Diagnosing Ocean Feedbacks to the BSISO: SST‐Modulated Surface Fluxes and the Moist Static Energy Budget

Abstract

The oceanic feedback to the atmospheric boreal summer intraseasonal oscillation (BSISO) is examined by diagnosing the sea surface temperature (SST) modification of surface fluxes and moist static energy on intraseasonal scales. SST variability affects intraseasonal surface latent heat (LH) and sensible heat (SH) fluxes, through its influence on air‐sea moisture and temperature gradients (∆q and ∆T, respectively). According to bulk formula decomposition, LH is mainly determined by wind‐driven flux perturbations, while SH is more sensitive to thermodynamic flux perturbations. SST fluctuations tend to increase the thermodynamic flux perturbations over active BSISO regions, but this is largely offset by the wind‐driven flux perturbations. Enhanced surface fluxes induced by intraseasonal SST anomalies are located ahead (north) of the convective center over both the Indian Ocean and the western Pacific, favoring BSISO northward propagation. Analysis of budgets of column‐integrated moist static energy (⟨m⟩) and its time rate of change (∂⟨m⟩/∂t) shows that SST‐modulated surface fluxes can influence the development and propagation of the BSISO, respectively. LH and SH variability induced by intraseasonal SSTs maintain 1–2% of ⟨m⟩ day−1 over the equatorial western Indian Ocean, Arabian Sea, and Bay of Bengal but damp about 1% of ⟨m⟩ day−1 over the western North Pacific. The contribution of intraseasonal SST variability to ∂⟨m⟩/∂t can reach 12–20% over active BSISO regions. These results suggest that SST variability is conducive, but perhaps not essential, for the propagation of convection during the BSISO life cycle.

Boreal summer intraseasonal oscillation in a superparameterized GCM: effects of air-sea coupling and ocean mean state

“…DeMott et al 2016and Gao et al (2019) provide a useful diagnostic method to quantitatively assess the contribution of intraseasonal SST variability to the MSE budget of ISO in boreal winter and boreal summer, respectively. We employ the same approach to understand oceanic feedbacks to the simulated BSISO, and compare these feedbacks to those in reanalysis data.…”

“…These SST-modulated fluxes are then projected onto column-integrated MSE and its tendency (〈m〉 and ∂〈m〉/ ∂t) to reveal the oceanic feedback to the maintenance and propagation of BSISO convection, respectively. Further details can be found in DeMott et al 2016and Gao et al (2019).…”

“…The daily variables used include: precipitation, outgoing longwave radiation (OLR), 850-hPa wind, surface variables related to latent heat and sensible heat (LH and SH) fluxes, and 3-D variables associated with moist static energy (MSE; Maloney, 2009). All the variables have the horizontal resolution of 1.5°×1.5°; we analyse the period of 1986-2016 for compatibility with Gao et al (2019).…”

“…As revealed by Gao et al (2019), intraseasonal SST can affect the BSISO by modulating surface LH and SH fluxes. The temporal and spatial distributions of SST-modulated fluxes are highly coherent with those of SST anomalies themselves (Gao et al, 2019;their Figs. 14 and 15).…”

Influences of ENSO on boreal summer intraseasonal oscillation over the western Pacific in decaying summer

“…Besides the internal atmospheric processes, the effects of air-sea interaction in promoting the NP of BSISO have also been demonstrated by many studies (Kemball-Cook and Wang 2001;DeMott et al 2013DeMott et al , 2015Wang et al 2018b;Gao et al 2019). BSISO convection influences the sea surface temperature (SST) through regulating solar radiation, heat fluxes, and mixed-layer entrainment, but the feedback effects of SST anomalies (heating, moistening, and destabilizing the low-level atmosphere) also have significant impacts on the BSISO activity (Kemball-Cook and Wang 2001;Fu et al 2003;Fu and Wang 2004;Wang et al 2018b).…”