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Abstract
The NCEP–NCAR reanalysis together with the outgoing longwave radiation anomalies (OLRAs) and a gridded daily precipitation over the United States were used to analyze precipitation over California on intraseasonal timescales. The intraseasonal (10–90 days) filtered OLRAs were subjected to singular spectrum analysis, which identifies nonlinear oscillations in noisy time series. There are two dominant oscillatory modes associated with California rainfall with periods near 36–40 and 20–25 days.
The 36–40-day mode is related to the Madden–Julian Oscillation (MJO) in the Tropics. Enhanced tropical convection propagates from the western Pacific to the central Pacific. A three-cell pattern with negative OLRAs in California and positive anomalies in the eastern Pacific and the Pacific Northwest starts to develop 4 days later and rainfall starts in California.
Anomalies associated with the 20–25-day mode are responsible for alternating wet and dry episodes over California with periods shorter than the timescales of the MJO. The 20–25-day mode is the leading mode in the 7–30-day band and is related to tropical convection in the Pacific. In the extratropics, cloud bands propagate northward along the west coast of North America from the eastern Pacific just north of the ITCZ through California to the Pacific Northwest. The 200-hPa streamfunction anomaly composites associated with the 20–25-day mode reveal a westward propagating wave train dominated by a zonal wavenumber 2. This mode has a spatial structure similar to the traveling pattern described by Branstator.
Abstract
The NCEP–NCAR reanalysis together with the outgoing longwave radiation anomalies (OLRAs) and a gridded daily precipitation over the United States were used to analyze precipitation over California on intraseasonal timescales. The intraseasonal (10–90 days) filtered OLRAs were subjected to singular spectrum analysis, which identifies nonlinear oscillations in noisy time series. There are two dominant oscillatory modes associated with California rainfall with periods near 36–40 and 20–25 days.
The 36–40-day mode is related to the Madden–Julian Oscillation (MJO) in the Tropics. Enhanced tropical convection propagates from the western Pacific to the central Pacific. A three-cell pattern with negative OLRAs in California and positive anomalies in the eastern Pacific and the Pacific Northwest starts to develop 4 days later and rainfall starts in California.
Anomalies associated with the 20–25-day mode are responsible for alternating wet and dry episodes over California with periods shorter than the timescales of the MJO. The 20–25-day mode is the leading mode in the 7–30-day band and is related to tropical convection in the Pacific. In the extratropics, cloud bands propagate northward along the west coast of North America from the eastern Pacific just north of the ITCZ through California to the Pacific Northwest. The 200-hPa streamfunction anomaly composites associated with the 20–25-day mode reveal a westward propagating wave train dominated by a zonal wavenumber 2. This mode has a spatial structure similar to the traveling pattern described by Branstator.
Abstract
The intraseasonal rainfall variability over North America is examined using singular spectrum analysis (SSA) and composites of outgoing longwave radiation anomalies (OLRAs), 200-hPa divergence and a gridded rainfall dataset over the United States. The evolution of the Arizona and New Mexico (AZNM) monsoon based on composites indicates that rainfall anomalies propagate eastward from the North Pacific through AZNM, the Great Plains, to the eastern United States. During summer, the wet and dry periods of the AZNM monsoon are modulated by an oscillatory mode with a period of 22–25 days (22-day mode). This is also the dominant mode associated with rainfall events over the Great Plains. The influence of the Madden–Julian Oscillation (MJO) on the AZNM monsoon is secondary. The strongest impact of the MJO is on precipitation over Mexico. SSA performed on the 200-hPa divergence and OLRAs averaged over Mexico show only one oscillatory mode with a period of about 36–40 days.
The 22–25-day mode also exists in the vertically integrated moisture fluxes over the Great Plains. During the wet periods of the AZNM monsoon, more moisture is transported from both the Gulf of Mexico and the Gulf of California to AZNM. The situation reverses when the oscillation reaches the other phase. The 22-day mode is linked to tropical convection. When rainfall associated with the 22-day mode travels eastward from AZNM to the Great Plains, the OLRA composites show westward propagating waves just north of the equator. When enhanced convection reaches the western Pacific, rainfall diminishes over AZNM. When convection in the western Pacific is suppressed and enhanced convection is located in the central Pacific, rainfall intensifies over AZNM.
