Low-latitude Circulation Research Articles

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.

Cloud Radiative Forcing of the Low-Latitude Tropospheric Circulation: Linear Calculations

Abstract The role of clouds for low-latitude atmospheric circulations is examined in a linearized calculation forced by diabatic heating rates. A comparison of the circulation calculated from total diabatic heating, obtained from reanalysis data, with observed fields determines which aspects of the calculation are realistic and which are not. The role of clouds is quantified by the circulation calculated from atmospheric cloud radiative forcing, which, in turn, has been calculated with the National Center for Atmospheric Research radiative transfer model using cloud properties observed in the International Satellite Cloud Climatology Project. In general, cloud radiative forcing contributes about 20% to the magnitude of low-latitude circulations. It typically reinforces the circulation that is driven by convective latent heating. Cloud radiative forcing tends to have a stronger influence in the lower troposphere than at upper levels. It influences local circulations more than remote ones. In particular, cl...

Vertical Wind Shear Influences on Tropical Cyclone Formation and Intensification during TCM-92 and TCM-93

Abstract Vertical wind shears between 200 and 850 mb are calculated from operational analyses and special interactive analyses for Tropical Storm Steve during the Tropical Cyclone Motion (TCM-93) field experiment and for Typhoon Omar at the end of the TCM-92 experiment. The operational Fleet Numerical Meteorology and Oceanography Center (FNMOC) analyses have strong 200-mb winds crossing over the intensifying storms, which leads to vertical wind shears exceeding the 12.5 m s−1 threshold value believed to prevent tropical cyclone intensification. Interactive analyses are produced with the multiquadric interpolation technique that blends composited cloud-drift winds and aircraft reports between 1800 and 0000 UTC and between 0600 and 1200 UTC, sets of synthetic observations to represent missing or mislocated tropical circulations, and the FNMOC analyses that are used as a first-guess field. These interactive analyses indicate that the high winds at 200 mb associated with low-latitude circulations such as mons...

Satellite observations of interaction between tropics and mid-latitudes

In certain seasons and over certain locations, the mid-latitude westerlies invade subtropical and tropical areas. Short wave perturbations moving in the broad mid-latitude westerlies amplify the. long wave troughs creating new baroclinic zones in relatively southern latitudes. These. baroclinic zones Interact .with the low-latitude circulations thus leading to development of new circulation pattern .In which low level easterlies extend northward over the Peninsula, central and northwest .India. The paper describes the role of short waves in the interaction between tropics and mid-latitudes and presents satellite data of a few sequences In which such Interactions have actually taken place.

Intraseasonal Variations in the Southern Hemisphere Circulation

Abstract Intraseasonal variations in the Southern Hemisphere atmospheric circulation have been examined through the application of a 10–50–day bandpass filter to daily ECMWF analyses for 1980–88. Variations on this time scale contribute more than 40% of the daily variance in 500 hPa geopotential over much of the middle and high latitudes of the Southern Hemisphere. EOF analysis of the unnormalized variance for all seasons shows that 49% of this variance can be explained by zonal wave trains centered on the South Pacific and southern Atlantic/Indian oceans, a high-latitude mode of global extent, and a wavenumber 3 pattern at midlatitudes. These modes are essentially equivalent barotropic but slope westward with height so that the patterns at 100 hPa typically lag those at 1000 hPa by around 1o° longitude. There appears to be little interaction with the low-latitude circulation. All of the leading modes propagate eastward but the most consistent movement is shown by the South Pacific wave train represented ...

