The anomalous southeastward currents during the warm phase and northwestward currents during the cool phase are seen in more detail in the TIW composite shown in Fig. 4a. Thermal wind shear is, by definition, parallel to the front, with cool water to the left and warm water to the right of the shear. The mean thermal wind shear composite is strongest and oriented to the southwest on the leading edge of the TIW cusp as the TIW transitions to its cool phase (Fig. 4c). At this stage of the wave, the southeasterly trade winds are most oblique to the front. During the subsequent cool phase, the mean thermal wind shear is somewhat westward. Then, during the transition to the warm phase of the TIW, on the trailing edge of the cusp, the thermal wind shear becomes northwestward. At this stage of the wave, the winds are nearly parallel to the front.
Fig. 4. Mean tropical instability wave composite (1 Nov 2004–28 Feb 2005) of (a) anomalous temperature (color), anomalous velocity (black vectors), and wind (blue vectors); (b) temperature relative to 28.3 m (color) and velocity relative to 20 m (black vectors); (c) as in (b) but for geostrophic currents relative to 20 m (green vectors); and (d) as in (b) but for ageostrophic currents relative to 20 m (brown vectors). The vector scale is shown at bottom.
During the warm phase, the southeasterly trades are more zonal and observed shears are nearly westward; while during the cool phase and the transition to the warm phase, observed shears are oriented to the northwest, roughly in the direction of the southeasterly trades (Fig. 4b). The small increase in the easterly trade winds prior to the transition to the TIW cool phase has been noted by several authors (e.g., Chelton et al. 2001). For most of the TIW cycle, however, the winds are steady.
On the basis of the classical Ekman model (4), one might expect that, if the winds are steady, the ageostrophic shears should be steady too. However, the ageostrophic shears have a strong TIW signal, with fluctuations of roughly ±4 cm s−1 over 15 m (Fig. 4d). In particular, the mean ageostrophic shear is largest and oriented northwestward during the transition to the cool phase when the geostrophic thermal wind is oriented southwestward, oblique to the wind. The ageostrophic shear also is relatively large and oriented in the direction of the wind during the transition to the warm phase when the wind blows along the front and the stratification is strong.
The observed currents relative to 20 m tend to be aligned with the wind as expected from the surface boundary condition (2b). Thus, in contrast to the large TIW swings in the geostrophic and ageostrophic shears, the observed currents relative to 20 m (Fig. 4b), like the winds, have a relatively steady orientation throughout most of the TIW. The magnitude of the shear, however is not steady; instead, its variation appears to be associated with changes in the stratification. The observed flow relative to 20 m (Fig. 4b) is weakest during the warm phase on day 5 when the stratification is weakest and has a local maximum when stratification is strongest: on day 15 during the cool phase and on day 24 during the transition to warm phase. Mixing and restratification likely affect the turbulent viscosity of the fluid and thus the shear that can be supported for a given stress. We note, though, that the maximum composite stratification is less than 0.07°C over 27.3 m, and that by most standards the upper 20 m would be considered always well-mixed throughout the TIW cycle.
In summary, as expected from the surface boundary condition (2b) with relatively steady wind stress, variations in the near-surface shear appear to be primarily due to variations in the turbulent viscosity as inferred from the stratification. Both geostrophic and ageostrophic components contribute strongly to the total shear with the geostrophic component determined by the orientation and strength of the front. The ageostrophic shear thus arises from the imbalance between the wind stress and the stress associated with the thermal wind shear. When the winds are nearly aligned with the front during the transition to the warm phase (on the trailing edge of the TIW cusp), the thermal wind shear is too weak to balance the wind stress, resulting in a near-surface residual ageostrophic shear in the direction of the wind. Likewise, the front, and thus the thermal wind shear, is strongest and most oblique to the southeasterly trade winds during the transition from the warm to cool phase (on the leading edge of the cusp). Consequently, a large northward ageostrophic shear must be combined with the thermal wind shear to satisfy the surface stress boundary condition (2b). The ageostrophic shear depends on the orientation and relative strength of the wind to the front rather than strictly upon the wind itself.
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