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- Author or Editor: George N. Kiladis x
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Abstract
Composite temperature and precipitation anomalies during various stages of an event in the Southern Oscillation (SO) have been computed for several hundred stations across the globe. Large regions of coherent, significant signals are shown to exist for both extremes of the SO, with warm event signals generally opposite to those during cold events. In addition, during the year preceding the development of an event in the SO (year −1), climatic anomalies tend to be opposite to those during the following year (year 0). This confirms that the biennial tendency of the SO over the Pacific/Indian ocean sectors is also present in more remote regions with climatic signals related to the SO. Many of the signals are consistent enough from event to event to be useful for extended range forecasting purposes.
Abstract
Composite temperature and precipitation anomalies during various stages of an event in the Southern Oscillation (SO) have been computed for several hundred stations across the globe. Large regions of coherent, significant signals are shown to exist for both extremes of the SO, with warm event signals generally opposite to those during cold events. In addition, during the year preceding the development of an event in the SO (year −1), climatic anomalies tend to be opposite to those during the following year (year 0). This confirms that the biennial tendency of the SO over the Pacific/Indian ocean sectors is also present in more remote regions with climatic signals related to the SO. Many of the signals are consistent enough from event to event to be useful for extended range forecasting purposes.
Abstract
The interannual variability of transient waves and convection over the central and eastern Pacific is examined using 30 northern winters of NCEP–NCAR reanalyses (1968/69–1997/98) and satellite outgoing longwave radiation data starting in 1974. There is a clear signal associated with the El Niño–Southern Oscillation, such that differences in the seasonal-mean basic state lead to statistically significant changes in the behavior of the transients and convection (with periods less than 30 days), which then feed back onto the basic state.
During a warm event (El Niño phase), the Northern Hemisphere subtropical jet is strengthened over the central Pacific; the region of upper-tropospheric mean easterlies over the tropical western Pacific expands eastward past the date line, and the upper-tropospheric mean “westerly duct” over the tropical eastern Pacific is weakened. The transients tend to propagate along the almost continuous waveguide of the subtropical jet; equatorward propagation into the westerly duct is reduced. The transient convective events over the ITCZ typically observed to be associated with these equatorward-propagating waves are subsequently reduced both in number and magnitude, leading to a seasonal-mean net negative diabatic heating anomaly over the central Pacific from 10° to 20°N, which then feeds back onto the basic state. During a cold event (La Niña phase), the situation is reversed. The different propagation characteristics of the transients in El Niño and La Niña basic states are well simulated in initial value experiments with a primitive equation model.
Abstract
The interannual variability of transient waves and convection over the central and eastern Pacific is examined using 30 northern winters of NCEP–NCAR reanalyses (1968/69–1997/98) and satellite outgoing longwave radiation data starting in 1974. There is a clear signal associated with the El Niño–Southern Oscillation, such that differences in the seasonal-mean basic state lead to statistically significant changes in the behavior of the transients and convection (with periods less than 30 days), which then feed back onto the basic state.
During a warm event (El Niño phase), the Northern Hemisphere subtropical jet is strengthened over the central Pacific; the region of upper-tropospheric mean easterlies over the tropical western Pacific expands eastward past the date line, and the upper-tropospheric mean “westerly duct” over the tropical eastern Pacific is weakened. The transients tend to propagate along the almost continuous waveguide of the subtropical jet; equatorward propagation into the westerly duct is reduced. The transient convective events over the ITCZ typically observed to be associated with these equatorward-propagating waves are subsequently reduced both in number and magnitude, leading to a seasonal-mean net negative diabatic heating anomaly over the central Pacific from 10° to 20°N, which then feeds back onto the basic state. During a cold event (La Niña phase), the situation is reversed. The different propagation characteristics of the transients in El Niño and La Niña basic states are well simulated in initial value experiments with a primitive equation model.
Abstract
The Madden–Julian oscillation (MJO) has been implicated as a major source of the wind stress variability that generates basin-scale Kelvin waves in the equatorial Pacific. One source of debate concerning this relationship is the apparent difference in the frequencies of the two processes.
