Seasonality of submesoscale dynamics in the Kuroshio Extension

Geophysical Research Letters RESEARCH LETTER 10.1002/2016GL071349 Key Points: • Upper ocean submesoscale (here 10-100 km) turbulence and inertia-gravity waves undergo strong seasonal cycles that are out of phase • Submesoscale turbulence dominates the horizontal velocity and sea surface height variability in late winter/early spring • Submesoscale inertia-gravity waves dominate the horizontal velocity and sea surface height variability in late summer/early fall Supporting Information: • Supporting Information S1 Correspondence to: C. B. Rocha, crocha@ucsd.edu Citation: Rocha, C. B., S. T. Gille, T. K. Chereskin, and D. Menemenlis (2016), Seasonality of submesoscale dynamics in the Kuroshio Extension, Geophys. Res. Lett., 43, 11,304–11,311, doi:10.1002/2016GL071349. Received 16 JUL 2016 Accepted 19 OCT 2016 Accepted article online 25 OCT 2016 Published online 8 NOV 2016 Seasonality of submesoscale dynamics in the Kuroshio Extension Cesar B. Rocha 1 , Sarah T. Gille 1 , Teresa K. Chereskin 1 , and Dimitris Menemenlis 2 1 Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, USA, 2 Earth Sciences Division, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA Abstract Recent studies show that the vigorous seasonal cycle of the mixed layer modulates upper ocean submesoscale turbulence. Here we provide model-based evidence that the seasonally changing upper ocean stratiﬁcation in the Kuroshio Extension also modulates submesoscale (here 10–100 km) inertia-gravity waves. Summertime restratiﬁcation weakens submesoscale turbulence but enhances inertia-gravity waves near the surface. Thus, submesoscale turbulence and inertia-gravity waves undergo vigorous out-of-phase seasonal cycles. These results imply a strong seasonal modulation of the accuracy of geostrophic velocity diagnosed from submesoscale sea surface height delivered by the Surface Water and Ocean Topography satellite mission. 1. Introduction Recent interest in upper ocean dynamics has focused on the strong seasonal cycle of shallow baroclinic insta- bilities and their role in submesoscale (roughly 1–100 km) turbulence and mesoscale (roughly 100–300 km) modulation [Sasaki et al., 2014; Qiu et al., 2014; Brannigan et al., 2015; Callies et al., 2015; Thompson et al., 2016; Buckingham et al., 2016]. Contemporary studies have also shown that inertia-gravity waves (IGWs) contribute signiﬁcantly to the near-surface variability at submesoscales [Richman et al., 2012; Buhler et al., 2014; Rocha et al., 2016], but their seasonality has not been investigated. The partition between geostrophic and ageostrophic ﬂows across the submesoscales and their seasonality have immediate applications for the planning of the Surface Water and Ocean Topography (SWOT) satellite mission. SWOT will deliver sea surface height (SSH) with about 15 km resolution, thereby providing the ﬁrst global SSH measurements at submesoscales [Fu and Ubelmann, 2014]. Thus, the projection of energetic sub- mesoscale ageostrophic ﬂows, such as inertia-gravity waves (IGWs), onto SSH represents a challenge and an opportunity for the SWOT community. The challenge is how to diagnose submesoscale surface velocity from SSH. The opportunity is stimulation of research on interactions between geostrophic and ageostrophic ﬂows and their projection onto SSH. Using the output of two comprehensive global numerical simulations, here we show that IGWs undergo a strong near-surface seasonal cycle in the Kuroshio Extension region. Interestingly, the seasonal cycle of IGWs is out of phase with the seasonal cycle of submesoscale turbulence. Consistent with previous studies, sub- mesoscale turbulence is strongest in winter [Sasaki et al., 2014; Qiu et al., 2014]. IGWs with horizontal scales between 10 and 100 km (hereafter submesoscale IGWs), however, peak in late summer/early fall, when the upper ocean is strongly stratiﬁed. Thus, there exists a strong seasonal modulation of upper ocean subme- soscale dynamics: submesoscale turbulence dominates the upper ocean dynamics in late winter/early fall, whereas submesoscale IGWs prevail in late summer/early fall. 2. The LLC Numerical Simulations ©2016. American Geophysical Union. All Rights Reserved. ROCHA ET AL. We use results of two latitude-longitude polar cap (LLC) comprehensive numerical simulations. The out- puts analyzed here, LLC2160 (nominal resolution 1/24 ∘ ≈ 4.7 km; eﬀective resolution ≈ 20 km) and LLC4320 (nominal resolution 1/48 ∘≈ 2.3 km; eﬀective resolution ≈ 8 km), are forward solutions of the Massachussetts Institute of Technology general circulation model (MITgcm) [Marshall et al., 1997] on a LLC grid [Forget et al., 2015] with 90 vertical levels. The coarser-resolution LLC simulation was spun up from an Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) [Menemenlis et al., 2008] adjoint-method state estimate, constrained to millions of observations from 2009 to 2011. SEASONALITY AT SUBMESOSCALES (10-100 KM)