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Paul S. Schopf

Abstract

A numerical model is used to mechanistically simulate the oceans’ seasonal cross-equatorial heat transport, and the results of Oort and Vonder Haar (1976). The basic process of Ekman pumping and drift is found to be able to account for a large amount of the cross-equatorial flux. Increased easterly wind stress in the winter hemisphere causes Ekman surface drift poleward, while decreased easterly stress allows a reduction in the poleward drift in the summer hemisphere. When the annual mean flow is removed, a net flow at the surface from summer to winter hemispheres is noted. The addition of planetary and gravity waves to this model does not alter the net cross-equatorial flow, although the planetary waves are clearly seen. On comparison with Oort and Vonder Haar (1976), this adiabatic advective redistribution of heat is seen to be plausible up to 10–20°N, beyond which other dynamics and thermodynamics are indicated.

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Paul S. Schopf

Abstract

Dynamics readjustment of a stratified ocean model to wind perturbations leads to variations in sea surface temperature (SST) related to the early phases of the observed interannual warming of the tropical Pacific known as El Niño. The role that the atmosphere plays in determining the extent and strength of the SST warming is examined through numerical experiments with varying parameterizations for the atmospheric thermal response to SST anomalies.

A prior specification of the atmospheric temperature (even as a function of space and time) amounts to assuming infinite heat capacity for the atmosphere. A zero-heat capacity atmospheric model is constructed, in which the surface air temperature is balanced between the SST and a radiative equilibrium temperature. In the latter model, SST perturbations are damped through radiative relaxation from the atmosphere, rather than through direct cooling to the atmosphere. This greatly increases the lifetime of SST anomalies and increases their areal extent.

The effect that the atmospheric parameterization has on an upper ocean model for El Niño is examined. The model tests are conducted by imposing wind perturbation on simple mean states driven by constant winds. Westerly wind perturbations in the western part of the model basin excite Kelvin waves that propagate to the east. Under southerly mean winds, this Kelvin wave propagates to the east without any signal in the SST, but large SST anomalies are generated upon reflection of the Rossby waves. Much weaker changes in the southerly winds near the eastern coast produce SST anomalies that mimic those generated by the westerly wind changes. Such a counter-example to remotely forced Kelvin wave theories for El Niño also arises when the southerly stress anomaly is held off the coast by 200 km. Sea-level changes associated with the westerly and southerly wind perturbations are markedly different. The rapid adjustment of the atmosphere to the ocean appears to be a necessary conditions for successful simulations of the El Niño warming.

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Paul S. Schopf and Andrew Loughe

Abstract

A global isopycnal ocean model is presented for the study of interannual to interdecadal variability in the global ocean. The model treats the primitive equations on a sphere with a generalized vertical coordinate. This coordinate is designed to represent a turbulent well-mixed surface layer and nearly isopycnal deeper layers. Disappearing isopycnics are treated through the quasi-isopycnal technique, in which the coordinate separates from the isopycnic in order to maintain a minimum layer thickness. A reduced gravity treatment is made, with the deepest interface at a mean depth of 2300 m. Coastal topography is represented, but the reduced gravity treatment precludes the use of variable bottom depth. The model is used for hindcast studies of El Niño during the decade from 1982 through 1991 using a combination of climatological wind forcing and wind anomalies derived from various sources. In order to carry out the hindcast experiments, a technique is developed for constructing a mean climatological surface heat flux using the model, climatological wind forcing, and climatological surface temperatures. In the hincast runs, the climatological winds and heat flux are augmented by the wind anomalies and a weak damping of surface temperature anomalies. A series of tests compares different data products for the wind anomalies. The first product is obtained from the Florida State University (FSU) wind analysis. The second and third wind products are obtained from global climate GCM simulations run over observed sea surface temperatures (SST). Although the wind products appear quite similar, the model results show large differences in hindcast skill, reflecting the fact that subtle features of the winds can have large impacts on ocean simulations and can be seen as a primary cause of wide differences in coupled GCM performance. The model maintains a sharp thermocline and a strong equatorial undercurrent in the center of the ocean basin. The heat flux needed to keep the model near the observed temperatures appears consistent with observational studies of the mean heat flux. When measured in terms of the skill in simulating the Niño-3 SST, the NASA Coupled Climate Dynamics Group (CCDG) model and FSU wind products provide the highest skill.

