surface wind response to oceanic fronts 1 the response of surface winds to ocean fronts characterized by sharp gradients in both sea surface temperature sst and ocean currents was analyzed using scatterometer nscat and quikscat wind data and gulf stream path positions in conjunction with simulations made with the pennsylvania state university psu national center for atmospheric research ncar mesoscale model mm5 all match ups between each scatterometer pass and the gulf stream path were visually examined and only those for which the wind field was free of atmospheric fronts or large curvature over a reasonably straight segment of the gulf stream were selected ten match ups met these criteria for the period studied from 16 september 1996 to 29 june 1997 for nscat and from 24 july 1999 to 31 december 2000 for quikscat changes in the modeled surface wind field across the front in each of the ten cases agree well with changes in the observed winds our findings suggest that the perturbation pressure gradient resulting from the thermal forcing by the front accounts for the decrease in wind speed when moving from warm to cold water and the increase observed in the converse in the cases examined the adjustment of the surface wind to the front occurred as a result of the vertical motion induced by horizontal divergence convergence and advection in the marine atmospheric boundary layer mabl the dynamical forcing associated with strong surface currents is also shown to modify scatterometer derived winds finally the numerical simulations suggest that the dynamical and thermal effects are very nearly additive

identification of cyclone track regimes in the north atlantic a langrangian type climatology of north atlantic cyclones is established based on the high resolution european centre for medium range weather forecasts data set of the 1000 hpa height field first an algorithm is introduced to identify mid latitude cyclones and cyclone paths with as few constraints as possible cluster analysis of relative cyclone displacements yields three types of cyclone tracks characterizing stationary north eastward and zonally travelling storms the internal lagrangian statistics of these cyclone track types reveal representative life cycles for central pressure and geopotential height gradients and a power law scaling behaviour of cyclone displacements finally a basic climatology of north atlantic cyclone track regimes is deduced in terms of a time series and circulation statistics

benthic foraminiferal recovery in the oslofjord norway responses to capping and re oxygenation abstract since 2001 the semi enclosed bekkelag basin inner oslofjord has been re oxygenated following decades of predominantly anoxic conditions additionally in 2007 parts of the polluted organic rich sediments in the basin were capped with uncontaminated post glacial clay this offered a unique opportunity to monitor the response recovery and colonization of the benthic community to re oxygenation and capping with uncontaminated sediments two sites 165 m apart at 50 m water depth in the bekkelag basin were regularly sampled for benthic foraminiferal and geochemical analyses between march 2008 and september 2010 one site rc5 capped was located in the middle and one rc9 re oxygenated uncapped just outside the capped area at the beginning of the study period the former had been exposed to colonization for one year and the latter for seven years maximum abundance of benthic foraminifera occurred in the surface 0 5 cm of sediment at both sites 60 occurred in the surface 0 1 cm and the majority of these were small 63 125 m forms the same taxa i e stainforthia fusiformis bulimina marginata textularia earlandi organic walled allogromiids were common at both sites indicating that the colonizers had rapidly immigrated from neighboring environments however their abundance showed pronounced differences between sites the dominance was lower and the species richness was significantly higher at the capped compared to the uncapped site throughout the study period these faunal characteristics probably reflect the fundamentally different environmental properties of the capping clay versus those of the formerly anoxic sediments nevertheless the absolute abundance increased at both sites and the assemblage compositions converged with time these results show that benthic foraminifera are well suited for monitoring ecological recovery and that colonization at the uncapped site where recovery was due to re oxygenation was slower than at the capped site

sea level pressure minimum along the kuroshio and its extension abstractatmospheric effects of sea surface temperature sst fronts along the kuroshio and kuroshio extension k ke are investigated by examining spatial characteristics of the climatological sea level pressure slp surface winds and surface heat flux q fields based on an in situ observation dataset a hydrostatic effect of the sst front is observed during the northwesterly monsoon characterized by a westward extending low slp wedge trough slightly south of the peak in q along the k ke ageostrophic surface westerlies crossing slp isobars toward a trough center are found north of the low slp wedge apparently because of the eastward acceleration by the vertical mixing effect this feature of the ageostrophic winds is less pronounced south of it a north south asymmetry arising as vertical mixing accelerates decelerates eastward ageostrophic winds north south of it while the slp trough near the sst front is found nearly year round except for summer its meridional location seasonally migrates pr

