Chris Landsea
Subject: [Tropical-storms] TUTT feature in Atlantic/ Double ITCZ in Indian Ocean
> To
all you peopel who live and breathe US and Atlantic
>
meteorology, this
>
is presumably a common feature. However, I have never heard of it
or
>
noticed it before. Is there anything much known about these
tropical
>
Atlantic upper level troughs?
There
is some knowledge about these systems. These TUTTs are very common
features to the Atlantic, though more so in the July to September timeframe.
While they can be reinforced by PV (mainly V) injections from sharp troughs
in the westerlies, TUTTs are cold-cored primarily driven by radiation balance-induced
subsidence. (Lack of deep cloudiness/convection in the subtropical
latitudes coupled with radiational cooling causes strong subsidence near
the
tropopause
and the formation of a stratospheric warm core.) In the past (Fitzpatrick
et al. 1995, _W&F_), the AVN model would fill these systems much
too quickly, leading to easterly wind biases in the forecast field
of the upper troposphere. The latest generation of global models
seem to do a better job with holding on the TUTTS, but may still
have a slight bias.
In years of El Nino events and cool Atlantic/Caribbean SSTs, the TUTT becomes semi-permanent during the hurricane season with a quite extended longitudinal reach (similar to what you have posted). They force high vertical shear in the main development region and can completely shut down the Atlantic season from African easterly wave developments. (This wasn't completely the case in 2002, but it's likely that we'll end up with slightly below season overall.)
Best
regards,
chris
***********************************************************************
Chris
Landsea
NOAA
AOML/Hurricane Research Division
John
McBride
Gedday
all,
I have had a quick look at the very nice paper Chris and coauthors wrote on systematic errors/model biases asscoiated with these Atlantic TUTTS (Fitzpatrick et al, Weather and Forecasting, June 1995). I have also tracked back from the references in the back of their paper on earlier studies on the dynamics and forcing of upper level TUTT type troughs.
I have
to admit I am a little sceptical on the proposed generation/maintenance
mechanism being radiational cooling in the
cloud-free
subtropics. I suppose it is always possible that once you have large-scale
sinking and associated upper-level convergence, the vorticity can organise
itself into an ocean-wide filament simply through barotropic dynamics...
It is by no means obvious though as to why the radiational processes would
zero in on a long skinny region of cyclonic vorticity.
To me it seems more likely that large scale barotropic dynamics are responsible, somehow as addressed in a paper by Colton in the Journal of the Atmospheric Sciences, 1973.
Dean Churchill
On
way in which the radiational cooling might be associated with a "long skinny
region of cyclonic vorticity" is that there may be enhanced water vapor
content that migrates with the high vorticity air. The radiative
cooling in the upper troposphere and lower stratosphere might be enhanced
where the vapor content is higher than in surrounding air masses of different
PV. Although the absolute water vapor content may be very low, there can
be enough vapor (left over from some cirrus perhaps
a
long time ago) to cause enhanced cooling and subsidence over a period of
days.
JunIchi
Yano
now
TUTT, in addition to CISK, WISHE, etc.....
some
time ago, I have concluded that this kind of arguments, based on dominant
balance (in quasi-stationary sense), tend to be misleading: the conclusions
mostly depend on what variable one looks at, and probably true maintenance
mechanism is hidden in higher-order balance.
John
Knaff
John,
Good to hear from you. Thank you for digging up this 1973 paper by Colton. This young man lacking in culture, was not aware of it. The proposed mechanism however seems to fit well with what we see during the daily weather discussions during the hurricane season. What we see is a series of cold lows breaking off of the westerly jet being advected first to the east and south and then propagating through the middle North Atlantic towards the south west. So the initial formation of the cold low and its propagation may be very well explained by barotropic dynamics, but the maintenance of these cold ows/TUTT cell is another issue altogether. Radiation will work to decrease the amplitude of the upper warm core and weaken these systems within a few days. Something must maintain the warming near the tropopause. Therefore, I still think it is sinking which offsets the radiational cooling and allows for these cold lows to exist for many days.
This
is further supported by the the observation by Neil Frank and others that
convection in the center of these cold lows weakens them and that they
do not persist very long over land in the summer or during the winter (when
oceanic convection is more plentiful). The land response often manifest
itself as a intense precipitation event (Whitfield and Lyons 1992, Wea.
Forecasting), but the cold low weakens or goes away completely. I think
the winter observation is key as the instability in the upper atmosphere
likely still exists, be it shifted toward the equator and likely stronger.
Similar mid-oceanic troughs are evident in the southern hemisphere during
the austral summer in the central Pacific, Atlantic and to a lesser extent
in the Southern Indian Ocean, which are also located near the center of
the subtropical highs. Coincidence?
Also
observed is that when the summertime pressure is higher in the tropical
Atlantic the TUTT is stronger (Knaff 1997, J. Climate), resulting in stronger
westerly winds at 200mb in this region.
John
John
Knaff
Research
Scientist
CIRA
Colorado
State University
Fort
Collins, CO 80523
Steve
Lyons
Steve
Lyons
Tropical
Program Manager
The
Weather Channel
slyons@weather.com
770-226-2537
You
may want to look at the paper listed below for at least "one realization"
of a TUTT low. Steve
Whitfield, M. B. and S. W. Lyons, 1992: The tropical upper tropospheric low: A synoptic-scale source of summer precipitation in Texas. We. and Forecasting, 7, 89-106.
Mark Lander
Hi John K. John McB, and TC group,
Regarding the structure and maintenance of the mid-ocean TUTTs and TUTT
cells, it is of interest to point out that the TUTT does not sit over the
mid-ocean surface subtropical ridge, but over the trades approximately
mid-way between the surface ridge axis and the ITCZ/monsoon trough.
Probably lesser known, the 200 mb subtropical ridge (of summer) is what
sits directly over the surface ridge/subtropical high. This
suggests subsidence in both the upper ridge and TUTT systems. Ageostrophic
flow must be an important contribution to such a distribution of subsidence
in a vertically aligned ridge system.
Summer ocean atmosphere in a picture:
* TUTT
* * * *
* * *
200 mb height profile
* STR
*
* *
* * *
1000 mb height profile
* * *
STR ITCZ
Regards,
Mark Lander