Search Results
You are looking at 1 - 10 of 101 items for
- Author or Editor: Michael J. McPhaden x
- Refine by Access: All Content x
An El Niño of moderate intensity developed in the tropical Pacific in 2002/03. This event, though not as strong as the 1997/98 El Niño, had significant impacts on patterns of weather variability worldwide. The evolution of the 2002/03 El Niño is documented through comprehensive satellite and in situ observations from the El Niño-Southern Oscillation (ENSO) Observing System. These observations underscore the importance of both episodic atmospheric forcing and large-scale low-frequency ocean–atmosphere interactions in the development of the event.
An El Niño of moderate intensity developed in the tropical Pacific in 2002/03. This event, though not as strong as the 1997/98 El Niño, had significant impacts on patterns of weather variability worldwide. The evolution of the 2002/03 El Niño is documented through comprehensive satellite and in situ observations from the El Niño-Southern Oscillation (ENSO) Observing System. These observations underscore the importance of both episodic atmospheric forcing and large-scale low-frequency ocean–atmosphere interactions in the development of the event.
Abstract
The purpose of this study is to document the zonal evolution of processes affecting sea surface temperature (SST) variability on intraseasonal timescales in the equatorial Pacific Ocean. Data primarily from the Tropical Atmosphere Ocean (TAO) array of moored buoys are used, focusing on four sites along the equator with decade-long time series. These sites are located in the western Pacific warm pool (165°E), the eastern Pacific equatorial cold tongue (110° and 140°W), and the transition zone between these two regions (170°W). Results indicate that SST variability on intraseasonal timescales is most significantly influenced by local surface heat fluxes in the western Pacific (165°E), zonal advection in the central Pacific (170°W), and vertical advection and entrainment in the eastern Pacific (110° and 140°W). East of the date line, oceanic equatorial Kelvin waves strongly mediate dynamical processes controlling intraseasonal SSTs variations, while surface fluxes tend to damp these dynamically generated SSTs at a rate of about 20 W m−2 °C−1. The details of coupling between Kelvin wave dynamics and mixed layer processes make for complicated SST phasing along the equator. While thermocline temperatures propagate eastward at Kelvin wave speeds in the central and eastern Pacific, SSTs can develop in phase over thousands of kilometers, or may even appear to propagate westward. Implications of these results for understanding the dynamical connection between intraseasonal and interannual variability are discussed.
Abstract
The purpose of this study is to document the zonal evolution of processes affecting sea surface temperature (SST) variability on intraseasonal timescales in the equatorial Pacific Ocean. Data primarily from the Tropical Atmosphere Ocean (TAO) array of moored buoys are used, focusing on four sites along the equator with decade-long time series. These sites are located in the western Pacific warm pool (165°E), the eastern Pacific equatorial cold tongue (110° and 140°W), and the transition zone between these two regions (170°W). Results indicate that SST variability on intraseasonal timescales is most significantly influenced by local surface heat fluxes in the western Pacific (165°E), zonal advection in the central Pacific (170°W), and vertical advection and entrainment in the eastern Pacific (110° and 140°W). East of the date line, oceanic equatorial Kelvin waves strongly mediate dynamical processes controlling intraseasonal SSTs variations, while surface fluxes tend to damp these dynamically generated SSTs at a rate of about 20 W m−2 °C−1. The details of coupling between Kelvin wave dynamics and mixed layer processes make for complicated SST phasing along the equator. While thermocline temperatures propagate eastward at Kelvin wave speeds in the central and eastern Pacific, SSTs can develop in phase over thousands of kilometers, or may even appear to propagate westward. Implications of these results for understanding the dynamical connection between intraseasonal and interannual variability are discussed.
Abstract
The equatorial subsurface countercurrents (SSCC are strong, steady, geostrophically balanced eastward flows situated below the high speed core of the Equatorial Undercurrent (EUC) at ∼3–5°N and S. The dynamics of these currents are explored using a continuously stratified, vertically diffusive, linear, steady state ocean model forced by zonal winds with effectively no wind stress curl. Model results agree favorably with observations in that both EUC- and SSCC-like structures are generated.