air sea interaction effects in the lower troposphere across the north wall of the gulf stream abstract significant atmospheric and oceanographic differences have often been observed in the regions lying on either side of the north wall of the gulf stream satellite visible range sensors have revealed changes in reflectivity across the gulf stream believed to be due to changes in sea state that are related to these atmospheric and oceanic differences in addition lines of low level cloud paralleling the edge of the gulf stream have been observed to persist in satellite imagery for several days at a time anomalous radar propagation also has been noted to occur in these same regions in an attempt to document the interrelation of these phenomena the lower atmosphere over the north wall of the gulf stream was studied on two separate occasions sea state photography and measurements of sea surface temperature air temperature dew point temperature and atmospheric pressure were made by an instrumented rp 3a aircraft flying at various altitudes from the gulf stream across the north wall and onto

atmospheric circulations induced by a midlatitude sst front a gcm study the atmospheric effects of sea surface temperature sst anomalies over and near western boundary currents are a matter of renewed interest the general circulation model gcm of the laboratoire de meteorologie dynamique lmd z has a zooming capability that allows a regionally increased resolution this gcm is used to analyze the impact of a sharp sst front in the north atlantic ocean two simulations are compared one with climatological ssts and the other with an enhanced gulf stream front the results corroborate the theory developed previously by the present team to explain the impact of oceanic fronts in this theory the vertical velocity at the top of the atmospheric boundary layer has two components mechanical and thermal it is the latter that is dominant in the tropics while in midlatitudes both play a role in determining the wind convergence above the boundary layer the strengthened sst front does generate the previously predicted stronger ascent above the warmer water south of the front and stronger descent above the colder waters to the north in the gcm simulations the ascent over the warm anomalies is deeper and more intense than the descent

influence of small scale north atlantic sea surface temperature patterns on the marine boundary layer and free troposphere a study using the atmospheric arpege model a high resolution global atmospheric model is used to investigate the influence of the representation of small scale north atlantic sea surface temperature sst patterns on the atmosphere during boreal winter two ensembles of forced simulations are performed and compared in the first ensemble hres the full spatial resolution of the sst is maintained while small scale features are smoothed out in the gulf stream region for the second ensemble smth the model shows a reasonable climatology in term of large scale circulation and air sea interaction coefficient when compared to reanalyses and satellite observations respectively the impact of small scale sst patterns as depicted by differences between hres and smth shows a strong meso scale local mean response in terms of surface heat fluxes convective precipitation and to a lesser extent cloudiness the main mechanism behind these statistical differences is that of a simple hydrostatic pressure adjustment related to increased sst and marine atmospheric boundary layer temperature gradient along the north atlantic sst front the model response to small scale sst patterns also includes remote large scale effects upper tropospheric winds show a decrease downstream of the eddy driven jet maxima over the central north atlantic while the subtropical jet exhibits a significant northward shift in particular over the eastern mediterranean region significant changes are simulated in regard to the north atlantic storm track such as a southward shift of the storm density off the coast of north america towards the maximum sst gradient a storm density decrease is also depicted over greenland and the nordic seas while a significant increase is seen over the northern part of the mediterranean basin changes in rossby wave breaking frequencies and weather regimes spatial patterns are shown to be associated to the jets and storm track changes

satellite measurements reveal persistent small scale features in ocean winds four year averages of 25 kilometer resolution measurements of near surface wind speed and direction over the global ocean from the quikscat satellite radar scatterometer reveal the existence of surprisingly persistent small scale features in the dynamically and thermodynamically important curl and divergence of the wind stress air sea interaction over sea surface temperature fronts throughout the world ocean is evident in both the curl and divergence fields as are the influences of islands and coastal mountains ocean currents such as the gulf stream generate distinctive patterns in the curl field these previously unresolved features have important implications for oceanographic and air sea interaction research

coupled ocean atmospheric waves on the equatorial front striking 20 30 day sea surface temperature waves observed along the equatorial front in the later half of the year are generally believed to be of an oceanic origin here we report the detection of atmospheric waves that are unambiguously tied to these oceanic waves using new satellite measurements of surface winds a general circulation model simulation reveals that these atmospheric waves have a shallow vertical structure trapped in the planetary boundary layer pbl unlike el nino southern oscillation where changes in deep convection are the cause of anomalous winds vertical wave motion penetrates well above the pbl and is likely to impact the distribution and transport of climatically important gas species such as ozone and dimethyl sulfide

