Higher tropical SSTs strengthen the tropical upwelling via deep convection

Kurzfassung

Recent observations show a distinct cooling of the tropical lower stratosphere, and chemistry-climate models (CCMs) suggest a link to a strengthening tropical upwelling, arguably related to increases in greenhouse gas concentrations from anthropogenic activity. The present study explores the strengthening of tropical upwelling by comparing ensemble realisations of two different transient scenarios with the CCM E39/C. Both scenarios share the same boundary conditions, including concentrations of ozone-depleting substances, but differ in their climate forcing via prescribed sea surface temperatures (SSTs) and well-mixed greenhouse gas concentrations. In the summer hemisphere tropics, higher SSTs amplify deep convection locally and hence the convective excitation of quasistationary waves. These waves propagate upward through the region of easterly winds while dissipating, but still carry enough of the signal into the low-latitude lower stratosphere to induce an anomalous low-latitude Brewer-Dobson (BD) cell. The transport change in turn increases the flux of ozone-poor tropospheric air into the tropical lower stratosphere.