Fig. 5. Tropical (30°S-30°N; at 82 hPa in SWOOSH and at 70 hPa in ERA5) and global average trends of SWV for SWOOSH, and ERA5. Orange bars represent raw trends and green bars represent trends that have been removed from IPWP. Values of trends are presented around each bar. Red texts represent the trends caused by IPWP and their proportion of raw trends. One asterisk denotes significance at the 90% confidence level and two asterisks denote significance at the 99% confidence level. A five-year running average is used before calculating trends.
IPWP warming impacts in models
It is possible that the signal of IPWP warming is tangling with other internal variabilities, for example, the long-term trend of other oceans, which can make it difficult to provide quantification and a clear physical mechanism for the IPWP warming impacts. An alternative approach is to isolate the IPWP warming signal with the aid of models. So, here we employ WACCM4 and CAM5 to carry out transient experiments to isolate the IPWP warming signal. The details of the transient experiments can be found in Table. 1 E2 and E3.
The effect of IPWP warming on the tropical temperature is visualized in Fig. 6 which shows a cross-section of temperature trends and Fig. 7, which shows decadal changes in the tropical temperature profiles. As expected, tropical air in the troposphere becomes warmer due to latent heat caused by intensified convection, with a heating center at about 200 hPa (Fig. 6). Air in the stratosphere, however, becomes cooler due to adiabatic cooling caused by enhanced large scale ascent. Both WACCM4 and CAM5 share the general pattern in tropical temperature long-term changes, but WACCM4 has a relatively larger gradient in the upper troposphere and lower stratosphere (UTLS) region (seen as dense contours near the tropopause). We also note that there are zonal asymmetries in the temperature trend in the tropics, with both the heating center in the troposphere and the cooling center in the UTLS being located over the IPWP region (Figs. 6c, 6d). The two out-of-phase temperature changes in the troposphere and the stratosphere, together, lead to the change in the shape of the tropical temperature profile and a lower coldest point temperature (Fig. 7). The coldest point temperature at 85 hPa drops from 196.055 K to 195.343 K in WACCM4, with an estimated decrease rate at about 0.17 K per decade. In CAM5, however, the cooling rate is about half of the rate of WACCM4.