Principal Investigator


At a Glance

Surface waves at the atmosphere-ocean interface have important implications for climate and weather modeling. This research focuses on two topics related to surface waves. The first is improved coupled model performance through explicit consideration of physical processes related to surface gravity waves, including upper ocean turbulent mixing and interfacial fluxes of heat, momentum, and gases. The second is the investigation of changing surface wave characteristics in an evolving climate.

 


Research Highlight

A present research focus within the first topic is the role of surface waves in upper ocean mixing. Earth Systems Models have historically suffered from biases in under-predicting upper-ocean mixing. One region where this bias is particularly notable is in the Southern Ocean, where simulated mean mixed layer depth using GFDL’s MOM6 (Figure 1.3b) average approximately 50 m, approximately 30 m shallower than the observations of Hosoda et al., 2010 (Figure 1.3a).

This discrepancy motivated a high-resolution process model investigation to understand and parameterize the role of surface waves in ocean mixing via a wave-turbulence interaction mechanism called Langmuir turbulence (LT). The result was a modified parameterization for upper ocean mixing that significantly improves the simulated mixed layer depth in the Southern Ocean relative to observations (Figure 1.3c). The implications of this are significant for improving climate models through simulating atmosphere-ocean exchange and ocean uptake of properties such as heat and carbon.

Figure 1.3. Summer mixed layer depths from [a] observed climatology by Hosoda et al., 2010, [b] GFDL CM4/MOM6 prototype model with no wave-driven mixing (original) and [c] the same model with the new wave-driven mixing parameterization (Langmuir Turbulence, LT). The mixed layer depth is determined by a threshold vertical density change of 0.03 kg/m3.
Another finding of this study is the failure of the wave-driven parameterization to improve mixed layer depth biases in the Equatorial region (Figure 1.3a-c). The model bias in this region is therefore likely unrelated to missing wave processes, motivating future work to improve parameterized turbulent mixing driven by other sources (e.g., due to vertical current shear).

The second topic has driven a separate research focus on trends in surface wave properties over the historical record and projections for the future. The importance of this work lies largely in societal impacts. Marine development and offshore operations rely on accurately knowing the local ocean environment and how the environment will evolve over several decades. Possibly the most relevant environmental parameter for these concerns is the ocean wave height, both on average and during extreme weather events such as hurricanes. Trends in wave statistics can be investigated over the historical record to understand how conditions may change in the future. Furthermore, using a wave model coupled to a climate model designed for projection allows another method for predicting how waves will respond to evolving environmental conditions.

A critical next step for both research paths will be explicitly coupling a surface wave model into NOAA/GFDL’s climate modeling framework to improve model capabilities and better understand impacts in a changing climate. Presently, the uncoupled simulations with prescribed (observed) wind fields allow the role of waves to be understood only in a historical context. Predicting the role of surface waves in a changing climate requires the ability to model how the waves will respond to changing forcing, and thus clearly merges both research branches. Furthermore, research has recently begun on improved model performance considering other aspects of wave effects, such as air entrainment, which will allow for better estimates of heat, gas, and momentum flux within the model.

 


References

Reichl, B.G. and R. Hallberg. An Energetically Constrained Planetary Boundary Layer (ePBL) Approach for Ocean Climate Simulation. In revision.

Reichl, B.G., A. Adcroft, S.M. Griffies, R.W. Hallberg, Q. Li, and B. Fox-Kemper. Impact of Langmuir Turbulence on Energetic Constraints of the Ocean Surface Boundary Layer. In prep.

Hosoda, S., T. Ohira, K. Sato, and T. Suga, 2010. Improved description of global mixed-layer depth using Argo profiling floats. J. Oceanogr. 66(6): 773-787. doi:10.1007/s10872-010-0063-3.