Surface Divergent Eddy Heat Fluxes and Their Impacts on Mixed Layer Eddy‐Mean Flow Interactions

Abstract

In this study, the global surface divergent eddy heat flux (EHF) is estimated using remote sensing observations of sea surface height (SSH) and sea surface temperature (SST) over two decades (1993–2017). These results are used as a metric to assess model fidelity in a mesoscale eddy‐resolving version of the Community Earth System Model. The estimated EHFs show that the midlatitudes significantly contribute to the poleward transport of heat because of strong regional variability in SSH and SST. A Helmholtz decomposition is performed on the global EHFs to remove prominent nondynamic rotational wave‐like structures that appear in these fluxes. The dynamic divergent EHFs are responsible for driving cross‐frontal exchange and positive Baroclinic Conversion (BC) rates, suggesting a conversion of mean potential energy to eddy potential energy within the mixed layer. The results show that the model captures the same spatial patterns of EHFs and BC rates, but with relatively higher values in the midlatitudes than observations. The mixed layer geostrophic meridional eddy heat transport reaches maximums of 0.07 PW and 0.1 PW in the midlatitudes in the observations and climate model, respectively. The global integrated BC rate is 0.11 TW in the observations, which is ∼30% weaker than the climate model (0.16 TW). A cross‐spectral analysis further shows that the model has higher energy in low‐frequency bands for periods greater than 10 months in the northern hemisphere western boundary currents, but can capture the major spectral peaks in EHFs that are seen in observations.

Increasing Trends in Oceanic Surface Poleward Eddy Heat Flux Observed Over the Past Three Decades

“…However, the weakened EHF magnitude found in this region, which is mostly in the upstream region, is generally away from the jet core; and is almost entirely attributable to the negative signal that appears in the term associated with the $${\overline{{v}^{\prime 2}}}^{m}$$ trend (Figure 4g). The uniformly increasing signal in the $${\overline{{T}^{\prime 2}}}^{m}$$‐related term (Figure 4f) is generally concentrated in the north of the jet path and aligns with locations where strong dynamic baroclinic energy conversion (Guo & Bishop, 2022) and thermal dissipation of EPE due to air‐sea interactions (Bishop et al., 2020) occur. These enhanced $${\overline{{T}^{\prime 2}}}^{m}$$ hotspots offset the jet core and contribute to the increasing trend in EHF.…”

“…However, it remains unclear how EHF has changed in response to the observed increasing trend in surface EKE in recent decades. In this study we examine the trends in EHF throughout the altimeter record (1993–2020), following the methods outlined in Abernathey and Wortham (2015) and Guo and Bishop (2022) for estimating ocean surface EHF from altimeter geostrophic velocity and sea surface temperature (SST). This paper is organized as follows.…”

“…Mesoscale phenomena, including eddies, fronts, and meanders, are key players in modulating eddy‐mean flow interactions, maintaining large‐scale ocean circulation (Bishop et al., 2016; Guo et al., 2022; Jing et al., 2020), and poleward transport of heat (Guo & Bishop, 2022; Jayne & Marotzke, 2002; Volkov et al., 2008). Poleward eddy heat transport has been found to play an important role in the global heat balance where it accounts for ∼one third of the variability in ocean heat transport (Volkov et al., 2008).…”

Mesoscale Variability Linked to Interannual Displacement of Gulf Stream

“…The Helmholtz decomposition on EHF is performed on global ocean at each time step, considering the indeterminacy of the solution in a bounded regional domain (Fox‐Kemper et al., 2003). More discussions on the decomposition can be found in Guo and Bishop (2022). In this work, we only focus on the meridional component of the divergent EHF considering the prominent equator‐to‐pole thermal gradient in the ocean.…”

“…As studied in Marshall and Shutts (1981), the EHF field comprises two components–rotational EHF and divergent EHF, which can be distinguished through a Helmholtz decomposition. It has been demonstrated that the rotational EHF generally fluctuates along mean isopycnal surfaces and has no dynamic importance, which is necessary to be removed in local analysis in energetic regions (Bishop et al., 2013; Guo & Bishop, 2022; Jayne & Marotzke, 2002). The Helmholtz decomposition on EHF is performed on global ocean at each time step, considering the indeterminacy of the solution in a bounded regional domain (Fox‐Kemper et al., 2003).…”