The 1997-98 summer rainfall season in southern Africa. Part I: Observations

Following the onset of the strong El Niño of 1997–98 historical rainfall teleconnection patterns and dynamical model predictions both suggested an enhanced likelihood of drought for southern Africa, but widespread dry conditions failed to materialize. Results from a diagnostic study of NCEP–NCAR reanalysis data are reported here demonstrating how the large- and regional-scale atmospheric circulations during the 1997–98 El Niño differed from previous events. Emphasis is placed on the January–March 1998 season and comparisons with the strong 1982–83 El Niño, although composites of eight events occurring between 1950 and 2000 are also considered. In a companion paper, simulation runs from three atmospheric general circulation models (AGCMs), and forecasts from three fully coupled models are employed to investigate the extent to which the anomalous atmospheric circulation patterns during the 1997–98 El Niño may have been anticipated. Observational results indicate that the 1997–98 El Niño displayed significant differences from both the 1982–83 episode and the composite event. An unusually strong Angola low, exceptionally high sea surface temperatures (SSTs) in the western Indian and eastern tropical South Atlantic Oceans, and an enhanced northerly moisture flux from the continental interior and the western tropical Indian Ocean all appear to have contributed to more seasonal rainfall in 1997–98 over much of the southern Africa subcontinent than in past El Niño events

The 1997-98 summer rainfall season in southern Africa. Part II: model simulations and coupled model forecasts

This is the second of a two-part investigation of rainfall in southern Africa during the strong El Nino of 1997/98. In Part I it was shown that widespread drought in southern Africa, typical of past El Nino events occurring between 1950 and 2000, generally failed to materialize during the 1997/98 El Nino, most notably during January–March (JFM) 1998. Here output from three atmospheric general circulation models (AGCMs) forced with observed sea surface temperatures (SSTs) and seasonal forecasts from three coupled models are examined to see to what extent conditions in JFM 1998 could have potentially been anticipated. All three AGCMs generated widespread drought conditions across southern Africa, similar to those during past El Nino events, and did a generally poor job in generating the observed rainfall and atmospheric circulation anomaly patterns, particularly over the eastern and southern Indian Ocean. In contrast, two of the three coupled models showed a higher probability of wetter conditions in JFM 1998 than for past El Nino events, with an enhanced moisture flux from the Indian Ocean, as was observed. However, neither the AGCMs nor the coupled models generated anomalous stationary wave patterns consistent with observations over the South Atlantic and Pacific. The failure of any of the models to reproduce an enhanced Angola low (favoring rainfall) associated with an anomalous wave train in this region suggests that the coupled models that did indicate wetter conditions in JFM 1998 compared to previous El Nino episodes may have done so, at least partially, for the wrong reasons. The general inability of the climate models used in this study to generate key features of the seasonal climate over southern Africa in JFM 1998 suggests that internal atmospheric variability contributed to the observed rainfall and circulation patterns that year. With the caveat that current climate models may not properly respond to SST boundary forcing important to simulating southern Africa climate, this study finds that the JFM 1998 rainfall in southern Africa may have been largely unpredictable on seasonal time scales

The seasonally-varying influence of ENSO on rainfall and tropical cyclone activity in the Philippines

An observational study covering the period 1950–2002 examines a seasonal reversal in the ENSO rainfall signal in the north-central Philippines. In boreal Summer of El Niño (La Niña) events, above (below) average rainfall typically occurs in this area. Rainfall anomalies of opposite sign develop across the country in the subsequent fall. This study investigates the seasonal evolution of the anomalous atmospheric circulation over the western North Pacific (WNP) during both El Niño and La Niña and places these features in the context of the large-scale evolution of ENSO events, including an analysis of changes in tropical cyclone activity affecting the Philippines. The results show that during boreal summer of El Niño (La Niña) events, a relatively narrow, zonally elongated band of enhanced (reduced) low-level westerlies develops across the WNP which serves to increase (decrease) the summer monsoon flow and moisture flux over the north-central Philippines and is associated with an increase (decrease) in the strength of the WNP monsoon trough via the anomalous relative vorticity. Tropical cyclone activity is shown to be enhanced (reduced) in the study region during boreal summer of El Niño (La Niña) events, which is related to the increase (decrease) of mid-level atmospheric moisture, as diagnosed using a genesis potential index. The subsequent evolution shows development of an anomalous anticyclone (cyclone) over the WNP in El Niño (La Niña) and the well-known tendency for below (above) average rainfall in the fall. Prolonged ENSO events also exhibit seasonal rainfall sign reversals in the Philippines with a similar evolution in atmospheric circulation

