Capotondi, A., and M. A. Alexander, 2001: The influence of thermocline topography on the oceanic response to fluctuating winds: A case study in the tropical North Pacific. In Advances in Mathematical Modelling of Atmosphere and Ocean Dynmanics, P. F. Hodnett (Ed.), Kluwer Academic, 119-124.


Thermocline processes likely play an important role in climate variability at decadal timescales. Surface anomalies subducting at midlatitudes propagate in the main thermocline toward the tropics with a timescale close to decadal. Thermocline variability is also associated with baroclinic Rossby waves, a fundamental agent in the adjustment of the ocean circulation, whose propagation time can introduce a delay of several years in the oceanic feedback to atmospheric forcing.

In the Pacific the thermocline exhibits three centers of variability, as indicated by temperature variance at 200m depth, one in the Kuroshio extension around 40°N, the second between 10°N and 15°N, and the third around 10°S. In this paper we will focus on the variability in the 10°-15°N latitude band (Figure 1). What is special about this region? Is the enhanced variance associated with a stronger surface forcing, or is the ocean response more pronounced in this area? Capotondi and Alexander (2000, CA hereafter) have analyzed the variability at 10°-15°N using the results from the National Center for Atmospheric Research (NCAR) ocean General Circulation Model (GCM) (Gent et al. 1998) referred to in Figure 1b. CA have shown that the band of enhanced variance around 13°N is associated with Rossby waves forced by anomalous Ekman pumping east of approximately 180°, while west of this longitude the waves appear to be freely propagating. Although several aspects of the waves can be explained in terms of characteristics of the surface forcing, the reason for the particularly large variance in the 10°-15°N latitude band remains unclear. In this paper we examine the possibility that spatial changes in mean thermocline depth may be responsible for the amplitude of the oceanic response to variable wind forcing. We will see that meridional movements of the thermocline can account for a large fraction of the temperature variability at 13°N.

This paper is organized as follows: in section 2 we summarize the major findings of CA concerning the nature of variability at 13°N, while in section 3 we examine the importance of the thermocline "topography" for determining the amplitude of the variability. We conclude in section 4.