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.