Coupled Impacts of the Diurnal Cycle of Sea Surface Temperature on the Madden–Julian Oscillation

Abstract

This study quantifies, from a systematic set of regional ocean-atmosphere coupled model simulations employing various coupling intervals, the effect of subdaily sea surface temperature (SST) variability on the onset and intensity of Madden-Julian oscillation (MJO) convection in the Indian Ocean. The primary effect of diurnal SST variation (dSST) is to raise time-mean SST and latent heat flux (LH) prior to deep convection. Diurnal SST variation also strengthens the diurnal moistening of the troposphere by collocating the diurnal peak in LH with those of SST. Both effects enhance the convection such that the total precipitation amount scales quasi-linearly with preconvection dSST and time-mean SST. A column-integrated moist static energy (MSE) budget analysis confirms the critical role of diurnal SST variability in the buildup of column MSE and the strength of MJO convection via stronger time-mean LH and diurnal moistening. Two complementary atmosphere-only simulations further elucidate the role of SST conditions in the predictive skill of MJO. The atmospheric model forced with the persistent initial SST, lacking enhanced preconvection warming and moistening, produces a weaker and delayed convection than the diurnally coupled run. The atmospheric model with prescribed daily-mean SST from the coupled run, while eliminating the delayed peak, continues to exhibit weaker convection due to the lack of strong moistening on a diurnal basis. The fact that time-evolving SST with a diurnal cycle strongly influences the onset and intensity of MJO convection is consistent with previous studies that identified an improved representation of diurnal SST as a potential source of MJO predictability.

Physics of diurnal warm layers : turbulence, internal waves, and lateral mixing

“…Sverdrup et al, 1942;Saunders, 1967;Price et al, 1986), but new research continues to show the importance of resolving the diurnal cycle for processes on large range of time-scales; for example, the Madden-Julian Oscillation time-scale (Bernie et al, 2005;Woolnough et al, 2007;Bernie et al, 2008;Seo et al, 2014). Diurnal cycling in models has recently been argued to be important to air-sea coupling (Wang, 2001;Danabasoglu et al, 2006), and daytime stratification of the upper ocean directly affects the nighttime convection of the ocean (Brainerd and Gregg, 1993a,b).…”

“…The inclusion of a diurnally-varying sea surface temperature (compared to a daily-average or temperature below the diurnal warm layer) in air-sea turbulent flux calculations has been shown to dramatically change the instantaneous fluxes by up to 100%, as well as annually-averaged flux by up to 10% in the tropics (Clayson and Bogdanoff, 2013;Weihs and Bourassa, 2014). Seo et al (2014) argued that including the diurnal cycle is important to properly resolve the onset and intensity of the Madden-Julian Oscillation. In the upper ocean, the descent of the DWL at night has been shown to be a mechanism for the onset of turbulence and mixing deeper in the ocean (Smyth et al, 2013).…”

Coupled ocean–atmosphere modeling and predictions

“…Precipitation anomalies were found to be forced by surface wind convergence patterns that are driven by mesoscale SST gradients, indicating the importance of the ocean forcing the atmosphere at this scale. Such deep atmospheric response is expected to modify the downstream development of weather systems in the North Pacific, thus potentially affecting the remote circulation (e.g., Seo et al 2014;Ma et al 2016). , no eddy-current coupling, and no total-current coupling in the wind stress calculations.…”

“…This model has been employed in several studies of ocean-atmosphere interaction in various regions with intense thermal ocean fronts such as in the Kuroshio Extension, where mesoscale eddies were shown to affect the winter precipitation (Putrasahan et al 2013a) and synoptic extreme rainfall events associated with the African Easterly Waves in the Atlantic ocean (Seo et al 2008a). Recently SCOAR2, using the Weather Research and Forecasting (WRF) model (Skamarock et al 2008) instead of RSM, was developed by Seo et al (2014).…”

Air–sea fluxes in a climate model using hourly coupling between the atmospheric and the oceanic components

“…The cold bias in the eastern tropical Pacific is reduced (Misra et al 2008). For the variability on intra-seasonal and inter annual scales, including the diurnal cycle in the SST strongly influences the onset and intensity of MJO convection (Seo et al 2014) and leads to a distinct improvement of the simulated intra-seasonal oscillation signal (Ham et al 2014). The simulated MJO is stronger and more coherent (Bernie et al 2008).…”