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Abstract
The purpose of this study is to evaluate the suitability of using linear drag as a proxy for surface friction in the observed climatological-mean momentum balance over the tropical Pacific Ocean. The linear drag parameterization of kinetic energy dissipation in the planetary boundary layer is widely used in simplified models of the tropical atmosphere, and in numerous observational studies of the surface momentum balance. Climatological seasonal-mean fields of sea level pressure and surface wind from the Comprehensive Ocean-Atmosphere Data Set are used to calculate the pressure gradient, Coriolis, and acceleration terms in the momentum budget; friction is derived as a residual. It is found that when friction is parameterized as a linear dissipation of kinetic energy, the damping time scale for the meridional wind is ∼2−3 times faster than the damping time for the zonal wind. The preceding formulation fits the observations well, especially in the trade-wind regions. It is suggested that the different damping coefficients for the zonal (u) and meridional (v) winds are, in part, a reflection of the different vertical profiles of u and v in the planetary boundary layer.
A realistic simulation of the tropical surface wind field from the observed sea level pressure field is obtained using a linear momentum balance with unequal damping lime scales for u and v. With equal damping times, the meridional component of the surface flow is too strong. Nonlinear advection improves the zonal wind simulation in limited regions of the northeast trades equatorial easterlies, and off South America, but only by ∼0.5 m S1.
Abstract
The purpose of this study is to evaluate the suitability of using linear drag as a proxy for surface friction in the observed climatological-mean momentum balance over the tropical Pacific Ocean. The linear drag parameterization of kinetic energy dissipation in the planetary boundary layer is widely used in simplified models of the tropical atmosphere, and in numerous observational studies of the surface momentum balance. Climatological seasonal-mean fields of sea level pressure and surface wind from the Comprehensive Ocean-Atmosphere Data Set are used to calculate the pressure gradient, Coriolis, and acceleration terms in the momentum budget; friction is derived as a residual. It is found that when friction is parameterized as a linear dissipation of kinetic energy, the damping time scale for the meridional wind is ∼2−3 times faster than the damping time for the zonal wind. The preceding formulation fits the observations well, especially in the trade-wind regions. It is suggested that the different damping coefficients for the zonal (u) and meridional (v) winds are, in part, a reflection of the different vertical profiles of u and v in the planetary boundary layer.
A realistic simulation of the tropical surface wind field from the observed sea level pressure field is obtained using a linear momentum balance with unequal damping lime scales for u and v. With equal damping times, the meridional component of the surface flow is too strong. Nonlinear advection improves the zonal wind simulation in limited regions of the northeast trades equatorial easterlies, and off South America, but only by ∼0.5 m S1.
Abstract
Hourly measurements from 51 moored buoys in the Tropical Atmosphere Ocean array (9°N–8°S, 165°E–95°W) during 1993–2004 are used to document the climatological seasonal and annual mean patterns of diurnal and semidiurnal near-surface wind variability over the tropical Pacific Ocean. In all seasons, the amplitude of the semidiurnal harmonic is approximately twice as large as the diurnal harmonic for the zonal wind component, while the diurnal harmonic is at least 3 times as large as the semidiurnal harmonic for the meridional wind component, both averaged across the buoy array. Except for the eastern equatorial Pacific, the semidiurnal zonal wind harmonic exhibits uniform amplitude (∼0.14 m s−1) and phase [maximum westerly wind anomalies ∼0325/1525 local time (LT)] across the basin in all seasons. This pattern is well explained by atmospheric thermal tidal theory. The semidiurnal zonal wind signal is diminished over the cold surface waters of the eastern equatorial Pacific where it is associated with enhanced boundary layer stability. Diurnal meridional wind variations tend to be out of phase north and south of the equator (maximum southerly wind anomalies ∼0700 LT at 5°N and ∼1900 LT at 5°S), while a noon southerly wind anomaly maximum is observed on the equator in the eastern Pacific particularly during the cold season (June–November). The diurnal meridional wind variations result in enhanced divergence along the equator and convergence along the southern border of the intertropical convergence zone ∼0700 LT (opposite conditions ∼1900 LT); the amplitude of the divergence diurnal cycle is ∼5 × 10−7 s−1. The diurnal meridional wind variations are largely consistent with the diurnal pressure gradient force.
