scholarly journals Diagnostics for Near-Surface Wind Response to the Gulf Stream in a Regional Atmospheric Model*

2014 ◽  
Vol 28 (1) ◽  
pp. 238-255 ◽  
Author(s):  
Kohei Takatama ◽  
Shoshiro Minobe ◽  
Masaru Inatsu ◽  
R. Justin Small
Keyword(s):  
Surface Layer ◽  
Seasonal Variation ◽  
Gulf Stream ◽  
Surface Wind ◽  
Atmospheric Model ◽  
Minor Role ◽  
Western Boundary ◽  
Near Surface ◽  
Adjustment Mechanism ◽  
Regional Atmospheric Model

Abstract The mechanisms acting on near-surface winds over the Gulf Stream are diagnosed using 5-yr outputs of a regional atmospheric model. The diagnostics for the surface-layer momentum vector, its curl, and its convergence are developed with a clear separation of pressure adjustment from downward momentum inputs from aloft in the surface-layer system. The results suggest that the downward momentum mixing mechanism plays a dominant role in contributing to the annual-mean climatological momentum curl, whereas the pressure adjustment mechanism plays a minor role. In contrast, the wind convergence is mainly due to the pressure adjustment mechanism. This can be explained by the orientation of background wind to the sea surface temperature front. The diagnostics also explain the relatively strong seasonal variation in surface-layer momentum convergence and the small seasonal variation in curl. Finally, the surface-layer response to other western boundary currents is examined using a reanalysis dataset.

scholarly journals Diagnostics for near-surface wind convergence/divergence response to the Gulf Stream in a regional atmospheric model

2011 ◽  
Vol 13 (1) ◽  
pp. 16-21 ◽  
Author(s):  
Kohei Takatama ◽  
Shoshiro Minobe ◽  
Masaru Inatsu ◽  
R. Justin Small
Keyword(s):  
Gulf Stream ◽  
Surface Wind ◽  
Atmospheric Model ◽  
Near Surface ◽  
Regional Atmospheric Model

scholarly journals Mechanisms shaping wind convergence under extreme synoptic situations over the Gulf Stream region

2021 ◽  
pp. 1-53
Author(s):  
Victor Rousseau ◽  
Emilia Sanchez-Gomez ◽  
Rym Msadek ◽  
Marie-Pierre Moine
Keyword(s):  
Atmospheric Circulation ◽  
Gulf Stream ◽  
Surface Wind ◽  
Vertical Mixing ◽  
Atmospheric Model ◽  
Heat Fluxes ◽  
Turbulent Heat ◽  
Atmospheric Conditions ◽  
Near Surface ◽  
Wind Regimes

AbstractAir-sea interaction processes over the Gulf Stream have received particular attention over the last decade. It has been shown that sea surface temperature (SST) gradients over the Gulf Stream can alter the near surface wind divergence through changes in the marine atmospheric boundary layer (MABL). Two mechanisms have been proposed to explain the response: the Vertical Mixing Mechanism (VMM) and the Pressure Adjustment Mechanism (PAM). However, their respective contribution is still under debate. It has been argued that the synoptic perturbations over the Gulf Stream can provide more insight on the MABL response to SST fronts. We analyze the VMM and PAM under different atmospheric conditions obtained from a classification method based on the deciles of the statistical distribution of winter turbulent heat fluxes over the Gulf Stream. Lowest deciles are associated with weak air-sea interactions and anticyclonic atmospheric circulation over the Gulf Stream, whereas highest deciles are related to strong air-sea interactions and a cyclonic circulation. Our analysis includes the low and high-resolution versions of the ARPEGEv6 atmospheric model forced by observed SST, and the recently released ERA5 global reanalysis. We find that the occurrence of anticyclonic and cyclonic perturbations associated with different anomalous wind regimes can locally modulate the activation of the VMM and the PAM. In particular, the PAM is predominant in anticyclonic conditions, whereas both mechanisms are equally present in most of the cyclonic conditions. Our results highlight the role of the atmospheric circulation and associated anomalous winds in the location, strength and occurrence of both mechanisms.