A diagnosis of the model momentum, vorticity and continuity balances at various depths and latitudes reveals that the SSCC lie outside a vertically diffusive equatorial momentum boundary layer so that both components of velocity are geostrophically balanced. They are, however, located at the poleward of a broader diffusive equatorial vorticity boundary layer. Within this boundary layer, cyclonic vorticity associated with the EUC diffuses to the level of the SSCC where it is balanced by poleward advection of planetary vorticity. Outside this boundary layer, the induced planetary vorticity advection is balanced by vortex stretching that weakens the temperature stratification to generate a thermostad-like structure. The SSCC are in turn geostrophically balanced by the meridional pressure gradients associated with this structure.
Abstract
The equatorial subsurface countercurrents (SSCC are strong, steady, geostrophically balanced eastward flows situated below the high speed core of the Equatorial Undercurrent (EUC) at ∼3–5°N and S. The dynamics of these currents are explored using a continuously stratified, vertically diffusive, linear, steady state ocean model forced by zonal winds with effectively no wind stress curl. Model results agree favorably with observations in that both EUC- and SSCC-like structures are generated.
A diagnosis of the model momentum, vorticity and continuity balances at various depths and latitudes reveals that the SSCC lie outside a vertically diffusive equatorial momentum boundary layer so that both components of velocity are geostrophically balanced. They are, however, located at the poleward of a broader diffusive equatorial vorticity boundary layer. Within this boundary layer, cyclonic vorticity associated with the EUC diffuses to the level of the SSCC where it is balanced by poleward advection of planetary vorticity. Outside this boundary layer, the induced planetary vorticity advection is balanced by vortex stretching that weakens the temperature stratification to generate a thermostad-like structure. The SSCC are in turn geostrophically balanced by the meridional pressure gradients associated with this structure.
Abstract
Two linearized, vertically diffusive steady-state models are formulated on an equatorial β-plane. The purpose is (a) to investigate the vertical boundary-layer structure in a continuously stratified ocean spanning the equator and (b) to test the sensitivity of the results to different turbulence parameterizations. Both models are analytically tractable in a horizontally unbounded basin. One is characterized by Newtonian cooling, the other has biharmonic friction. For either model, the equations are analogous to the well-known equations governing equatorial wave motion. This analogy is exploited in both obtaining and interpreting the solutions.
In both models, zonal wind forcing leads to features such as the Equatorial Undercurrent, South Equatorial Current and Equatorial Intermediate Current. Structures resembling the recently discovered subsurface countercurrents are also generated. The depth, velocity and other scales are model dependent but the basic dynamics are not. Specifically, near the equator, Ekman layers are well behaved due to the presence of baroclinic meridional pressure gradients while zonal flow below the equatorial Ekman layer is geostrophic and vertically diffusive.
The response to a zonally varying sea surface temperature anomaly is two orders of magnitude stronger in the equatorial ocean than at higher latitudes. Moreover, near the equator, the thermally forced solution is comparable in both magnitude and spatial structure to the wind-forced solution. This suggests an important role for the surface mixed layer in determining subsurface equatorial flow patterns.
Abstract
Two linearized, vertically diffusive steady-state models are formulated on an equatorial β-plane. The purpose is (a) to investigate the vertical boundary-layer structure in a continuously stratified ocean spanning the equator and (b) to test the sensitivity of the results to different turbulence parameterizations. Both models are analytically tractable in a horizontally unbounded basin. One is characterized by Newtonian cooling, the other has biharmonic friction. For either model, the equations are analogous to the well-known equations governing equatorial wave motion. This analogy is exploited in both obtaining and interpreting the solutions.
In both models, zonal wind forcing leads to features such as the Equatorial Undercurrent, South Equatorial Current and Equatorial Intermediate Current. Structures resembling the recently discovered subsurface countercurrents are also generated. The depth, velocity and other scales are model dependent but the basic dynamics are not. Specifically, near the equator, Ekman layers are well behaved due to the presence of baroclinic meridional pressure gradients while zonal flow below the equatorial Ekman layer is geostrophic and vertically diffusive.
