Showing papers in "Monthly Weather Review in 1984"
TL;DR: In this article, seasonal hurricane frequency as related to E1 Nino events during 1900-82 and to the equatorial Quasi-Biennial Oscillation (QBO) of stratospheric zonal wind from 1950 to 1982 is discussed.
Abstract: This is the first of two papers on Atlantic seasonal hurricane frequency In this paper, seasonal hurricane frequency as related to E1 Nino events during 1900–82 and to the equatorial Quasi-Biennial Oscillation (QBO) of stratospheric zonal wind from 1950 to 1982 is discussed It is shown that a substantial negative correlation is typically present between the seasonal number of hurricanes, hurricane days and tropical storms, and moderate or strong (15 cases) El Nin off the South American west coast A similar negative anomaly in hurricane activity occurs when equatorial winds at 30 mb are from an easterly direction and/or are becoming more easterly with time during the hurricane season This association of Atlantic hurricane activity with El Nino can also be made with the Southern Oscillation Index By contrast, seasonal hurricane frequency is slightly above normal in non-El Nino years and substantially above normal when equatorial stratospheric winds blow from a westerly direction and/or are bec
806 citations
TL;DR: In this paper, the authors compared the North Atlantic Oscillation (NAO) and Southern Oscillations (SO) from the standpoint of their association with Northern Hemisphere winter mean distributions of sea-level pressure (SLP) and 500 mb height.
Abstract: The North Atlantic Oscillation (NAO) and Southern Oscillation (SO) are compared from the standpoint of their association with Northern Hemisphere winter mean distributions of sea-level pressure (SLP) and 500 mb height. The NAO and SO are associated with significant SLP differences over much of the hemisphere except for Siberia and western North America. Significant SLP and 500 mb height differences occur in the NAO over the Atlantic Ocean and near Baja California, while in the SO they occur over the Pacific Ocean, India and the western Atlantic. Only over the latter region do large pressure and height variations consistently occur in the extremes of both oscillations; these are also associated with winter temperature variability over the southeastern United States. For example, during winter 1982–83, when the two oscillations simultaneously reached extremes, the NAO was associated with record December warmth east of the Mississippi River, but during January and February the SO dominated the heigh...
669 citations
TL;DR: In this paper, the utility of a simple index for monitoring the Southern Oscillation signal is explored in detail based upon sea level pressure data at the two stations Tahiti (T) and Darwin (D), in the sense that it combines the Southern oscillation variance into one series is the combination [Tn + Dn] where the subscript n denotes normalization by the overall standard deviation of each series.
Abstract: The utility of a simple index for monitoring the Southern Oscillation signal is explored in detail. Based upon sea level pressure data at the two stations Tahiti (T) and Darwin (D), an optimal index, in the sense that it combines the Southern Oscillation variance into one series is the combination [Tn + Dn] where the subscript n denotes normalization by the overall standard deviation of each series. A direct measure of the noise due to small-scale or transient phenomena that are not a part of the large-scale coherent Southern Oscillation fluctuations is the index [Tn + Dn]. It is recommended that this index of noise also should be monitored in order to determine the representativeness of the Southern Oscillation index. The signal-to-noise ratio is shown to depend upon the cross correlation between Darwin and Tahiti, and can be increased by applying weighted moving average low-pass filters to the data. Monthly data exhibit a signal-to-noise ratio, defined as the ratio of the standard deviations, o...
542 citations
TL;DR: In this article, the effects of directionally varying wind shear on convective storm structure and evolution over a wide range of shear magnitudes were investigated using a three-dimensional numerical cloud model.
Abstract: Using a three-dimensional numerical cloud model, we investigate the effects of directionally varying wind shear on convective storm structure and evolution over a wide range of shear magnitudes. As with a previous series of experiments using unidirectional wind shear profiles (Weisman and Klemp), the current results evince a spectrum of storm types ranging from short lived single cells at low shears, multicells at intermediate shears, to supercells at high shears. With a clockwise curved hodograph, the supercellular growth is confined to the right flank of the storm system while multicellular growth is favored on the left flank. An analysis of the dynamic structure of the various cells reveals that the quasi-steady supercell updrafts are strongly enhanced by dynamically induced pressure gradients on the right flank of the storm system. We use this feature along with other related storm characteristics (such as updraft rotation) to propose a dynamically based storm classification scheme. Following...
