Zonal Speed Research Articles

Quasi-geostrophic monopoles in sheared zonal jets and multiple-jet flows

This work continues our earlier studies of the interaction between a monopolar vortex and a sheared zonal flow in the framework of a 1.5-layer quasi-geostrophic model, based on numerical experiments with singular vortices. Earlier examination of flows with shears of fixed sign showed that the interaction depends strongly on the latitudinal distribution of the gradient of background potential vorticity b(y) (y being the latitude). The latitude y0 at which b(y) changes sign turns out to be of particular importance. In the vicinity of y0, under certain conditions, there arises the zonal-strip region, which attracts (repels) prograde (retrograde) vortices. This effect is examined here for the zonal flows in the form of individual jets as well as for the systems of alternating zonal jets; in all these cases, the background-flow velocity shear and the parameter b(y) can change sign depending on y. It is shown that the vortex drifts to the nearest latitude y0 on the prograde side of the zonal flow, and the meridional speed of the trapped vortex almost vanishes, but its zonal speed is directed westward and approaches the Rossby-wave drift velocity.

Measurements of Titan’s Stratospheric Winds during the 2009 Equinox with the eSMA

Saturn’s moon Titan possesses stratospheric zonal winds that places it among a sparse class of planetary bodies known to have superrotation in their atmospheres. Few measurements have been made of these speeds in the upper stratosphere, leaving their seasonal variations still not well understood. We examined observations made with the extended Submillimeter Array in 2009 March (L s = 355°) and 2010 February (L s = 5°), shortly before and after Titan's northern spring equinox. Cassini observations and atmospheric models find equinoctial periods to be especially dynamic. Zonal wind calculations, derived from the Doppler frequency shift of CH3CN near 349.4 GHz, yielded speeds of 128 ± 27 m s−1 in 2009 and 209 ± 48 m s−1 in 2010. We estimated the measured emission to originate from vertical altitudes of 336−88+112 km, equivalent to pressures of 3.8−3.4+19.2 Pa, commensurate with Titan’s upper stratosphere/lower mesosphere. This suggests a possible increase in zonal speeds during this period. The results are then compared to those from previous Cassini-inferred and direct-interferometric observations of winds, as well as general circulation model simulations, to form a more complete picture of the seasonal cycle of stratospheric zonal winds.

Eddy trains and eddy jets tracked by constellated altimetry

Systematic eddy propagation with a westward dominance is a well-documented feature in the global ocean, thanks to the availability of constellated altimeter measurement of sea level anomaly with a centimeter-level accuracy. According to their zonal speed relative to the phase speed of the first baroclinic Rossby wave, we propose that eddies can be divided into two modes: “eddy trains” and “eddy jets”. The former refers to isolated eddies intrinsically guided by the long Rossby wave theory in zonal propagation, meanwhile significantly advected by energetic extratropical currents (via Doppler-like effect) and equatorial planetary waves (basically Kelvin waves). The latter refers mostly to “geared” eddy dipoles (also known as “modons” governed by the equivalent barotropic vorticity equation) that propagate with a “super” speed escaping from the Rossby wave guide. In this study, departure and reversal of eddy trains from the Rossby theory are analyzed and compensated, meanwhile the signature and mechanism of eddy jets are identified and revealed. As a result, three distinct regimes are observed in oceanic eddy propagation: A primary Rossby regime (∼66%, westward), a secondary anti-Rossby regime (∼33%, eastward, modulated by Doppler-like and Kelvin wave effects), and a third minor non-Rossby regime (∼1%, zonally neutral, of dipolar nature with gear effect). The first two regimes combine to form the eddy trains, while the third regime belongs to the eddy jets. A clear understanding of the mechanism and pattern of the complex eddy propagations is critical to marine science in general, and to eddy science in particular.

