scholarly journals A “Cold Path” for the Gulf Stream–Troposphere Connection

2017 ◽  
Vol 30 (4) ◽  
pp. 1363-1379 ◽  
Author(s):  
Benoît Vannière ◽  
Arnaud Czaja ◽  
Helen Dacre ◽  
Tim Woollings
Keyword(s):  
Boundary Layer ◽  
Gulf Stream ◽  
Surface Wind ◽  
Extratropical Cyclone ◽  
Convective Precipitation ◽  
Dynamical Response ◽  
Sst Front ◽  
Simple Boundary ◽  
Pressure Anomaly ◽  
Sst Gradient

Abstract The mechanism by which the Gulf Stream sea surface temperature (SST) front anchors a band of precipitation on its warm edge is still a matter of debate, and little is known about how synoptic activity contributes to the mean state. In the present study, the influence of the SST front on precipitation is investigated during the course of a single extratropical cyclone using a regional configuration of the Met Office Unified Model. The comparison of a control run with a simulation in which SST gradients were smoothed brought the following conclusions: a band of precipitation is reproduced for a single extratropical cyclone, and the response to the SST gradient is dominated by a change of convective precipitation in the cold sector of the storm. Several climatological features described by previous studies, such as surface wind convergence on the warm edge or a meridional circulation cell across the SST front, are also reproduced at synoptic time scales in the cold sector. Based on these results, a simple boundary layer model is proposed to explain the convective and dynamical response to the SST gradient in the cold sector. In this model, cold and dry air parcels acquire more buoyancy over a sharp SST gradient and become more convectively unstable. The convection sets a pressure anomaly over the entire depth of the boundary layer that drives wind convergence. This case study offers a new pathway by which the SST gradient can anchor a climatological band of precipitation.

Numerical Simulation of Boundary Layer Structure and Cross-Equatorial Flow in the Eastern Pacific*

2005 ◽  
Vol 62 (6) ◽  
pp. 1812-1830 ◽  
Author(s):  
R. Justin Small ◽  
Shang-Ping Xie ◽  
Yuqing Wang ◽  
Steven K. Esbensen ◽  
Dean Vickers
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Layer Structure ◽  
Surface Wind ◽  
Eastern Pacific ◽  
Vertical Shear ◽  
Boundary Layer Structure ◽  
Sst Front ◽  
Flow Adjustment ◽  
Sst Gradient

Abstract Recent observations from spaceborne microwave sensors have revealed detailed structure of the surface flow over the equatorial eastern Pacific in the boreal fall season. A marked acceleration of surface wind across the northern sea surface temperature (SST) front of the cold tongue is a prominent feature of the regional climate. Previous studies have attributed the acceleration to the effect of enhanced momentum mixing over the warmer waters. A high-resolution numerical model is used to examine the cross-frontal flow adjustment. In a comprehensive comparison, the model agrees well with many observed features of cross-equatorial flow and boundary layer structure from satellite, Tropical Atmosphere Ocean (TAO) moorings, and the recent Eastern Pacific Investigation of Climate Processes (EPIC) campaign. In particular, the model simulates the acceleration across the SST front, and the change from a stable to unstable boundary layer. Analysis of the model momentum budget indicates that the hydrostatic pressure gradient, set up in response to the SST gradient, drives the surface northward acceleration. Because of thermal advection by the mean southerly flow, the pressure gradient is located downstream of the SST gradient and consequently, divergence occurs over the SST front, as observed by satellite. Pressure gradients also act to change the vertical shear of the wind as the front is crossed. However, the model underpredicts the changes in vertical wind shear across the front, relative to the EPIC observations. It is suggested that the vertical transfer of momentum by mixing, a mechanism described by Wallace et al. may also act to enhance the change in shear in the observations, but the model does not simulate this effect. Reasons for this are discussed.

scholarly journals Deep Atmospheric Response to the Spring Kuroshio over the East China Sea*

2011 ◽  
Vol 24 (18) ◽  
pp. 4959-4972 ◽  
Author(s):  
Haiming Xu ◽  
Mimi Xu ◽  
Shang-Ping Xie ◽  
Yuqing Wang
Keyword(s):  
Wind Speed ◽  
East China Sea ◽  
Surface Wind ◽  
Convective Precipitation ◽  
East China ◽  
Atmospheric Response ◽  
The East China Sea ◽  
Sst Front ◽  
China Sea ◽  
The Kuroshio

