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Jon M. Schrage and Andreas H. Fink

Abstract

Some spatiotemporal characteristics and possible mechanisms controlling the onset of the widespread, low-level nocturnal stratiform clouds that formed during May–October 2006 over southern tropical West Africa are investigated using cloudiness observations from surface weather stations, data from various satellite platforms, and surface-based remote sensing profiles at Nangatchori in central Benin. It is found that the continental stratus is lower than the maritime stratus over the Gulf of Guinea and persists well into the noon hours. For the study period, a clear seasonal cycle was documented, as well as a dependence on latitude with the cloudiest zone north of the coastal zone and south of approximately 9°N. It is also shown that nonprecipitating clear and cloudy nights observed at Nangatchori in central Benin often reflect clearer and cloudier than normal conditions over a wide region of southern West Africa. At Nangatchori, on average the stratus developed at 0236 UTC (about local time) with an extremely low cloud base at 172 m (above ground level) when averaged over all cloudy nights. About 2–3 h before cloudiness onset, a distinct nighttime low-level jet formed that promoted static destabilization and a low Richardson number flow underneath it. The ensuing vertical upward mixing of moisture that accumulated under the near-surface inversion after sunset caused the cloud formation. It is argued that a strong shear underneath the nighttime low-level jet is the major process for cloud formation, but the low-level static stability and the time scale of the shear-driven mixing are other potential factors.

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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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Jon M. Schrage and Andreas H. Fink

Abstract

The West African squall line is a key quasi-linear storm system that brings much of the precipitation observed in the data-poor Sudanian climate zone. Squall lines propagate at a wide range of speeds and headings, but the lack of operational radar stations in the region makes quantifying the propagation of the squall lines difficult. A new method of estimating the propagation rate and heading for squall lines is proposed. Based on measurements of the time of onset of precipitation (OOP) at a network of rain gauge stations, an estimate of the propagation characteristics of the squall line can be inferred. By combining estimates of propagation rate with upper-air observations gathered at a nearby radiosonde station, the impact of various environmental factors on the propagation characteristics of West African squall lines is inferred. Results suggest that the propagation speed for West African squall lines is related to the conditions at midtropospheric levels, where dry air and an enhanced easterly flow favor faster propagation. Northerly anomalies at these levels are also associated with faster propagation. When applied to West African squall lines, the correlations between these environmental factors and the speed of propagation are significantly higher than those of methods developed for mesoscale convective systems in other parts of the world.

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Andreas H. Fink, Jon M. Schrage, and Simone Kotthaus

Abstract

For years, various indices of seasonal West African precipitation have served as useful predictors of the overall tropical cyclone activity in the Atlantic Ocean. Since the mid-1990s, the correlation unexpectedly deteriorated. In the present study, statistical techniques are developed to describe the nonstationary nature of the correlations between annual measures of Atlantic tropical cyclone activity and three selected West African precipitation indices (namely, western Sahelian precipitation in June–September, central Sahelian precipitation in June–September, and Guinean coastal precipitation in the preceding year’s August–November period). The correlations between these parameters are found to vary over the period from 1921 to 2007 on a range of time scales. Additionally, considerable year-to-year variability in the strength of these correlations is documented by selecting subsamples of years with respect to various meteorological factors. Broadly, in years when the environment in the main development region is generally favorable for enhanced tropical cyclogenesis (e.g., when sea surface temperatures are high, when there is relatively little wind shear through the depth of the troposphere, or when the relative vorticity in the midtroposphere is anomalously high), the correlations between indices of West African monsoon precipitation and Atlantic tropical cyclone activity are considerably weaker than in years when the overall conditions in the region are less conducive. Other more remote climate parameters, such as the phase of the Southern Oscillation, are less effective at modulating the nature of these interactions.

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Dayton G. Vincent, Jon M. Schrage, and L. David Sliwinski

Abstract

The importance of the “warm pool” region of the western Pacific on in situ and global-scale circulations has gained wide recognition in the last decade with the advent of TOGA and, more recently, with the field experiment TOGA-COARE. The objectives of this study are two fold: 1)to provide a climatology of the kinematic properties of the atmosphere over the tropical western Pacific and adjacent areas, based on 1985–90 analyses., and 2) to focus on a detailed diagnosis of the four-month period, November–February, since the intensive observing period of TOGA-COARE occurred during those months. The dataset used in this study is the WCRP/TOGA archive II analyses produced by ECMWF. The analyses contain uninitialized gridpoint values of several variables at 2.5° lat/long and at 14 mandatory pressure levels. The dataset also includes a full surface package at the same horizontal resolution. The variables examined are mean sea level pressure, zonal and meridional wind components, vertical velocity, and relative vorticity. Many well-known features of the circulation are reproduced by the analyses. In particular, there is a good documentation of the differences between the El Niño event of 1986–1987 and the La Niña event of 1988–1989. During the four-month period, the circulation features across the large-scale array of TOGA-COARE, as well as those associated with the Australian monsoon and SPCZ, show well-defined patterns, both temporally and spatially.