Amateur Observers Witness the Return of Venus’ Cloud Discontinuity

Firstly identified in images from JAXA’s orbiter Akatsuki, the cloud discontinuity of Venus is a planetary-scale phenomenon known to be recurrent since, at least, the 1980s. Interpreted as a new type of Kelvin wave, this disruption is associated to dramatic changes in the clouds’ opacity and distribution of aerosols, and it may constitute a critical piece for our understanding of the thermal balance and atmospheric circulation of Venus. Here, we report its reappearance on the dayside middle clouds four years after its last detection with Akatsuki/IR1, and for the first time, we characterize its main properties using exclusively near-infrared images from amateur observations. In agreement with previous reports, the discontinuity exhibited temporal variations in its zonal speed, orientation, length, and its effect over the clouds’ albedo during the 2019/2020 eastern elongation. Finally, a comparison with simultaneous observations by Akatsuki UVI and LIR confirmed that the discontinuity is not visible on the upper clouds’ albedo or thermal emission, while zonal speeds are slower than winds at the clouds’ top and faster than at the middle clouds, evidencing that this Kelvin wave might be transporting momentum up to upper clouds.

Atmospheric Drivers of Oceanic North Swells in the Eastern Caribbean

Large wintertime ocean swells in the Caribbean, known as north swells, generate high surf and expose communities, ecosystems, and infrastructure to hazardous conditions. Empirical orthogonal functions and cluster analyses using ERA5 reanalysis swell data are performed to characterize north swells in the eastern Caribbean and to establish a ranked list of historical events. ERA5 atmospheric and swell data are used to create basin-scale sea-level pressure, surface wind and swell composites for north swell events of different magnitudes. Additionally, storm events are identified in the mid-latitude North Atlantic Ocean. North swells are predominantly generated by storms that intensify off the North American east coast. However, there is a subset of moderately sized swells associated with a westward-located high-pressure system in the North Atlantic. While lower sea-level pressure and stronger surface winds are important for generating larger swells, the location of the low-pressure center and storm track as well the zonal speed of the storm are critical in the development of large eastern Caribbean north swells. The largest such events are associated with storms located comparatively further southeast, with a more zonal trajectory, and slower zonal speed. Large storms located further northwest, with a more southwest to northeast trajectory, and faster zonal speeds are associated with weaker north swells or in many cases, no significant north swell in the eastern Caribbean.

Seasonal Behaviour of Mesoscale Eddy Trajectories in the North Indian Ocean Based on Satellite Altimetry

In the north Indian Ocean (NIO), maps of sea level anomaly from satellite altimetry were analysed from January-1995 to December-2000. The study attempted to trace the trajectories of the individual mesoscale anomalies manually and to understand seasonal changes in terms of phase speed. Mesoscale anomalies are detected as concentric circular shapes and diameters of ~90 km to 600 km and the minimum 30 days life cycle. Relatively higher eddy kinetic energy was noticed in the northwestern region of the NIO. Individual mesoscale anomalies, namely positive (warm, anticyclonic eddies) and negative (cold, cyclonic eddies) showing travelling direction westward in the NIO basins. In autumn, the number of negative anomalies detected is more than positive anomalies and vice versa during summer. The westward propagating positive (negative) anomalies in the Arabian Sea start appearing in winter (spring) along (away from) the west coast of India and west of 65°E; individual anomalies move to the west in spring/summer/autumn and collide along Somalia’s & Arabian coast. A group of positive (negative) anomalies trajectories appears as a tail at the southern tip of India are located west of the Laccadive ridge in winter (summer to autumn) associated with LH (LL). The Bay of Bengal (BB) trajectories show southwestward in northern BB, westward in central BB and northwestward in southern BB; individual anomalies are appearing along the west coast of Andaman & Nicobar ridge. The zonal phase speed decreases away from the equator, and the magnitude varies longitudinally in each season in the form of a wave-like pattern propagating westward from autumn to summer; the life cycle of the wave is almost 365 days (a year). The theoretical phase speed of the first mode of the baroclinic Rossby waves is quite similar to that of averaged zonal speed. Therefore mesoscale anomalies (eddies) are embedded into the large waves like phenomenon (Rossby waves), responsible for creating high variability and EKE in the region of NIO along the western boundaries.

