Search Results

You are looking at 1 - 10 of 100 items for

  • Author or Editor: Michael McPhaden x
  • Refine by Access: All Content x
Clear All Modify Search
Michael J. McPhaden
Full access
Kunio Kutsuwada and Michael McPhaden

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.

Full access
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.

Full access
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.

Full access
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.

Full access
Michael J. McPhaden

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.

Full access
Chidong Zhang and Michael J. McPhaden

Abstract

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.

Full access
Lisan Yu and Michael J. McPhaden

Abstract

An in-depth data analysis was conducted to understand the occurrence of a strong sea surface temperature (SST) front in the central Bay of Bengal before the formation of Cyclone Nargis in April 2008. Nargis changed its course after encountering the front and tracked along the front until making landfall. One unique feature of this SST front was its coupling with high sea surface height anomalies (SSHAs), which is unusual for a basin where SST is normally uncorrelated with SSHA. The high SSHAs were associated with downwelling Rossby waves, and the interaction between downwelling and surface fresh waters was a key mechanism to account for the observed SST–SSHA coupling.

The near-surface salinity field in the bay is characterized by strong stratification and a pronounced horizontal gradient, with low salinity in the northeast. During the passage of downwelling Rossby waves, freshening of the surface layer was observed when surface velocities were southwestward. Horizontal convergence of freshwater associated with downwelling Rossby waves increased the buoyancy of the upper layer and caused the mixed layer to shoal to within a few meters of the surface. Surface heating trapped in the thin mixed layer caused the fresh layer to warm, whereas the increase in buoyancy from low-salinity waters enhanced the high SSHA associated with Rossby waves. Thus, high SST coincided with high SSHA.

The dominant role of salinity in controlling high SSHA suggests that caution should be exercised when computing hurricane heat potential in the bay from SSHA. This situation is different from most tropical oceans, where temperature has the dominant effect on SSHA.

Full access
Motoki Nagura and Michael J. McPhaden

Abstract

The number of in situ observations in the Indian Ocean has dramatically increased over the past 15 years thanks to the implementation of the Argo profiling float program. This study estimates the mean circulation in the Indian Ocean using hydrographic observations obtained from both Argo and conductivity–temperature–depth (CTD) observations. Absolute velocity at the Argo float parking depth is used so there is no need to assume a level of no motion. Results reveal previously unknown features in addition to well-known currents and water masses. Some newly identified features include the lack of an interior pathway to the equator from the southern Indian Ocean in the pycnocline, indicating that water parcels must transit through the western boundary to reach the equator. High potential vorticity (PV) intrudes from the western coast of Australia in the depth range of the Subantarctic Mode Water, which leads to a structure similar to a PV barrier. The subtropical anticyclonic gyre retreats poleward with depth, as happens in the subtropical Atlantic and Pacific. An eastward flow was found in the eastern basin along 15°S at the depth of the Antarctic Intermediate Water—a feature expected from property distributions but never before detected in velocity estimates. Meridional mass transport indicates about 10 Sv (1 Sv ≡ 106 m3 s−1) southward flow at 6°S and 18 Sv northward flow at 20°S, which results in meridional convergence of currents and thermocline depression at about 16°–20°S. These estimated absolute velocities agree well with those of an ocean reanalysis, which lends credibility to the strictly databased analysis.

Full access
Xuri Yu and Michael J. McPhaden

Abstract

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.

Full access