J. David Neelin and Hui Su
J. Climate, 18, 3928-3950, 2005.
Paper (PDF 2.4 MB)
© Copyright 2005 by the American Meteorological Society.
Abstract.
Teleconnections have traditionally been studied for the case of dry dynamical
response to a given diabatic heat source. Important anomalies often occur
within convective zones, for instance, in the observed remote response to El
Niño. The reduction of rainfall and teleconnection propagation
in deep convective regions poses theoretical challenges because
feedbacks involving convective heating and cloud
radiative effects come into play. Land surface
feedbacks, including variations of land surface temperature,
and ocean surface layer temperature response must be taken
into account. During El Niño, descent and negative
precipitation anomalies often extend across equatorial South America
and the Atlantic intertropical convergence zone.
Analysis of simulated mechanisms in a case study of the 1997-98 El Niño
is used to illustrate the general principals of teleconnections occurring
in deep convective zones, contrasting land and ocean regions.
Similar behavior is noted in examination of other events.
Tropospheric temperature and wind anomalies are spread eastward
by wave dynamics modified by interaction with the moist convection
zones. The traditional picture would have gradual descent balanced by
radiative damping but this scenario misses the
most important balances in the moist static energy (MSE) budget.
A small "zoo" of mechanisms is active in producing strong regional
descent anomalies and associated drought.
Factors common to several mechanisms include the role of convective
quasi-equilibrium (QE) in linking low-level moisture
anomalies to free tropospheric temperature anomalies in a
two-way interaction referred to as QE mediation.
Convective heating feedbacks change the net static stability to
a gross moist stability (GMS) M.
The large cloud-radiative feedback terms may be manipulated to
appear as a modified static stability 
,
under approximations that are quantified for
the quasi-equilibrium tropical circulation model used here.
The relevant measure of differs between land,
where surface energy flux balance applies, and short time scales over ocean.
For the time scale of an onsetting El Niño, a mixed layer
ocean response is similar to a fixed SST case, with surface fluxes
lost into the ocean and substantially
reduced over ocean enhancing descent anomalies. Use of
aids analysis of
terms that act as the initiators of descent anomalies.
Apparently modest terms in the MSE budget can be acted on by
the GMS multiplier effect which yields substantial precipitation
anomalies due to the large ratio of the moisture convergence
to the MSE divergence.
Advection terms enter in several mechanisms, with the leading
effects here due to advection by mean winds in both MSE and
momentum balances. A Kelvinoid solution is presented
as a prototype for how easterly flow enhances moist wave decay
mechanisms, permitting relatively small damping terms by surface
drag and radiative damping to produce the substantial eastward
temperature gradients seen in observations and simulations
and contributing to precipitation anomalies.
The leading mechanism for
drought in eastern equatorial South America is the upped-ante
mechanism in which QE-mediation of teleconnected tropospheric temperature
anomalies tends to produce moisture gradients between the
convection zone, where low-level moisture increases toward QE,
and the neighboring non-convective region. Over the
Atlantic ITCZ, the upped-ante mechanism is a substantial contributor
but on short time scales several mechanisms
referred to jointly as troposphere/SST disequilibrium mechanisms
are important. While SST is adjusting during passive
SST (coupled ocean mixed-layer) experiments, or for fixed SST,
heat flux to the ocean is lost to the atmosphere and
these mechanisms can induce descent and precipitation anomalies,
although they disappear when SST equilibrates.
In simulations here, cloud-radiative feedbacks,
surface heat fluxes induced by teleconnected
wind anomalies, and surface fluxes induced by QE-mediated
temperature anomalies are significant disequilibrium contributors.
At time scales of several months or longer,
remaining Atlantic ITCZ rainfall reductions are maintained by
the upped-ante mechanism.
Citation. J. D. Neelin and H. Su, 2005: Moist teleconnection mechanisms for the tropical South American and Atlantic sector during El Niño. J. Climate, 18, 3928-3950.