Sub‐seasonal teleconnections between convection over the Indian Ocean, the East African long rains and tropical Pacific surface temperatures

Since 1999, the increased frequency of dry conditions over East Africa, particularly during the March–May (MAM) season, has heightened concerns in a region already highly insecure about food. The underlying mechanisms, however, are still not yet fully understood. This article analyses a proxy for daily convection variations over a large region encompassing East Africa and the whole Indian Ocean basin by applying a cluster analysis to more than 30 years of daily outgoing longwave radiation (OLR). Focusing on the MAM season to investigate relationships with East African long rains, four recurrent convection regimes associated with wet/dry conditions in East Africa are identified. Interestingly, all four regimes are related to western/central Pacific sea surface temperatures (SSTs) and rainfall. Wet regimes are associated with cool and dry/warm and wet conditions over the Maritime Continent (MC)/tropical Pacific east of the date line. Dry regimes exhibit opposite SST/rainfall dipole patterns in the Pacific compared to wet regimes, with the Indian Ocean found to modulate impacts on East African rainfall. Significant relationships between off‐equatorial warming in the west Pacific and a more frequent dry regime in May since 1998–1999 suggest an earlier onset of the monsoon and Somali jet, consistent with the recent abrupt shift observed in East African long rains and their modulation at multi‐decadal time scales of the Pacific

Water Shortages, Development, and Drought in Rockland County, New York

Knowledge of the historical variability of regional climate is an essential element of successful water resource planning. Lacking such perspective, planners and managers can be deceived as to the severity of a recent climate extreme, such as drought, and place a disproportionate blame on the climate, not the integrity of the supply system should water restrictions become necessary to avoid shortages. Presented here is a vivid example of how development, a lack of adequate planning, and climate variability have converged to produce three water emergencies in Rockland County, New York, since 1995. An examination of climate data over the past century indicates that the severity of the recent droughts was well within the range of past variability. Rather than climate alone, the recent water emergencies have highlighted a significant mismatch between supply and demand that has been developing in Rockland County over the past three decades. Substantial development, largely in the form of single-family homes, has not been matched with a corresponding enhancement of the county's water system. Realistic plans for meeting current water demand will require cooperation among all stakeholders, beginning with an acknowledgement that climate variations are inevitable, not the sole source of blame when water shortages arise

Subseasonal convection variability over the Intra-American Seas simulated by an AGCM and sensitivity to CMIP5 SST biases and projections

The influence of coupled model sea surface temperature (SST) climatological biases and SST projections on daily convection over the Intra-American Seas (IAS) during the May–November rainy season are examined by clustering (k − means) daily OLR anomalies in ECHAM5 atmospheric global climate model (AGCM) experiments. The AGCM is first forced by 1980–2005 observed SSTs (GOGA), then with climatological, multi-model mean monthly climato- logical SST bias from 31 CMIP5 coupled models (HIST) and projected SST changes for 2040–2059 (PROJ) and 2080–2099 (PROJ2) imposed on top of observed values. A typology of seven recurrent convection regimes is identified and consists of three dry and four wet regimes, including three regimes charac- terized by tropical-midlatitude interactions between surface convection cells across the IAS and Rossby wave in the upper-troposphere, and a regime of broad wettening typical of the ITCZ. Compared to an earlier observational study, all seven regimes are reasonably well reproduced in the HIST runs. However, the latter exhibit drier dry regimes, a less wet ITCZ-like wet regime and a southeastward shift of convective anomalies developing across the IAS in the three other regimes, all result in a drier simulated IAS climate compared to GOGA. ECHAM5 projection runs for PROJ and PROJ2 are both character- ized by the inhibition of the broad ITCZ-like wet regime, indicating a signifi- cant trend towards more frequent dry weather. Meanwhile, convection anomalies related to tropical-midlatitude interactions are shifted further east of the Caribbean as lead increases. These results suggest more frequent and intense extreme rainfall over the tropical Atlantic and the southeast US, while parts of the Caribbean are projected to experience drier climate. The projected drying, however, is of the same order of magnitude as results from historical SST biases, suggesting that the latter need to be considered in model projec- tions, which might underestimate future IAS drying

The Drought and Humanitarian Crisis in Central and Southwest Asia: A Climate Perspective

