Results of an investigation to study the interaction between ocean and atmosphere on the annual to decadal time scale are reported. Separate studies of the ocean response to atmospheric forcing, and of atmospheric response to ocean forcing were also conducted. The main findings are the dynamics of sea surface temperature anomalies, the role of short time scale weather fluctuations in the seasonal cycle of the upper ocean variability, and the planetary wave response to sea surface temperature anomalies. A numerical model of the ocean atmosphere continent system and a two layer quasi-geostropic ocean model is discussed

Frankignoul, C.

English

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•	 (NASA-C6-173834) MECHANISMS OF INTERANNUAL	 N84-30595
OCLAN -ATMOSPHERR INTEEALTICHS Final
Technical Report (Massachusetts Inst. of
Tech.) 5 p HC A02/MF 801 	 CSCL 04B	 ULclas
63/47 20094
Final Technical Report
to the
National Aeronautics and Space Administration
for the Grant NAG 5-137
entitled
"Mechanisms of Interannual Ocean-Atmosphere Interactions"
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Principal Investigator: Claude Frankignoul
Department of Earth, Atmospheric and Planetary Sciences
Massachusetts Institute of Technology
Cambridge, MA 02139
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Grant NAG 5-137 covered the period 2/1/81- 31/5/83 and was extended at
no cost through 7/31/84. Partial support has been continued under NASA
Grant NGR-22009721.
This research has been undertaken to in-prove our understanding of the
interaction between ocean and atmosphere on the annual-to-decadal time
scale. The main goal was to develop a simplified numerical model of the
ocean-atmosphere-continent system and to perform basic theoretical studies
of the coupling and feedback between the different constituents in the
middle latitudes. Separate studies of the oceanic response to atmospheric
forcing, and of the atmospheric response to oceanic forcing were also
conducted. The main findings are the following:
1. Dynamics of sea surface temperature anomalies. A first study was
aimed at interpreting monthly sea surface temperature anomaly data over the
North Pacific in terms of a stochastically forced oceanic mixed-layer
model. The results are given in:
C. Frankignoul and R. W. Reynolds, 1983. Testing a dynamical model
for mid-latitude sea surface temperature anomalies. JPO, 13, 1131-
1145. Abstract:
A slab model of the oceanic mixed layer is used to predict the
statistical characteristics of the sea surface temperature anomalies
that are forced by day-to-day changes in air-sea fluxes in the
presence of a mean current. Because of the short time scale of the
atmospheric fields, the model validity can be tested without quanti-
tative information on the atmospheric forcing. A procedure is
developed for the case where the mean current is given. It is
applied to sea surface temperature (SST) anomaly data from the North
Pacific using ship drift data as estimates of the mean ocean currents.
At the 95% level of significance the model is consistent with the
data over more than 85% of the investigated region. The results in-
dicate tl.at the atmospheric forcing acts as a white noise forcing; in
regions of large currents, advection effects are important at low
frequencies. However, SST anomaly autospectra are equally well re-
presented by a local model where advection is neglected.
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The available meteorological data are then used to estimate
the forcing due to heat flux and Ekman advection anomalies. This
forcing compares well with the stochastic forcing estimated from the
SST data over most of the North Pacific. It is found that heat flux
anomalies play a more important role than advection by anomalous
Ekman currents; direct wind forcing and the resulting mixed-layer
depth variability seem important at high latitudes but could not be
estimated here. Finally, the cross-correlations between the SST
anomaly and the atmospheric forcing fields are consistent with the
stochastic forcing model and suggest that heat exchanges also contrib-
ute to the SST anomaly damping, thereby acting as a negative feedback.
A second study was aimed at determining the role of the short time
scale weather fluctuations in the seasonal cycle of the upper ocean
variability. A numerical one-dimensional mixed-layer model was used for
long-term simulations of the oceanic conditions at Weathership N (30*N,
140*W) during a 17-year period. This work, which was done in collaboration
with M. Cane, MIT, has been completed and a paper is in preparation. The
main findings are: (1) to simulate correctly the seasonal cycle, the
short-time scale weather fluctuations must be taken into account
explicitly, (2) changes in surface heat flux and wind stress are correlated
and induce similar mixed-layer changes, (3) the weather forcing acts as a
vertical diffusion for the averaged seasonal changes, but true diffusion is
needed for long-term simulations, and (4) advection plays a role at low
frequencies.
2. Planetary wave response to sea surface temperature anomalies. The
linear wave response to sea surface temperature anomalies in the
midlstitudes has been investigated analytically, using a simple -plane
model. The emphasis was on the relationship between sea surface
temperature and heating anomalies, and on the back interaction of the
atmosphere onto the ocean. This work is being continued under grant
NGR-22--9727 and a paper entitled "Sea surface temperature anomalies,
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planetary waves and air-sea feedback in the middle latitudes" is in
preparation. The results suggest in particular that the forced atmospheric
response to typical sea surface temperature anomalies should be of moderate
magnitude, and that it should contribute to the damping of the oceanic
anomalies. At low wavenumbers, the damping should be weak and the
planetary wave should act as an eastward propagator. At high wavenumbers,
the damping should be st-ong. This is consistent with the observed scale
dependence of the persistence of sea surface temperature anomalies.
3. Numerical model of the ocean-atmosphere continent system. A first
version of a simplified midlatitude ocean-atmosphere model has been
designed in collaboration with E. Kalnay and K. C. Mo, NASA/Goddard Space
Flight Center, Greenbelt. 1L includes a two-layer quasi-geostrophic
atmospheric model, a continent and a copper plate oceanic mixed layer. The
model is on a a-plane, has dimension 10,000 x 9,000 km, and is periodic in
the zonal direction. The system is forced by seasonally changing solar
radiation, and the atmospheric thermodynamics is related to the oceanic
temperature via a simple parameterization of the air-sea heat exchanges. A
mountain has been included in some of the simulations. Several five-year
runs of the model have been made in a variety of conditions to investigate
the effect of geometry and land-sea contrast (land only, ocean only, ocean
and '-ind with and without mountain). It has been found that the presence
of land to the north of the ocean had an overwhelming influence on the
position and strength of the jet stream, and that a more realistic seasonal
cycle was achieved by using an ocean covering the eastern half of the
domain. 7 4ever, the model is not baroclinically unstable under these
latter circumstances, for realistic solar radiation. Since the short time
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scale variability of the atmospheric fluxes will be the primal source for
the climate variability in the model, further tuning is needed.
In parallel, a two-layer quasi-geostrophic ocean model has been
designed. The model will respond to the wind forcing and it will be used
to advect the oceanic mixed-layer. Thus, oceanic and atmospheric
circulations will be coupled on the gyre time scale. The oceanic model has
been tested with different spatial resolution and it is ready for coupling
to the atmospheric component. This work is being continued under grant 	 , t
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NGR-22009"27.
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Mechanisms of interannual ocean-atmosphere interactions

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