Abstract
Hourly measurements from 51 moored buoys in the Tropical Atmosphere Ocean array (9°N–8°S, 165°E–95°W) during 1993–2004 are used to document the climatological seasonal and annual mean patterns of diurnal and semidiurnal near-surface wind variability over the tropical Pacific Ocean. In all seasons, the amplitude of the semidiurnal harmonic is approximately twice as large as the diurnal harmonic for the zonal wind component, while the diurnal harmonic is at least 3 times as large as the semidiurnal harmonic for the meridional wind component, both averaged across the buoy array. Except for the eastern equatorial Pacific, the semidiurnal zonal wind harmonic exhibits uniform amplitude (∼0.14 m s−1) and phase [maximum westerly wind anomalies ∼0325/1525 local time (LT)] across the basin in all seasons. This pattern is well explained by atmospheric thermal tidal theory. The semidiurnal zonal wind signal is diminished over the cold surface waters of the eastern equatorial Pacific where it is associated with enhanced boundary layer stability. Diurnal meridional wind variations tend to be out of phase north and south of the equator (maximum southerly wind anomalies ∼0700 LT at 5°N and ∼1900 LT at 5°S), while a noon southerly wind anomaly maximum is observed on the equator in the eastern Pacific particularly during the cold season (June–November). The diurnal meridional wind variations result in enhanced divergence along the equator and convergence along the southern border of the intertropical convergence zone ∼0700 LT (opposite conditions ∼1900 LT); the amplitude of the divergence diurnal cycle is ∼5 × 10−7 s−1. The diurnal meridional wind variations are largely consistent with the diurnal pressure gradient force.
Abstract
In the early 1970s, Namias and Born speculated that ocean temperature anomalies created over the deep mixed layer in winter could be preserved in the summer thermocline and reappear at the surface in the following fall or winter. This hypothesis is examined using upper-ocean temperature observations and simulations with a mixed layer model. The data were collected at six ocean weather stations in the North Atlantic and North Pacific. Concurrent and lead-lag correlations are used to investigate temperature variations associated with the seasonal cycle in both the observations and the model simulations.
Concurrent correlations between the surface and subsurface temperature anomalies in both the data and the model indicate that the penetration of temperature anomalies into the ocean is closely tied to the seasonal cycle in mixed layer depth: high correlations extend to relatively deep (shallow) depths in winter (summer). Lead-lag correlations in both the data and the model, at some of the stations, indicate that temperature anomalies beneath the mixed layer in summer are associated with the temperature anomalies in the mixed layer in the previous winter/spring and following fall/winter but are unrelated or weakly opposed to the temperature anomalies in the mixed layer in summer. These results suggest that vertical mixing processes allow ocean temperature anomalies created over a deep mixed layer in winter to be preserved below the surface in summer and reappear at the surface in the following fall, confirming the Namias–Born hypothesis.
Abstract
In the early 1970s, Namias and Born speculated that ocean temperature anomalies created over the deep mixed layer in winter could be preserved in the summer thermocline and reappear at the surface in the following fall or winter. This hypothesis is examined using upper-ocean temperature observations and simulations with a mixed layer model. The data were collected at six ocean weather stations in the North Atlantic and North Pacific. Concurrent and lead-lag correlations are used to investigate temperature variations associated with the seasonal cycle in both the observations and the model simulations.