507 citations
TL;DR: A review of the various methods used to compute both the fluxes and the rate of heating and cooling due to atmospheric radiation for use in numerical models of atmospheric circulation is presented in this paper.
Abstract: This paper presents a review of the various methods used to compute both the fluxes and the rate of heating and/or cooling due to atmospheric radiation for use in numerical models of atmospheric circulation. The paper does not follow, step by step, the solution to the relevant radiative transfer problem but rather concentrates on providing the reader with the physical basis underlying the various methods. The paper discusses, separately, the various parameterizations for the absorptions by water vapor, carbon dioxide and ozone and for the scattering and absorption associated with cloud (and hazes) and also provides some indication of their accuracy.
338 citations
TL;DR: In this paper, the large-scale heat and moisture budgets over the Tibetan Plateau and surrounding area during a 40-day period from late May to early July 1979 were studied using the FGGE Level II-b data.
Abstract: The large-scale heat and moisture budgets over the Tibetan Plateau and surrounding area during a 40-day period from late May to early July 1979 are studied using the FGGE Level II-b data. During this period the general circulation over East Asia underwent a distinct seasonal change characterizing the onset of the summer monsoon circulation. The analyses of the horizontal distributions of the vertically integrated heat source and moisture sink reveal the major heat source regions and their different degrees of association with precipitation. The 40-day mean distributions show intense heat sources of 150–300 W m−2 with moisture sinks of nearly equal magnitude over the Assam–Bengal region and in a broad belt extending over the China Plain along the Mei-yu front. The heat source of ∼100–150 W m−21 over the eastern Tibetan Plateau is accompanied by a moisture sink with a magnitude about half as large. The heat sources over the western Plateau and the Takla Makan Desert are not accompanied by appreciab...
331 citations
TL;DR: In this paper, general circulation mechanisms instrumental in both annual cycle and interannual variability of rainfall are studied with reference to key regions of the tropical Americas and Africa, including the Central American-Caribbean area, northern Northeast Brazil, Subsaharan Africa, the Angola coast and the zaire (Congo) and Amazon basins.
Abstract: General circulation mechanisms instrumental in both annual cycle and interannual variability of rainfall are studied with reference to key regions of the tropical Americas and Africa, including the Central American–Caribbean area, northern Northeast Brazil, Subsaharan Africa, the Angola coast and the zaire (Congo) and Amazon basins. For most of these regions, rainfall anomalies tend to be associated with departures in the large-scale atmospheric and oceanic fields that correspond to the pattern changes in the annual alternation of dry and rainy seasons. The interannual variability of climate and circulation thus appears largely as enhancement and reduction of the annual cycle.
302 citations
TL;DR: In this paper, the authors examined the climatology and structure of tropical cyclones of different sizes and suggested that cyclones grow as a result of an increased convergence of angular momentum forced by their environment.
Abstract: In this paper the climatology and structure of, and possible reasons for, tropical cyclones of different sizes are examined. The climatology of tropical cyclone sizes confirms that tropical cyclones of the western North Pacific are characteristically twice as large as their Atlantic counterparts, and also reveals that the typical size of tropical cyclones varies seasonally and regionally and is only weakly correlated with cyclone intensity (maximum surface wind or minimum surface pressure). Rawinsonde composities of large and small tropical cyclones show that large cyclones have much more relative angular momentum (RAM) than small cyclones, while the differences in RAM between intense and weaker cyclones of equivalent size are less. Some of the implications of this observance are discussed, and a hypothesis that cyclones grow as a result of an increased convergence of angular momentum forced by their environment is presented.
288 citations
TL;DR: In this article, the authors describe a series of numerical experiments simulating the effect of large-scale irrigation on short-term changes of hydrology and climate, using a simple general circulation model, with a limited computational domain and idealized geography.