Abstract The atmospheric response to the spring Kuroshio Front over the East China Sea is investigated using a suite of high-resolution satellite data and a regional atmospheric model. The atmospheric response appears to extend beyond the marine atmospheric boundary layer, with frequent occurrence of cumulus convection. In spring, Quick Scatterometer (QuikSCAT) wind speed shows a clear effect of sea surface temperature (SST), with high (low) wind speed observed over the warm (cold) tongue. This in-phase relationship between SST and surface wind speed is indicative of SST influence on the atmosphere. Wind convergence is found on the warmer flank of the Kuroshio Front, accompanied by a narrow rainband. The analysis of satellite-borne radar measurements indicates that deep convection appears over the Kuroshio warm tongue in the spring season, with enhanced convective precipitation, frequent occurrence of cumulus convection, and increased precipitation (cloud) tops in altitude. These deep convective activities along the Kuroshio warm tongue are further supported by enhanced lightning flash rate observed by satellite and atmospheric heating estimated by a Japanese reanalysis. The Weather Research and Forecasting (WRF) model is used to investigate the precipitation response to the spring Kuroshio SST front over the East China Sea. Forced by observed SST [control (CTL)], the model well simulates a narrow band of precipitation, high wind speed, and surface wind convergence that closely follows the Kuroshio warm current, consistent with satellite observations. This narrow rainband completely disappears in the model when the SST front is removed by horizontally smoothed SST (SmSST). The results show that it is convective precipitation that is sensitive to the Kuroshio SST front. A case study for an eastward-moving extratropical cyclone indicates that convective precipitation increases its intensity and duration in the CTL run compared to the SmSST run. Local enhancement of upward sensible and latent heat fluxes and convective instability in the lower atmosphere are the key to anchoring the narrow band of convective precipitation that closely follows the Kuroshio.

Atmospheric response to Gulf Stream SST front shifting: impact of horizontal resolution in an ensemble of global climate models

2021 ◽  
Author(s):  
Luca Famooss Paolini ◽  
Alessio Bellucci ◽  
Paolo Ruggieri ◽  
Panos Athanasiadis ◽  
Silvio Gualdi
Keyword(s):  
High Resolution ◽  
Gulf Stream ◽  
General Circulation ◽  
Diabatic Heating ◽  
Storm Track ◽  
General Circulation Models ◽  
Horizontal Resolution ◽  
Atmospheric Response ◽  
Sst Front ◽  
Pressure Anomaly

<p>Western boundary currents transport a large amount of heat from the Tropics toward higher latitudes; furthermore they are characterized by a strong sea surface temperature (SST) gradient, which anchors zones of intense upward motion extending up to the upper-troposphere and shapes zones of intense baroclinic eddy activity (storm tracks). For such reasons they have been shown to be fundamental in influencing the climate of the Northern Hemisphere and its variability, and a potentially relevant source of atmospheric predictability. </p><p> </p><p>General circulation models show deficiencies in simulating the observed atmospheric response to SST front variability. The atmospheric horizontal resolution has been recently proposed as a key element in understanding such differences. However, the number of studies on this subject is still limited. Furthermore, a multi-model analysis to systematically investigate differences between low-resolution and high-resolution atmospheric response to oceanic forcing is still lacking. </p><p> </p><p>The present work has the objective to fill this gap, analysing the atmospheric response to Gulf Stream SST front shifting using data from recent High Resolution Model Intercomparison Project (HighResMIP). This project was designed with the specific objective of investigating the impact of increased model horizontal resolution on the representation of the observed climate. Ensembles of historical simulations performed with three atmospheric general circulation models (AGCMs) have been analysed, each conducted with a low-resolution (LR, about 1°) and a high-resolution (HR, about 0.25°) configuration. AGCMs have been forced with observed SSTs (HadISST2 dataset), available at daily frequency on a 0.25° grid, during 1950–2014. </p><p><br>Results show atmospheric responses to the SST-induced diabatic heating anomalies that are strongly resolution dependent. In LR simulations a low-pressure anomaly is present downstream of the SST anomaly, while the diabatic heating anomaly is mainly balanced by meridional advection of air coming from higher latitudes, as expected for an extra-tropical shallow heat source. In contrast, HR simulations generate a high-pressure anomaly downstream of the SST anomaly, thus driving positive temperature advection from lower latitudes (not balancing diabatic heating). Along the vertical direction, both in LR and HR simulation, the diabatic heating in the interior of the atmosphere is balanced by upward motion south of GS SST front and downward motion north and further south of the Gulf Stream. Finally, LR simulations show a reduction in storm-track activity over the North Atlantic, whereas HR simulations show a meridional displacement of the storm-track considerably larger (yet in the same direction) than that of the SST front. HR simulations reproduce the atmospheric response obtained from observations, albeit weaker. This is a hint for the existence of a positive feedback between ocean and atmosphere, as proposed in previous studies. These findings are qualitatively consistent with previous results in literature and, leveraging on recent coordinated modelling efforts, shed light on the effective role of atmospheric horizontal resolution in modelling the atmospheric response to extra-tropical oceanic forcing.</p>

