Saturday, September 20, 2014

New paper describes another potential solar amplification mechanism

A new paper published in Atmospheric Chemistry and Physics finds that sudden stratospheric warming (SSW) events, which have been linked to solar activity and may act as a solar amplification mechanism, can have significant effects on weather/climate of the lower atmosphere [troposphere] via effects on convection, convective cloud formation, and water vapor feedback. 

According to the authors,
"It is generally believed that such changes in the stratosphere do not affect the troposphere, due to the difference in air density between the two" and "the influence from above (i.e., from the stratosphere) is generally neglected"
but the authors instead find this 'settled' 'consensus' belief to be incorrect and that changes in the stratosphere (some of which relate to solar activity) can have profound influences on the weather/climate of the troposphere via effects on cloud formation and deep convection. 

Sudden stratospheric warming (SSW) events have also been linked to changes in the the quasi-biennial oscillation [QBO], and the QBO is also linked to changes in solar activity. According to the authors, the "Madden–Julian Oscillation (MJO) has a significant influence on tropical convective activity" similar to that found in this paper, although they did not find a direct link between the MJO and sudden stratospheric warming (SSW) events. The MJO has also been linked to solar activity.

The paper joins many others describing potential solar amplification mechanisms and illustrates the complexity of determining indirect but large-scale effects from tiny changes in solar activity on weather and climate.

Sudden Stratospheric Warming Split the Polar Vortex in Two



Weather forecasting in tropical regions is challenging due to the unstable nature of
the atmosphere there and its sensitivity to various extratropical disturbances. The impact
of the extratropical circulation on the tropics, such as the lateral propagation of
tropospheric Rossby waves, has been studied previously (e.g., Kiladis and Weickmann,
1992; Funatsu and Waugh, 2008). The influence from above (i.e., from the
stratosphere) is generally neglected, but under certain circumstances, such as during
a sudden stratospheric warming (SSW) event, stratospheric meridional circulation
change can modify convective activity as will be shown later.

Early satellite measurements showed that enhanced poleward eddy heat fluxes in
the extratropical stratosphere induce tropical cooling through changes in the mean
meridional circulation (Fritz and Soules, 1970; Plumb and Eluszkiewicz, 1999; Randel
et al., 2002). It is generally believed that such changes in the stratosphere do not
affect the troposphere, due to the difference in air density between the two. Indeed,
tropical temperature change induced by the intraseasonal mean meridional circulation
is apparent 5 only in the layer around 70 hPa and above (Ueyama et al., 2013).
However, this does not imply that the stratospheric meridional circulation has no
impact on the atmosphere below the 70 hPa level. A possible impact of stratospheric
meridional circulation on cumulus heating has been suggested by Thuburn and Craig
(2000) in a simplified general circulation model experiment. Stratospheric upwelling
effects on tropical convection is also confirmed by a more realistic general circulation
model forecast study (Kodera et al., 2011a). These models make use of cumulus
parameterization to account for the effect of convection into large scale circulation.
Therefore, model sensitivity should be dependent on the parameterization [fudge factor] used. 
Stratospheric effect on tropical convection is also found in non-hydrostatic models that treat
the convection explicitly.

Although it is not fully understood yet how stability influences anvil cloud-top height,
Chae and Sherwood (2010) showed with observational data and a regional nonhydrostatic
model experiment that the variation of static stability near the tropopause
due to a change in the stratospheric upwelling, influences cloud height even the cloud
height peaks only near 12 km (or 200 hPa). Using a global non-hydrostatic model simulation,
Eguchi et al. (2014) also found that increased tropical upwelling due to a SSW
event reduces the static stability in the upper Tropical Tropopause layer (TTL), which
leads to an increase of deep convective activity in the troposphere.
Temperature response to stratospheric upwelling becomes unclear in the region
lower than the tropopause because clouds form in response to adiabatic cooling associated
with upwelling. Stratospheric temperature decrease, but minimal temperature
change in the TTL, results in a decrease in static stability in the upper TTL (Li and
Thompson, 2013). In the regions where deep convective clouds are frequent, stratospheric
influence further penetrates deeper in the troposphere (Eguchi and Kodera,
2010; Kodera et al., 2011b). Once the distribution of convective clouds is modified,
this effect can be amplified within the troposphere through a feedback involving water
vapour transport (Eguchi and Kodera, 2007).

Here, we focus on the role of overshooting and deep convective clouds in
stratosphere–troposphere dynamical coupling in the tropics, and present case studies
of two of the recent largest SSW events in January 2009 and January 2010 (Harada
et al., 2010; Ayarzagüena et al., 2011). It should be noted, however, that not all major
SSW events necessarily have large tropical impacts, as this depends on the latitude of
the wave breaking (Taguchi, 2011).

Summary and discussion

The results of our analysis of changes in tropical circulation associated with large
SSWs [Stratospheric Sudden Warmings] during January 2009 and January 2010 can be
summarized as follows.