This work utilizes data from the Tropical Atmosphere Ocean (TAO) array of moored buoys along with outgoing longwave radiation data to show by means of a multiple linear regression model and case studies that the frequency discrepancy is due to a systematic decrease in the phase speeds of the Kelvin waves and an increase in the period of the waves toward the east as conditions adjust toward El Niño. Among the potential contributing factors to this phase speed decrease is an apparent air–sea interaction that enhances the wind forcing of some of the Kelvin waves, allowing them to continue to amplify because the propagating wind stress anomaly decelerates to the speed of the developing Kelvin wave instead of the significantly faster speed of the typical MJO. Kelvin waves appear to be most effectively amplified during periods when the temperature gradient above the thermocline across the equatorial central Pacific is strong, the thermocline shoals steeply toward the east in the central Pacific, and/or when the phase speed of the propagating wind stress forcing is closest to that of the Kelvin wave. These conditions tend to occur as the ocean adjusts toward El Niño. Since Kelvin waves are instrumental to the development of El Niño events, isolating the detailed relationship between the waves and the MJO will lead to a better understanding of interannual ocean–atmosphere interactions.
Abstract
The Madden–Julian oscillation (MJO) has been implicated as a major source of the wind stress variability that generates basin-scale Kelvin waves in the equatorial Pacific. One source of debate concerning this relationship is the apparent difference in the frequencies of the two processes.
This work utilizes data from the Tropical Atmosphere Ocean (TAO) array of moored buoys along with outgoing longwave radiation data to show by means of a multiple linear regression model and case studies that the frequency discrepancy is due to a systematic decrease in the phase speeds of the Kelvin waves and an increase in the period of the waves toward the east as conditions adjust toward El Niño. Among the potential contributing factors to this phase speed decrease is an apparent air–sea interaction that enhances the wind forcing of some of the Kelvin waves, allowing them to continue to amplify because the propagating wind stress anomaly decelerates to the speed of the developing Kelvin wave instead of the significantly faster speed of the typical MJO. Kelvin waves appear to be most effectively amplified during periods when the temperature gradient above the thermocline across the equatorial central Pacific is strong, the thermocline shoals steeply toward the east in the central Pacific, and/or when the phase speed of the propagating wind stress forcing is closest to that of the Kelvin wave. These conditions tend to occur as the ocean adjusts toward El Niño. Since Kelvin waves are instrumental to the development of El Niño events, isolating the detailed relationship between the waves and the MJO will lead to a better understanding of interannual ocean–atmosphere interactions.
Abstract
Intraseasonal oceanic Kelvin waves are the dominant mode of variability in the thermocline of the equatorial Pacific. Dynamic height data from the Tropical Atmosphere Ocean (TAO) Array of buoys moored in the tropical Pacific offer a convenient grid on which to study the waves but can only be effectively applied to study basinwide wave activity since about 1988 because of insufficient data at earlier times. Kelvin wave signals are also present in sea level data from island and coastal sites from the University of Hawaii Sea Level Center, some of which are available from before 1970 and up to 2003. This work describes a technique for reconstructing equatorial dynamic height data back to 1974, by utilizing regression relationships between the TAO data and daily sea level time series from 11 stations in the tropical Pacific. The reconstructed data are analyzed for skill in approximating Kelvin wave signals when TAO data are available. Reconstructed Kelvin wave signals prior to the TAO period are then analyzed for consistency with the wind stress anomalies that are responsible for generating the waves.
A regression analysis showing intraseasonal patterns of convection and winds that occur during periods of adjustment toward El Niño conditions is applied during the period 1974–87 for comparison with an earlier result calculated from TAO data for 1988–2005. Systematic changes in Kelvin wave phase speed with respect to ENSO documented for the latter period are confirmed in the earlier reconstructed dataset.
Abstract
Intraseasonal oceanic Kelvin waves are the dominant mode of variability in the thermocline of the equatorial Pacific. Dynamic height data from the Tropical Atmosphere Ocean (TAO) Array of buoys moored in the tropical Pacific offer a convenient grid on which to study the waves but can only be effectively applied to study basinwide wave activity since about 1988 because of insufficient data at earlier times. Kelvin wave signals are also present in sea level data from island and coastal sites from the University of Hawaii Sea Level Center, some of which are available from before 1970 and up to 2003. This work describes a technique for reconstructing equatorial dynamic height data back to 1974, by utilizing regression relationships between the TAO data and daily sea level time series from 11 stations in the tropical Pacific. The reconstructed data are analyzed for skill in approximating Kelvin wave signals when TAO data are available. Reconstructed Kelvin wave signals prior to the TAO period are then analyzed for consistency with the wind stress anomalies that are responsible for generating the waves.