scholarly journals Processes Shaping the Frontal-Scale Time-Mean Surface Wind Convergence Patterns around the Kuroshio Extension in Winter

2020 ◽  
Vol 33 (1) ◽  
pp. 3-25
Author(s):  
Ryusuke Masunaga ◽  
Hisashi Nakamura ◽  
Bunmei Taguchi ◽  
Takafumi Miyasaka
Keyword(s):  
Kuroshio Extension ◽  
Surface Wind ◽  
Convective Precipitation ◽  
Western Boundary ◽  
Boundary Currents ◽  
Sst Front ◽  
Atmospheric Structure ◽  
Mean Convergence ◽  
Daily Scale ◽  
The Kuroshio

AbstractHigh-resolution satellite observations and numerical simulations have revealed that climatological-mean surface wind convergence and precipitation are enhanced locally around the midlatitude warm western boundary currents (WBCs) with divergence slightly to their poleward side. While steep sea surface temperature (SST) fronts along the WBCs have been believed to play an important role in shaping those frontal-scale atmospheric structures, the mechanisms and processes involved are still under debate. The present study explores specific daily scale atmospheric processes that are essential for shaping the frontal-scale atmospheric structure around the Kuroshio Extension (KE) in winter, taking advantage of a new product of global atmospheric reanalysis. Cluster analysis and case studies reveal that a zonally extending narrow band of surface wind convergence frequently emerges along the KE, which is typically observed under the surface northerlies after the passage of a developed synoptic-scale cyclone. Unlike its counterpart around the cyclone center and associated cold front, the surface convergence tends to be in moderate strength and more persistent, contributing dominantly to the distinct time-mean convergence/divergence contrast across the SST front. Accompanying ascent and convective precipitation, the band of convergence is a manifestation of a weak stationary atmospheric front anchored along the SST front or generation of a weak meso-α-scale cyclone. By reinforcing the ascent and convergence, latent heating through convective processes induced by surface convergence plays an important role in shaping the frontal-scale atmospheric structure around the KE.

Precipitation Response to the Gulf Stream in an Atmospheric GCM*

2010 ◽  
Vol 23 (13) ◽  
pp. 3676-3698 ◽  
Author(s):  
Akira Kuwano-Yoshida ◽  
Shoshiro Minobe ◽  
Shang-Ping Xie
Keyword(s):  
Gulf Stream ◽  
General Circulation ◽  
Narrow Band ◽  
Deep Convection ◽  
Surface Wind ◽  
Lower Troposphere ◽  
Circulation Model ◽  
Convective Precipitation ◽  
Precipitation Response ◽  
Upward Velocity

Abstract The precipitation response to sea surface temperature (SST) gradients associated with the Gulf Stream is investigated using an atmospheric general circulation model. Forced by observed SST, the model simulates a narrow band of precipitation, surface convergence, and evaporation that closely follows the Gulf Stream, much like satellite observations. Such a Gulf Stream rainband disappears in the model when the SST front is removed by horizontally smoothing SST. The analysis herein shows that it is convective precipitation that is sensitive to SST gradients. The Gulf Stream anchors a convective rainband by creating surface wind convergence and intensifying surface evaporation on the warmer flank. Deep convection develops near the Gulf Stream in summer when the atmosphere is conditionally unstable. As a result, a narrow band of upward velocity develops above the Gulf Stream throughout the troposphere in summer, while it is limited to the lower troposphere in other seasons.

