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Antonio J. Busalacchi and Frédérique Blanc

Abstract

A series of model experiments Of the tropical Atlantic Ocean is Performed to demonstrate the effect of various closed and open boundary configurations on spinup and steady state time sclaes. In response to forcing by idealized winds and a realistic seasonal cycle, the large-scale interior solutions are relatively insensitive to the choice of boundary condition. However, the choice of boundary condition does have an important impact on the structure and variability of the western boundary circulation, and its relation to the interior flow field. As expected, transient effects during spinup produced significant differences in western boundary coastal upwelling and downwelling regimes based on the implementation of closed or open boundaries. More importantly, the mere existence of certain western boundary features such as coastally trapped jet was strongly dependent on the choice of boundaries under mean and seasonal equilibrium conditions. In particular only those solutions with an open western boundary were able to simulate the continuous northwestward coastal flow of the North Brazil Current during spring and the complete eastward veering of this current into the North Equatorial Countercurrent during the fall.

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Antonio J. Busalacchi and Joël Picaut

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A numerical model incorporating a single baroclinic mode and realistic coastline geometry is used to analyze the linear, dynamic response to estimates of the seasonal wind field over the tropical Atlantic Ocean. The forced periodic response consists of a spatially dependent combination of a locally forced response, Kelvin waves, Rossby waves and multiple wave reflections. The seasonal displacements of the model pycnocline are compared with observed dynamic height. Annual and semiannual fluctuations dominate the seasonal signal throughout the basin. In general, the distribution of amplitude and phase are similar for annual changes in dynamic height and pycnocline depth. Major features of the seasonal response are reproduced, e.g., east-west changes in pycnocline depth about a nodal point at the equator, the seasonal pycnocline movement along the northern and southern coast of the Guinea Gulf, and a significant changes of phase in the ocean variability north and south of the ITCZ. The relative importance between local and remote forcing is determined for several parts of the model basin. The wind-driven annual signal in the idealized Gulf of Guinea is due to equatorial zonal wind stress fluctuations west of the Gulf. The semi-annual response in the Gulf of Guinea is a result of zonal and meridional wind stress fluctuations in the eastern half of the tropical Atlantic. The seasonal response in the western equatorial and northernmost parts of the model basin are primarily local.

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Ragu Murtugudde and Antonio J. Busalacchi

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Interannual variability of the tropical Indian Ocean is studied with a reduced gravity, primitive equation, ocean general circulation model (OGCM). The OGCM is coupled to an atmospheric mixed layer model for surface heat flux computation. The seasonal simulation of sea surface temperatures (SST), current, and thermocline structures are in good agreement with observations and other models. The seasonal cycle of SST along the equator exhibits an eastward propagation with larger variability in the west. The interannual simulations are carried out over 1980–95 with interannual wind stresses and wind speeds but climatological data for solar radiation and cloudiness. The SST anomalies are smaller than 1°C over most of the basin and the leading EOF shows an ENSO-related warming. However, the correlation between the Southern Oscillation index and the time series of the leading EOF is only −0.51 and SST anomalies of similar magnitudes as an El Niño year appear in other years too. ENSO-related equatorial winds determine the SST anomalies along the coast of Sumatra and this anomaly in the eastern southern tropical Indian Ocean (STIO) is typically opposite in sign to the anomaly in the western STIO. The western STIO has some of the largest SSTA because of a shallow thermocline and the entrainment effects associated with wind stress curl anomalies in the region. The quasi-biennial oscillation in the thermocline and the SST gradient in the STIO is correlated with the Somali jet, which in turn is correlated with the Indian summer monsoon. An experiment with climatological wind stresses but interannual wind speeds demonstrates that the wind-driven variations in SST are larger than previously estimated with relaxation type heat fluxes. A parallel experiment with climatological wind speeds but interannual wind stresses shows that there are regions where heat fluxes contribute significantly to SST variability. Another simulation with interannual data for radiation and cloudiness shows that model simulation is affected significantly in some regions by the use of climatological data for solar radiation and cloudiness. A model experiment with an open eastern boundary provides a simplistic illustration of the effects of the Indonesian Throughflow (ITF). The main influence of the ITF is to warm the Indian Ocean and reduce the effect of upwelling on SST.

