), and upper-ocean circulation (e.g., Schiller and Godfrey 2003;Masumoto et al 2005;Chen et al 2015Chen et al , 2019. Intraseasonal SST variability is actively coupled to the MJO's atmospheric variabilities (Hendon and Glick 1997;Wang and Xie 1998;Woolnough et al 2001).…”

Semiannual Variations in 1,000‐dbar Equatorial Indian Ocean Velocity and Isotherm Displacements from Argo Data

“…Some currents, such as the seasonally reversing Wyrtki Jets (Wyrtki, 1973), are found nowhere else. Although observations and models indicate that equatorial Indian Ocean currents vary across a broad range of timescales (Chen et al, 2015;Chen et al, 2019;McPhaden et al, 2015;Nagura & McPhaden, 2008, 2010a, 2010b, 2016Nyadjro & McPhaden, 2014;Sengupta et al, 2007;Visbeck & Schott, 1992;Zhang et al, 2014), it is the monsoonally driven, semiannual variations that are particularly robust in this region, including the strong eastward flowing Wyrtki jets during the spring and autumn monsoon transitions and the seasonally reversing western boundary current off Somalia (e.g., Davis, 2005;Han et al, 1999;Huang, McPhaden, et al, 2018;Luyten & Roemmich, 1982;Reverdin, 1987;Schott et al, 2009;Schott & McCreary, 2001;Visbeck & Schott, 1992;Wyrtki, 1973).…”

A Decade of Eastern Tropical Indian Ocean Observation Network (TIOON)

“…Studies based on the TIOON mooring array suggest that there are important dynamic and energetic relationships between the currents in the equatorial region and those in the near-equatorial region. The contributions of the equatorial remote forcing, local forcing and inter-basin connection are quantitatively assessed by analyzing the TIOON mooring array data together with model simulations (Chen et al, 2015a(Chen et al, , 2015b(Chen et al, , 2016a(Chen et al, , 2017(Chen et al, , 2018(Chen et al, , 2019(Chen et al, , 2020Huang et al 2018aHuang et al , 2018bHuang et al , 2019Huang et al , 2020aHuang et al , 2020b.…”

“…We found significant differences in the generation and maintenance mechanisms of the Equatorial Undercurrent in different regions based on measurements from TIOON moorings Q2, Q4 and Q5. In the western Indian Ocean, the Equatorial Undercurrent is directly forced by the equatorial easterly winds and mainly results from equatorial Kelvin and Rossby waves (Chen et al, 2015a(Chen et al, , 2019). In the central and eastern Indian Ocean, the reflected Rossby waves from the eastern boundary play a key role in the Equatorial Undercurrent (Chen et al, 2015).…”