scholarly journals High-Resolution Satellite Measurements of the Atmospheric Boundary Layer Response to SST Variations along the Agulhas Return Current

2005 ◽  
Vol 18 (14) ◽  
pp. 2706-2723 ◽  
Author(s):  
Larry W. O’Neill ◽  
Dudley B. Chelton ◽  
Steven K. Esbensen ◽  
Frank J. Wentz
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Wind Stress ◽  
Large Scale ◽  
Surface Wind ◽  
Sensible Heat Flux ◽  
Wind Stress Curl ◽  
Return Current ◽  
Short Scale ◽  
Sst Gradient

Abstract The marine atmospheric boundary layer (MABL) response to sea surface temperature (SST) perturbations with wavelengths shorter than 30° longitude by 10° latitude along the Agulhas Return Current (ARC) is described from the first year of SST and cloud liquid water (CLW) measurements from the Advanced Microwave Scanning Radiometer (AMSR) on the Earth Observing System (EOS) Aqua satellite and surface wind stress measurements from the QuikSCAT scatterometer. AMSR measurements of SST at a resolution of 58 km considerably improves upon a previous analysis that used the Reynolds SST analyses, which underestimate the short-scale SST gradient magnitude over the ARC region by more than a factor of 5. The AMSR SST data thus provide the first quantitatively accurate depiction of the SST-induced MABL response along the ARC. Warm (cold) SST perturbations produce positive (negative) wind stress magnitude perturbations, leading to short-scale perturbations in the wind stress curl and divergence fields that are linearly related to the crosswind and downwind components of the SST gradient, respectively. The magnitudes of the curl and divergence responses vary seasonally and spatially with a response nearly twice as strong during the winter than during the summer along a zonal band between 40° and 50°S. These seasonal variations closely correspond to seasonal and spatial variability of large-scale MABL stability and surface sensible heat flux estimated from NCEP reanalysis fields. SST-induced deepening of the MABL over warm water is evident in AMSR measurements of CLW. Typical annual mean differences in cloud thickness between cold and warm SST perturbations are estimated to be about 300 m.

scholarly journals Idealized Modeling of the Atmospheric Boundary Layer Response to SST Forcing in the Western Indian Ocean

2019 ◽  
Vol 76 (7) ◽  
pp. 2023-2042 ◽  
Author(s):  
Adam V. Rydbeck ◽  
Tommy G. Jensen ◽  
Matthew R. Igel
Keyword(s):  
Boundary Layer ◽  
Indian Ocean ◽  
Atmospheric Boundary Layer ◽  
Rossby Waves ◽  
Surface Wind ◽  
Western Indian Ocean ◽  
Overturning Circulation ◽  
Low Level ◽  
Sst Gradient ◽  
Equatorial Rossby Waves

Abstract The atmospheric response to sea surface temperature (SST) variations forced by oceanic downwelling equatorial Rossby waves is investigated using an idealized convection-resolving model. Downwelling equatorial Rossby waves sharpen SST gradients in the western Indian Ocean. Changes in SST cause the atmosphere to hydrostatically adjust, subsequently modulating the low-level wind field. In an idealized cloud model, surface wind speeds, surface moisture fluxes, and low-level precipitable water maximize near regions of strongest SST gradients, not necessarily in regions of warmest SST. Simulations utilizing the steepened SST gradient representative of periods with oceanic downwelling equatorial Rossby waves show enhanced patterns of surface convergence and precipitation that are linked to a strengthened zonally overturning circulation. During these conditions, convection is highly organized, clustering near the maximum SST gradient and ascending branch of the SST-induced overturning circulation. When the SST gradient is reduced, as occurs during periods of weak or absent oceanic equatorial Rossby waves, convection is much less organized and total rainfall is decreased. This demonstrates the previously observed upscale organization of convection and rainfall associated with oceanic downwelling equatorial Rossby waves in the western Indian Ocean. These results suggest that the enhancement of surface fluxes that results from a steepening of the SST gradient is the leading mechanism by which oceanic equatorial Rossby waves prime the atmospheric boundary layer for rapid convective development.

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.

