Fig. 2. EEMD for the tropical (30°S-30°N) averaged SWV entry from 1984
to 2020 (units: ppmv; at 82 hPa in SWOOSH, at 70 hPa in ERA5). (a) and
(b) are the original time series. (c), (d), (e), (f), (g) and (h) are
the first three IMFs. (i) and (j) are the final residuals after seven
IMFs extracted.
In sum, a robust drying trend in the lower tropical stratosphere is
confirmed by both the linear and nonlinear long-term trend analysis. The
freezing-dry model is generally applicable in the decrease rate,
indicating the dominating role of the tropical pathway in the process.
Decreasing SWV entry associated with
IPWP
As a key factor modulating the transport through the tropical pathway,
the SST long-term variations in tropical oceans, especially the tropical
Pacific and tropical Indian Ocean, are further investigated on its
linkage to the drying trend in the lower stratosphere. Fig. 3a shows the
zonally resolved correlation coefficients between tropical SST anomalies
and the tropical tropopause temperature and SWV. To remove the strong
interannual variations, we apply a 5-year running mean on tropical
tropopause temperature and SWV time series before the linear
relationship analysis. A strong negative linear relationship is shown
between the long-term change of SST of the IPWP and the tropical
tropopause temperature, with a weak positive relationship over the Niño3
region. Because tropical tropopause temperature is a great indicator of
SWV, the SST in IPWP also has a greatly negative correlation with SWV,
for both the tropical and global averages from both data sources. During
1984-2020, the IPWP has been substantially warming, at a higher rate
(0.12 oC per decade) than the eastern Pacific (Figs.
3b, 3c). The strong correlation between the SST in IPWP and SWV
indicates that there may be some connection between the warming IPWP and
the drying stratosphere.