Abstract

The eastern Pacific and Atlantic have a curious climatic asymmetry relative to the equator. Whereas the intertropical convergence zone (ITCZ) characterized by persistent and heavy rainfall and the warmest surface waters reside north of the equator, a cold tongue in sea surface temperature (SST) occurs at and south of the equator even though the time-mean solar radiation is approximately symmetric about the equator. In this paper the author investigates the relative role of three types of coupled ocean–atmosphere interaction processes—the meridional wind–SST feedback, the evaporation–wind feedback, and the low-level stratus cloud–SST feedback—in determining the climatic asymmetry relative to the equator.

This study has two components. First, a simple analytical model is constructed in which the aforementioned three positive-feedback mechanisms are all included in a unified dynamic framework. The author’s stability analysis indicates that in a reasonable parameter regime the growth rates associated with the three coupled instabilities are of the same order of magnitude, suggesting that they are all important in contributing to the climatic asymmetry. Because of the dependence of the three feedback mechanisms on the existence of a shallow oceanic mixed layer that, in turn, is a result of equatorial easterlies, the existence of the equatorial easterlies is essential for the amplification of the climatic asymmetry.

Next, a hybrid coupled general circulation model is used in which a realistic continental and coastal geometry is presented. The model starts from an ideal symmetric condition forced only by the annual-mean insolation at the top of the atmosphere which is approximately symmetric about the equator. In the presence of the three air–sea interaction mechanisms, the coupled model is capable of reproducing a realistic asymmetric time-mean state in the eastern Pacific and Atlantic. The fundamental cause of the asymmetry in the eastern Pacific is the tilt of the western coast of the Americas, which perturbs SST in the vicinity of the coastal region through a so-called coastal wind-upwelling mechanism. The asymmetry in the Atlantic, on the other hand, results from the land–ocean thermal contrast between the bulge of northwestern Africa and the ocean to the south. The ocean–atmosphere interactions act as an amplifier to amplify the asymmetry set up by the continental or coastal asymmetry. Numerical experiments presented here demonstrate the importance of the geographic asymmetries and the ocean–atmosphere interactions in determining the preferred climatic position for the ITCZ.

Abstract

A stationary SST mode is proposed to understand the physical mechanisms responsible for the phase transition of the El Niño–Southern Oscillation. This stationary SST mode differs from the original delayed oscillator mode and the slow SST mode in the sense that it considers both balanced and unbalanced thermocline depth variations and does not take into account the zonal propagation of SST. Within this mode, the Walker circulation acts as a positive feedback mechanism to amplify and maintain an existing interannual SST anomaly, whereas the Hadley circulation acts as a negative feedback mechanism that dismisses the original anomaly and causes the phase shift from a warm (cold) to a cold (warm) episode.

The key to the cause of interannual oscillations in the stationary SST mode lies in the zonal-mean thermocline depth variation that is not in equilibrium with the winds. Because of the nonequilibrium, this part of the thermocline depth anomaly tends to have a phase lag with the wind (or SST) anomaly and therefore holds a key for the interannual oscillation. The zonally asymmetric part of the thermocline depth anomaly, on the other hand, is always in Sverdrup balance with the winds. Such a phase relationship agrees well with observations and with GCM simulations.

The stationary SST mode strongly depends on the basin width, on the air–sea coupling strength, and on the seasonal-cycle basic state. For a reasonable parameter regime, it depicts an interannual oscillation with a period of 2–7 years. This stationary SST mode is also season dependent: it has a maximum growth rate during the later part of the year and a negative growth rate during the northern spring, which may explain the occurrence of the mature phases of the El Niño in the northern winter and a rapid drop of the lagged correlation of the Southern Oscillation index in the boreal spring.

Abstract

The development and movement of the tropical intraseasonal system (TIS) exhibit remarkable annual variations. It was hypothesized that spatial and temporal variation in sea surface temperature (SST) is one of the primary climatic factors that are responsible for the annual variation of TISs. This paper examines possible influences of SST on the TIS through numerical experiments with a 2.5-layer atmospheric model on an equatorial β plane, in which SST affects atmospheric heating via control of the horizontal distribution of moist static energy and the degree of convective instability.

The gradient of the antisymmetric (with respect to the equator) component of SST causes a southward propagation of the model TIS toward northern Australia in boreal winter and a northward propagation over the Indian and western Pacific Oceans in boreal summer. The phase speed of the meridional propagation increases with the magnitude, of antisymmetric SST gradients. The poleward propagation of the equatorial disturbance takes the form of moist antisymmetric Rossby modes and influences the summer monsoon.

During May when SST is most symmetric in the western Pacific, a disturbance approaching the date line may evolve into westward-moving, double cyclonelike, symmetric Rossby modes due to the suppression of the moist Kelvin mode by the cold ocean surface cast of the date line. The disturbance over the equatorial Indian Ocean, however, may evolve into an eastward-moving, moist Kelvin–Rossby wave packet; meanwhile, a cyclonic circulation may be induced over the Gulf of Thailand and Malaysia, drifting slowly westward into the Indian subcontinent.

