Fig. 1. (a) Time series of
tropical (30°S-30°N) averaged SWV entry anomalies (solid lines) and
their linear trends (dashed lines) based on SWOOSH (black lines, 82 hPa)
and ERA5 (red lines, 70 hPa) from 1984 to 2020. The trends are shown in
the dashed lines legend, and one asterisk denotes significance at the
90% confidence level and two asterisks denote significance at the 99%
confidence level. The
uncertainties are expressed by 2σ errors. The number in the top
left-hand corner and top
right-hand corner are the averages of anomalies over the first five
years and the last five years. (b) The same as (a), but for the tropical
tropopause temperature (30°S-30°N, 70 hPa) based on ERA5 (red lines) and
JRA55 (black lines). (c) Vertical profiles of global SWV trend using
SWOOSH (red line) and ERA5 (black line). (d) The same as (c), but for
meridional variations of the global SWV (100-10 hPa) trend. Bold lines
in (c) and (d) mean significance at the 90% confidence level.
The cooling trend in the lower stratosphere is captured by the tropical
tropopause temperature (Fig. 1b). The linear cooling trends are 0.328 ±
0.083 K per decade in ERA5, while JRA55 has a relatively larger rate at
0.253 ± 0.08 K per decade. Coherently, the temperature average value
also supports the SWV entry decrease but with a larger value in ERA5
than in JRA55. The free-drying regime works if it is assumed all air
entering from the troposphere to the stratosphere passes through the
extremely cold temperature. Thus, the decreased rate of SWV entry
predicted by the Clausius-Clapeyron equation, for the linear cooling
trend of the tropical tropopause at the value of 0.328 K per decade, is
about 0.09 ppmv per decade, which is very close but slightly smaller
than the observed value (0.106 ppmv per decade).
As the tropical SWV entry decreases, we can speculate that the global
SWV may have the same change. So, the vertical and meridional resolved
zonal-mean SWV trends are shown in Fig. 1c and 1d, respectively. The
decreasing trends based on the two datasets are generally in agreement
in the lower stratosphere, showing significant dehydration in the lower
stratosphere for the period 1984-2020 (Fig. 1c). Above 30 hPa, however,
the positive trend is only seen in SWOOSH. This increase in water vapour
is possibly induced by the increase of methane (le Texier et al. 1988)
and its production of water vapour via oxidization. The meridional
variation shows that the drying trend in the southern hemisphere is
stronger than that in the northern hemisphere. This hemispheric
difference has not been clarified yet. In general, the linear trends of
SWV based on observations show a consistent decreasing trend for the
period 1984-2020. And it is similar to the tropical SWV entry.
However, the tropical SWV entry time series show large interannual
variability, which can degrade the linear estimation of its long-term
trend. So, we apply a nonlinear algorithm, EEMD, to confirm the drying
trend. Fig. 2 shows the original tropical SWV entry time series in the
tropics and its decomposed modes using EEMD analysis. In the original
time series, the tropical SWV entry shows large interannual variability
with the magnitudes up to about 1.5 ppmv (Fig. 2 (a) and (b)). The first
three components are corresponding to the irregular oscillations, annual
cycles, and interannual variations, with the peak-to-peak amplitudes at
around 0.2 ppmv, 0.6 ppmv, and 0.2 ppmv, respectively (Fig. 2 (c), (d),
(e), (f), (g) and (h)). With the number of EEMD modes increasing, the
frequency of the oscillations becomes lower. Finally, the residual term
has only one extremum which can be used to diagnose the nonlinear
long-term change (Fig. 2 (i) and (j)). The residual component shows a
general decreasing trend in the tropical SWV entry and the level around
2020 is still much lower than that around 1980. And, one thing that
should be noted is that ERA5 exhibits strong non-linearity in its
residual with a turnaround in 2005. This turnaround may be related to
the preceding inconsistency between the amplitude of the drop and the
estimated linear trend. And this turnaround makes decreasing trends much
weaker in ERA5. We also applied EEMD on the tropopause temperature time
series, and the results are in good agreement with a residual term
showing a decades-long cooling trend (Fig. S3).