Meridional Field Research Articles

Zeeman Doppler imaging ofξBoo A and B

Aims.We present a magnetic-field surface map for both stellar components of the young visual binaryξBoo AB (A: G8V, B: K5V).Methods.We employed high-resolution Stokes-Vspectra obtained with the Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) at the Large Binocular Telescope (LBT). We inverted StokesVline profiles with ouriMAP software and compared them with previous inversions. We employed an iterative regularization scheme without the need for a penalty function and incorporated a three-component description of the surface magnetic-field vector. The spectral resolution of our data is 130 000 (0.040–0.055 Å) and we obtain a signal-to-noise ratio (S/N) of up to 3000 per pixel depending on wavelength. We used a singular-value decomposition (SVD) of a total of 1811 spectral lines to average Stokes-Vprofiles. Our mapping is accompanied by a residual bootstrap error analysis.Results.We constructed magnetic flux densities of the radial field component forξBoo A andξBoo B of up to plus or −115±5 G and 55±3 G, respectively. The magnetic morphology ofξBoo A is characterized by a very high latitude, nearly polar spot of negative polarity and three low-to-mid-latitude spots of positive polarity, while that ofξBoo B is characterized by four low-to-mid-latitude spots of mixed polarity. No polar magnetic field is reconstructed for the coolerξBoo B star. Both our maps are dominated by the radial field component, containing 86% and 89% of the magnetic energy ofξBoo A and B, respectively. We find only weak azimuthal and meridional field densities on both stars (plus or −15–30 G), about a factor two weaker than what was seen previously forξBoo A. The phase averaged longitudinal field component and dispersion is +4.5±1.5 G for ξ Boo A and −5.0±3.0 G forξBoo B.

Angular momentum transport by magnetoconvection and the magnetic modulation of the solar differential rotation

In order to explain the variance of the solar rotation law during the activity minima and maxima, the angular momentum transport by rotating magnetoconvection is simulated in a convective box penetrated by an inclined azimuthal magnetic field. Turbulence-induced kinetic and magnetic stresses and the Lorentz force of the large-scale magnetic background field are the basic transporters of angular momentum. Without rotation, the sign of the magnetic stresses naturally depends on the signs of the field components as positive (negative) BθBϕ transport the angular momentum poleward (equatorward). For fast enough rotation, however, the turbulence-originated Reynolds stresses start to dominate the transport of the angular momentum flux. The simulations show that positive ratios of the two meridional magnetic field components to the azimuthal field reduce the inward radial as well as the equatorward latitudinal transport, which result from hydrodynamic calculations. Only for BθBϕ > 0 (generated by solar-type rotation laws with an accelerated equator) does the magnetic-influenced rotation at the solar surface prove to be flatter than the nonmagnetic profile together with the observed slight spin-down of the equator. The latter phenomenon does not appear for antisolar rotation with polar vortex as well as for rotation laws with prevailing radial shear.

Magnetic boundary layers in numerical dynamos with heterogeneous outer boundary heat flux

It has been proposed that magnetic flux expulsion due to outer core fluid upwellings may affect the geomagnetic secular variation on the core-mantle boundary (Bloxham, 1986). In this process intense horizontal field lines are concentrated below the outer boundary, introducing small radial length scales and consequently strong radial diffusion. Here we explore such magnetic boundary layers in numerical dynamo simulations with heterogeneous outer boundary heat flux inferred from a tomographic model of lower mantle seismic shear waves velocity anomalies. Our scheme associates magnetic boundary layers to peak horizontal magnetic fields at the top of the shell. In our models mean magnetic boundary layer thickness ranges ≈200 − 400 km and decreases with increasing magnetic Reynolds number. Extrapolation or interpolation to Earth's core conditions based on total core flow amplitude or its poloidal part gives mean magnetic boundary layer thickness of ≈220 and ≈260 − 330 km, respectively. We find magnetic boundary layers associated with the azimuthal field at the equatorial region, whereas magnetic boundary layers associated with the meridional field are found at mid latitudes. Negative outer boundary heat flux anomalies yield preferred locations of expulsion of azimuthal field below Africa and the Pacific, while positive outer boundary heat flux anomalies yield preferred locations of expulsion of meridional field below the Americas and East Asia. Furthermore, we find a tendency of the azimuthal field to low latitudes of the Northern Hemisphere. Our results suggest that the local diffusion time is on the order of several kyr and the local magnetic Reynolds number is on the order of ≈10, both much smaller than classical estimates.