Factors Controlling the Diversities of MJO Propagation and Intensity

AbstractThe diversity of the Madden-Julian Oscillation (MJO) in terms of its maximum intensity, zonal extent and phase speed was explored using a cluster analysis method. The zonal extent is found to be significantly correlated to the phase speed. A longer zonal extent is often associated with a faster phase speed.The diversities of zonal extent and speed are connected with distinctive interannual sea surface temperature anomaly (SSTA) distributions and associated moisture and circulation patterns over the equatorial Pacific. An El Niño–like background SSTA leads to enhanced precipitation over the central Pacific, allowing a stronger vertically overturning circulation to the east of the MJO. This promotes both a larger east-west asymmetry of column-integrated moist static energy (MSE) tendency and a greater boundary-layer moisture leading, serving as potential causes of the faster phase speed. The El Niño–like SSTA also favors the MJOs intruding further into the Pacific, causing a larger zonal extent.The intensity diversity is associated with the interannual SSTA over the Maritime Continent and background moisture condition over the tropical Indian Ocean. An observed warm SSTA over the Maritime Continent excites a local Walker cell with a subsidence over the Indian Ocean, which could decrease the background moisture, weakening the MJO intensity. The intensity difference between strong and weak events would be amplified due to distinct intensity growth speed. The faster intensity growth of a strong MJO is attributed to a greater longwave radiative heating and a greater surface latent heat flux, as both of which contribute to a greater total time change rate of the column-integrated MSE.

The Predictability of the Extratropical Flow Response to Recurving Atlantic Tropical Cyclones

AbstractGiven the ability of recurving Atlantic tropical cyclones (TCs) to disturb the amplitude of Rossby waves on the extratropical jet, this study investigates whether the predictability of the synoptic-scale flow is significantly modified from climatology downstream from and after TC recurvature events. Predictability is evaluated as the standard deviation of isentropic potential vorticity among a 50-member ensemble and is compared to a model climatology. It is shown that forecast uncertainty is dependent upon the relative location of the nearest trough at the time of recurvature and the relative zonal speed between the aforementioned trough and the TC in the 72 hours after recurvature. Predictability is significantly degraded when recurvature occurs downstream of a trough; the elevated uncertainty subsequently propagates downstream along with the trough axis. Furthermore, this study evaluates predictability in spectral space in order to distinguish between uncertainty tied to the exact location of troughs and ridges and uncertainty in Rossby wave amplitude. The wavelet analysis demonstrates that the increase in uncertainty is not solely limited to the trough location, as there is also significantly elevated uncertainty in the Rossby wave amplitude that originates from the upstream trough and spans across downstream troughs and ridges. Uncertainty is also increased near the recurvature longitude in the subset of cases in which the Rossby wave train propagates zonally slower than the TC after recurvature, which is hypothesized to be linked to baroclinic growth processes.

Aporte sobre la variabilidad espacial de la corriente de Cromwell en el Pacífico ecuatorial, entre 170°o y 85°o en el promedio de 1993 -2017