A persistent multi-year drought in Central and Southwest Asia has affected close to 60 million people as of November 2001. Chronic political instability in many parts of this region and the recent military action in Afghanistan have further complicated the situation. This report provides a climatic perspective on the severity and spatial extent of the ongoing drought and its social and economic impacts. The target audience for this report includes national, regional and international policymakers, humanitarian relief agencies, members of the research community as well as others with a general interest in Central and Southwest Asia and the causes and consequences of the persistent drought in the region. The report discusses underlying climatic mechanisms that might explain the causes for the persistent drought, and presents seasonal climate forecasts and their implications for the region. The principal conclusions of this report are as follows: Central and Southwest Asia represents the largest region of persistent drought over the past three years anywhere in the world. From a regional perspective, the ongoing drought is the most severe in the past several decades. Significant shortfalls in precipitation have led to widespread social and economic impacts, particularly in Iran, Afghanistan, Western Pakistan, Tajikistan, Uzbekistan and Turkmenistan. Agriculture, animal husbandry, water resources, and public health have been particularly stressed throughout the region. Preliminary analysis suggests that the drought is related to large-scale variations in the climate across the Indian and Pacific Oceans, including the recent “La Niña” in the eastern Pacific. Current seasonal climate forecasting skill in Central and Southwest Asia is modest. IRI seasonal forecasts for the period November 2001-April 2002 are consequently for climatology or equal likelihood of above-, near-, or below-normal precipitation in the region. While not indicative of any pronounced trends, a climatology forecast is less dire than one indicating enhanced probabilities for below normal precipitation

The East African Long Rains in Observations and Models

Decadal variability of the East African precipitation during the season of March–May (long rains) is examined and the performance of a series of models in simulating the observed features is assessed. Observational results show that the drying trend of the long rains is associated with decadal natural variability associated with sea surface temperature (SST) variations over the Pacific Ocean. Empirical orthogonal function (EOF), linear regression, and composite analyses all show the spatial pattern of the associated SST field to be La Niña like. The SST-forced International Research Institute for Climate and Society (IRI) forecast models are able to capture the East African precipitation climatology, the decadal variability of the long rains, and the associated SST anomaly pattern but are not consistent with observations from the 1970s. The multimodel mean of the SST-forced models from the Coupled Model Intercomparison Project phase 5 (CMIP5) Atmospheric Model Intercomparison Project (AMIP) experiment captures the climatology and the drying trend in recent decades. The fully coupled models from the CMIP5 historical experiment, however, have systematic errors in simulating the East African precipitation climatology by underestimating the long rains while overestimating the short rains. The multimodel mean of the historical simulations of the long rains anomalies, which is the best estimate of the radiatively forced change, shows a weak wetting trend associated with anthropogenic forcing. The SST anomaly pattern associated with the long rains has large discrepancies with the observations. The results herein suggest caution in projections of East African precipitation from CMIP5 or the relationship between the East African precipitation and the SST spatial pattern found in paleoclimate studies with coupled climate models

Finescale Evaluation of Drought in a Tropical Setting: Case Study in Sri Lanka

In regions of climatic heterogeneity, finescale assessment of drought risk is needed for policy making and drought management, mitigation, and adaptation. The relationship between drought relief payments (a proxy for drought risk) and meteorological drought indicators is examined through a retrospective analysis for Sri Lanka (1960–2000) based on records of district-level drought relief payments and a dense network of 284 rainfall stations. The standardized precipitation index and a percent-of-annual-average index for rainfall accumulated over 3, 6, 9, and 12 months were used, gridded to a spatial resolution of 10 km. An encouraging correspondence was identified between the spatial distribution of meteorological drought occurrence and historical drought relief payments at the district scale. Time series of drought indices averaged roughly over the four main climatic zones of Sri Lanka showed statistically significant (p < 0.01) relationships with the occurrence of drought relief. The 9-month cumulative drought index provided the strongest relationships overall, although 6- and 12-month indicators provided generally similar results. Some cases of appreciable drought without corresponding relief payments could be attributed to fiscal pressures, as during the 1970s. Statistically significant relationships between drought indicators and relief payments point to the potential utility of meteorological drought assessments for disaster risk management. In addition, the study provides an empirical approach to testing which meteorological drought indicators bear a statistically significant relationship to drought relief across a wide range of tropical climates

NMME Monthly Seasonal Forecasts for NASA SERVIR Applications Science

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