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Michael J. McPhaden
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Michael J. McPhaden

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.

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Michael J. McPhaden

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.

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Michael J. McPhaden

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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.

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Michael J. McPhaden

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.

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Xuebin Zhang and Michael J. McPhaden

Abstract

Vertical advection of temperature is the primary mechanism by which El Niño–Southern Oscillation (ENSO) time-scale sea surface temperature (SST) anomalies are generated in the eastern equatorial Pacific. Variations in vertical advection are mediated primarily by remote wind-forced thermocline displacements, which control the temperature of water upwelled to the surface. However, during some ENSO events, large wind stress variations occur in the eastern Pacific that in principle should affect local upwelling rates, the depth of the thermocline, and SST. In this study, the impact of these wind stress variations on the eastern equatorial Pacific is addressed using multiple linear regression analysis and a linear equatorial wave model. The regression analysis indicates that a zonal wind stress anomaly of 0.01 N m−2 leads to approximately a 1°C SST anomaly over the Niño-3 region (5°N–5°S, 90°–150°W) due to changes in local upwelling rates. Wind stress variations of this magnitude occurred in the eastern Pacific during the 1982/83 and 1997/98 El Niños, accounting for about 1/3 of the maximum SST anomaly during these events. The linear equatorial wave model also indicates that depending on the period in question, zonal wind stress variations in the eastern Pacific can work either with or against remote wind stress forcing from the central and western Pacific to determine the thermocline depth in the eastern Pacific. Thus, zonal wind stress variations in the eastern Pacific contribute to the generation of interannual SST anomalies through both changes in local upwelling rates and changes in thermocline depth. Positive feedbacks between the ocean and atmosphere in the eastern Pacific are shown to influence the evolution of the surface wind field, especially during strong El Niño events, emphasizing the coupled nature of variability in the region. Implications of these results for understanding the character of event-to-event differences in El Niño and La Niña are discussed.

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Chidong Zhang and Michael J. McPhaden

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Three idealized models for the surface structure of the Madden–Julian oscillation (MJO) were summarized from observations, numerical simulations, and theories, which demonstrate contrasting phase relationships of surface variables. To explore which model represents the most commonly observed features of the MJO in the western Pacific, in situ observations from moored buoys and satellite data were used to construct composite time series for components of surface fluxes of heat, momentum, and buoyancy during intraseasonal cooling episodes, defined as periods when the ocean loses heat through the surface. The composites show a near in-phase relationship among maxima in net surface cooling, latent heat flux, precipitation rate, wind stress, westerly wind, and minima in solar radiation flux and net buoyancy flux. The phase of net buoyancy flux is determined by the net heat flux, whereas its magnitude is substantially compensated by freshwater flux. During the composite cooling episode, both the oceanic isothermal layer and mixed layer become deeper, the barrier layer becomes thinner, and the sea surface becomes cooler. The strength of atmospheric forcing and the oceanic response increases with the length of the cooling episodes. The phase relationships found in the composites are consistent with one of the three MJO models and with some previous studies based on observations and global model analyses but are inconsistent with others. Possible reasons for the disagreement among different studies and the implications of the disagreement are discussed.

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Michael J. McPhaden and Hartmut Peters

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A diurnal cycle in temperature and vertical displacement variance has been observed in the stratified region below the surface mixed layer using moored time-series data at 0°, 140°W for there periods: November 1984, April 1987, and May–June 1987. The November 1984 and April 1987 periods coincided with the TROPIC HEAT and TROPIC HEAT-2 experiments, during which direct measurements of turbulent dissipation rates were made near the mooring site. In May–June 1987, a special set of moored time series were collected between 30-m and 61-m depth with 1-minute temporal resolution in addition to standard measurements at 15-minute resolution. The high-resolution data indicated the existence of a diurnal cycle in variance that was most pronounced at frequencies of 10–30 cph and that was coherent over the 31-m extent of the vertical array. It is likely that this diurnal cycle in variance was due in part to internal waves remotely generated at the base of the nighttime mixed layer and that the appearance of internal waves in the thermocline at frequencies higher than the local Väisälä frequency (about 2–7 cph) in May–June 1987 was due to Doppler shifting by the Equatorial Undercurrent. It is also likely that part of the observed diurnal cycle in variance was due to local shear instabilities in the Equatorial Undercurrent that may have been triggered by the diurnally modulated internal wave field. More coarsely resolved 15-minute moored time-series data from November 1984, April 1987, and May–June 1987 indicated the presence of a diurnal cycle after averaging over at least 30 days of data. Largest diurnal ranges in temperature and vertical displacement variance were typically observed in November 1984 when wind speed, zonal wind stress, and mean vertical shear were largest and when the mean gradient Richardson number was smallest. The diurnal cycle in turbulent dissipation rate had a larger amplitude in November 1984 than in April 1987, consistent with a dynamical connection between internal wave variability and turbulence in the equatorial thermocline.

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Xuri Yu and Michael J. McPhaden

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Five to nine years of observations from the Tropical Atmosphere Ocean array in the tropical Pacific are used to document the seasonal cycle of surface winds, zonal currents, SST, thermocline depth, and dynamic height. Along the equator, the normally westward surface zonal current reverses direction in late boreal spring and early summer. This seasonal variation in the zonal surface current propagates westward, as do seasonal variations in the Equatorial Undercurrent and zonal surface winds at the equator. At 5°N and 5°S, the seasonal variations in the 20°C isotherm and dynamic height also propagate westward. Conversely at the equator in the eastern and central Pacific, the variations in 20°C isotherm depth and dynamic height propagate eastward.

These seasonal variations are interpreted by means of a simple dynamical model based on linear equatorial wave theory. Model results indicate that seasonal variability between 5°N and 5°S is dominated by wind-forced equatorial Kelvin waves and first meridional mode Rossby waves of the first and second baroclinic modes. The sum of the Rossby waves and Kelvin waves results in westward propagation in the equatorial zonal currents and off-equatorial thermal structure and, yet, eastward propagation in thermocline depth and dynamic height along the equator in the eastern and central Pacific.

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Xuebin Zhang and Michael J. McPhaden

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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.

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