The response to a zonally varying sea surface temperature anomaly is two orders of magnitude stronger in the equatorial ocean than at higher latitudes. Moreover, near the equator, the thermally forced solution is comparable in both magnitude and spatial structure to the wind-forced solution. This suggests an important role for the surface mixed layer in determining subsurface equatorial flow patterns.
Abstract
A combination of satellite and in situ datasets is used to investigate the impact of interannual changes in atmospheric dust content on the sea surface temperature (SST) of the tropical North Atlantic Ocean. Throughout most of the region the authors find, in agreement with previous studies, that positive anomalies of dust are associated with a significant reduction in surface shortwave radiation (SWR), while negative anomalies of dust are associated with an enhancement of SWR. Statistical analysis for 1984–2000 suggests that changes in dustiness in the tropical North Atlantic (10°–25°N, 20°–60°W) explained approximately 35% of the observed interannual SST variability during boreal summer, when climatological dust concentrations are highest. Measurements from a long-term moored buoy in the central tropical North Atlantic are used to investigate the causes of anomalously cool SST that occurred in conjunction with a period of enhanced dustiness at the start of the unexpectedly quiet 2006 hurricane season. It is found that surface SWR varied out of phase with dustiness, consistent with historical analyses. However, most of the anomalous cooling occurred prior to the period of enhanced dustiness and was driven primarily by wind-induced latent heat loss, with horizontal oceanic heat advection and SWR playing secondary roles. These results indicate that dust-induced changes in SWR did not play a major direct role in the cooling that led up to the 2006 Atlantic hurricane season.
Abstract
A combination of satellite and in situ datasets is used to investigate the impact of interannual changes in atmospheric dust content on the sea surface temperature (SST) of the tropical North Atlantic Ocean. Throughout most of the region the authors find, in agreement with previous studies, that positive anomalies of dust are associated with a significant reduction in surface shortwave radiation (SWR), while negative anomalies of dust are associated with an enhancement of SWR. Statistical analysis for 1984–2000 suggests that changes in dustiness in the tropical North Atlantic (10°–25°N, 20°–60°W) explained approximately 35% of the observed interannual SST variability during boreal summer, when climatological dust concentrations are highest. Measurements from a long-term moored buoy in the central tropical North Atlantic are used to investigate the causes of anomalously cool SST that occurred in conjunction with a period of enhanced dustiness at the start of the unexpectedly quiet 2006 hurricane season. It is found that surface SWR varied out of phase with dustiness, consistent with historical analyses. However, most of the anomalous cooling occurred prior to the period of enhanced dustiness and was driven primarily by wind-induced latent heat loss, with horizontal oceanic heat advection and SWR playing secondary roles. These results indicate that dust-induced changes in SWR did not play a major direct role in the cooling that led up to the 2006 Atlantic hurricane season.
Abstract
Previous studies have described the impacts of wind stress variations in the eastern Pacific on sea surface temperature (SST) anomalies associated with the El Niño–Southern Oscillation (ENSO) phenomenon. However, these studies have usually focused on individual El Niño events and typically have not considered impacts on La Niña—the cold phase of the ENSO cycle. This paper examines effects of wind stress and heat flux forcing on interannual SST variations in the eastern equatorial Pacific from sensitivity tests using an ocean general circulation model over the period 1980–2002. Results indicate that in the Niño-3 region (5°N–5°S, 90°–150°W) a zonal wind stress anomaly of 0.01 N m−2 leads to about 1°C SST anomaly and that air–sea heat fluxes tend to damp interannual SST anomalies generated by other physical processes at a rate of about 40 W m−2 (°C)−1. These results systematically quantify expectations from previous event specific numerical model studies that local forcing in the eastern Pacific can significantly affect the evolution of both warm and cold phases of the ENSO cycle. The results are also consistent with a strictly empirical analysis that indicates that a wind stress anomaly of 0.01 N m−2 leads to ∼1°C SST anomaly in the Niño-3 region.
