Observation of an anti-cyclonic mesoscale eddy in the subtropical northwestern Pacific Ocean from altimetry and Argo profiling floats
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Abstract: The comprehensive three-dimensional structures of an anti-cyclonic mesoscale eddy (AE) in the subtropical northwestern Pacific Ocean were investigated by combining the Argo floats profiles with enhanced vertical and temporal sampling and satellite altimetry data. The AE originated near the Kuroshio Extension and then propagated westward with mean velocity of 8.9 cm/s. Significant changes and evolutions during the AE’s growing stage (T1) and further growing stage (T2) were revealed through composite analysis. In the composite eddy core, maximum temperature (T) and salinity (S) anomalies were of 1.7 (1.9)°C and 0.04 (0.07) psu in T1 (T2) period, respectively. The composite T anomalies showed positive in almost whole depth, but the S anomalies exhibited a sandwich-like pattern. The eddy’s intensification and its influence on the intermediate ocean became more significant during its growth. The trapping depth increased from 400×104 Pa to 580×104 Pa while it was growing up, which means more water volume, heat and salt content in deeper layers can be transported. The AE was strongly nonlinear in upper oceans and can yield a typical mean volume transport of 0.17×106 m3/s and a mean heat and salt transport anomaly of 3.6×1011 W and –2.1×103 kg/s during the observation period. The Energy analysis showed that eddy potential and kinetic energy increased notably as it propagated westward and the baroclinic instability is the major energy source of the eddy growth. The variation of the remained Argo float trapped within the eddy indicated significant water advection during the eddy’s propagation.
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Figure 1. Mean sea surface height (cm) of the northwestern Pacific Ocean from Rio et al. (2009) the contour interval is 10 cm and the blue rectangle indicates the regions where the anti-cyclonic eddy (AE) was observed (a), and the locations of the Argo floats (gray stars) deployed in the blue rectangle (b). The contours denote the altimeter sea level anomaly (SLA) field on that day. The contour interval of the SLA is 2 cm, and the negative, zero, and positive values are marked by the gray dashed, thick solid, thin solid lines, respectively. The green solid line denotes the trajectory of the AE and the purple dots’ interval is two weeks.
Figure 2. Evolutions of the AE detected by altimeter SLA from March 27 to July 25, 2014. The contours denote the altimeter SLA field on that day. The contour interval of the SLA is 3 cm, and the negative, zero, and positive values are marked by the gray dashed, thick solid, thin solid lines, respectively. The purple stars denote the locations of the Argo floats. The gray rectangle denotes the field of AE during T1 period; the red rectangle denotes the field of AE during T2 period.
Figure 3. Daily evolution of radius (a), amplitude (b) , EKE (c) of the AE. The red and green rectangles indicate the T1 and T2 period, respectively.
Figure 4. Vertical profiles of temperature anomaly (a), salinity anomaly (b), density anomaly (c), and vorticity (maximum within the eddy core) (d) inside the composite AE. The blue (red) lines denote the values in T1 (T2) period.
Figure 7. Vertical sections of the zonal geostrophic current anomaly
$V'$ (m/s) of the composite AE at Δdx=0 during T1 (a) and T2 (b) period (The contour interval is 0.01 m/s and the dashed lines indicate the zero contours), and vertical sections of the density (black contours; in kg/m3) and density anomaly (color shading; in kg/m3) of the composite AE at Δdx=0 during T1 (c) and (d) period (The black contours’ interval is 0.2 kg/m3).
Figure 5.
$T'$ fields (°C) of the composite AE at 50×104, 250×104, 450×104, 600×104, 750×104 and 900×104 Pa during periods T1 (a) to T2 (b). The contour interval is 0.01°C and the units of the X-axes (Δdx) and Y-axes (Δdy) are both kilometer.
Figure 6.
$S'$ fields (psu) of the composite AE at 50×104, 250×104, 450×104, 600×104, 750×104 and 900×104 Pa during periods T1 (a) to T2 (b). The contour interval is 0.008 and the units of the X-axes (Δdx) and y-axes (Δdy) are both kilometer.
Figure 8. Horizontal fields of geostrophic velocity anomaly (vectors; in m/s) and
$DH'$ (color shading; in m2/s2) of the composite AE during T1 (a) and T2 (b) period. The units of the X-axes (Δdx) and Y-axes (Δdy) are both km.
Figure 9. Vertical profile of the rotational speed of AE (solid line) and its errors (shadings) in T1 (a) and T2 (b) period. The dashed line indicates the mean propagation speed of the AE (0.08 m/s).
Figure 10. Mean available heat anomaly (a) and salt anomaly (b) , inside composite AE during period T1 (blue line) and T2 (red line).
Figure 11. Mean vertical profiles of the EPE (a), EKE (b) and BCI (C) inside the composite AE during T1 period (blue line) and T2 period (red line).
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