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  1. Abstract

    Tropical instability waves (TIWs) are identified in three multiyear equatorial mooring records in Pacific and Atlantic cold tongues to evaluate how TIWs modulate turbulence. At 0°, 140°W in the Pacific, TIWs are present in 43% of observations, and are associated with elevated vertical shear and a 40% average increase in turbulence dissipation rates (ϵ) above the Equatorial Undercurrent. Zonal shear is greatest when currents are southward while buoyancy is greatest later in the TIW cycle, leading to greater potential for instability and elevated turbulence before and during the southward flow maximum. This suggests that TIW vortex stretching contributes to enhanced shear and turbulence. In the Atlantic, TIWs are found in 38% of observations at 0°, 23°W and 16% of observations at 0°, 10°W. TIWs at 23°W increaseϵby 18% where turbulence is likely modulated by vortex stretching and, near the surface, by the seasonally wind‐forced equatorial roll. At 23°W and 140°W, TIWs with strong meridional velocity fluctuations are associated with the strongest turbulence. Contributions of seasonal variations are removed by considering only periods when TIWs are climatically active. During these periods, mean values ofϵin the presence of strong TIWs are elevated by 61% at 140°W, 29% at 23°W, and 36% at 10°W. At 10°W, where our identification scheme may include wind‐forced oscillations in the same frequency band, increases inϵare not consistent in the presence of TIWs and do not contribute significantly to multiyear averages.

     
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  2. Turbulence-enhanced mixing of upper ocean heat allows interaction between the tropical atmosphere and cold water masses that impact climate at higher latitudes thereby regulating air–sea coupling and poleward heat transport. Tropical cyclones (TCs) can drastically enhance upper ocean mixing and generate powerful near-inertial internal waves (NIWs) that propagate down into the deep ocean. Globally, downward mixing of heat during TC passage causes warming in the seasonal thermocline and pumps 0.15 to 0.6 PW of heat into the unventilated ocean. The final distribution of excess heat contributed by TCs is needed to understand subsequent consequences for climate; however, it is not well constrained by current observations. Notably, whether or not excess heat supplied by TCs penetrates deep enough to be kept in the ocean beyond the winter season is a matter of debate. Here, we show that NIWs generated by TCs drive thermocline mixing weeks after TC passage and thus greatly deepen the extent of downward heat transfer induced by TCs. Microstructure measurements of the turbulent diffusivity ( κ ) and turbulent heat flux ( J q ) in the Western Pacific before and after the passage of three TCs indicate that mean thermocline values of κ and J q increased by factors of 2 to 7 and 2 to 4 (95% confidence level), respectively, after TC passage. Excess mixing is shown to be associated with the vertical shear of NIWs, demonstrating that studies of TC–climate interactions ought to represent NIWs and their mixing to accurately capture TC effects on background ocean stratification and climate. 
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    Free, publicly-accessible full text available June 27, 2024
  3. Abstract

    Observations in the Pacific Equatorial Undercurrents (EUC) show that the nighttime deep-cycle turbulence (DCT) in the marginal-instability (MI) layer of the EUC exhibits seasonal variability that can modulate heat transport and sea surface temperature. Large-eddy simulations (LES), spanning a wide range of control parameters, are performed to identify the key processes that influence the turbulent heat flux at multiple time scales ranging from turbulent (minutes to hours) to daily to seasonal. The control parameters include wind stress, convective surface heat flux, shear magnitude, and thickness of the MI layer. In the LES, DCT occurs in discrete bursts during the night, exhibits high temporal variability within a burst, and modulates the mixed layer depth. At the daily time scale, turbulent heat flux generally increases with increasing wind stress, MI-layer shear, or nighttime convection. Convection is found to be important to mixing under weak wind, weak shear conditions. A parameterization for the daily averaged turbulent heat flux is developed from the LES suite to infer the variability of heat flux at the seasonal time scale. The LES-based parameterized heat flux, which takes into account the effects of all control parameters, exhibits a seasonal variability that is similar to the observed heat flux from theχ-pods.

