How does the air-sea coupling frequency affect convection during the MJO passage?

Abstract

The importance of air-sea coupling in the simulation and prediction of the Madden-Julian Oscillation (MJO) has been well established. However, it remains unclear how air-sea coupling modulates the convection and related oceanic features on the subdaily scale. Based on a regional cloud-permitting coupled model, we evaluated the impact of the air-sea coupling on the convection during the convectively active phase of the MJO by varying the coupling frequency. The model successfully reproduced the atmospheric and oceanic variations observed by satellite and in situ measurements but with some quantitative biases. According to the sensitivity experiments, we found that stronger convection was mainly caused by the higher sea surface temperatures (SSTs) generated in high-frequency coupled experiments, especially when the coupling frequency was 1 hr or shorter. A lower coupling frequency would generate the phase lags in the diurnal cycle of SST and related turbulent heat fluxes. Our analyses further demonstrated that the phase-lagged diurnal cycle of SST suppressed deep convection through a decrease in daytime moistening in the lower troposphere. Meanwhile, in the upper ocean, the high-frequency air-sea coupling helped maintain the shallower mixed and isothermal layers by diurnal heating and cooling at the sea surface, which led to a higher mean SST. In contrast, the low-frequency coupled experiments underestimated the SST and therefore convective activities. Overall, our results demonstrated that high-frequency air-sea coupling (1 hr or shorter) could improve the reproducibility of the intensity and temporal variation in both diurnal convection and upper ocean processes. Plain Language SummaryThe Madden-Julian Oscillation (MJO) is one of the most important sources of atmospheric variability in tropical regions; however, even the modern numerical models could not well reproduce the MJO. We believe that the underestimation of air-sea coupling may cause some parts of such biases in simulating/predicting the MJO. Therefore, our study is aimed to uncover the impact of the air-sea coupling on convection and the related oceanic feature during the MJO. By varying the air-sea coupling frequency, our results showed that the 1-hr or higher-frequency coupled experiments had better performance due to the well-reproduced sea surface temperature, while suppressed convection was found in low-frequency coupled experiments. In general, our study suggested that the high-frequency air-sea coupling could improve the reproducibility of both convection and upper ocean features during the MJO.

Fifty Years of Research on the Madden‐Julian Oscillation: Recent Progress, Challenges, and Perspectives

“…Under strongly suppressed conditions, a thin ocean mixed layer combined with intense diurnal surface heating can induce diurnal SST perturbations of 1–3 K. Nighttime surface cooling drives convective overturning of the ocean mixed layer, mixing some of the daytime‐accumulated heat below the mixed layer; the remaining heat yields a warmer upper ocean at the next day's sunrise than the previous day's sunrise (Anderson et al, 1996). Thus, the SST diurnal cycle rectifies onto the intraseasonal scale (e.g., Bernie et al, 2005; Zhao & Nasuno, 2020).…”

“…L. Zhou and Murtugudde (2020) found that SST anomalies up to +0.6 K northwest of Australia during MJO suppressed conditions generate anomalous cyclonic circulations and moisture advection that promote the MJO convective “detour” south of the MC. In regional coupled simulations, Zhao and Nasuno (2020) found that the rectification of diurnal SST variability associated with ocean mixed‐layer shoaling onto intraseasonal SST perturbations was more important for MJO propagation than the diurnal SST itself.…”

“…Furthermore, this framework can help reveal the sensitivity of the MJO, and convection in general, to diurnal SST fluctuations (e.g., Matthews et al, 2014) that are captured only with fine oceanic vertical resolution (~1 m in the upper ocean) and frequent (approximately hourly) ocean‐atmosphere coupling (e.g., J.‐Y. Hsu et al, 2019; Y. Li et al, 2013; Zhao & Nasuno, 2020). A collection of simulations, including fully coupled, atmosphere‐only, and 1‐D ocean coupled, can help identify the time scales of coupled feedbacks that most strongly enable or inhibit MJO fidelity and focus efforts to improve oceanic or atmospheric processes most relevant to those scales during model development cycles.…”

Validation of Satellite-Based Sea Surface Temperature Products Against in Situ Observations Off the Western Coast of Sumatra

“…Years of the Maritime Continent (YMC 1 ) is a multiyear (from 2015 to present) international program with participants from more than 15 countries. Its goal is to improve the understanding of the local oceanic and atmospheric multiscale variability of the Indo-Paci c maritime continent (MC), and the initial results have been published [2][3][4][5][6][7][8][9] .…”