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Stefan Hastenrath

Abstract

Heat budget estimates for the global tropics are derived from recent calculations of the oceanic heat budget and satellite measurements of net radiation at the top of the atmosphere. Annual mean heat export from the zone 30°N–30°S amounts to ∼101 × 1014 W (=100 units). Of this total 39 and 61 units are performed within the oceanic water body and the atmospheric column over sea and land, respectively. In the zone 0–10°N, to which the planetary cloud band (ITCZ) is essentially limited throughout the year, atmospheric heat export reaches only 13 units, as compared to an oceanic export of 18 units from the zone 0–10°S. In particular, oceanic export in the belt 0–5°S alone contributes 11 units which is 90% of the net radiative heat gain at the top of the atmosphere in this latitude zone. Accordingly, the atmospheric heat export from the realm of the ITCZ related to hot tower mechanisms seems to play a more, modest relative role in the global heat budget than heretofore believed. By comparison, oceanic export from the cold water zones immediately to the south of the Atlantic and Pacific equator emerges as an important factor in global energetics.

Oceanic meridional heat transport in the Pacific is directed from the tropics into either hemisphere; in the Atlantic it is northward from high southern latitudes all the way to the arctic; and it is directed south-ward in the Indian Ocean. Oceanic heat gain in the Pacific offsets deficits in the higher southern latitudes of the Atlantic and Indian Ocean sectors, as well as in the Atlantic as a whole. Meridional heat transport for all oceans combined is largest around 30°N and 25°S, where it accounts for 53 and 35% of the total poleward transport. Atmospheric transport is largest and effects the bulk of the total transport in midlatitudes.

Appreciably different estimates of net radiation at the top of the atmosphere, and of oceanic and atmospheric heat export must be regarded as compatible within the broad error limits indicated at present for all three terms.

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Stefan Hastenrath

Abstract

The NCEP–NCAR 1958–97 upper-air dataset and surface observations have been analyzed for evidence of zonal–vertical circulations along the Indian Ocean equator and their role in climatic variability. The long-term mean upper-tropospheric circulation is dominated in boreal winter by divergent outflow from the southern Indian Ocean northwestward into southern Asia, and in summer from southern Asia southwestward into the Southern Hemisphere. In boreal autumn only, divergent easterlies blow from Indonesia along the equator into an upper-tropospheric convergence band over East Africa, and only then a closed zonal–vertical circulation cell materializes along the Indian Ocean equator, between the centers of ascending motion over Indonesia and of subsidence over equatorial East Africa, and featuring westerlies in the lower layers. The boreal autumn zonal–vertical circulation varies interannually. A regime of intense circulation features accelerated equatorial surface westerlies, enhanced subsidence, and deficient rainfall at the coast of East Africa. In the high phase of the Southern Oscillation [anomalously high (low) pressure at Tahiti (Darwin)] this regime is preferred. The regime of weak zonal circulation has the opposite departure characteristics.

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Stefan Hastenrath

Abstract

Large-scale departure maps of sea level pressure (SLP) and sea surface temperature (SST) are presented for the tropical Atlantic and eastern Pacific Oceans, as obtained by stratification with respect to extreme climatic events in key regions of the tropical Americas. Drought in the Central American-Caribbean region is characterized by an equatorward expansion of the North Atlantic high, a band of anomalously cold water extending across the North Atlantic and a positive SST anomaly in the eastern Pacific. Drought in northeast Brazil is associated with high SLP over the South Atlantic and low SLP over the North Atlantic, cold water in the South Atlantic, a band of positive SST anomalies across the North Atlantic, and positive SST departures in the eastern Pacific. During the Ecuador/Peru El Nin̄o, SLP in the eastern Pacific is low and SST high, and positive SLP departures dominate the tropical Atlantic.

Independently, preferred modes of departure configurations are identified from principal component analysis of SLP (1942–71) and SST (1948–71). The first four principal components of SLP explain 43, 26, 14 and 10%, and the first four SST components 42, 24, 10 and 6% of the variance. The first principal components of SLP and SST, and the sea temperature along the Ecuador/Peru coast are highly correlated. This ensemble of departure configurations closely replicates the ones characteristic of the Ecuador/Peru El Nin̄o. The second principal components of SLP and SST are correlated, as is the third SLP with the fourth SST component. However, the departure patterns obtained by stratification with respect to regional climate anomalies provide no overall analogy to these pattern ensembles. The fourth principal SLP and the third SST components are highly correlated. Both possess a high correlation of one sign with rainfall in the Central American-Caribbean region, and a high correlation of opposite sign with precipitation in northeast Brazil. This pattern ensemble offers an excellent replication for the two sets of departure patterns obtained by stratification with respect to drought in the Central American-Caribbean region and north-east Brazil. An additional principal component analysis was performed in which SLP, SST, and the three aforementioned regional hydrometeorological time series simultaneously served as input. Results corroborate the separate SLP and SST principal component analyses.

