Interpreting Observed Interactions Between Near-Inertial Waves and Mesoscale Eddies

Table of Links

Abstract and Intro

Observations

Models

Results

Discussion

Conclusion

APPENDIX A

APPENDIX B

APPENDIX C

References

ABSTRACT

The evolution of wind-generated near-inertial waves (NIWs) is known to be influenced by the mesoscale eddy field, yet it remains a challenge to disentangle the effects of this interaction in observations. Here, the model of Young and Ben Jelloul (YBJ), which describes NIW evolution in the presence of slowly evolving mesoscale eddies, is compared to observations from a mooring array in the Northeast Atlantic Ocean. The model captures the evolution of both the observed NIW amplitude and phase much more accurately than a slab mixed layer model.

The YBJ model allows for the identification of specific physical processes that drive the observed evolution. It reveals that differences in the NIW amplitude across the mooring array are caused by the refractive concentration of NIWs into anticyclones.

Advection and wave dispersion also make important contributions to the observed wave evolution. Stimulated generation, a process by which mesoscale kinetic energy acts as a source of NIW potential energy, is estimated to be 20 µW m−2 in the region of the mooring array, which is two orders of magnitude smaller than the global average input to mesoscale kinetic energy and likely not an important contribution to the mesoscale kinetic energy budget in this region. Overall, the results show that the YBJ model is a quantitatively useful tool to interpret observations of NIWs.

1. Introduction

Near-inertial waves (NIWs), internal waves with a frequency close to the inertial frequency 𝑓 , are resonantly excited by atmospheric winds exerting a stress on the ocean’s surface. It has long been recognized that these waves can interact with mesoscale eddies and that this interaction may be important in the life cycle of wind-generated NIWs.

Observational evidence of NIW–mesoscale interactions is accumulating, but interpreting the observed NIW evolution in the presence of mesoscale eddies remains challenging. Here, we employ the theoretical framework of Young and Ben Jelloul (1997, from hereon YBJ) to identify and interpret such interactions in mooring observations in the Northeast Atlantic Ocean.

NIWs are associated with a large vertical shear, which can result in shear instabilities and vertical mixing (for a review, see Alford et al. 2016). These shear instabilities are a key mechanism by which atmospheric storms can cause the surface mixed layer to deepen. Jochum et al. (2013) showed that an improved representation of NIWs in a climate model led to a deepening of the mixed layer on average, which in turn resulted in significant changes in sea surface temperatures, winds, and precipitation.

It has also been proposed that NIWs not only react to the presence of mesoscale eddies but feed back on the eddies and affect their evolution. Approximately 80% of the ocean’s kinetic energy exists as mesoscale motions (Ferrari and Wunsch 2009). The geostrophic constraint on mesoscale eddies traps energy at large scales, and it is not entirely clear how the energy input into mesoscale motion is balanced by dissipation (M¨uller et al. 2011).

A number of mechanisms by which mesoscale eddies lose energy are known, including dissipation in bottom boundary layers (e.g., Arbic and Flierl 2004), the generation of dissipative lee waves (e.g., Nikurashin et al. 2013), energy loss near western boundaries (e.g., Zhai et al. 2010), and the top drag arising from the current dependence of the wind stress (e.g., Dewar and Flierl 1987; Renault et al. 2016).

The extraction of energy from mesoscale eddies by NIWs presents another possibility (Xie and Vanneste 2015; Rocha et al. 2018). Given the great importance of mesoscale eddies to the transport of heat and carbon (e.g., Jayne and Marotzke 2002; Gnanadesikan et al. 2015), even small changes (see discussion in Asselin and Young 2020) to the mesoscale eddy field caused by NIWs may be significant to the impact of the ocean on climate.

Despite its successes in capturing some aspects of the observed NIW signal, the PM model cannot explain the propagation of NIWs out of the mixed layer. If the NIW field is initially uniform, it will remain so. The model captures neither 𝛽-refraction nor the interaction with mesoscale eddies.

This interpretation of the Ocean Storms Experiment was pursued further by Balmforth et al. (1998), who ran spin-down simulations of the YBJ equation. NIWs were initialized in the mixed layer and evolved in the presence of an idealized, barotropic mesoscale eddy field. Qualitative comparisons between the simulations and observations showed that YBJ dynamics were not inconsistent with the observed time for NIWs to escape the mixed layer.

Balmforth and Young (1999) showed that including the 𝛽-effect improved the agreement with observations. Because an idealized eddy field was used, however, no quantitative conclusions could be drawn about the ability of YBJ to capture the observed evolution.

In this study, we aim to bridge the gap between theory and observations by using the YBJ framework to interpret the observed evolution of NIWs in the Northeast Atlantic Ocean. We use an array of nine moorings to capture some of the mesoscale variations in the NIW field. The YBJ framework allows us to attribute the observed NIW evolution to a set of well-defined physical processes. We integrate the three-dimensional YBJ equation using observational inputs for the wind forcing, mesoscale streamfunction, and stratification, and we compare these simulations to simpler slab models.

We show that the YBJ model offers significant improvements in modeling NIW evolution, without the need for any tuning. We use the YBJ energy budgets to provide a dynamical interpretation of spatial and temporal variations in the NIW field and quantify the relative importance of the various physical processes involved. We also provide an estimate for the importance of stimulated generation in this region.