Abstract
The intraseasonal rainfall variability over North America is examined using singular spectrum analysis (SSA) and composites of outgoing longwave radiation anomalies (OLRAs), 200-hPa divergence and a gridded rainfall dataset over the United States. The evolution of the Arizona and New Mexico (AZNM) monsoon based on composites indicates that rainfall anomalies propagate eastward from the North Pacific through AZNM, the Great Plains, to the eastern United States. During summer, the wet and dry periods of the AZNM monsoon are modulated by an oscillatory mode with a period of 22–25 days (22-day mode). This is also the dominant mode associated with rainfall events over the Great Plains. The influence of the Madden–Julian Oscillation (MJO) on the AZNM monsoon is secondary. The strongest impact of the MJO is on precipitation over Mexico. SSA performed on the 200-hPa divergence and OLRAs averaged over Mexico show only one oscillatory mode with a period of about 36–40 days.
The 22–25-day mode also exists in the vertically integrated moisture fluxes over the Great Plains. During the wet periods of the AZNM monsoon, more moisture is transported from both the Gulf of Mexico and the Gulf of California to AZNM. The situation reverses when the oscillation reaches the other phase. The 22-day mode is linked to tropical convection. When rainfall associated with the 22-day mode travels eastward from AZNM to the Great Plains, the OLRA composites show westward propagating waves just north of the equator. When enhanced convection reaches the western Pacific, rainfall diminishes over AZNM. When convection in the western Pacific is suppressed and enhanced convection is located in the central Pacific, rainfall intensifies over AZNM.
Abstract
A statistical model based on the combination of singular spectrum analysis (SSA) and the maximum entropy method (MEM) is applied to monitor and forecast outgoing longwave radiation anomalies (OLRAs) in the intraseasonal band over the Indian–Pacific sector and in the pan-American region. SSA is related to empirical orthogonal function analysis (EOF) but is applied to time series. The leading SSA modes (T-EOFs) are orthogonal and they are determined from the training period before filtering. The OLRA time series can be projected onto T-EOFs to obtain the principal components (T-PCs). To obtain fluctuations in any frequency band, one can partially sum up a chosen subset of T-EOFs and the related T-PCs in that band. The filter based on the SSA modes is data adaptive and there is no loss of end points. It is well suited for real-time monitoring of intraseasonal oscillations.
In the Pacific and the pan-American region, there are three leading modes (T-EOFs) of oscillations with periods near 40, 22, and 18 days. The T-PCs associated with these modes are quasiperiodic and they can be modeled by an autoregressive process. To perform forecasts, the MEM is used to determine the autoregressive coefficients from the training period. These coefficients are used to advance T-PCs. The summation of T-EOFs and T-PCs related to three preferred modes gives the predicted OLRAs. For 5-day mean OLRAs, the averaged correlation between the predicted and the observed anomalies is 0.65 at the lead times of four pentads (20 days). The SSA–MEM method is effective for any time series containing large oscillatory components. The deficiency of this method is that the forecasted magnitudes of anomalies are usually weaker than observations.
Abstract
A statistical model based on the combination of singular spectrum analysis (SSA) and the maximum entropy method (MEM) is applied to monitor and forecast outgoing longwave radiation anomalies (OLRAs) in the intraseasonal band over the Indian–Pacific sector and in the pan-American region. SSA is related to empirical orthogonal function analysis (EOF) but is applied to time series. The leading SSA modes (T-EOFs) are orthogonal and they are determined from the training period before filtering. The OLRA time series can be projected onto T-EOFs to obtain the principal components (T-PCs). To obtain fluctuations in any frequency band, one can partially sum up a chosen subset of T-EOFs and the related T-PCs in that band. The filter based on the SSA modes is data adaptive and there is no loss of end points. It is well suited for real-time monitoring of intraseasonal oscillations.
In the Pacific and the pan-American region, there are three leading modes (T-EOFs) of oscillations with periods near 40, 22, and 18 days. The T-PCs associated with these modes are quasiperiodic and they can be modeled by an autoregressive process. To perform forecasts, the MEM is used to determine the autoregressive coefficients from the training period. These coefficients are used to advance T-PCs. The summation of T-EOFs and T-PCs related to three preferred modes gives the predicted OLRAs. For 5-day mean OLRAs, the averaged correlation between the predicted and the observed anomalies is 0.65 at the lead times of four pentads (20 days). The SSA–MEM method is effective for any time series containing large oscillatory components. The deficiency of this method is that the forecasted magnitudes of anomalies are usually weaker than observations.