Concurrent correlations between the surface and subsurface temperature anomalies in both the data and the model indicate that the penetration of temperature anomalies into the ocean is closely tied to the seasonal cycle in mixed layer depth: high correlations extend to relatively deep (shallow) depths in winter (summer). Lead-lag correlations in both the data and the model, at some of the stations, indicate that temperature anomalies beneath the mixed layer in summer are associated with the temperature anomalies in the mixed layer in the previous winter/spring and following fall/winter but are unrelated or weakly opposed to the temperature anomalies in the mixed layer in summer. These results suggest that vertical mixing processes allow ocean temperature anomalies created over a deep mixed layer in winter to be preserved below the surface in summer and reappear at the surface in the following fall, confirming the Namias–Born hypothesis.
Abstract
Precipitation is often quantified by the amount that falls over a given period of time but not the rate at which most of it falls or the rate associated with the most frequent events. Here, three metrics are introduced to distill salient characteristics of typical daily precipitation accumulation based on the full distribution of rainfall: rain amount peak (the rain rate at which the most rain falls), rain frequency peak (the most frequent nonzero rain rate), and rain amount width (a measure of the variability of typical precipitation accumulation). These metrics are applied to two observational datasets to describe the climatology of typical daily precipitation accumulation: GPCP 1° daily (October 1996–October 2015) and TMPA 3B42 (January 1998–October 2015). Results show that the rain frequency peak is similar to total rainfall in terms of geographical pattern and seasonal cycle and varies inversely with rain amount width. In contrast, the rain amount peak varies distinctly, reaching maxima on the outer edges of the regions of high total precipitation, and with less seasonal variation. Despite that GPCP and TMPA 3B42 are both merged satellite–gauge precipitation products, they show substantial differences. In particular, the rain amount peak and rain amount width are uniformly greater in TMPA 3B42 compared to GPCP, and there are large discrepancies in their rain frequency distributions (peak and width). Issues relating to model evaluation are highlighted using CESM1 as an illustrative example and underscore the need for observational datasets incorporating measurements of light rain.
Abstract
Precipitation is often quantified by the amount that falls over a given period of time but not the rate at which most of it falls or the rate associated with the most frequent events. Here, three metrics are introduced to distill salient characteristics of typical daily precipitation accumulation based on the full distribution of rainfall: rain amount peak (the rain rate at which the most rain falls), rain frequency peak (the most frequent nonzero rain rate), and rain amount width (a measure of the variability of typical precipitation accumulation). These metrics are applied to two observational datasets to describe the climatology of typical daily precipitation accumulation: GPCP 1° daily (October 1996–October 2015) and TMPA 3B42 (January 1998–October 2015). Results show that the rain frequency peak is similar to total rainfall in terms of geographical pattern and seasonal cycle and varies inversely with rain amount width. In contrast, the rain amount peak varies distinctly, reaching maxima on the outer edges of the regions of high total precipitation, and with less seasonal variation. Despite that GPCP and TMPA 3B42 are both merged satellite–gauge precipitation products, they show substantial differences. In particular, the rain amount peak and rain amount width are uniformly greater in TMPA 3B42 compared to GPCP, and there are large discrepancies in their rain frequency distributions (peak and width). Issues relating to model evaluation are highlighted using CESM1 as an illustrative example and underscore the need for observational datasets incorporating measurements of light rain.
Abstract
Internal variability in twenty-first-century summer Arctic sea ice loss and its relationship to the large-scale atmospheric circulation is investigated in a 39-member Community Climate System Model, version 3 (CCSM3) ensemble for the period 2000–61. Each member is subject to an identical greenhouse gas emissions scenario and differs only in the atmospheric model component's initial condition.
September Arctic sea ice extent trends during 2020–59 range from −2.0 × 106 to −5.7 × 106 km2 across the 39 ensemble members, indicating a substantial role for internal variability in future Arctic sea ice loss projections. A similar nearly threefold range (from −7.0 × 103 to −19 × 103 km3) is found for summer sea ice volume trends.