A regression analysis showing intraseasonal patterns of convection and winds that occur during periods of adjustment toward El Niño conditions is applied during the period 1974–87 for comparison with an earlier result calculated from TAO data for 1988–2005. Systematic changes in Kelvin wave phase speed with respect to ENSO documented for the latter period are confirmed in the earlier reconstructed dataset.
Abstract
The importance of the presence of South America and Australia to the existence and orientation of the South Pacific Convergence Zone (SPCZ) during January is explored using the ECMWF T21 model. Each of the continents is removed from the model and replaced with an ocean surface, and the resulting precipitation and circulation associated with the SPCZ are then compared to a perpetual January control run. Results show that the presence of South America and the equatorial Pacific upwelling zone does not appear to be crucial to the SPCZ, but that the removal of Australia destroys the southern monsoon and substantially weakens the western part of the SPCZ. This suggests that the northwest-southeast orientation of the SPCZ during southern summer is more dependent on interactions with the midlatitude westerlies over the South Pacific than on the distribution of sea surface temperature and land over the Southern Hemisphere.
Abstract
The importance of the presence of South America and Australia to the existence and orientation of the South Pacific Convergence Zone (SPCZ) during January is explored using the ECMWF T21 model. Each of the continents is removed from the model and replaced with an ocean surface, and the resulting precipitation and circulation associated with the SPCZ are then compared to a perpetual January control run. Results show that the presence of South America and the equatorial Pacific upwelling zone does not appear to be crucial to the SPCZ, but that the removal of Australia destroys the southern monsoon and substantially weakens the western part of the SPCZ. This suggests that the northwest-southeast orientation of the SPCZ during southern summer is more dependent on interactions with the midlatitude westerlies over the South Pacific than on the distribution of sea surface temperature and land over the Southern Hemisphere.
Abstract
This paper presents an investigation of the mechanisms giving rise to the main intraseasonal mode of convection in the African monsoon during northern summer, here identified as the quasi-biweekly zonal dipole (QBZD). The QBZD is primarily characterized by a quasi-stationary zonal dipole of convection whose dimension is larger than the West African monsoon domain, with its two poles centered along the Guinean coast and between 30° and 60°W in the equatorial Atlantic. The QBZD dynamical processes within the Atlantic–Africa domain are examined in some detail. The QBZD has a dipole pattern associated with a Walker-type circulation in the near-equatorial zonal plane. It is controlled both by equatorial atmospheric dynamics through a Kelvin wave–like disturbance propagating eastward between its two poles and by land surface processes over Africa, inducing combined fluctuations in surface temperatures, surface pressure, and low-level zonal winds off the coast of West Africa. When convection is at a minimum over central and West Africa, a lack of cloud cover results in higher net shortwave flux at the surface, which increases surface temperatures and lowers surface pressures. This creates an east–west pressure gradient at the latitude of both the ITCZ (10°N) and the Saharan heat low (20°N), leading to an increase in eastward moisture advection inland. The arrival from the Atlantic of the positive pressure signal associated with a Kelvin wave pattern amplifies the low-level westerly wind component and the moisture advection inland, leading to an increase in convective activity over central and West Africa. Then the opposite phase of the dipole develops. Propagation of the QBZD convective envelope and of the associated 200 high-level velocity potential anomalies is detected from the eastern Pacific to the Indian Ocean. When the effect of the Kelvin wave propagation is removed by filtering, the stationary character of the QBZD is highlighted. The impact of the QBZD in combination with a Kelvin wave is illustrated by a case study of the monsoon onset in 1984.
