Low-latitude Wind Research Articles

Mean winds at the cloud top of Venus obtained from two-wavelength UV imaging by Akatsuki

Venus is covered with thick clouds. Ultraviolet (UV) images at 0.3–0.4 microns show detailed cloud features at the cloud-top level at about 70 km, which are created by an unknown UV-absorbing substance. Images acquired in this wavelength range have traditionally been used to measure winds at the cloud top. In this study, we report low-latitude winds obtained from the images taken by the UV imager, UVI, onboard the Akatsuki orbiter from December 2015 to March 2017. UVI provides images with two filters centered at 365 and 283 nm. While the 365-nm images enable continuation of traditional Venus observations, the 283-nm images visualize cloud features at an SO2 absorption band, which is novel. We used a sophisticated automated cloud-tracking method and thorough quality control to estimate winds with high precision. Horizontal winds obtained from the 283-nm images are generally similar to those from the 365-nm images, but in many cases, westward winds from the former are faster than the latter by a few m/s. From previous studies, one can argue that the 283-nm images likely reflect cloud features at higher altitude than the 365-nm images. If this is the case, the superrotation of the Venusian atmosphere generally increases with height at the cloud-top level, where it has been thought to roughly peak. The mean winds obtained from the 365-nm images exhibit local time dependence consistent with known tidal features. Mean zonal winds exhibit asymmetry with respect to the equator in the latter half of the analysis period, significantly at 365 nm and weakly at 283 nm. This contrast indicates that the relative altitude may vary with time and latitude, and so are the observed altitudes. In contrast, mean meridional winds do not exhibit much long-term variability. A previous study suggested that the geographic distribution of temporal mean zonal winds obtained from UV images from the Venus Express orbiter during 2006–2012 can be interpreted as forced by topographically induced stationary gravity waves. However, the geographic distribution of temporal mean zonal winds we obtained is not consistent with that distribution, which suggests that the distribution may not be persistent.

Spatial distribution of turbulence characteristics in the inner solar wind

Data on the spatial distributions of turbulence characteristics in the inner solar wind are reported. Spectral indices for the outer and inner turbulence scales have been obtained in radio occultation experiments using signals from several spacecraft at different phases of the solar cycle. The characteristics of turbulence in the slow, low-latitude solar wind remain, on average, constant during the solar cycle. The outer turbulence scale in the fast, high-latitude solar wind appreciably exceeds that of the slow, low-latitude wind at the solar minimum. The new data confirm that the transition from the acceleration region to the steady-flow region is accompanied by a change in the turbulence regime. This change in the turbulence regime takes place at greater distances from the Sun for the fast than for the slow solar wind.

Dynamical response of low-latitude middle atmosphere to major sudden stratospheric warming events

The zonally averaged UK Meteorological Office (UKMO) zonal mean temperature and zonal winds for the latitudes 8.75°N and 60°N are used to investigate the low-latitude dynamical response to the high latitude sudden stratospheric warming (SSW) events that occurred during winter of the years 1998–1999, 2003–2004 and 2005–2006. The UKMO zonal mean zonal winds at 60°N show a short-term reversal to westward winds in the entire upper stratosphere and lower mesosphere and the low-latitude winds (8.75°N) show enhanced eastward flow in the upper stratosphere and strong westward flow in the lower mesosphere during the major SSW events at high latitudes. The mesosphere and lower thermosphere (MLT) zonal winds acquired by medium frequency (MF) radar at Tirunelveli (8.7°N, 77.8°E) show a change of wind direction from eastward to westward several days before the onset of SSW events and these winds decelerate and weak positive (eastward) winds prevail during the SSW events. The time variation of zonal winds over Tirunelveli is nearly similar to the one reported from high latitudes, except that the latter shows intense eastward winds during the SSW events. Besides, the comparison of daily mean meridional winds over Tirunelveli with those over Collm (52°N, 15°E) show that large equatorial winds are observed over Tirunelveli during the 2005–2006 event and over Collm during the 1998–1999 events. The variable response of MLT dynamics to different SSW events may be explained by the variability of gravity waves.

QBO and QTO of atmospheric trace elements nitrous oxide and chlorofluorocarbons: An update

An analysis of N2O (nitrous oxide) and CFC (chlorofluorocarbon) concentrations (monthly values) at various locations during 1978-2001 indicated seasonal variations. When these were minimized by calculating 12-month running means (12m) and 3-year running means (37m) were subtracted from 12m, the residues (12m-37m) showed oscillations with peak spacings in the QBO (quasi-biennial oscillation, 2-3 year) and in the QTO (quasi-triennial oscillation, 3-4 year) regions. The QBO at high northern latitudes roughly resembled the QBO of stratospheric low latitude winds and the QTO at low latitudes resembled the QTO of ENSO (El Niño/Southern oscillation).Key words – QBO, QTO, ENSO, Chlorofluorocarbon.

