Impacts of Regional Mixing on the Temperature Structure of the Equatorial Pacific Ocean. Part 2: Depth-Dependent Vertical Diffusion

Abstract:

In this study, we use an ocean model to explore how vertical mixing influences temperature in the eastern equatorial Pacific Ocean (EEPO). Our approach is to change the background diffusion coefficient from a constant value $\kappa_{b}$ to $\kappa_{b}+\delta\kappa(z)$ in various subregions of the tropical Pacific, and then to determine the resulting temperature changes in the near-equilibrium response. In a companion paper (Furue et al., 2014, submitted), we consider the impacts of depth-independent $\kappa_b$ anomalies. Here, we examine the impacts of depth-dependent anomalies that are confined above, or centered on, the mid-depth of the pycnocline.

During the first year of adjustment, solutions develop a local temperature response that results largely from the one-dimensional balance, $\delta T_{t} = (\delta\kappa T_{z})_{z} = \delta\kappa_{z}T_{z}+\delta\kappa T_{zz}$, with a similar equation for salinity. At this stage, $\delta\kappa$ generates temperature and salinity anomalies that are either associated with a density change (dynamical anomalies) or without one (spiciness anomalies). Subsequently, dynamical and spiciness anomalies spread to remote regions by wave radiation and advection, respectively.

For positive $\delta\kappa$ anomalies confined above the mid-pycnocline, $\delta\kappa_{z}T_{z}$ ($\delta\kappa T_{zz}$) tends to produce positive (negative) temperature anomalies, which spread to the equator dynamically (by wave radiation) and are still apparent in near-equilibrium solutions. For $\delta\kappa$ anomalies confined within the pycnocline (with monopole, dipole, and tripole profiles), the response is dominated by $\delta\kappa_{z}T_{z}$, owing to $\delta\kappa$ having a smaller vertical scale than $T$ and to $\delta\kappa$ not overlapping well with $T_{zz}$; the resulting temperature anomalies tend to shift the pycnocline vertically (dipole profile) or to alter its thickness (monopole and tripole profiles).

Positive anomalies from all subregions contribute to an increase of near-surface (upper 50 m) temperature in the EEPO, the amplitude and location of the warming depending on the depth range of the warming band generated within the locally forced subregion.