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Dayton G. Vincent

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.

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Dayton G. Vincent
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Dayton G. Vincent

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.

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Perry G. Ramsey
and
Dayton G. Vincent

Abstract

An important quantity whose magnitude has not been thoroughly examined is the vertical distribution of heating in the Tropics. The details of the vertical distribution of heating have a significant impact on a number of phenomena, including the 30–60 day oscillation, sometimes known as the intraseasonal oscillation. Prior attempts to establish the structure of the heating relied on limited field data or assimilated data, coupled with climatological radiative heating parameters. The availability of high quality global-scale datasets has made it possible to make more accurate calculations than were possible a few years ago.

An important component of the apparent heat budget is the longwave radiative cooling, which in this paper is found by using the ECMWF/WCRP/TOGA Archive II and ISCCP C1 datasets, together with a well-established parameterization scheme. A method is developed that can be used to estimate the vertical structure of cloud amounts based on top-of-atmosphere cloud observations, and the results are used with a wide-band long-wave parameterization to produce longwave cooling rates over the tropical Pacific Ocean.

Outgoing longwave radiation is calculated and compared to ERBE results. The calculated values are generally higher than those from ERBE, though the spatial distributions are similar. Some significant problems exist with the ECMWF upper-tropospheric water vapor amounts, which could imply uncertainties of 0.5°C day−1 in the calculated cooling rates. This is comparable to the differences associated with the minimum or random overlap assumptions used to generate cloud profiles.

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Dayton G. Vincent
and
Robert G. Waterman

Abstract

A diagnostic analysis is presented of the large-scale atmospheric conditions in the Caribbean area during the 4-day period of Hurricane Carmen's (1974) development and intensification. Data taken during the Global Atmospheric Research Program's (GARP) Atlantic Tropical Experiment (GATE) form the basis for the study and consist primarily of rawinsonde reports at standard pressure levels. Analyses of the following observed and computed quantities are performed at 12 h intervals: wind direction and speed, u and v wind components, geopotential height, air temperature, equivalent potential temperature, sea surface temperature, relative humidity, vertical motion, horizontal divergence, relative and absolute vorticity, and vorticity budget terms.

It is seen that several features of the large-scale circulation persist in Camen's vicinity throughout the 4-day period. These include warm sea surface temperatures, low-level conditional instability, a deep moisture-rich layer, low-level confluence and upper level diffluence, upward vertical motion, low and mid-tropospheric. cyclonic vorticity with upper tropospheric anticyclonic vorticity, and deep, easterly flow with weak vertical shear.

Results are also examined separately for Carmen's preburricane and hurricane stages. It is indicated that, as Carmen intensifies, its circulation has an impact on the large-scale flow. The upper level westerlies ahead of Carmen become easterlies as the hurricane approaches the western Caribbean and it appears that deep cumulus convection associated with Carmen plays an important role in producing large-scale vorticity.

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Dayton G. Vincent
and
Andreas H. Fink

Abstract

This study uses a 1° × 1° lat–long dataset, extracted from ECMWF reanalyses for the 15-yr period 1979–93 (ERA-15), to composite environmental characteristics and flow features in the vicinity of named tropical cyclones (TCs) in the eastern and western North Pacific Ocean basins. Tropical cyclones are partitioned into one of four classifications as they pass by selected locations along the axes of maximum frequency and TC tracks: weak (W), strong (S), intensifying (I), or dissipating (D).

Results of this study show that peak values of rising motion, within the same classification, are greater for TC composites in the western Pacific than for those in the eastern Pacific. The level of maximum rising motion was at or above the 500-hPa level for all locations and classifications, except for the Ss at our northernmost point (25°N, 130°E) in the western Pacific. Their maximum upward motion occurred at 700 hPa. It is also found that the latter systems, contrary to all other points, were located in a region of minimum large-scale convective instability. As one cause of stabilization, large-scale advection of drier air from the East China Sea into the western and southern vicinity of the composite storm is identified.

Anomalies of precipitable water (PW) were found to be related to the intensity of the storm, but not to the amount of available climatological “background” moisture. In the eastern Pacific, the monsoontype southwesterly moisture flow across 10°N was much stronger and deeper for Is than for Ws at point B (17.5°N, 112.5°W). On the other hand, the eastern Pacific Ws were more impacted by westward transports originating off the Central American coast. When integrated from the surface to 700 hPa, the net effect was a change in the direction of moisture transport vectors and, therefore, in the major source region. Such a distinct directional change between classifications was exceptional to point B, and was not found for the three points in the western Pacific and South China Sea.