Diagnosing the Annual Cycle of the Equatorial Undercurrent in the Atlantic Ocean from a General Circulation Model

Abstract Ten-year-long output series from a general circulation model forced by daily realistic winds are used to analyze the annual cycle of the Equatorial Undercurrent (EUC) in the Atlantic Ocean. Two well-defined transport maxima are found: One, present during boreal summer and autumn in the central part of the basin, is generally recognized and regarded as a near-equilibrium response to the equatorial easterly trades that culminate in this period. Another one, most pronounced near the western boundary, occurs in April–May when the trades relax. This second maximum is less patent in the observations, but concomitant signals in previously published analyses of the North Brazil Current and surface velocity seasonal variations might be indirect manifestations of its reality. Because this intensification appears at periods when the boundary between the tropical and equatorial gyres nears the equator, the authors relate its existence to wind stress curl variations at subequatorial latitudes. A link between the interannual variability of the spring transport maximum and that of the low-latitude wind stress curl is, indeed, found in the model. This diagnostic approach suggests that two different dynamical regimes shape up the EUC seasonal cycle: in summer and autumn, local forcing by the equatorial zonal wind component and main supply from the ocean interior; in winter and spring, remote forcing by the low-latitude rotational wind component and supply from the western boundary currents.

QBO and QTO of the atmospheric trace element carbon monoxide during 1989–2001: An update

Abstract An analysis of the 12-month running means of CO (carbon monoxide) concentrations at various latitude belts during the 13 years 1989–2001 indicated oscillations with peak spacings in the QBO, QTO (quasi-biennial and quasi-triennial oscillations, 2–3 and 3–4 year) ranges. For middle and high northern latitudes, the QBOs of CO almost resembled the QBO of stratospheric low latitude winds. For other latitudes, there was no similarity with stratospheric winds. CO had a significant QTO in the range 3.0–3.3 years, but it did not match well with the major periodicity of ENSO (El Nino/Southern oscillation) at 4.3 years. The relative importance of the two mechanisms proposed by Hamilton and Fan [Journal of Geophysical Research 105 (2000) 20581–20587], namely the transport mechanism (dynamic stratospheric QBO modulating the interannual variations of long-lived tropospheric constituents) and the ozone mechanism (stratospheric QBO, affecting stratospheric ozone, affecting the UV radiation filtering down, affecting OH radical in the troposphere, affecting CO) is discussed. Both mechanisms seem to be partly operative in some latitudes.

QBO and QTO of the atmospheric trace element carbon dioxide?an update

Abstract An analysis of CO 2 (carbon dioxide) concentrations (monthly values and their 12-month running means) at various locations during 1970–2001 indicated a general increase of ∼10% in 2–3 decades. It was not uniform and was smaller (only ∼1.5% in 5 years) during 1990–1995. When the long-term (3 years or more) variation was estimated by calculating 3-year running means and was subtracted from the 12-month running means, small oscillations (range less than 0.2%) were noticed with peak spacings in the QBO, QTO (quasi-biennial and quasi-triennial oscillations, 2–3 and 3–4 year) ranges. In latitudes north of ∼30°N, the peaks resembled the QBO of stratospheric low latitude winds (peak spacings of ∼28 months). For latitudes south of 30°N, CO 2 changes seemed to be associated with ENSO phenomenon, though the major El Ninos of 1982–1983 and 1997–1998 seemed to have caused abnormal variations (∼exceeding 0.2%) only in one or two latitude belts.

