Abstract

Annual distribution and phase propagation of tropical convection are delineated using harmonic and amplitude-phase characteristics analysis of climatological pentad mean outgoing longwave radiation and monthly frequencies of highly reflective cloud.

An annual eastward propagation of peak rainy season along the equator from the central Indian Ocean (60°E) to Arafura Sea (130°E) is revealed. This indicates a transition from the withdrawal of the Indian summer monsoon to the onset of the Australian summer monsoon. Significant bimodal variations are found around major summer monsoon regions. These variations originate from the interference of two adjacent regimes.

The convergence zones over the eastern North Pacific, the South Pacific, and the southwest Indian Ocean are identified as a marine monsoon regime that is characterized by a unimodal variation with a concentrated summer rainfall associated with the development of surface westerlies equatorward of a monsoon trough. Conversely, the central North Pacific and North Atlantic convergence zones between persistent northeast and southeast trades are classified as trade-wind convergence zones, which differ from the marine monsoon regime by their persistent rainy season and characteristic bimodal variation with peak rainy seasons occurring in late spring and fall.

The roles of the annual march of sea surface temperature in the phase propagation and formation of various climatic regimes of tropical convection are also discussed.

Abstract

The latest two Pacific basinwide warm episodes (1982–83 and 1986–87) exhibit some common features in their development and vertical structure. These features are examined by multivariate empirical orthogonal function analysis of the interannual variability of the ocean-atmosphere system along equatorial Indian and Pacific oceans.

The updraft and downdraft branches of the anomalous Walker circulation originate over the western Pacific and the eastern Indian Ocean, respectively. The early development of basinwide warming is characterized by the strengthening of a cross-equatorial low-level southerly component over the eastern Pacific and the enhancement of convection and boundary-layer westerlies over the western Pacific.

The structure of the ENSO anomaly mode changes from the cold to the warm phase of the Southern Oscillation. This is attributed to its eastward migration and the intrinsic longitudinal dependence of the vertical structure. The latter results from the east–west contrast of the air–sea interaction processes. Over Indonesia and the western Pacific, the land–sea thermal contrast and high SST maintain a semipermanent convective action center, whose intensity is sensitively modulated by small SST fluctuation. Since moist static ability is small, the surface pressure responds sensitively to the heating, so that the anomalous low pressure and associated zonal wind convergence in the boundary layer are in phase with the enhanced convection. In contrast, over the central-eastern Pacific, large SST gradient-induced pressure gradient force drives boundary-layer flows whose beta convergence determines atmospheric heating, while the feedback of the free atmosphere to boundary-layer flows is weak due to large static stability. The enhanced convection is thus nearly in phase with anomalous boundary-layer westerlies, positive zonal SST gradient, and negative zonal surface pressure gradient.

It is possible that an individual ENSO event may result from different combinations of various sets of coupled processes, especially with regard to those that work in the eastern Pacific cold tongue and those in the western Pacific warm pool.

Abstract

In spite of the fundamental difficulties in interpreting the growth of tropical storms, the basic idea of CISK remains valuable in understanding the instability resulting from the interaction between cumulus convection and large-scale flows. A generalized solution of a quasi-balanced continuous model, which can be applied to various types of vertical heating distribution and basic-state stratification, is derived and used to explore the behaviors of the CISK mode.

In the absence of cumulus momentum mixing, the CISK solution exhibits, in general, a scale selection. However, two types of unbounded growth rates associated with different closure assumptions may exist. Both of them take place in a common situation that is characterized by local warming at the top of the moist convergence layer in the area of rising motion. In these circumstances, the direct coupling between the heating and the large-scale moisture supply through the divergent wind component dominates over the indirect coupling through the rotational component. It is suggested that, for a feasible Ekman CISK mechanism, the dominant feedback of the convective heating to the low-level moisture convergence must be of an indirect nature. In this feedback process, planetary vorticity and/or preexisting relative vorticity play an essential role in converting heating-induced divergent kinetic energy to rotational kinetic energy, thus accelerating the spin-up of a large-scale vortex.

The cumulus momentum mixing destabilizes short waves by enhancing cyclonic circulation at the top of the Ekman layer and by reducing the vertical extent of the temperature disturbance; meanwhile, it stabilizes long waves by weakening the anticyclonic circulation in the upper levels.