Interannual Modulations of Oceanic Imprints on the Wintertime Atmospheric Boundary Layer under the Changing Dynamical Regimes of the Kuroshio Extension

2016 ◽  
Vol 29 (9) ◽  
pp. 3273-3296 ◽  
Author(s):  
Ryusuke Masunaga ◽  
Hisashi Nakamura ◽  
Takafumi Miyasaka ◽  
Kazuaki Nishii ◽  
Bo Qiu
Keyword(s):  
Boundary Layer ◽  
High Resolution ◽  
Atmospheric Boundary Layer ◽  
Kuroshio Extension ◽  
Surface Wind ◽  
Vertical Wind Shear ◽  
Heat Fluxes ◽  
Convective Precipitation ◽  
Positive Anomaly ◽  
The Kuroshio

Abstract The Kuroshio Extension (KE) fluctuates between its different dynamic regimes on (quasi) decadal time scales. In its stable (unstable) regime, the KE jet is strengthened (weakened) and less (more) meandering. The present study investigates wintertime mesoscale atmospheric structures modulated under the changing KE regimes, as revealed in high-resolution satellite data and data from a particular atmospheric reanalysis (ERA-Interim). In the unstable KE regime, a positive anomaly in sea surface temperature (SST) to the north of the climatological KE jet accompanies positive anomalies in upward heat fluxes from the ocean, surface wind convergence, and cloudiness. As revealed in the atmospheric reanalysis, these positive anomalies coincide with local lowering of sea level pressure, weaker vertical wind shear, warming and thickening of the marine atmospheric boundary layer (MABL), anomalous ascent, and convective precipitation. In the stable KE regime, by contrast, the corresponding imprints of sharp SST gradients across the KE and Oyashio fronts on the wintertime MABL are separated more distinctly, and so are the surface baroclinic zones along those two SST fronts. In the ERA-Interim data, such mesoscale imprints of the KE variability as above are not well represented in a period during which the resolution of SST data prescribed is relatively low. The present study thus elucidates the importance of high-resolution SST data prescribed for atmospheric reanalysis in representing modulations of the MABL structure and air–sea fluxes by the variability of oceanic fronts and/or jets, including the modulations occurring with the changing KE regimes through the hydrostatic pressure adjustment and vertical mixing mechanisms.

Evolution of Marine Atmospheric Boundary Layer Structure across the Cold Tongue–ITCZ Complex

2005 ◽  
Vol 18 (5) ◽  
pp. 737-753 ◽  
Author(s):  
Hollis E. Pyatt ◽  
Bruce A. Albrecht ◽  
Chris Fairall ◽  
J. E. Hare ◽  
Nicholas Bond ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Layer Structure ◽  
Surface Wind ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Cold Tongue ◽  
Marine Atmospheric Boundary Layer ◽  
Cold Side ◽  
Sst Front

Abstract The structure of the marine atmospheric boundary layer (MABL) over the tropical eastern Pacific Ocean is influenced by spatial variations of sea surface temperature (SST) in the region. As the MABL air is advected across a strong SST gradient associated with the cold tongue–ITCZ complex (CTIC), substantial changes occur in the thermodynamic structure, surface fluxes, and cloud properties. This study attempts to define and explain the variability in the MABL structure and clouds over the CTIC. Using data collected on research cruises from the fall seasons of 1999–2001, composite soundings were created for both the cold and warm sides of the SST front to describe the mean atmospheric boundary layer (ABL) structure and its evolution across this front. The average difference in SST across this front was ∼6°C; much of this difference was concentrated in a band only ∼50 km wide. During the fall seasons, on the cold side of the gradient, a well-defined inversion exists in all years. Below this inversion, both fair-weather cumulus and stratiform clouds are observed. As the MABL air moves over the SST front to warmer waters, the inversion weakens and increases in height. The MABL also moistens and eventually supports deeper convection over the ITCZ. Both the latent and sensible heat fluxes increase dramatically across the SST front because of both an increase in SST and surface wind speed. Cloudiness is variable on the cold side of the SST front ranging from 0.2 to 0.9 coverage. On the warm side, cloud fraction was quite constant in time, with values generally greater than 0.8. The highest cloud-top heights (>3 km) are found well north of the SST front, indicating areas of deeper convection. An analysis using energy and moisture budgets identifies the roles of various physical processes in the MABL evolution.

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