The Role of Oceanic Processes in the Initiation of Boreal Winter Intraseasonal Oscillations Over the Indian Ocean

Abstract

Observational analyses and ocean general circulation model experiments were performed to understand the influence of oceanic processes on warm sea surface temperature anomalies (SSTAs) prior to convection initiation of boreal winter intraseasonal oscillations (ISOs), including the Madden‐Julian Oscillation (MJO), in the tropical Indian Ocean. We found 39 strong ISOs that passed over the Indian Ocean Warm Pool (WP) region during the November–April season of the 2001–2012 period. Seventeen of them (44%) initiated over the Seychelles‐Chagos Thermocline Ridge (SCTR) before propagating eastward to the WP and Maritime Continent. Including only global‐scale MJOs, 71% (24%) of the remaining MJO events initiated from the SCTR (WP). Four (seven) SCTR (WP) ISO events were preceded by SSTAs that were strongly influenced by wind stress‐driven oceanic processes; all four (2/7) SCTR (WP) events were MJOs. Composite analyses of the four oceanic process influenced SCTR MJO events showed that wind stress‐driven oceanic processes contributed about 30% (between 0.1 and 0.2 °C) of the preconvection warming in the SCTR, which coincides with the Intertropical Convergence Zone with mean SST > 28 °C. Reduced upwelling and entrainment played a more important role than horizontal advection in elevating the +SSTAs. Case studies revealed that the two ocean process‐influenced primary MJO SCTR events were also associated with oceanic equatorial Rossby waves. Furthermore, results of a linear atmospheric mixed layer model indicated that boundary layer processes related to SSTAs contributed approximately half of the total convergence during MJO initiation, relative to the effect of the free troposphere, for the Rossby wave‐influenced events.

Reciprocity in the Indian Ocean: Intraseasonal Oscillation and Ocean Planetary Waves

“…This indicates that while the downwelling ER waves identified in this analysis might assist the rapid strengthening of the ISO, they are not typically responsible for its initiation. This does not rule out the possibility of ISO convective initiation by downwelling ER waves as shown in previous work (Rydbeck & Jensen, 2017; Rydbeck, Jensen, & Flatau, 2019; Rydbeck, Jensen, & Igel, 2019; Webber et al., 2010; Webber, Matthews, et al., 2012; Webber, Stevens, et al., 2012; West et al., 2018, 2020), but suggests that this behavior is atypical as indicated by compositing over many ER wave events. It should be noted that ER waves discussed here are just one factor amongst many that have shown to vary the intensity of the ISO.…”

“…These waves then arrive in the western Indian Ocean ∼90 days later and force a subsequent ISO event by increasing surface latent heat fluxes and converging low‐level moisture (Rydbeck & Jensen, 2017) which humidifies the atmospheric boundary layer and prompts convection initiation. The hypothesis has been supported using observations (Rydbeck & Jensen, 2017; Webber, Matthews, et al., 2012; Webber, Stevens, et al., 2012; West et al., 2018, 2020) and a cloud resolving model (Rydbeck, Jensen, & Igel, 2019). These studies suggest that ER waves are partly responsible for the episodic nature of the ISO and may help to explain its broadband behavior, particularly its low‐frequency tail.…”

“…Over the last decade, important roles for the second category, the lagged response, have been discovered. These include intraseasonal contributions from large‐scale ocean equatorial waves that trail the initial ISO forcing, adding greater complexity to intraseasonal atmosphere‐ocean interactions (Azaneu et al., 2021; Rydbeck & Jensen, 2017; Rydbeck et al., 2017; Rydbeck, Jensen, Smith, et al., 2019; Webber et al., 2010; Webber, Matthews, et al., 2012; Webber, Stevens, et al., 2012; West et al., 2018, 2020). These ocean waves arise from the integrated and somewhat lagged ocean response to forcing by ISO wind stress anomalies.…”

“…These studies suggest that ER waves are partly responsible for the episodic nature of the ISO and may help to explain its broadband behavior, particularly its low‐frequency tail. However, this mechanism of ISO initiation is relatively infrequent, occurring once or twice every couple of years (Rydbeck & Jensen, 2017; Webber et al., 2010; Webber, Matthews, et al., 2012; Webber, Stevens, et al., 2012; West et al., 2018, 2020).…”

Oceanic Rossby wave predictability in ECMWF's subseasonal‐to‐seasonal reforecasts

“…Equatorial Kevin and Rossby waves dominate upper-ocean intraseasonal and interannual variability in the tropics (Hendon et al, 1998;McPhaden, 1999). This implies a dynamical control of sea surface temperature (SST) and upper-ocean heat content due to equatorial Rossby waves that has been linked to an enhancement or suppression of air-sea interactions and atmospheric convection (Battisti, 1988;McCreary, 1983;Rydbeck et al, 2017;Rydbeck et al, 2019;Webber et al, 2010;Webber et al, 2011;West et al, 2020). Webber et al (2010) first hypothesized a positive feedback loop between the intraseasonal oscillation (ISO) (Madden & Julian, 1972), the leading mode of atmospheric intraseasonal variability in the tropics, and OERWS in the IO.…”