About

James McWilliams is a professor in the Department of Atmospheric and Oceanic Sciences at UCLA, holding the Louis B. Slichter Professorship of Earth Sciences since 1994. He received his B.S. in Applied Mathematics with honors from Caltech in 1968, followed by a M.S. in 1969 and a Ph.D. in 1971 from Harvard. His early career included a Research Fellowship in Geophysical Fluid Dynamics at Harvard and a position as a Senior Scientist at the National Center for Atmospheric Research (NCAR), where he worked in the Oceanography Section. In 2002, he was elected to the National Academy of Sciences. McWilliams' primary scientific research focuses on the fluid dynamics of Earth's oceans and atmosphere, encompassing their theory and computational modeling. His work includes studying the maintenance of general circulations, climate dynamics, vortex dynamics, planetary boundary layers, thermohaline convection, and the roles of coherent structures in turbulent flows within geophysical and astrophysical regimes. Recently, he has contributed to developing a three-dimensional simulation model of the U.S. West Coast that integrates physical oceanography, biogeochemistry, and sediment transport to interpret coastal phenomena, analyze historical variability, and assess future scenarios.

Research topics

• Geology
• Oceanography
• Environmental science
• Ecology
• Biology
• Meteorology
• Physics
• Computer Science
• Geography
• Environmental resource management
• Climatology
• Atmospheric sciences

Selected publications

• Comparing the life cycles of a turbulent front and dense filament in the oceanic surface boundary layer

  Journal of Fluid Mechanics · 2026-01-02

  articleOpen access

  Submesoscale processes, typically shaped by intricate interactions between frontal dynamics and turbulence, have significant impacts on the transport of momentum, heat and biogeochemical tracers in the ocean. This study employs large-eddy simulations to investigate submesoscale frontogenesis and arrest in the ocean surface boundary layer. We compare a single-sided front with a dense filament, which can be viewed as a two-sided front. Both cases exhibit a similar life cycle, including frontogenesis driven by secondary circulation, frontal arrest due to the growth of instability and turbulence, and eventual frontal decay. One major difference is that the filament remains stationary throughout its life cycle, while the front propagates towards the denser side. Another distinction lies in the relative contributions of horizontal and vertical turbulent fluxes. In the filament case, horizontal (cross-front) turbulent flux dominates and effectively counteracts the frontogenetic tendency induced by secondary circulation, leading to frontal arrest. In contrast, both vertical and horizontal turbulent fluxes are crucial for the arrest of the single-sided front. Horizontal shear production is the primary source of turbulence in the filament, associated with the emergence of horizontal coherent eddies and consistent with the characteristics of horizontal shear instability. For the front, the development of horizontal eddies is less pronounced, and vertical shear production plays a more important role. This study reveals the similarities and differences between the dynamics of submesoscale fronts and filaments, as well as the role of turbulence in their evolution, providing insights for improved representation of these processes in ocean models.

  PublisherDOI

• Near-inertial echoes of ageostrophic instability in submesoscale filaments

  Journal of Fluid Mechanics · 2025-07-17

  articleOpen access

  Ocean submesoscales, flows with characteristic size $10\,\text{m}{-}10\,\text{km}$ , are transitional between the larger, rotationally constrained mesoscale and three-dimensional turbulence. In this paper, we present simulations of a submesoscale ocean filament. In our case, the filament is strongly sheared in both vertical and cross-filament directions, and is unstable. Instability indeed dominates the early behaviour with a fast extraction of kinetic energy from the vertically sheared thermal wind. However, the instability that emerges does not exhibit characteristics that match the perhaps expected symmetric or Kelvin–Helmholtz instabilities, and appears to be non-normal in nature. The prominence of the transient response depends on the initial noise, and for large initial noise amplitudes, saturates before symmetric instability normal modes are able to develop. The action of the instability is sufficiently rapid – with energy extraction from the mean flow emerging and peaking within the first inertial period ( $\sim\! 18\ \text{h}$ ) – that the filament does not respond in a geostrophically balanced sense. Instead, at all initial noise levels, it later exhibits vertically sheared near-inertial oscillations with higher amplitude as the initial minimum Richardson number decreases. Horizontal gradients strengthen only briefly as the fronts restratify. These unstable filaments can be generated by strong mixing events at pre-existing stable structures; we also caution against inadvertently triggering this response in idealised studies that start in a very unstable state.