Abstract: This paper describes a series of numerical experiments simulating the effect of large-scale irrigation on short-term changes of hydrology and climate. This is done through the use of a simple general circulation model, with a limited computational domain and idealized geography. The soil at three latitude bands, namely 30°N–60°N, 0–30°N, and 15°S–15°N is initially saturated with moisture. The results from these experiments indicate that irrigation affects not only the distribution of evaporation but also that of large-scale precipitation. It is found that the anomalies of soil moisture created by irrigation of these respective latitude zones can persist for at least several months due to increased evaporation and precipitation. Furthermore, it the irrigated region is located under a rainbelt, precipitation in that rainbelt is enhanced. Conversely, if the irrigated region is not located under a rainbelt, much of the additional moisture is transported to a rainbelt outside this area. Thus the moist...
283 citations
TL;DR: In this paper, the effect of soil moisture availability on the thermally induced upslope flow during daylight hours is examined, and the results indicate that in flat terrain, which has a nonhomogeneous moisture availability, the intensity of thermal circulations approaches that of sea-breeze circulations for the case of large contrasts.
Abstract: The current study provides an evaluation of thermally induced circulation over flat terrain due to nonuniform horizontal distribution of soil moisture availability. Additionally, the effect of soil moisture availability on the thermally induced upslope flow during the daylight hours is examined. The results indicate that in flat terrain, which has a nonhomogeneous moisture availability, the intensity of thermal circulations approaches that of sea-breeze circulations for the case of large contrasts. Even small amounts of water in thesoil, adjacent to dry areas, can still result in a significant mesoscale flow. The upslope flow has been indicated to be perturbed significantly when the moisture availability ii~creases or when the horizontal contrast of soil moisture availability becomes large.
252 citations
TL;DR: In this article, a detailed analysis of the temperature and pressure fields of the planetary boundary layer (PBL) and their seasonal variability over the western United States and over the Plateau of Tibet (Qinghai-Xizang Plateau).
Abstract: Detailed analyses are presented of the temperature and pressure fields of the planetary boundary layer (PBL) and their seasonal variability over the western United States and over the Plateau of Tibet (Qinghai-Xizang Plateau). Over the United States these analyses rely on 850 mb data, augmented by surface data. Over Tibet the 600 mb surface adequately describes the PBL. The effects of a “plateau monsoon” appear prominently during winter and summer over both plateaus. Together with continental monsoon effects they help to shape prominent circulation features, such as the low-level jet stream (LLJ) over Texas and Oklahoma. The complex, seasonal characteristics of precipitation regimes over the North American continent can be explained, to a large extent, by considering these monsoonal changes in the PBL, especially over the mountains.
TL;DR: In this article, the authors discussed the association of El Nino and the phases of the stratospheric Quasi-Biennial Oscillation (QBO) of equatorial zonal wind with Atlantic seasonal hurricane variability.
Abstract: This is the second of two papers on Atlantic seasonal hurricane activity. It is an extension of Part I, which discussed the association of El Nino and the phases of the stratospheric Quasi-Biennial Oscillation (QBO) of equatorial zonal wind with Atlantic seasonal hurricane variability. It is shown how the addition of regional sea-level pressure data from Caribbean basin meteorological stations can be combined with the more global El Nino and QBO information to form a forecast scheme for Atlantic seasonal hurricane activity. Seasonal forecasts might be issued on 1 June of each year and updated prior to the commencement of the most active part of the hurricane season on 1 August. Although this forecast scheme, of necessity, has been developed on dependent data, the expected forecast skill degradation when applied to independent data sets has been estimated. It appears not to be large enough to significantly negate the rather substantial degree of potential forecast skill that is evident in the deve...
TL;DR: In this article, the authors analyzed the impact of the seasonality of the time series of meteorological variables on the performance of time series statistics, and showed that the presence of an annual cycle means that statistics are nonstationary unless computed from multi-year samples of limited duration.