Simulations of an observed stratospheric warming with quasigeostrophic refractive index as a model diagnostic

A three-dimensional primitive equation model of the stratosphere and mesosphere is described. The model was forced at its lower boundary by observed 100 mb height fields for the wavenumber 2 stratospheric warming period of February 1979, and correctly simulated the reversal of the high latitude circulation. This behaviour contrasts with earlier model simulations of wavenumber 2 warmings in which forcing of a climatological zonally symmetric initial circulation by a stationary wave perturbation led to an initial reversal of the circulation in low latitudes. In a series of idealized experiments we show that wave-wave interaction played no essential role in this simulation and that the ingredients leading to its success were firstly the non-climatological initial wind structure and secondly an imposed longitudinal phase speed for the upward propagating planetary wave at the lower boundary. These studies also demonstrate that in the prewarming period the direction of propagation of planetary wave activity, as represented by integral curves of the so-called Eliassen-Palm flux, may be qualitatively described by the WKBJ limit of the quasi-geostrophic potential vorticity equation. In this limit, the Eliassen-Palm flux is simply related to the zonal mean refractive index, leading to two complementary diagnostics for studying wave, mean-flow interaction.

Simulations of an observed stratospheric warming with quasigeostrophic refractive index as a model diagnostic

AbstractA three‐dimensional primitive equation model of the stratosphere and mesosphere is described. The model was forced at its lower boundary by observed 100 mb height fields for the wavenumber 2 stratospheric warming period of February 1979, and correctly simulated the reversal of the high latitude circulation. This behaviour contrasts with earlier model simulations of wavenumber 2 warmings in which forcing of a climatological zonally symmetric initial circulation by a stationary wave perturbation led to an initial reversal of the circulation in low latitudes. In a series of idealized experiments we show that wave‐wave interaction played no essential role in this simulation and that the ingredients leading to its success were firstly the non‐climatological initial wind structure and secondly an imposed longitudinal phase speed for the upward propagating planetary wave at the lower boundary. These studies also demonstrate that in the prewarming period the direction of propagation of planetary wave activity, as represented by integral curves of the so‐called Eliassen‐Palm flux, may be qualitatively described by the WKBJ limit of the quasi‐geostrophic potential vorticity equation. In this limit, the Eliassen‐Palm flux is simply related to the zonal mean refractive index, leading to two complementary diagnostics for studying wave, mean‐flow interaction.

Large Mesoscale Convection and Sea Breeze Circulation. Part II: Nonlinear Numerical Model

The nonlinear equations are applied to study the interactions between the sea breeze circulation and trapeze instability in low latitudes. The well-developed sea breeze circulation produced by the nonlinear model at coast is not so affected by the trapeze instability as shown in Part I of this study. However, the waves over the land are quite similar to those of Part I, but with a smaller growth rate. This study suggests that a strong diurnal temperature variation over the land and/or the latent heat may be required to produce the observed cloud bands in the tropical continental region.

The low-latitude circulation of Mars

A simple linear, baroclinic and primitive equation model which includes both parameterized dissipation and a fairly realistic basic state is used to study the seasonal response of the Martian atmosphere to the steady-state influence of the orography of Mars. It is argued that the orography possesses a thermal and mechanical influence upon the state of the atmosphere. The thermal influence, which has a maximum at low latitudes, is a result of the temperature anomaly introduced into the atmosphere throughout the troposphere by the orographic feature. The resultant heat sources are shown to possess time scales which are much longer than diurnal, thus allowing a steady-state background circulation to develop. Using thermal and mechanical forcing derived from simple laws, the model is solved numerically to provide seasonal distributions of the steady-state circulation. The steady-state solutions are dominated by the thermal forcing of the Tharsis Ridge region and to a lesser degree, by that of the Olympus Mons region. Mechanical orographic forcing appears to possess an insignificant role in determining the low-latitude circulation. The states of the winter midlatitudes and tropics and the summer midlatitudes are very different, with the former region the most energetic. In the winter midlatitudes the kinetic energy is seen to increase with height with the excitation of large-scale and geostrophic near-barotropic eddies. In the tropics, the kinetic energy decreases with height and the response is nearly completely confined to the longitude-height plane. The transition between these two states occurs abruptly in the subtropics. Some of these features are similar to the planetary scale and long-period circulation of the low latitudes of the terrestrial atmosphere. Other features require consideration of properties inherent in the Martian atmosphere. To study these a simple, continuous analytic model is introduced which contains strong dissipation of time scales characteristic of Mars. It is shown that one solution, the equatorial Kelvin wave, is modified considerable by the strong damping and that it dominates the low-latitude circulation. Besides decaying rapidly with height, the vertical wave scale is stretched considerably with height by the dissipative processes. Such a stretching is shown to be scale selective and the longest horizontal modes are stretched the most in the vertical. Besides allowing an explanation of some features of the Martian atmosphere, the predominant vertical scale of the equatorial Kelvin wave allows some confidence in the choice of a two-layer model for the numerical study.