Abstract
Tropical intraseasonal variations in the Pacific are related to the tropical storm activity in the Atlantic basin using outgoing longwave radiation anomalies (OLRAs) and circulation anomalies from the NCEP–NCAR reanalysis. Tropical storms are most likely to develop and maintain in the Atlantic, when enhanced convection associated with the tropical intraseasonal oscillations (TIOs) is located over the Indian Ocean and convection in the Pacific is suppressed. Tropical storm activity decreases when the TIO shifts to the opposite phase.
The dominant signal associated with the TIO is the Madden–Julian oscillation. The atmospheric response in the Tropics is a dipole pattern in the 200-hPa streamfunction anomalies just north of the equator. Positive OLRA propagates eastward from the Indian Ocean to the central Pacific. The dipole moves eastward in concert with OLRAs. When enhanced convection is located in the Indian Ocean and convection in the Pacific is suppressed, positive 200-hPa streamfunction anomalies as a part of the dipole extend from Central America to the central Atlantic. There are more upper-tropospheric easterly wind anomalies over the Caribbeans and the tropical Atlantic. The vertical wind shear decreases. These conditions are favorable for tropical storms to development and enhance. When the TIO shifts to the opposite phase with enhanced convection in the Pacific, the wind shear in the tropical Atlantic increases and the occurrence of tropical storms decreases.
Abstract
Tropical intraseasonal variations in the Pacific are related to the tropical storm activity in the Atlantic basin using outgoing longwave radiation anomalies (OLRAs) and circulation anomalies from the NCEP–NCAR reanalysis. Tropical storms are most likely to develop and maintain in the Atlantic, when enhanced convection associated with the tropical intraseasonal oscillations (TIOs) is located over the Indian Ocean and convection in the Pacific is suppressed. Tropical storm activity decreases when the TIO shifts to the opposite phase.
The dominant signal associated with the TIO is the Madden–Julian oscillation. The atmospheric response in the Tropics is a dipole pattern in the 200-hPa streamfunction anomalies just north of the equator. Positive OLRA propagates eastward from the Indian Ocean to the central Pacific. The dipole moves eastward in concert with OLRAs. When enhanced convection is located in the Indian Ocean and convection in the Pacific is suppressed, positive 200-hPa streamfunction anomalies as a part of the dipole extend from Central America to the central Atlantic. There are more upper-tropospheric easterly wind anomalies over the Caribbeans and the tropical Atlantic. The vertical wind shear decreases. These conditions are favorable for tropical storms to development and enhance. When the TIO shifts to the opposite phase with enhanced convection in the Pacific, the wind shear in the tropical Atlantic increases and the occurrence of tropical storms decreases.
Abstract
Pattern correlations between daily anomalies have been used to study the persistence of the Southern Hemisphere circulations. The dataset consists of daily Australian analyses of 500 mb heights and sea level pressure for the period from 1972 to 1983. Compared to the Northern Hemisphere, the pattern correlations are much lower and more variable in the Southern Hemisphere. The mean one-day lag autocorrelation is only 0.57, compared to 0.81 in the Northern Hemisphere. The correlations increase significantly for the filtered anomalies, which consist of the planetary wavenumbers from 0 to 4.
Subjective criteria based on the pattern correlations are used to select quasi-stationary events. A series of 5 or more daily maps is defined to be quasi-stationary if the pattern correlations between all pairs of five consecutive maps in this time series are larger than or equal to 0.5. In winter, quasi-stationary events can be classified in terms of wavenumbers. Waves 3 and 4 are by far the dominant waves. More than half of the events have wave 3 amplitude with geographically fixed orientations.
Abstract
Pattern correlations between daily anomalies have been used to study the persistence of the Southern Hemisphere circulations. The dataset consists of daily Australian analyses of 500 mb heights and sea level pressure for the period from 1972 to 1983. Compared to the Northern Hemisphere, the pattern correlations are much lower and more variable in the Southern Hemisphere. The mean one-day lag autocorrelation is only 0.57, compared to 0.81 in the Northern Hemisphere. The correlations increase significantly for the filtered anomalies, which consist of the planetary wavenumbers from 0 to 4.