Meridional component of the large-scale magnetic field at minimum and characteristics of the subsequent solar activity cycle

The polar magnetic field near the cycle minimum is known to correlate with the height of the next sunspot maximum. There is reason to believe that the hemispheric coupling can play an important role in forming the next cycle. The meridional component of the large-scale magnetic field can be one of the hemispheric coupling indices. For our analysis we have used the reconstructed data on the large-scale magnetic field over 1915–1986. We show that in several cycles not only the height but also the general course of the cycle can be described in this way about 6 years in advance. This coupling has been confirmed by the currently available data from 1976 to 2016, but the ratio of the meridional field to the total absolute value of the field vector has turned out to be a more promising parameter. In this paper it was calculated at a height of ∼70 Mm above the photosphere. The date of the forthcoming minimum is estimated using this parameter to be mid-2018; using the global field as a forecast parameter gives a later date of the minimum, early 2020.

Aberrations of soft x-ray and vacuum ultraviolet optical systems with orthogonal arrangement of elements

The wave aberration of plane-symmetric optical systems is expressed with the aperture-ray coordinates on the reference exit wavefront in the paper; the defocus aberration caused by the meridional field of source is analyzed in detail. Based on the expressions of the wave aberration and the defocus, the aberrations of the soft x-ray and ultraviolet (XUV) optical systems with an orthogonal arrangement of elements are studied as a whole. The resultant aberration expressions are used to calculate the aberrations of two design examples; the images are compared to the ray-tracing results with Shadow to validate the aberration expressions. The study shows that the accuracy of the aberration expressions is satisfactory. The analytical analysis of aberrations is helpful in the design and optimization of the XUV optical systems with an orthogonal arrangement of elements.

Magnetic fields of young solar twins

The goal of this work is to study the magnetic fields of six young solar-analogue stars both individually, and collectively, to search for possible magnetic field trends with age. If such trends are found, they can be used to understand magnetism in the context of stellar evolution of solar-like stars and, the past of the Sun and the solar system. This is also important for the atmospheric evolution of the inner planets, Earth in particular. We used Stokes IV data from two different spectropolarimeters, NARVAL and HARPSpol. The least-squares deconvolution multi-line technique was used to increase the signal-to-noise ratio of the data. We then applied a modern Zeeman-Doppler imaging code in order to reconstruct the magnetic topology of all stars and the brightness distribution of one of our studied stars. Our results show a significant decrease in the magnetic field strength and energy as the stellar age increases from 100Myr to 250Myr while there is no significant age dependence of the mean magnetic field strength for stars with ages 250-650Myr. The spread in the mean field strength between different stars is comparable to the scatter between different observations of individual stars. The meridional field component has the weakest strength compared to the radial and azimuthal field components in 15 out of the 16 magnetic maps. It turns out that 89-97% of the magnetic field energy is contained in l=1-3. There is also no clear trend with age and distribution of field energy into poloidal/toroidal and axisymmetric/non-axisymmetric components within the sample. The two oldest stars in this study do show a twice as strong octupole component compared to the quadrupole component. This is only seen in one out of 13 maps of the younger stars. One star, chi1 Ori displays two field polarity switches during almost 5 years of observations suggesting a magnetic cycle length of either 2, 6 or 8 years.

Rotational Flow in Centrifugal Pump Meridian Using Curvilinear Coordinates

A new model for calculation of the velocity field in a pump meridian is presented in this paper. It is a parameterized rotational flow model providing the pump designer with an option to simulate various flow conditions at the blade leading edge using a single parameter, which leads to different blade designs. Despite being rotational, this flow model does not allow for creation of helical swirl structures, a feature that is usually sought to be suppressed in a pump impeller. A condition for a flow pattern suitable for a pump hydraulic design is derived using curvilinear coordinates. The variation of the meridional velocity for different choices of the parameter is demonstrated, and the meridional flow field is compared against the turbulent flow. The resulting velocity field is easy to obtain and provides a unified approach to determine the distribution of the blade inlet angles along the leading edge for a range of design requirements.

