Modes of tropical convection and their roles in transporting moisture and moist static energy: contrast between deep and shallow convection

Abstract

The dominant modes of convection (vertical motion) in the tropical convective regions were explored using the EOF analysis method and the ERA-interim reanalysis data, with an emphasis on comparing their roles in transporting moisture and moist static energy (MSE). Two major types of convection orthogonal to each other were identified in the deep tropics, including deep and shallow convection. The deep convection, derived from the 1st EOF mode of vertical motion, shows different vertical structures (e.g., top-heavy versus bottom-heavy) under different thermodynamic environments (e.g., west versus east Pacific); while the shallow convection, derived from the 2nd EOF mode of vertical motion, exhibits opposite signs of vertical structure (e.g., positive and negative) between upper and lower troposphere. The two subtypes of deep convection exhibit distinct tendencies in moisture and MSE transport, i.e., while both import moisture into the atmosphere, the top-heavy one tends to export MSE, leading to a stabilized atmosphere; whereas the bottom-heavy counterpart inclines to import MSE, resulting in a destabilized atmosphere. On the contrary, the two subtypes of shallow convection display a consistent tendency in moisture and MSE transport, i.e., the positive shallow convection imports moisture and MSE; whereas the negative counterpart exports moisture and MSE. More importantly, the MSE transport by shallow convection (including positive and negative ones) can be comparable in size with the MSE transport by deep convection (including top-heavy and bottom-heavy ones), implying a non-negligible role played by shallow convection in charging and discharging the atmosphere in addition to the commonly recognized deep convection.

Appendix A: Implications to ENSO variability

The alternative occurrences of positive and negative shallow convection (${\omega }_{2}$) shown in EPCZ (Fig. 6c) is likely to have strong implications to climate variability of interannual timescale such as ENSO (El Nino/Southern Oscillation). To elaborate, Fig. 

Fig. 11

Timeseries of ONI (Oceanic Nino Index) and ${\omega }_{2}$ at 850 hPa (in $\text{Pa}\,\text{s}^{-1}$) over the EPCZ. The correlation coefficient (r) between the two timeseries and the statistical p-value are also displayed at the top-left corner

11 shows the timeseries of ONI (Oceanic Nino Index) obtained from NOAA’s CPC (Climate Prediction Centre), along with the timeseries of the second mode of vertical motion (${\omega }_{2}$) at 850 hPa, which is approximately the level of peak vertical motion for shallow convection. As shown in Fig. 11, the two timeseries are significantly correlated ($r=0.63\mathrm{ and }p<0.05$), with positive values of ${\omega }_{2}$ (i.e., negative shallow mode) occurring during the positive ONI years (e.g., 2002, 2004, 2006 and 2009). When combining ${\omega }_{2}$ with ${\omega }_{1}$, this implies that the convection becomes deeper in EPCZ during the El Nino years of 2002, 2004, 2006 and 2009 (see Fig. 6a–c), which is generally consistent with enhanced precipitation intensity in EPCZ during these warm El Nino years (see Fig. 6d).