Subjective criteria based on the pattern correlations are used to select quasi-stationary events. A series of 5 or more daily maps is defined to be quasi-stationary if the pattern correlations between all pairs of five consecutive maps in this time series are larger than or equal to 0.5. In winter, quasi-stationary events can be classified in terms of wavenumbers. Waves 3 and 4 are by far the dominant waves. More than half of the events have wave 3 amplitude with geographically fixed orientations.
Abstract
Atmospheric circulation features and convection patterns associated with two leading low-frequency modes in the Southern Hemisphere (SH) are examined in multiyear global reanalyses produced by NCEP–NCAR and NASA–DAO. The two leading modes, referred to as the Pacific–South American (PSA) modes, are represented by the first two EOF patterns. The two patterns are in quadrature with each other and are dominated by wavenumber 3 in midlatitudes with large amplitudes in the Pacific–South American sector. In the Pacific, anomalies in the subtropics and in the midlatitudes are opposite in phase. Taken together, the two PSA modes represent the intraseasonal oscillation in the SH with periods of roughly 40 days. The evolution of the PSA modes shows a coherent eastward propagation.
A composite analysis was conducted to study the evolution of tropical convection and the corresponding circulation changes associated with the PSA modes. Outgoing longwave radiation (OLR) anomaly composites during the mature phase of the PSA modes resemble the first two leading EOFs of OLR anomalies (OLRA) in the Tropics. Composites of OLRA show an east–west dipole structure roughly 5–10 days prior to the onset of persistent PSA events. The PSA 1 mode is associated with enhanced convection in the Pacific between 140°E and 170°W and suppressed convection over the Indian Ocean. The PSA 2 mode is linked to tropical heating anomalies in the central Pacific extending from 160°E to 150°W just south of the equator and suppressed convection in the western Pacific with a maximum at 20°N. Contributions are from both interannual and intraseasonal bands.
Abstract
Atmospheric circulation features and convection patterns associated with two leading low-frequency modes in the Southern Hemisphere (SH) are examined in multiyear global reanalyses produced by NCEP–NCAR and NASA–DAO. The two leading modes, referred to as the Pacific–South American (PSA) modes, are represented by the first two EOF patterns. The two patterns are in quadrature with each other and are dominated by wavenumber 3 in midlatitudes with large amplitudes in the Pacific–South American sector. In the Pacific, anomalies in the subtropics and in the midlatitudes are opposite in phase. Taken together, the two PSA modes represent the intraseasonal oscillation in the SH with periods of roughly 40 days. The evolution of the PSA modes shows a coherent eastward propagation.
A composite analysis was conducted to study the evolution of tropical convection and the corresponding circulation changes associated with the PSA modes. Outgoing longwave radiation (OLR) anomaly composites during the mature phase of the PSA modes resemble the first two leading EOFs of OLR anomalies (OLRA) in the Tropics. Composites of OLRA show an east–west dipole structure roughly 5–10 days prior to the onset of persistent PSA events. The PSA 1 mode is associated with enhanced convection in the Pacific between 140°E and 170°W and suppressed convection over the Indian Ocean. The PSA 2 mode is linked to tropical heating anomalies in the central Pacific extending from 160°E to 150°W just south of the equator and suppressed convection in the western Pacific with a maximum at 20°N. Contributions are from both interannual and intraseasonal bands.
Abstract
Time series of outgoing longwave radiation (OLR) fields and various gridded reanalysis products are used to identify and describe periods with abundant and deficient rainfall over South America during summer. Empirical orthogonal function analyses of OLR anomalies filtered to retain variations longer than 10 days reveal a meridional seesaw of dry and wet conditions over tropical and subtropical South America. It appears that intensification of the South Atlantic convergence zone (SACZ) is associated with rainfall deficits over the subtropical plains of South America. In contrast, when the SACZ weakens, precipitation over these plains is abundant. These results are in agreement with those of Kousky and Casarin.
This seesaw pattern appears to be a regional component of a larger-scale system, possibly related to the 30–60-day oscillation in the Tropics, with the southward extension and strengthening of the SACZ found with enhanced tropical convection over the central and eastern Pacific and dry conditions over the western Pacific and the Maritime Continent. At the same time, convection is suppressed in the region of the South Pacific convergence zone, over the Gulf of Mexico, and in the ITCZ over the North Atlantic.
