Equatorial wave circulation associated with subseasonal convective variability over the subtropical western North Pacific in boreal summer

Abstract. Atmospheric convection over the subtropical western North Pacific (SWNP) during boreal summer varies with a lifespan around 10 days, with significant effects on both local and remote circulation. Among the less understood effects is the coupling between SWNP convection variability and variability in equatorial wave circulation. This paper quantifies equatorial wave perturbations and their evolution throughout the SWNP convection lifespan, using wave space regression between outgoing longwave radiation over the SWNP region and spectral expansion coefficients of global tropospheric circulation from ERA5 reanalyses. The regression distinguishes between convection-coupled Rossby and Kelvin waves, and mixed Rossby-gravity (MRG) and inertia-gravity (IG) waves. The former two correspond to the Gill solution of tropical wave response to asymmetric heating. The results show that MRG and IG waves exhibit amplitudes comparable to those of the Gill response component in the upper troposphere. In particular, MRG and IG waves dominate the cross-equatorial northerly flow over the Maritime Continent, with MRG waves being more important. These findings suggest caution in applying the Gill solution to interpret circulation responses to asymmetric heating sources in model simulations.

As SWNP convection intensifies, the MRG wave northerly winds across the equator strengthen, while IG waves represent enhanced upper-tropospheric outflow over the SWNP region. By contrast, the combined effect of Kelvin and Rossby waves reinforces circulation on the equatorward flank of the anticyclone over the SWNP region, with Rossby wave easterlies being about three times stronger than those associated with Kelvin waves. The Rossby wave signal resembles the n = 1 Rossby wave, with its Southern Hemisphere (SH) subtropical anticyclonic gyre forming over the southern Indian ocean during the decay phase of the SWNP convection. This subtropical Rossby wave gyre, combined with the IG meridional flow in the SH, acts as a bridge between the SWNP convection and extratropical circulation during austral winter.