Documentation for the Coupled Pacific Ocean Global Atmosphere model
(CPOGA)
The CPOGA is a 4.5-layer reduced gravity Pacific Ocean model
anomaly-coupled to the NCAR AGCM CAM3. The only
modification made to the atmospheric model is a land mask consistent
with the ocean model. The ocean model is nested into the atmospheric
model by updating SSTs in the Pacific basin with output from the ocean
model while all other ocean surfaces are updated using monthly mean
climatological SSTs. The climatological SSTs are a composite of the
annual cycle for the period 1981-2001, using the global HadISST OI
dataset prior to 1981 and the Smith/Reynolds EOF dataset post-1981. A
land surface model calculates fields over land (the NCAR CLM2). The
atmospheric model is run at T42 resolution with 26 vertical levels (the
standard resolution for CAM3). The ocean model extends from 50°S to
60°N and is run at 1 deg X 0.5 deg resolution.
The ENSO variability of the
updated model is consistent with the documentation provided below.
The ocean model is first run using the CAM3 boundary layer model
forced with climatological monthly mean surface air
temperature and winds. The main parameters that are varied to get a
realistic
seasonally-varying climate are ABL, Ocean Mixing, Ocean Diffusion.
Two 150-year CPOGA integrations, with and without air-sea coupling in
the extratropical Pacific have been completed.
The first simulation is coupled from 55°S to 50°N in the
Pacific basin and is referred to as the control integration (CNT). The
second simulation is coupled from 15°S to 15°N in the tropical
Pacific and is referred to as the tropics-alone integration (TP). In
the extratropical Pacific Ocean, TP is forced by heat fluxes and wind
stress calculated from climatological surface air temperature and wind
(atmospheric anomalies are not passed to the ocean).
The control integration (CNT) compared to Observations
The ocean model consists of four active layers overlying a deep, inert
layer of temperature T5=0ºC where the pressure gradient field is
assumed to vanish. Each of the layers corresponds primarily to a single
water mass type, namely, mixed layer water
(T1), upper-thermocline water (T2), lower-thermocline
water (T3), upper-intermediate water (T4) in layers-1, -2, -3, and
-4,
respectively. These layers have thicknesses; H1, H2, H3,and H4. The two
figures compare the climatological seasonal means
from CNT to LEVITUS94 observations. Observed ocean temperatures in the
lower-thermocline are estimated as the average ocean temperatures
between H1+H2 and H1+H2+H3.
Four active layers allows for the simulation of both the Tropical Cell,
where water recirculates within the tropics in layers-1 and -2, and the
Subtropical-Tropical Cell, where water of extratropical origin flows to
the equatorial thermocline (layers-2 and -3). Meridional transport in layer-2 shows
the equatorward transport of water within 5 degrees of the equator, the
lower
branch of the Tropical Cell.
Meridional transport in layer-3 shows
that subtropical water is transported to the tropics from both the
North and South Pacific. Note the poleward transport in the eastern
subtropical North Pacific in both CNT and observations that blocks the
transport of water to the
equator through an interior pathway. Observed transports are taken
from monthly mean NOAA Pacific Hindcast data from
1980-2004. Observed meridional transports are calculated from
meridional velocities integrated over the depth of layers-2 and -3,
H2 and H3, respectively.
Mixed-layer physics is parameterized using a Kraus-Turner bulk mixed
layer model (Kraus and Turner 1967, Tellus). The adequacy of this
parameterization in simulating the mixed layer temperature of a large
eddy simulation model was demonstrated by Wang (2003, GRL). CPOGA MAM depth of
first model layer, H1, compared to observed LEVITUS94 May mixed layer depth
(the depth of the first CPOGA layer is plotted. Model mixed layer depth
is deeper in regions where T1 is approximately equal to T2).