the basic ingredients of the north atlantic storm track part i land sea contrast and orography understanding and predicting changes in storm tracks over longer time scales is a challenging problem particularly in the north atlantic this is due in part to the complex range of forcings land sea contrast orography sea surface temperatures etc that combine to produce the structure of the storm track the impact of land sea contrast and midlatitude orography on the north atlantic storm track is investigated through a hierarchy of gcm simulations using idealized and semirealistic boundary conditions in a highresolution version of the hadley centre atmosphere model hadam3 this framework captures the largescale essence of features such as the north and south american continents eurasia and the rocky mountains enabling the results to be applied more directly to realistic modeling situations than was possible with previous idealized studies the physical processes by which the forcing mechanisms impact the large scale flow and the midlatitude storm tracks are discussed the characteristics of the north american continent are found to be very important in generating the structure of the north atlantic storm track in particular the southwest northeast tilt in the upper tropospheric jet produced by southward deflection of the westerly flow incident on the rocky mountains leads to enhanced storm development along an axis close to that of the continent s eastern coastline the approximately triangular shape of north america also enables a cold pool of air to develop in the northeast intensifying the surface temperature contrast across the eastern coastline consistent with further enhancements of baroclinicity and storm growth along the same axis

variability of the observed midlatitude storm tracks in relation to low frequency changes in the circulation pattern abstract the principal modes of month to month variability of the wintertime storm tracks over the north pacific and north atlantic are identified by empirical orthogonal function analysis of the root mean square statistics of bandpass 2 5 6 day filtered geopotential height data for 19 yr one of the two leading modes depicts fluctuations in the level of synoptic scale activity without any noticeable spatial displacement of the storm track axes whereas the other mode is associated with meridional shifts of the storm tracks from their time averaged positions higher order modes are indicative of diversion or truncation of cyclone tracks in particular geographical regions it is demonstrated that the leading storm track modes are linked to some of the best known monthly averaged teleconnection patterns the dipolar western pacific and western atlantic patterns for the monthly mean flow are seen to be accompanied by marked changes in the intensity of the storm tracks over the western oceans whereas the m

western boundary currents and frontal air sea interaction gulf stream and kuroshio extension abstract in the northern hemisphere midlatitude western boundary current wbc systems there is a complex interaction between dynamics and thermodynamics and between atmosphere and ocean their potential contribution to the climate system motivated major parallel field programs in both the north pacific kuroshio extension system study kess and the north atlantic climate variability and predictability clivar mode water dynamics experiment climode and preliminary observations and analyses from these programs highlight that complexity the gulf stream gs in the north atlantic and the kuroshio extension ke in the north pacific have broad similarities as subtropical gyre wbcs but they also have significant differences which affect the regional air sea exchange processes and their larger scale interactions the 15 yr satellite altimeter data record which provides a rich source of information is combined here with the longer historical record from in situ data to describe and compare the curr

sensitivity of cyclogenesis to sea surface temperature in the northwestern atlantic during the intensive observation period 15 13 15 february 1997 of the fastex experiment a major cyclone crossed the atlantic ocean from the newfoundland basin to southern iceland its surface low center deepened by 17 hpa in 7 h when the perturbation crossed the north atlantic current nac from cold 38c to warm water 158c to elucidate the role of sea surface temperature sst and air sea fluxes in the dynamics of oceanic cyclones three nonhydrostatic mesoscale simulations were performed the first one is a control experiment with a realistic sst field describing in detail the oceanic front associated with the nac system the two following simulations are sensitivity experiments where the sst front is removed the first one uses a uniformly cold sst equal to 38c and the second one uses a uniformly warm sst equal to 158c the frontogenetic function and the vertical velocity sources in the lower atmospheric layers of the three simulations were diagnosed in the control simulation the surface heat fluxes were found to be negative in the perturbation warm sector and positive in the region behind the cold front as reported by numerous authors this pattern of surface heating and cooling did not intensify the cyclone except in the occlusion when the phasing with the sst front occurs this configuration enhances the horizontal gradient of surface buoyancy flux across the occlusion which increases the buoyancy flux source of vertical velocity w when the sst front is removed the surface heat fluxes are strongly affected in magnitude and in spatial variability the marine atmospheric boundary layer mabl stability the convective activity the warm advection in the core of the wave and the heating depth are strongly affected by the different surface flux fields there are several consequences i the uniform ssts tend to decrease the cold front intensity of the wave ii a weaker buoyancy flux source of vertical velocity is found above a uniform cold sst across the occlusion in comparison with the control case and iii surprisingly a weaker w buoyancy flux source is also found above a uniform warm sst because of a higher heating depth vertical velocity depends not only on the buoyancy flux forcing but also on the thermal wind the turbulent momentum and the thermal wind imbalance forcings the thermal wind forcing and the thermal wind imbalance forcing were the most sensitive to the sst compared to the turbulent momentum forcing this result means that i the feed back of the ageostrophic circulation induced by the surface is greater on the kinematic forcings than on the turbulent forcings and ii the turbulent momentum forcing does not play a crucial role in cyclogenesis above a uniform warm sst the strongest intensity of the occlusion is due to the strongest w thermal wind forcing and w thermal wind imbalance forcing in the mabl in spite of a weaker w buoyancy flux forcing than in the control case this result is explained by the convective activity that increases the low level convergence and vorticity spinup this point means that latent heat release and baroclinicity are in tight interaction in the first 12 h and at the scale of the simulation domain the three cyclones evolve similarly but at a small scale their internal structures diverge strongly and rapidly the scale at which the surface turbulent fluxes act on the dynamics of marine cyclones is therefore important finally the cyclone simulated in the warm sst case developed more rapidly than those simulated in the real and the cold sst cases this behavior is attributed to the strong positive surface heat fluxes because they preconditioned the mabl by moistening and heating the low levels during the incipient stage of the cyclone development

