Jon M. Schrage

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...

On the Potential Causes of the Nonstationary Correlations between West African Precipitation and Atlantic Hurricane Activity

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 dev...

Nocturnal Continental Low-Level Stratus over Tropical West Africa: Observations and Possible Mechanisms Controlling Its Onset

AbstractSome 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 a...

2-3-day convective variability in the tropical western Pacific

Abstract This paper is an examination of 2–3-day convective variability in the tropical Pacific region. The initial focus of the paper is on the western tropical Pacific during the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) intensive observation period (IOP); high spatial and temporal resolution outgoing longwave radiation (OLR) data and sea surface temperatures are filtered to eliminate diurnal and lower-frequency variability. The propagation of the 2–3-day convective variability is also studied. Westward propagation appears to be favored in some regions, indicative of the events being influenced by westward-propagating inertio-gravity waves. However, many regions have 2–3-day events that divide fairly equally into eastward and westward propagations, indicating that both eastward- and westward-propagating inertio-gravity waves are influencing the oscillation. The SST data during the 4-month IOP dataset show evidence of a 2–3-day variability during those con...

Three MCS Cases Occurring in Different Synoptic Environments in the Sub-Sahelian Wet Zone during the 2002 West African Monsoon

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 Oueme 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 EC...

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...

Subtropical Jet Streaks over the South Pacific

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...

Use of a Rain Gauge Network to Infer the Influence of Environmental Factors on the Propagation of Quasi-Linear Convective Systems in West Africa

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 easterl...

Recent Climatology of Kinematic Variables in the TOGA-COARE Region

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 re...