The stratification and principal component analyses are complementary approaches, in that they yield realistic and physically plausible patterns. It is hypothesized that mass exchanges on the scale of the near-global tropics dominate the pressure pattern and are related to regional circulation changes and climate anomalies.

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Stefan Hastenrath

Abstract

This study expands recent research into the predictability of Indian monsoon rainfall anomalies. In addition to the April latitude position of the 500 mb ridge over India, and Darwin pressure tendency, the May surface resultant wind speed in a strategic area of the jet axis over the western equatorial Indian Ocean (0–10°S, 45°–50°E) is used as predictor. Regression models developed on 20-yr portions of the 1939–83 record are employed to predict the summer monsoon rainfall anomalies of the 25 yr 1959–83. Correlation, root-mean-square error, bias, and absolute error are presented as measures of forecast performance on the “independent” dataset. It is found that the model constraint optimal for predictive purposes is to be ascertained empirically. “Updating” is not necessarily superior to the use of a fixed regression base period. Relationships between preseason indicators and monsoon rainfall were, strongest in the early and late portions of the 1939–83 record, and weakest in the 1950s and 1960s. Antecedent departures in the late-scale circulation setting allowed prediction of more than 60% of the interannual variance of Indian monsoon rainfall for the period 1959–83, and nearly 80% for 1969–83.

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Stefan Hastenrath

Abstract

Departure characteristics of the general circulation in the tropical Atlantic and eastern Pacific are studied in relation to extreme climatic events identified from collectives of long-term rainfall stations and other hydrometeorological parameters, with emphasis to the Central American-Caribbean region (CARIB). Ship observations during 1911–72 compiled with a 1° square latitude-longitude resolution and extending between 30°N–30°S from the African coast to the eastern Pacific form a major observational basis.

The quality of the rainy season in CARIB has a large negative correlation with annual precipitation in the U.S. Central Great Plains, and with rainfall and sea surface temperature along the Ecuadorian/Peruvian littoral; in addition there are weak negative correlations with northeastern Brazil rainfall and with discharge and water level in northern tributaries of the Amazon, and a strong positive correlation with hydrometeorological events in Subsaharan Africa. Northeastern Brazil rainfall is strongly negatively correlated with sea surface temperature along the Eucador/Peru coast, but not with rainfall.

During extreme dry years in CARIB the North Atlantic high expands equatorward, meridional pressure gradients steepen, and the trades are stronger, albeit in a somewhat more southward location; at the same time, the ITCZ over the eastern Pacific stays farther south and the South Atlantic high contracts on its equatorward side. For extreme wet years, the reverse departure patterns from the 1911–70 mean maps are characteristic. Sea surface temperature anomalies reflect a response to variations in the subtropical highs and major ocean currents: advection of cold waters in the eastern part of the oceans is favored by equatorward expansion of the subtropical high in the respective hemisphere; the wintertime Gulf Stream system has a distinct signature concomitant with departures in the equatorial and South Atlantic; and in the equatorial eastern Pacific departures reverse from the winter preceding toward the height of an extreme rainy season, warm waters in July/August being characteristic for drought in CARIB. In view of the strong spatial correlations, departure patterns constructed from stratification according to extreme events in CARIB are expected to have more general validity. Anomalous rainy season conditions are signaled in advance by large-scale departure patterns in January/February, thus offering the prospect of foreshadowing extreme rainy season behavior from the setup of low-latitude circulation during the preceding northern winter.

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Stefan Hastenrath

Abstract

General circulation mechanisms instrumental in both annual cycle and interannual variability of rainfall are studied with reference to key regions of the tropical Americas and Africa, including the Central American–Caribbean area, northern Northeast Brazil, Subsaharan Africa, the Angola coast and the zaïre (Congo) and Amazon basins. For most of these regions, rainfall anomalies tend to be associated with departures in the large-scale atmospheric and oceanic fields that correspond to the pattern changes in the annual alternation of dry and rainy seasons. The interannual variability of climate and circulation thus appears largely as enhancement and reduction of the annual cycle.

