9 Determining Ocean Circulation and Sea Level from Satellite Altimetry
149
strengths, etc.) exhibit discrepancies. The dynamic mechanisms for the striations
are not yet well understood. Using a kinematic model of randomly distributed field
of eddies, Schlax and Chelton (2008) showed that such striations could be caused
by the migration of eddies that were not completely averaged out over a finite time
period. The residual speed of the eddy currents after 10-year averaging is on the
order of 1 cm/s, comparable to the results of Maximenko et al. (2008). As the residual currents go down with 1/T, where T is the averaging time, a data set of multiple
decades is needed to average out the eddy effects.
A testimony of the quality of the surface mean dynamic topography of Niiler
et al. (2003) was the study of the vorticity balance of the Antarctic Circumpolar
Current (ACC) by Hughes (2005). Estimating the vorticity of the flow by differentiating the dynamic topography, Hughes (2005) discovered two modes of flow
behaviors: Meanders in which a balance was achieved between the advection of relative and planetary vorticity as in a stationary equivalent-barotropic Rossby wave,
and a flow in which the advection of total vorticity was related to bottom topographic
steering.
9.3 Large Scale Low Frequency Variability
A series of El Niño Southern Oscillation events in the 1990s including the phenomenal event of 1997–1998 provided a focus for demonstrating the power of satellite
altimetry to study large-scale climate variability. Fu and Smith (1996) demonstrated
an early comparison of altimetry observation with a model simulation of a Pacific
warming event. The success of satellite altimetry in providing global ocean observations was a major motivation for the advancement in global ocean modeling and
data assimilation in the 1990s (Stammer et al., 1996, 2002). This development has
establish a new framework for performing ocean reanalysis using modern state estimation approach by integrating data from a observing network into ocean general
circulation models (Wunsch et al., 2009).
As the altimetry data record extended into its second decade, oceanographers
for the first time had a continuous global data set for studying ocean variability
beyond the seasonal-to-interannual scales. Hakkinen and Rhines (2004) reported
a slow-down of the subpolar gyre circulation of the North Atlantic Ocean from
analysis of T/P data in combination with earlier altimeter data. They attributed
this change to weakened thermohaline forcing. Based on satellite altimetry data
in combination with a variety of in-situ observations, Roemmich et al. (2007)
discovered a decadal intensification of the subtropical gyre of the South Pacific
Ocean from 1993 to 2004. The gyre circulation increased by 20%, resulting from a
decadal strengthening of wind forcing east of New Zealand as part of a circumpolar
change of climatic state. On the other hand, Lee (2004) found that the upper ocean
overturning circulation of the Indian Ocean decreased by 70% from 1992 to 2000,
caused by the weakening of the trade winds. Subsequently Lee and McPhaden
(2008) found a larger-scale linkage of the decadal variability of the Indian and
Pacific Oceans.
149
strengths, etc.) exhibit discrepancies. The dynamic mechanisms for the striations
are not yet well understood. Using a kinematic model of randomly distributed field
of eddies, Schlax and Chelton (2008) showed that such striations could be caused
by the migration of eddies that were not completely averaged out over a finite time
period. The residual speed of the eddy currents after 10-year averaging is on the
order of 1 cm/s, comparable to the results of Maximenko et al. (2008). As the residual currents go down with 1/T, where T is the averaging time, a data set of multiple
decades is needed to average out the eddy effects.
A testimony of the quality of the surface mean dynamic topography of Niiler
et al. (2003) was the study of the vorticity balance of the Antarctic Circumpolar
Current (ACC) by Hughes (2005). Estimating the vorticity of the flow by differentiating the dynamic topography, Hughes (2005) discovered two modes of flow
behaviors: Meanders in which a balance was achieved between the advection of relative and planetary vorticity as in a stationary equivalent-barotropic Rossby wave,
and a flow in which the advection of total vorticity was related to bottom topographic
steering.
9.3 Large Scale Low Frequency Variability
A series of El Niño Southern Oscillation events in the 1990s including the phenomenal event of 1997–1998 provided a focus for demonstrating the power of satellite
altimetry to study large-scale climate variability. Fu and Smith (1996) demonstrated
an early comparison of altimetry observation with a model simulation of a Pacific
warming event. The success of satellite altimetry in providing global ocean observations was a major motivation for the advancement in global ocean modeling and
data assimilation in the 1990s (Stammer et al., 1996, 2002). This development has
establish a new framework for performing ocean reanalysis using modern state estimation approach by integrating data from a observing network into ocean general
circulation models (Wunsch et al., 2009).
As the altimetry data record extended into its second decade, oceanographers
for the first time had a continuous global data set for studying ocean variability
beyond the seasonal-to-interannual scales. Hakkinen and Rhines (2004) reported
a slow-down of the subpolar gyre circulation of the North Atlantic Ocean from
analysis of T/P data in combination with earlier altimeter data. They attributed
this change to weakened thermohaline forcing. Based on satellite altimetry data
in combination with a variety of in-situ observations, Roemmich et al. (2007)
discovered a decadal intensification of the subtropical gyre of the South Pacific
Ocean from 1993 to 2004. The gyre circulation increased by 20%, resulting from a
decadal strengthening of wind forcing east of New Zealand as part of a circumpolar
change of climatic state. On the other hand, Lee (2004) found that the upper ocean
overturning circulation of the Indian Ocean decreased by 70% from 1992 to 2000,
caused by the weakening of the trade winds. Subsequently Lee and McPhaden
(2008) found a larger-scale linkage of the decadal variability of the Indian and
Pacific Oceans.