On the Origin of Intrinsic Narrow Absorption Lines inz ≲ 1 QSOs

We present an exhaustive statistical analysis of the associated (Δvabs < 5000 km s-1), high-ionization (C IV, N V, O VI) narrow absorption line (NAL) systems in a sample of 59 QSOs defined from the Hubble Space Telescope (HST) QSO Absorption Line Key Project. The goals of the research were twofold: (1) to determine the frequency of associated NALs at low redshift and in low luminosity QSOs, and (2) to address the question of what QSO properties either encourage or inhibit the presence of associated NAL gas. To that end, we have compiled the QSO rest-frame luminosities at 2500 Å, 5 GHz, and 2 keV, spectral indices at 2500 Å and 5 GHz, the Hβ emission-line FWHM, and the radio core fraction at observed 5 GHz. In addition, we have measured the C IV emission-line FWHM. We find 17 associated NALs (16 selected by C IV and one selected by O VI) toward 15 QSOs, of which ~10 are statistically expected to be intrinsic. From a multivariate clustering analysis, we find that the QSOs group together (in parameter space) based primarily on radio luminosity, followed (in order of importance) by radio spectral index, C IV emission-line FWHM, and soft X-ray luminosity. We find that radio-loud QSOs that have compact radio morphologies, flat radio spectra [α(5 GHz) > -0.5], and mediocre C IV FWHM (≲6000 km s-1) do not have detectable associated NALs, down to Wr(C ) = 0.35 Å. We also find that broad absorption line (BAL) QSOs have an enhanced probability of hosting detectable NAL gas. In addition, we find that the velocity distribution of associated NALs is peaked around the emission redshifts rather than the systemic redshifts of the QSOs. Finally, we find only one strong NAL [Wr(C ) ≳ 1.5 Å] in our low-redshift sample. A comparison with previous higher redshift surveys reveals evolution in the number of strong NAL systems with redshift. We interpret these results in the context of an accretion disk model. We propose that NAL gas hugs the streamlines of the faster, denser, low-latitude wind, which has been associated with BALs. In the framework of this scenario, we can explain the observational clues as resulting from differences in orientation and wind properties, the latter presumably associated with the QSO radio properties.

Low-Latitude Circulation and Mass Transport Pathways in a Model of the Tropical Atlantic Ocean*

An eddy-resolving numerical ocean circulation model is used to investigate the pathways of low-latitude intergyre mass transport associated with the upper limb of the Atlantic meridional overturning cell (MOC). Numerical experiments with and without applied wind stress and an imposed MOC exhibit significant differences in intergyre transport, western boundary current intensity, and mesoscale ring production. The character of interaction between low-latitude wind- and overturning-driven circulation systems is found to be predominantly a linear superposition in the annual mean, even though nonlinearity in the form of diapycnal transport is essential to some segments of the mean pathway. Within a mesoscale band of 10‐100 day period, significant nonlinear enhancement of near-surface variability is observed. In a realistically forced model experiment, a 14 Sv upperocean MOC return flow is partitioned among three pathways connecting the equatorial and tropical wind-driven gyres. A frictional western boundary current with both surface and intermediate depth components is the dominant pathway and accounts for 6.8 Sv of intergyre transport. A diapycnal pathway involving wind-forced equatorial upwelling and interior Ekman transport is responsible for 4.2 Sv. Translating North Brazil Current rings contribute approximately 3.0 Sv of intergyre transport.

大気大循環モデルでシミュレートされた北極振動の解析

The annular mode in the Northern Hemisphere (NH) extratropical circulation, which has an equivalent barotoropic structure from the surface to the lower stratosphere, is called the Arctic Oscillation (AO) by Thompson and Wallace (1998). The AO is the leading empirical orthogonal function (EOF) mode of the sea level pressure field, and it can be seen throughout the year. It is more dominant in winter. It is characterized as a seesaw pattern of mass, between the Arctic region and the midlatitude belt. It is also a seesaw of the mean zonal wind, between the region poleward of 40°N and that equatorward of it In this study, an analysis of the simulated AO is made, to confirm that the AO is an atmospheric interna. mode, and for studying the transition mechanism to the high/low polarity of the AO. The seasonal and perpetual February runs were made by using the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) atmospheric general circulation model (AGCM). In both runs. the AO signature is dominant. Its magnitude is similar to the observed one. Thus, it is confirmed that the AO is an internal mode of the atmosphere. Using data from a perpetual February integration, a composite analysis of the transition to the high/low polarity of the AO is made. Based upon the transformed Eulerian mean equation for the mean zonal wind, it is investigated which term is responsible for the transition. The results indicate that the wave forcing term contributes to the transition and the term of the residual meridional circulation acts to restore the transition. A decomposition of the wave forcing to zoual wave component indicates that planetary-scale wavenumbers 2 and 3 contribute most to the transition. The synoptic-scale waves contribute partly to the low-latitude wind change. In the perpetual February run, a slowly propagating AO-related stratosphere-troposphere coupled mode, is detected. Its oscillation period is 4-6 months. The anomaly of the zonal-mean, zonal wind first appears in the subtropical stratosphere and propagates poleward. Once the anomaly reaches high latitude, it becomes large and propagates into the troposphere.