Abstract: Time series of meteorological variables typically exhibit a pronounced annual cycle and persistence and samples are of finite size. This paper analyses the impact of these complicating features on certain statistics computed from the time series. The presence of an annual cycle means that statistics are nonstationary unless computed from multiyear samples of limited duration. Persistence leads to lack of independence of observations. Large amplitude weather (high frequency) events induce natural variability at low frequencies, known as climatic noise, that is enhanced by the presence of persistence. This natural variability should be, taken into account when estimating population statistics from a finite sample, but generally this has not been done in meteorology. A number of studies in meteorology have computed statistics from daily data by 1) removing the annual cycle; 2) computing second moment statistics over each individual season; and 3) averaging the second moment statistics over all years...
TL;DR: In this article, an analysis of a special boundary layer mission flown during the WMO/ICSU Summer Monsoon Experiment leads us to believe that partial deceleration of the monsoon flow by upstream blocking effects of the Western Ghats Mountains initiates and maintains a vertical and horizontal motion field that could support the observed convection.
Abstract: Seven-year averaged values of percent frequency of occurrence of highly reflective cloud for the months June, July, and August indicate that offshore convection is a major component of the cloudiness of the southwest monsoon. Principal areas of convection occur off of the western coats of India, Burma, Thailand, and the Philippines. This study concentrates on the area upstream of the Western Ghats Mountains of India. Analysis of a special boundary layer mission flown during the WMO/ICSU Summer Monsoon Experiment leads us to believe that partial deceleration of the monsoon flow by upstream blocking effects of the mountains initiates and maintains a vertical and horizontal motion field that could support the observed convection. Data obtained on this mission allow a large-scale momentum budget computation for the subcloud layer, which shows pressure deceleration to be significant. The budget, dominated by advection, predicts an increase of average wind speed which is observed. The pressure decelera...
TL;DR: In this paper, the environmental similarities and differences between fast-moving (line speed VL > 7 m s−1) and slow-moving VL < 3 m s −1) mesoscale convective cloud lines were examined.
Abstract: Rawinsonde data from GATE are composited to examine the environmental similarities and differences between fast-moving (line speed VL > 7 m s−1) and slow-moving VL < 3 m s−1) mesoscale convective cloud lines. Thermodynamic structures for the two types of line are similar with respect to the mixed-layer values of θe and convective available potential energy, but the fast-moving lines have a more pronounced minimum in θe at ∼700 mb. Kinematic structure shows that the vertical shows of the horizontal wind is normal to the leading edge of convention in the fast-moving convective lines but parallel to the leading edge of the slow-moving cloud lines. The results suggest that the type of line (fast versus slow) which forms may be affected by the initial environmental conditions.
TL;DR: In this paper, a scheme for a semi-staggered grid which exactly reflects the Arakawa theory for nondivergent flow is obtained for the first time, which is achieved by conservation of energy and enstrophy as defined on the staggered grid.
Abstract: A common problem with nonlinear advection schemes is the false accumulation of energy at the smallest resolvable scales. To keep this process under control, following Arakawa (1966), a number of energy and enstrophy conserving schemes for staggered and semi-staggered grids have been designed. In this paper, it is demonstrated that, in contrast to the staggered grid, the conservation of energy and enstrophy on the semi-staggered gods does not guarantee that the erroneous transport of energy from large to small scales will be effectively restricted. Using a new approach to the application of the Arakawa Jacobian, a scheme for a semi-staggered grid which exactly reflects the Arakawa theory for nondivergent flow is obtained for the first time. This is achieved by conservation of energy and enstrophy as defined on the staggered grid. These two quantities are of higher accuracy and cannot be calculated directly from the dependent variables on the semi-staggered grid. It is further demonstrated that the...
TL;DR: In this article, a statistical analysis of 12 and 24 hour deepening rates for all surface lows analyzed on at least two successive NMC 12 hourly “front half” hemispheric surface charts was performed for one year of data.
