Role of Air–Sea Interaction in the 30–60-Day Boreal Summer Intraseasonal Oscillation over the Western North Pacific

Abstract

This study investigates the role of air–sea interaction in the 30–60-day boreal summer intraseasonal oscillation (BSISO) over the western North Pacific with daily outgoing longwave radiation (OLR), CFSR, and OAFlux datasets for 1985–2009. The BSISO events are identified with the first principal component of 30–60-day bandpass filtered OLR anomalies. Composite analysis of these events reveals that during the northward migration of BSISO, the convection can interact with underlying sea surface temperature (SST). A near-quadrature phase relationship exists between the convection and SST anomalies. An active (a suppressed) convection tends to induce a cold (warm) underlying SST anomaly by reducing (increasing) downward solar radiation but a warm SST anomaly in its northern (southern) portion by reducing near-surface wind and upward latent and sensible heat fluxes, resulting in a 10-day delayed maximized warm SST anomaly ahead of the active convection. In turn, this warm SST anomaly tends to increase upward surface sensible and latent heat fluxes via amplifying sea–air temperature and humidity differences. This oceanic feedback acts to heat, moisten, and destabilize the low-level atmosphere, favoring the trigger of shallow convection, which can further develop into deep convection. The maximum warm SST anomaly lies in the southern (northern) portion of the convectively suppressed (enhanced) area, which weakens the anomalous descending motion in the southern portion of convectively suppressed area and preconditions the boundary layer to promote convection development in the northern portion of convectively enhanced area. Such a spatial and temporal phase relationship between the convection and SST anomalies suggest that air–sea interaction can play a delayed negative feedback role in the BSISO cycle and provide an alternative mechanism responsible for its northward propagation.

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

“…The southerlies become much stronger over the Bay of Bengal (Figure 1a), which transport the enhanced hmi near the convective center to the north (Figure 10). À v·∂m 0 =∂y h ialso enhances northward propagation over the South China Sea and western Pacific, as revealed by Wang et al (2018), in which they reported the importance of the anomalous moisture by mean meridional wind (À v·∂q 0 =∂y h i ) to the northward propagation of convection. The anomalous hmi advection by the anomalous zonal wind (Àu 0 · ∂m 0 /∂x) is negative over the South China Sea, largely inhibiting propagation.…”

“…Therefore, our "SST effect" may be slightly underestimated. Wang et al (2018) found that the intraseasonal SST anomalies could induce boundary layer moisture convergence and increase surface turbulent flux anomalies, which tend to heat and moisten the boundary layer, favoring the northward propagation of BSISO. Based on our diagnostics, intraseasonal SST variability contributes up to 20% to the propagation of convection (∂hmi/∂t).…”

“…SST fluctuations also promote MJO eastward propagation, with a contribution of up to 10% of ∂ ⟨ m ⟩/ ∂t across the Warm Pool. Recently, Wang et al () showed that warm SST anomalies to the north of convection tend to increase surface turbulent fluxes by amplifying the air‐sea temperature and moisture gradients over the western North Pacific, which promotes BSISO northward propagation by preconditioning the boundary layer for convection.…”

“…SST anomalies and their gradients may induce anomalous boundary layer convergence (Lindzen & Nigam, 1987) and destabilize the atmosphere ahead of convection on intraseasonal scales (Hsu & Li, 2012). The anomalous SST can also affect the atmospheric ISO convection by modifying surface fluxes (DeMott et al, 2016;Wang et al, 2018). DeMott et al (2016) analyzed the effect of intraseasonal SST variability on the MJO by computing surface flux component terms using daily mean and 61-day running mean SST.…”

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). Fu and Wang (2004) found that the BSISO activity and its relationship with the underlying SST are more realistic in a coupled model and the ISO is approximately 50% stronger in a coupled model than in the atmosphere-only model (Fu et al 2003).…”