atmospheric turbulence structure in the vicinity of an oceanic front fast response data taken aboard the national oceanic and atmospheric administration wp 3d aircraft are used to determine the structure of atmospheric boundary layer turbulence on either side of a well developed sea surface temperature front southwest of bermuda the data were taken on february 17 1986 as part of the frontal air sea interaction experiment fasinex a broad region of low humidity air extending from 15 km to 35 km south of the front is probably due to the presence of a frontally induced secondary circulation evidence for a secondary flow is found in both the time series of atmospheric variables and the statistics obtained from conditionally sampled updrafts and downdrafts in the transect across the sst front larger sea air temperature and humidity differences on the warm south side of the front give rise to surface layer sensible and latent heat and buoyancy fluxes that are larger than those on the cold side turbulence structure appears to be influenced as much by the presence of strong wind shear at the top of the boundary layer as by differing conditions at the surface on either side of the front a larger rate of entrainment on the warm side of the front is indicated by the greater influence of low momentum air from the overlying shear layer on updrafts in the upper part of the mixed layer as well as the more frequent overturning of cool moist updrafts and warm dry downdrafts it is conjectured that the larger entrainment rate is due to the interaction between the inversion layer and more energetic updrafts produced by greater surface forcing on the warm side of the front

low frequency variability in the midlatitude atmosphere induced by an oceanic thermal front abstract this study examines the flow induced in a highly idealized atmospheric model by an east west oriented oceanic thermal front the model has a linear marine boundary layer coupled to a quasigeostrophic equivalent barotropic free atmosphere the vertical velocity at the top of the boundary layer drives the flow in the free atmosphere and produces an eastward jet parallel to the oceanic front s isotherms a large gyre develops on either side of this jet cyclonic to the north and anticyclonic to the south of it as the jet intensifies during spinup from rest it becomes unstable the most unstable wave has a length of about 500 km it evolves into a meander and eddies detach from the eastern edge of each gyre the dependence of the atmospheric dynamics on the strength t of the oceanic front is studied the gulf stream and kuroshio fronts correspond roughly in the scaling used here to t 7 c for weak fronts t 4 c the circulation is steady and exhibits two large antisymmetric gyres sepa

observed patterns of month to month storm track variability and their relationship to the background flow month to month storm track variability is investigated via eof analyses performed on era 40 monthlyaveraged high pass filtered daily 850 hpa meridional heat flux and the variances of 300 hpa meridional wind and 500 hpa height the analysis is performed both in hemispheric and sectoral domains of the northern and southern hemispheres patterns characterized as pulsing and latitudinal shifting of the climatologicalmean storm tracks emerge as the leading sectoral patterns of variability based on the analysis presented storm track variability on the spatial scale of the two northern hemisphere sectors appears to be largely but perhaps not completely independent pulsing and latitudinally shifting storm tracks are accompanied by zonal wind anomalies consistent with eddy forced accelerations and geopotential height anomalies that project strongly on the dominant patterns of geopotential height variability the north atlantic oscillation nao northern hemisphere annular mode nam is associated with a pulsing of the atlantic storm track and a meridional displacement of the upper tropospheric jet exit region whereas the eastern atlantic ea pattern is associated with a latitudinally shifting storm track and an extension or retraction of the upper tropospheric jet analogous patterns of stormtrack and upper tropospheric jet variability are associated with the western pacific wp and pacific north america pna patterns wave mean flow relationships shown here are more clearly defined than in previous studies and are shown to extend through the depth of the troposphere the southern hemisphere annular mode sam is associated with a latitudinally shifting storm track over the south atlantic and indian oceans and a pulsing south pacific storm track the patterns of storm track variability are shown to be related to simple distortions of the climatological mean upper tropospheric jet

