Effects of Sea State and Small-Scale Currents on Air-Sea Fluxes in the Northwest Tropical Atlantic Ocean

2021 ◽  
Author(s):  
Cesar Sauvage ◽  
Hyodae Seo ◽  
Carol Anne Clayson ◽  
Jim Edson
Keyword(s):  
Wind Stress ◽  
Wave Model ◽  
Heat Fluxes ◽  
Ocean Currents ◽  
Small Scale ◽  
Turbulent Heat ◽  
Sea State ◽  
Trade Winds ◽  
Low Level ◽  
Directional Alignment

<p>The Northwest Tropical Atlantic is characterized by the strong North Brazilian Current (NBC), its rings, and numerous mesoscale eddies, which ceaselessly interact with the persistent trade winds and trade cumuli. Near the coast, the ocean stratification is maintained by the Amazon and Orinoco river discharges, which control the vertical mixing and the near-shore circulation dynamics. Breaking waves and swells are ubiquitous under the trade winds, and hence, the wave-induced mixing and wave-mediated air-sea fluxes are expected to modulate the eddy variability and low-level clouds. Our study aims to enhance understanding of the air-sea fluxes mediated by the mesoscale ocean currents and surface waves and evaluate their impacts on the ocean and atmosphere.</p><p>High-resolution ocean model (ROMS) and wave model (WW3) simulations are conducted for the period of the ATOMIC/EUREC4A experiments. The model surface state variables are used to compute offline the air-sea heat and momentum fluxes using the latest COARE v3.6 bulk flux algorithm under various sea state conditions induced by surface waves, ocean currents, and their interaction. The results demonstrate that considering the spatial variability in sea states via wave slope and wave age (e.g., swells and wind-seas) leads to enhanced spatial variability in drag coefficient and wind stress. Comparison to wind stress estimated using the wind-speed dependent formulation, meaning that COARE makes sea state assumptions under given wind, indicates that, at any given time, wind and wave in fact, rarely match those assumptions. The swells (wind-seas) decreases (increases) the sea surface roughness length, drag coefficient, and wind stress by 10-15%. However, we find that the sea state impact on turbulent heat flux is negligible.</p><p>More importantly, we also show that considering the ocean currents in the COARE algorithm yields much stronger spatio-temporal variations in not just the wind stress but also turbulent heat fluxes. The intense and small-scale current fields in this region are associated with the NBC and its rings, smaller mesoscale eddies, and filamentary density fronts associated with the freshwater plumes. The surface currents associated with these small-scale energetic features alter the relative wind speed and thus the air-sea fluxes depending on the directional alignment between the wind and current; the increase (decrease) in both the wind and heat fluxes by ~20% is found with the current and wind are in the opposite (same) direction wind. Moreover, this relative wind effect appears to be reinforced by wave direction as well, also via the directional alignment between waves and currents, since the waves are mainly aligned with the trade wind in this region.</p><p>Further analyses are underway to examining the seasonality of the modulation by the wave-current interaction, quantifying the role of the freshwater distribution, and exploring the time-mean influence on the low-level clouds. The results from the ocean and wave modeling efforts will guide our ongoing fully coupled ocean-atmosphere (and wave) model simulations to quantify their impacts on the atmosphere, including low-level clouds.</p>

scholarly journals Air–Sea Turbulent Heat Fluxes in Climate Models and Observational Analyses: What Drives Their Variability?

2019 ◽  
Vol 32 (8) ◽  
pp. 2397-2421 ◽  
Author(s):  
R. Justin Small ◽  
Frank O. Bryan ◽  
Stuart P. Bishop ◽  
Robert A. Tomas
Keyword(s):  
Heat Flux ◽  
Climate Models ◽  
Climate Model ◽  
Turbulent Heat Flux ◽  
Heat Fluxes ◽  
Small Scale ◽  
Turbulent Heat ◽  
Realistic Representation ◽  
Frontal Zones ◽  
The Tropics

