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Chidong Zhang

Abstract

In response to perturbations in surface wind and energy fluxes associated with the atmospheric Madden–Julian oscillation (MJO), the thermal structure of the upper ocean (surface to 300 m) in the equatorial western Pacific exhibits prominent and intriguing intraseasonal variability. Distinct features occur in two regimes: the surface regime from the surface to 75 m and the thermocline from 100 to 300 m. The intraseasonal variability in temperature is highly coherent within each regime. The two regimes, however, are generally decoupled in the sense that the mechanisms for the intraseasonal variability in each are different and the temperature perturbations in one regime do not significantly affect those in the other. Intraseasonal perturbations in the surface regime are closely related to fluctuations in surface wind speed and wind work, implying the effects of air–sea heat fluxes and turbulent mixing. Intraseasonal surface warming and cooling may penetrate to 75 m or deeper, where the thermal stratification can be strong. In the thermocline regime, variations in temperature result from vertical displacement of the thermocline induced by zonal wind stress forcing. The intraseasonal variations in the two regimes and their phase relations are subjected to considerable modulations by the mean zonal wind. Outstanding intraseasonal signals in the upper ocean are also found for isotherm depths and ocean heat content. These observations are based on a 7-yr dataset collected from the Tropical Atmosphere–Ocean buoy moored at 0° and 165°E. The implications of these observations to ocean mixed layer thermodynamics and heat balance of the western Pacific warm pool are discussed.

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Chidong Zhang

Abstract

A particular pattern of intraseasonal perturbations in sea surface temperature (SST) is observed in the eastern Pacific Ocean following events of strong surface winds associated with the Madden–Julian oscillation (MJO). This intraseasonal SST pattern straddles at the equator with its longitudinal scales of 2–5 × 103 km and meridional scales of about 500 km. The amplitude of the perturbations is 0.5°C or greater. Positive and negative perturbations sometimes follow one another. They show tendencies of both eastward and westward movement. Such equatorially elongated perturbations in SST in the eastern Pacific are hypothesized to be caused by intraseasonal oceanic Kelvin waves forced by the MJO over the western/central Pacific. As the Kelvin waves propagate eastward, changes in the vertical temperature gradient in the upper ocean due to the fluctuations in the depth of the thermocline modify the thermal effect of the equatorial upwelling. As a result, mixed layer and surface temperatures may fluctuate. The observational basis for this hypothesis is presented through an empirical analysis of intraseasonal perturbations in SST, surface wind forcing, the depth of the thermocline, and the vertical temperature gradient of the upper ocean along the equator. The intraseasonal components of these fields fluctuate in coherence on interannual timescales. A possible implication of the observations to the interannual variability in the Pacific is proposed.

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Chidong Zhang

The Madden–Julian oscillation exerts broad influences on global weather and climate as its center of convection moves from the tropical Indian Ocean into the Pacific. Weather events under the influence of the MJO include precipitation, surface temperature, tropical cyclones, tornadoes, flood, wildfire, and lightning, among others. Several climate phenomena are also affected by the MJO. They are the monsoons, El Niño–Southern Oscillation, the North Atlantic Oscillation, the Pacific and North American pattern, the Arctic and Antarctic Oscillations or northern and southern annual modes, the Indian Ocean dipole, the Wyrtki jets, and the Indonesian Through-flow. This article provides a brief summary of the connections between the MJO and these weather and climate phenomena. These connections demonstrate the critical role of the MJO in the weather–climate continuum and its prediction.

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Chidong Zhang

Abstract

Empirical relationships between tropical sea surface temperature (SST) and atmospheric deep convection are examined. Large-scale features of tropical deep convection are estimated from two independent satellite datasets: monthly mean outgoing longwave radiation of 15 years and high-resolution pentad (5 day) fractional coverage of infrared radiation histograms of 5 years. Results based on the two datasets lead to the same conclusions.

The relationships are addressed from two aspects: how deep convection varies with changing SST and how it varies at constant SST. Deep convection remains weak and rarely observed for SST <26°C; the frequency and mean intensity of deep convection substantially increase with SST from 26°C up to about 29.5°−30°C, and then decay for further increasing SST. Meanwhile, in the warm pool region with SST >27°C, situations of no deep convection and vigorous deep convection can both be observed; the areas coverage of convectively related high clouds is always dominated by that of clear sky and low clouds. The variability of deep convection, thus becomes larger for higher SST. The large variability of deep convection at constant high SST is found to be attributable to the differences in mean spatial distributions and in the annual variations of SST and deep convection. The annual variations in areal coverages by warm sea surfaces and active deep convection are out of phase in the Indian and western Pacific oceans. The tendency of an increase in deep convection with SST is more identifiable in January but less clear in April. In general, the relationship is less apparent for the intertropical convergence zone than the other regions of the tropical oceans. Therefore, neither the increase in deep convection with SST nor the large variability of deep convection at constant high SST is generally representative of the relationship between the two fields in the tropics. The empirical relationship varies in space and time.

It is argued that, in the warm pool region, the absence of deep convection at particular locations and times and the large variability of deep convection do not imply changes in high SST have little effect on deep convection. Rather, they reflect that other factors can sometimes be dominantly unfavorable to deep convection. The influence of SST on deep convection in the warm pool region is revealed by the fact that, as SST increases, deep convection becomes more frequent and, when it occurs, tends to be more intense on average, regardless of other factors. This increase in deep convection with SST is found to be smooth and continuous with no abrupt change at any particular SST.