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Stefan Hastenrath

The term “dipole” implies a seesaw, inverse variations of an element at the extremities of an oscillation system. The Southern Oscillation and North Atlantic Oscillation are examples of such mainly standing oscillations in pressure. By contrast, for the sea surface temperature (SST) fields in the tropical Atlantic and equatorial Indian Oceans, the SST gradient was found to be closely associated with climatic anomalies, but there is no seesaw. Use the term dipole is misleading.

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Stefan Hastenrath

Climate-prediction research in the 1980s has shown particular promise for methods based on (a) general circulation and statistics, and (b) numerical modeling. Empirically based methods of predicting seasonal rainfall anomalies have been presented for India, Java, Kenya, Sahel, and Northeast Brazil. For some of these regions, about half of the interannual rainfall variability can be predicted from antecedent departures in the large-scale circulation. River discharge in northern South America, as well as atlantic tropical storm activity have proven highly predictable on empirical grounds. Numerical modeling has been used to advantage for the prediction of El Niño. Numerical modeling efforts are underway, directed to the forecasting of Sahel rainfall anomalies. Remarkable progress has been made towards the empirical prediction of food-grain production. A sound diagnostic understanding is crucial for the development of both empirical and numerical prediction methods. Among the most important tasks pending are the maintenance and timely processing of reliable, continuously functioning conventional raingauge networks; documentation of methods and verification of forecasts; and enhancement of contacts with the prospective users of climate prediction.

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Stefan Hastenrath

Climatic disasters are common in many tropical regions, and rainfall anomalies in particular have a severe human impact. Accordingly, both the World Climate Programme and the U.S. National Climate Program have identified climate prediction as a major objective. Approaches can be grouped into five major categories: (i) the extrapolation of empirically or theoretically deduced periodicities; (ii) the assessment of statistical relationships between rainfall and various meteorological elements; (iii) the relation between rainfall in the preseason and at the height of the rainy season; (iv) comprehensive diagnostic studies of climate and circulation anomalies combined with statistical methods; and (v) numerical modeling.

Methods pertaining to (iv) indicate the feasibility of empirically based climate prediction for certain tropical regions. For the drought-prone region of Northeast Brazil and Indonesia, in particular, it has been demonstrated on independent data sets that almost half of the interannual rainfall variability can be explained from antecedent departures in the large-scale circulation. Application of these methods on an operational basis involves two simultaneous input data requirements: 1) they must be available in quasi real time; 2) long (>10 years) homogeneous reference series of internally consistent parameters are needed, while absolute calibration is not essential. The practical benefit of climate forecasts appears to hinge on societal and economic factors.

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Stefan Hastenrath

Abstract

A complex of anomalies in the premonsoon large-scale circulation setting heralds the interannual variability of India summer monsoon rainfall. The most prominent precursors of precipitation anomalies are the latitude position of the upper-air ridge over India, apparently reflecting the persistence of the boreal winter wind regime and its consequence for the establishment of the summer upper-air circulation; the temperature in southern Asia and the adjacent North Indian Ocean waters, a factor instrumental in heat-low development and hence the establishment of meridional pressure gradients and lower-tropospheric airstreams from the Southern Hemisphere; and indices of the Southern Oscillation, capturing pressure departure patterns spanning the global tropics. Stepwise multiple regression is used to extract from this “anomaly complex” the variance most pertinent to the interannual variability of Southwest monsoon rainfall, observations of pertinent elements being available for the period 1939–81. Regression models developed on a portion of this record are then used to predict the summer monsoon rainfall anomalies of the years 1966–81.

The correlations between the various precursors and the rainfall anomalies vary in the course of 1939–81, being, on the whole, strongest in the 1950s and 1960s. While the April latitude position of the 500 mb ridge along 75°E proves to be the strongest predictor, performance is improved by inclusion of other elements representing preseason temperature and the Southern Oscillation. Correlation, root-mean-square error, bias, and absolute error are used as measures of forecast performance. A set of experiments with the dependent dataset, ending in 1965, indicates that a regression base period of about 20 yr is optimal for predictions into the independent portion of the record. Another set of experiments, in which the regression base periods are successively updated to the year immediately preceding the year to be forecast, shows no improvement of predictions over the fixed regression base periods. “Cross-validation” is not found less demanding than prediction proper. It is demonstrated that about half of the interannual variance in monsoon rainfall can be predicted from antecedent anomalies in the large-scale circulation setting.

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