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A comparison of the Atlantic EUC at N,
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23 W simulated by two recent versions of
the Brazilian Earth System Model E. Giarolla*, P. Nobre, M. Bottino, L. Siqueira, M. Malagutti *Ins&tuto Nacional de Pesquisas Espaciais. Av. dos Astronautas, 1758, São José dos Campos, SP, 12227-­‐010, Brazil. e-­‐mail: [email protected] Abstract: This study compares the Atlantic Equatorial Undercurrent (EUC) simulated by two versions of the Brazilian
Earth System Model (BESM). The mean seasonal variation of the zonal current is represented in a time–depth diagram
at approximately 0o N, 23o W, because at this location observed current data from a PIRATA ADCP (“Acoustic Doppler
Current Profiler”) is available. Considering the EUC core as the region of maximum positive values of zonal current, the
CTRL experiment (with the older version of the model) and the NCCS experiment (with the newer version) can
reproduce relatively well the seasonal depth migration of the core. However, in both models, the EUC core is deeper
than observations in March-April. In November, the CTRL experiment core depth is closer to the observed one by the
ADCP. But apart from this, other features show that the NCCS experiment simulates better the zonal current profile. 1. INTRODUCTION The first version of the Brazilian Earth System Model (BESM), a cooperative effort of many institutions in Brazil, was called OceanAtmosphere 2.3 (OA2.3). This version was submitted to the CMIP5 (“Coupled Model Intercomparison Project”, phase 5) community, and
documented in Nobre et al. (2013, J.Clim 26, doi:10.1175/JCLI-D-12-00580.1).
The BESM has continuously been upgraded. Bottino and Nobre (2013, in prep.) later revised the cloud cover scheme and optical property
coefficients of the OA2.3 version, changing the radiative balance of the model, and this new version was then named OA2.3.1. The
changes brought significant improvement in the atmospheric circulation and precipitation over the tropical Atlantic, as discussed by
Bottino and Nobre (2013), but some improvements in the ocean circulation were noticed as well, possibly linked to the shift of the wind
pattern, on the Atlantic Ocean, into a more realistic position by the new version. This study compares the Equatorial Undercurrent (EUC),
of those two versions of the BESM, with observations from an ADCP (“Acoustic Doppler Current Profiler”) of the buoy at 0o N, 23o W.
2. MODEL AND DATA SETS The BESM OA2.3 is described in details in Nobre et al. (2013). The oceanic component is the MOM4p1 from the Geophysical Fluid
Dynamics Laboratory (NOAA/GFDL). The horizontal grid resolution varies uniformly:1o long x 1/4o lat in the tropical region (10o S – 10o
N);1o long x 1o lat at 45o S/N;1o long x 2o lat at 90o S/N. In the vertical axis, 50 levels, increasing gradually:10-m resolution in the upper
220 m; 370-m at deeper regions. Two sets of experiments were performed with constant atmospheric CO2, a “control” set (CTRL), with
the OA2.3 version, and the NCCS (from “new cloud cover scheme”) set, with the OA2.3.1 configuration. The first 30 years, of an
ensemble member which ran for 100+ years starting from 2007, were used. The model results are compared to observations from the
PIRATA current data at 0o N, 23o W, GODAS ocean reanalysis, and the available experiments of two known CMIP5 models, the NCAR’s
(“National Center for Atmospheric Research”) CCSM4, and the Hadley Centre’s HadCM3, both starting around 1960-1961.
3. RESULTS Figure 1: Time-depth diagrams of seasonal zonal current at 0o N, 23o W. Contours represent (a) CTRL
and (b) NCCS experiments, computed from simulation years 2-30; (c) and (d) are the CCSM4 and
HadCM3 counterparts respectively. The shading colors in all diagrams were obtained from available
ADCP records of the PIRATA buoy moored at the same location, from 2002 to 2009. Values in cm s-1.
Both CTRL (Figure 1a) and NCCS (Figure
1b) simulations reproduce the seasonal
depth migration of the EUC core, despite a
weaker and deeper than observed EUC
simulation during March-April. In the CTRL
experiment, the deeper eastward currents
anomalously revert from mid-April to midAugust, reaching levels up to 90 m
approximately. Both experiments do not
precisely represent the surfacing of the
EUC, which is observed between midMarch to April. In the CTRL experiment it
starts earlier, in the beginning of March,
while in NCCS experiment the surfacing
happens after mid-April. Nevertheless, the
time period of the EUC surfacing, about
1.5 months, is better simulated by the
NCCS experiment, while it lasts more than
4 months in the CTRL experiment. The
CCSM4 model has the best representation
of the EUC seasonal variation and core
depth (Figure 1c). But the undercurrent
surfacing is delayed and lasts approximately 3 months. Finally, the EUC seasonal variation of the HadCM3 experiment has a spurious
intrusion of westward (negative) flow from deeper levels, from January to June. From June to December, a more realistic simulation of
the EUC core is noticed, though with weaker magnitudes.
The mean annual Atlantic EUC along the longitudes is shown in
Figure 2. Comparing both BESM experiments, the NCCS EUC core
(Figure 2b) is deeper and stronger than the CTRL one (Figure 2a).
In contrast, the CCSM (Figure 2c) and HadCM3 (Figure 2d) models
have the strongest and the weakest EUC cores respectively. It is
noticed that none of the models compared in this study shows a
narrow and strong EUC core such as the one obtained with the
GODAS ocean reanalysis (Figure 2e), which shows its maximum
approximately between 35o and 25o W, at 90 m depth.
4. CONCLUSIONS The changes, in the cloud cover scheme and optical properties of
the BESM OA2.3.1, brought significant improvement in the
atmospheric circulation and precipitation over the tropical Atlantic,
and these improvements in their turn resulted in a more realistic
simulation of the Atlantic EUC seasonal variability, as discussed in
this work with Figures 1 and 2.
Figure 2: Annual mean of the zonal current profile at the equatorial Atlantic,
for (a) CTRL, (b) NCCS, (c) CCSM4 and (d) HadCM3 model experiments
(computed between years 2–30), and for GODAS analysis (2001-2009, the
same period as the PIRATA ADCP). Only positive values were drawn, in m s-1.