Interannual variability in the North Pacific meridional overturning circulation

We analyzed the temporal and spatial variation, and interannual variability of the North Pacific meridional overturning circulation using an empirical orthogonal function method, and calculated mass transport using Simple Ocean Data Assimilation Data from 1958-2008. The meridional streamfunction field in the North Pacific tilts N-S; the Tropical Cell (TC), Subtropical Cell (STC), and Deep Tropical Cell (DTC) may be in phase on an annual time scale; the TC and the STC are out of phase on an interannual time scale, but the interannual variability of the DTC is complex. The TC and STC interannual variability is associated with ENSO (El Nio-Southern Oscillation). The TC northward, southward, upward, and downward transports all weaken in El Nios and strengthen in La Nias. The STC northward and southward transports are out of phase, while the STC northward and downward transports are in phase. Sea-surface water that reaches the middle latitude and is subducted may not completely return to the tropics. The zonal wind anomalies over the central North Pacific, which control Ekman transport, and the east-west slope of the sea level may be major factors causing the TC northward and southward transport interannual variability and the STC northward and southward transports on the interannual time scale. The DTC northward and southward transports decrease during strong El Nios and increase during strong La Nias. DTC upward and downward transports are not strongly correlated with the Nio-3 index and may not be completely controlled by ENSO.

Magnetic induction maps in a magnetized spherical Couette flow experiment

The DTS experiment is a spherical Couette flow experiment with an imposed dipolar magnetic field. Liquid sodium is used as a working fluid. In a series of measurement campaigns, we have obtained data on the mean axisymmetric velocity, the mean induced magnetic field and electric potentials. All these quantities are coupled through the induction equation. In particular, a strong omega-eff ect is produced by di fferential rotation within the fluid shell, inducing a significant azimuthal magnetic field. Taking advantage of the simple spherical geometry of the experiment, I expand the azimuthal and meridional fields into Legendre polynomials and derive the expressions that relate all measurements to the radial functions of the velocity field for each harmonic degree. For small magnetic Reynolds numbers Rm the relations are linear, and the azimuthal and meridional equations decouple. Selecting a set of measurements for a given rotation frequency of the inner sphere (Rm = 9.4), I invert simultaneously the velocity and the magnetic data and thus reconstruct both the azimuthal and the meridional fields within the fluid shell. The results demonstrate the good internal consistency of the measurements, and indicate that turbulent non-axisymmetric fluctuations do not contribute significantly to the axisymmetric magnetic induction.

Initial MST radar observations of upper tropospheric-lower stratospheric duct-like structures over Jicamarca, Peru

Abstract. Persistent wind jet structures along zonal and meridional fields, believed to be caused by stationary gravity waves, were detected in February 1999 in mesosphere-stratosphere-troposphere (MST) radar wind measurements of the troposphere and lower stratosphere over Jicamarca, Peru. Over a continuous seven day span of MST-data analyzed in this study, two days of observations showed signatures of wave-like structures in the upper troposphere/lower stratosphere wind jets associated with the phases of the stationary gravity waves. We believe these wave-like structures are ducted gravity waves. We present these initial observations, their characteristics, and the results of simple numerical simulations used in an attempt to mimic these observed features. Although a fair replication of the observed ducted structure in the numerical model is found, the observed period of ~90 min is nonetheless much longer than what is traditionally observed. As a result, the specific physical nature of the observed structures is not fully established. Nevertheless, given the high quality of the observations, we demonstrate here that continued analysis of this data set and concurrent modeling efforts will allow for a better understanding of Doppler ducts at high spatial and temporal resolution, and the results presented here can ultimately be applied to studies of middle atmospheric fronts, ducts, and bores.

Deflection flows ahead of ICMEs as an indicator of curvature and geoeffectiveness

We examine the upstream meridional deflection flows of interplanetary coronal mass ejections (ICMEs) in an effort to investigate their cross‐sectional shape and the magnetic field orientation in their sheath regions. Eight out of 11 magnetic clouds (MCs) near solar minimum identified for the curvature study are concave outward as indicated by the elevation angle of the MC normal with respect to the solar equatorial plane; an inverse correlation is observed between the meridional deflection flow and the spacecraft latitude for these concave‐outward MCs, which suggests that the upstream plasma is deflected toward the equatorial plane. MHD simulations, however, show that the meridional deflection flow moves poleward for a concave‐outward CME. The poleward flow deflection is observed only ahead of convex‐outward MCs. Possibilities leading to this discrepancy are discussed. The deflection flow speed in sheath regions of ICMEs increases with the ICME speed relative to the ambient solar wind, which together with the coupling between the meridional magnetic field and deflection flow yields a positive linear correlation between the sheath meridional field and the ICME relative speed. This empirical relationship could predict the sheath meridional field based on the observed CME speed, which may be useful for space weather forecasting as ICME sheaths are often geoeffective. Implications of the deflection flows and ICME curvature are also discussed in terms of magnetic reconnection and particle acceleration in ICME sheaths.

