The tropical tropopause inversion layer: variability and modulation by equatorial waves
The tropical tropopause layer (TTL) acts as a transition layer between the troposphere and the stratosphere over several kilometers, where air has both tropospheric and stratospheric properties. Within this region, a fine-scale feature is located: the tropopause inversion layer (TIL), which consi...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-09-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/16/11617/2016/acp-16-11617-2016.pdf |
| Summary: | The tropical tropopause layer (TTL) acts as a transition layer between the
troposphere and the stratosphere over several kilometers, where air has both
tropospheric and stratospheric properties. Within this region, a fine-scale
feature is located: the tropopause inversion layer (TIL), which consists of a
sharp temperature inversion at the tropopause and the corresponding high
static stability values right above, which theoretically affect the
dispersion relations of atmospheric waves like Rossby or inertia–gravity
waves and hamper stratosphere–troposphere exchange (STE). Therefore, the TIL
receives increasing attention from the scientific community, mainly in the
extratropics so far. Our goal is to give a detailed picture of the
properties, variability and forcings of the tropical TIL, with special
emphasis on small-scale equatorial waves and the quasi-biennial oscillation
(QBO).<br><br>We use high-resolution temperature profiles from the COSMIC satellite
mission, i.e., ∼ 2000 measurements per day globally, between 2007 and
2013, to derive TIL properties and to study the fine-scale structures of
static stability in the tropics. The situation at near tropopause level is
described by the 100 hPa horizontal wind divergence fields, and the vertical
structure of the QBO is provided by the equatorial winds at all levels, both
from the ERA-Interim reanalysis.<br><br>We describe a new feature of the equatorial static stability profile: a
secondary stability maximum below the zero wind line within the easterly QBO
wind regime at about 20–25 km altitude, which is forced by the descending
westerly QBO phase and gives a double-TIL-like structure. In the lowermost
stratosphere, the TIL is stronger with westerly winds. We provide the first
evidence of a relationship between the tropical TIL strength and
near-tropopause divergence, with stronger (weaker) TIL with near-tropopause
divergent (convergent) flow, a relationship analogous to that of TIL strength
with relative vorticity in the extratropics.<br><br>To elucidate possible enhancing mechanisms of the tropical TIL, we quantify
the signature of the different equatorial waves on the vertical structure of
static stability in the tropics. All waves show, on average, maximum cold
anomalies at the thermal tropopause, warm anomalies above and a net TIL
enhancement close to the tropopause. The main drivers are Kelvin,
inertia–gravity and Rossby waves. We suggest that a similar wave modulation
will exist at mid- and polar latitudes from the extratropical wave modes. |
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| ISSN: | 1680-7316 1680-7324 |