Higher rates of summer Arctic sea ice loss in CCSM3 are associated with enhanced transpolar drift and Fram Strait ice export driven by surface wind and sea level pressure patterns. Over the Arctic, the covarying atmospheric circulation patterns resemble the so-called Arctic dipole, with maximum amplitude between April and July. Outside the Arctic, an atmospheric Rossby wave train over the Pacific sector is associated with internal ice loss variability. Interannual covariability patterns between sea ice and atmospheric circulation are similar to those based on trends, suggesting that similar processes govern internal variability over a broad range of time scales. Interannual patterns of CCSM3 ice–atmosphere covariability compare well with those in nature and in the newer CCSM4 version of the model, lending confidence to the results. Atmospheric teleconnection patterns in CCSM3 suggest that the tropical Pacific modulates Arctic sea ice variability via the aforementioned Rossby wave train. Large ensembles with other coupled models are needed to corroborate these CCSM3-based findings.
Abstract
Internal variability in twenty-first-century summer Arctic sea ice loss and its relationship to the large-scale atmospheric circulation is investigated in a 39-member Community Climate System Model, version 3 (CCSM3) ensemble for the period 2000–61. Each member is subject to an identical greenhouse gas emissions scenario and differs only in the atmospheric model component's initial condition.
September Arctic sea ice extent trends during 2020–59 range from −2.0 × 106 to −5.7 × 106 km2 across the 39 ensemble members, indicating a substantial role for internal variability in future Arctic sea ice loss projections. A similar nearly threefold range (from −7.0 × 103 to −19 × 103 km3) is found for summer sea ice volume trends.
Higher rates of summer Arctic sea ice loss in CCSM3 are associated with enhanced transpolar drift and Fram Strait ice export driven by surface wind and sea level pressure patterns. Over the Arctic, the covarying atmospheric circulation patterns resemble the so-called Arctic dipole, with maximum amplitude between April and July. Outside the Arctic, an atmospheric Rossby wave train over the Pacific sector is associated with internal ice loss variability. Interannual covariability patterns between sea ice and atmospheric circulation are similar to those based on trends, suggesting that similar processes govern internal variability over a broad range of time scales. Interannual patterns of CCSM3 ice–atmosphere covariability compare well with those in nature and in the newer CCSM4 version of the model, lending confidence to the results. Atmospheric teleconnection patterns in CCSM3 suggest that the tropical Pacific modulates Arctic sea ice variability via the aforementioned Rossby wave train. Large ensembles with other coupled models are needed to corroborate these CCSM3-based findings.
Abstract
The spatial and temporal patterns of interannual temperature variability within the main thermocline (200–400-m depth) of the Pacific (30°S–60°N) during 1968–97 are documented in two observational datasets and an ocean general circulation model forced with observed winds and air temperatures. Analysis of the processes responsible for the subsurface temperature variance is used to verify the performance of the model and as a basis for assessing the realism of the two observational archives. The subsurface temperature variance is largest in the western portion of the basin, with maxima along the Kuroshio Current Extension and along the equatorward flanks of the subtropical gyres in both hemispheres. In the latter regions, approximately half of the temperature variability may be attributed to local wind-induced Ekman pumping fluctuations one season earlier. A contribution from westward-propagating Rossby waves is also evident in the band 10°–20°N. In contrast, subsurface temperature fluctuations along the Kuroshio Current Extension exhibit little relation to local Ekman pumping variations. Rather, they are linked to basin-scale wind stress curl changes ∼4 yr earlier. Similarities and differences between the two observational subsurface temperature archives are discussed.
Abstract
The spatial and temporal patterns of interannual temperature variability within the main thermocline (200–400-m depth) of the Pacific (30°S–60°N) during 1968–97 are documented in two observational datasets and an ocean general circulation model forced with observed winds and air temperatures. Analysis of the processes responsible for the subsurface temperature variance is used to verify the performance of the model and as a basis for assessing the realism of the two observational archives. The subsurface temperature variance is largest in the western portion of the basin, with maxima along the Kuroshio Current Extension and along the equatorward flanks of the subtropical gyres in both hemispheres. In the latter regions, approximately half of the temperature variability may be attributed to local wind-induced Ekman pumping fluctuations one season earlier. A contribution from westward-propagating Rossby waves is also evident in the band 10°–20°N. In contrast, subsurface temperature fluctuations along the Kuroshio Current Extension exhibit little relation to local Ekman pumping variations. Rather, they are linked to basin-scale wind stress curl changes ∼4 yr earlier. Similarities and differences between the two observational subsurface temperature archives are discussed.
