Response of the Atmosphere to Climate Variability in the Tropical Atlantic By Alfredo Ruiz

1
Response of the Atmosphere to Climate Variability
in the Tropical AtlanticByAlfredo
RuizBarradas1, James A. Carton, and Sumant
NigamUniversity of Maryland--------o--------A
GU Spring MeetingWashington, DC1alfredo_at_atmos.um
d.edu
May
2831, 2002

GC51B-09





1. Introduction. This study explores variability
of the lower troposphere in the tropical Atlantic
sector through a series of modeling simulations
with a diagnostic primitive equation model. The
focus is on two main distribution of anomalies
important at interannual and longer time scales
that for their surface temperature
characteristics are referred as interhemispheric
and equatorial distribution of anomalies. Such
distributions, with maximum amplitude during
boreal spring (MAM) and summer (JJA)
respectively, have been identified previously
through a rotated joint empirical orthogonal
function analysis (RuizBarradas et al., 2000 ).
3. Local forcing.
4. Extrabasin interactions.
5. Conclusions.
Control Simulations
Global view of control simulations

• Realistic diabatic heating anomalies are the most
  important term forcing the tropical circulation.
•  
• Transients have not effect in the deep tropics.
• The near-surface wind response is dominated by a
  given source of forcing (surface or elevated)
  without a particular dominance on a given wind
  component.
•  
• Anomalies of surface latent heat flux induced by
  the simulated wind anomalies of the distributions
  suggest positive feedbacks in the deep tropics on
  both sides of the equator to the west of 20?W.
• The extratropical atmosphere responds to the
  tropical Atlantic forcing during boreal spring
  and summer.
•  
• Two paths link ENSO events with variability of
  the atmosphere in the tropical Atlantic impacting
  over the Hadley cell
• one along the equator through the intensification
  of the Atlantic Walker circulation with anomalous
  upper level westerlies along the equator and
  near-surface anomalous equatorial trades.
• the other is through the northern tropics where
  the extratropical response is associated with an
  upper level trough and a low sea level pressure
  over the subtropical Atlantic.

Regressed anomalies on the time series associated
to the tropical Atlantic distributions

• Forcing confined to the tropical Atlantic is able
  to induce the surface pressure and wind anomalies
  characteristics of the distributions, although
  weaker than observed.

Feedback

• Wave-like patterns at surface and upper levels
  propagate from the source region toward the
  extratropics
• in a southwest-northeast direction for the
  interhemispheric distribution during MAM.
• in a northwest-southeast direction for the
  equatorial distribution during JJA.

• Observed anomalies of 850/1000 mb thickness and
  diabatic heating at lower levels (P780mb) follow
  the sea surface temperature anomalies of the
  interhemispheric and equatorial distributions.
• Diabatic heating at middle and upper levels
  (P690mb) are shifted following deep convection
  zones.

ENSOs influence over the tropical Atlantic

• The induced wind anomalies, via release of latent
  heat, suggest the existence of positive feedbacks
  westward of 20W in both distributions.

2. Model and data. The diagnostic model used here
solves the steady sigma coordinate primitive
equations obtained from linearization about a
zonally symmetric basic state. The resulting
linear equations model the eddy component of the
circulation (Nigam, 1997). While the vertical
structure is discretized using 18 sigma levels
(with 5 below 850 mb), the horizontal structure
is represented with a 2.5 meridional resolution
and a 30 zonal wave number. Thermal and momentum
diffusion processes in the boundary layer are
represented by a linear representation of
vertical diffusion terms, which require the
specification of additional vertical boundary
conditions (Nigam, 1994). In addition the
horizontal momentum and thermodynamic equations
use a horizontal ?2 diffusive damping, and a
sponge layer near the model top. The model is
forced three ways, through diabatic heating and
transient eddy heat fluxes, through eddy momentum
fluxes, and through surface temperature and
topography. Monthly data are obtained from the
NCEP/NCAR Reanalysis data set (Kalnay et al.,
1996 ) while orography is from the ECMWF.
Preliminary experiments show little orographic
effects in the region of interest and it is left
out in the rest of the study.
Sensitivity Analysis

6. References.

• Kalnay, E. and co?authors, 1996 The NMC/NCAR
  40?Year Reanalysis Project. Bull. Amer. Meteor.
  Soc., 77, 437?431.
• MestasNuñez, A. M., and D. B. Enfield, 2001
  Eastern equatorial Pacific SST variability ENSO
  and non-ENSO components and their climatic
  associations. J. Climate, 14, 391-402.
• Nigam, S., 1994 On the dynamical basis for the
  Asian summer-monsoon rainfall-El Nino
  relationship. J. Climate, 7, 1750-1771.
• Nigam, S., 1997 The annual warm to cold phase
  transition in the eastern equatorial Pacific
  diagnosis of the role of stratus cloud-top
  cooling. J. Climate, 10, 2447-2467.
• RuizBarradas, A., J. A. Carton and S. Nigam,
  2000 Structure of interannual-to-decadal climate
  variability in the tropical Atlantic sector. J.
  Climate, 13, 3285-3297.

• Transients have not effect in the deep tropics.
• Middle and upper heating (P690mb) force surface
  circulation in the subtropics.
• Surface temperature and lower level heating
  (P780mb) force it in the deep tropics

Correlations between simulated and observed wind
anomalies

• Correlation between simulated and observed wind
  anomalies for the 1958-1993 period show higher
  values over the ocean than over the cool waters
  and continents.
• The model reasonably simulates the annual
  progression of the ITCZ as well as the trade wind
  systems but has some problems along the coastal
  regions of South America and Africa.

• This influence from the tropical Pacific in the
  tropical Atlantic is reflected in the atmosphere
  as
• weakening of the northeasterly trades, and
• weakening of the Hadley cell (MestasNuñez and
  Enfield, 2001).
• The link between both basins is through two
  paths
• A tropical connection through an enhanced
  Atlantic Walker circulation forced by heating in
  the Pacific to the east of 140W.
• An extratropical link through the wave-like
  pattern forced by heating in the central Pacific.

Heating over Continents

• Forcing confined to the continents induces
  anomalous winds over the ocean in the
  interhemispheric distribution only.