# Seasonal Variation of Intra-Seasonal Eddy Kinetic Energy along the East Australian Current

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Data and Methods

#### 2.1. Observation Data

#### 2.2. ECCO2 State Estimate

## 3. Verification of ECCO2 Simulation

## 4. Seasonal EKE Modulation along the EAC

^{2}/s

^{2}) in January and a minimum (110 cm

^{2}/s

^{2}) in September, the seasonality of EKE along the EAC is clearly dominated by the local EKE signals around the EAC separation latitude.

## 5. Processes Governing the Seasonal EKE Variability

^{2}), ${\rho}_{0}$ is the background potential density (${\rho}_{0}$ = 1025 kg/m

^{3}). It should be noted that the barotropic conversion rate (BTR) represents the conversion process between EKE and mean kinetic energy (MKE). A positive BTR represents the conversion of energy from the MKE to the EKE, and reflects barotropic instability of the background mean flow. The baroclinic conversion rate (BCR) represents the transformation between eddy potential energy (EPE) and EKE. A positive baroclinic conversion rate represents the energy transformation from EPE to EKE as a result of baroclinic instability.

^{−4}cm

^{2}/s

^{3}) and smallest in September (0.4 × 10

^{−4}cm

^{2}/s

^{3}). It can be seen that the seasonal variation of BTR is similar to the seasonal cycle of EKE, except for the drop-out value in February. Over all, this result reveals the high correlation between EKE and BTR.

^{−4}cm

^{2}/s

^{3}) in August and smallest (0.04 × 10

^{−4}cm

^{2}/s

^{3}) in February, which is opposite to the seasonal BTR variation. In general, the seasonal variation of BCR is much smaller than that of BTR, indicating the leading role of barotropic instability in the EAC’s EKE evolution.

## 6. Discussion and Summary

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Climatology absolute dynamic topography (ADT, colors) and surface velocities (black arrows) in the western Pacific from (

**a**) AVISO data and (

**b**) ECCO2 state estimate outputs. Both results are averaged between January 1993 and October 2019.

**Figure 2.**Climatology eddy kinetic energy (EKE) distributions calculated from surface velocities of (

**a**) AVISO and (

**b**) ECCO2. (

**c**) shows the distribution of AVISO EKE minus ECCO2 EKE. Red box indicates the area in which EKE time series and budget are calculated.

**Figure 3.**Surface EKE time series averaged in 150−160° E, 25−38° S as a function of months. EKE is calculated by using surface velocities of AVISO (red line) and ECCO2 (black line).

**Figure 4.**Variance-preserved power spectra as a function of periods of the sea surface height from AVISO (red line) and ECCO2 (blue line) data in the area of 150−160° E, 25−38° S.

**Figure 5.**Blue line: EKE averaged in 150–160° E,25–38° S as a function of depth from the ECCO2; the EKE level has been normalized by its surface value: 〈E(t, z)〉/〈E(t, 0)〉. Red line: EKE coherence level as a function of depth with respect to the surface EKE values; see Equation (3) for the definition of coherence level.

**Figure 6.**Monthly mean climatological EKE distributions in the upper 500 m layer. Here EKE is calculated from the 60–180-day bandpass filtered velocity data from the ECCO2 (1992–2019).

**Figure 7.**Monthly mean climatological barotropic conversion rate (BTR) distribution in the upper 500 m from the ECCO2. See Equation (4) for the definition of BTR.

**Figure 8.**Monthly mean climatological baroclinic conversion rate (BCR) distribution averaged in the upper 500 m layer. See Equation (5) for the definition of BCR.

**Figure 9.**Monthly mean climatological eddy kinetic energy (EKE, black line), barotropic conversion rate (BTR, red solid line), baroclinic conversion rate (BCR, red dash line) in the upper 500 m averaged over (150–160° E, 25–38° S). Green line denotes the monthly mean climatological variations of the southward velocity in the upper 500 m averaged over (150–155° E, 25–38° S), which has been 180-day low-pass filtered.

**Figure 10.**Monthly mean climatological background current distributions averaged in the upper 500 m layer, and color indicates velocity magnitude anomalies which have subtracted the annual mean value. Here velocities are based on the 180-day low-pass filtered ECCO2 outputs.

**Figure 11.**Monthly mean climatological lateral rate of strain (LRS) anomalies distributions averaged in the upper 500 m layer, which have subtracted the annual mean value. Here velocities are based on the 180-day low-pass filtered ECCO2 outputs.

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**MDPI and ACS Style**

Xu, Z.; Yang, C.; Chen, X.; Qi, Y. Seasonal Variation of Intra-Seasonal Eddy Kinetic Energy along the East Australian Current. *Water* **2022**, *14*, 3725.
https://doi.org/10.3390/w14223725

**AMA Style**

Xu Z, Yang C, Chen X, Qi Y. Seasonal Variation of Intra-Seasonal Eddy Kinetic Energy along the East Australian Current. *Water*. 2022; 14(22):3725.
https://doi.org/10.3390/w14223725

**Chicago/Turabian Style**

Xu, Zhipeng, Chengcheng Yang, Xiao Chen, and Yiquan Qi. 2022. "Seasonal Variation of Intra-Seasonal Eddy Kinetic Energy along the East Australian Current" *Water* 14, no. 22: 3725.
https://doi.org/10.3390/w14223725