Abstract
This study examines the contribution of tropical sea surface temperature (SST) forcing to the 1976/77 climate transition of the winter atmospheric circulation over the North Pacific using a combined observational and modeling approach. The National Center for Atmospheric Research (NCAR) Community Atmospheric Model version 3 (CAM3) simulates approximately 75% of the observed 4-hPa deepening of the wintertime Aleutian low from 1950–76 to 1977–2000 when forced with the observed evolution of tropical SSTs in a 10-member ensemble average. This response is driven by precipitation increases over the western half of the equatorial Pacific Ocean. In contrast, the NCAR Community Climate Model version 3 (CCM3), the predecessor to CAM3, simulates no significant change in the strength of the Aleutian low when forced with the same tropical SSTs in a 12-member ensemble average. The lack of response in CCM3 is traced to an erroneously large precipitation increase over the tropical Indian Ocean whose dynamical impact is to weaken the Aleutian low; this, when combined with the response to rainfall increases over the western and central equatorial Pacific, results in near-zero net change in the strength of the Aleutian low. The observed distribution of tropical precipitation anomalies associated with the 1976/77 transition, estimated from a combination of direct measurements at land stations and indirect information from surface marine cloudiness and wind divergence fields, supports the models’ simulated rainfall increases over the western half of the Pacific but not the magnitude of CCM3’s rainfall increase over the Indian Ocean.
Abstract
This study examines the contribution of tropical sea surface temperature (SST) forcing to the 1976/77 climate transition of the winter atmospheric circulation over the North Pacific using a combined observational and modeling approach. The National Center for Atmospheric Research (NCAR) Community Atmospheric Model version 3 (CAM3) simulates approximately 75% of the observed 4-hPa deepening of the wintertime Aleutian low from 1950–76 to 1977–2000 when forced with the observed evolution of tropical SSTs in a 10-member ensemble average. This response is driven by precipitation increases over the western half of the equatorial Pacific Ocean. In contrast, the NCAR Community Climate Model version 3 (CCM3), the predecessor to CAM3, simulates no significant change in the strength of the Aleutian low when forced with the same tropical SSTs in a 12-member ensemble average. The lack of response in CCM3 is traced to an erroneously large precipitation increase over the tropical Indian Ocean whose dynamical impact is to weaken the Aleutian low; this, when combined with the response to rainfall increases over the western and central equatorial Pacific, results in near-zero net change in the strength of the Aleutian low. The observed distribution of tropical precipitation anomalies associated with the 1976/77 transition, estimated from a combination of direct measurements at land stations and indirect information from surface marine cloudiness and wind divergence fields, supports the models’ simulated rainfall increases over the western half of the Pacific but not the magnitude of CCM3’s rainfall increase over the Indian Ocean.
Abstract
The approximately century-long instrumental record of precipitation over land reflects a single sampling of internal variability. Thus, the spatiotemporal evolution of the observations is only one realization of “what could have occurred” given the same climate system and boundary conditions but different initial conditions. While climate models can be used to produce initial-condition large ensembles that explicitly sample different sequences of internal variability, an analogous approach is not possible for the real world. Here, we explore the use of a statistical model for monthly precipitation to generate synthetic ensembles based on a single record. When tested within the context of the NCAR Community Earth System Model version 1 Large Ensemble (CESM1-LE), we find that the synthetic ensemble can closely reproduce the spatiotemporal statistics of variability and trends in winter precipitation over the extended contiguous United States and that it is difficult to infer the climate change signal in a single record given the magnitude of the variability. We additionally create a synthetic ensemble based on the Global Precipitation Climatology Centre (GPCC) dataset, termed the GPCC-synth-LE; comparison of the GPCC-synth-LE with the CESM1-based ensembles reveals differences in the spatial structures and magnitudes of variability, highlighting the advantages of an observationally based ensemble. We finally use the GPCC-synth-LE to analyze three water resource metrics in the upper Colorado River basin: frequency of dry, wet, and whiplash years. Thirty-one-year “climatologies” in the GPCC-synth-LE can differ by over 20% in these key water resource metrics due to sampling of internal variability, and individual ensemble members in the GPCC-synth-LE can exhibit large near-monotonic trends over the course of the last century due to sampling of internal variability alone.