The equatorial undercurrent (EUC), also known as the Cromwell current, is important in the distribution of nutrients, heat and its impact on the fishery resources of the Equatorial Pacific Ocean. However, being a subsurface current it does not have time series of measurements in situ long enough to describe in detail its behavior. For this reason, the use of reanalysis productsis presented as an alternative to these information shortcomings. The objective of this work is to contribute to the general knowledge of the EUC in order to establish its behavior through the Equatorial Pacific basin. This research analyzed 25 years of monthly averages of temperature, salinity and current fromitszonal and meridional components. The main statisticalparameters of the zonal velocity of the current along 170°W, 140°W, 110°W and 85°W were described, including their respective depths of 15°C and 20°C isotherms. The average, standard deviation of the sample, estimation error, maximum and minimum were calculated as the main statistical variables of the core of the zonal component of the current along 170°W, 140°W, 110°W and 85°W. The average and the 90% confidence interval, for the average zonal speed at 170°W was 0.18 ± 0.02m/s centered at 176 ± 13 m between 0.375°S and 2.125°N. At 140°W the current was presented with a velocity of 0.13 ± 0.015 m/s, more superficial than the last meridian(between75 and 200 m) and a less extensive nucleus (between 0.375°S and1.875°N). At 110°W the maximum was found at 0.08 ± 0.010 m/s, with a corebetween 50 and 125 m deep and between 0.125 and 1,125°N. Finally, Eastof the Galapagos Islands (85°W), the EUC was presented with a coredeployed between 0.375°S and 1.875°N, more superficial (between 50 and150 m) and at a lower zonal speed (0.07 ± 0.004 m/s). The EUC was notfound centered at the 0° latitude and its depth along the ocean basin was notconstant, it was framed between the isotherms of 20 ° C and 15 ° C, but west of 140 ° O the nucleus left these limits presenting a higher temperaturerange. Quasi-zonal equatorial subsurface currents were notoriouslyidentified in the Central Pacific, while approaching the Eastern Pacific thespeed decreased making it difficult to define the core of the EUC.Themeridional component of the EUC is much smaller than the zonalcomponent, and shows its northward maximums around 120 ° W andsouthward maximums near the Galapagos Islands (95 °W).

Analysis and Evaluation of Data-Adaptive Spectral Estimation Algorithms for Processing MST Radar Data

The National Atmospheric Research Laboratory (NARL) situated at Gadanki, Andhra Pradesh, constitutes the primary source for Indian MST radar data (Mesosphere-Stratosphere-Troposphere). The primary purpose of the MST radar is to analyze and explore the atmospheric dynamics. The MST radar is developed with an array antenna in the active phase and consists of 1024 Yagi-Uda antennas being operated at a frequency of 53 MHz. The NARL provides atmospheric wind data by using MST radar. The wind speed parameters are calculated from the radar echo signals by using spectral estimation. In this paper, the analysis and evaluation of data-adaptive spectral estimation algorithms namely nonparametric iterative adaptive approach (IAA) and semiparametric methods—sparse learning via iterative minimization (SLIM) and sparse iterative covariance-based estimation (SPICE)—have been presented. These algorithms exhibit high resolution and low sidelobe levels. The detectability of the radar signals is improved at low signal-to-noise (SNR) conditions. The zonal speed (U), meridional speed (V) and wind speed (W) are computed and are validated using the Global Positioning System (GPS) radiosonde data. From the results, the SPICE algorithm outperforms the other two data-adaptive algorithms and the existing periodogram method.

First Conjugate Observations of Medium‐Scale Traveling Ionospheric Disturbances (MSTIDs) in the Europe‐Africa Longitude Sector

AbstractAll‐sky imagers located in Asiago, Italy (45.87oN, 11.53oE; 40.7o magnetic latitude) and Sutherland, South Africa (32.37oS, 20.81oE; −40.7o magnetic latitude) are used to study magnetically conjugate medium scale traveling ionospheric disturbances (MSTIDs). We present initial results from the first year of joint Asiago‐Sutherland data sets from July 2016 to June 2017. The 630.0‐nm airglow perturbations showing different kinds of waves were frequently observed. Some of these wave events resemble MSTIDs propagating south‐westward in Asiago, typical direction observed at other longitude sectors in the northern hemisphere. They are mostly observed as single bands propagating through the field of view of the all‐sky imagers. We select and analyze five cases of magnetically conjugate bands associated with MSTIDs. The bands observed at Sutherland move mainly westward, noticeably different from the north‐west direction of propagation of MSTIDs observed in the southern hemisphere. We compare the MSTIDs propagation speeds and find that three cases show larger values at Sutherland. When we compare the zonal speeds all the cases show larger values at Sutherland. On average, the propagation speed at Sutherland is 20% larger and the zonal speed is ~35% larger. The westward motion at Sutherland is explained by taking onto account how its magnetic declination (~24oW) affects the orientation of the bands. The larger speed at Sutherland is due to the weaker Earth's magnetic field in the southern hemisphere and the particular configuration of the magnetic field lines in this longitude sector.

