Abstract
Vertical mixing is an important factor in determining the temperature, sharpness and depth of the equatorial Pacific thermocline, which are critical to the development of El Ninõ and Southern Oscillation (ENSO). Yet, properties, dynamical causes and large-scale impacts of vertical mixing in the thermocline are much less understood than that nearer the surface. Here, based on Argo float and the Tropical Ocean and Atmosphere (TAO) mooring measurements, we identify a large number of thermocline mixing events occurring down to the lower half of the thermocline and the lower flank of the Equatorial Undercurrent (EUC), in particular in summer to winter. The deep-reaching mixing events occur more often and much deeper during periods with tropical instability waves (TIWs) than those without and under La Niña than under El Niño conditions. We demonstrate that the mixing events are caused by lower Richardson numbers resulting from shear of both TIWs and the EUC.
Highlights
Vertical mixing is an important factor in determining the temperature, sharpness and depth of the equatorial Pacific thermocline, which are critical to the development of El Ninoand Southern Oscillation (ENSO)
Measurements and model studies suggests that turbulence and mixing below the mixed layer base of the equatorial Pacific are attributed to the vertical velocity gradient between the eastward flowing Equatorial Undercurrent (EUC) and the westward flowing South Equatorial Current[7,8,9,10,11,12,13,14,15], which is likely to be further modulated by the wind stress[16,17,18]
The instabilities in the lower part of the thermocline may be caused by absorption and saturation of wave energy at critical levels[19], whereas the mixing in the upper part of the thermocline is found to be related to baroclinic inertialgravity waves[20], Kelvin waves[14] and, in particular, the tropical instability waves (TIWs)[13]
Summary
Vertical mixing is an important factor in determining the temperature, sharpness and depth of the equatorial Pacific thermocline, which are critical to the development of El Ninoand Southern Oscillation (ENSO). The vigorous deepreaching mixing are attributed to additional shear provided by the meridional velocity of the TIW above the EUC core[13] If this identified relationship between TIW and enhanced deep thermocline mixing is largely representative, it implies that after a long duration of TIWs the associated thermocline mixing may have the potential to alter the structure of the thermocline and the subsurface temperature of the Pacific cold tongue, which may further have an impact on the large-scale oceanic-atmospheric dynamics, such as El Nino and Southern Oscillation (ENSO)[6] and the global climate at large[33]
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