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Peter J. Lamb
and
Randy A. Peppler

Abstract

Sub-Saharan West Africa (10°–20°N) receives rainfall from westward-propagating disturbance lines that have their base within and receive most of their moisture from the low-level, wedge-shaped, southwest monsoonal flow off the tropical Atlantic. This paper builds on earlier research to further identify the tropical Atlantic surface atmospheric and oceanic patterns that accompany drought in sub-Saharan West Africa. Patterns for the four driest years since 1940 (1972, 1977, 1983, 1984) are compared with counterparts for the wettest of the last 20 years (1975) and 60-year (1911–70) average fields.

The key results for the rainy season (July-September) of three of the four severe sub-Saharan drought years (1972, 1977, 1984) duplicate those obtained earlier. They include (i) a distinctive basinwide sea surface temperature (SST) anomaly pattern (positive departure to the south of ∼10°N; negative departures between 10°–25°N); (ii) a concomitant southward displacement (relative to the 1911–70 mean) of the zone of maximum SST by 250–500 km; (iii) the North (South) Atlantic subtropical high extending farther (less) equaterward than in the 60-year mean; and (iv) associated southward displacements (by 200–350 km) of the near-equatorial pressure trough, wind direction discontinuity between Northern and Southern hemisphere tmdm and zones of maximum rainfall frequency and total cloud amount. These results offer further evidence that very deficient sub-Saharan rainy seasons tend to coincide with the southwesterly surface monmonal flow not extending as far north along the West African coast as in the 60-year mean and, by extension, a reduced northward penetration of the monsoon wedge into West Africa. Also consistent with earlier findings is that only the SST patterns of the aforementioned results show evidence of evolving during preceding seasons. This further underlines the potential for tropical Atlantic SST to provide the basis for the prediction of sub-Saharan rainy season quality several months in advance.

These results were not characteristic of the other extremely deficient sub-Saharan rainy season investigated (1983) or the nondrought rainy season studied for comparative purposes (1975), During July-September 1983, the SST departures were positive over much of the tropical Atlantic, and most of the aforementioned near-equatorial atmospheric-oceanic features were in close to their 1911–70 average positions. The latter was also true of July-September 1975, when the SST anomaly pattern was rather fragmented.

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M. Issa Lélé
and
Peter J. Lamb

Abstract

The Intertropical Front (ITF) is a fundamental feature of the atmospheric circulation over West Africa. It separates the wedge of warm moist southwesterly monsoon flow off the tropical Atlantic from much hotter and very dry northeasterly wind from the Sahara Desert. Here, the daily temperature, humidity, and rainfall data for 1974–2003 are analyzed to document the northward advance and southward retreat of the ITF between boreal spring and autumn, and assess its role in Sudan–Sahel (10°–20°N) rainfall patterns. Using largely dekadal (10 day) and monthly resolutions, analyses are performed for the 30-yr-average seasonal time scale and sets of extreme years, with a major focus on concurrent monthly ITF–rainfall relations. The seasonal rainfall predictive potential of the early season ITF latitude is also investigated, as is the secular variation of ITF latitude–weather system–rainfall associations during 1974–2003.

The northward advance of the ITF across the Sudan–Sahel from April to early August is relatively slow, averaging 0.8° latitude dekad−1 (8.8 km day−1). The southward ITF retreat between mid-August and mid-November is almost twice as fast, averaging 1.4° latitude dekad−1 (15.5 km day−1). Coupled with the ITF advance, the monsoon rainbelt migrates northward and intensifies. However, its northern boundary (1 mm day−1 monthly average isohyet) lags 100–250 km south of the ITF, while the most useful rainfall for society (>3 or 4 mm day−1 monthly average) generally occurs more than 400 km south of the ITF. There, the monsoon wedge is thickest and the horizontal velocity and moisture convergence are maximized in a regional ITCZ. The rapid ITF retreat during September–October is preceded by a similar rainbelt displacement. During both ITF advance and retreat, rainfall over the Sudan–Sahel region is positively related to the ITF’s latitude. The association is strongest during the early (April–June) and late (October) rainy season months (linear correlation, r = +0.74 to +0.81), when the ITF is located to the south and rainfall is low. It is weaker during the July–September rainy season core when the ITF is farthest north (r = +0.50 to +0.58). This concurrent rainfall dependence on ITF latitude is established further by contingency analyses for the 30-yr study period and by investigation of several extremely dry and wet individual seasons. The April ITF latitude anomaly is a moderately consistent indicator of the subsequent ITF latitude and associated rainfall anomaly through the first core rainy season month (July). This seasonal prediction potential does not persist into the rainy season peak (August), when the concurrent ITF–rainfall relationship is weakest (r = +0.50), the monsoon wedge is thickest, and rain-producing mesoscale dynamical processes are developed fully. However, because the ITF tends to retreat early (late) in seasons when it advanced early (late), the April ITF latitude specification of the September–October ITF latitude and rainfall (negative) is almost as consistent as that for July (positive). The secular variation of ITF latitude during 1974–2003 strongly influenced mesoscale weather systems and rainfall variability on decadal time scales.