Abstract A traditional view is that the ocean outside of the tropics responds passively to atmosphere forcing, which implies that air–sea heat fluxes are mainly driven by atmosphere variability. This paper tests this viewpoint using state-of-the-art air–sea turbulent heat flux observational analyses and a climate model run at different resolutions. It is found that in midlatitude ocean frontal zones the variability of air–sea heat fluxes is not predominantly driven by the atmosphere variations but instead is forced by sea surface temperature (SST) variations arising from intrinsic oceanic variability. Meanwhile in most of the tropics and subtropics wind is the dominant driver of heat flux variability, and atmosphere humidity is mainly important in higher latitudes. The predominance of ocean forcing of heat fluxes found in frontal regions occurs on scales of around 700 km or less. Spatially smoothing the data to larger scales results in the traditional atmosphere-driving case, while filtering to retain only small scales of 5° or less leads to ocean forcing of heat fluxes over most of the globe. All observational analyses examined (1° OAFlux; 0.25° J-OFURO3; 0.25° SeaFlux) show this general behavior. A standard resolution (1°) climate model fails to reproduce the midlatitude, small-scale ocean forcing of heat flux: refining the ocean grid to resolve eddies (0.1°) gives a more realistic representation of ocean forcing but the variability of both SST and of heat flux is too high compared to observational analyses.

The importance of including sea surface current when estimating air-sea turbulent heat fluxes and wind stress in the Gulf Stream region

2020 ◽  
Author(s):  
Xiangzhou Song
Keyword(s):  
Heat Flux ◽  
Wind Stress ◽  
Gulf Stream ◽  
Turbulent Heat Flux ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
Surface Currents ◽  
Mean Flow ◽  
Geostrophic Currents ◽  
Turbulent Heat Fluxes

AbstractUsing buoy observations from 2004 to 2010 and newly released atmospheric reanalysis and satellite altimetry-derived geostrophic currents from 1993 to 2017, the quantitative contribution of daily mean surface currents to air-sea turbulent heat flux and wind stress uncertainties in the Gulf Stream (GS) region is investigated based on bulk formulas. At four buoy stations, the daily mean latent (sensible) heat flux difference between the estimates with and without surface currents ranges from -18 (-4) to 20 (4) Wm-2, while the daily mean wind stress difference ranges from -0.04 to 0.02 Nm-2. The positive values indicate higher estimates with opposite directions between surface currents and absolute winds. The transition between positive and negative differences is significantly associated with synoptic-scale weather variations. The uncertainties based on buoy observations are approximately 7% and 3% for wind stress and turbulent heat fluxes, respectively. The new reanalysis and satellite geostrophic currents confirm the uncertainties identified by buoy observations with acceptable discrepancies and provide a spatial view of the uncertainty fields. The mean geostrophic currents are aligned with the surface wind along the GS; therefore, the turbulent heat fluxes and wind stress will be ‘underestimated’ with surface currents included. However, on both sides of the GS, the surface flow can be upwind due to possible mechanisms of eddy-mean flow interactions and recirculations, resulting in higher turbulent heat flux estimations. The wind stress and turbulent heat flux uncertainties experience significant seasonal variations and show long-term trends.

scholarly journals Wind Stress Dynamics in Chesapeake Bay: Spatiotemporal Variability and Wave Dependence in a Fetch-Limited Environment

2015 ◽  
Vol 45 (10) ◽  
pp. 2679-2696 ◽  
Author(s):  
Alexander W. Fisher ◽  
Lawrence P. Sanford ◽  
Steven E. Suttles
Keyword(s):  
Chesapeake Bay ◽  
Wind Stress ◽  
Wave Model ◽  
Heat Fluxes ◽  
Spatiotemporal Variability ◽  
Surface Heat Fluxes ◽  
Wave Measurements ◽  
Variable Surface ◽  
Ultrasonic Anemometer ◽  
Variable Wind

AbstractThe spatiotemporal variability of wind stress dynamics in Chesapeake Bay has been investigated using a combination of observations and numerical modeling. Direct measurements of momentum and surface heat fluxes were collected using an ultrasonic anemometer deployed on a fixed tower in the middle reaches of Chesapeake Bay in the spring of 2012 along with collocated wave measurements. These measurements were compared to bulk estimates of wind stress using wave-dependent formulations of the Charnock parameter (alpha). Results indicate that a constant alpha value of 0.018 reasonably represents observed stress values, but estimates can be improved by the inclusion of surface wave information in the parameterization of alpha. Using a wave age formulation of alpha in combination with an optimally interpolated 10-m neutral wind field, a third-generation numerical wave model, Simulating Waves Nearshore (SWAN), was employed to investigate the spatiotemporal variability of wind stress across the estuary. Alpha values were found to be wind speed dependent and displayed spatial distributions that ranged between open-ocean values and strongly fetch-limited values. Model results suggest that variable wind stress dynamics stemming from a combination of variable surface winds and fetch-limited wave growth may result in the 10-m neutral drag coefficient varying by a factor of 2 across the estuary. Up to 20% of these changes can be directly attributed to the effects of variable waves.

