Abstract
AbstractThe 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.
Highlights
The intraseasonal oscillation (ISO) is one of the dominant modes of variability in the tropics
Influenced by the Asian summer monsoon, the low-level circulation is dominated by strong southwesterlies over the Indian Ocean and southeasterlies over the western Pacific
This study extends the diagnostic framework of DeMott et al (2016) to the boreal summer intraseasonal oscillation (BSISO); it is useful to compare our results for the BSISO with those from DeMott et al (2016) for the Madden-Julian Oscillation (MJO)
Summary
The intraseasonal oscillation (ISO) is one of the dominant modes of variability in the tropics. The ISO during boreal summer (BSISO) has a more complex structure than the MJO, due to its intense interaction with the Asian summer monsoon (Krishnamurti & Subrahmanyam, 1982; Yasunari, 1979). It exhibits multiple propagation pathways: east and north in the Indian monsoon region (Gadgil & Srinivasan, 1990; Hartmann & Michelsen, 1989; Murakami & Nakazawa, 1985) and northwest over the western North Pacific (Chen & Murakami, 1988; Lau & Chan, 1986; Murakami, 1980). As the BSISO propagates north from the equator into monsoon areas, its activity substantially affects monsoon rainfall (Webster et al, 1998; Wu & Wang, 2001; Zhou & Chan, 2005), tropical cyclones (Hartmann & Maloney, 2001; Straub & Kiladis, 2003), and the occurrence of extreme events (Hsu et al, 2017; Li et al, 2015; Ren et al, 2013; Sun et al, 2016)
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