ocean frontal effects on the vertical development of clouds over the western north pacific in situ and satellite observations abstract a suite of shipboard and satellite observations are analyzed and synthesized to investigate the three dimensional structure of clouds and influences from sea surface temperature fronts over the western north pacific sharp transitions are observed across the kuroshio extension ke front in the marine atmospheric boundary layer mabl and its clouds the ocean s influence appears to extend beyond the mabl with higher cloud tops in altitude along the ke front than the surroundings in winter intense turbulent heat release from the ocean takes place on the southern flank of the ke front where the cloud top penetrates above the mabl and reaches the midtroposphere in this band of high cloud tops frequent lightning activity is observed the results of this study suggest a sea level pressure mechanism for which the temperature gradient in the mabl induces strong surface wind convergence on the southern flank of the ke front deepening the clouds there in early summer sea fog frequently occurs on

the atmospheric response to weak sea surface temperature fronts abstractthe response of the atmospheric boundary layer to fronts of sea surface temperature sst is characterized by correlations between wind stress divergence and the downwind component of the sst gradient and between the wind stress curl and the crosswind component of the sst gradient the associated regression or coupling coefficients for the wind stress divergence are consistently larger than those for the wind stress curl to explore the underlying physics the authors introduce a linearized model of the atmospheric boundary layer response to sst induced modulations of boundary layer hydrostatic pressure and vertical mixing in the presence of advection by a background ekman spiral model solutions are a strong function of the sst scale and background advection and recover observed characteristics the coupling coefficients for wind stress divergence and curl are governed by distinct physics wind stress divergence results from either large scale winds crossing the front or from a thermally direct

the gulf stream convergence zone in the time mean winds abstractthe distribution of surface divergence in the northwest atlantic is investigated using 10 years of satellite wind observations from quikscat and a 1 yr simulation from the coamps atmospheric model a band of time mean surface convergence overlies the gulf stream called here the gulf stream convergence zone gscz and has been attributed previously to a local boundary layer response to gulf stream sst gradients however this analysis shows that the gscz results mainly from the aggregate impacts of strong convergence anomalies associated with storms propagating along the storm track which approximately overlies the gulf stream storm surface convergence anomalies are one to two orders of magnitude greater than the time mean convergence and produce a highly asymmetric divergence distribution skewed toward convergent winds the sensitivity of the sign and magnitude of the time mean divergence to extreme weather events is demonstrated through analysis using an extreme value filter conditional samp

on the role of sea surface temperature gradients in forcing low level winds and convergence in the tropics abstract we examine the importance of pressure gradients due to surface temperature gradients to low level p 700 mb flow and convergence in the tropics over time scales 1 month the latter plays a crucial role in determining the distribution of cumulonimbus convection and rainfall our approach is to consider a simple one layer model of the trade cumulus boundary layer wherein surface temperature gradients are mixed vertically consistent with ecmwf analyzed data the top of the layer is taken at 700 mb the influence from higher levels is intentionally suppressed by setting horizontal pressure gradients and frictional stresses to zero at the top of the layer horizontal convergence within the layer is taken up by cumulonimbus mass flux however the development of the cumulonimbus mass flux is associated with a short relaxation time o hr roughly the development time for such convection during this short time horizontal convergence acts to redistribute mass so as to reduce horizontal pressur