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Jon M. Schrage, Carol Anne Clayson, and Brian Strahl

Abstract

Using 9 yr of IR brightness temperature values as a proxy for tropical convection, the climatology and statistics of the 2–3-day convective oscillation signal are examined. These higher-frequency oscillations are shown to have a very different spatial distribution than the signal associated with the 25–70-day intraseasonal oscillation, making it quite unlikely that these oscillations simply represent a form of noise in the time series.

The characteristics of the oscillations are shown to be strong functions of latitude, season, and strength of the oscillation itself. Only the strongest of the 2–3-day oscillations prove to be in good agreement with the properties shown in previous studies, which were based on smaller spatial and temporal domains. These strong oscillations manifest a strong association with the intraseasonal oscillation and westerly wind bursts, particularly in the deep Tropics. Weaker oscillations, on the other hand, are found to be more stochastic and less dependent on larger-scale atmospheric structures.

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Dayton G. Vincent, Ken-Chung Ko, and Jon M. Schrage

Abstract

The main objective of this study is to obtain a better understanding of the upper-tropospheric subtropical westerly wind maxima over the Australian–South Pacific region in the summer half of the year, which have been documented in previous papers to occur with a periodicity of 1–2 weeks. The focus of the study is to quantify the relative importance of tropical versus nontropical forcing during the acceleration phase of the aforementioned westerly wind maxima. Outgoing longwave radiation, wind data, and kinetic energy budgets, partitioned into rotational and divergent components, are used to examine the significance of the forcing mechanisms during the 6-month summer periods from 1985 to 1989. Criteria are developed to identify strong episodes of zonal wind accelerations. In all, 40 cases were found that met these criteria, or approximately 10 cases per year.

In summary, 17 of the 40 cases suggested that tropical forcing was primarily responsible for the observed increase in the rotational kinetic energy of the jet streaks. In contrast, in 13 cases it appeared that little or no connection occurred between tropical convective heat sources and the accelerating jets. In fact, it seemed that midlatitude wave activity was the important factor during the acceleration phase of most of these 13 cases. For the remaining 10 cases, it was difficult to conclude whether tropical forcing was more important than middle latitude forcing; however, it appeared that tropical forcing, albeit weaker than the 17 aforementioned cases, did play a forcing role.

An examination of the case composites in each of these three categories revealed that the energy cycle for the tropically forced cases consisted of a generation of divergent kinetic energy, a conversion of divergent to rotational kinetic energy, and a loss of rotational kinetic energy due to horizontal export and frictional dissipation. Except for the loss of rotational kinetic energy by dissipation, the main energy cycle for the nontropically forced accelerations was the reverse of that for tropically forced jets. Finally, for those 10 cases where the primary region of forcing was uncertain, the composited energy cycle generally consisted of a compromise between the tropically and nontropically forced composites, although there was a significant generation of divergent kinetic energy, as well as a conversion of divergent to rotational kinetic energy, as for all tropically forced cases.

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Dayton G. Vincent, Andreas Fink, Jon M. Schrage, and Peter Speth

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 Niño–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 independent from the MJO, such as easterly waves, monsoon depressions, typhoons, or circulations involved in tropical–extratropical interactions. Over the equatorial eastern Indian Ocean, strong activity in both bands persists throughout the year, but the bands are found to be anticorrelated, regardless of the ENSO phase.

The effect of ENSO timescales is further examined by looking at December–February anomalies for five El Niño and two La Niña events during this 20-yr sample. A well-defined response of the two bands is restricted to the northwestern and central Pacific. Over the northwestern Pacific Ocean, the two bands complement one another with suppressed (enhanced) convection occurring during El Niño (La Niña) events. Both bands also complement each other over the equatorial central Pacific but are out-of-phase with those in the western Pacific on ENSO timescales. In contrast, over the Australian monsoon region and the eastern Indian Ocean, neither band shows a uniform response in terms of anomalous activity when the latest five ENSO warm events, 1977–78, 1982–83, 1986–87, 1991–92, and 1992–93, are considered.

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Jon M. Schrage, Andreas H. Fink, Volker Ermert, and Epiphane D. Ahlonsou

Abstract

Three mesoscale convective systems (MCSs) occurring in the sub-Sahelian wet zone of West Africa are examined using observations from the 2002 Integrated Approach to the Efficient Management of Scarce Water Resources in West Africa (IMPETUS) field campaign, the European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, and Meteosat infrared imagery. These datasets enable the analysis of the synoptic-scale environment in which the MCSs were embedded, along with a high-resolution monitoring of surface parameters during the systems’ passages. The available data imply that cases I and II were of a squall-type nature. Case I propagated into a moderately sheared and rather moist lower and middle troposphere over the Upper Ouémé Valley (UOV). In contrast, case II was associated with a well-sheared and dry lower troposphere and a large, moist instability. In either case, behind the convective cluster a westward-propagating cyclonic vorticity maximum that was likely captured by the ECMWF analysis as a result of the special upper-air station at Parakou (Benin). In case I, the fast-moving vorticity signal slowed down over the Guinean Highlands where convection dissipated. Farther downstream, it might have played a role in the consolidation of an African easterly waves (AEW) trough over the West African coast and the eastern Atlantic. Case III proved to be a more stationary pattern of convection associated with a vortex in the monsoon flow. It also exhibited a moist and low shear environment.

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