Abstract
This paper presents an investigation of the mechanisms giving rise to the main intraseasonal mode of convection in the African monsoon during northern summer, here identified as the quasi-biweekly zonal dipole (QBZD). The QBZD is primarily characterized by a quasi-stationary zonal dipole of convection whose dimension is larger than the West African monsoon domain, with its two poles centered along the Guinean coast and between 30° and 60°W in the equatorial Atlantic. The QBZD dynamical processes within the Atlantic–Africa domain are examined in some detail. The QBZD has a dipole pattern associated with a Walker-type circulation in the near-equatorial zonal plane. It is controlled both by equatorial atmospheric dynamics through a Kelvin wave–like disturbance propagating eastward between its two poles and by land surface processes over Africa, inducing combined fluctuations in surface temperatures, surface pressure, and low-level zonal winds off the coast of West Africa. When convection is at a minimum over central and West Africa, a lack of cloud cover results in higher net shortwave flux at the surface, which increases surface temperatures and lowers surface pressures. This creates an east–west pressure gradient at the latitude of both the ITCZ (10°N) and the Saharan heat low (20°N), leading to an increase in eastward moisture advection inland. The arrival from the Atlantic of the positive pressure signal associated with a Kelvin wave pattern amplifies the low-level westerly wind component and the moisture advection inland, leading to an increase in convective activity over central and West Africa. Then the opposite phase of the dipole develops. Propagation of the QBZD convective envelope and of the associated 200 high-level velocity potential anomalies is detected from the eastern Pacific to the Indian Ocean. When the effect of the Kelvin wave propagation is removed by filtering, the stationary character of the QBZD is highlighted. The impact of the QBZD in combination with a Kelvin wave is illustrated by a case study of the monsoon onset in 1984.
Abstract
The dominant mode of convectively coupled Kelvin waves has been detected over the Atlantic and Africa during northern summer by performing composite analyses on observational fields based on an EOF reconstructed convection index over West Africa. Propagating eastward, many waves originate from the Pacific sector, interact with deep convection of the marine ITCZ over the Atlantic and the continental ITCZ over West and central Africa, and then weaken over East Africa and the Indian Ocean. It has been shown that they are able to modulate the life cycle and track of individual westward-propagating convective systems. Their mean kinematic characteristics comprise a wavelength of 8000 km, and a phase speed of 15 m s−1, leading to a period centered on 6 to 7 days. The African Kelvin wave activity displays large seasonal variability, being highest outside of northern summer when the ITCZ is close to the equator, facilitating the interactions between convection and these equatorially trapped waves. The convective and dynamical patterns identified over the Atlantic and Africa show some resemblance to the theoretical equatorially trapped Kelvin wave solution on an equatorial β plane. Most of the flow is in the zonal direction as predicted by theory, and there is a tendency for the dynamical fields to be symmetric about the equator, even though the ITCZ is concentrated well north of the equator at the full development of the African monsoon. In the upper troposphere and the stratosphere, the temperature contours slope sharply eastward with height, as expected from an eastward-moving heat source that forces a dry Kelvin wave response. It is finally shown that the mean impact of African Kelvin waves on rainfall and convection is of the same level as African easterly waves.
Abstract
The dominant mode of convectively coupled Kelvin waves has been detected over the Atlantic and Africa during northern summer by performing composite analyses on observational fields based on an EOF reconstructed convection index over West Africa. Propagating eastward, many waves originate from the Pacific sector, interact with deep convection of the marine ITCZ over the Atlantic and the continental ITCZ over West and central Africa, and then weaken over East Africa and the Indian Ocean. It has been shown that they are able to modulate the life cycle and track of individual westward-propagating convective systems. Their mean kinematic characteristics comprise a wavelength of 8000 km, and a phase speed of 15 m s−1, leading to a period centered on 6 to 7 days. The African Kelvin wave activity displays large seasonal variability, being highest outside of northern summer when the ITCZ is close to the equator, facilitating the interactions between convection and these equatorially trapped waves. The convective and dynamical patterns identified over the Atlantic and Africa show some resemblance to the theoretical equatorially trapped Kelvin wave solution on an equatorial β plane. Most of the flow is in the zonal direction as predicted by theory, and there is a tendency for the dynamical fields to be symmetric about the equator, even though the ITCZ is concentrated well north of the equator at the full development of the African monsoon. In the upper troposphere and the stratosphere, the temperature contours slope sharply eastward with height, as expected from an eastward-moving heat source that forces a dry Kelvin wave response. It is finally shown that the mean impact of African Kelvin waves on rainfall and convection is of the same level as African easterly waves.
Abstract
We show by means of a general circulation model experiment that the atmospheric circulation over the South Pacific Ocean is sensitive to sea surface temperature anomalies in the tropical and subtropical regions of the South Pacific convergence zone. The possible implications for understanding the life cycle of an extreme event in the Southern Oscillation are discussed.