Variations in Low-Latitude Circulation and Extreme Climatic Events in the Tropical Americas

Departure characteristics of the general circulation in the tropical Atlantic and eastern Pacific are studied in relation to extreme climatic events identified from collectives of long-term rainfall stations and other hydrometeorological parameters, with emphasis to the Central American-Caribbean region (CARIB). Ship observations during 1911–72 compiled with a 1° square latitude-longitude resolution and extending between 30°N–30°S from the African coast to the eastern Pacific form a major observational basis. The quality of the rainy season in CARIB has a large negative correlation with annual precipitation in the U.S. Central Great Plains, and with rainfall and sea surface temperature along the Ecuadorian/Peruvian littoral; in addition there are weak negative correlations with northeastern Brazil rainfall and with discharge and water level in northern tributaries of the Amazon, and a strong positive correlation with hydrometeorological events in Subsaharan Africa. Northeastern Brazil rainfall is strongly negatively correlated with sea surface temperature along the Eucador/Peru coast, but not with rainfall. During extreme dry years in CARIB the North Atlantic high expands equatorward, meridional pressure gradients steepen, and the trades are stronger, albeit in a somewhat more southward location; at the same time, the ITCZ over the eastern Pacific stays farther south and the South Atlantic high contracts on its equatorward side. For extreme wet years, the reverse departure patterns from the 1911–70 mean maps are characteristic. Sea surface temperature anomalies reflect a response to variations in the subtropical highs and major ocean currents: advection of cold waters in the eastern part of the oceans is favored by equatorward expansion of the subtropical high in the respective hemisphere; the wintertime Gulf Stream system has a distinct signature concomitant with departures in the equatorial and South Atlantic; and in the equatorial eastern Pacific departures reverse from the winter preceding toward the height of an extreme rainy season, warm waters in July/August being characteristic for drought in CARIB. In view of the strong spatial correlations, departure patterns constructed from stratification according to extreme events in CARIB are expected to have more general validity. Anomalous rainy season conditions are signaled in advance by large-scale departure patterns in January/February, thus offering the prospect of foreshadowing extreme rainy season behavior from the setup of low-latitude circulation during the preceding northern winter.

インド-赤道太平洋領域の降雨量と南半球中・低緯度大循環の年々変動

いわゆる赤道太平洋乾燥帯における降雨は,約12,000kmにも及ぶ長大な帯状領域の全域にわたって,年々の変動が特異であることで知られている.筆者は,ドイツ海洋気象台発行のDie Witterung in Übersee(1953- )に示された毎年の世界降雨分布とその偏差図を調査して,前記乾燥帯で年雨量が極端に多い年には,ニューギニアから西にかけてインドに至る領域で広域の年雨量減少の発現することを見いだした.そのような年は1957,1958,1965および1966である.また,乾燥帯でより少なく,インド周辺でより多いというような年として,1955, 1956および1962があげられる.インドの洪水・干ばつの資料および乾燥帯のOcean島とFanning島の永年降雨資料を調べると,前述のようなインド-赤道太平洋領域間の逆位相の降雨変動が,さらに過去にさかのぼっても存在したことがわかる.このように特異な広域雨量変動の発生には,南東貿易風の変動が介在するが,同時に南半球中緯度偏西風の変動も考慮されるべきである.偏西風の年々変動はかなり大きく,弱い年には冬(7月)でも夏(1月)より弱いことがある.その顕著な例としては1957および1958の場合がある.WALKER(1924)の提唱したsoutheron oscillation は, BJERKNES(1969)によってWalker circuIationという東西方向の循環モデルに発展したが,南半球偏西風の変動をこのモデルの吟味に加えることが可能である.