Abstract
The approximately century-long instrumental record of precipitation over land reflects a single sampling of internal variability. Thus, the spatiotemporal evolution of the observations is only one realization of “what could have occurred” given the same climate system and boundary conditions but different initial conditions. While climate models can be used to produce initial-condition large ensembles that explicitly sample different sequences of internal variability, an analogous approach is not possible for the real world. Here, we explore the use of a statistical model for monthly precipitation to generate synthetic ensembles based on a single record. When tested within the context of the NCAR Community Earth System Model version 1 Large Ensemble (CESM1-LE), we find that the synthetic ensemble can closely reproduce the spatiotemporal statistics of variability and trends in winter precipitation over the extended contiguous United States and that it is difficult to infer the climate change signal in a single record given the magnitude of the variability. We additionally create a synthetic ensemble based on the Global Precipitation Climatology Centre (GPCC) dataset, termed the GPCC-synth-LE; comparison of the GPCC-synth-LE with the CESM1-based ensembles reveals differences in the spatial structures and magnitudes of variability, highlighting the advantages of an observationally based ensemble. We finally use the GPCC-synth-LE to analyze three water resource metrics in the upper Colorado River basin: frequency of dry, wet, and whiplash years. Thirty-one-year “climatologies” in the GPCC-synth-LE can differ by over 20% in these key water resource metrics due to sampling of internal variability, and individual ensemble members in the GPCC-synth-LE can exhibit large near-monotonic trends over the course of the last century due to sampling of internal variability alone.
Abstract
Empirical orthogonal function analysis of winter sea surface temperature (SST) anomalies over the Pacific domain (60°N–20°S) reveals an El Niño-Southern Oscillation (ENSO) mode that is linked to the eastern North Pacific, and a North Pacific mode that is linearly independent of ENSO. The North Pacific mode exhibits maximum amplitude and variance explained along ∼40°N, west of ∼170°W. SSTs in this region have decreased by ∼1.5°C from 1950 to 1987. The cooling in winter has been associated with a strengthening of the overlying westerly winds.
Abstract
Empirical orthogonal function analysis of winter sea surface temperature (SST) anomalies over the Pacific domain (60°N–20°S) reveals an El Niño-Southern Oscillation (ENSO) mode that is linked to the eastern North Pacific, and a North Pacific mode that is linearly independent of ENSO. The North Pacific mode exhibits maximum amplitude and variance explained along ∼40°N, west of ∼170°W. SSTs in this region have decreased by ∼1.5°C from 1950 to 1987. The cooling in winter has been associated with a strengthening of the overlying westerly winds.
Abstract
Ship observations of sea surface temperature (SST), sea level pressure and surface wind, and satellite measurements of outgoing longwave radiation (OLR) (an indicator of deep tropical convection) are used to describe the large-scale atmospheric circulation over the tropical Pacific during composite warm and cold episodes. Results are based on linear regression analysis between the circulation parameters and an index of SST in the tropical Pacific during the period 1946–85 (1974–89 for OLR). Warm episodes along the Peru coast (i.e., El Niño events) and basin-wide warmings associated with the Southern Oscillation are examined separately. Charts of the total as well as anomalous fields of SST, sea level pressure, surface wind and OLR for both warm and cold episodes are presented.
