On the energetics of convectively coupled Kelvin Waves: contrast between Indian and Pacific Basins

Abstract

Using the fifth generation European Center for Medium-range Weather Forecasts (ERA5) reanalysis data, we present a detailed examination of the climatological features of convectively coupled Kelvin waves (CCKWs) over the Indian and Pacific basins. The composited horizontal structure of Indian CCKWs resembles the theoretical Kelvin waves, with a maximum wave response at the equator. In contrast, the Pacific counterpart exhibits a very different pattern, characterized by a significant northward shift of the convective center, along with enhanced meridional winds and a relatively stronger wave response. The moist static energy (MSE) budget analysis is conducted to elucidate the physical factors that control the energetics of CCKWs. Despite the marked contrast in horizontal structure between Pacific and Indian CCKWs, the energy cycle and the physical factors that maintain this cycle are rather similar. During the recharge period (days -2 and -1), the column process (including vertical MSE advection, apparent heat source and moisture sink) functions to destabilize the atmosphere by importing the MSE; while the horizontal MSE advection tends to destabilize the atmosphere on day -2 but starts to stabilize the atmosphere earlier on day -1. During the discharge and transition period (from days 0 to + 2), the column process functions to stabilize the atmosphere by exporting the MSE; while the horizontal MSE advection inclines to stabilize the atmosphere on days 0 and + 1 but again starts to destabilize the atmosphere earlier on day + 2. The leading of horizontal MSE advection to the recharge-discharge cycle clearly points out the importance of the former in driving the eastward propagation of CCKWs. Both the horizontal MSE advection and column process are vital in maintaining the energy cycle of CCKWs, as they often take turns leading the role in recharging and discharging the atmosphere.

Appendix: Horizontal structure of CCKWs at 200 hPa

For comparison with Fig. 3, Fig. 11 displays the composited horizontal structures of CCKW-filtered OLR, geopotential height, wind and geopotential anomalies in the Indian and Pacific basins at 200 hPa, where the amplitudes of CCKW wind anomalies are the strongest. We note that the CCKW-filtered wind and geopotential height anomalies at 200 hPa are about twice as large as those at 850 hPa, and the wind and geopotential height anomalies at 200 hPa are generally opposite to those at 850 hPa, which are typical characteristics of convectively-coupled waves (Inoue et al. 2020; Nakamura and Takayabu 2022). Moreover, we also notice that the in-phase relation between zonal wind and geopotential height anomalies (a typical feature of dry Kelvin waves) is more obvious at 200 hPa than at 850 hPa due to the much smaller condensation heating near the tropopause.

Fig. 11

Same as Fig. 3, but for the composited horizontal structures of CCKW-filtered OLR anomalies (color shadings; interval: 1 $\mathrm{W }{{\text{m}}}^{-2}$), geopotential height anomalies (contours; interval: 0.5 ${\text{m}}$), and horizontal winds anomalies (vectors; unit: $\mathrm{m }{{\text{s}}}^{-1}$) at 200 hPa for (a) Indian and (b) Pacific CCKWs at day 0. It is noted that the scales for geopotential height and wind anomalies shown here are different from those of Fig. 3