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 on timescales of around 10 d, with significant effects on both local and remote circulation. One of less understood effects of this variability is its coupling with equatorial wave dynamics. This paper quantifies equatorial wave perturbations and their evolution throughout the lifecycle of SWNP convection 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 linear Rossby and Kelvin waves, and mixed Rossby-gravity (MRG) and inertia-gravity (IG) waves. The former two correspond to the Gill-Type tropical circulation 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 playing the larger role. These findings suggest caution when applying the Gill solution to interpret circulation responses to asymmetric heating 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 nCombining double low line1 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 gyre, together with the SH IG meridional flow provides a dynamical bridge linking SWNP convection and extratropical circulation during austral winter.