Diurnal and Seasonal Lightning Variability over the Gulf Stream and the Gulf of Mexico

2015 ◽  
Vol 72 (7) ◽  
pp. 2657-2665 ◽  
Author(s):  
Katrina S. Virts ◽  
John M. Wallace ◽  
Michael L. Hutchins ◽  
Robert H. Holzworth
Keyword(s):  
Gulf Of Mexico ◽  
Diurnal Cycle ◽  
Gulf Stream ◽  
Land Surface ◽  
Southeastern United States ◽  
Surface Wind ◽  
Tropical Rainfall Measuring Mission ◽  
Sea Breeze ◽  
Near Surface ◽  
Early Evening

Recent observations from the World Wide Lightning Location Network (WWLLN) reveal a pronounced lightning maximum over the warm waters of the Gulf Stream that exhibits distinct diurnal and seasonal variability. Lightning is most frequent during summer (June–August). During afternoon and early evening, lightning is enhanced just onshore of the coast of the southeastern United States because of daytime heating of the land surface and the resulting sea-breeze circulations and convection. Near-surface wind observations from the Quick Scatterometer (QuikSCAT) satellite indicate divergence over the Gulf of Mexico and portions of the Gulf Stream at 1800 LT, at which time lightning activity is suppressed there. Lightning frequency exhibits a broad maximum over the Gulf Stream from evening through noon of the following day, and QuikSCAT wind observations at 0600 LT indicate low-level winds blowing away from the continent and converging over the Gulf Stream. Over the northern Gulf of Mexico, lightning is most frequent from around sunrise through late morning. During winter, lightning exhibits a weak diurnal cycle over the Gulf Stream, with most frequent lightning during the evening. Precipitation rates from a 3-hourly gridded dataset that incorporates observations from Tropical Rainfall Measuring Mission (TRMM), as well as other satellites, exhibit a diurnal cycle over the Gulf Stream that lags the lightning diurnal cycle by several hours.

scholarly journals Pacific Shallow Meridional Overturning Circulation in a Warming Climate

2011 ◽  
Vol 24 (24) ◽  
pp. 6424-6439 ◽  
Author(s):  
Daiwei Wang ◽  
Mark A. Cane
Keyword(s):  
Surface Layer ◽  
Large Scale ◽  
Climate Model ◽  
Surface Wind ◽  
Meridional Overturning Circulation ◽  
First Century ◽  
Western Boundary ◽  
Warming Climate ◽  
Meridional Overturning ◽  
Twenty First Century

Abstract By analyzing a set of the Coupled Model Intercomparison Project phase 3 (CMIP3) climate model projections of the twenty-first century, it is found that the shallow meridional overturning of the Pacific subtropical cells (STCs) show contrasting trends between two hemispheres in a warming climate. The strength of STCs and equivalently the STC surface-layer transport tend to be weakening (strengthening) in the Northern (Southern) Hemisphere as a response to large-scale surface wind changes over the tropical Pacific. The STC pycnocline transport convergence into the equatorial Pacific Ocean from higher latitudes shows a robust weakening in the twenty-first century. This weakening is mainly through interior pathways consistent with the relaxation of the zonal pycnocline tilt, whereas the transport change through western boundary pathways is small and not consistent across models. It is found that the change of the western boundary pycnocline transport is strongly affected by the shoaling of the pycnocline base. In addition, there is a robust weakening of the Indonesian Throughflow (ITF) transport in a warming climate. In the multimodel ensemble mean, the response to greenhouse warming of the upper-ocean mass balance associated with the STCs is such that the weakening of the equatorward pycnocline transport convergence is balanced by a weakening of the poleward surface-layer transport divergence and the ITF transport of similar amounts.

scholarly journals Covariability of Near-Surface Wind Speed Statistics and Mesoscale Sea Surface Temperature Fluctuations