  PublisherOA PDFDOI

• Second-Order Velocity Structure Functions at Submesoscales

  Journal of Physical Oceanography · 2025-11-24

  article

  Abstract Observations of ocean surface currents from the JPL Doppler Scatterometer (DopplerScatt) during the Submesoscale Ocean Dynamics Experiment (S-MODE) campaigns reveal unexpectedly shallow second-order velocity structure function (SF) slopes at submesoscale separation scales ( r < 10 km), deviating from classical turbulence theory and prior modeling results. This discrepancy suggests missing physics in current submesoscale-resolving numerical ocean models or an incomplete interpretation of the DopplerScatt observations. To investigate this, we analyze high-resolution Regional Ocean Modeling System (ROMS) simulations across a range of configurations that isolate the influence of model resolution, season, high-frequency forcings, and surface gravity wave effects on currents. We find that high-frequency motions associated with near-inertial waves reduce the transverse SF amplitude, driving the ratio of longitudinal to transverse SFs close to unity at submesoscales independently of the season. Additionally, the inclusion of wave–current interactions, often omitted in standard submesoscale-resolving models, can produce energetic small-scale motions, leading to broadband shallow structure function slopes. These results reveal a broader mechanism by which shallow structure function slopes can emerge: Any process that injects kinetic energy at small scales over a narrow range of wavenumbers will appear broadband in structure function space and produce shallow scalings. Wave effects are one such candidate and offer a plausible interpretation of the DopplerScatt observations under energetic wave conditions. However, under low wave conditions, other processes with similar spectral characteristics are required to account for the observed shallowness. Finally, the relatively large transverse-to-longitudinal SF ratio in DopplerScatt may reflect its lateral averaging over part of an inertial period, a sampling strategy not replicated in models and warranting further study.

  PublisherDOI

• Spontaneous emission of internal waves by a radiative instability

  2025-03-15

  preprintOpen access

  The spontaneous emission of internal waves (IWs) from balanced mesoscale eddies has been proposed as a source of oceanic IW kinetic energy (KE). This study investigates the mechanisms leading to the spontaneous radiation of spiral-shaped IWs from an anticyclonic eddy with an order-one Rossby number, using a high-resolution numerical simulation of a flat-bottomed, wind-forced, reentrant channel flow configured to resemble the Antarctic Circumpolar Current. It is shown that the IWs are spontaneously generated due to a loss of balance process that occurs at the edge of the edge and radiates radially outward. A 2D linear stability analysis of the eddy reveals that the spontaneous emission arises from a radiative instability, which involves an interaction between a vortex Rossby wave supported by the radial gradient of potential vorticity and an outgoing IW. This particular instability occurs when the perturbation frequency is superinertial. This finding is supported by a KE analysis of the unstable modes and the numerical solution, demonstrating that the horizontal shear production provides the source of perturbation KE. Additionally, the horizontal length scale and frequency of the most unstable mode from the stability analysis closely correspond to those of the spontaneously emitted IWs in the numerical solution.