Abstract: A statistical analysis of 12 and 24 hour deepening rates for all surface lows analyzed on at least two successive NMC 12 hourly “front half” hemispheric surface charts was performed for one year of data. Both 12 and 24 hour deepening distributions showed statistically significant (at the 5% level) departures from normality, with the largest deviations occurring along the tail of the distribution associated with most rapid deepening. The sum of two normal curves of different means and standard deviations was successfully fitted to the deepening distributions, suggesting that most cases of explosive cyclogenesis are the result of some additional physical mechanism distinct from ordinary baroclinic instability. The climatology of explosive cyclones (Sanders and Gyakum) was updated to include the 1979–82 cold seasons, and compared to the previous three-year sample. In addition, a climatology of formation positions, maximum deepening positions and dissipation positions for all cyclones in a one-year d...
TL;DR: The most probable local or mesoscale distribution of a climatic variable when only the large-scale value is given may be viewed as a sort of climate inversion problem as discussed by the authors.
Abstract: The estimation of the most probable local or mesoscale distribution of a climatic variable when only the large-scale value is given may be viewed as a sort of climate inversion problem. As an initial statistical study of this question, the monthly-averaged surface temperature and monthly total precipitation for stations in Oregon are analyzed for the purpose of relating their most probable mesoscale distributions to the large-scale monthly anomalies. The first empirical orthogonal mode of the covariance matrix of mesoscale transient departures explains 78.2 and 80.8% of the total variance of temperature and precipitation, respectively. The time structure of the first mode is predominantly seasonal and is in phase with the large-scale anomalies, and the correlation coefficient between this oscillation and the large-scale anomaly is 0.96 for temperature and 0.95 for precipitation. The most probable mesoscale distribution as specified by only the first empirical orthogonal function is predictable wi...
TL;DR: In this article, the authors used heat and moisture budget studies of tropical mesoscale convective systems to partition the total heating in tropical disturbances into cumulus and mesoscales components.
Abstract: Results from recent heat and moisture budget studies of tropical mesoscale convective systems have been used to partition the total heating in tropical disturbances into cumulus and mesoscale components The mesoscale component refer to that part of tropical cloud systems which contains mesoscale anvil circulations, viz., a mesoscale updraft in an upper-tropospheric cloud shield extending from near the 0°C level to near the 0°C level to near the tropopause and a mesoscale downdraft in the lower troposphere. The cumulus component, which is determined as a residual, consists of contributions from cumulus elements of all sizes, ranging from deep cumulonimbus to shallow cumulus; however, deep cumulus effects predominate in the tropical disturbances studied here. The method of partitioning requires an estimate of the fraction of f of the total rainfall in tropical disturbances produced by mesoscale anvil systems. The total apparent heat source Q1, and moisture sink Q2 of Yanai and others and the mesosc...
TL;DR: The equivalence of taking an isotropic, moving, spatial average of a two-dimensional field on the sphere to multiplying the coefficients in its spherical harmonics representation with factors that depend only on the total wavenumber n is discussed in this paper.
Abstract: The equivalence of taking an isotropic, moving, spatial average of a two-dimensional field on the sphere to multiplying the coefficients in its spherical harmonics representation with factors that depend only on the total wavenumber n is discussed. Equivalent spatial averaging operators for several such spectral filters are displayed.
TL;DR: In this paper, the authors examined the AVE-SESAME 11 data set of 19 April 1979 to determine the thermodynamic conditions prior to the onset of deep convection in western Kansas and found that the convective region was characterized locally by substantial potential convective energy and low convective inhibition above the boundary layer.
Abstract: The AVE-SESAME 11 data set of 19 April 1979 is examined to determine the thermodynamic conditions prior to the onset of deep convection in western Kansas. The observations indicate that the convective region was characterized locally by substantial potential convective energy and low convective inhibition above the boundary layer. One-dimensional modeling shows that the convection occurred where the convective inhibition was a minimum, but not where the potential convective energy was a maximum. The model results suggest that the convective inhibition above the boundary layer was almost zero when the first echoes were observed.