     
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  4. Abstract Uncertainties in ocean-mixing parameterizations are primary sources for ocean and climate modeling biases. Due to lack of process understanding, traditional physics-driven parameterizations perform unsatisfactorily in the tropics. Recent advances in the deep-learning method and the new availability of long-term turbulence measurements provide an opportunity to explore data-driven approaches to parameterizing oceanic vertical-mixing processes. Here, we describe a novel parameterization based on an artificial neural network trained using a decadal-long time record of hydrographic and turbulence observations in the tropical Pacific. This data-driven parameterization achieves higher accuracy than current parameterizations, demonstrating good generalization ability under physical constraints. When integrated into an ocean model, our parameterization facilitates improved simulations in both ocean-only and coupled modeling. As a novel application of machine learning to the geophysical fluid, these results show the feasibility of using limited observations and well-understood physical constraints to construct a physics-informed deep-learning parameterization for improved climate simulations. 
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  5. Abstract

    Several years of moored turbulence measurements fromχpods at three sites in the equatorial cold tongues of Atlantic and Pacific Oceans yield new insights into proxy estimates of turbulence that specifically target the cold tongues. They also reveal previously unknown wind dependencies of diurnally varying turbulence in the near-critical stratified shear layers beneath the mixed layer and above the core of the Equatorial Undercurrent that we have come to understand as deep cycle (DC) turbulence. Isolated by the mixed layer above, the DC layer is only indirectly linked to surface forcing. Yet, it varies diurnally in concert with daily changes in heating/cooling. Diurnal composites computed from 10-min averaged data at fixedχpod depths show that transitions from daytime to nighttime mixing regimes are increasingly delayed with weakening wind stressτ. These transitions are also delayed with respect to depth such that they follow a descent rate of roughly 6 m h−1, independent ofτ. We hypothesize that this wind-dependent delay is a direct result of wind-dependent diurnal warm layer deepening, which acts as the trigger to DC layer instability by bringing shear from the surface downward but at rates much slower than 6 m h−1. This delay in initiation of DC layer instability contributes to a reduction in daily averaged values of turbulence dissipation. Both the absence of descending turbulence in the sheared DC layer prior to arrival of the diurnal warm layer shear and the magnitude of the subsequent descent rate after arrival are roughly predicted by laboratory experiments on entrainment in stratified shear flows.

    Significance Statement

    Only recently have long time series measurements of ocean turbulence been available anywhere. Important sites for these measurements are the equatorial cold tongues where the nature of upper-ocean turbulence differs from that in most of the world’s oceans and where heat uptake from the atmosphere is concentrated. Critical to heat transported downward from the mixed layer is the diurnally varying deep cycle of turbulence below the mixed layer and above the core of the Equatorial Undercurrent. Even though this layer does not directly contact the surface, here we show the influence of the surface winds on both the magnitude of the deep cycle turbulence and the timing of its descent into the depths below.

     
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  6. Abstract In low winds (≲2 m s −1 ), diurnal warm layers form but shear in the near-surface jet is too weak to generate shear instability and mixing. In high winds (≳8ms −1 ), surface heat is rapidly mixed downward and diurnal warm layers do not form. Under moderate winds of 3–5 m s −1 , the jet persists for several hours in a state that is susceptible to shear instability. We observe low Richardson numbers of Ri ≈ 0.1 in the top 2 m between 10:00 and 16:00 local time (from 4 h after sunrise to 2 h before sunset). Despite Ri being well below the Ri = 1/4 threshold, instabilities do not grow quickly, nor do they overturn. The stabilizing influence of the sea surface limits growth, a result demonstrated by both linear stability analysis and two-dimensional simulations initialized from observed profiles. In some cases, growth rates are sufficiently small (≪1 h −1 ) that mixing is not expected even though Ri < 1/4. This changes around 16:00–17:00. Thereafter, convective cooling causes the region of unstable flow to move downward, away from the surface. This allows shear instabilities to grow an order of magnitude faster and mix effectively. We corroborate the overall observed diurnal cycle of instability with a freely evolving, two-dimensional simulation that is initialized from rest before sunrise. 
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  7. Abstract