  PublisherDOI

• Urban eutrophication enhances domoic acid production by Pseudo-nitzschia in the Southern California Bight

  Harmful Algae · 2025-11-20 · 2 citations

  article
  PublisherDOI

• Development of a Mesoscale- and Tide-Resolving Ocean-Biogeochemistry Model for the Pacific: Tidal Enhancement of the Biological Carbon Pump

  2025-09-11

  preprintOpen accessSenior author

  This study presents a new physical-biogeochemical simulation of the Pacific Ocean that resolves mesoscale dynamics and explicitly includes tidal forcing. The primary objective is to develop and document a modeling framework that serves both as a detailed record of model configuration and forcing preparation, and as a reference for future regional downscaling. The model is extensively evaluated against physical and biogeochemical datasets and successfully reproduces large-scale circulation patterns and key biogeochemical features. The second objective is to assess the impact of explicitly including tidal forcing on primary production and carbon export, thereby clarifying the biogeochemical consequences of tidal dynamics. While tides are known to energize high-frequency motions, their influence on ocean biogeochemistry remains insufficiently constrained. By comparing simulations with and without tidal forcing, we show that tides enhance net primary production and particulate organic carbon export, particularly in coastal zones and the eastern equatorial Pacific. Basin-wide, tidal forcing increases carbon uptake by approximately 0.45 PgC yr-1 and export by 0.09 PgC yr-1. This enhancement is primarily driven by tide-induced mixing, which increases nutrient supply to the surface and stimulates biological productivity. These findings underscore the importance of accounting for tidal processes in ocean biogeochemical models to improve estimates of the global carbon cycle and refine projections of future oceanic carbon uptake.

  PublisherOA PDFDOI

• An Investigation of Coupled Atmospheric and Oceanic Boundary Layers Using Large-Eddy Simulation

  Journal of the Atmospheric Sciences · 2025-03-14 · 1 citations

  articleOpen access

  Abstract The marine atmospheric boundary layer (ABL) and oceanic boundary layer (OBL) are a two-way coupled system. At the ocean surface, the ABL and OBL share surface fluxes of momentum and buoyancy that incorporate variations in sea surface temperature (SST) and currents. To investigate the interactions, a coupled ABL–OBL large-eddy simulation (LES) code is developed and exercised over a range of atmospheric stability. At each time step, the coupling algorithm passes oceanic currents and SST to the atmospheric LES, which in turn computes surface momentum, temperature, and humidity fluxes driving the oceanic LES. Equations for each medium are time advanced using the same time step but utilize different grid resolutions: the horizontal grid resolution in the ocean is approximately four times finer, e.g., (Δ x o , Δ x a ) = (1.22, 4.88) m. Interpolation and anterpolation (its adjoint) routines connect the atmosphere and ocean surface layers. In the simplest setup of a statistically horizontally homogeneous flow, the largest scale ABL turbulent shear-convective rolls leave an imprint on the OBL currents in the upper layers. This result is shown by comparing simulations that use coupling rules that are applied either instantaneously at every x – y grid point or averaged across an x – y plane. The spanwise scale of the ABL turbulence is ∼1000 m, while the depth of the OBL is ∼20 m. In these homogeneous, fully coupled cases, the large-scale spatially intermittent turbulent structures in the ABL modulate SST, currents, and the connecting momentum and buoyancy fluxes, but the mean profiles in each medium are only slightly different.

  PublisherOA PDFDOI

• A Submesoscale Cascade‐Driven Mesoscale Seasonal Cycle in the Subtropics

  Geophysical Research Letters · 2025-11-12

  articleOpen access

  Abstract We show that the submesoscale inverse cascade almost entirely drives the mesoscale kinetic energy (KE) seasonal cycle in the interior subtropical gyre. Using a coarse‐graining framework to diagnose cross‐scale energy fluxes (), we show the forward cascade remains confined to <20 km within the mixed layer, peaking January–March. The inverse cascade exhibits a dramatic upscale shift: its peak scale expands from ∼30 km in January to ∼200 km by June while penetrating vertically below the mixed layer by March. The observed cascade timescale (∼180 days) far exceeds predictions from classical turbulence theory (∼40 days), revealing fundamental departures from idealized quasi‐geostrophic dynamics. This horizontal and vertical expansion establishes the pathway whereby submesoscale eddies energize mesoscale motions. Lead‐lag analysis reveals potential energy conversion precedes frontogenesis by 7–21 days, submesoscale eddies by 9–23 days, and peak by 20–90 days, which in turn leads large‐scale KE by 30–70 days.