TL;DR: In this article, a simple ad hoc model representing a stochastically-forced, seasonally-varying interaction between the atmosphere and the ocean in the Indonesian region can product simulated anomalies at Darwin pressure and Indonesian sea surface temperature that reproduce the observed statistical behavior of them anomalies.
Abstract: The relationship of the Southern Oscillation and El Nino phenomena to sea surface temperature anomalies in the Indonesian region is investigated. The three are closely related and the relationship has a strong annual cycle. The Indonesian sea surface temperature anomalies show arena persistence approximately from January through October with a tendency to dissipate or change sign around November. Changes of Indonesian sea surface temperature anomalies lead by about a season changes in the Southern Oscillation and east Pacific sea surface temperature. It is demonstrated that a simple ad hoc model representing a stochastically-forced, seasonally-varying interaction between the atmosphere and the ocean in the Indonesian region can product simulated anomalies at Darwin pressure and Indonesian sea surface temperature that reproduce the observed statistical behavior of them anomalies without the inclusion of the effects of oceanic and atmospheric events external to the Indonesian region. It suggested t...
TL;DR: In this article, the boundary layer structure which accompanies the development of daytime local wind systems in a broad mountain valley, as revealed by cross sections of potential temperature, is described, and the authors describe how this structure leads to the occurrence of a region of convergence to the downwind side of mountains.
Abstract: This paper presents the boundary layer structure which accompanies the development of daytime local wind systems in a broad mountain valley, as revealed by cross sections of potential temperature. It describes how this structure leads to the occurrence of a region of convergence to the downwind side of mountains. Previous studies, based primarily on one-dimensional sounding of potential temperature and horizontal winds, have shown that profiles of static stability and the presence of winds aloft have an important effect on the manner in which daytime, thermally-forced wind systems develop. In the present study, two-dimensional cross sections obtained from aircraft data, vertical soundings and surface mesonet data show several relevant features. In mid to late morning near the surface, for example, upslope winds form in a shallow mixed layer at the underside of the nocturnal inversion layer (cold pool); at elevations above the top of this cold pool, convectively-mixed surface winds exist. The spat...
TL;DR: In this article, the stability and accuracy of multiply-upstream, semi-Lagrangian method of integrating the advective equation in two dimensions is examined for four different interpolation schemes; namely, bilinear, biquadratic, bicubic and biquartic.
Abstract: The stability and accuracy of the multiply-upstream, semi-Lagrangian method of integrating the advective equation in two dimensions is examined for four different interpolation schemes; namely, bilinear, biquadratic, bicubic and biquartic. All are shown to be consistent and unconditionally stable for constant advecting velocity. Their respective amplitude and phase errors are discussed. They are then used to integrate the test problem of a cone being advected about the plane at constant angular velocity. The merits of the schemes relative to each other and relative to a well tried Eulerian scheme am examined with particular regard to accuracy and computation time.
TL;DR: A simple advective model for tropical ocean surface warming can produce anomalous sea surface temperature (SST) fields similar to those of the first several months of the 1982 El Nino as discussed by the authors.
Abstract: A simple advective model for tropical ocean surface warming can produce anomalous sea surface temperature (SST) fields similar to those of the first several months of the 1982 El Nino. The physics invoked assumes that the existing SST field is advected by anomalous currents to produce the anomalous warming, and that the anomalous currents are those induced subsequent to the passage of downwelling Kelvin wave fronts. Numerical experiments with a fully nonlinear and thermally active ocean model support the physical relevance of the more idealized study. The anomalous horizontal advection model also accounts for the initial SST evolution during the more common type of El Nino event.
TL;DR: In this paper, the authors compare the results of the first and second years of the Zierikzec experiment with the results obtained during the first year of the experiment and evaluate the performance of the forecasters during the second year.