rain ratio variation in the tropical ocean tests with surface sediments in the eastern equatorial pacific the organic carbon to calcite flux ratio rain ratio has a profound effect on the preservation of carbonates in the deep sea and may influence atmospheric pco2 over millennia unfortunately the degree to which the rain ratio varies in the more productive regions of the oceans is not well determined with sediment trap data the rain ratio in the upper ocean appears dominantly linked to diatom productivity which is not necessarily directly linked to total production and may be regionally variable however ballasting and protection of organic carbon by calcareous particles in the deeps may limit ratio variability at the seafloor sediment trap data do not exist for the regional determination of rain ratios in key highly productive areas like the eastern equatorial pacific eep to overcome this we turn to surface sediment composition and accumulation rates as a representation of modern ratio variation we present 230 thorium 230 th normalized carbonate opal organic carbon and detrital matter accumulation rates from core top samples in the eep we demonstrate a novel approach for estimating modern rain ratios from sedimentary proxies by 1 calculating vertical calcite flux from 230 th normalized carbonate accumulation rates cars with correction for preservation and 2 calculating organic carbon fluxes with multiple algorithms that depend in varying degrees on ballasting we find that organic carbon flux estimates from algorithms with and without a ballasting function produce results different from one another sediment accumulation rates for opal reflect the likely pattern of diatom production by contrast the organic carbon accumulation rate does not correlate well with surface ocean productivity or any of our algorithm based organic carbon flux estimates instead it correlates with the detrital component of the sediments suggesting an allochthonous input to sedimentary organic carbon accumulation in the eep which reduces its value as a productivity tracer however our calcite and multiple satellite based organic carbon fluxes allow estimation of the rain ratio and demonstrate a common regional pattern with moderate to strong variability in the rain ratio across the eep this variability is significant and is transmitted into the deeps leaving a sedimentary record regardless of the algorithm chosen to calculate organic carbon fluxes furthermore we provide evidence suggesting that the rain ratio in the eep may be driven by wind supplied iron availability which would regionally enhance nutrient use and promote diatom growth r 2007 elsevier ltd all rights reserved

the effects of sst induced surface wind speed and direction gradients on midlatitude surface vorticity and divergence abstract the effects of surface wind speed and direction gradients on midlatitude surface vorticity and divergence fields associated with mesoscale sea surface temperature sst variability having spatial scales of 100 1000 km are investigated using vector wind observations from the seawinds scatterometer on the quick scatterometer quikscat satellite and sst from the advanced microwave scanning radiometer for earth observing system amsr e aqua satellite the wind sst coupling is analyzed over the period june 2002 august 2008 corresponding to the first 6 years of the amsr e mission previous studies have shown that strong wind speed gradients develop in response to persistent mesoscale sst features associated with the kuroshio extension gulf stream south atlantic and agulhas return current regions midlatitude sst fronts also significantly modify surface wind direction the surface wind speed and direction responses to typical sst differences of about 2 4 c are on average about 1 2 m s 1 and 4

dynamical analyses of marine atmospheric boundary layer structure near the gulf stream oceanic front the effects of the sea surface temperature s s t front at the edge of the gulf stream on the marine atmospheric boundary layer mabl are investigated using a numerical model to study the modification effects of an oceanic front on the mabl structure the situation simulated is flow from over cold shelf water to over the warm water of the gulf stream the initial temperature and humidity profiles of the air are specified to be near neutral over the cold water and are therefore typical of undisturbed conditions the differential in s s t across the oceanic front creates a horizontal variation in the surface perturbation pressure and the stability the surface perturbation pressure and turbulent fluxes modulate the flow and produce horizontal variations in horizontal wind components with associated vertical motions a thermally direct cell is produced as a result of the s s t difference across the front the isotherms slope upward towards the warm water entrainment of inversion layer air and upward vertical motion over the warm water cause the mabl to be deeper there a layer of cloud forms over warm water and is associated with mixed layer deepening rather than lowering of the condensation level turbulent fluxes in the mabl show considerable spatial variation surface stress is much larger over the front and over the warm water than over the cold water this is mostly caused by wind speed changes associated with the front changes in the drag coefficient due to changes in surface roughness and stability are much less important mean budgets for temperature and total water indicate that there is a balance between horizontal advection and turbulent flux divergence the u momentum budget shows that once the geostrophic balance terms are subtracted the balance is mainly between the pressure gradient force associated with the induced temperature field and turbulent friction with horizontal advection and the coriolis force acting on the geostrophic departure playing minor roles the v momentum budget shows a balance between horizontal advection coriolis force and friction although there are few data for comparison the results are in qualitative agreement with observations in the area this study shows that the s s t front at the gulf stream edge produces marked local changes in the nearby atmospheric surface layer

