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- Author or Editor: Michael McPhaden x
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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
Time series data from the Tropical Atmosphere–Ocean (TAO) array of moored buoys and data from other sources are used to document the oceanic dynamical response to intraseasonal (periods centered near 60 days) wind stress forcing in the equatorial Pacific. We focus on the period from October 1996 to December 1997, encompassing the time just prior to and during the onset of the 1997–98 El Niño, when both atmospheric forcing and ocean response were exceptionally strong.
Analysis reveals that the oceanic response to intraseasonal wind forcing is characterized by two district regimes. West of the international date line, the response is dominated by reversing zonal jets in both time and depth in the upper 200 m. East of the date line, variability is controlled primarily by first baroclinic mode equatorial Kelvin waves propagating eastward at phase speeds of 2.4–2.7 m s−1. However, amplitude structures in the eastern Pacific deviate from those expected for first baroclinic Kelvin waves in a resting ocean, suggesting that the wave field may be sensitive to the presence of a zonally sloping thermocline and the Equatorial Undercurrent. In addition, upward phase propagation, consistent with downward energy propagation, suggests the presence of some higher baroclinic mode wind-forced wave energy in the eastern Pacific. The implications of these results for understanding the evolution of the 1997–98 El Niño are discussed.
Abstract
Time series data from the Tropical Atmosphere–Ocean (TAO) array of moored buoys and data from other sources are used to document the oceanic dynamical response to intraseasonal (periods centered near 60 days) wind stress forcing in the equatorial Pacific. We focus on the period from October 1996 to December 1997, encompassing the time just prior to and during the onset of the 1997–98 El Niño, when both atmospheric forcing and ocean response were exceptionally strong.
Analysis reveals that the oceanic response to intraseasonal wind forcing is characterized by two district regimes. West of the international date line, the response is dominated by reversing zonal jets in both time and depth in the upper 200 m. East of the date line, variability is controlled primarily by first baroclinic mode equatorial Kelvin waves propagating eastward at phase speeds of 2.4–2.7 m s−1. However, amplitude structures in the eastern Pacific deviate from those expected for first baroclinic Kelvin waves in a resting ocean, suggesting that the wave field may be sensitive to the presence of a zonally sloping thermocline and the Equatorial Undercurrent. In addition, upward phase propagation, consistent with downward energy propagation, suggests the presence of some higher baroclinic mode wind-forced wave energy in the eastern Pacific. The implications of these results for understanding the evolution of the 1997–98 El Niño are discussed.
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.
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
Recent observations have shown evidence of intraseasonal oscillations (with periods of approximately 1–2 months) in the northern and southern tropical Atlantic trade winds. In this paper, the oceanic response to the observed intraseasonal wind variability is addressed through an analysis of the surface mixed layer heat balance, focusing on three locations in the northwestern tropical Atlantic where in situ measurements from moored buoys are available (14.5°N, 51°W; 15°N, 38°W; and 18°N, 34°W). It is found that local heat storage at all three locations is balanced primarily by wind-induced latent heat loss, which is the same mechanism that is believed to play a dominant role on interannual and decadal time scales in the region. It is also found that the intraseasonal wind speed oscillations are linked to changes in surface wind convergence and convection over the western equatorial Atlantic warm pool. These atmospheric circulation anomalies and wind-induced SST anomalies potentially feed back on one another to affect longer time-scale variability in the region.
Abstract
Recent observations have shown evidence of intraseasonal oscillations (with periods of approximately 1–2 months) in the northern and southern tropical Atlantic trade winds. In this paper, the oceanic response to the observed intraseasonal wind variability is addressed through an analysis of the surface mixed layer heat balance, focusing on three locations in the northwestern tropical Atlantic where in situ measurements from moored buoys are available (14.5°N, 51°W; 15°N, 38°W; and 18°N, 34°W). It is found that local heat storage at all three locations is balanced primarily by wind-induced latent heat loss, which is the same mechanism that is believed to play a dominant role on interannual and decadal time scales in the region. It is also found that the intraseasonal wind speed oscillations are linked to changes in surface wind convergence and convection over the western equatorial Atlantic warm pool. These atmospheric circulation anomalies and wind-induced SST anomalies potentially feed back on one another to affect longer time-scale variability in the region.
Abstract
This paper describes observed changes in surface winds, sea surface temperature (SST), and the volume of water warmer than 20°C (WWV) in the equatorial Pacific Ocean for the period 1980–99. The purpose is to test recent hypotheses about the relationship between variations in WWV and the El Niño–Southern Oscillation (ENSO) cycle. The results confirm inferences based on theory, models, and previous empirical analyses using proxy data (namely sea level) that ENSO involves a recharge and discharge of WWV along the equator and that the cyclic nature of ENSO results from a disequilibrium between zonal winds and zonal mean thermocline depth. The authors also find that the magnitude of ENSO SST anomalies is directly related to the magnitude of zonal mean WWV anomalies. Furthermore, for a given change in equatorial WWV, the corresponding warm El Niño SST anomalies are larger than the corresponding cold La Niña anomalies. This asymmetry between the warm and cold phases of the ENSO cycle implies differences in the relative importance of physical processes controlling SST during El Niño and La Niña events.
Abstract
This paper describes observed changes in surface winds, sea surface temperature (SST), and the volume of water warmer than 20°C (WWV) in the equatorial Pacific Ocean for the period 1980–99. The purpose is to test recent hypotheses about the relationship between variations in WWV and the El Niño–Southern Oscillation (ENSO) cycle. The results confirm inferences based on theory, models, and previous empirical analyses using proxy data (namely sea level) that ENSO involves a recharge and discharge of WWV along the equator and that the cyclic nature of ENSO results from a disequilibrium between zonal winds and zonal mean thermocline depth. The authors also find that the magnitude of ENSO SST anomalies is directly related to the magnitude of zonal mean WWV anomalies. Furthermore, for a given change in equatorial WWV, the corresponding warm El Niño SST anomalies are larger than the corresponding cold La Niña anomalies. This asymmetry between the warm and cold phases of the ENSO cycle implies differences in the relative importance of physical processes controlling SST during El Niño and La Niña events.
