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- Author or Editor: George N. Kiladis x
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Abstract
The dynamics of the 2-day wave, a type of convectively coupled disturbance that frequents the equatorial western Pacific, is examined using observations and a linear primitive equation model. A statistical composite of the wave's kinematic and thermodynamic structure is presented. It is shown that 1) the wave's wind and temperature perturbations can be modeled as linear responses to convective heating and cooling, and 2) the bulk of the wave's dynamical and convective structure can be represented with two vertical modes. The observations and model results suggest that the 2-day wave is an n = 1 westward-propagating inertio–gravity wave with a shallow equivalent depth (14 m) that results from the partial cancelation of adiabatic temperature changes due to vertical motion by convective heating and cooling.
Abstract
The dynamics of the 2-day wave, a type of convectively coupled disturbance that frequents the equatorial western Pacific, is examined using observations and a linear primitive equation model. A statistical composite of the wave's kinematic and thermodynamic structure is presented. It is shown that 1) the wave's wind and temperature perturbations can be modeled as linear responses to convective heating and cooling, and 2) the bulk of the wave's dynamical and convective structure can be represented with two vertical modes. The observations and model results suggest that the 2-day wave is an n = 1 westward-propagating inertio–gravity wave with a shallow equivalent depth (14 m) that results from the partial cancelation of adiabatic temperature changes due to vertical motion by convective heating and cooling.
Abstract
Observations of the horizontal and vertical structure of convectively coupled Kelvin waves are presented and are compared with the predicted structures of moist Kelvin (or gravity) waves in three simple models of coupled wave instability: wave–conditional instability of the second kind (CISK), wind-induced surface heat exchange (WISHE), and stratiform instability. The observations are based on a linear regression analysis of multiple years of ECMWF reanalysis and station radiosonde data. Results suggest that both the wave-CISK and stratiform instability theories successfully predict many important features of observed moist Kelvin waves, but that unrealistic aspects of these models limit their ability to provide comprehensive explanations for the dynamics of these waves. It is suggested that an essential component of any theory for moist Kelvin waves is the second baroclinic mode heat source associated with stratiform precipitation.
Abstract
Observations of the horizontal and vertical structure of convectively coupled Kelvin waves are presented and are compared with the predicted structures of moist Kelvin (or gravity) waves in three simple models of coupled wave instability: wave–conditional instability of the second kind (CISK), wind-induced surface heat exchange (WISHE), and stratiform instability. The observations are based on a linear regression analysis of multiple years of ECMWF reanalysis and station radiosonde data. Results suggest that both the wave-CISK and stratiform instability theories successfully predict many important features of observed moist Kelvin waves, but that unrealistic aspects of these models limit their ability to provide comprehensive explanations for the dynamics of these waves. It is suggested that an essential component of any theory for moist Kelvin waves is the second baroclinic mode heat source associated with stratiform precipitation.
Abstract
Space–time spectral analysis of tropical cloudiness data shows strong evidence that convectively coupled n = 0 mixed Rossby–gravity waves (MRGs) and eastward inertio-gravity waves (EIGs) occur primarily within the western/central Pacific Ocean. Spectral filtering also shows that MRG and EIG cloudiness patterns are antisymmetric with respect to the equator, and they propagate coherently toward the west and east, respectively, with periods between 3 and 5 days, in agreement with Matsuno’s linear shallow-water theory. In contrast to the spectral approach, in a companion paper it has been shown that empirical orthogonal functions (EOFs) of 2–6-day-filtered cloudiness data within the tropical Pacific Ocean also suggest an antisymmetric pattern, but with the leading EOFs implying a zonally standing but poleward-propagating oscillation, along with the associated tropospheric flow moving to the west. In the present paper, these two views are reconciled by applying an independent approach based on a tracking method to assess tropical convection organization. It is shown that, on average, two-thirds of MRG and EIG events develop independently of one another, and one-third of the events overlap in space and time. This analysis also verifies that MRG and EIG cloudiness fields tend to propagate meridionally away from the equator. It is demonstrated that the lack of zonal propagation implied from the EOF analysis is likely due to the interference between eastward- and westward-propagating disturbances. In addition, it is shown that the westward-propagating circulation associated with the leading EOF is consistent with the expected theoretical behavior of an interference between MRGs and EIGs.