A self-similar magnetohydrodynamic model for ball lightnings

Ball lightning is modeled by magnetohydrodynamic (MHD) equations in two-dimensional spherical geometry with azimuthal symmetry. Dynamic evolutions in the radial direction are described by the self-similar evolution function y(t). The plasma pressure, mass density, and magnetic fields are solved in terms of the radial label η. This model gives spherical MHD plasmoids with axisymmetric force-free magnetic field, and spherically symmetric plasma pressure and mass density, which self-consistently determine the polytropic index γ. The spatially oscillating nature of the radial and meridional field structures indicate embedded regions of closed field lines. These regions are named secondary plasmoids, whereas the overall self-similar spherical structure is named the primary plasmoid. According to this model, the time evolution function allows the primary plasmoid expand outward in two modes. The corresponding ejection of the embedded secondary plasmoids results in ball lightning offering an answer as how they come into being. The first is an accelerated expanding mode. This mode appears to fit plasmoids ejected from thundercloud tops with acceleration to ionosphere seen in high altitude atmospheric observations of sprites and blue jets. It also appears to account for midair high-speed ball lightning overtaking airplanes, and ground level high-speed energetic ball lightning. The second is a decelerated expanding mode, and it appears to be compatible to slowly moving ball lightning seen near ground level. The inverse of this second mode corresponds to an accelerated inward collapse, which could bring ball lightning to an end sometimes with a cracking sound.

A differential rotation driven dynamo in a stably stratified star

We present numerical simulations of a self-sustaining magnetic field in a differentially rotating non-convective stellar interior. A weak initial field is wound up by the differential rotation; the resulting azimuthal field becomes unstable and produces a new meridional field component, which is then wound up anew, thus completing the `dynamo loop'. This effect is observed both with and without a stable stratification. A self-sustained field is actually obtained more easily in the presence of a stable stratification. The results confirm the analytical expectations of the role of Tayler instability.

Meridional thermal field of a coupled ocean-atmosphere system: a conceptual model

This paper constitutes the author’s continuing effort in the construction of a minimal theory of the earth’s climate. In an earlier paper published in the Journal of Climate in 2001, this author has derived the global-mean fields of an aquatic planet forced by the solar insolation, which provide the necessary constraints for the present derivation of the meridional thermal field. The model closure invokes maximized entropy production (MEP), a thermodynamic principle widely used in turbulence and climate studies.Based on differing convective regimes of the ocean and atmosphere, both fluids are first reduced two thermal masses with aligned fronts, consistent with a minimal description of the observed field. Subjected to natural bounds, a robust solution is then found, characterized by an ice-free ocean, near-freezing cold fluid masses, mid-latitude fronts, and comparable ocean and atmosphere heat transports. The presence of polar continents, however, sharply reduces the ocean heat transport outside the tropics, but leaves the thermal field largely unchanged.Given the limitation of an extremely crude model, the deduced thermal field nonetheless seems sensible, suggesting that the model has captured the physics for a minimal account of the observed field. Together with the above-mentioned paper, the model reinforces the pre-eminent role of the triple point of water in stabilizing the surface temperature – against changing external condition. Such internal control is made possible by the turbulent nature of the climate fluids, which necessitates a selection rule based on extremization.

Turbulent dynamo near tachocline and reconstruction of azimuthal magnetic field in the solar convection zone

In order to extend the abilities of the αΩ dynamo model to explain the observed regularities and anomalies of the solar magnetic activity, the negative buoyancy phenomenon and the magnetic quenching of the α effect were included in the model, as well as newest helioseismically determined inner rotation of the Sun were used. Magnetic buoyancy constrains the magnitude of toroidal field produced by the Ω effect near the bottom of the solar convection zone (SCZ). Therefore, we examined two “antibuoyancy” effects: i) macroscopic turbulent diamagnetism and ii) magnetic advection caused by vertical inhomogeneity of fluid density in the SCZ, which we call the ∇ρ effect. The Sun's rotation substantially modifies the ∇ρ effect. The reconstruction of the toroidal field was examined assuming the balance between mean-field magnetic buoyancy, turbulent diamagnetism and the rotationally modified ∇ρ effect. It is shown that at high latitudes antibuoyancy effects block the magnetic fields in the deep layers of the SCZ, and so the most likely these deep-rooted fields could not become apparent at the surface as sunspots. In the near-equatorial region, however, the upward ∇ρ effect can facilitate magnetic fields of about 3000 – 4000 G to emerge through the surface at the sunspot belt. Allowance for the radial inhomogeneity of turbulent velocity in derivations of the helicity parameter resulted in a change of sign of the α effect from positive to negative in the northern hemisphere near the bottom of the SCZ. The change of sign is very important for direction of the Parker's dynamo-waves propagation and for parity of excited magnetic fields. The period of the dynamo-wave calculated with allowance for the magnetic quenching is about seven years, that agrees by order of magnitude with the observed mean duration of the sunspot cycles. Using the modern helioseismology data to define dynamo-parameters, we conclude that north-south asymmetry should exist in the meridional field. At low latitudes in deep layers of the SCZ, the αΩ dynamo excites most efficiency the dipolar mode of the meridional field. Meanwhile, in high-latitude regions a quadrupolar mode dominates in the meridional field. The obtained configuration of the net meridional field is likely to explain the magnetic anomaly of polar fields (the apparent magnetic “monopole”) observed near the maxima of solar cycles. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of anisotropic heat transport in the earth's core on the geodynamo