2018 ◽  
Vol 48 (3) ◽  
pp. 465-478 ◽  
Author(s):  
Johannes Gemmrich ◽  
Adam Monahan
Keyword(s):  
Wind Speed ◽  
Cold Water ◽  
Surface Wind ◽  
Surface Wind Speed ◽  
Western Boundary ◽  
Near Surface ◽  
Water Entrainment ◽  
Speed Fluctuations ◽  
Coupling Processes

AbstractThe atmospheric (ABL) and ocean (OBL) boundary layers are intimately linked via mechanical and thermal coupling processes. In many regions over the world’s oceans, this results in a strong covariability between anomalies in wind speed and SST. At oceanic mesoscale, this coupling can be driven either from the atmosphere or the ocean. Gridded SST and wind speed data at 0.25° resolution show that over the western North Atlantic, the ABL mainly responds to the OBL, whereas in the eastern North Pacific and in the Southern Ocean, the OBL largely responds to wind speed anomalies. This general behavior is also verified by in situ buoy observations in the Atlantic and Pacific. A stochastic, nondimensional, 1D coupled air–sea boundary layer model is utilized to assess the relative importance of the coupling processes. For regions of little intrinsic SST fluctuations (i.e., most regions of the world’s oceans away from strong temperature fronts), the inclusion of cold water entrainment at the thermocline is crucial. In regions with strong frontal activities (e.g., the western boundary regions), the coupling is dominated by the SST fluctuations, and the frontal variability needs to be included in models. Generally, atmospheric and ocean-driven coupling lead to an opposite relationship between SST and wind speed fluctuations. This effect can be especially important for higher wind speed quantiles.

scholarly journals Processes Shaping the Frontal-Scale Time-Mean Surface Wind Convergence Patterns around the Gulf Stream and Agulhas Return Current in Winter

2020 ◽  
Vol 33 (21) ◽  
pp. 9083-9101
Author(s):  
Ryusuke Masunaga ◽  
Hisashi Nakamura ◽  
Bunmei Taguchi ◽  
Takafumi Miyasaka
Keyword(s):  
High Resolution ◽  
Gulf Stream ◽  
Kuroshio Extension ◽  
Surface Wind ◽  
Western Boundary ◽  
Primary Role ◽  
Return Current ◽  
Atmospheric Fronts ◽  
Potential Factors ◽  
Moderate Magnitude

ABSTRACTHigh-resolution satellite observations and numerical experiments have revealed local enhancement of time-mean surface wind convergence along the axes of warm western boundary currents and divergence slightly poleward. A recent study has suggested that frequent occurrence of persistent atmospheric fronts and sea level pressure (SLP) troughs along a sea surface temperature (SST) front are responsible for shaping the frontal-scale wind convergence and divergence contrast as seen in the wintertime climatology near the Kuroshio Extension (KE). These events tend to induce surface wind convergence with moderate magnitude. Through atmospheric reanalysis with high-resolution SST, the present study reveals that, as in the vicinity of the KE, surface wind convergence with moderate magnitude and divergence with moderate-to-extreme magnitude are found to play a primary role in shaping the climatological-mean wind convergence–divergence contrasts across the SST fronts near the Gulf Stream (GS) and Agulhas Return Current (ARC) in winter. In contrast, strong-to-extreme convergence events associated with synoptic-scale atmospheric disturbances are found to yield horizontally uniform time-mean wind convergence. Furthermore, cluster analysis and case studies suggest that persistent atmospheric fronts and SLP troughs are responsible for inducing moderate wind convergence also near the GS and ARC. Thus, these features are consistent with their counterpart near the KE, but the impacts of the ARC tend to be substantially weaker, probably due to its cooler SST among other potential factors.