Abstract
Previous studies have described the impacts of wind stress variations in the eastern Pacific on sea surface temperature (SST) anomalies associated with the El Niño–Southern Oscillation (ENSO) phenomenon. However, these studies have usually focused on individual El Niño events and typically have not considered impacts on La Niña—the cold phase of the ENSO cycle. This paper examines effects of wind stress and heat flux forcing on interannual SST variations in the eastern equatorial Pacific from sensitivity tests using an ocean general circulation model over the period 1980–2002. Results indicate that in the Niño-3 region (5°N–5°S, 90°–150°W) a zonal wind stress anomaly of 0.01 N m−2 leads to about 1°C SST anomaly and that air–sea heat fluxes tend to damp interannual SST anomalies generated by other physical processes at a rate of about 40 W m−2 (°C)−1. These results systematically quantify expectations from previous event specific numerical model studies that local forcing in the eastern Pacific can significantly affect the evolution of both warm and cold phases of the ENSO cycle. The results are also consistent with a strictly empirical analysis that indicates that a wind stress anomaly of 0.01 N m−2 leads to ∼1°C SST anomaly in the Niño-3 region.
Abstract
Measurements from three long-term moored buoys are used to investigate the impact of barrier layer thickness (BLT) on the seasonal cycle of sea surface temperature (SST) in the central tropical North Atlantic Ocean. It is found that seasonal variations of the BLT exert a considerable influence on SST through their modulation of the vertical heat flux at the base of the mixed layer, estimated as the residual in the mixed layer heat balance. Cooling associated with this term is strongest when the barrier layer is thin and the vertical temperature gradient at the base of the mixed layer is strong. Conversely, thick barrier layers are associated with a significant reduction in the vertical temperature gradient at the base of the mixed layer, which suppresses the upward transfer of cooler water into the mixed layer. Forced ocean and coupled ocean–atmosphere models that do not properly simulate the barrier layer may have difficulty reproducing the observed seasonal cycle of SST in the tropical North Atlantic.
Abstract
Measurements from three long-term moored buoys are used to investigate the impact of barrier layer thickness (BLT) on the seasonal cycle of sea surface temperature (SST) in the central tropical North Atlantic Ocean. It is found that seasonal variations of the BLT exert a considerable influence on SST through their modulation of the vertical heat flux at the base of the mixed layer, estimated as the residual in the mixed layer heat balance. Cooling associated with this term is strongest when the barrier layer is thin and the vertical temperature gradient at the base of the mixed layer is strong. Conversely, thick barrier layers are associated with a significant reduction in the vertical temperature gradient at the base of the mixed layer, which suppresses the upward transfer of cooler water into the mixed layer. Forced ocean and coupled ocean–atmosphere models that do not properly simulate the barrier layer may have difficulty reproducing the observed seasonal cycle of SST in the tropical North Atlantic.
Abstract
The authors use a new and novel heat balance formalism for the upper 50 m of the Niño-3 region (5°N–5°S, 90°–150°W) to investigate the oceanographic processes underlying interannual sea surface temperature (SST) variations in the eastern equatorial Pacific. The focus is on a better understanding of the relationship between local and remote atmospheric forcing in generating SST anomalies associated with El Niño–Southern Oscillation (ENSO) events. The heat balance analysis indicates that heat advection across 50-m depth and across 150°W are the important oceanic mechanisms responsible for temperature variations with the former being dominant. On the other hand, net surface heat flux adjusted for penetrative radiation damps SST. Jointly, these processes can explain most of interannual variations in temperature tendency averaged over the Niño-3 region. Decomposition of vertical advection across the bottom indicates that the mean seasonal advection of anomalous temperature (the so-called thermocline feedback) dominates and is highly correlated with 20°C isotherm depth variations, which are mainly forced by remote winds in the western and central equatorial Pacific. Temperature advection by anomalous vertical velocity (the “Ekman feedback”), which is highly correlated with local zonal wind stress variations, is smaller with an amplitude of about 40% on average of remotely forced vertical heat advection. These results support those of recent empirical and modeling studies in which local atmospheric forcing, while not dominant, significantly affects ENSO SST variations in the eastern equatorial Pacific.