Abstract

The low-level cyclonic vortices which form over the Tibetan Plateau in the summer monsoon season are major rain-producing systems and have the potential to trigger cyclogenesis on the lee side when they move off the plateau. Two cases of the plateau vortices which occurred in July 1979 are studied. The characteristics of the vertical structure in their developing stage, and the circulation condition favorable for the eastward movement in the mature stage are diagnosed and presented by use of FGGE IIIb datasets. Numerical simulations with real data were performed using the GFDL limited-area mesoscale simulation model. Results suggest that the latent heating is an essential driving force for the development of the vortices studied here.

The analysis of a continuous CISK model with a basic state resembling that actually observed over the summer plateau shows that the predicted unstable mode has a preferred scale, growth rate and vertical structure, all of which are qualitatively comparable to observations. The instability in the plateau environment is mainly attributed to 1) the relatively shallow vertical extent of heating located in the upper troposphere in which the heat capacity of the air column per unit surface area is relatively small; 2) the dramatic reduction of the static stability due to surface sensible heat flux; and 3) the significant increase of moisture content in the plateau boundary layer due to surface evaporation and monsoon transport of water vapor. Most of these favorable conditions are referred to as the dynamic and thermal effects of the elevated plateau terrain. In this sense, the development of the plateau vortices during the rainy season may be regarded as resulting from the interaction between the lane-sca1e circulation and the plateau topographic effect and from the release of convective latent heat.

Abstract

The characteristics of the onset of the Pacific basin-wide warming have experienced notable changes since the late 1970s. The changes are caused by a concurrent change in the background state on which El Niño evolves.

For the most significant warm episodes before the late 1970s (1957, 1965, and 1972), the atmospheric anomalies in the onset phase (November to December of the year preceding the El Niño) were characterized by a giant anomalous cyclone over east Australia whose eastward movement brought anomalous westerlies into the western equatorial Pacific, causing development of the basin-wide warming. Meanwhile, the trades in the southeastern Pacific (20°S–0°, 125°–95°W) relaxed back to their weakest stage, resulting in a South American coastal warming, which led the central Pacific warming by about three seasons. Conversely, in the warm episodes after the late 1970s (1982, 1986–87, and 1991), the onset phase was characterized by an anomalous cyclone over the Philippine Sea whose intensification established anomalous westerlies in the western equatorial Pacific. Concurrently, the trades were enhanced in the southeastern Pacific, so that the coastal warming off Ecuador occurred after the central Pacific warming.

It is found that the atmospheric anomalies occurring in the onset phase are controlled by background SSTs that exhibit a significant secular variation. In the late 1970s, the tropical Pacific between 20°S and 20°N experienced an abrupt interdecadal warming, concurrent with a cooling in the extratropical North Pacific and South Pacific and a deepening of the Aleutian Low. The interdecadal change of the background state affected El Niño onset by altering the formation of the onset cyclone and equatorial westerly anomalies and through changing the trades in the southeast Pacific, which determine whether a South American coastal warming leads or follows the warming at the central equatorial Pacific.

Abstract

abstract not available.

Abstract

Stability of the equatorial atmosphere to a quasi-zonal, low frequency (order of 10⁻⁶ s⁻¹) disturbance is investigated, using a model that consists of a two-layer free atmosphere and well-mixed boundary layer. The inclusion of boundary layer convergence leads to a circulation-dependent heating more nearly in accord with the behavior of numerically simulated low-frequency waves.

Slowly eastward moving unstable waves were found in a parameter regime stable to inviscid wave-CISK. The instability depends crucially upon the vertical distribution of the moist static energy of the basic state. A derived instability criterion suggests that amplification occurs when the condensational heating supported jointly by interior wave convergence and frictional convergence dominates over the dissipations due to longwave radiation and boundary layer viscosity.

The unstable waves exhibit preferred planetary scales. Since the vertical distribution of moist static energy of the basic state is closely related to sea surface temperature (SST), with increasing SST the growth rate increases for all wavelength, but the preferred scales shifts to shorter wavelength. The boundary layer convergence plays an important role in spatial scale selection. It not only suppresses unbounded growth of short waves, but also couples barotropic and baroclinic components in such a way that the generation of wave available potential energy is most efficient for planetary scales.

In the presence of boundary layer friction, the amplitude of an unstable wave in the zonal wind (or geopotential) at the upper level is significantly larger than that at the lower level. The phases between the two levels differ by about 180°. The slow eastward movement results mainly from the interior wave-induced reduction of static stability and thermal damping.