Intermediate dynamos are axisymmetric, spherical models that evade Cowling's theorem by invoking an α-effect to create the meridional magnetic field from the zonal. Usually the energy source maintaining the motions is a specified thermal wind, but here the dynamo is driven by the buoyancy created by a uniform distribution of heat sources. It has been argued by Braginsky and Meytlis (this journal, vol. 55, 1990) that, in a rapidly rotating, strongly magnetic system such as the Earth's core, heat is transported principally by a small-scale turbulence that is highly anisotropic. They conclude that the diffusion of heat parallel to the rotation axis is then significantly greater than it is in directions away from that axis. A preliminary study of the consequences of this idea is reported here. Solutions are derived numerically using both isotropic and non-isotropic thermal diffusivity tensors, and the results are compared. It is shown that even a small degree of anisotropy can materially alter the character of the dynamo.

On the Nearly Viscometric Torsional Motion of Viscoelastic Liquids Between Shrouded Rotating Disks

Flow of a viscoelastic liquid in a cylindrical cavity, driven by rotating finite disks is investigated. The cylindrical sidewall is fixed and the covers rotate with different angular velocities either in the same or in opposite directions. A regular perturbation in terms of the angular velocity of the caps is used. The flow field is resolved into a primary azimuthal stratified viscometric field and a weaker secondary meridional field. Results are presented for a range of cylinder aspect and cap rotation ratios and viscoelastic parameters. Interesting instabilities of the fluid of second grade are discussed. The controversy surrounding the sign of the first Rivlin-Ericksen constant is completely irrelevant to the discussion. It is shown qualitatively that loss of stability occurs repeatedly and bifurcating flows exist for critical values of an elasticity parameter at fixed aspect and cap rotation ratio. Branching flows also occur at a fixed value of the elasticity parameter for critical values of the cap rotation ratio, when the aspect ratio is fixed.

Noise-Induced Interhemispheric Particle Transport—Stochastic Resonance in a Hamiltonian System

A Lagrangian model is employed to study the characteristics of a horizontal cross-equatorial flow. The Coriolis force and the mean meridional pressure field assumed here render the dynamics of particle flow across the equator a nonlinear Hamiltonian system of a bistable potential that has a local maximum at the equator. In the absence of any additional forces this local maximum at the equator prohibits particles from flowing from one hemisphere to the other. When all other (i.e., in addition to the mean meridional pressure gradient) forces are introduced into the system as stochastic forcing, modeled by Gaussian white noise, anomalous diffusion up the mean pressure gradient occurs and particles launched in one hemisphere can reach the other. Spectral estimations of equator crossing events show that at low noise intensity the spectral peak is low, narrow, and situated at low frequencies, and that as the amplitude of the noise increases, the peak becomes higher, wider, and shifts toward higher frequencies. At very large noise intensities the spectral peak flattens out, which implies that the process of equator crossing becomes noise dominated. The results demonstrate the existence of an optimal noise intensity where the signal-to-noise ratio of equator crossings exhibits a sharp maximum, and this optimal noise intensity is insensitive to the precise value of the mean meridional pressure gradient. These findings are applicable to the terrestrial atmosphere where the mean meridional geopotential height gradient and the (zonal and temporal) deviations from it are of the same order. These results demonstrate, for the first time, the occurrence of stochastic resonance in a Hamiltonian system.