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Antonio J. Busalacchi and James J. O'Brien

Abstract

A simple linear model of the tropical Pacific Ocean is used to simulate the oceanic response to time-dependent wind stress forcing. A linear, one-layer, reduced-gravity transport model on an equatorial beta-plane is incorporated. The non-rectangular model basin extends from 18°N to 12°S. Bottom topography, thermohaline and thermodynamic effects are neglected.

The equatorial response, particularly at the eastern boundary, is studied along the same lines as Kindle. Annual and semiannual harmonics of the zonal equatorial wind stress calculated by Meyers are used to force the model. The east-west slope of the model pycnocline is compared with depth observations of the 14°C isotherm. The linear model generates a semiannual eastern boundary response remote from any region with strong second harmonies of the zonal wind stress. This response supports Meyers' hypothesis that at the eastern boundary the semiannual displacement of the thermocline is due to remote forcing.

The major application of the model is forced by mean monthly wind stresses based on 10 years of observations over the tropical Pacific. The resulting meridional profile of the pycnocline depth is similar to Wyrtki's profile of dynamic height. The equatorial system of troughs and ridges is evident in the pycnocline profile. The seasonal variation of the major equatorial surface currents is compared with the observations. An annual Rossby wave emanating from the eastern boundary is found to modify the location and variability of the Countercurrent Trough. The presence of an anomalous eastward flow centered south of the equator in the eastern equatorial Pacific is supported by Tsuchiya's maps of the dynamic topography of this region.

The results of the two model applications indicate that the dynamics inherent in linear theory are capable of simulating some of the major features of the equatorial response and those of the equatorial surface current system.

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Rong-Hua Zhang and Antonio J. Busalacchi

Abstract

The impacts of freshwater flux (FWF) forcing on interannual variability in the tropical Pacific climate system are investigated using a hybrid coupled model (HCM), constructed from an oceanic general circulation model (OGCM) and a simplified atmospheric model, whose forcing fields to the ocean consist of three components. Interannual anomalies of wind stress and precipitation minus evaporation, (PE), are calculated respectively by their statistical feedback models that are constructed from a singular value decomposition (SVD) analysis of their historical data. Heat flux is calculated using an advective atmospheric mixed layer (AML) model. The constructed HCM can well reproduce interannual variability associated with ENSO in the tropical Pacific. HCM experiments are performed with varying strengths of anomalous FWF forcing. It is demonstrated that FWF can have a significant modulating impact on interannual variability. The buoyancy flux (QB) field, an important parameter determining the mixing and entrainment in the equatorial Pacific, is analyzed to illustrate the compensating role played by its two contributing parts: one is related to heat flux (QT) and the other to freshwater flux (QS). A positive feedback is identified between FWF and SST as follows: SST anomalies, generated by El Niño, nonlocally induce large anomalous FWF variability over the western and central regions, which directly influences sea surface salinity (SSS) and QB, leading to changes in the mixed layer depth (MLD), the upper-ocean stability, and the mixing and the entrainment of subsurface waters. These oceanic processes act to enhance the SST anomalies, which in turn feedback to the atmosphere in a coupled ocean–atmosphere system. As a result, taking into account anomalous FWF forcing in the HCM leads to an enhanced interannual variability and ENSO cycles. It is further shown that FWF forcing is playing a different role from heat flux forcing, with the former acting to drive a change in SST while the latter represents a passive response to the SST change. This HCM-based modeling study presents clear evidence for the role of FWF forcing in modulating interannual variability in the tropical Pacific. The significance and implications of these results are further discussed for physical understanding and model improvements of interannual variability in the tropical Pacific ocean–atmosphere system.

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Rong-Hua Zhang and Antonio J. Busalacchi

Abstract

The role of subsurface temperature variability in modulating El Niño–Southern Oscillation (ENSO) properties is examined using an intermediate coupled model (ICM), consisting of an intermediate dynamic ocean model and a sea surface temperature (SST) anomaly model. An empirical procedure is used to parameterize the temperature of subsurface water entrained into the mixed layer (Te) from sea level (SL) anomalies via a singular value decomposition (SVD) analysis for use in simulating sea surface temperature anomalies (SSTAs). The ocean model is coupled to a statistical atmospheric model that estimates wind stress anomalies also from an SVD analysis. Using the empirical Te models constructed from two subperiods, 1963–79 (T 63–79 e) and 1980–96 (T 80–96 e), the coupled system exhibits strikingly different properties of interannual variability (the oscillation period, spatial structure, and temporal evolution). For the T 63–79 e model, the system features a 2-yr oscillation and westward propagation of SSTAs on the equator, while for the T 80–96 e model, it is characterized by a 5-yr oscillation and eastward propagation. These changes in ENSO properties are consistent with the behavior shift of El Niño observed in the late 1970s. Heat budget analyses further demonstrate a controlling role played by the vertical advection of subsurface temperature anomalies in determining the ENSO properties.