Abstract
The authors use a new and novel heat balance formalism for the upper 50 m of the Niño-3 region (5°N–5°S, 90°–150°W) to investigate the oceanographic processes underlying interannual sea surface temperature (SST) variations in the eastern equatorial Pacific. The focus is on a better understanding of the relationship between local and remote atmospheric forcing in generating SST anomalies associated with El Niño–Southern Oscillation (ENSO) events. The heat balance analysis indicates that heat advection across 50-m depth and across 150°W are the important oceanic mechanisms responsible for temperature variations with the former being dominant. On the other hand, net surface heat flux adjusted for penetrative radiation damps SST. Jointly, these processes can explain most of interannual variations in temperature tendency averaged over the Niño-3 region. Decomposition of vertical advection across the bottom indicates that the mean seasonal advection of anomalous temperature (the so-called thermocline feedback) dominates and is highly correlated with 20°C isotherm depth variations, which are mainly forced by remote winds in the western and central equatorial Pacific. Temperature advection by anomalous vertical velocity (the “Ekman feedback”), which is highly correlated with local zonal wind stress variations, is smaller with an amplitude of about 40% on average of remotely forced vertical heat advection. These results support those of recent empirical and modeling studies in which local atmospheric forcing, while not dominant, significantly affects ENSO SST variations in the eastern equatorial Pacific.
Abstract
This study compares the Tropical Rainfall Measuring Mission (TRMM) microwave imager (TMI) and precipitation radar (PR) rainfall measurements to self-siphoning rain gauge data from 14 open-ocean buoys located in heavy-rain areas of the tropical Pacific and Atlantic Oceans. These 14 buoys are part of the Tropical Atmosphere–Ocean (TAO) array and Pilot Research Moored Array in the Tropical Atlantic (PIRATA). Differences between buoy and TRMM monthly and seasonal rainfall accumulations are calculated from satellite data within 0.1° × 0.1°–5.0° × 5.0° square areas centered on the buoys. Taking into account current best estimates of sampling and instrumental errors, mean differences between the buoy and TMI rainfall are not significant at the 95% confidence level, assuming no wind-induced undercatch by the buoy gauges. Mean differences between the buoy and PR monthly and seasonal accumulations for these spatial scales suggest that the PR underestimates these accumulations by about 30% in comparison with the buoys. If the buoy rain rates are corrected for wind-induced undercatch, TMI accumulations fall systematically and significantly below buoy values, with underestimates of up to 22% for both monthly and seasonal data. Also the PR underestimates, relative to wind-corrected buoy values, increase to up to 40% for both monthly and seasonal data. Regional and rain-rate dependencies of these comparisons are also investigated.
Abstract
This study compares the Tropical Rainfall Measuring Mission (TRMM) microwave imager (TMI) and precipitation radar (PR) rainfall measurements to self-siphoning rain gauge data from 14 open-ocean buoys located in heavy-rain areas of the tropical Pacific and Atlantic Oceans. These 14 buoys are part of the Tropical Atmosphere–Ocean (TAO) array and Pilot Research Moored Array in the Tropical Atlantic (PIRATA). Differences between buoy and TRMM monthly and seasonal rainfall accumulations are calculated from satellite data within 0.1° × 0.1°–5.0° × 5.0° square areas centered on the buoys. Taking into account current best estimates of sampling and instrumental errors, mean differences between the buoy and TMI rainfall are not significant at the 95% confidence level, assuming no wind-induced undercatch by the buoy gauges. Mean differences between the buoy and PR monthly and seasonal accumulations for these spatial scales suggest that the PR underestimates these accumulations by about 30% in comparison with the buoys. If the buoy rain rates are corrected for wind-induced undercatch, TMI accumulations fall systematically and significantly below buoy values, with underestimates of up to 22% for both monthly and seasonal data. Also the PR underestimates, relative to wind-corrected buoy values, increase to up to 40% for both monthly and seasonal data. Regional and rain-rate dependencies of these comparisons are also investigated.