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Aondover Tarhule
and
Peter J. Lamb

Beginning in response to the disastrous drought of 1968–73, considerable research and monitoring have focused on the characteristics, causes, predictability, and impacts of West African Soudano–Sahel (10°–18°N) rainfall variability and drought. While these efforts have generated substantial information on a range of these topics, very little is known of the extent to which communities, activities at risk, and policy makers are aware of, have access to, or use such information. This situation has prevailed despite Glantz's provocative BAMS paper on the use and value of seasonal forecasts for the Sahel more than a quarter century ago. We now provide a systematic reevaluation of these issues based on questionnaire responses of 566 participants (in 13 communities) and 26 organizations in Burkina Faso, Mali, Niger, and Nigeria. The results reveal that rural inhabitants have limited access to climate information, with nongovernmental organizations (NGOs) being the most important source. Moreover, the pathways for information flow are generally weakly connected and informal. As a result, utilization of the results of climate research is very low to nonexistent, even by organizations responsible for managing the effects of climate variability. Similarly, few people have access to seasonal climate forecasts, although the vast majority expressed a willingness to use such information when it becomes available. Those respondents with access expressed great enthusiasm and satisfaction with seasonal forecasts. The results suggest that inhabitants of the Soudano–Sahel savanna are keen for changes that improve their ability to cope with climate variability, but the lack of information on alternative courses of action is a major constraint. Our study, thus, essentially leaves unchanged both Glantz's negative “tentative conclusion” and more positive “preliminary assessment” of 25 years ago. Specifically, while many of the infrastructural deficiencies and socioeconomic impediments remain, the great yearning for climate information by Soudano–Sahalians suggests that the time is finally ripe for fostering increased use. Therefore, a simple model for improved dissemination of climate research and seasonal climate forecast information is proposed. The tragedy is that a quarter century has passed since Glantz's clarion call.

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Peter J. Lamb
and
Andrew F. Bunker

Abstract

This paper documents the annual march of the following processes for the 70°N–20°S region of the Atlantic, including the Gulf of Mexico and Caribbean Sea: net surface heat gains (monthly mean time-scale), subsurface heat storage change (bimonthly), divergence of the “vertically and zonally integrated net meridional heat transport” VZINMHT; (bimonthly). Results for the first three parameters are presented as averages for 10° (5°) zones of the extratropics (tropics)l the VZINMHT's are for the zones' bounding latitude circles.

The net surface heat gain is residually-estimated from sea-air heat exchange calculations. The extratropical North Atlantic is a net loser of heat to the atmosphere for the year as a whole. It experiences a very short period (May–August) of surface heat uptake, during which the maximum rate is as high as 110–130 W m−2, and a more lengthy surface heat loss, much of which exceeds 100 W m−2 and has a 190–250 W m−2 extreme. The tropical Atlantic undergoes a more subdued, and sometimes more irregular, annual march of this process. Between 20°N–5°S the ocean surface gains heat throughout all or almost all of the year, but generally at much lower rates than in the extratropics. April–September surface heat losses between 5–20°S are balanced by October–March gains. Estimation of the subsurface heat storage change is made using 233 957 soundings for the decade 1967–76, a 5° latitude-longitude square spatial resolution, and 14 oceanic layers between the surface and 500 m. Extratropical warming is largely confined to May–August, appears to reach 400 m in some zones, and generally totals 150–225 W m−2. The maximum cooling in this region tends to occur in November–December and, with the exception of 30–40°N, extends to 500 m and totals 250–350 W m−2. Between 30–40°N the storm change is strongly concentrated above 100 m. The annual cycle of this process is more varied and irregular, and of smaller amplitude, in the tropical belt.

