Lagrangian Timescales of Southern Ocean Upwelling in a Hierarchy of Model Resolutions

Abstract

In this paper we study upwelling pathways and timescales of Circumpolar Deep Water (CDW) in a hierarchy of models using a Lagrangian particle tracking method. Lagrangian timescales of CDW upwelling decrease from 87 years to 31 years to 17 years as the ocean resolution is refined from 1° to 0.25° to 0.1°. We attribute some of the differences in timescale to the strength of the eddy fields, as demonstrated by temporally degrading high‐resolution model velocity fields. Consistent with the timescale dependence, we find that an average Lagrangian particle completes 3.2 circumpolar loops in the 1° model in comparison to 0.9 loops in the 0.1° model. These differences suggest that advective timescales and thus interbasin merging of upwelling CDW may be overestimated by coarse‐resolution models, potentially affecting the skill of centennial scale climate change projections.

Cold vs. warm water route – sources for the upper limb of the Atlantic Meridional Overturning Circulation revisited in a high-resolution ocean model

“…That is, we released particles in the NBC at 6 • S for years 1960-1969 and 2000-2009 (highAL) and then traced them backwards towards the predefined source sections by looping through the velocity data of each period for a maximum of 40 years (instead of making use of the whole simulation period 1958-2009). Even though this looping technique has already been employed by various authors (e.g., Döös et al, 2008;Rühs et al, 2013;Thomas et al, 2015;Berglund et al, 2017;Drake et al, 2018;Durgadoo et al, 2017), the obtained results have to be interpreted with caution. Looping may introduce unphysical jumps in the velocity and tracer fields and, consequently, also in the volume transport pathways and along-track tracer changes.…”

A Lagrangian study of the contribution of the Canary coastal upwelling to the open North Atlantic nitrogen budget

“…However, both upwelling depth and the depth of euphotic zone vary in space and time. Yet, Drake et al (2018) show that defining upwelling with varying depth criteria yield results that are also well captured by experiments that use fixed depths. Furthermore, a variable upwelling depth definition can make it difficult to distinguish between upwelling variations driven by the variability of atmospheric forcing and those caused by changes in the analysis depth.…”

A Deep Eastern Boundary Current Carrying Indian Deep Water South of Australia

“…The Agulhas rings may present a unique situation; however, a similar, albeit weaker, westward eddy track exists around the southwest corner of Australia (Cresswell & Griffin, ) (Figure b), which suggests that eddy PV fluxes could play a role in the Indian pathway. Additionally, the lack of an IDW pathway along the southern coast of Australia in Lagrangian experiments using non‐eddy‐resolving velocities from the GFDL model suite indicates that eddy dynamics are necessary for this pathway (Drake et al, ). In the southeast Pacific eastern boundary poleward pathway, there are regions where the PV balance is nonconservative.…”

“…Although the presence of this DEBC was suggested from the oxygen, chlorofluorocarbon, and carbon signatures as measured during WOCE, the scarcity of observations of deep ocean properties and velocities south of Australia has hindered further investigation into the structure of the Indian DEBC that connects the Indian Ocean to the ACC via the southern coast of the Australian mainland and Tasmania (Figure ). However, recent evidence from eddying models shows the importance of this pathway for upwelling of deep waters from the Indian Ocean to the ACC (Drake et al, ; Tamsitt et al, ), indicating that this is a previously unrecognized key route for delivering carbon‐rich deep water to the surface of the Southern Ocean.…”