Nonlinear adjustment of a rotating homogeneous atmosphere to zonal momentum forcing

Idealized numerical simulations using a simple shallow water model are performed to study ageneralized Rossby adjustment problem which focuses on the nonlinear response of a rotating, uniform, homogeneous, barotropic zonal flow to meso-α and β scale zonal momentum forcing. The prescribed forcings propagate downstream at a speed (c) which is less than the basic stateflow speed (U), and represent the local effects of momentum deposition/redistribution attributableto a variety of physical processes. For small Rossby number flow and t≤ τ =2a/(U — c), the near-field response to meso-α scale forcing in the moving frame of reference is to producelocalized zonal jets of finite longitudinal and latitudinal extent whose geometries are similar tothe imposed forcing structure. The perturbation mass (height) field adjusts to the wind fieldassociated with these disturbances. Although the free surface vertical motion is dominated bytransient inertia-gravity waves at early times, well-defined localized vertical motions also formin the vicinity of the forcing center. For isolated forcing, ascending and descending verticalmotion occurs south and north of the forcing center, respectively. For dipole forcing, a fourcellpattern of vertical motion characterized by ascent in the southwest and northeast quadrantsand descent in the northwest and southeast quadrants flanks the forcing center where a pairof easterly and westerly jets form. For t >t, the exit region of the localized zonal jet producedby isolated forcing is advected downstream, carrying portions of the meridional perturbationwinds and free surface displacement fields with it. The long term asymptotic response is azonally elongated, synoptic scale jet due to the temporally continuous relative vorticity generatedby the zonal momentum forcing. A divergent cross-stream ageostrophic flow in the jetentrance region produces an isolated region of ascending vertical motion which is compensatedby weaker regions of descent to the east and west of the forcing center. The easterly jet producedby flow deceleration in the exit region of the dipole forcing is advected downstream during thesame time period. A four-cell pattern of vertical motion accompanies this easterly jet. Theresponse in the vicinity of the forcing center is an isolated meso-α scale westerly jet, withmeridionally confluent flow in its entrance region and meridionally diffluent flow in its exitregion. The ageostrophic circulation produces rising motion in the jet entrance region andsinking motion in the jet exit region. For moderately large Rossby number flow and meso-bscale dipole forcing, a mesoscale cyclone forms in response to fluid parcels being displacedsouthward into deeper fluid around a ridge in the height field. The moderately strong meso-bscale zonal wind maximum which is produced has associated vertical motions whose geometryis similar to those produced by larger meso-α scale dipole forcing. Stronger nonlinear advectionallows the meso-β scale jet to form four times sooner than the westerly jet produced by smallerRossby number meso-α scale dipole forcing.

Intercomparison of the seasonal cycle of tropical surface stress in 17 AMIP atmospheric general circulation models

The mean state of the tropical atmosphere is important as the nature of the coupling between the ocean and the atmosphere depends nonlinearly on the basic state of the coupled system. The simulation of the annual cycle of the tropical surface wind stress by 17 atmospheric general circulation models (AGCMs) is examined and intercompared. The models considered were part of the Atmospheric Model Intercomparison Project (AMIP) and were integrated with observed sea surface temperature (SST) for the decade 1979–1988. Several measures have been devised to intercompare the performance of the 17 models on global tropical as well as regional scales. Within the limits of observational uncertainties, the models under examination simulate realistic tropical area-averaged zonal and meridional annual mean stresses. This is a noteworthy improvement over older generation low resolution models which were noted for their simulation of surface stresses considerably weaker than the observations. The models also simulate realistic magnitudes of the spatial distribution of the annual mean surface stress field and are seen to reproduce realistically its observed spatial pattern. Similar features are observed in the simulations of the annual variance field. The models perform well over almost all the tropical regions apart from a few. Of these, the simulations over Somali are interesting. Over this region, the models are seen to underestimate the annual mean zonal and meridional stresses. There is also wide variance between the different models in simulating these quantities. Large model-to-model variations were also seen in the simulations of the annual mean meridional stress field over equatorial Indian Ocean, south central Pacific, north east Pacific and equatorial eastern Pacific oceans. It is shown that the systematic errors in simulating the surface winds are related to the systematic errors in simulating the Inter-Tropical Convergence Zone (ITCZ) in its location and intensity. Weaker than observed annual mean southwesterlies simulated by most models over Somali is due to weaker than observed southwesterlies during the Northern Hemisphere summer. This is related to the weaker than observed land precipitation simulated by most models during the Northern Hemisphere summer. The diversity in simulation of the surface wind over Somali and equatorial Indian ocean is related to the diversity of AGCMs in simulating the precipitation zones in these regions.