150
L.-L. Fu
In the Pacific Ocean, sea surface temperature, wind stress, and ocean circulation
are involved in a decadal variability called the Pacific Decadal Oscillation (PDO).
Qiu and Chen (2009) analyzed 16 years’ worth of altimetry data and noted that
the phase transition of PDO triggered westward baroclinic Rossby waves, which
affected the stability of the Kuroshio Extension upon arrival in that region. Shifting
between stable and unstable regimes, the eddy energy and its interaction with the
Kuroshio Extension is linked to the larger-scale PDO.
Westward propagation of large scale variability is a ubiquitous phenomenon well
documented by numerous papers since the seminal paper by Chelton and Schlax
(1996). Fu and Chelton (2001) provided a review of the subject. The conclusion
then was that the predominant westward propagation was associated with baroclinic Rossby waves (see the Chapter 12 by Cipollini et al., this volume). Fu (2004)
discussed the latitudinal variation of the frequency content of the propagation and
identified cases in which the wave frequency was higher than allowed by the conventional Rossby wave theory. Some of the cases were attributable to barotropic
Rossby waves.
9.4 Large Scale High Frequency Variability
A big surprise when the T/P data were first analyzed was the presence of large-scale
variability of the ocean at periods on the order of 10 days. These turned out to be
barotropic response of the ocean to rapid changes in wind forcing. Chao and Fu
(1995) showed that the observed variability could be simulated by ocean general
circulation models. The results were further confirmed by Fu and Smith (1996). A
modeling study by Fukumori et al. (1998) suggested that up to 50% of the variance
of such large-scale variability could have periods shorter than 20 days, the Nyquist
period of T/P. This raised concerns for aliasing these high-frequency signals to low
frequencies in altimetry data. Model simulations forced by good quality wind were
then used to de-alias the high-frequency signals in altimetry data (e.g., Stammer
et al., 2000; Carrere and Lyard, 2003).
To a large extent the high-frequency variability is influenced by bottom topography. Fu et al. (2001) found a 25-day oscillation of the Argentine Basin over the
Zapiola Rise and explained it as a free barotropic mode of the basin (also see Weijer
et al., 2007). Using a simple wind-driven linear vorticity model, Fu (2003) illustrated
the intraseasonal variability of the Southern Ocean and the North Pacific Ocean
could be explained as a balance between wind stress curl and relative vorticity with
bottom friction. In the Indian Ocean, the highly periodic monsoon wind generates
intraseasonal variability at periods of 180, 120, 90, 75 days (Fu, 2007). Some of
these could be explained as resonant basin modes.
9.5 Mesoscale Eddies
Mesoscale variability was observed even by the GEOS-3 altimeter (Huang et al.,
1978) with a noise level of 25 cm. The strength of the signals and the relatively
L.-L. Fu
In the Pacific Ocean, sea surface temperature, wind stress, and ocean circulation
are involved in a decadal variability called the Pacific Decadal Oscillation (PDO).
Qiu and Chen (2009) analyzed 16 years’ worth of altimetry data and noted that
the phase transition of PDO triggered westward baroclinic Rossby waves, which
affected the stability of the Kuroshio Extension upon arrival in that region. Shifting
between stable and unstable regimes, the eddy energy and its interaction with the
Kuroshio Extension is linked to the larger-scale PDO.
Westward propagation of large scale variability is a ubiquitous phenomenon well
documented by numerous papers since the seminal paper by Chelton and Schlax
(1996). Fu and Chelton (2001) provided a review of the subject. The conclusion
then was that the predominant westward propagation was associated with baroclinic Rossby waves (see the Chapter 12 by Cipollini et al., this volume). Fu (2004)
discussed the latitudinal variation of the frequency content of the propagation and
identified cases in which the wave frequency was higher than allowed by the conventional Rossby wave theory. Some of the cases were attributable to barotropic
Rossby waves.
9.4 Large Scale High Frequency Variability
A big surprise when the T/P data were first analyzed was the presence of large-scale
variability of the ocean at periods on the order of 10 days. These turned out to be
barotropic response of the ocean to rapid changes in wind forcing. Chao and Fu
(1995) showed that the observed variability could be simulated by ocean general
circulation models. The results were further confirmed by Fu and Smith (1996). A
modeling study by Fukumori et al. (1998) suggested that up to 50% of the variance
of such large-scale variability could have periods shorter than 20 days, the Nyquist
period of T/P. This raised concerns for aliasing these high-frequency signals to low
frequencies in altimetry data. Model simulations forced by good quality wind were
then used to de-alias the high-frequency signals in altimetry data (e.g., Stammer
et al., 2000; Carrere and Lyard, 2003).
To a large extent the high-frequency variability is influenced by bottom topography. Fu et al. (2001) found a 25-day oscillation of the Argentine Basin over the
Zapiola Rise and explained it as a free barotropic mode of the basin (also see Weijer
et al., 2007). Using a simple wind-driven linear vorticity model, Fu (2003) illustrated
the intraseasonal variability of the Southern Ocean and the North Pacific Ocean
could be explained as a balance between wind stress curl and relative vorticity with
bottom friction. In the Indian Ocean, the highly periodic monsoon wind generates
intraseasonal variability at periods of 180, 120, 90, 75 days (Fu, 2007). Some of
these could be explained as resonant basin modes.
9.5 Mesoscale Eddies
Mesoscale variability was observed even by the GEOS-3 altimeter (Huang et al.,
1978) with a noise level of 25 cm. The strength of the signals and the relatively