Search Results
You are looking at 1 - 10 of 42 items for
- Author or Editor: Stefan Hastenrath x
- Refine by Access: All Content x
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Abstract
A substantial portion of the interannual variability of rainfall at Jakarta, Java, can be predicted from antecedent pressure anomalies at Darwin, northern Australia; the pressure persistence, the concurrent correlation of pressure and rainfall, and the predictability of rainfall from antecedent pressure are all largest during the “east” monsoon (June-November). Because of the relatively simple large-scale circulation setting, warranting a single predictor (Darwin pressure), this region is chosen for a series of experiments aimed at exploring the seasonality and secular variations of predictability, optimal length of dependent record, and updating of the regression base period used for predictions on the independent data set.
The major features of pressure-rainfall relationships are common through much of the 1911–83 record, namely sign and general magnitude of correlations and the closer relationships during the east, as compared to the west monsoon. Considerable differences are, however, apparent between decades. Them may stem from both sampling deficiencies (noise) and real long-term changes of the pressure-rainfall couplings due to secular alterations in the large-scale circulation setting. The competition between these two factors is relevant concerning the optimal length of the dependent record used for predictions into the independent data set, as well as the updating of the regression base period.
Abstract
A substantial portion of the interannual variability of rainfall at Jakarta, Java, can be predicted from antecedent pressure anomalies at Darwin, northern Australia; the pressure persistence, the concurrent correlation of pressure and rainfall, and the predictability of rainfall from antecedent pressure are all largest during the “east” monsoon (June-November). Because of the relatively simple large-scale circulation setting, warranting a single predictor (Darwin pressure), this region is chosen for a series of experiments aimed at exploring the seasonality and secular variations of predictability, optimal length of dependent record, and updating of the regression base period used for predictions on the independent data set.
The major features of pressure-rainfall relationships are common through much of the 1911–83 record, namely sign and general magnitude of correlations and the closer relationships during the east, as compared to the west monsoon. Considerable differences are, however, apparent between decades. Them may stem from both sampling deficiencies (noise) and real long-term changes of the pressure-rainfall couplings due to secular alterations in the large-scale circulation setting. The competition between these two factors is relevant concerning the optimal length of the dependent record used for predictions into the independent data set, as well as the updating of the regression base period.
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.
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.
Abstract
Updated estimates of meridional heat transport in the Atlantic Ocean water body derived from the surface energy badget agree with the evaluation of hydrographic sections, annual mean northward transports being about 8 and 11 × 1014 W at 30°8 and 25°N, respectively. For the World Ocean as a whole, a southward transport of 6 × 1014 W is obtained at 60°S. Concerning the satellite-derived not radiation at the top of the atmosphere, the five data sets published in the literature are adjusted to form a zero annual mean for the globe as a whole; the required adjustments are ±10 W m−2 for the various sets. Even so, the required poleward heat transport in the atmosphere-ocean system obtained from the five data sets differs conspicuously, with a range of 20 × 1014 W at 30°N. This range may reflect observational errors and real interannual variability. The comfortable numerical agreement notwithstanding, plausible error tolerances far exceed the differences between the heat transport estimates derived by various independent methods.
Abstract
Updated estimates of meridional heat transport in the Atlantic Ocean water body derived from the surface energy badget agree with the evaluation of hydrographic sections, annual mean northward transports being about 8 and 11 × 1014 W at 30°8 and 25°N, respectively. For the World Ocean as a whole, a southward transport of 6 × 1014 W is obtained at 60°S. Concerning the satellite-derived not radiation at the top of the atmosphere, the five data sets published in the literature are adjusted to form a zero annual mean for the globe as a whole; the required adjustments are ±10 W m−2 for the various sets. Even so, the required poleward heat transport in the atmosphere-ocean system obtained from the five data sets differs conspicuously, with a range of 20 × 1014 W at 30°N. This range may reflect observational errors and real interannual variability. The comfortable numerical agreement notwithstanding, plausible error tolerances far exceed the differences between the heat transport estimates derived by various independent methods.
Abstract
The interannual variability of tropical convection related to the Southern Oscillation (SO) and regional climate anomalies is studied from satellite-derived estimates of highly reflective clouds (HRC) during 1971–87. The novel HRC data bank provides a particularly useful measure of tropical convection for the purposes of climate diagnostics, because of its length and continuity of record. For the first time, maps are presented of the patterns of correlation between the SO, as well as regional rainfall anomalies, and convection over the global tropics.
Throughout the year, the SO (high SO phase defined by anomalously high/low pressure at Tahiti/Darwin) exhibits a highly significant negative correlation with HRC in the equatorial Pacific but a much weaker positive correlation with Indonesia. The SO is correlated positively with HRC in the Amazon basin in boreal winter but negatively with HRC over central Africa throughout most of the year. The three equatorial convection centers tend to vary in unison, in particular those over the Amazon basin and central Africa, while the positive correlations of any of these centers with the SO are much weaker. Copious precipitation during the March-April rainy season of northeast Brazil is associated with a southward displaced low-pressure trough and embedded wind confluence, as well as a southward shift of the convection belt in the sector extending from South America across the Atlantic into equatorial Africa. During abundant Nordeste rainy seasons, as in the high SO phase, convective activity tends to be enhanced over Indonesia but reduced in the equatorial Pacific. Copious rainfall in Subsaharan West Africa (Sahel) tends to be associated with the high SO phase and thus intense convection over Indonesia and reduced convective activity in the equatorial central Pacific. Another new finding is the strong inverse relationship of Sahel rainfall with the convection over central Africa. Abundant Indian summer monsoon rainfall is accompanied by enhanced convective activity over the Indian Ocean and Indonesia and reduced convection in the equatorial central Pacific, characteristics of the high SO phase.
Abstract
The interannual variability of tropical convection related to the Southern Oscillation (SO) and regional climate anomalies is studied from satellite-derived estimates of highly reflective clouds (HRC) during 1971–87. The novel HRC data bank provides a particularly useful measure of tropical convection for the purposes of climate diagnostics, because of its length and continuity of record. For the first time, maps are presented of the patterns of correlation between the SO, as well as regional rainfall anomalies, and convection over the global tropics.
Throughout the year, the SO (high SO phase defined by anomalously high/low pressure at Tahiti/Darwin) exhibits a highly significant negative correlation with HRC in the equatorial Pacific but a much weaker positive correlation with Indonesia. The SO is correlated positively with HRC in the Amazon basin in boreal winter but negatively with HRC over central Africa throughout most of the year. The three equatorial convection centers tend to vary in unison, in particular those over the Amazon basin and central Africa, while the positive correlations of any of these centers with the SO are much weaker. Copious precipitation during the March-April rainy season of northeast Brazil is associated with a southward displaced low-pressure trough and embedded wind confluence, as well as a southward shift of the convection belt in the sector extending from South America across the Atlantic into equatorial Africa. During abundant Nordeste rainy seasons, as in the high SO phase, convective activity tends to be enhanced over Indonesia but reduced in the equatorial Pacific. Copious rainfall in Subsaharan West Africa (Sahel) tends to be associated with the high SO phase and thus intense convection over Indonesia and reduced convective activity in the equatorial central Pacific. Another new finding is the strong inverse relationship of Sahel rainfall with the convection over central Africa. Abundant Indian summer monsoon rainfall is accompanied by enhanced convective activity over the Indian Ocean and Indonesia and reduced convection in the equatorial central Pacific, characteristics of the high SO phase.