Abstract: Subjective probability forecasts of wind speed, visibility and precipitation events for six-hour periods have been prepared on an experimental basis by forecasters at Zierikzec in The Netherlands since October 1980. Results from the first year of the experiment were encouraging, but they revealed a substantial amount of overforecasting (i.e., a strong tendency for forecast probabilities to exceed observed relative frequencies) for all events, periods and forecasters. Moreover, this overforecasting was reflected in a rapid deterioration in the skill of the forecast as a function of lead time. In October 1981 the forecasters were given extensive feedback concerning their individual and collective performance during the first year of the experimental program. The purpose of this paper is to compare the results of the first and second years of the experiment. Evaluation of the forecasts formulated in the fist and second years of the Zierikzee experiment reveals marked improvements in reliability (i.e...
TL;DR: In this article, the evolution of extratropical transient waves as they propagate eastward from the Eurasian land mass toward the Pacific during selected cold surge events in the winter Monsoon Experiment (MONEX) is studied.
Abstract: The evolution of extratropical transient waves as they propagate eastward from the Eurasian land mass toward the Pacific during selected cold surge events in the winter Monsoon Experiment (MONEX) is studied. The outstanding cold surge episodes during MONEX are first identified, and the salient synoptic features related to these events are described using composite streamline charts. The structure of rapidly varying disturbances accompanying the cold surges and the associated energetics are examined, and the behavior of those fluctuations over relatively longer time scales is addressed.
TL;DR: In this article, a detailed analysis of the sub-subtropical jet streak revealed by an analysis of a Presidents' Day cyclone of February 18-19, 1979 is presented.
Abstract: A detailed examination is undertaken of the subtropical jet (STJ) streak revealed by an analysis of the Presidents' Day cyclone of February 18-19, 1979. During its 24 h period, the STJ's flow became increasingly supergeostrophic and apparently unbalanced, while ageostrophic wind speeds increased to more than 30 m/sec in association with a significant cross-contour component, directed toward lower values of the Montgomery streamfunction, as the flow along the STJ became more divergent with time. These phenomena are linked to the increasing confluence in the entrance region of the jet streak and the decreasing wavelength of the trough-ridge system in which the jet streak was embedded. The upper level divergence and upward vertical motion near the axis of the STJ are found to be important factors in the development of the cyclone's first area of heavy snow.
TL;DR: In this paper, the interannual variability of surface meteorological fields over the Indian Ocean during the period 1954-76 using 2 million ship reports obtained from different sources was studied.
Abstract: The interannual variability of surface meteorological fields over the Indian Ocean during the period 1954–76 is studied using 2 million ship reports obtained from different sources. Monthly mean fields of wind, pressure, air temperature, mixing ratio, cloud cover and sea surface temperature (SST) on a grid mesh of 2° have been determined by computing the monthly mean values of the data around each grid point. Because data coverage is poor over certain areas, an objective analysis based on the successive correction method was performed. Seasonal anomaly fields are presented for the Northern Hemisphere summers of 1956 and 1972, with the two summers having opposite extreme features. During 1956 (1972), the intensity of the trade winds in the Southern Hemisphere and the summer cross-equatorial flow were above (below) normal. Sea surface temperature was below (above) normal over the entire Indian Ocean in 1956 (1972). Time series of the anomalies over different key areas show that weaker than normal t...
TL;DR: In this paper, a scale analysis of the equations of motion of the wind field in straits is performed, where the wind fields are in approximate geostrophic balance with an imposed cross-channel pressure gradient.
Abstract: The complicated wind regimes in straits which develop in response to difFerent large-scale pressure fields are investigated by scale analysis of the equations of motion. Adjustment of the mass and motion fields in straits O(lOs km) in width is governed by four nondimensional numbers: separate along- and cross-strait Rossby numbers, a strait drag coefficient, and a stratification parameter, which relates the internal Rossby radius of deformation to the width of the strait. The wind field is in approximate geostrophic balance with an imposed cross-channel pressure gradient. An along-channel pressure gradient is primarily balanced by ageostrophic acceleration of the wind field down the axis of the strait (the gap wind). Vertical motion and the accompanying horizontal divergence in the near-surface wind field can be large even for moderately stable stratification; as a consequence, there may be particularly abrupt transitions of the surface wind field at the exits of straits, where there is a rapid c...