Dayton G. Vincent

The South Pacific Convergence Zone (SPCZ): A Review

Abstract The circulation features associated with the South Pacific convergence zone (SPCZ) and its accompanying cloud band are reviewed and discussed. The paper focuses on the following topics: location, structure, and characteristics of the SPCZ; theories and observations concerning its existence; the significance and scope of the SPCZ in global-scale circulation patterns; quasi-periodic changes in its location and strength; and synoptic-scale features within its regional influence (e.g., cyclones, subtropical jets). It concludes with some challenging problems for the future.

High- and Low-Frequency Intraseasonal Variance of OLR on Annual and ENSO Timescales

Abstract Using 20 yr of outgoing longwave radiation observations, the complex behavior of the higher- (6–25-day) and lower- (25–70-day) frequency bands of tropical intraseasonal convective oscillations is investigated. Emphasis is given to the mean annual cycle and interannual variability of both bands and to the interaction between the two bands. The focus with regard to the interannual variability within each band is on the warm and cold events associated with the El Nino–Southern Oscillation (ENSO) cycle. The study encompasses the tropical and subtropical Indian and Pacific Oceans (including Australasia). The strongest intraseasonal signals are, for the most part, aligned with the intertropical convergence zone (ITCZ) and South Pacific convergence zone. In some cases, the 6–25-day signal is not collocated with the Madden–Julian oscillation (MJO) signal and/or occurs remotely from the ITCZ. In these cases, the higher-frequency intraseasonal convective perturbations are associated with phenomena independ...

Mean State and Wave Disturbances during Phases I, II, and III of GATE Based on ERA-40

Abstract Using ECMWFs second-generation reanalysis, ERA-40, the large-scale mean state and synoptic-scale features associated with African easterly wave disturbances (AEWs) are examined over West Africa and the adjacent eastern Atlantic Ocean during the three 21-day observing periods of the Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment (GATE) in 1974 (Phase I, 26 June–16 July; Phase II, 28 July–17 August; Phase III, 30 August–19 September). Results are partitioned into four geographical boxes, in order to highlight differences among the AEW vortices as they propagate westward along two tracks (northern and southern) over West Africa (land) and the adjacent eastern Atlantic Ocean (water). This marks the first time that a detailed diagnosis of the northerly track AEWs has been conducted. Results are also compared to previous GATE studies and a 30-yr climatology is extracted from ERA-40. In general, the subjectively analyzed wind fields presented in earlier studies compare favorabl...

Circulation Features Over the South Pacific During 10–18 January 1979

Abstract A diagnostic study of the time-averaged large-scale circulation features over the equatorial and South Pacific Ocean during 10–18 January 1979, FGGE SOP-1, is presented. Results are based on ECMWF Level III-b analyses for an area bounded by 10°N, 105°W, 42.5°S and 170°E. During the nine day period, the South Pacific Convergence Zone (SPCZ) and its accompanying cloud band were quasi-stationary features in the area of study. Analyses of surface pressure, geopotential height, horizontal winds, temperature, relative humidity, vertical p-velocity (ω), horizontal divergence and relative vorticity are presented and discussed. Particular attention is devoted to examining the structure of the SPCZ through the use of vertical cross sections along and across both the west-east (zonal) and northwest-southeast (diagonal) portions of the zone.

Zonal and Eddy Components of the Synoptic-Scale Energy Budget during Intensification of Hurricane Carmen (1974)

Abstract A diagnostic analysis is presented of the partitioned kinetic and available potential energy budgets of the synoptic-scale circulation that occurs in conjunction with Hurricane Carmen (1974) during its intensification in the Caribbean from a tropical depression to a major hurricane. The zonal and eddy energy equations developed by Muench (1965) for the Northern Hemisphere stratosphere are derived in a more general form so that they can be applied to an area of limited latitudinal and longitudinal extent. Standard level rawinsonde and surface data are subjectively analyzed over the Caribbean area and grid-point values are extracted at increments of 2.5° latitude and longitude at 11 levels for eight consecutive synoptic times, 12 h apart, beginning 0000 GMT 30 August. To complete the energy budgets, vertical motion, momentum and heat transports and most of the diabatic heating terms are computed; however, frictional effects are neglected. Results are composited into Carmens.pre-hurricane and hurri...