Small-Scale Signal in Mean Dynamic Topographies Applying Combined Geoid Models

2020 ◽  
Author(s):  
Frank Siegismund ◽  
Xanthi Oikonomidou ◽  
Philipp Zingerle
Keyword(s):  
High Resolution ◽  
Gulf Stream ◽  
Ocean Surface ◽  
Ocean Dynamics ◽  
Small Scale ◽  
Western Boundary ◽  
Low Resolution ◽  
Geoid Model ◽  
Near Surface ◽  
Dependent Signal

<p>The Dynamic ocean Topography (DT) describes the deviation of the true ocean surface from a hypothetical equilibrium state ocean at rest forced by gravity alone. With the geostrophic surface currents obtained from its gradients the DT is an essential parameter for describing the ocean dynamics. Observation-based global temporal Mean geodetic DTs (MDTs) are obtained from the difference of altimetric Mean Sea Surface (MSS) and the geoid height, that equipotential surface of gravity closest to the ocean surface.</p><p>The geoid is provided either as a satellite-only, or a combined model including in addition gravity anomalies derived from satellite altimetry and ground data. In recent years the focus was on satellite-only models, produced from new space-born observations obtained from the Gravity Recovery and Climate Experiment (GRACE) and Gravity field and Ocean Circulation Explorer (GOCE) satellite missions. Moreover, combined geoid models are only cautiously used for MDT calculation, since it is still in question to what extent the gravity information obtained from altimetry is distorted by the MDT information included therein and how this translates into errors of the MDT.</p><p>Here we want to concentrate on MDT models based on recent combined geoid models. An assessment is provided based on comparisons to near-surface drifter data from the Global Drifter Program (GDP). Besides providing a general, global assessment, we focus on signal content on small scales, addressing mainly two questions: 1) Do MDTs obtained from combined geoid models contain signal for scales smaller than resolvable by the<br>satellite-only models? 2) Is there a maximum resolution beyond which no signal is detectable?</p><p>Until recently, these questions couldn't be answered since low resolution MDTs usually contained strong wavy-structured errors and thus needed a strong spatial filtering thereby killing the smallest scales resolved in the MDT. These errors, which worsen with lower resolution, are caused by Gibbs effects provoked by imperfections in bringing the high resolution ocean-only MSS models into spectral consistency with the much lower resolved global geoid model. A new methodology, however, improves the necessary globalization of the MSS. After subtraction of the geoid model, subsequent cutting-off the signal beyond a specific spherical harmonic degree and order (d/o) results in an MDT without any Gibbs effects, also for low resolution models.</p><p>To answer the questions posed above applying the new methodology, the scale-dependent signal in MDTs for different geoid models is presented for a list of cut off d/os. To minimize the influence of noise on the results we concentrate on strong signal Western Boundary Currents like the Gulf Stream and the Kuroshio. For the Gulf Stream results of a high resolution hydrodynamic model are available and presented as an independent method to estimate the scale dependent signal.</p>

scholarly journals Influence of Soil Moisture on Urban Microclimate and Surface-Layer Meteorology in Oklahoma City

2014 ◽  
Vol 53 (1) ◽  
pp. 83-98 ◽  
Author(s):  
Syed Zahid Husain ◽  
Stéphane Bélair ◽  
Sylvie Leroyer
Keyword(s):  
Surface Layer ◽  
Soil Moisture ◽  
Wind Speed ◽  
Urban Soil ◽  
Surface Wind ◽  
Surface Wind Speed ◽  
Oklahoma City ◽  
Near Surface ◽  
Model Simulations ◽  
Urban Surface Layer

AbstractThe influence of soil moisture on the surface-layer atmosphere is examined in this paper by analyzing the outputs of model simulations for different initial soil moisture configurations, with particular emphasis on urban microclimate. In addition to a control case, four different soil moisture distributions within the urban and surrounding rural areas are considered in this study. Outputs from the Global Environmental Multiscale atmospheric model simulations are compared with observations from the Joint Urban 2003 experiment held in Oklahoma City, Oklahoma, and the relevant conclusions drawn in this paper are therefore valid for similar medium-size cities. In general, high soil moisture is found to be associated with colder near-surface temperature and lower near-surface wind speed, whereas drier soil resulted in warmer temperatures and enhanced low-level wind. Relative to urban soil moisture content, rural soil conditions are predicted to have larger impacts on both rural and urban surface-layer meteorological conditions. Dry rural and wet urban soil configurations are shown to have a strong influence on the urban–rural temperature contrast and resulted in city-induced secondary circulations that considerably affect the near-surface wind speed. Dry rural soil in particular is found to intensify the nocturnal low-level jet and significantly affect the thermal stability of nocturnal near-neutral urban surface layer by altering both thermal and mechanical generation of turbulence.