In the opposite phase there is a strong influx of moisture from the Tropics into central Argentina and southern Brazil. The moisture influx is enhanced by a strong low-level jet (LLJ) east of the Andes. The LLJ displays a marked diurnal oscillation and characteristics similar to the well-documented LLJs over the Great Plains of North America.
Abstract
Time series of outgoing longwave radiation (OLR) fields and various gridded reanalysis products are used to identify and describe periods with abundant and deficient rainfall over South America during summer. Empirical orthogonal function analyses of OLR anomalies filtered to retain variations longer than 10 days reveal a meridional seesaw of dry and wet conditions over tropical and subtropical South America. It appears that intensification of the South Atlantic convergence zone (SACZ) is associated with rainfall deficits over the subtropical plains of South America. In contrast, when the SACZ weakens, precipitation over these plains is abundant. These results are in agreement with those of Kousky and Casarin.
This seesaw pattern appears to be a regional component of a larger-scale system, possibly related to the 30–60-day oscillation in the Tropics, with the southward extension and strengthening of the SACZ found with enhanced tropical convection over the central and eastern Pacific and dry conditions over the western Pacific and the Maritime Continent. At the same time, convection is suppressed in the region of the South Pacific convergence zone, over the Gulf of Mexico, and in the ITCZ over the North Atlantic.
In the opposite phase there is a strong influx of moisture from the Tropics into central Argentina and southern Brazil. The moisture influx is enhanced by a strong low-level jet (LLJ) east of the Andes. The LLJ displays a marked diurnal oscillation and characteristics similar to the well-documented LLJs over the Great Plains of North America.
Abstract
Eighteen 30-day integrations with the NMC global atmospheric model (T40 resolution) were performed in order to test the impact of sea surface temperature anomalies (SSTAs) on 30-day forecasts for the Northern Hemisphere early summer. The years considered—1987, 1988, and 1989—correspond to a warm El Niño-Southern Oscillation (ENSO) event, a cold ENSO event, and a normal (non-ENSO year), respectively. For each year, 30-day forecasts were started on three successive days around 22 May, using climatological SSTs, and repeated using SSTAs fixed at their initial values.
The results indicate that SSTAs have a clear positive impact on the tropical forecasts and surface fluxes. The impacts on the extratropical forecasts, on the other hand, tend to be positive but small. Larger positive impacts in midlatitudes are obtained only in a case in which the atmospheric anomalous circulation is apparently driven by the ocean anomalies. A simple rule of thumb to distinguish whether quasi-stationary atmospheric anomalies are the cause or the result of SSTAs is discussed. It is also found that ensemble averaging results in a modest improvement in forecast skill. Moreover, in areas where the ensemble forecast anomalies are found to be significantly different from zero in a statistical sense, the anomalies tend to verify well, suggesting a method to estimate a priori regional skill. Overall, the Southern Hemisphere forecasts are more skillful than those in the Northern Hemisphere, perhaps because of a seasonal effect.
Abstract
Eighteen 30-day integrations with the NMC global atmospheric model (T40 resolution) were performed in order to test the impact of sea surface temperature anomalies (SSTAs) on 30-day forecasts for the Northern Hemisphere early summer. The years considered—1987, 1988, and 1989—correspond to a warm El Niño-Southern Oscillation (ENSO) event, a cold ENSO event, and a normal (non-ENSO year), respectively. For each year, 30-day forecasts were started on three successive days around 22 May, using climatological SSTs, and repeated using SSTAs fixed at their initial values.
The results indicate that SSTAs have a clear positive impact on the tropical forecasts and surface fluxes. The impacts on the extratropical forecasts, on the other hand, tend to be positive but small. Larger positive impacts in midlatitudes are obtained only in a case in which the atmospheric anomalous circulation is apparently driven by the ocean anomalies. A simple rule of thumb to distinguish whether quasi-stationary atmospheric anomalies are the cause or the result of SSTAs is discussed. It is also found that ensemble averaging results in a modest improvement in forecast skill. Moreover, in areas where the ensemble forecast anomalies are found to be significantly different from zero in a statistical sense, the anomalies tend to verify well, suggesting a method to estimate a priori regional skill. Overall, the Southern Hemisphere forecasts are more skillful than those in the Northern Hemisphere, perhaps because of a seasonal effect.
