Modulation of Cross-Isothermal Velocities with ENSO in the Tropical Pacific Cold Tongue

Abstract

The tropical Pacific cold tongue (CT) plays a major role in the global climate system. The strength of the CT sets the zonal temperature gradient in the Pacific that couples with the atmospheric Walker circulation. This coupling is an essential component of the El Niño Southern Oscillation (ENSO). The CT is supplied with cold water by the equatorial undercurrent that follows the thermocline as it shoals toward the east, adiabatically transporting cold water towards the surface. As the thermocline shoals, its water is transformed through diabatic processes producing water mass transformation (WMT) that allows water to cross mean isotherms. Here, we examine WMT in the cold tongue region from a global high resolution ocean simulation with saved budget terms that close its heat budget exactly. Using the terms of the heat budget, we quantify each individual component of WMT (vertical mixing, horizontal mixing, eddy fluxes, solar penetration), and find that vertical mixing is the single most important contribution in the thermocline, while solar heating dominates close to the surface. Horizontal diffusion is much smaller. During El Niño events, vertical mixing, and hence cross-isothermal flow as a whole, is much reduced, while during La Niña periods strong vertical mixing leads to strong WMT, thereby cooling the surface. This analysis demonstrates the enhancement of diabatic processes during cold events, which in turn enhances cooling of the CT from below the surface.

Seasonal Modulation of Dissolved Oxygen in the Equatorial Pacific by Tropical Instability Vortices

“…Figure 1 compares the climatological annual and monthly mean surface velocities and their meridional shear in CESM‐LR and CESM‐HR to the NOAA Global Drifter Program product (Laurindo et al., 2017). In both configurations, CESM simulates relatively well the structure and location of the westward flowing SEC and NEC, and eastward flowing NECC (Figures 1a–1c), though the magnitude of the NECC is noticeably weaker in both simulations, likely due to deficiencies in the wind forcing as also found in a recent eddy‐resolving simulation of CESM (Deppenmeier et al., 2021). We also note stronger shear along the equator in CESM‐LR compared to CESM‐HR associated with more defined north and south SEC branches at the lower resolution.…”

The Impact of Indonesian Throughflow Constrictions on Eastern Pacific Upwelling and Water‐Mass Transformation

“…At temperatures above 21°C, strong cool‐to‐warm WMT due to surface forcing dominates (red dotted line in Figure 7b), partially offset by vertical and numerical mixing‐driven cooling. This regime represents transformation occurring near the surface, particularly in the eastern equatorial Pacific upwelling regions, where surface heat fluxes and mixing‐driven transformation are strong and largely opposing (Deppenmeier et al., 2021; Holmes, Zika, England, 2019; Holmes, Zika, Ferrari, et al., 2019; Huguenin et al., 2020; Moum et al., 2013). An equatorial longitude‐depth slice of Control diathermal velocities due to surface fluxes illustrates a strong warming transformation above the thermocline in the eastern equatorial Pacific (Figure 8a).…”

Frequency Dependence of Ocean Kinetic Energy and Respective Changes Over the Period 1983–2018

“…We cite here a few details of the model configuration of specific relevance to the present study. Further model configuration details and applications to salinity budget and energy studies can be found in Johnson et al (2016), Köhler et al (2018), Deppenmeier et al (2021), andGuo et al (2022). An overview of the performance of the simulation relative to other high-resolution ocean models integrated under the same protocol can be found in Chassignet et al (2020).…”