Kinetic Energy Budget Analysis During Interaction of Tropical Storm Candy (1968) with an Extratropical Frontal System

Abstract A kinetic energy budget over the central United States is constructed using the quasi-Lagrangian scheme of Vincent and Chang (1973), during the 12 h time period 0000–1200 GMT 25 June 1968 when Tropical Storm Candy interacted with a developing extratropical frontal system. Although it is difficult to specifically establish some of the interacting physical mechanisms without consideration of an available potential energy budget, the kinetic energy results, in addition to computations of the release of available potential energy, suggest that there is an interaction taking place between the two cyclone systems. The major results of the study were as follows: 1) there is a large export of kinetic energy, primarily in the upper troposphere, from the extratropical baroclinic zone toward the tropical system; 2) the tropical system remains warm core during and after interaction with the cold-air-advecting baroclinic zone; 3) the release of available potential energy by the tropical system increases durin...

Generation of Available Potential Energy of an Extratropical Cyclone System

Abstract Generation of available potential energy of an extratropical cyclone system moving across the United States is computed for the period 20–21 March 1962 using standard rawinsonde data. A quasi-Lagrangian frame of reference is used and only the baroclinic component of generation, which is based on specification of a reference state and efficiency factors for the cyclone system, is considered. Parameterization schemes are used to estimate diabatic heating distributions due to convective and stable latent heat release and long-wave radiation. The heating fields are found to be in good agreement with observed weather patterns and computed kinematic parameters. Available potential energy generation estimates appear to be more representative than those from other similar studies of extratropical cyclones. Longwave radiation is a dominant source of available potential energy generation during the systems early development in the southwestern United States. Latent heat release rapidly becomes the major s...

An Application of Two Tropical Parameterization Schemes of Convective Latent Heat Release in Middle Latitudes

Abstract Models developed by Krishnamurti et al. (1973) and Kuo (1974) for parameterizing the convective latent heat release in synoptic–scale tropical systems are slightly modified and applied to a case study in which Tropical Storm Candy (1968) interacts with a developing extratropical baroclinic frontal system over the United States. Stable latent heat release is also computed, although much of the reported weather activity is unstable. The convective and stable components of latent heat are converted to precipitation rates and compared to the observed precipitation rate. The results show that Krishnamurtis scheme does not produce reliable patterns. Reasons for this failure are discussed. Kuos scheme, on the other hand, shows better agreement with observed precipitation rates, although discrepancies still exist.

Tropical Convection on 7–21-Day Timescales over the Western Pacific

Abstract Evidence is presented to demonstrate that the oscillations of convection on 7–21-day timescales are an important component of the intraseasonal variability over the region spanning the equatorial western Pacific to the subtropical South Pacific. In that area of the world, these oscillations are largely confined to regions with high sea surface temperatures (SSTS) or SST gradients. Consequently, the patterns of 7–21-day variability of convection undergo significant changes, as the El Nin˜o/Southern Oscillation reconfigures the distributions of SST. A test is developed that detects episodes in which the 7–21-day oscillation of outgoing longwave radiation (OLR) is particularly well defined for several cycles. Applying this test, 29 episodes of high 7–21-day variability were defined. Based on this information, the annual and longitudinal distribution of 7–21-day variability is discussed. The 7–21-day oscillations of convection found at subtropical southern latitudes tend to have stronger wind shear i...

Relationship between Tropical Heating and Subtropical Westerly Maxima in the Southern Hemisphere during SOP-1, FGGE

Abstract FGGE Level III-b analyses, produced by the Goddard Laboratory for Atmospheres, NASA, are used to investigate the relationship between tropical heating and subtropical westerly maxima in the Southern Hemisphere during SOP-1 (5 January-4 March 1979). The mean state of two 15-day periods is examined, as well as day-to-day variations for the entire 59-day period. In Period 1 (6–20 January), the central South Pacific was extremely active convectively, while in Period 2 (3–17 February), convective activity over the western Indian Ocean was enhanced. Episodes of strong outflow in the tropics, as measured by the upper tropospheric velocity potential, were found to be well correlated with the strengthening and propagation of westerly wind maxima in the subtropics. The average location of the westerly maximum over the South Pacific and Indian oceans oceans about 16° latitude south, and slightly east, of its corresponding heat source. For a cyclone case study which is presented, however, this distance was c...