    Based on velocity data from a long‐term moored observatory located at 0°N, 23°W we present evidence of a vertical asymmetry during the intraseasonal maxima of northward and southward upper‐ocean flow in the equatorial Atlantic Ocean. Periods of northward flow are characterized by a meridional velocity maximum close to the surface, while southward phases show a subsurface velocity maximum at about 40 m. We show that the observed asymmetry is caused by the local winds. Southerly wind stress at the equator drives northward flow near the surface and southward flow below that is superimposed on the Tropical Instability Wave (TIW) velocity field. This wind‐driven overturning cell, known as the Equatorial Roll, shows a distinct seasonal cycle linked to the seasonality of the meridional component of the south‐easterly trade winds. The superposition of vertical shear of the Equatorial Roll and TIWs causes asymmetric mixing during northward and southward TIW phases.

     
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  8. Abstract

    Freshwater lenses (FWLs) deposited by rain exhibit local anomalies in surface salinity and temperature. The resulting patchiness in near‐surface density and sea surface temperature influence upper ocean dynamics and air‐sea fluxes of heat. Understanding lens formation and evolution has been a focus of recent observational and modeling efforts. The work presented here integrates near‐surface ocean and atmosphere time series with remote sensing of sea surface disturbances (X‐band radar) to describe properties and kinematics of FWLs in the equatorial Indian Ocean. Twenty‐eight FWLs were observed with diverse temperature‐salinity properties and structure. Fresh salinity anomalies were as large as −1.35 psu at 3 m depth. Associated temperature anomalies ranged from −0.80 to +0.59°C. Ship‐based radar imagery allowed quantification of propagation speeds of 10 FWL fronts. In the reference frame of the moving fluid, the observed speeds are consistent with the linear long wave speed of. These results offer a novel perspective on the evolution of FWLs as gravity currents whose dynamics need to be properly accounted for to assess lens longevity, including persistence of salinity and temperature anomalies, as well as influences on air‐sea interaction.

     
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  9. Abstract

    Multiyear turbulence measurements from oceanographic moorings in equatorial Atlantic and Pacific cold tongues reveal similarities in deep cycle turbulence (DCT) beneath the mixed layer (ML) and above the Equatorial Undercurrent (EUC) core. Diurnal composites of turbulence kinetic energy dissipation rate,ϵ, clearly show the diurnal cycles of turbulence beneath the ML in both cold tongues. Despite differences in surface forcing, EUC strength and core depth DCT occurs, and is consistent in amplitude and timing, at all three sites. Time‐mean values ofϵat 30 m depth are nearly identical at all three sites. Variations of averaged values ofϵin the deep cycle layer below 30 m range to a factor of 10 between sites. A proposed scaling in depth that isolates the deep cycle layers and ofϵby the product of wind stress and current shear collapses vertical profiles at all sites to within a factor of 2.

     
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  10. Abstract

    A decade‐long time series of mixing in the equatorial Pacific cold tongue at 0°, 140°W reveals how mixing changes on El Niño–Southern Oscillation (ENSO) time scales. Separated into phase transitions to and from the neutral state, we find that mixing is most intense during the perturbation from the neutral state to peak La Niña when sea surface temperature cools and weakest during the perturbation from the neutral state to peak El Niño when sea surface temperature warms. Intermediate levels of mixing occur during relaxations back to the neutral state. Heating and cooling rates due to the divergence of turbulence heat flux across the mixed layer, where the net surface heat flux is the value of the turbulence heat flux at the sea surface, have the same amplitude and sign as sea surface heating and cooling rates during ENSO phase transitions. We suggest that the basic Bjerknes feedback must include mixing.

     
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