SST and surface wind anomalies associated with warm episodes are consistent with the results of Rasmusson and Carpenter (1982). El Niño events are characterized by strong positive SST anomalies along the coasts of Ecuador and Peru and along the equator eastward of 130°W, and by an equatorward expansion and intensification of the Inter Tropical Convergence Zone (ITCZ) over the eastern Pacific. Basin-wide warm episodes exhibit positive SST anomalies along the equator eastward of 170°E, a southward expansion and intensification of the ITCZ, and an eastward shift and strengthening of the Indonesian convective zone. The movements of the precipitation zones are in good agreement with anomalous large scale surface wind convergence, Meridional wind anomalies dominate the anomalous surface convergence throughout the tropical Pacific.
Surface winds are consistent with the sea level pressure distribution, with down-gradient flow near the equator, and with Ekman balance in the subtropics. A center of below normal sea level pressure over the equatorial eastern Pacific, distinct from the negative pressure anomalies over the subtropical southeast Pacific, is observed during basin-wide warm episodes. This equatorial feature is highly correlated with local SST and appears to be a boundary layer phenomenon.
There is a net increase in deep convection over the tropical Pacific during warm episodes. Enhanced convection in the ITCZ during warm years is not accompanied by a net increase in surface wind convergence. A comparison between precipitation and surface wind convergence suggests that moisture convergence extends through a deeper layer in the equatorial western Pacific than in the ITCZ over the eastern Pacific.
The contrasting distributions of surface relative humidity, total cloudiness and air-sea temperature difference over the eastern tropical Pacific during basin-wide warm and cold episodes are described in the context of boundary layer processes.
Abstract
Ship observations of sea surface temperature (SST), sea level pressure and surface wind, and satellite measurements of outgoing longwave radiation (OLR) (an indicator of deep tropical convection) are used to describe the large-scale atmospheric circulation over the tropical Pacific during composite warm and cold episodes. Results are based on linear regression analysis between the circulation parameters and an index of SST in the tropical Pacific during the period 1946–85 (1974–89 for OLR). Warm episodes along the Peru coast (i.e., El Niño events) and basin-wide warmings associated with the Southern Oscillation are examined separately. Charts of the total as well as anomalous fields of SST, sea level pressure, surface wind and OLR for both warm and cold episodes are presented.
SST and surface wind anomalies associated with warm episodes are consistent with the results of Rasmusson and Carpenter (1982). El Niño events are characterized by strong positive SST anomalies along the coasts of Ecuador and Peru and along the equator eastward of 130°W, and by an equatorward expansion and intensification of the Inter Tropical Convergence Zone (ITCZ) over the eastern Pacific. Basin-wide warm episodes exhibit positive SST anomalies along the equator eastward of 170°E, a southward expansion and intensification of the ITCZ, and an eastward shift and strengthening of the Indonesian convective zone. The movements of the precipitation zones are in good agreement with anomalous large scale surface wind convergence, Meridional wind anomalies dominate the anomalous surface convergence throughout the tropical Pacific.
Surface winds are consistent with the sea level pressure distribution, with down-gradient flow near the equator, and with Ekman balance in the subtropics. A center of below normal sea level pressure over the equatorial eastern Pacific, distinct from the negative pressure anomalies over the subtropical southeast Pacific, is observed during basin-wide warm episodes. This equatorial feature is highly correlated with local SST and appears to be a boundary layer phenomenon.
There is a net increase in deep convection over the tropical Pacific during warm episodes. Enhanced convection in the ITCZ during warm years is not accompanied by a net increase in surface wind convergence. A comparison between precipitation and surface wind convergence suggests that moisture convergence extends through a deeper layer in the equatorial western Pacific than in the ITCZ over the eastern Pacific.
The contrasting distributions of surface relative humidity, total cloudiness and air-sea temperature difference over the eastern tropical Pacific during basin-wide warm and cold episodes are described in the context of boundary layer processes.