scholarly journals U.S. West Coast Surface Heat Fluxes, Wind Stress, and Wind Stress Curl from a Mesoscale Model

2005 ◽  
Vol 133 (11) ◽  
pp. 3202-3216 ◽  
Author(s):  
T. Haack ◽  
S. D. Burk ◽  
R. M. Hodur
Keyword(s):  
Relative Humidity ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Wind Stress ◽  
Mesoscale Model ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Wind Stress Curl ◽  
Low Level

Abstract Monthly averages of numerical model fields are beneficial for depicting patterns in surface forcing such as sensible and latent heat fluxes, wind stress, and wind stress curl over data-sparse ocean regions. Grid resolutions less than 10 km provide the necessary mesoscale detail to characterize the impact of a complex coastline and coastal topography. In the present study a high-resolution mesoscale model is employed to reveal patterns in low-level winds, temperature, relative humidity, sea surface temperature as well as surface fluxes, over the eastern Pacific and along the U.S. west coast. Hourly output from successive 12-h forecasts are averaged to obtain monthly mean patterns from each season of 1999. The averages yield information on interactions between the ocean and the overlying atmosphere and on the influence of coastal terrain forcing in addition to their month-to-month variability. The spring to summer transition is characterized by a dramatic shift in near-surface winds, temperature, and relative humidity as offshore regions of large upward surface fluxes diminish and an alongshore coastal flux gradient forms. Embedded within this gradient, and the imprint of strong summertime topographic forcing, are small-scale fluctuations that vary in concert with local changes in sea surface temperature. Potential feedbacks between the low-level wind, sea surface temperature, and the wind stress curl are explored in the coastal regime and offshore waters. In all seasons, offshore extensions of colder coastal waters impose a marked influence on low-level conditions by locally enhancing stability and reducing the wind speed, while buoy measurements along the coast indicate that sea surface temperatures and wind speeds tend to be negatively correlated.

scholarly journals Large-scale drivers of the mistral wind: link to Rossby wave life cycles and seasonal variability

2021 ◽  
Vol 2 (3) ◽  
pp. 609-630
Author(s):  
Yonatan Givon ◽  
Douglas Keller Jr. ◽  
Vered Silverman ◽  
Romain Pennel ◽  
Philippe Drobinski ◽  
...  
Keyword(s):  
Rossby Wave ◽  
Clustering Algorithm ◽  
Life Cycles ◽  
Heat Fluxes ◽  
Extreme Weather Events ◽  
Turbulent Heat ◽  
Low Level ◽  
Lee Cyclogenesis ◽  
The Mediterranean ◽  
Upper Level

Abstract. The mistral is a northerly low-level jet blowing through the Rhône valley in southern France and down to the Gulf of Lion. It is co-located with the cold sector of a low-level lee cyclone in the Gulf of Genoa, behind an upper-level trough north of the Alps. The mistral wind has long been associated with extreme weather events in the Mediterranean, and while extensive research focused on the lower-tropospheric mistral and lee cyclogenesis, the different upper-tropospheric large- and synoptic-scale settings involved in producing the mistral wind are not generally known. Here, the isentropic potential vorticity (PV) structures governing the occurrence of the mistral wind are classified using a self-organizing map (SOM) clustering algorithm. Based upon a 36-year (1981–2016) mistral database and daily ERA-Interim isentropic PV data, 16 distinct mistral-associated PV structures emerge. Each classified flow pattern corresponds to a different type or stage of the Rossby wave life cycle, from broad troughs to thin PV streamers to distinguished cutoffs. Each of these PV patterns exhibits a distinct surface impact in terms of the surface cyclone, surface turbulent heat fluxes, wind, temperature and precipitation. A clear seasonal separation between the clusters is evident, and transitions between the clusters correspond to different Rossby-wave-breaking processes. This analysis provides a new perspective on the variability of the mistral and of the Genoa lee cyclogenesis in general, linking the upper-level PV structures to their surface impact over Europe, the Mediterranean and north Africa.