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Antonio J. Busalacchi and Mark A. Cane

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A formalism is developed to examine the effect of zonally varying stratification on equatorial wave phenomena; an effect present in the real ocean but neglected from standard linear theory. The approach utilized involves the application of a matching condition to equatorial waves incident on a single zonal discontinuity in the density field of a shallow water system. Transmission and reflection coefficients are sought for the projection of an incoming wave onto the entire set of resultant vertical and horizontal wave modes of a general continuously stratified fluid. The limiting case of a meridional density front is extended, in a manner analogous to radiative transfer problems, to a series of discrete density intervals. These techniques are applied to specific choices of stratification ranging from a zonal jump discontinuity in the density field to density changes with zonal scales large with respect to the waves in question, i.e., a WKB limit. The results demonstrate that zonally varying stratification does not produce substantial changes in the energy flux of propagating equatorial waves. However, as a result of changes to the equatorial radius of deformation, the amplification of equatorial zonal velocity can be appreciable. A corresponding decrease in pressure, albeit smaller, may also be non-negligible.

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Antonio J. Busalacchi and Mark A. Cane

Abstract

The sea level signature for the onset of the 1982–83 El Niño is hindcasted using a linear wind-driven model that consists of four vertical modes. The hindcast solution is compared with sea level time series from 18 island and coastal stations throughout the tropical Pacific. The response of the first and second baroclinic modes, when summed, account for a significant portion of the phase and amplitude of the observed sea level. The comparisons with observed sea level are best in the eastern tropical Pacific along the equator and South American coast where the solutions are a function of integrals of the zonal wind field to the west. The skill of the hindcast is notably less away from the equator.

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Kristopher B. Karnauskas and Antonio J. Busalacchi

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Satellite- and gauge-based precipitation and sea surface temperature (SST) are analyzed to understand the role of SST in the east Pacific warm pool (EPWP) in the interannual variability of Central American rainfall. It is shown that, during the rainy season following the mature phase of an El Niño event, an anomalously warm EPWP can cause a rapid enhancement of the eastern Pacific intertropical convergence zone (EP ITCZ), which directly leads to a positive rainfall anomaly over Central America. Moreover, the timing and amplitude of the SST-enhanced EP ITCZ depends on the persistence of the El Niño event. The longer the equatorial SST anomaly persists, the longer the EPWP is subject to anomalous shortwave heating, and thus the stronger (and later) the subsequent SST enhancement of the EP ITCZ. The implications for regional climate monitoring and predictability are explored; potential predictability of seasonal rainfall is demonstrated 4 months in advance using an SST-based index designed to capture the essence of the above-mentioned mechanism.

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Kristopher B. Karnauskas and Antonio J. Busalacchi

Abstract

In comparison with the western and equatorial Pacific Ocean, relatively little is known about the east Pacific warm pool (EPWP). Observations indicate that the interannual variability of sea surface temperature (SST) in the EPWP is highly correlated (0.95) with the El Niño–Southern Oscillation (ENSO). In this paper, an ocean general circulation model (OGCM) of the tropical Pacific Ocean and various atmospheric and oceanic observations are used to diagnose the physical processes governing the interannual variability of SST in the EPWP. Atmospheric forcings for the OGCM are derived purely from satellite observations between 1988 and 2004.

Shortwave heating is identified as playing a dominant role in the interannual SST tendency of the EPWP. The high correlation between SST in the EPWP and eastern equatorial Pacific is therefore explained not by ocean processes, but by an atmospheric link. ENSO-driven equatorial SST anomalies modify the distribution of the overlying atmospheric vertical motions and therefore cloud cover and ultimately shortwave heating. During an El Niño event, for example, the ITCZ is equatorward displaced from its normal position over the EPWP, resulting in anomalously large shortwave heating over the EPWP. Analysis of poleward ocean heat transport and coastal Kelvin waves confirms that oceanic processes are not sufficient to explain the interannual variability of the EPWP.

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