Enhanced stratospheric wave activity produced a cooling in the tropical stratosphere
through a strengthening of the BD circulation. This influence penetrated downward into
the troposphere through a change in the cloud formation. Among the variables representing
different convective activity, COV shows the highest correlation with the lower
stratospheric vertical velocity. This result is reasonable because the COV clouds can
penetrate above the tropopause and interact directly with the stratospheric circulation.
The reason of low correlation of the OLR [outgoing longwave radiation] with stratospheric
upwelling originates from the fact that the tropospheric variation lags by about a week (Fig. 1).

The results obtained from our two case studies are consistent with earlier results
from an independent composite analysis of the winters between 1979 and 2001
(Kodera, 2006), which revealed that the tropospheric convective activity in the equatorial
SH is enhanced following the stratospheric equatorial upwelling induced by upward
propagation of planetary waves during the NH winter.

As for a process which relates tropical stratospheric upwelling and the tropospheric
convective activity, investigation between the diabatic heating in the TTL and the 
stratospheric vertical velocity is crucial. Direct measurement of such quantities are difficult,
but a global non-hydrostatic model study (Eguchi et al., 2014) confirmed the relationship
suggested in the present result.

The characteristics of the convective activity changed following the stratospheric
event. When stratospheric upwelling was suppressed before onset of the event, convection
tended to cluster around the equatorial Maritime Continent or western Pacific
region depending on the phase of ENSO. When the stratospheric upwelling increased,
convection expanded over a wide range of longitudes in the tropical summer hemisphere.
In other words, tropical circulation changed from a more Walker like (east–
west) configuration to a more Hadley (north–south) type.

The Madden–Julian Oscillation (MJO) (Madden and Julian, 1994) has a significant
influence on tropical convective activity. One would ask whether or not the present
phenomenon is associated with the MJO. The features of the MJO in January 2009
and 2010 differed significantly as can be seen in Fig. 5. A convective centre remained
stationary over the Maritime Continent prior to the onset of the 2009 stratospheric
event, after which an eastward propagation was initiated from the Indian Ocean. In
contrast, an eastward propagating convective centre became almost stationary over
the western Pacific after the onset in January 2010. In spite of the differences in the
MJO in January 2009 and 2010, circulation changes related to the stratospheric events
showed similar features during both winters, suggesting that the present phenomenon
is independent of the MJO.

Atmos. Chem. Phys. Discuss., 14, 23745-23761, 2014

K. Kodera1,2, B. M. Funatsu3,4, C. Claud4, and N. Eguchi5
1Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan
2Climate and Ecosystems Dynamics Division, Mie University, Tsu, Japan
3LETG-Rennes COSTEL, Université Rennes 2, Rennes, France
4Laboratoire de Météorologie Dynamique, Ecole Polytechnique, Palaiseau, France
5Research Institute for Applied Mechanics, Kyushu University, Kasuga, Japan

Abstract. This paper investigates the role of deep convection and overshooting convective clouds in stratosphere–troposphere dynamical coupling in the tropics during two large major stratospheric sudden warming events in January 2009 and January 2010. During both events, convective activity and precipitation increased in the equatorial Southern Hemisphere as a result of a strengthening of the Brewer–Dobson circulation induced by enhanced stratospheric planetary wave activity. Correlation coefficients between variables related to the convective activity and the vertical velocity were calculated to identify the processes connecting stratospheric variability to the troposphere. Convective overshooting clouds showed a direct relationship to lower stratospheric upwelling at around 70–50 hPa. As the tropospheric circulation change lags behind that of the stratosphere, outgoing longwave radiation shows almost no simultaneous correlation with the stratospheric upwelling. This result suggests that the stratospheric circulation change first penetrates into the troposphere through the modulation of deep convective activity.

1 comment:

  1. I don't swallow the entire thinking:
    1- Whether hot oceanic surface temperature provides excess troposphere heat, OK.
    2- Whether this hot wet bubble pops up towards upper troposphere limit, still remaining warmer than ordinary air there, OK: this is so far nothing else than to make latent heat (from water vapour) free as pressure declines.
    3- Whether the heated stratosphere occurs, OK, this is consequence of former item.
    4 -But whether the upper warmer stratospheric bubble "falls in troposphere and warms it" NO WAY, because it is against basic laws of Physics: In same pressure conditions, warmer dry air cannot sink into a colder almost dry air !!!!!!!!!! It is as if a bottle cork would sink down to sea floor..!
    Thence, what occurs with this warmer stratospheric air? I am not a climatologist at all, so I let specialists to answer. I "feel" that this warm air creates an inversion layer above troposhere up it cools down by ozone irradiance. Thus the entire phenomenum would be a further cooling process, alike a thermostat, not a positive feed-back.
    This is only a personnal and non validated idea....