scholarly journals Effects of Periodic Tidal Elevations on the Air-Sea Momentum and Turbulent Heat Fluxes in the East China Sea

Atmosphere ◽  
2022 ◽  
Vol 13 (1) ◽  
pp. 90
Author(s):  
Yuting Han ◽  
Yuxin Liu ◽  
Xingwei Jiang ◽  
Mingsen Lin ◽  
Yangang Li ◽  
...  
Keyword(s):  
East China Sea ◽  
Wind Stress ◽  
Heat Fluxes ◽  
Surface Elevation ◽  
East China ◽  
Turbulent Heat ◽  
Sensible Heat ◽  
The East China Sea ◽  
Turbulent Heat Fluxes ◽  
The Mean

Using bulk formulas, two-year platform (fastened to the seabed) hourly observations from 2016 to 2017 in the East China Sea (121.6° E, 32.4° N) are used to investigate the role of the tide-induced surface elevation in changing the fixed observational height and modifying the momentum and air-sea turbulent heat fluxes. The semidiurnal tide-dominated elevation anomalies ranging from −3.6 to 3.9 m change the fixed platform observational height. This change causes hourly differences in the wind stress and latent and sensible heat fluxes between estimates with and without considering surface elevation, with values ranging from −1.5 × 10−3 Nm−2, −10.2 Wm−2, and −3.6 Wm−2 to 2.2 × 10−3 Nm−2, 8.4 Wm−2, and 4.6 Wm−2, respectively. More significant differences occur during spring tides. The differences show weak dependence on the temperature, indicating weak seasonal variations. The mean (maximum) difference percentage relative to the mean magnitude is approximately 3.5% (7%), 1.5% (3%), and 1.5% (3%) for the wind stress and latent and sensible heat fluxes, respectively. The boundary layer stability (BLS) can convert from near-neutral conditions to stable and unstable states in response to tide-induced changes in the observational height, with a probability of occurrence of 2%. Wind anomalies play dominant roles in determining the hourly anomalies of the latent heat flux, regardless of the state of the BLS. Extreme cases, including the cold air outbreak in 2016, tropical cyclones Meranti in 2016, and Ampil in 2018, are also examined. This study will facilitate future observation-reanalysis comparisons in the studied coastal region where ocean–atmosphere-land interactive processes are significant.

Northern Hemisphere Extratropical Turbulent Heat Fluxes in ASRv2 and Global Reanalyses

2019 ◽  
Vol 32 (7) ◽  
pp. 2145-2166 ◽  
Author(s):  
Flavio Justino ◽  
Aaron B. Wilson ◽  
David H. Bromwich ◽  
Alvaro Avila ◽  
Le-Sheng Bai ◽  
...  
Keyword(s):  
Northern Hemisphere ◽  
Vegetation Cover ◽  
Large Scale ◽  
Intraseasonal Variability ◽  
Horizontal Resolution ◽  
Heat Fluxes ◽  
Small Scale ◽  
Turbulent Heat ◽  
Area Index ◽  
Wind Speeds

Abstract Large-scale objectively analyzed gridded products and satellite estimates of sensible (H) and latent (LE) heat fluxes over the extratropical Northern Hemisphere are compared to those derived from the regional Arctic System Reanalysis version 2 (ASRv2) and a selection of current-generation global reanalyses. Differences in H and LE among the reanalyses are strongly linked to the wind speed magnitudes and vegetation cover. Specifically, ASRv2 wind speeds match closely with observations over the northern oceans, leading to an improved representation of H compared to the global reanalyses. Comparison of evaporative fraction shows that the global reanalyses are characterized by a similar H and LE partitioning from April through September, and therefore exhibit weak intraseasonal variability. However, the higher horizontal resolution and weekly modification of the vegetation cover based on satellite data in ASRv2 provides an improved snow–albedo feedback related to changes in the leaf area index. Hence, ASRv2 better captures the small-scale processes associated with day-to-day vegetation feedbacks with particular improvements to the H over land. All of the reanalyses provide realistic dominant hemispheric patterns of H and LE and the locations of maximum and minimum fluxes, but they differ greatly with respect to magnitude. This is especially true for LE over oceanic regions. Therefore, uncertainties in heat fluxes remain that may be alleviated in reanalyses through improved representation of physical processes and enhanced assimilation of observations.