The Study of Zonal Circulation of Venus’s Atmosphere by Data Analysis of Radio Occultation Measurements Made by Venera-15 and -16 Satellites

The vertical profiles of temperature and pressure obtained from radio occultation measurements made by satellites Venera-15 and -16 from October 1983 to September 1984 are used for the analysis of wind speed in Venus’s atmosphere. The altitude and latitude dependences of zonal wind speed in the middle atmosphere of the planet’s northern and southern hemispheres are identified at altitudes from 50 to 80 km over the latitude interval of 60°–85°. Zonal speeds were determined assuming cyclostrophic atmosphere balance. The existence of jet flows, whose maximal speed is ~100 m/s and axis is placed at an altitude near 60 km within the latitude zone of 73°–75° N, is confirmed in the northern polar atmosphere of the planet. The results of wind speed measurements in the southern hemisphere clearly show that the jet flows exist at an altitude around 62 km at latitudes from 70° to 72° S, and their zonal speed reaches a maximum of ~115 m/s. It is found that these jets at high latitudes are stipulated by negative latitudinal temperature gradients at altitudes below the jet flow axes in Venus’s polar atmosphere.

Precipitation and Mesoscale Convective Systems: Explicit versus Parameterized Convection over Northern Africa

Abstract Precipitating systems are analyzed during a dust event from 9 to 14 June 2006 over northern Africa. A common analysis is applied to satellite observations and two Meso-NH simulations: one convection permitting (grid spacing x = 2.5 km) and the other with parameterized convection (x = 20 km). The precipitating systems are identified as cloud objects and classified as deep convective clouds (DCCs) or other clouds according to their infrared signature. Large DCCs [hereafter named mesoscale convective systems (MCSs)] are tracked, characterized in terms of precipitation and thermodynamic profiles, and analyzed in southern West Africa (SWA), central Africa, and Ethiopia. Precipitation is mostly observed along 0°–15°N, with 71% of the total precipitation produced by all DCCs and 55% by long-lived MCSs. It shows a marked diurnal cycle with a peak in the evening, mainly due to long-lived MCSs, which are characterized by an increase in size, zonal speed, and duration from east to west, with the largest, fastest, and longest-lived ones found over SWA. This is due to an enhanced African easterly jet (AEJ) and monsoon flow leading to stronger shear and greater conditional instability. The simulation with parameterized convection fails to distribute precipitation correctly. The convection-permitting simulation captures most of the observed precipitation features, but lacks the increase in organization of the long-lived MCSs over SWA. Excess moisture in a too zonal AEJ flow suggests that the long-lived MCSs in SWA are poorly located with respect to African easterly waves. The convection-permitting model improves the representation of precipitation but without fully resolving the long-lived MCSs.

Stationary waves and slowly moving features in the night upper clouds of Venus

At the cloud top level of Venus (65-70 km altitude) the atmosphere rotates 60 times faster than the underlying surface, a phenomenon known as superrotation. Whereas on Venus's dayside the cloud top motions are well determined and Venus general circulation models predict a mean zonal flow at the upper clouds similar on both day and nightside, the nightside circulation remains poorly studied except for the polar region. Here we report global measurements of the nightside circulation at the upper cloud level. We tracked individual features in thermal emission images at 3.8 and 5.0 $\mathrm{\mu m}$ obtained between 2006 and 2008 by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS-M) onboard Venus Express and in 2015 by ground-based measurements with the Medium-Resolution 0.8-5.5 Micron Spectrograph and Imager (SpeX) at the National Aeronautics and Space Administration Infrared Telescope Facility (NASA/IRTF). The zonal motions range from -110 to -60 m s$^{-1}$, consistent with those found for the dayside but with larger dispersion. Slow motions (-50 to -20 m s$^{-1}$) were also found and remain unexplained. In addition, abundant stationary wave patterns with zonal speeds from -10 to +10 m s$^{-1}$ dominate the night upper clouds and concentrate over the regions of higher surface elevation.