CPOGA simulates the observed "re-emergence" of wintertime sea surface
temperature anomalies in the North Pacific. This is seen in lead-lag correlations between
temperature anomalies in the summertime seasonal thermocline and
temperature anomalies between the surface and 150
meters. Correlations are shown for the western North Pacific
(38-42°N, 160-180°E),
central North Pacific (26-42°N, 164-148°W), and eastern North
Pacific
26-42°N, 132-116°W). Reemergence is
clearly seen in all three regions with correlations similar to those
observed
(Figure 12 Alexander et al. 1999, JC). Correlations greater than 0.7
move downward in time, suggesting that
temperature anomalies are being subducted into the permanent
thermocline.
Transport streamlines in layer-2 (red
for transport initiated in the North Pacific between 15-30°N, black
for
transport initiated in the South Pacific between 30-15°S) show the
"potential
vorticity barrier" in the North Pacific that blocks transport to the
equator through the interior pathway, forcing water the flow around
this barrier. To the west of these streamlines, water flows to the
western boundary and then to the equator. To the west of these
streamlines, water recirculates within the subtropcal gyre and never
reaches the tropics. In the South Pacific the flow of water to the equator
through interior pathways is unobstructed. Transport streamlines in layer-3 show that water transported from
the subtropical South Pacific crosses the equator while water
transported from the subtropical North Pacific reaches the Tropics but
never flows to the equator in this layer. These streamlines capture
the observed differences between pathways in the North and South
Pacific, as well as, differences in transport initiated in the eastern and
western North Pacific (e.g. Liu and Huang 1998, JPO).
The ocean model includes a mean inflow of 10 Sv (1 Sv = 106 m3s-1)
in layer-4 in the southwest corner of the basin and an outflow of 10 Sv
divided evenly among layers-1 through layer-3 at the western boundary
between 4N-8N to simulate the Indonesian Throughflow. This is required
to produce mean transport pathways where intermediate water enters the
South Pacific from the Southern Ocean, crosses the equator, and exits
the basin in the Indonesian Throughflow, potentially by circuitous
paths.
The NINO3 index is calculated by averaging SST anomalies over
(150-90°W, 5°S-5°N).
CNT has significant power on a broad
range of interannual timescales
similar to observations. The observed
NINO3 index was constructed by the NCAR Climate Analysis Section. A
wavelet analysis using 50 years from the CNT integration is plotted to
allow for direct comparison with observations.
The spatial structure of ENSO is seen in Regressions
of SST anomalies
in the tropical Pacific to the CNT NINO3 timeseries. The spatial
structure of El Niño in the CNT integration, seen at zero lag in
the upper row in Figure 3, is similar to observed with anomalies
extending to the eastern boundary and south along the South American
coast, as well as, in the off-equatorial South Pacific. Also similar to
observations, the La Niña or neutral conditions extend for
approximately three years after an El Niño event and have
smaller amplitude.
The Tropics-alone run (TP) compared to the control integration (CNT)
Limiting air-sea feedbacks to the tropical Pacific has little impact
on the seasonal mean ocean temperatures; mixed
layer water (T1), lower-thermocline water
(T3), and meridional
transports; layer-2 (VH2), layer-3 (VH3).
The impact of excluding extratropical coupling on the frequency of ENSO
can be readily seen by comparing the CNT
and TP NINO3 time averaged wavelet
spectrum.
When extratropical coupling is excluded the period of ENSO is
essentially locked at 2 years.
Regressions of SST anomalies in the tropical
Pacific to the TP NINO3 timeseries demonstrate that the spatial
structure of ENSO has changed as well. However, when extratropical
coupling is excluded, SST anomalies at the peak of an ENSO event
extend farther to the west on the equator, are more meridionally
confined in the central equatorial Pacific, are absent along the South
American coast, and occur with a predominantly two-year period.
Even though the frequency and spatial structure of ENSO changes when
extratropical coupling is excluded, the climate mean SSTs are
relatively unchanged.
TP vs CNT Sea surface
temperatures. The main difference is that wintertime SSTs in the
eastern South Pacific are colder when air-sea coupling is allowed in
this region. In addition, summertime SSTs are warmer in the western
North Pacific when air-sea feedbacks are allowed in this region.