scholarly journals Evaluation of Near-Surface Parameters in the Two Versions of the Atmospheric Model in CESM1 using Flux Station Observations

2013 ◽  
Vol 26 (1) ◽  
pp. 26-44 ◽  
Author(s):  
Jenny Lindvall ◽  
Gunilla Svensson ◽  
Cecile Hannay
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Diurnal Cycle ◽  
Surface Wind ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
Seasonal Temperature ◽  
Near Surface ◽  
Surface Parameters ◽  
Low Level

Abstract This paper describes the performance of the Community Atmosphere Model (CAM) versions 4 and 5 in simulating near-surface parameters. CAM is the atmospheric component of the Community Earth System Model (CESM). Most of the parameterizations in the two versions are substantially different, and that is also true for the boundary layer scheme: CAM4 employs a nonlocal K-profile scheme, whereas CAM5 uses a turbulent kinetic energy (TKE) scheme. The evaluation focuses on the diurnal cycle and global observational and reanalysis datasets are used together with multiyear observations from 35 flux tower sites, providing high-frequency measurements in a range of different climate zones. It is found that both model versions capture the timing of the diurnal cycle but considerably overestimate the diurnal amplitude of net radiation, temperature, wind, and turbulent heat fluxes. The seasonal temperature range at mid- and high latitudes is also overestimated with too warm summer temperatures and too cold winter temperatures. The diagnosed boundary layer is deeper in CAM5 over ocean in regions with low-level marine clouds as a result of the turbulence generated by cloud-top cooling. Elsewhere, the boundary layer is in general shallower in CAM5. The two model versions differ substantially in their representation of near-surface wind speeds over land. The low-level wind speed in CAM5 is about half as strong as in CAM4, and the difference is even larger in areas where the subgrid-scale terrain is significant. The reason is the turbulent mountain stress parameterization, only applied in CAM5, which acts to increase the surface stress and thereby reduce the wind speed.

scholarly journals The Modulation of Equatorial Turbulence by Tropical Instability Waves in a Regional Ocean Model

2015 ◽  
Vol 45 (4) ◽  
pp. 1155-1173 ◽  
Author(s):  
R. M. Holmes ◽  
L. N. Thomas
Keyword(s):  
Turbulent Mixing ◽  
Large Scale ◽  
Ocean Model ◽  
Heat Fluxes ◽  
Small Scale ◽  
Turbulent Heat ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
Regional Ocean Model ◽  
The Impact

AbstractSmall-scale turbulent mixing in the upper Equatorial Undercurrent (EUC) of the eastern Pacific cold tongue is a critical component of the SST budget that drives variations in SST on a range of time scales. Recent observations have shown that turbulent mixing within the EUC is modulated by tropical instability waves (TIWs). A regional ocean model is used to investigate the mechanisms through which large-scale TIW circulation modulates the small-scale shear, stratification, and shear-driven turbulence in the EUC. Eulerian analyses of time series taken from both the model and the Tropical Atmosphere Ocean (TAO) array suggest that increases in the zonal shear of the EUC drive increased mixing on the leading edge of the TIW warm phase. A Lagrangian vorticity analysis attributes this increased zonal shear to horizontal vortex stretching driven by the strain in the TIW horizontal velocity field acting on the existing EUC shear. To investigate the impact of horizontal vortex stretching on the turbulent heat flux averaged over a TIW period the effects of periodic TIW strain are included as forcing in a simple 1D mixing model of the EUC. Model runs with TIW forcing show turbulent heat fluxes up to 30% larger than runs without TIW forcing, with the magnitude of the increase being sensitive to the vertical mixing scheme used in the model. These results emphasize the importance of coupling between the large-scale circulation and small-scale turbulence in the equatorial regions, with implications for the SST budget of the equatorial Pacific.

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