Magnitude and Phase of Diurnal SST Variations in the ACCESS‐S1 Model During the Suppressed Phase of the MJOs

Abstract

Diurnal variations of sea surface temperature (DV SST) can affect the heat and moisture fluxes at the air-sea interface during the suppressed phase of the MJO, thereby conditioning a favorable atmosphere environment for the subsequent MJO active phase. In order to better understand the causes and impacts of DV SST, and the modeling requirements for faithfully simulating these processes, we explore the DV SST and their influences on the air-sea fluxes during the suppressed phases of three MJO events using a global coupled model, ACCESS-S1, which has a 1-m vertical resolution near the ocean surface and coupling frequency of 1 hr. Compared with the MTSAT-1R satellite SST measurements, ACCESS-S1 is able to simulate the magnitude of the DV SST reliably but has a phase delay in simulated peak SST of~3 hr. Imposing an instant vertical mixing in the upper 10 m in ACCESS-S1 would suppress the DV SST by >1°K and decrease the peak (>30 W m −2 ) and daily averaged latent heat flux. On the other hand, changing the atmosphere-ocean coupling frequency within several hours mostly affects the DV SST phase instead of magnitude. Extreme DV SST (>1°K) occurs only when ocean mixing layer depth = 1 m. It implies that vertical resolution of ≤1 m near the sea surface is important for the coupled models to simulate both the magnitude and phase of DV SST accurately and thereby the air-sea heat fluxes in future model forecast of the MJO. Key Points:• Using the vertical resolution of 1 m near the ocean surface affects DV SST more than using high coupling frequency, e.g., 1 hr • Global coupled model ACCESS-S1can predict the DV SST magnitude reliably but with the phase shift of 3 hr • Effects of DV SST magnitude on the air-sea heat fluxes are more significant than the DV SST phase Supporting Information:• Supporting Information S1

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

“…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.…”

Assessment of the sea surface temperature diurnal cycle in CNRM-CM6-1 based on its 1D coupled configuration

“…In 3D climate models, the individual component time-steps are larger than in this single AOSCM configuration and do not allow us to couple at such high frequency. Hsu et al (2019) show that with a 1-h coupling, they still have deficiencies in the phasing of the SST maximum but could not go far beyond in their AOGCM. Here the AOSCM configuration allows us to tackle this question with more details.…”

“…We may wonder if the simulated SST would better match the observed skin SST if the model resolution was increased near the surface. Besides, the role of vertical discretization has been raised in many studies (Bernie et al, 2005;Hsu et al, 2019;Ge et al, 2017) but these studies generally test resolution between 1 m and 10 m. Hsu et al (2019) assess the diurnal SST representation in the ACCESS-S1 model which also uses NEMO at a 1-m vertical resolution. They suggest that flaws in representing the diurnal warming may be due to insufficient vertical resolution or to deficiencies in vertical mixing in the NEMO model.…”

“…We thus extend the analysis already made in Bernie et al (2005) to higher vertical resolution. The way the vertical coordinate is defined in CNRM-CM6-1 (ln zco case in Madec et al, 2017) makes it complicated to change the ocean vertical resolution (as also raised in Hsu et al, 2019). To ease the change in vertical grid scale, we have moved to a uniform grid representation and limited the depth to 135 m. To check the effect of changing the vertical grid formulation and reducing the ocean depth, we first perform an experiment in which the vertical resolution is set uniformly to 1 m (Ocean v1m) as in the first layer of the reference experiment.…”

Variability of Diurnal Sea Surface Temperature during Short Term and High SST Event in the Western Equatorial Pacific as Revealed by Satellite Data

“…Gas exchange, such as carbon dioxide flux at the sea surface [12], is also affected by diurnal SST cycle, and it has also been suggested that including the diurnal cycle in the calculations of climate model could improve representations of climate variability [7]. Moreover, the diurnal air-sea interaction may play an essential role in the longer time scale of climate variabilities such as Madden-Julian oscillation (MJO), and El Niño and Southern oscillation (ENSO) [13][14][15][16][17]. For the shorter scale, δSST variation determines the formation of the short period and high SST phenomena, called hot event (HE) [18][19][20].…”