air sea fluxes and surface layer turbulence around a sea surface temperature front the observed effects of sharp changes in sea surface temperature sst on the air sea fluxes surface roughness and the turbulence structure in the surface layer and the marine atmospheric boundary layer are discussed in situ flux and turbulence observations were carried out from three aircraft and two ships within the fasinex framework three other aircraft used remote sensors to measure waves microwave backscatter and lidar signatures of cloud tops descriptions of the techniques intercomparison of aircraft and ship flux data and use of different methods for analyzing the fluxes from the aircraft data are described changing synoptic weather on three successive days yielded cases of wind direction both approximately parallel and perpendicular to a surface temperature front for the wind perpendicular to the front wind over both cold to warm and warm to cold surface temperatures occurred model results consistent with the observations suggest that an internal boundary layer forms at the sst

storm track response to ocean fronts in a global high resolution climate model synoptic atmospheric eddies are affected by lower tropospheric air temperature gradients and by turbulent heat fluxes from the surface in this study we examine how ocean fronts affect these quantities and hence the storm tracks we focus on two midlatitude regions where ocean fronts lie close to the storm tracks the north west atlantic and the southern ocean an atmospheric climate model of reasonably high resolution 50 km is applied in a climate length 60 year simulation in order to obtain stable statistics simulations with frontal structure in the sea surface temperature sst in one of the regions are compared against simulations with globally smoothed sst we show that in both regions the ocean fronts have a strong influence on the transient eddy heat and moisture fluxes not just in the boundary layer but also in the free troposphere local differences in these quantities between the simulations reach 20 40 of the maximum values in the simulation with smoothed sst averaged over the entire region of the storm track over the ocean the corresponding differences are 10 20 the effect on the transient eddy meridional wind variance is strong in the boundary layer but relatively weak above that the potential mechanisms by which the ocean fronts influence the storm tracks are discussed and our results are compared against previous studies with regional models aquaplanet models and coarse resolution coupled models

skewness of low frequency fluctuations in the tropospheric circulation during the northern hemisphere winter abstract departures of lowpass filtered 500 mb height and sea level pressure anomalies from a gaussian distribution have been examined based on northern hemisphere operational analyses for 30 winters at each gridpoint we evaluated several indices that are measures of the differences between the amplitudes and frequencies of the persistent anomalies with positive and negative polarities and differences in the temporal variability observed within extended periods of positive and negative anomalies the spatial patterns for these indices are all quite similar and they resemble the distribution of the moment coefficient of skewness documented by white positive skewness is observed to the north of the stormtrack latitudes and the negative to the south of them the skewness pattern in the sea level pressure field is weaker particularly at high latitudes and it exhibits a bias toward negative skewness anomalies with amplitudes larger than two standard deviations which occur about 5 of the time account

the signature of the midlatitude tropospheric storm tracks in the surface winds abstract storm track analysis is applied to the meridional winds at 10 m and 850 hpa for the winters of 1999 2006 the analysis is focused on the north atlantic and north pacific ocean basins and the southern ocean spanning the region south of the indian ocean the spatial patterns that emerge from the analysis of the 850 hpa winds are the typical free tropospheric storm tracks the spatial patterns that emerge from the analysis of the surface winds differ from the free tropospheric storm tracks the spatial differences between the surface and free tropospheric storm tracks can be explained by the influence of the spatial variability in the instability of the atmospheric boundary layer strongly unstable boundary layers allow greater downward mixing of free tropospheric momentum momentum mixing and this may be the cause of the stronger surface storm tracks in regions with greater instability in the time mean principal component analysis suggests that the basin scale variability that is reflected in th

numerical simulation of boundary layer structure and cross equatorial flow in the eastern pacific 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

on the existence of storm tracks abstract given that middle latitude weather systems transport heat in a manner such as to weaken the baroclinicity that is thought to be crucial to their growth it is perhaps surprising that concentrated regions of such eddy activity i e storm tracks are found in the northern hemisphere winter the existence and possible self maintenance of storm tracks is investigated using a linear stationary wave model with storm track region forcings taken from data averaged over a number of winters it is found that the direct thermal effect of the eddies does indeed act against the existence of the storm track their vorticity fluxes lead to some reduction of this effect it is argued that the mean diabatic heating in the storm track region is an indirect eddy effect this heating is found to maintain the mean maximum in baroclinicity in the region further the mean low level flow induced by the eddy effects is such as to enhance the warm western oceanic boundary currents that are crucial to the existence of t