Abstract
Space–time spectral analysis of tropical cloudiness data shows strong evidence that convectively coupled n = 0 mixed Rossby–gravity waves (MRGs) and eastward inertio-gravity waves (EIGs) occur primarily within the western/central Pacific Ocean. Spectral filtering also shows that MRG and EIG cloudiness patterns are antisymmetric with respect to the equator, and they propagate coherently toward the west and east, respectively, with periods between 3 and 5 days, in agreement with Matsuno’s linear shallow-water theory. In contrast to the spectral approach, in a companion paper it has been shown that empirical orthogonal functions (EOFs) of 2–6-day-filtered cloudiness data within the tropical Pacific Ocean also suggest an antisymmetric pattern, but with the leading EOFs implying a zonally standing but poleward-propagating oscillation, along with the associated tropospheric flow moving to the west. In the present paper, these two views are reconciled by applying an independent approach based on a tracking method to assess tropical convection organization. It is shown that, on average, two-thirds of MRG and EIG events develop independently of one another, and one-third of the events overlap in space and time. This analysis also verifies that MRG and EIG cloudiness fields tend to propagate meridionally away from the equator. It is demonstrated that the lack of zonal propagation implied from the EOF analysis is likely due to the interference between eastward- and westward-propagating disturbances. In addition, it is shown that the westward-propagating circulation associated with the leading EOF is consistent with the expected theoretical behavior of an interference between MRGs and EIGs.
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
Lagged cross correlations between outgoing longwave radiation (OLR) and National Meteorological Center global analyses are utilized to isolate the preferred upper-level and surface circulation anomalies associated with tropical convection during northern winter. Three intraseasonal time scales are studied: 30–70, 14–30, and 6–14 days. In the 30–70-day band, the upper-level circulation signals are zonally elongated, with zonal wavenumbers 0–2 dominant. Higher-frequency signals are dominated by zonal wavenumbers 5 and 6. In the 14–30-day band, convection over the eastern hemisphere is associated with upper-level anticyclones in the subtropics and appears to be linked in some cases to midlatitude wave trains. The strongest signals are for convection over Africa, Australia, and the eastern Indian Ocean. Only weak signals are seen for convection over Indonesia. In these regions of upper-level easterlies, OLR anomalies peak prior to the maximum anomalies in wind, suggesting forcing of the circulation by tropical heating.
In contrast, 14–30-day and 6–14-day convection over the eastern tropical Pacific, eastern South America, and central South Pacific is primarily associated with the intrusion of troughs in the westerlies originating in the extratropics. These are regions of mean upper level westerly flow, or where upper-westerlies lie adjacent to tropical convergence zones overlain by only weak easterly flow aloft. The large amplitude of these troughs prior to the OLR anomaly is indicative of the forcing of the convection by these disturbances.
Abstract
Lagged cross correlations between outgoing longwave radiation (OLR) and National Meteorological Center global analyses are utilized to isolate the preferred upper-level and surface circulation anomalies associated with tropical convection during northern winter. Three intraseasonal time scales are studied: 30–70, 14–30, and 6–14 days. In the 30–70-day band, the upper-level circulation signals are zonally elongated, with zonal wavenumbers 0–2 dominant. Higher-frequency signals are dominated by zonal wavenumbers 5 and 6. In the 14–30-day band, convection over the eastern hemisphere is associated with upper-level anticyclones in the subtropics and appears to be linked in some cases to midlatitude wave trains. The strongest signals are for convection over Africa, Australia, and the eastern Indian Ocean. Only weak signals are seen for convection over Indonesia. In these regions of upper-level easterlies, OLR anomalies peak prior to the maximum anomalies in wind, suggesting forcing of the circulation by tropical heating.
In contrast, 14–30-day and 6–14-day convection over the eastern tropical Pacific, eastern South America, and central South Pacific is primarily associated with the intrusion of troughs in the westerlies originating in the extratropics. These are regions of mean upper level westerly flow, or where upper-westerlies lie adjacent to tropical convergence zones overlain by only weak easterly flow aloft. The large amplitude of these troughs prior to the OLR anomaly is indicative of the forcing of the convection by these disturbances.
Abstract
Statistical evidence is presented to support the notion that tropical convection in the eastern Pacific and Atlantic intertropical convergence zone (ITCZ) during northern winter can be forced by disturbances originating in the extratropics. The synoptic-scale transients in these regions are characterized at upper levels by strong positive tilts in the horizontal and appear to induce vertical motions ahead of troughs as in midlatitude baroclinic systems. Two case studies of such interactions are examined, one for the eastern North Pacific ITCZ and another somewhat different type of interaction for the South Pacific convergence zone (SPCZ) over the western South Pacific.