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Zuojun Yu and Paul S. Schopf

Abstract

In this study, the authors explore how vertical-mixing parameterizations influence the structure of zonal currents in the eastern equatorial Pacific using an isopycnal ocean model that contains an explicit surface mixed layer. The mixing parameterizations considered are the schemes that depend on the Richardson number (Ri). One of the schemes (the Step scheme) consists of high (ν c) and low (ν b) values of mixing coefficients, depending on whether Ri is less than or greater than a critical value. In simulations using the Step scheme, there is a region of large vertical shear just beneath the mixed layer where Ri is low and the mixing coefficient is ν c; this high mixing controls the depth and strength of the westward surface drift. Near the undercurrent core, Ri is high and the mixing coefficient is ν b; this low mixing is nevertheless dynamically important in that it affects the strength of the undercurrent. For the Ri-dependent schemes investigated, it is demonstrated that the extrema attained by mixing coefficients at low and high Ri are the crucial factor rather than the detailed structure of the Ri-dependent functions.

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Paul S. Schopf and Max J. Suarez

Abstract

A reexamination of the coupled delayed-action oscillator model of Suarez and Schopf for the El Nino/Southern Oscillation (ENSO) phenomenon is made, by deriving it using a parameterized atmosphere and explicit linear ocean wave dynamics. The derivation attempts to clarify the role of boundary reflections, damping, and scale sensitivity in determining the characteristic timescale of the model. Making the assumption that SST anomalies are related to thermocline perturbations in the central to eastern part of the basin, and that wind anomalies are related to SST anomalies, ocean wave dynamics are invoked to solve for the relationship between wind anomalies and the relevant thermocline displacement.

A perturbation to SST causes wind anomalies which drive Kelvin waves eastward, thereby increasing the SST perturbation. The wind perturbations also generate Rossby waves in the ocean, which propagate westward, eventually reflecting from the western boundary as Kelvin waves. The thermocline displacements of these waves have the opposite sense to those of the directly driven Kelvin waves and form the basis for the delayed, negative feedback in the delayed-action oscillator. By solving the wave dynamics explicitly, we are able to conclude that: 1) The delayed action oscilator—in its simplest form with no eastern boundary reflection and a single Rossby wave reflected from the western boundary—forms the basic oscillator mechanism. 2) Very little of the Rossby wave energy propagating to the western boundary needs to be reflected into the Kelvin wave in order for the system to oscillate (as little as 20% in some cases). 3) The zonal extent of the wind field response to SST anomalies has almost no influence on the solutions. 4) Broadening of the meridional shape of the winds, which inparts more of the Rossby wave energy to higher meridional modes has the net effect of lengthening the delay without a large impact on the amplitude of the returning signal. 5) The role of the eastern boundary is relatively unimportant.

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Roxana C. Wajsowicz and Paul S. Schopf

Abstract

The annual mean and seasonal cycle in latent heating over the Indian Ocean are investigated using a simple, analytical ocean model and a 3D, numerical, ocean model coupled to a prescribed atmosphere, which permits interaction through sea surface temperature (SST). The role of oceanic divergence in determining the seasonal cycle in evaporation rate is reexamined from the viewpoint that the amount of rainfall over India during the southwest monsoon is a function of the amount of water evaporated over the “monsoon streamtube” as well as orographically induced convective activity.

Analysis of Comprehensive Ocean–Atmosphere Dataset (COADS) shows that nearly 90% of the water vapor available to precipitate over India during the southwest monsoon results from the annual mean evaporation field. The seasonal change in direction of airflow, which opens up a pathway from the southern Indian Ocean to the Arabian Sea, rather than the change in evaporation rate is key to explaining the climatological cycle, though the change in latent heating due to seasonal variations is similar to that needed to account for observed interannual-to-interdecadal variability in monsoon rainfall. The simple model shows that net oceanic heat advection is not required to sustain vigorous evaporation over the southern tropical Indian Ocean; its importance lies in ensuring that the maximum evaporation occurs during boreal summer. Also shown with the simple model is that evaporation over the Arabian Sea cannot increase sufficiently to make up for the loss of water vapor accumulated over the southern Indian Ocean should there be a change in circulation such that the Southern Ocean is no longer part of the monsoon streamtube.

