1. Response of the Atmosphere to Climate Variability in the Tropical Atlantic By Alfredo Ruiz–Barradas 1, James A. Carton, and Sumant Nigam University of Maryland --------o-------- AGU Spring Meeting Washington, DC 1 alfredo@atmos.umd.edu May 28–31, 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 (Ruiz– Barradas et al., 2000 ). Regressed anomalies on the time series associated to the tropical Atlantic distributions Observed anomalies of 850/1000 mb thickness and diabatic heating at lower levels (P≥780mb) follow the sea surface temperature anomalies of the interhemispheric and equatorial distributions. Diabatic heating at middle and upper levels (P≤690mb) are shifted following deep convection zones. 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. 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. 3. Local forcing. Control Simulations 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 The induced wind anomalies, via release of latent heat, suggest the existence of positive feedbacks westward of 20°W in both distributions. Sensitivity Analysis Transients have not effect in the deep tropics. Middle and upper heating (P≤690mb) force surface circulation in the subtropics. Surface temperature and lower level heating (P≥780mb) force it in the deep tropics Heating over Continents Forcing confined to the continents induces anomalous winds over the ocean in the interhemispheric distribution only. 4. Extra–basin interactions. Global view of control simulations 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. ENSO’s influence over the tropical Atlantic 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 (Mestas–Nuñez 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 140°W. An extratropical link through the wave-like pattern forced by heating in the central Pacific. 5. Conclusions. 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. 6. References. 1.Kalnay, E. and co  authors, 1996: The NMC/NCAR 40  Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437  431. 2.Mestas–Nuñez, 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. 3.Nigam, S., 1994: On the dynamical basis for the Asian summer- monsoon rainfall-El Nino relationship. J. Climate, 7, 1750-1771. 4.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. 5.Ruiz–Barradas, 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.