Finally, based on ERA-15 model-produced rain rates, it is found that, in the western Pacific, total precipitation rates for this study were compatible with those of earlier research by W. M. Frank, who used large-scale data. The fraction of ERA-15 stratiform precipitation to the total precipitation varies from 25% to 47% in composite samples used here. The representation of convective and stratiform rain in the ERA-15 model obviously favors the former when the systems are stronger and have a more intense and broader secondary circulation.

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David J. Karoly
and
Dayton G. Vincent

Abstract

No Abstract available.

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Matthew D. Eastin
and
Dayton G. Vincent

Abstract

The climatology of the kinetic energy associated with the subtropical jet over the Australian–South Pacific region is investigated for a 6-yr period, January 1985–December 1990, using monthly mean data. The total kinetic energy (TKE) is partitioned into vertically averaged mean kinetic energy (KM) and level-by-level departure from the mean, or so-called shear kinetic energy (KS). A comparison of the two components during the annual cycle reveals that KM within the region of the subtropical jet is usually greater than KS. An out-of- phase relationship between the annual cycle of TKE and the annual cycle of the percentage of TKE represented by KS is found. A higher percentage of KS occurs in the summer season, when the jet is weakest. During late summer, KS dominates in the entrance region of the jet over Australia and the western Pacific. This appears to coincide with the annual strengthening of the jet. During winter, when the jet reaches its maximum intensity, KM dominates. It also dominates throughout the year in the exit region of the jet.

In addition, a comparison of TKE during an El Niño–Southern Oscillation cycle is made. Results indicate an increase of kinetic energy during El Niño over the central Pacific coupled with a decrease over Australia, indicating eastward movement of the jet. Subsequently, during La Niña, an opposite pattern is observed as the jet moves westward. The results of this climatological study, which appear to be in good agreement with the previous seasonal studies of the subtropical jet, could be beneficial to seasonal or year-to-year forecasting.

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Jon M. Schrage
and
Dayton G. Vincent

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 in the vertical column than oscillations detected in the equatorial Tropics. Vertical motion maxima were generally found at lower levels of the atmosphere in the subtropical episodes than in those found along the equator. As predicted by other studies, the subtropical latitude cases appear to be caused by the passage of a series of baroclinic waves.

Two of the 29 episodes are described in detail. The atmospheric state is composited with respect to the active and inactive phases of the 7–21-day oscillation of OLR. Contrasting events when the OLR values are low and high reveals patterns of circulation features both upstream and downstream from the convection. Composite profiles of vertical velocity and horizontal divergence, as well as maps of divergence and geopotential height anomalies at 200 hPa, were consistent with an atmosphere that had alternately enhanced and suppressed convective activity.

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Ken-Chung Ko
and
Dayton G. Vincent

Abstract

The behavior of subtropical jet streaks over the Australian–South Pacific region is investigated for four consecutive 6-month summer seasons, November to April, during the period 1985–89. The study is an extension of a recent one by Ko and Vincent (hereafter referred to as KV), in which jet streaks during the summer season of 1984–85 were examined. Ko and Vincent found that the upper-tropospheric zonal wind behaved in a quasi-periodic manner, with maxima recurring over the western South Pacific at intervals between one and two weeks. In the present study, wind maxima are found to exhibit a 1–2-week periodic oscillation over the South Pacific in all four years, thus substantiating the finding by KV. In addition, a second set of jet streaks, with a similar periodicity, is found over the southern Australian region. It is also seen that the mean summertime position of the South Pacific jet and its accompanying jet streaks are much farther cast during the El Niño event of 1986–87 than they are in the other three years. The jet is also strongest in 1986–87.

Based on point-to-point correlations of the 5–20-day filtered winds, it appears that the South Pacific jet streak form over eastern and sometimes northern Australia, and propagate east-southeastward at about 10° longitude per day (12 m s−1). The jet streaks over southern Australia originate over the eastern Indian Ocean and propagate eastward at about the same phase speed as the South Pacific jets. These two tracks of wind maxima appear to repeat their patterns approximately every 12 days. In some years, the South Pacific and southern Australian jet streaks appear to be distinctly separate, while in other years it seems as though the South Pacific jets are a continuation of the jets that originated over the Indian Ocean and propagated across southern Australia. Finally, it is found that the jet streaks in both regions are advected eastward in close proximity to upper-tropospheric midlatitude troughs, which also have a statistically significant period between one and two weeks. Consequently, these results could impact on medium-range, as well as short-term intraseasonal, forecasting.

In the concluding remarks, the similarity is noted between the results found in this study and those found by other authors who have investigated jet streak activity in the Northern Hemisphere. A hypothesis is suggested for the behavior of the jet streaks diagnosed in this study.

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