The VZINMHT divergence is obtained as the difference between the rates of net surface heat gain and subsurface heat storage change. The extratropical zones import heat throughout all or almost all of January–October, generally at rates of 50–150 W m−2. Only in November–December (40–70°N) and January–February (40–50°N) is this region suggested to export, heat, a result that is rather uncertain. The tropical VZINMHT divergence pattern is dominated by export, especially between 25°N–10°S. The VZINMHT is estimated by successive southward integration of its divergence from assumed near-zero 70°N boundary conditions, a procedure whose uncertainty increases in the same direction and becomes large in the tropics. Northward VZINMHTs prevail throughout the study region during all or almost all of January–October. They tend to be largest in the tropics (150–250×1013 W), especially during July–October, and experience pronounced extratropical decreases, often between 30–50°N. The November–December VZINMHT is suggested to be directed southward throughout much or even all of the study region, and to increase in this direction to a 10–20°S maximum of almost 300×1013 W. However, this result is considered extremely tentative. The annual average VZINMHT is accordingly directed northward at all latitudes. It increases from 50–80×1013 W at 20°S to a 107–115×1013 W maximum in the northern tropics, and then decreases strongly poleward of 30°N, especially between 30–40°N.

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Stefan Hastenrath
and
Peter J. Lamb

Abstract

The heat budget of the atmosphere-ocean-land system in the Indian Ocean region (30°N–30°S, 30–120°E) is studied on the basis of ocean surface heat flux calculations from long-term ship observations and satellite-derived estimates of net radiation at the top of the atmosphere.

The hydrosphere to the north of the equator exports heat at rates of 5 × 1014 W for the year as a whole, and more than 8 × 1014 W during the northern summer (May–October) half-year, respectively. In contrast, the heat budget of the Southern Hemisphere water is dominated by the seasonal storage/depletion of heat transferred through the ocean surface. Oceanic heat export/import is small for this region during both the November–April and May–October half-years, and near zero for the year as a whole. The mean annual net meridional oceanic heat transport is directed southward throughout the study area, reaching a maximum of 8 × 1014 W at 10–15°S. From heat balance considerations, the annual average upwelling north of the equator is calculated to be ∼6 × 10−7 m s−1. Most of the compensatory down-welling must occur outside the tropical Indian Ocean.

Residually determined heat export by the atmosphere north of the equator averages 18 and 4 × 10−14 W during the northern summer and winter half-years, respectively. South of the equator the atmosphere exports heat at a mean annual rate of 19 × 10−14 W, with little seasonal variation. During northern summer, the atmospheric energy export from the southern tropical Indian Ocean is largely in the form of latent heat and is directed northward across the equator. The southern tropical Indian Ocean is the major source of the atmospheric water vapor carried across the coastline of southern Asia during the northern summer southwest monsoon. The larger water vapor flux divergence south of the equator at this time is fed by strong evaporation. This is supported by a combination of the seasonal depletion of the local oceanic heat content and oceanic heat import from north of the equator, in addition to the surface net radiation.

South of about 10°S, the atmosphere must dispose of both the net radiative heat input at the top of the system and the heat imported within the oceanic water body. In contrast, to the north the atmosphere and hydrosphere make similar contributions to the lateral energy export.

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Peter J. Lamb
and
Stanley A. Changnon Jr.

Abstract

Historical (1901–79) temperature and precipitation data for four Illinois stations were used to determine the frequency with which summer and winter averages for periods of various length (i.e., different climatic normals) are closest to the value for the next year, and hence its best predictor. The normal achieving the highest frequency in this regard is considered the best for characterizing the recent climate for a given point in time and assessing the abnormality of the following year.

Normals for 5, 10, 15, 20 and 25 years were investigated, along with the 30-year ones generally used. Five-year normals most frequently provided the closest estimate of the next year's value for both parameters in both seasons. Ten-year normals also have a high probability of being the best predictors, whereas 20-year normals have a particularly low probability of such success. The standard 30-year normals also perform poorly in this regard. These results contrast strongly with earlier suggestions that 15–25 year normals are “optimum” for prediction because they possess the minimum extrapolation variance when normals are employed as predictors. This difference between the two sets of results indicated that 5-year normals tend to possess larger prediction errors when they are not the best predictors, than do other normals on the greater number of occasions they are not the best predictors. The present findings were used by the Illinois Commerce Commission in evaluating weather normalization rate adjustments proposed by utility companies in 1979–80.

