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[1] This study examines the synoptic scale impacts of African dust on easterly waves in the tropical northeast Atlantic. Moderate Resolution Imaging Spectrometer aerosol optical depth (AOD), National Oceanic and Atmospheric Administration products, and National Center for Environmental Prediction reanalysis fields in the Atlantic main hurricane development region (MDR) form the basis for statistical analysis of a limited set of cases objectively selected for the 2000–2008 hurricane seasons when thresholds are exceeded for sea surface temperature (SST), easterly wind shear, cyclonic vorticity, and upward motion. After ranking African easterly waves by AOD, the top (dusty) and bottom (clean) cases are studied as composite differences. African dust and subsidence cause temperatures to warm ∼3°C in the 700 hPa layer, while SSTs cause temperatures to cool, stabilizing the atmosphere. Increased AOD and strong (10 m s−1) 600 hPa easterly winds limit cloud efficiency through shear and oversupply of condensation nuclei. Vertical section composites demonstrate that warm dry subsident air coincides with the African dust plume in the latitudes 18°N–30°N. Hurricane reanalysis data indicate that higher AOD in the MDR reduces chances for the intensification of African easterly waves.

[1] Climatic variables in the period from 1951 to 2000 are analyzed across the tropics from the East Pacific to Africa. A quasi-decadal mode is isolated using singular value decomposition (SVD) applied to monthly smoothed and detrended rainfall, sea surface temperature (SST), sea level pressure (SLP), and 200 hPa zonal (U) wind anomalies. Seven- to 10-year cycles in Caribbean rainfall are revealed as the dominant mode (50% variance) and are found to be related to a mode of tropical variability that is distinct from previously known global signals. Sources of this signal include east Atlantic SST and the northern subtropical ridge, which modulate upwelling off Venezuela. SVD analysis of daily rainfall suggests interaction between annual and quasi-decadal signals, with northern summer convection as a driver. The second mode of Caribbean rainfall variability derives from the east Pacific El Nino Southern Oscillation (ENSO, 21% variance) that is expressed as an east-west dipole of convection across the Caribbean. Composite analysis of rainfall for high and low phases of the quasi-decadal cycle reveals a corresponding signal that extends from the eastern Pacific Ocean across the Caribbean and West Africa to India. The southern Hadley cell spins up during wet phase, and the ITCZ migrates northward. This hemispheric-scale anomaly brings pulses of convection to the Caribbean. Impacts of the quasi-decadal cycle on socioeconomic resources are investigated.

[1] Low-frequency climate variability across the American continents and surrounding oceans is analyzed by application of singular value decomposition (SVD) to gauge-based rainfall and environmental anomaly fields in the period 1901–2002. A 5-year filter is used to maintain a focus on interdecadal cycles. The rainfall regime of particular interest (mode 1) is when West Africa and the Caribbean share positive loading and North and South America share negative loading. Wavelet cospectral energy is found at ∼8, 24, and 50 years for Caribbean/West African zones and 16 and 32 years for North/South America. West Africa and South America exhibit antiphase multidecadal variability, while North America and the Caribbean rainfall exhibit quasi-decadal cycles. The rainfall associations are nonstationary. In the early 1900s, Caribbean and South American rainfall were antiphase. Since 1930 low-frequency oscillations of North American (West African) rainfall have been positively (negatively) associated with South America. Low-frequency oscillations of North American rainfall have been consistently antiphase with respect to Caribbean rainfall; however, West Africa rainfall fluctuations have been in phase with the Caribbean more in the period 1920–1950 than at other times. Hemispheric-scale environmental SVD patterns and scores were compared with the leading rainfall modes. The north-south gradient modes in temperature are influential in respect of mode 1 rainfall, while east-west gradients relate to mode 2 (northern Brazil) rainfall. The ability of the GFDL2.1 coupled (ocean-atmosphere) general circulation model to represent interdecadal rainfall modes in the 20th century was evaluated. While mode 2 is reproduced, mode 1 remains elusive.

[1] Thirty-five meteorological stations encompassing the Caribbean region (Cuba, Bahamas, Jamaica, Dominican Republic, Puerto Rico, US Virgin Islands, St. Maarten, and Barbados) were analyzed over the time interval 1951–1981 to assess regional precipitation patterns and their relationships with the North Atlantic Oscillation (NAO) and El Nino-Southern Oscillation (ENSO). Application of factor analysis to these series revealed the existence of four geographically distinct precipitation regions, (C1) western Cuba and northwestern Bahamas, (C2) Jamaica, eastern Cuba, and southeastern Bahamas, (C3) Dominican Republic and northwestern Puerto Rico, and (C4) eastern Puerto Rico, US Virgin Islands, St. Maarten, and Barbados. This regionalization is related to different annual cycles and interannual fluctuations of rainfall. The annual cycle is more unimodal and largest in the northwest Caribbean (C1) and becomes increasingly bimodal toward lower latitudes (C4) as expected. Year-to-year variations of precipitation are compared with two well-known climatic indices. The ENSO relationship, represented by Nino 3.4 sea surface temperatures (SST), is positive and stable at all lags, but tends to reverse over the SE Caribbean (C4) in late summer. The NAO influence is weak and seasonally dependent. Early summer rainfall in the northwest Caribbean (C1) increases under El Nino conditions. Clusters 2 and 3 are less influenced by the global predictors and more regional in character.

