Interannual Variations in Stratospheric Water Vapor

1
Interannual Variations in Stratospheric Water
Vapor
J.N. Lee1, Y. Kawatani2, and K. Hamilton3 1NASA
Goddard Space Flight Center, 2JAMSTEC RIGC , 3IPRC
Specific humidity mean annual cycle (ppmv)
The water vapor content of air entering the
tropical stratosphere is determined by the
saturation value of the coldest temperature in
the tropical tropopause (around 17 km or 90 hPa).
Satellite observations have shown that the annual
temperature cycle at the tropopause imposes a
cycle in water content, and this signal in the
annual water cycle is advected upwards by the
slow mean rising motion (Fig. 1). Using the
long record now available from the Microwave Limb
Sounder (MLS) on the NASA AURA satellite we are
able to examine the height-time structure of the
interannual variations in water content. Fig. 2
displays the expected upward propagation below
10 hPa, but apparent downward propagation at
higher levels. We explain this surprising result
as a dynamical effect of the largest interannual
component in the circulation in this region, the
Quasi-biennial Oscillation (QBO), and have shown
that this effect is reproduced in global
circulation models (Fig. 3).
6 4 3
Fig. 1. Height-time diagram of the long-term mean
annual cycle of water vapor mixing ratio (parts
per million by volume) in the equatorial
stratosphere.
Specific Humidity (ppmv) 12S12N
Fig. 3. Composite of the height-time diagram of
the water vapor mixing ratio based on the phase
of the QBO in zonal wind. Results from AURA MLS
(left) and from simulation in the MIROC global
model (right). Note boomerang pattern in the
anomaly fields with upward propagation below and
downward propagation above 10 hPa . The upward
propagation is due to the mean advection of
interannual water content anomalies induced at
the tropopause. The downward propagation is due
to the advection of the mean vertical gradient of
water content by the interannual fluctuations in
the vertical wind.
Fig. 2. Interannual anomalies in the water vapor
mixing ratio in the equatorial stratosphere from
105 months of NASA AURA MLS observations.