Mesoscale Eddy-Induced Ocean Dynamic and Thermodynamic Anomalies in the North Pacific

Jiang Zhou,Guidi Zhou,Xuhua Cheng,Zhuhua Li,Hailong Liu

doi:10.3389/fmars.2021.756918

Abstract

Oceanic mesoscale eddies are associated with large thermodynamic anomalies, yet so far they are most commonly studied in terms of surface temperature and in the sense of composite mean. Here we employ an objective eddy identification and tracking algorithm together with a novel matching and filling procedure to more thoroughly examine eddy-induced thermodynamic anomalies in the North Pacific, their relationship with eddy amplitude (SSH), and the percentage of variability they explain on various timescales from submonthly to interannual. The thermodynamic anomalies are investigated in terms of sea surface temperature (SST), isothermal layer depth (ITD), and upper ocean heat content (HCT). Most eddies are weak in amplitude and are associated with small thermodynamic anomalies. In the sense of composite mean, anticyclonic eddies are generally warm eddies with deeper isothermal layer and larger heat content, and the reverse is true for cyclonic eddies. A small fraction of eddies, most probably subsurface eddies, exhibits the opposite polarities. Linear relationships with eddy amplitude are found for each of the thermodynamic parameters but with different level of scatter and seasonality. HCT-amplitude relation scatters the least and has the smallest seasonal difference, ITD-amplitude relation has the largest scatter and seasonality, while SST-amplitude relation is in between. For the Kuroshio and Oyashio Extension region, the most eddy-rich region in the North Pacific, eddies are responsible for over 50% of the total SSH variability up to the intra-seasonal scale, and ITD and HCT variability up to interannual. Eddy-induced SST variability is the highest along the Oyashio Extension Front on the order of 40–60% on submonthly scales. These results highlight the role of mesoscale eddies in ocean thermodynamic variability and in air-sea interaction.

Highlights

Mesoscale eddies are ubiquitous in the global oceans
The trajectories of eddies longer than 180 days are shown in Figure 1C, which reveals that eddies travel longer distances near the equator, as a result of the poleward decreasing Rossby wave speed
Active eddy generation is noticeable along the eastern boundary, where coastal Kelvin waves from the tropics are transformed into Rossby waves propagating westward away from the boundary (Clarke and Shi, 1991; Fu and Qiu, 2002)

Summary

Introduction

Mesoscale eddies are ubiquitous in the global oceans. They are coherent rotating vortices on horizontal scales of O(100 km) and temporal scales of O(100 days). In the North Pacific, the Kuroshio Extension and the Subtropical Counter Current are two of the most eddy-energetic regions with dynamic interactions between eddies and the time-mean flow. Other regions, such as along the eastern boundary and in the Gulf of Alaska, are abundant with eddies, there eddies are generally weaker (Cheng et al, 2014)

Objectives

Results

Conclusion