Analytical, periodic solutions of the linearized heat balance equation for the simple model are presented under the assumption that the residual of net surface heat flux minus rate of change of heat content (DIV) is considered to be an external periodic forcing independent of SST to first order. These solutions, expressed as functions of the amplitude and phase of DIV, lie in two regimes. The first regime is characterized by increases (decreases) in the amplitude of DIV resulting in an increase (decrease) in the amplitude of the solution. In contrast, in the second regime, the amplitude of the solution decreases (increases) as the amplitude of DIV increases (decreases). It is noteworthy that the regime boundaries for SST and latent heating do not necessarily coincide. For the present climate, as determined from COADS, the southern Indian Ocean’s annual harmonics of latent heating and SST lie in the second regime near the border, and so their tendencies are sensitive to the nature of the perturbation to the harmonic in DIV. The southern Indian Ocean’s semiannual harmonic of latent heating lies in the first regime, and so its tendency is robust to the nature of the perturbation to the harmonic in DIV; that of SST lies in the second regime near the border.

Contrasting runs of the 3D numerical model, in which the Indonesian throughflow differs by less than 4 × 106 m3 s−1 in the annual mean and less than ±2 × 106 m3 s−1 in seasonal variability, provides new estimates for its potential role in the Indian Ocean heat balance. Net surface heat flux differences of over 20 W m−2 are found along the length and breadth of the southwest monsoon streamtube: particularly noteworthy regions are over the Somali jet and to the east of Madagascar. These changes can be explained in part by the changes in oceanic meridional transport generated by the throughflow as well as by its heat input. Spatial resolution and upper ocean physics are sufficient for the throughflow to retain its zonal jet character across the Indian Ocean and so inhibit meridional overturning. Significantly, its presence reduces the amount of heat imported into the Southern Ocean from the Arabian Sea during boreal summer, so making SSTs in the Arabian Sea higher.

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D. E. Harrison and Paul S. Schopf

Abstract

The initial surface warming of the 1982 El Niño event was of quite different timing and pattern from that associated with most El Niño events; strong anomalous warming occurred first in July along the equator and subsequently along the South American coast. We show here that a simple advective model for tropical ocean surface warming can produce anomalous sea-surface temperature (SST) fields like those found in the first few months of the 1982 El Niño. The model physics assumes that the existing SST field is advected by anomalous currents to produce the anomalous warming, and that the anomalous currents are those induced subsequent to the passage of downwelling Kelvin wave front(s). With the initial SST field taken to be that of July 1982, the anomalous eastward currents of the model lead to a satisfactory prediction of the evolution of anomalous SST for several months. Numerical experiments with a fully nonlinear and thermally active ocean model support the physical relevance of the more idealized study.

The anomalous horizontal advection model can also account for the initial SST evolution during the more common type of El Niño event. The reason that a similar anomalous current field can produce two such different warming patterns is that the gradients of SST along the equator have strong seasonal variation. If anomalous eastward currents are generated along the equator between February and April, when the climatological zonal SST gradient is small, little equatorial warming will occur and so coastal warming is observed first; this is the case in most El Niño events. But if the same anomalous currents occur later in the year, when there is typically a strong zonal temperature gradient, strong equatorial surface warming will occur prior to coastal warming, as happened in 1982. The pattern of SST changes resulting from remote westerly wind changes in the tropical Pacific thus is very strongly linked to the annual cycle of SST.

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Paul S. Schopf and Mark A. Cane

Abstract

We describe a new numerical model designed to study the interactions between hydrodynamics and thermodynamics in the upper ocean. The model incorporates both primitive equation dynamics and a parameterization of mixed layer physics. There is a consistent treatment of mixed layer structure for all physical processes.

In order to study interplay between dynamics and mixed layer physics in the equatorial ocean, we carried out a series of numerical experiments with simple patterns of wind stress and surface heating. In some cases stratification and/or mixed layer physics were suppressed. On the basis of these experiments we reached the following conclusions:

The vertical circulation at the equator is so vigorous that surface heating is essential if stratification is to be maintained for periods longer than a few months. Without stratification to inhibit mixed layer deepening momentum will be mixed uniformly to the main thermocline and the equatorial undercurrent will disappear.

Vertical transfers of momentum due to vertical advection and mixed layer entrainment are essential features of equatorial dynamics. These process influence currents, SST and upwelling rates more than changes in sea surface elevation. Consequently, the overall mass field adjustments of equatorial oceans are more nearly linear than are the currents or SST variations.