The solar wind alpha‐particle content as a clue for the origin of slow flows

Some aspects of the variation of the alpha‐particle abundance in the solar wind are studied using Ulysses data. We focus on the first mid‐latitude phase of the mission, when the spacecraft intermittently enters the fast high‐latitude wind, and on the region around the first perihelion passage, when low‐latitude wind in conditions of low solar activity is observed. In particular, we look for the existence of a relationship between the alpha‐particle abundance and the solar wind velocity. No evidence of correlation is found for the midlatitude wind, while at low latitudes a positive correlation emerges. This difference may be indicative of a different relevance, in the two regions, of the processes that can be at the origin of low‐speed solar wind. At mid latitudes the low velocity of the wind would be mainly due to the topology of the solar magnetic field, with a large expansion factor. Near the equator this effect would become weaker and a dominant role would be played by source region features related to the alpha‐particle content.

Quasi-biennial and quasi-triennial oscillations in geomagnetic activity indices

Abstract. Data for geomagnetic activity index aa for 1868–1994 were subjected to spectral analysis for 12 intervals each of 11 consecutive years. In each interval, QBO and QTO (quasi-biennial and quasi-triennial oscillations) were observed at ~2.00, 2.15, 2.40, 2.70 y and ~3.20, 3.40 y, but not all in all intervals. These fluctuations are absent near (2–3 y before and after) the sunspot minima and are present only as 2 or 3 peaks in aa indices, one near or before the sunspot maximum and the other (one or two, generally the larger ones) in the declining phase of the sunspot cycle. Comparison with the solar wind (1965 onwards) showed a fairly good match, indicating that the aa variations were mostly due to similar variations in the solar wind, which must have their origin in solar physical processes. A few aa variations did not match with solar wind. When compared with terrestrial phenomena, no match was found with stratospheric low-latitude zonal wind QBO; but some QTO in aa matched QTO in ENSO (El Nino/Southern Oscillation). This may or may not be a chance coincidence and needs further exploration.

Evaluation of a Scheme for Improving the Acceptance of Low-Latitude Data in Atmospheric Data Assimilation

A scheme of iterated multivariate optimum interpolation and nonlinear normal-mode initialization is applied to a shallow-water model in an attempt to reduce the rejection of low-latitude height and divergent wind data. The amount of rejection and the performance of the iterative scheme are evaluated as a function of the adjustable analysis parameters in the Tropics. Results typically indicate a modest convergence of the scheme, with analysis-and initialization-produced changes to height and velocity potential showing some decrease. Allowing the normalized observation error to vary as a function of iteration significantly accelerates this decrease because the increasing ratio of observation error to background error results in smaller analysis increments. In all cases, however, the final initialized state is not greatly different from the first guess in the Tropics. The behavior of the scheme is relatively insensitive, at least qualitatively, to the value of any of the analysis parameters. Increasing (decreasing) the value of the geostrophic (divergent) parameter yields better-balanced analyses, meaning analyses with less rejection, as well as improved convergence of the iterative scheme. Varying other analysis parameters, as well as including topography and diffusion, has little qualitative impact. One finding of this study is that retaining some mass-wind coupling in the Tropics may be advantageous, since it tends to produce slightly improved balance in the analysis. The main conclusion, however, is that the combined analysis-initialization scheme appears only marginally useful at low latitudes, at least for current operational prediction systems and data sources.

Nonlinearities in Low-Frequency Equatorial Waves

Abstract The nonlinear response to low-latitude, temporally periodic forcing centered on the equator is studied. Comparisons are made with a linear solution containing no advections; for both solutions the model consists of a spectral, equatorial β-plane shallow-water system. The imposed forcing period is 40 or 16 days, designed to simulate respectively the global-scale atmospheric oscillation and the 10–20 day monsoonal oscillation. Forcing zonal wavenumbers are M = 1 or 2 (40-day period) and M = 5 (16-day period). Results for all cases show that advection does not greatly influence the time evolution of the long-term response, although particular features at a given time may be noticeably affected. For the M = 1 (propagating forcing) experiments, the primary changes are in the geopotential field off the equator (consistent with gradient wind effects), and in the low-latitude zonal wind u. The low-latitude wind influences help cause significant displacement of divergence from forcing in the nonlinear, as...

An essay on low‐latitude wind analysis: Reply

An essay on low‐latitude wind analysis: Reply

An essay on low‐latitude wind analysis

An essay on low‐latitude wind analysis