On the mean meridional circulations in the Northern Hemisphere during the winter season

AbstractA new attempt is made to determine the mean meridional wind components of the Northern Hemisphere at different levels for the winter season December‐February, and to compute the mean mass circulation as a function of latitude. The computation shows the existence of a strong ‘direct’ Hadley circulation in low latitudes and a much weaker ‘indirect’ circulation in higher latitudes. From the mass circulation in low latitudes the meridional flux of water vapour is computed, and from the divergence of this flux the difference between precipitation and evaporation from the earth's surface is evaluated for different latitudinal belts between the Equator and 40°N.

Mean meridional circulations in the atmosphere

AbstractThe mean meridional circulation in winter and summer is derived as a function of latitude and height from an analysis of observed winds in the area 160°W‐0‐40°E, 75°N‐15°N. The results confirm the classical idea of a direct circulation in low latitudes, but the indirect circulation in middle latitudes is stronger than expected. An investigation of the energy released by the meridional circulation implies that it is of comparable importance to large‐scale disturbances in the energy balance of the general circulation.

Evidence of a mean meridional circulation in the atmosphere from surface wind observations

AbstractSurface‐wind observations in the Northern Hemisphere are analysed and fields of mean meridional and zonal motion obtained. From these, profiles of mean meridional motion as a function of latitude are deduced. The results are consistent with a direct meridional circulation in low latitudes and an indirect circulation in mid‐latitudes, both attaining speeds of 1 to 2 m/sec at the surface. This scheme is found to be consistent with a decrease of upward flux of westerly momentum with height (convergence) in the surface layers in low latitudes and a decrease of downward flux in mid‐latitudes.

Application of particle dynamics to derive a general circulation in low latitudes

In Section 1 are set out the reasons for the use of particle dynamics in this investigation. In Section 2 are solved the equations of motion for particles constrained to move along a horizontal surface under the action of a constant meridional pressure gradient force to obtain (a) velocities in spherical polar co-ordinates with naturally varying Coriolis terms, and (b) velocities and trajectories in plane cartesian co-ordinates in which the Coriolis parameter is assumed constant. Results obtained in each system are compared, showing that the plane co-ordinates as used are not significantly inaccurate, and trajectories of air particles obtained by application significantly inaccurate, and trajectories of air particles obtained by application of this system in steps of 5° of latitude are plotted to show the production of belts of convergence at some distance from the equator under the action of meridional pressure gradient forces.

Studies of Upper-Air Conditions in Low Latitudes . Part I: On the Formation of West Atlantic Hurricanes (Riehl); Part II: Relations between High- and Low-Latitude Circulations (Cressman). Herbert Riehl and George P. Cressman. Chicago: Univ. of Chicago Press, 1948. Pp. vi + 103. (Illustrated.) $2.00.

<i>Studies of Upper-Air Conditions in Low Latitudes</i> . Part I: <i>On the Formation of West Atlantic Hurricanes</i> (Riehl); Part II: <i>Relations between High- and Low-Latitude Circulations</i> (Cressman). Herbert Riehl and George P. Cressman. Chicago: Univ. of Chicago Press, 1948. Pp. vi + 103. (Illustrated.) $2.00.

Studies of Upper-Air Conditions in Low Latitudes . Part I: On the Formation of West Atlantic Hurricanes (Riehl); Part II: Relations between High- and Low-Latitude Circulations (Cressman). Herbert Riehl and George P. Cressman. Chicago: Univ. of Chicago Press, 1948. Pp. vi + 103. (Illustrated.) $2.00.

Studies of Upper-Air Conditions in Low Latitudes . Part I: On the Formation of West Atlantic Hurricanes (Riehl); Part II: Relations between High- and Low-Latitude Circulations (Cressman). Herbert Riehl and George P. Cressman. Chicago: Univ. of Chicago Press, 1948. Pp. vi + 103. (Illustrated.) $2.00.