The connection between changes in SST and dynamical quantities such as sea surface topography need not be straightforward. For example, increased upwelling will make the mixed layer shallower but will not reduce SST unless it induces increased entrainment of colder water. The influences of upwelling and down-welling on SST are highly asymmetric so that the influence of perturbations cannot be predicted without considering the mean vertical velocity.

The asymmetry in the interaction between vertical velocity and mixed layer physics can result in the formation of surface fronts. On the upwelling side of a w = 0 line the surface layer is cold and shallow while on the downwelling side it is warm and deep. Differential advection creates a temperature discontinuity at the depth discontinuity. It is suggested that the Galapagos Front has this character.

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Bohua Huang, Paul S. Schopf, and J. Shukla

Abstract

The tropical Atlantic variability is composed of three major patterns of significant importance for variability and predictability of climate in the Atlantic sector. They are the southern tropical Atlantic (STA) pattern with anomalous sea surface temperature (SST) fluctuations expanding from the Angolan coast to the central equatorial ocean, the northern tropical Atlantic (NTA) pattern centered near the northern African coast, and the southern subtropical Atlantic (SSA) pattern in the open subtropical ocean.

Previous studies have suggested that both the regional air–sea coupling and remote forcing from outside the basin may affect the formation of these patterns and their variability. A specially designed global coupled ocean– atmosphere general circulation model, which eliminates air–sea feedback outside the Atlantic, reproduces the major features of these observed patterns realistically. This suggests that these patterns originate from air–sea coupling within the Atlantic Ocean or by the oceanic responses to atmospheric internal forcing, in which there is no anomalous forcing external to the Atlantic Ocean. The effect of the Pacific El Niño–Southern Oscillation (ENSO) seems to modulate their temporal evolution through influencing atmospheric planetary waves propagating into the basin.

One of the problems of the model simulation is that the STA pattern as represented by the SST fluctuations centered at the Angolan coast is weak in the equatorial waveguide. Unlike the observations, the model SST fluctuations around the equator are largely unconnected with the changes in the southeastern part of the ocean. This lack of connection between these two parts of the tropical ocean is related to a model systematic bias of excessive southward shift of the model intertropical convergence zone to around 10°S in boreal spring. In the coupled model, the air–sea feedback forms an artifical “warm pool” to the south of the equator extending from the Brazilian coast nearly to the eastern boundary. This warm pool blocks the connection between the fluctuations in the equatorial and the southern part of the ocean. Due to this systematic bias, this model did not simulate the STA pattern adequately.

Several sensitivity experiments have been conducted to further examine the mechanisms of the anomalous SST patterns. The results demonstrate that both the NTA and SSA patterns are mainly associated with the thermodynamic air–sea interactions, while the STA pattern is likely more closely associated with the dynamical response of the equatorial and tropical ocean to the surface wind forcing. Moreover, results from a simulation with a time-independent correction term of the surface heat flux show that the simulated STA mode can be significantly strengthened and have a more realistic spatial structure if the model mean SST errors are reduced.

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Paul S. Schopf and D. E. Harrison

Abstract

We present results from three numerical model experiments designed to study the thermal and hydrodynamics changes associated with downwelling Kelvin wave passage and east coastal reflection along and near the equator. The model employs primitive equation dynamics in two active layers and a full thermodynamics equation, so that sea surface temperature, thermocline displacement and sea level are each independently predicted. Wind and thermal finding are used. The surface layer is a slab mixed layer using Kraus and Turner-style bulk physics. Kelvin waves are excited by introducing a westerly wind anomaly in the western part of the basin, and the temperature and current changes caused by the waves are studied as the wave fronts propagate through the circulation forced by three different mean wind fields: no mean winds, southerly men winds and easterly mean winds. The wave-induced changes depend strongly on the conditions that prevail when the waves are forced. Anomalous advection of the existing SST field is the primary SST change mechanism. The two internal Kelvin-wave modes allowed by the model sometimes induce comparable temperature changes near the east coast MA sometimes the effect of one mode substantially dominates that of the other. The shear mode wave does not always propagate to the east coast; it can be destroyed by nonlinear effects associated with the meridional circulation along the equator. Temperature changes near the east coast, similar in magnitude to those observed in the early stages of El Niño events, are caused in the mean southerly wind case, but no broad westward tongue appears latter on. The implications of these results on existing models of El Niño and for future model studies are examined.

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