  PublisherDOI

• Langmuir Mixing Schemes Based on a Modified K‐Profile Parameterization

  Journal of Advances in Modeling Earth Systems · 2025-04-01 · 8 citations

  articleOpen access

  Abstract Langmuir turbulence, a dominant process in the ocean surface boundary layer, drives substantial vertical mixing that influences temperature, salinity, mixed layer depth, and biogeochemical tracer distributions. While direct resolution of Langmuir turbulence in ocean and climate models remains computationally prohibitive, its effects are commonly parameterized, frequently within established turbulent mixing frameworks like the K‐profile parameterization (KPP). This study utilizes a modified KPP that determines boundary layer depth through an integral criterion, diverging from the conventional KPP's dependence on the bulk Richardson number. The modified KPP demonstrates markedly lower sensitivity to model vertical resolution than its conventional counterpart. Building upon this modified KPP framework, we introduce an innovative parameterization scheme for Langmuir mixing effects. We evaluate the performance of this new scheme against existing approaches using a one‐dimensional (1D) column model across four different scenarios, incorporating validation against both large eddy simulation (LES) results and field measurements. Our analysis reveals that the new Langmuir mixing scheme, explicitly designed for the modified KPP framework, performs competitively while maintaining reduced sensitivity to vertical resolution.

  PublisherOA PDFDOI

• What is Endangered now? Climate Science at the Crossroads

  2025-07-31

  preprintOpen access

  The greenhouse gas “endangerment finding” of the U.S. Environmental Protection Agency (EPA), established in 2009 after a 2006 U.S. Supreme Court case (Massachusetts vs EPA) in which we participated as amicus curiae (friends of the court) , has become the basis for U.S. regulation of greenhouse gases in the years since. The current Administration of President Donald Trump is now seeking its repeal. Here, we review the role climate science played in that 2006 case, and how the scientific evidence that undergirds the endangerment finding has gotten stronger in the 16 years since. Finally, we consider what will be the fate of the endangerment finding – and indeed that of role of science in contributing to policy – in light of the current challenging environment for science in the U.S.

  PublisherOA PDFDOI

Recent grants

• Collaborative Research: CMG: Mathematical Theory and Modeling of Wave-Current Interaction

  NSF · $324k · 2003–2009

• Collaborative Research: Topography, Boundary Currents and the Submesoscale

  NSF · $352k · 2010–2014

• Dynamical and Material Connectivity Across Continental Shelves

  NSF · $632k · 2014–2018

• Collaborative Research: Does Topography Control Mesocale Dissipation?

  NSF · $223k · 2006–2010

• CMG COLLABORATIVE RESEARCH: Wave Breaking Dissipation Modeling and Parametrization in Wave/Current Interactions

  NSF · $285k · 2007–2012

Frequent coauthors

• Lionel Renault

  Centre National de la Recherche Scientifique

  175 shared

• M. Jeroen Molemaker

  University of California, Los Angeles

  102 shared

• Peter P. Sullivan

  NSF National Center for Atmospheric Research

  83 shared

• Jonathan Gula

  Ifremer

  66 shared

• Alexander F. Shchepetkin

  65 shared

• Fayçal Kessouri

  60 shared

• Roy Barkan

  University of California, Los Angeles

  52 shared

• François Colas

  Institut de Recherche pour le Développement

  48 shared

Education

• B.S., Applied Mathematics

  Caltech

  1968

• M.S., Applied Mathematics

  Harvard

  1969

• Ph.D., Applied Mathematics

  Harvard

  1971

Awards & honors

• National Academy of Sciences (2002)
• Louis B. Slichter Professor of Earth Sciences at UCLA (1994)