Abstract

Diagnosis of the dynamic and thermodynamic balances using observed climatological monthly mean data reveals that 1) anisotropic, latitude-dependent Rayleigh friction coefficients lead to much improved modeling of the monthly mean surface wind field for a given monthly mean sea level pressure field, and 2) the annual variation of the vertically averaged lapse rate is important for modeling sea level pressure.

Based on the aforementioned observations, a thermodynamic equilibrium climate model for the tropical Pacific is proposed. In this model, the sea level pressure is thermodynamically determined from sea surface temperature (SST) through a vertically integrated hydrostatic equation in which the vertical mean lapse rate is a function of SST plus a time-independent correction. The surface winds are then computed from sea level pressure gradients through a linear surface momentum balance with anisotropic, latitude-dependent Rayleigh friction coefficients. The precipitation is finally obtained from a moisture budget by taking into account the effects of SST on convective instability.

Despite its simplicity, the model is capable of simulating realistic annual cycles as well as interannual variations of the surface wind, sea level pressure, and precipitation over the tropical Pacific. The success of the model suggests that the tropical atmosphere on a monthly mean time scale is, to the lowest-order approximation, in a thermodynamic equilibrium state in which sea level pressure is primarily controlled by SST and the effects of dynamic feedback on sea level pressure may be parameterized by an empirical SST-lapse rate relationship. Further studies are needed to establish a firm physical basis for the proposed parameterization.

Abstract

The tropical atmosphere model presented here is suitable for modeling both the annual cycle and short-term (monthly to decadal time scale) climate fluctuations in sole response to the thermal forcing from the underlying surface, especially the ocean surface. The present model consists of a well-mixed planetary boundary layer and a free troposphere represented by the gravest baroclinic mode. The model dynamics involves active interactions between the boundary-layer flow driven by the momentum forcing associated with sea surface temperature (SST) gradient and the free tropospheric flow stimulated by diabatic heating that is controlled by the thermal effects of SST. This process is demonstrated to be essential for modeling Pacific basinwide low-level circulations. The convective heating is parameterized by a SST-dependent conditional heating scheme based upon the proposition that the potential convective instability increases with SST in a nonlinear fashion.

The present model integrates the virtue of a Gill-type model with that of a Lindzen–Nigam model and is capable of reproducing both the shallow intertropical convergence zone (ITCZ) in the boundary layer and the deep South Pacific convergence zone (SPCZ) and monsoon troughs in the lower troposphere. The precipitation pattern and intensity, the trade winds and associated subtropical highs, and the near-equatorial trough can also be simulated reasonably well.

The thermal contrast between oceans and continents is shown to have a profound influence on the circulation near landmasses. Changes in land surface temperature, however, do not exert significant influence on remote oceanic regions. Both the ITCZ and SPCZ primarily originate from the inhomogeneity of ocean surface thermal conditions. The continents of South and North America contribute to the formation of these oceanic convergence zones through indirect boundary effects that support coastal upwelling changing the SST distribution. The diagnosis of observed surface wind and pressure fields indicates that the nonlinear advection of momentum is generally negligible, even near the equator, in the boundary-layer momentum balance. The large SST gradients in the subtropics play an important role in forcing rotational and cross-isobaric winds.

Abstract

Tropical boundary-layer flows interact with the free tropospheric circulation and underlying sea surface temperature, playing a critical role in coupling collective effects of cumulus heating with equatorial dynamics. In this paper a unified theoretical framework is developed in which convective interaction with large-scale circulation includes three mechanisms: convection–wave convergence (CWC) feedback, evaporation–wind (EW) feedback, and convection–frictional convergence (CFC) feedback. We examine the dynamic instability resulting from the convective interaction with circulation, in particular the role of CFC feedback mechanism.

CFC feedback results in an unstable mode that has distinctive characteristics from those occurring from CWC feedback or EW feedback in the absence of mean flow. The instability generated by CFC feedback is of low frequency with a typical growth rate on an order of 10⁻⁶ s⁻¹. The unstable mode is a multiscale wave packet; a global-scale circulation couples with a large-scale (several thousand kilometers) convective complex. The complex is organized by boundary-layer convergence and may consist of a few synoptic-scale precipitation cells. The heating released in the complex in turn couples the moist Kelvin wave and the Rossby wave with the gravest meridional structure, forming a dispersive system. The energy propagates slower than the individual cells within the wave packet. A transient boundary layer is shown to favor planetary-scale instability due to the fractionally created phase shift between the maximum vertical motion and the heating associated with boundary-layer convergence.

The implications of the theory to the basic dynamics of tropical intraseasonal oscillation are discussed.

