[Mitgcm-support] Wind adjustment and resulting sea level change

[mitgcm-support at dev.mitgcm.org]

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What adjustments of control variables that make the model to achieve the
improved fit to T/P sea level anomalies as well as TOGA-TAO temperature
and velocities? Analysis shows that the change of the estimated wind
stress from the prior values plays the dominant role. The change in
10-day averaged zonal wind stress in the tropical Pacific for the middle
of each month (in 1997) is shown in this plot[1] (these changes are
relatively small comparing to the total that's not shown). The
difference in 10-day averaged sea level between assimilation and
simulation is shown here[2] . Two features are evident: (1) Near 10N
(North Equatorial Current), the "string" of "red bumps" in the
correction of wind stress in the first few months of the year correspond
well to those in sea level, i.e., change in wind stress results in
equatorial Rossby-wave like feature in sea level there. (2) Near the
western-central equatorial Pacific, there is a large positive change in
zonal wind stress, which corresponds to the built-up of sea level
difference in the central-eastern equatorial Pacific, which, later in
the year, cause some reflection into Rossby waves on both sides of the
equator. The following plots further illustrate the correspondence of
wind correction and difference in sea level between assimilation and
simulation (and thus improves upon the simulation), longitude-time plots
of differences in wind stress curl (due to corrections of zonal and
meridional wind stresses) and sea level are shown for 0N[3] , 5N[4] ,
and 25N[5] in the Pacific. In these plots, panels on the left contain
the time mean, those on the right don't. At the equator, the positive
change in curl near the dateline (Ekman suction) during the first 2-3
months results in eastwards propagation of negative sea level anomaly
(upwelling Kelvin waves). Later in the year, the negative change in curl
(Ekman pumping) results in eastward propagation of downwelling Kelvin
waves. At 5N and 25N, one sees a general tendency of positive (negative)
change in curl resulting in westward propagation of negative (positive)
sea level anomaly (equatorial and mid-latitude Rossby waves,
respectively). There are some features in the sea level difference that
do not quite correspond to the change in curl, these are presumably due
to (1) meridional propagation of Rossby waves generated at other
latitudes, and (2) zonal propagation of Rossby waves reflected from the
eastern boundary. Therefore, accounting for errors in wind stress
properly has a substantial impact on the improvement of the model state
(in sea level, temperature, and current). The difference in subsurface
temperature between the assimilation and simulation, not discussed here,
may also shed light on (1) how many vertical modes is needed in the
Kalman filter assimilation, and (2) whether it's reasonable to simply
project sea level signal to the thermocline like the OI scheme used by
NCEP.

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[1] http://escher.jpl.nasa.gov:2000/hosts/escher/escher4/medea/tlee/FIG_FORUM/xx_tx_maps_trop_Pac.jpg
[2] http://escher.jpl.nasa.gov:2000/hosts/escher/escher4/medea/tlee/FIG_FORUM/h_maps_trop_Pac.jpg
[3] http://escher.jpl.nasa.gov:2000/hosts/escher/escher4/medea/tlee/FIG_FORUM/xx_curl_h_lontim_Pac_0N.jpg
[4] http://escher.jpl.nasa.gov:2000/hosts/escher/escher4/medea/tlee/FIG_FORUM/xx_curl_h_lontim_Pac_5N.jpg
[5] http://escher.jpl.nasa.gov:2000/hosts/escher/escher4/medea/tlee/FIG_FORUM/xx_curl_h_lontim_Pac_25N.jpg