Quasi-biweekly oscillations in the Bay of Bengal in observations and model simulations

Monitoring Intraseasonal Oscillations in the Indian Ocean Using Satellite Observations

“…In Figure 15a, the signal largely disappears over land, which can be due to both the cell moving northward out of the 10-20°N range and weakening over India before it reaches its demise in the AS. SSS has a weak northward propagation but remains fairly strong at both the coastal boundary and central BoB suggesting a coastally trapped signal as seen in SLA (Figure 15e), and Ekman pumping in the central BoB, as described in Roman-Stork et al (2019). The northern cell does not have a strong SLA signal outside of the coastal boundaries, suggesting a coastal wave propagation in response to 10-20-day wind forcing and equatorial wave activity at the coastal boundary as the dominant mode of SLA variability for this ISO, which is consistent with the 30-90-day ISO as well.…”

“…There are 10-20-day spectral peaks in atmospheric variability globally (Kikuchi & Wang, 2009); however, the most relevant to the southwest monsoon is most predominant over the BoB and ranges from the western Pacific to the Arabian Sea (AS). The 10-20-day mode in the Indian Ocean consists of a double cell structure of atmospheric convection, with the northern cell centered over 15-20°N and the southern cell positioned around the equator in the BoB (Chatterjee & Goswami, 2004;Chen & Chen, 1993;Krishnamurti & Ardanuy, 1980;Roman-Stork et al, 2019). The northern cell is generated by breaking Rossby waves in the upper atmosphere over the Pacific Ocean, which triggers convection over the South China Sea (Ortega et al, 2017).…”

“…The 30‐90‐day mode commonly describes the Madden‐Julian Oscillation (MJO), with a northward propagating phase referred to as either the monsoon intraseasonal oscillation (MISO) or boreal summer intraseasonal oscillation (BSISO; Ajayamohan et al, ; Girishkumar et al, ; Krishnamurti et al, ; Madden & Julian, ; Wheeler & Hendon, ; Zhang, ). The 10‐20‐day mode, or quasi‐biweekly oscillation, is a westward propagating system over the Bay of Bengal (BoB) and equatorial Indian Ocean (Chatterjee & Goswami, ; Kikuchi & Wang, ; Murakami, ; Roman‐Stork et al, ). The 3‐7‐day mode is due to oscillations in the monsoon trough and is chiefly responsible for the active and break cycles of the southwest monsoon that have spectral peaks between 4 and 5 days (Trott et al, ).…”

“…While the structure and dynamics of the 10‐20‐day mode in the atmosphere are reasonably well understood, the corresponding signature and dynamics in the ocean are less so, with few studies focused on the ocean (Chatterjee et al, ; Miyama et al, ; Roman‐Stork et al, ; Sengupta et al, ; Sengupta et al, ; Subrahmanyam et al, ; Trott et al, ; Trott & Subrahmanyam, ). Ocean currents and meridional surface winds in the eastern equatorial Indian Ocean were found to exhibit strong biweekly variability, with fluctuations in this wind stress exciting a westward wave response that is associated with a pattern of upwelling and downwelling along the coastal boundary (Sengupta et al, ; Sengupta et al, ).…”

“…Ocean currents and meridional surface winds in the eastern equatorial Indian Ocean were found to exhibit strong biweekly variability, with fluctuations in this wind stress exciting a westward wave response that is associated with a pattern of upwelling and downwelling along the coastal boundary (Sengupta et al, ; Sengupta et al, ). Within the BoB, SSS responses have been found to be predominantly forced by river runoff in the northern Bay and by wind‐induced Ekman processes in the central Bay for all ISOs, with the signal in the northern Bay being the strongest (Roman‐Stork et al, ; Subrahmanyam et al, ; Trott et al, ). Mixed Rossby Gravity (MRG) or Yanai waves in the Indian Ocean that also occur on biweekly timescales and have been shown to have the same structure as the 10‐20‐day mode (Chatterjee et al, ; Miyama et al, ; Sengupta et al, ).…”