Decadal Variability of Eddy Kinetic Energy in the South Pacific Subtropical Countercurrent in an Ocean General Circulation Model

Abstract

The eddy kinetic energy (EKE) associated with the Subtropical Countercurrent (STCC) in the western subtropical South Pacific is known to exhibit substantial seasonal and decadal variability. Using an eddypermitting ocean general circulation model, which is able to reproduce the observed, salient features of the seasonal cycles of shear, stratification, baroclinic production, and the associated EKE, the authors investigate the decadal changes of EKE. The authors show that the STCC region exhibits, uniquely among the subtropical gyres of the world's oceans, significant, atmospherically forced, decadal EKE variability. The decadal variations are driven by changing vertical shear between the STCC in the upper 300 m and the South Equatorial Current below, predominantly caused by variations in STCC strength associated with a changing meridional density gradient. In the 1970s, an increased meridional density gradient results in EKE twice as large as in later decades in the model. Utilizing sensitivity experiments, decadal variations in the wind field are shown to be the essential driver. Local wind stress curl anomalies associated with the interdecadal Pacific oscillation (IPO) lead to upwelling and downwelling of the thermocline, inducing strengthening or weakening of the STCC and the associated EKE. Additionally, remote wind stress curl anomalies in the eastern subtropical South Pacific, which are not related to the IPO, generate density anomalies that propagate westward as Rossby waves and can account for up to 30%-40% of the density anomalies in the investigated region.

South Pacific Decadal Climate Variability and Potential Predictability

“…Travis and Qiu (2017) argued that changes in oceanic stratification and vertical shear of horizontal velocities can cause decadal variability of baroclinic instability in the subtropical countercurrent (STCC) region over the South Pacific. Utilizing an ocean general circulation model, Rieck et al (2018) also show that the STCC region (i.e., 258-338S, 1538-1758W) exhibits significant decadal baroclinic instability.…”

The Nature of Eddy Kinetic Energy in the Labrador Sea: Different Types of Mesoscale Eddies, Their Temporal Variability, and Impact on Deep Convection

“…The mean velocities (u, y) are based on 1-yr averages to avoid interannual current variability from being considered EKE (cf. Penduff et al 2004;Rieck et al 2015). The observational estimate for EKE is derived from the gridded, delayed time geostrophic surface velocities made available by EU Copernicus Marine Service (CMEMS) and calculated as described above.…”

“…The long hindcast simulation enables us to investigate temporal variations beyond interannual time scales. This is important as, especially near strong currents, a large part of the temporal variability of the mesoscale is intrinsic to the ocean, that is, not attributable to a certain atmospheric forcing (Sérazin et al 2015;Rieck et al 2018), even at interannual time scales. After describing the model, data, and methods in section 2, we focus on investigating the properties, generation mechanisms, and potential impact on deep convection of Irminger Rings (section 3a), convective eddies (section 3b), and boundary current eddies (section 3c), summarizing and discussing the results in section 4.…”

“…As the ocean integrates the highfrequency atmospheric forcing into lower-frequency variability of the general circulation, links between yearto-year variations in the atmosphere and large-scale ocean circulation are hard to detect. In fact, oceanic EKE variability is largely intrinsic, that is, internally driven, in most regions on interannual time scales (Wilson et al 2015;Rieck et al 2018), which might explain the difficulties previous studies had relating the interannual EKE variability in the WGC to an external forcing (e.g., Luo et al 2011). In the following, we thus focus on variations on decadal time scales.…”

Response of Total and Eddy Kinetic Energy to the Recent Spinup of the Beaufort Gyre

“…The velocities can be further decomposed into a time-mean and an eddy part, (u, y) 5 (u 1 u 0 , y 1 y 0 ), from which we can decompose KE into a mean kinetic energy [MKE 5 0:5(u 2 1 y 2 )] and an eddy kinetic energy [EKE 5 0:5(u 02 1 y 02 )] component. Here we follow Rieck et al (2018) and use annual means to compute u and y as opposed to the long-term mean that is often considered (e.g., Hogg et al 2015). This allows us to account for interannual variations of the mean currents and in particular for the effects of the gyre spinup that occurs over the period considered (Fig.…”