Abstract

The role of the annual-mean climate on seasonal and interannual variability in the tropical Pacific is investigated by means of a coupled atmosphere–ocean general circulation model. The atmospheric component of this coupled model is the Naval Operational Global Atmospheric Prediction System and the oceanic component is the Geophysical Fluid Dynamics Laboratory Modular Ocean Model. Three sets of experiments are conducted. In case A, no annual-mean flux adjustment is applied so that the coupled model generates its own time-mean state. In case B, an annual-mean flux adjustment for SST is applied. In case C, both the annual-mean SST and surface wind are adjusted. It is found that a realistic simulation of both the seasonal and interannual variations can be achieved when a realistic annual-mean state is presented. The long-term (40 yr) simulations of the coupled GCM demonstrate the importance of the annual-mean climate on seasonal and interannual variability in the Tropics. The mechanism that causes an annual rather than a semiannual cycle at the equator is discussed. The authors particularly notice that the interannual oscillations in the model capture essentially all three ENSO phase transition modes: the delayed oscillator mode, the slow SST mode, and the stationary SST mode.

Abstract

Although the seasonal cycle of the equatorial Atlantic and Pacific Oceans have many similarities, for example, an annual signal is dominant at the equator even though the sun “crosses” the equator twice a year, different processes determine the seasonal cycles of the two oceans and in the Atlantic different processes are important in the east and west. In the Gulf of Guinea in the eastern equatorial Atlantic, the seasonal cycle of surface winds is primarily in response to seasonal variations in land temperatures so that annual changes in sea surface temperatures are, to a first approximation, the passive response of the ocean to the winds. The seasonal cycle of the western equatorial Atlantic has similarities with that of the equatorial Pacific—both are strongly influenced by ocean–atmosphere interactions in which the surface winds and sea surface temperature patterns depend on each other—but only in the western equatorial Atlantic are the seasonal variations in sea surface temperature influenced by vertical excursions of the thermocline. These results are obtained by means of a general circulation model of the atmosphere and a relatively simple coupled ocean–atmosphere model.

Abstract

Although the sun “crosses” the equator twice a year, the eastern equatorial Pacific has a pronounced annual cycle, in sea surface temperature and in both components of the surface winds for example. (This is in contrast to the Indian Ocean and western Pacific where a semiannual oscillation of the zonal wind is the dominant signal on the equator.) Calculations with a relatively simple coupled ocean-atmosphere model indicate that the principal reason for this phenomenon is the marked asymmetry, relative to the equator, of the time-averaged climatic conditions in the eastern tropical Pacific. The important asymmetries are in surface winds, oceanic currents, and sea surface temperature: The time-averaged winds and currents have northward components at the equator and the warmest waters are north of the equator. Because of those asymmetries, seasonally varying solar radiation that is strictly antisymmetric relative to the equator can force a response that has a symmetric component. The amplitude of the resultant annual cycle at the equator depends on interactions between the ocean and atmosphere, and on positive feedbacks that involve low-level stratus clouds that form over cold surface waters.

Abstract

An intermediate tropical Pacific Ocean model is developed to bridge the gap between anomaly models of El Niño and ocean general circulation models. The model contains essential physics for reproducing both the annual and interannual variations of sea surface temperature (SST). A new parameterization scheme for entrained water temperature is shown to work satisfactorily in both the cold tongues and warm pools. This scheme combines the Cane-Zebiak (CZ) model's dynamic framework and mixed layer physics, giving a more realistic description of the active tropical ocean.

Incorporation of the Niiler-Kraus scheme for turbulent entrainment enables the model to better simulate El Niño-Southern Oscillation in the central equatorial Pacific where the CZ model considerably underestimates observed SST variations. It also improves the model's performance on the seasonal cycle, especially in the central-eastern equatorial Pacific and the intertropical convergence zone (ITCZ). The potential energy generation induced by penetrative solar radiation tends to reduce entrainment in the central equatorial Pacific but to enhance mixing in the far eastern equatorial Pacific. Without this process, the model central (eastern) Pacific would be excessively cold (warm).

In response to an idealized sequential westerly burst located in the western equatorial Pacific, the CZ model produces SST oscillations in the eastern equatorial Pacific due to the thermocline oscillation associated with passages of Kelvin waves. In the present model, however, SST variation in the eastern Pacific is insignificant because local entrainment transcends the influence of thermocline oscillation; on the other hand, positive SST anomalies slowly amplify near the date line due to the reduction in wind-induced mixing and surface evaporation.

The annual variations of the oceanic momentum and heat transports associated with the annual march of the ITCZ are shown to have significant impacts on the annual mean state. On the other hand, including an annual mean heat flux correction in the present model does not strongly influence the amplitudes of annual and interannual SST variations. However, it does improve the phase structure of the annual cycle by providing a more accurate annual mean state.