Abstract
We show by means of a general circulation model experiment that the atmospheric circulation over the South Pacific Ocean is sensitive to sea surface temperature anomalies in the tropical and subtropical regions of the South Pacific convergence zone. The possible implications for understanding the life cycle of an extreme event in the Southern Oscillation are discussed.
Abstract
A dynamical model is constructed of the northern summertime global circulation, maintained by empirically derived forcing, based on the same dynamical code that has recently been used to study African easterly waves (AEWs) as convectively triggered perturbations (; ). In the configuration used here, the model faithfully simulates the observed mean distributions of jets and transient disturbances, and explicitly represents the interactions between them. This simple GCM is used to investigate the origin and intraseasonal intermittency of AEWs in an artificially dry (no convection) context. A long integration of the model produces a summertime climatology that includes a realistic African easterly jet and westward-propagating 3–5-day disturbances over West Africa. These simulated waves display intraseasonal intermittency as the observed AEWs also do. Further experiments designed to discern the source of this intermittency in the model show that the simulated waves are mainly triggered by dynamical precursors coming from the North Atlantic storm track. The model is at least as sensitive to this remote influence as it is to local triggering by convective heating.
Abstract
A dynamical model is constructed of the northern summertime global circulation, maintained by empirically derived forcing, based on the same dynamical code that has recently been used to study African easterly waves (AEWs) as convectively triggered perturbations (; ). In the configuration used here, the model faithfully simulates the observed mean distributions of jets and transient disturbances, and explicitly represents the interactions between them. This simple GCM is used to investigate the origin and intraseasonal intermittency of AEWs in an artificially dry (no convection) context. A long integration of the model produces a summertime climatology that includes a realistic African easterly jet and westward-propagating 3–5-day disturbances over West Africa. These simulated waves display intraseasonal intermittency as the observed AEWs also do. Further experiments designed to discern the source of this intermittency in the model show that the simulated waves are mainly triggered by dynamical precursors coming from the North Atlantic storm track. The model is at least as sensitive to this remote influence as it is to local triggering by convective heating.
Abstract
This study examines systematic biases in sea surface temperature (SST) under the stratus cloud deck in the southeast Pacific Ocean and upper-ocean processes relevant to the SST biases in 19 coupled general circulation models (CGCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The 20 years of simulations from each model are analyzed. Pronounced warm SST biases in a large portion of the southeast Pacific stratus region are found in all models. Processes that could contribute to the SST biases are examined in detail based on the computation of major terms in the upper-ocean heat budget. Negative biases in net surface heat fluxes are evident in most of the models, suggesting that the cause of the warm SST biases in models is not explained by errors in net surface heat fluxes. Biases in heat transport by Ekman currents largely contribute to the warm SST biases both near the coast and the open ocean. In the coastal area, southwestward Ekman currents and upwelling in most models are much weaker than observed owing to weaker alongshore winds, resulting in insufficient advection of cold water from the coast. In the open ocean, warm advection due to Ekman currents is overestimated in models because of the larger meridional temperature gradient, the smaller zonal temperature gradient, and overly weaker Ekman currents.
Abstract
This study examines systematic biases in sea surface temperature (SST) under the stratus cloud deck in the southeast Pacific Ocean and upper-ocean processes relevant to the SST biases in 19 coupled general circulation models (CGCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). The 20 years of simulations from each model are analyzed. Pronounced warm SST biases in a large portion of the southeast Pacific stratus region are found in all models. Processes that could contribute to the SST biases are examined in detail based on the computation of major terms in the upper-ocean heat budget. Negative biases in net surface heat fluxes are evident in most of the models, suggesting that the cause of the warm SST biases in models is not explained by errors in net surface heat fluxes. Biases in heat transport by Ekman currents largely contribute to the warm SST biases both near the coast and the open ocean. In the coastal area, southwestward Ekman currents and upwelling in most models are much weaker than observed owing to weaker alongshore winds, resulting in insufficient advection of cold water from the coast. In the open ocean, warm advection due to Ekman currents is overestimated in models because of the larger meridional temperature gradient, the smaller zonal temperature gradient, and overly weaker Ekman currents.