Neptune long-lived atmospheric features in 2013–2015 from small (28-cm) to large (10-m) telescopes

Since 2013, observations of Neptune with small telescopes (28–50 cm) have resulted in several detections of long-lived bright atmospheric features that have also been observed by large telescopes such as Keck II or Hubble. The combination of both types of images allows the study of the long-term evolution of major cloud systems in the planet. In 2013 and 2014 two bright features were present on the planet at southern mid-latitudes. These may have merged in late 2014, possibly leading to the formation of a single bright feature observed during 2015 at the same latitude. This cloud system was first observed in January 2015 and nearly continuously from July to December 2015 in observations with telescopes in the 2-10-m class and in images from amateur astronomers. These images show the bright spot as a compact feature at −40.1 ± 1.6° planetographic latitude well resolved from a nearby bright zonal band that extended from −42° to −20°. The size of this system depends on wavelength and varies from a longitudinal extension of 8000 ± 900 km and latitudinal extension of 6500 ± 900 km in Keck II images in H and Ks bands to 5100 ± 1400 km in longitude and 4500 ± 1400 km in latitude in HST images in 657 nm. Over July to September 2015 the structure drifted westward in longitude at a rate of 24.48 ± 0.03°/day or −94 ± 3 m/s. This is about 30 m/s slower than the zonal winds measured at the time of the Voyager 2 flyby. Tracking its motion from July to November 2015 suggests a longitudinal oscillation of 16° in amplitude with a 90-day period, typical of dark spots on Neptune and similar to the Great Red Spot oscillation in Jupiter. The limited time covered by high-resolution observations only covers one full oscillation and other interpretations of the changing motions could be possible. HST images in September 2015 show the presence of a dark spot at short wavelengths located in the southern flank (planetographic latitude −47.0°) of the bright compact cloud observed throughout 2015. The drift rate of the bright cloud and dark spot translates to a zonal speed of −87.0 ± 2.0 m/s, which matches the Voyager 2 zonal speeds at the latitude of the dark spot. Identification of a few other features in 2015 enabled the extraction of some limited wind information over this period. This work demonstrates the need of frequently monitoring Neptune to understand its atmospheric dynamics and shows excellent opportunities for professional and amateur collaborations.

Low‐latitude daytime F region irregularities observed in two geomagnetically quiet days by the Hainan coherent scatter phased array radar (HCOPAR)

AbstractHainan coherent scatter phased array radar (HCOPAR) located at low latitude of China has recorded the extremely rare daytime F region irregularities at noon of 22 July 2013 and 23 May 2016. The two field‐aligned irregularities (FAIs) appeared in the topside F2 layer and presented small Doppler velocities and narrow spectral widths. The fan sector maps show that the FAIs moved northward with almost no zonal speed. The irregularities emerged in the geomagnetically quiet condition and were irrelevant to the storm‐induced eastward electric field as other daytime cases. More than 2 h after the emergency of the daytime irregularities over Hainan, the Shaoyang digisonde situated ~870 km north to the HCOPAR recorded the spread‐F in ionospheric F1 layer. According to the echo altitudes, the spread‐F may connect the daytime bubbles via magnetic field line. The strong photoionization after sunrise made it difficult to generate the plasma bubbles in the sunlit ionosphere. Consequently, the two midday FAIs over Hainan may drift along the magnetic field lines from higher altitudes in the south and are most likely the remnant of previous night's bubbles.

The topography effect on the sudden deceleration of the mesoscale eddy propagation speed around the Dongsha Islands in northern South China Sea

Based on satellite altimeter data, it was found that the mesoscale eddy propagates along the continental slope in northern South China Sea. When the mesoscale eddy passed the Dongsha Islands, the propagation speed clearly slowed down. Additionally, the radius increased and strength decreased when the mesoscale eddy touched the Dongsha Islands. Using the analytic expression for the speed of the center of an isolated eddy, it was found that the sudden slowdown of the zonal propagation speed was accompanied by the sudden decrease of the meridional topography gradient, which was the main reason for the slowdown of the propagation speed.