(1) Interannual fluctuations of monsoons around Africa and the stability of associations with the El Nino-Southern Oscillation (ENSO) and African rainfall are studied. The statistical analysis employs sea surface temperature (SST), surface and upper winds, and surface pressure averaged over key monsoon areas of the tropical Atlantic and Indian Oceans. The time series span the period 1958-1998, and wavelet analysis is applied to localize relationships in time as well as in frequency and enable us to examine how the amplitude and time delay at interannual scales varies through the record. Comparisons are made with Nino3 SST and other known ENSO signals in the African hemisphere. It is found that upper zonal winds over the tropical Atlantic are an integral part of the global ENSO. Zonal winds are associated with SST changes in the equatorial east Atlantic, which are antiphase to those in the west-central Indian Ocean. A composite analysis of warm and cool events in the Indian Ocean reveals that evaporation, radiative fluxes, and wind curl interact constructively. Anticyclonic curl (depression of isotherms) leads warm events, while cool events may initiate from oceanic advection and are sustained by evaporative fluxes. Rainfall fluctuations across Africa are analyzed, and three coherent areas are identified: West (Sahel-Guinea), Southern (Kalahari-Zambezi), and East (Kenya- Tanzania). Multivariate regression algorithms are fitted to the continuous filtered rainfall series over the period 1958-1988. Using three monsoon indices in a multivariate model, about 40% of the variance is explained at zero lag. An influential variable for most African rainfall areas is the zonal wind over the tropical Atlantic. The north-south SST gradient in the tropical Atlantic modulates rainfall in West Africa as expected. At 6 month lead, surface pressure in the north Indian Ocean is a key determinant for West African climate. For southern African rainfall, SST in the southwest Indian Ocean and monsoon indices in the west-central Indian Ocean play significant roles. East African rainfall fluctuations are linked with zonal winds in the east Indian Ocean. The findings address current Climate Variability and Predictability program (CLIVAR) priorities for understanding how continental climate interacts with ENSO and other regional modes of variability. INDEX TERMS: 1620 Global Change: Climate dynamics (3309); 1635 Global Change: Oceans (4203); 3339 Meteorology and Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504); 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854); KEYWORDS: African climate, tropical Atlantic, Indian Ocean, monsoon

Aircraft cross-section observations across the northern edge of the warm Agulhas Current to the south of Africa are used to form composite results of mean and turbulent meteorological parameters over a 4-day period characterized by winter zonal westerlies. Across a sea surface temperature (SST) front of 8°C (40 km)−1 near 35°S, 23°E, surface heat fluxes and wind stress increase fivefold. The moist unstable marine atmospheric boundary layer is 400 m deeper on the warm side of the SST front, and turbulent variances increase with SST in a logarithmic correlation (r = +0.76). A jetlike core of accelerated winds (17 m s−1) over the axis of warmest water is found. Its existence is thought to be due to secondary circulations set up by enhanced pressure gradients and baroclinic effects. The sharp thermal front is quasi-stationary and semipermanent during synoptic prefrontal conditions and zonal flows and lies approximately over the 200-m isobath in association with the Agulhas Current.

The Subtropical Convergence Agulhas Retroflection Cruise, from February 12 to March 4, 1987, provided an opportunity for studying variability of surface heat fluxes and marine boundary layer modification within an ocean area where few atmospheric measurements have been made. From 3-hourly surface observations it has been ascertained that surface heat flux processes are enhanced under cold air outbreak, postfrontal synoptic conditions and over warmer ocean areas. A maximum turbulent heat loss of 828 W/m2, 4 times the climatological value, was found. Over the cooler Subtropical Convergence waters and over the Agulhas Bank, heat fluxes were comparatively low. Two transect lines were performed across the Subtropical Convergence-Agulhas SST front. In both cases, air temperatures, dew point temperatures, and particularly wind speeds increased over the warmer water owing to enhanced vertical mixing resulting from oceanic heat losses. The second transect revealed marked vertical shear of the zonal wind. The essential elements for cyclogenesis, namely, low static stability and strong low level baroclinicity, were found to be characteristic of the region.