Abstract
Monthly mean teleconnections during the northern winter between proxies for tropical heating (OLR and SST data) and Northern Hemisphere 700 mb circulation patterns (PNA, TNH, and WPO) are examined, principally with correlation analysis. In particular it is found that positive projections on all three patterns are highly probable during certain strong ENSO winters but the means to predict their relative strengths was not discovered, although the absolute strength of the TNH pattern is directly related to SST anomalies in the central Pacific. Other ENSO winters also have a tendency for positive PNA and WPO projections, but for a negative TNH projection. For other winters the importance of an area north of the equator and 25 degrees to the west of the date line is confirmed as a probable source region for the PNA pattern. Another area about 25 degrees to the east of the date line is singled out as a possible tropical response to the PNA pattern. Implications for current and future GCM experiments and long-range prediction are discussed.
Abstract
Monthly mean teleconnections during the northern winter between proxies for tropical heating (OLR and SST data) and Northern Hemisphere 700 mb circulation patterns (PNA, TNH, and WPO) are examined, principally with correlation analysis. In particular it is found that positive projections on all three patterns are highly probable during certain strong ENSO winters but the means to predict their relative strengths was not discovered, although the absolute strength of the TNH pattern is directly related to SST anomalies in the central Pacific. Other ENSO winters also have a tendency for positive PNA and WPO projections, but for a negative TNH projection. For other winters the importance of an area north of the equator and 25 degrees to the west of the date line is confirmed as a probable source region for the PNA pattern. Another area about 25 degrees to the east of the date line is singled out as a possible tropical response to the PNA pattern. Implications for current and future GCM experiments and long-range prediction are discussed.
Abstract
Simultaneous and lagged correlation statistics have been calculated between time series of seasonal height anomalies at selected stations and extratropical grid-point anomalies in both hemispheres. The tropical stations in two major tropical precipitation zones, the Indo-China maritime continent and Africa, are well correlated with each other. These stations are also correlated with stations in the North Pacific and Australia, but the coefficients are smaller. The correlations between height anomalies at any of these stations and Northern Hemisphere height anomalies show a well-defined global pattern. Depending upon the location of the stations, the pattern is either a Pacific North American (PNA), a Tropical Northern Hemisphere (TNH) pattern or a mixed pattern having both elements. All three patterns, PNA, TNH and WPO (Western Pacific Oscillation), have been linked to tropical variations. The correlations between height anomalies at these well-correlated stations and the Southern Hemisphere height anomalies at the 500 mb level give the summer teleconnection pattern of Mo and White (1985). The vertical structure of patterns indicate that they are approximately equivalent barotropic.
The TNH pattern tends to be associated more with tropical variability for time scales longer than one season, while the PNA pattern is present in both high- and low-pass filtered analyses, although weakly in the former. Moreover, its low frequency connection to the tropics appears to be confined to ENSO years.
During ENSO years both patterns appear in both simultaneous and lagged maps, but in non-ENSO years, the TNH is weak in simultaneous charts.
Abstract
Simultaneous and lagged correlation statistics have been calculated between time series of seasonal height anomalies at selected stations and extratropical grid-point anomalies in both hemispheres. The tropical stations in two major tropical precipitation zones, the Indo-China maritime continent and Africa, are well correlated with each other. These stations are also correlated with stations in the North Pacific and Australia, but the coefficients are smaller. The correlations between height anomalies at any of these stations and Northern Hemisphere height anomalies show a well-defined global pattern. Depending upon the location of the stations, the pattern is either a Pacific North American (PNA), a Tropical Northern Hemisphere (TNH) pattern or a mixed pattern having both elements. All three patterns, PNA, TNH and WPO (Western Pacific Oscillation), have been linked to tropical variations. The correlations between height anomalies at these well-correlated stations and the Southern Hemisphere height anomalies at the 500 mb level give the summer teleconnection pattern of Mo and White (1985). The vertical structure of patterns indicate that they are approximately equivalent barotropic.
The TNH pattern tends to be associated more with tropical variability for time scales longer than one season, while the PNA pattern is present in both high- and low-pass filtered analyses, although weakly in the former. Moreover, its low frequency connection to the tropics appears to be confined to ENSO years.
During ENSO years both patterns appear in both simultaneous and lagged maps, but in non-ENSO years, the TNH is weak in simultaneous charts.