Both cases are associated with upper-level troughs, strong cold advection deep into the tropics, and the formation of a frontal boundary at low levels. The ITCZ case is characterized by the advection of anomalously high isentropic potential vorticity air southward, a strong poleward flux of heat and westerly momentum, and the development of a subtropical jet downstream of the disturbance. The SPCZ disturbance is not strongly tilted, but is still accompanied by a strong poleward flux of heat and momentum. Evidence for the occurrence of cross-equatorial wave dispersion in the eastern Pacific during northern winter is also presented. These observations are consistent with theory and modeling of Rossby waves in a westerly basic state extending from the midlatitudes into the tropics.
Abstract
Statistical evidence is presented to support the notion that tropical convection in the eastern Pacific and Atlantic intertropical convergence zone (ITCZ) during northern winter can be forced by disturbances originating in the extratropics. The synoptic-scale transients in these regions are characterized at upper levels by strong positive tilts in the horizontal and appear to induce vertical motions ahead of troughs as in midlatitude baroclinic systems. Two case studies of such interactions are examined, one for the eastern North Pacific ITCZ and another somewhat different type of interaction for the South Pacific convergence zone (SPCZ) over the western South Pacific.
Both cases are associated with upper-level troughs, strong cold advection deep into the tropics, and the formation of a frontal boundary at low levels. The ITCZ case is characterized by the advection of anomalously high isentropic potential vorticity air southward, a strong poleward flux of heat and westerly momentum, and the development of a subtropical jet downstream of the disturbance. The SPCZ disturbance is not strongly tilted, but is still accompanied by a strong poleward flux of heat and momentum. Evidence for the occurrence of cross-equatorial wave dispersion in the eastern Pacific during northern winter is also presented. These observations are consistent with theory and modeling of Rossby waves in a westerly basic state extending from the midlatitudes into the tropics.
Abstract
Composite surface pressure, temperature, and precipitation anomalies are mapped over the Indian and Pacific sectors during the various stages of Warm and Cold Events in the Southern Oscillation. In the year before the development of positive sea surface temperature anomalies in the central and eastern equatorial Pacific (Year–1 of a Warm Event), a strong South Pacific High is associated with below normal surface pressure over Australia and the Indian Ocean. This occurs concurrently with a poleward displacement of the Pacific convergence zones, with above normal air temperature and precipitation over the subtropical Pacific, and opposite conditions along the equator. By the next year (Year 0) of the Warm Event, thew anomalies have the opposite sign. The sequence of anomalies during a Cold Event is inverse to that during a Warm Event but otherwise the anomaly patterns are remarkably similar.
It appears that enhanced convection and low surface pressure within the Pacific convergence zones contribute to the observed westerly wind anomalies in the western equatorial Pacific at the end of Year–1, which are in turn tied to the onset of above normal equatorial SST in the following year. The observed reversal in atmospheric anomalies over the Indian and Pacific oceans daring Warm Events is an extreme manifestation of a general biennial tendency in these anomalies, with Cold Events occupying the opposite extreme.
Abstract
Composite surface pressure, temperature, and precipitation anomalies are mapped over the Indian and Pacific sectors during the various stages of Warm and Cold Events in the Southern Oscillation. In the year before the development of positive sea surface temperature anomalies in the central and eastern equatorial Pacific (Year–1 of a Warm Event), a strong South Pacific High is associated with below normal surface pressure over Australia and the Indian Ocean. This occurs concurrently with a poleward displacement of the Pacific convergence zones, with above normal air temperature and precipitation over the subtropical Pacific, and opposite conditions along the equator. By the next year (Year 0) of the Warm Event, thew anomalies have the opposite sign. The sequence of anomalies during a Cold Event is inverse to that during a Warm Event but otherwise the anomaly patterns are remarkably similar.
It appears that enhanced convection and low surface pressure within the Pacific convergence zones contribute to the observed westerly wind anomalies in the western equatorial Pacific at the end of Year–1, which are in turn tied to the onset of above normal equatorial SST in the following year. The observed reversal in atmospheric anomalies over the Indian and Pacific oceans daring Warm Events is an extreme manifestation of a general biennial tendency in these anomalies, with Cold Events occupying the opposite extreme.