scholarly journals Feedback of mesoscale ocean currents on atmospheric winds in high-resolution coupled models and implications for the forcing of ocean-only models

2017 ◽  
Author(s):  
Rafael Abel ◽  
Claus W. Böning ◽  
Richard J. Greatbatch ◽  
Helene T. Hewitt ◽  
Malcolm J. Roberts
Keyword(s):  
Climate Model ◽  
Global Climate Model ◽  
Surface Wind ◽  
Ocean Currents ◽  
Global Ocean ◽  
Ocean Current ◽  
Western Boundary ◽  
Coupling Coefficients ◽  
Coupled Models ◽  
Near Surface

Abstract. The repercussions of surface ocean currents for the near-surface wind and the air-sea momentum flux are investigated in two versions of a global climate model with eddying ocean. The focus is on the effect of mesoscale ocean current features at scales of less than 150 km, by considering high-pass filtered, monthly-mean model output fields. We find a clear signature of a mesoscale oceanic imprint in the wind fields over the energetic areas of the oceans, particularly along the extensions of the western boundary currents and the Antarctic Circumpolar Current. These areas are characterized by a positive correlation between mesoscale perturbations in the curl of the surface currents and the wind curl. The coupling coefficients are spatially non-uniform and show a pronounced seasonal cycle. The positive feedback of mesoscale current features on the near-surface wind acts in opposition to their damping effect on the wind stress. A tentative incorporation of this feedback in the surface stress formulation of an eddy-permitting global ocean-only model leads to a gain in the kinetic energy of up to 10 %, suggesting a fundamental shortcoming of present ocean model configurations.

scholarly journals The Modulation of Gulf Stream Influence on the Troposphere by the Eddy-Driven Jet

2020 ◽  
Vol 33 (10) ◽  
pp. 4109-4120 ◽  
Author(s):  
Rhys Parfitt ◽  
Young-Oh Kwon
Keyword(s):  
Boundary Layer ◽  
North Atlantic ◽  
Gulf Stream ◽  
Surface Wind ◽  
Sensible Heat Flux ◽  
Temperature Gradients ◽  
Sensible Heat ◽  
Reanalysis Dataset ◽  
Near Surface ◽  
Atmospheric Fronts

AbstractThis study suggests that the Gulf Stream influence on the wintertime North Atlantic troposphere is most pronounced when the eddy-driven jet (EDJ) is farthest south and better collocated with the Gulf Stream. Using the reanalysis dataset NCEP-CFSR for December–February 1979–2009, the daily EDJ latitude is separated into three regimes (northern, central, and southern). It is found that the average trajectory of atmospheric fronts covaries with EDJ latitude. In the southern EDJ regime (~19% of the time), the frequency of near-surface atmospheric fronts that pass across the Gulf Stream is maximized. Analysis suggests that this leads to significant strengthening in near-surface atmospheric frontal convergence resulting from strong air–sea sensible heat flux gradients (due to strong temperature gradients in the atmosphere and ocean). In recent studies, it was shown that the pronounced band of time-mean near-surface wind convergence across the Gulf Stream is set by atmospheric fronts. Here, it is shown that an even smaller subset of atmospheric fronts—those associated with a southern EDJ—primarily sets the time mean, due to enhanced Gulf Stream air–sea interaction. Furthermore, statistically significant anomalies in vertical velocity extending well above the boundary layer are identified in association with changes in EDJ latitude. These anomalies are particularly strong for a southern EDJ and are spatially consistent with increases in near-surface atmospheric frontal convergence over the Gulf Stream. These results imply that much of the Gulf Stream influence on the time-mean atmosphere is modulated on synoptic time scales, and enhanced when the EDJ is farthest south.

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