An investigation also is made into the nature of the climatic variation occurring when each normal is the best predictor. Five-year normals tend to attain this position for precipitation when the difference from the preceding year and the departures from longer-term averages are all moderate-to-small. When 5-year normals are the best temperature predictors, in contrast, the departures from this normal (and hence prediction errors) are very large. The frequency with which various normals were the best predictors shows no marked temporal variation during the study period.

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Rainwatch

A Prototype GIS for Rainfall Monitoring in West Africa

Aondover Tarhule
,
Zakari Saley-Bana
, and
Peter J. Lamb

This paper describes Rainwatch, a stand-alone, prototype Geographic Information System (GIS) application that automates and streamlines key aspects of rainfall data management, processing, and visualization for West Africa. Rainwatch is an interactive Map Objects Visual Basic application that permits the tracking of critical rainfall attributes beneficial to farmers. Using the simple-to-understand concept of cumulative rainfall plots, the program allows users to compare rainfall for any year against six percentile thresholds for a historical reference period (1965-2000). These thresholds separate dry, normal, and wet conditions. Users also can compare rainfall data between stations for a given season or between seasons for a particular station, and spatially interpolate rainfall for a single event, defined period, or an entire season. The system is dynamic and automatically updates all charts and tables as new data are added to the database. Thus, for this poor and drought-prone region, Rainwatch can help reduce delay in rainfall data processing, facilitate communication between data collection agencies, and generally make rainfall data more accessible and meaningful.

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Andrew M. Carleton
and
Peter J. Lamb

The results of a pilot study to assess the feasibility of documenting the occurrence of jet contrails over the United States from high-resolution Defense Meteorological Satellite Program (DMSP) imagery are presented. They are strongly positive, suggesting that 1) contrails can be distinguished from natural cirrus on the imagery; 2) contrails are consistently identifiable; 3) contrails often occur in association with the natural cirrus and frequently spread; and 4) this spreading could extend the accompanying natural cirrus shield. The analyses also indicate that contrails tend to occur relatively frequently, that they more often cluster in groups than appear singly, and that they seem to show a preference for developing in (near) upper-tropospheric cold troughs (ridgelines). It is accordingly suggested that DMSP imagery can provide a basis for research into a contrail-cirrus-climate relationship.

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Peter J. Lamb
and
Randy A. Peppler

An outline of the concept of the North Atlantic Oscillation (NAO), along with some of its history is presented. This is followed by a brief presentation of the results and implications of an encouraging new application of the NAO to a regional climate problem—the interannual variation of Moroccan winter-semester precipitation. That precipitation is shown to be inversely related to the concurrent state of the NAO, and the relationship is relatively strong by the standards of recent research into the mechanisms of tropical and subtropical precipitation fluctuations. It is suggested that the NAO is of particular significance for the important issue of the long-range prediction of Moroccan (and probably also Spanish, Portuguese, and Algerian) winter precipitation, and that further research on this subject is warranted. Several specific recommendations in the latter regard are made.

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John F. Mejia
,
Michael W. Douglas
, and
Peter J. Lamb

Abstract

This paper describes aspects of a strong moisture surge over the Gulf of California that was observed during the 2004 North American Monsoon Experiment. Although a variety of special observation platforms aid the analyses, the authors focus on observations collected during two NOAA research aircraft flights made on 12 and 13 July. These flights sampled the initial and mature phases of a strong surge associated with Tropical Storm Blas. The first flight is identified by both a convective outflow and another feature, both deeper and with larger spatial scale, ahead of the outflow in association with the surge’s leading edge. The surge speed, ~18 m s−1, was identified from anomaly analysis of surface station pressure data. Observations show interesting multiscale features associated with the surge during its initial stages but do not allow for unambiguous identification of the surge’s forcing mechanism or dynamical properties. Data from the second flight were used to describe the along- and cross-gulf structure of the enhanced low-level flow associated with the surge event. The strongest winds were over the northernmost gulf, with weaker winds over the surrounding coastal areas. The kinematic moisture flux increased toward the northern gulf; wind speed is the main control on the flux as the moist layer shows only small horizontal gradients. Over the northern gulf, the combination of a very shallow moist layer and a shallow low-level jet yield maximum moisture fluxes near 950 hPa that are almost an order of magnitude larger than those at 850 hPa.

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