Solar large-scale flows obtained from the HMI time–distance data-analysis pipeline

Using the subsurface flow maps obtained from the HMI time–distance data-analysis pipeline, I examine the temporal evolution of torsional oscillation, meridional flow, and long-living large-scale structures in high-latitude areas. During the 5.5-year analysis period, both the torsional oscillation and meridional flow show strong hemispheric asymmetry while persisting the converging-flow patterns toward the activity belts. Meanwhile, for both hemispheres in the mid-latitude zone, the meridional-flow speed shows an anti-correlation with the magnetic flux being transported toward the pole, slowing down (speeding up) when following-polarity (leading-polarity) magnetic flux is transported. In the latitudinal band studied, the meridional-flow speed and magnetic field remained relatively unchanged from 2012 through 2015 in the northern hemisphere, but varied substantially during the same period in the southern hemisphere. Long-living large-scale structures, characterized by their low zonal speed, are observed in high-latitude areas, but the nature and cause of these structures are unknown.

An Observational and Model Characterization of Vertical Structure of Wind Fields over Eastern United States: A Case Study of Sterling, Virginia

The performance of twenty GCMs that participated in the Coupled Model Intercomparison Phase 5 (CMIP5) is evaluated at Sterling, Virginia, by comparing model outputs with radiosonde observational dataset and reanalysis dataset. We evaluated CMIP5 models in their ability to simulate wind climatology, seasonal cycle, interannual variability, and trends at the pressure levels from 850 hPa to 30 hPa. We also addressed the question of the number of years required to detect statistically significant wind trends using radiosonde wind measurements. Our results show that CMIP5 models and reanalysis successfully reproduced the observed climatological annual mean zonal wind and wind speed vertical distribution. They also capture the observed seasonal zonal, meridional, and wind speed vertical distribution with stronger (weaker) wind during the winter (summer) season. However, there is some disagreement in the magnitude of vertical profiles among CMIP5 models, reanalysis, and radiosonde observation. Overall, the number of years to obtain statistically significant trend decreases with increasing pressure level except for upper troposphere. Although the vertical profile of interannual variability of CMIP5 models and reanalysis agree with the radiosonde observation, the wind trend is not statistically significant. This indicates that detection of trends on local scale is challenging because of small signal-to-noise ratio problems.

Titan’s meridional wind profile and Huygens’ orientation and swing inferred from the geometry of DISR imaging

The altitude and zonal motion of the Huygens probe descending through Titan’s atmosphere was determined early under the assumption of no meridional motion (Bird et al. [2005]. Nature 438, 800–802). By comparing images taken during the descent, Karkoschka et al. (Karkoschka et al. [2007]. Planet. Space Sci. 55, 1895–1935) determined the meridional motion of Huygens, which was generally much smaller than its zonal motion. Here, we present a comprehensive geometrical analysis of all images taken during the descent that is four times more accurate than the previous study. The result is a meridional wind profile across Titan’s troposphere with northward winds by up to 0.4m/s with an average of 0.1m/s above 1km altitude, and southward winds below, peaking at 0.9m/s near 0.4km altitude. The imaging data extend down to 0.22km altitude, although additional information came from the horizontal impact speed near 0.8m/s southward (Schröder et al. [2012]. Planet. Space Sci. 73, 327–340). There is a region between 5 and 8km altitude with no significant meridional wind. In the stratosphere, the average meridional wind was 1.2±1.5m/s northward, and zero meridional motion is possible down to 15km altitude. We present the difference between the zonal speeds of Huygens and the wind that was ignored in previous publications and amounts to up to 7m/s. We determined the three rotational angles of Huygens for the times of each exposure that showed surface features. During 26 exposures, the swing speed of Huygens was fast enough to smear images. Inferred swing speeds were up to 20°/s during the calm phase of the descent, consistent with up to 40°/s swings reported before during the rough phase. The improved geometric calibration of images allowed identification of many features also seen in Cassini radar images. This comparison yields the location of the Huygens LandingSite as 192.34±02° West and 10.47±0.02° South.