Behaviors of model unstable waves appear to resemble the observed 40–50 day mode in many aspects, such as vertical structure, energy source, zonal phase propagations, characteristic zonal and meridional scales, and its relation to SST. Limitations of the model are also discussed.

Abstract

In the tropical eastern central Pacific Ocean, the annual cycle in sea surface temperature (SST), surface winds and pressure, and clouds are alternatively dominated by an antisymmetric (with respect to the equator) monsoonal mode in February and August and a quasi-symmetric equatorial-coastal mode in May and November, both having a period of one year. The monsoonal mode is forced by the differential insulation between the Northern and Southern Hemispheres. The surface wind variation of the monsoonal mode tends to lead SST variation in late spring/fall. The equatorial-coastal mode originates from atmosphere–ocean interaction. Its development is characterized by contemporaneous intensification and spatial expansion (westward and poleward phase propagation).

The interaction between the forced monsoonal mode and the coupled equatorial-coastal mode plays a critical role in the annual cycle. From October to February, the decline of the southern winter regime of the monsoonal mode initiates and sustains the amplification of the equatorial-coastal mode, causing annual weakening of the cold tongue. From April to June, the enhancement of the poleward SST gradient associated with the decay of the equatorial-coastal mode initiates the eastern North Pacific summer monsoon. Atmosphere-ocean interaction is directly responsible for the annual weakening and reestablishment of the cold tongue, whereas the annual cycle in insulation regulates the interaction indirectly through the forced monsoonal mode.

Abstract

A coupled atmosphere–ocean–coastline model driven by solar radiation is advanced to understand the essential physics determining the annual cycle of the intertropical convergence zone (ITCZ)–equatorial cold tongue (ECT) complex and associated latitudinal climate asymmetry. With a thermocline depth similar to that of the western Pacific, the aquaplanet climate is latitudinal symmetric and stable. The presence of an oceanic eastern boundary supports an east–west asymmetric climate and an ECT due to unstable air–sea interaction and counter stabilization provided by zonal differential surface buoyancy flux. Formation of latitudinal climate asymmetry requires the presence of the ECT.

The antisymmetric solar forcing due to annual variation of the solar declination angle can convert a stable latitudinal symmetric climate into a bistable-state latitudinal asymmetric climate by changing trade winds, which in turn control annual variations of the ECT. The ECT then interacts with ITCZ, providing a self-maintenance mechanism for ITCZ to linger in one hemisphere, either the northern or southern, depending on initial conditions. The establishment of the bistable-state asymmetry requires a delicate balance between counter effects of the antisymmetric solar forcing and self-maintenance. Two factors are critical for the latter: (i) The annual variation of ECT follows the SST of the ITCZ-free hemisphere and the meridional SST gradients between the ECT and ITCZ sustain moisture convergence, which prolongs residence of the ITCZ in summer hemisphere. (ii) The latent heat released in the ITCZ produces remarkable asymmetry in Hadley circulation and trades between the two hemispheres, and the stronger evaporation cooling in the ITCZ-free hemisphere delays and weakens the warming and convection development in that hemisphere.

The annual cycle of insolation due to the earth–sun distance variation may convert the bistable-state asymmetry into a preferred latitudinal asymmetric climate. The earth’s present orbit (with a minimum distance in December solstices) favors ITCZ staying north of the equator by compelling the ECT into a delayed in-phase variation with the Southern Hemisphere SST. With annual-mean solar forcing a tilted eastern boundary can support a weak preferred latitudinal asymmetry. Inclusion of the annual variation of insolation can dramatically amplify the asymmetry in the mean climate through the self-maintenance mechanism.

Abstract

The variability of El Niño–La Niña events was analyzed in a low-dimensional phase space, a concept derived from dynamic system theory. The space–time extended EOFs derived from the observed monthly mean SST field over tropical Pacific were used as the basis of the phase space that describes the time evolution of ENSO signals. It was shown that the essential features of the ENSO variability, such as the irregular oscillation, the phase locking to the annual cycle, and the interdecadal changes in its propagation and onset, can be effectively represented by a three-dimensional phase space. The typical El Niño–La Niña life cycle is four years with its mature phases in boreal winter. The intensity of the ENSO signal within one life cycle is closely linked to the frequency of its occurrence (onset). The interdecadal variability of the ENSO signals is characterized by both the intensity and the frequency of occurrence, displaying an irregularity with the gross feature comparable to the regime behavior and intermittency of some low-dimensional chaotic systems.