the influence of cross equatorial pressure gradients on the location of near equatorial convection tomas and webster note that in regions where there is a substantial cross equator surface pressure gradient there is locally anticyclonic absolute vorticity on the low pressure side of the equator and thus the flow in this region meets the parcel criterion for inertial instability they hypothesize that the atmospheric response to this absolute vorticity distribution is a convergence divergence doublet in the boundary layer the convergence centre results in strong convection and thus is very important in determining the strength and location of convection i e the intertropical convergence zones whereas the divergence centre results in suppressed convection in this work a zonally symmetric model of the boundary layer is developed to investigate this hypothesis further an analysis of a linearized version of the model indicates that although the observed flow in these regions meets the parcel criterion for instability it does not satisfy the linear stability criteria because of the stabilizing influence of dissipation and the finite vertical scale much greater shear than that observed is required for linear instability it is noted however that when the parcel criterion is met one of the nonlinear terms which was neglected in the linear analysis may have an important influence on the flow several experiments are performed integrating the full nonlinear model relaxing back to a pressure distribution having a cross equatorial gradient to test whether the observed vorticity wind and convergence distributions can be well simulated it is found that the simulations reach a steady state in which there is a region of locally anticyclonic absolute vorticity on the low pressure side of the equator this absolute vorticity distribution results in an anomalous acceleration of the meridional wind and a convergence divergence doublet similar to that observed by tomas and webster this response is explained as resulting from accelerations that act to bring the zonal flow out of geostrophic balance as it passes through the region of locally anticyclonic absolute vorticity the response of the meridional wind to the strength of the pressure gradient is quasi linear for small and somewhat larger values of forcing but between these values there is a range where the response is highly nonlinear i e large increases in the strength of the meridional wind result from small changes in the pressure gradient it is suggested that this nonlinear response may play a role in the observed sudden onset of the monsoon circulations the nonlinear model is also forced using pressure distributions taken from observations the resulting steady state simulated vorticity wind and convergence distributions closely match the observations for the cases of pressure distributions taken from the east pacific and east atlantic during july when forced using the pressure distribution from the indian ocean region during july the model produces an unrealistically strong meridional wind response some improvement is obtained by including the latitudinally dependent zonal pressure gradient force which is moderately strong in this region finally the model is forced using a pressure distribution taken from observations in the central pacific ocean during july in this case the cross equator pressure gradient is near zero

on the importance of midlatitude oceanic frontal zones for the mean state and dominant variability in the tropospheric circulation 1 observations indicate that midlatitude weather systems are organized into storm tracks near oceanic frontal zones with pronounced sea surface temperature sst gradients a pair of atmospheric general circulation model experiments with zonally uniform sst profiles prescribed show that their observed collocation is not fortuitous in one experiment a storm track is anchored around a midlatitude sst front that maintains near surface thermal gradients and energizes eddies westerly momentum transport by eddies produces a well defined polar front jet along the front even in winter when a subtropical jet stream intensifies in the other experiment removal of the sst front leads to a substantial weakening in eddy activity and the pfj especially in winter it also leads to a weakening of the annular mode the dominant mode of westerly jet variability and its notable structural distortion in winter though idealized our experiments suggest the importance of midlatitude oceanic fronts for the tropospheric circulation and its variability

observations of coupling between surface wind stress and sea surface temperature in the eastern tropical pacific satellite measurements of surface wind stress from the quikscat scatterometer and sea surface temperature sst from the tropical rainfall measuring mission microwave imager are analyzed for the three month period 21 july 20 october 1999 to investigate ocean atmosphere coupling in the eastern tropical pacific oceanic tropical instability waves tiws with periods of 20 40 days and wavelengths of 1000 2000 km perturb the sst fronts that bracket both sides of the equatorial cold tongue which is centered near 1 8s to the east of 1308w these perturbations are characterized by cusp shaped features that propagate systematically westward on both sides of the equator the space time structures of these sst perturbations are reproduced with remarkable detail in the surface wind stress field the wind stress divergence is shown to be linearly related to the downwind component of the sst gradient with a response on the south side of the cold tongue that is about twice that on the north side the wind stress curl is linearly related to the crosswind component of the sst gradient with a response that is approximately half that of the wind stress divergence response to the downwind sst gradient the perturbed sst and wind stress fields propagate synchronously westward with the tiws this close coupling between sst and wind stress supports the wallace et al hypothesis that surface winds vary in response to sst modification of atmospheric boundary layer stability