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Chidong Zhang

Abstract

High-resolution satellite observations are used to examine the annual cycle in mean areal coverage by highest, coldest clouds in the tropics. It is found that the mean fractional coverage by clouds with cloud-top temperatures below 200 K undergoes an annual cycle in phase with the annual cycle in tropopause height and temperature. The maximum mean fractional coverage by these clouds tends to occur during the northern winter when the tropopause is the highest and coldest throughout the tropics. The annual cycle in lower and warmer cirroform clouds is much less significant. This relationship is observed for various subregions of the tropics as well as for the tropics as a whole.

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Chidong Zhang

Abstract

Impacts of atmospheric mean zonal flows on equatorial perturbations laterally forced by extratropical mobile sources in a linear model are examined. An analytical solution of the model with a constant mean zonal flow reveals that amplitudes of forced waves can be significantly modulated by the mean zonal flow through its Doppler-shift effect on the forcing frequency. In general, amplitudes of westward-propagating waves, such as the Rossby and mixed Rossby–gravity waves, tend to be larger in mean westerlies than in mean easterlies for low-frequency forcing but smaller in mean westerlies for high-frequency forcing. The opposite dependence on the mean zonal flow applies to eastward-propagating waves, such as the Kelvin wave. The model numerical solutions show that the spatial structure of the laterally forced equatorial perturbation as a whole is sensitive to the mean zonal flow. Particularly, a substantial zonal variation in the equatorial perturbation occurs when the mean zonal flow varies in longitude. The main conclusion of this study emphasizes that the impact of the mean zonal flow on different equatorial waves is generally not the same and also varies with the forcing frequency. The study supports the speculation that the mean-flow impact is a contributing factor to the coherence between the longitudinal distributions of the atmospheric mean zonal wind field and laterally forced wave activity observed in the tropical upper troposphere.

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Chidong Zhang and Jian Ling

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This study explores the extent to which the dynamical structure of the Madden–Julian oscillation (MJO), its evolution, and its connection to diabatic heating can be described in terms of potential vorticity (PV). The signature PV structure of the MJO is an equatorial quadrupole of cyclonic and anticyclonic PV that tilts westward and poleward. This PV quadrupole is closely related to positive and negative anomalies in precipitation that are in a swallowtail pattern extending eastward along the equator and splitting into off-equatorial branches westward. Two processes dominate the generation of MJO PV. One is linear, involving MJO diabatic heating alone. The other is nonlinear, involving diabatic heating and relative vorticity of perturbations spectrally outside the MJO domain but spatially constrained to the MJO convective envelope. The MJO is thus partially a self-sustaining system and partially a consequence of scale interaction of MJO-constrained stochastic processes. Convective initiation of the MJO over the Indian Ocean features a swallowtail pattern of negative anomalous precipitation and associated anticyclonic PV anomalies at the early stage, and increasing cyclonic PV generation straddling the equator in the midtroposphere due to increasing positive anomalies in precipitation. These lead to the swallowtail pattern in positive anomalous precipitation and the associated PV quadrupole that signifies the fully developed MJO. The equatorial Kelvin and Rossby waves bear PV structures distinct from that of the MJO. They contribute insignificantly to the structure and generation of MJO PV. Solely based on the PV analysis, a hypothesis is proposed that the fundamental dynamics of the MJO depends on neither Kelvin nor Rossby waves.

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Atul Kapur and Chidong Zhang

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The Madden–Julian oscillation (MJO) is parameterized to study the role of the feedback it receives from sea surface temperature (SST) in its influence on El Niño–Southern Oscillation (ENSO). The parameterization describes MJO surface westerlies in terms of a few basic parameters that include amplitude, zonal propagation extent, propagation speed, and the interval between adjacent events. It is used to drive a coupled ocean–atmosphere model of intermediate complexity tuned to a marginally stable regime. The MJO parameters acquire values either additively (i.e., based on observed estimates of most probable value and stochasticity) or multiplicatively (i.e., modulated by an evolving model ENSO SST, albeit with some stochasticity). Simulations reveal that ENSO variance increases with the stochasticity of MJO amplitude but is insensitive to the stochasticity of zonal extent and speed, except that ENSO vanishes completely when the propagation speed is zero. Likewise, ENSO strengthens linearly with the SST modulation of MJO amplitude, but not of speed and zonal extent—even though the two are known to be significantly influenced by SST. Ensemble comparisons between simulations with and without SST feedback demonstrate that SST feedback to the MJO acting in a stable regime can be responsible for the observed ENSO variance. The multiplicative case has a larger ensemble spread than the additive case, which manifests in a larger interdecadal variability of ENSO. The results emphasize that ENSO reproduction in coupled models depends on correctly representing the MJO, especially its amplitude and SST feedback.

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Chidong Zhang and Jian Ling

Abstract

Explanations for the barrier effect of the Indo-Pacific Maritime Continent (MC) on the MJO should satisfy two criteria. First, they should include specific features of the MC, namely, its intricate land–sea distributions and elevated terrains. Second, they should include mechanisms for both the barrier effect and its overcoming by some MJO events. Guided by these two criteria, a precipitation-tracking method is applied to identify MJO events that propagate across the MC (MJO-C) and those that are blocked by the MC (MJO-B). About a half of MJO events that form over the Indian Ocean propagate through the MC. Most of them (>75%) become weakened over the MC. The barrier effect cannot be explained in terms of the strength, horizontal scale, or spatial distribution of MJO convection when it approaches the MC from the west. A distinction between MJO-B and MJO-C is their precipitation over the sea versus land in the MC region. MJO-C events rain much more over the sea than over land, whereas rainfall over the sea never becomes dominant for MJO-B. This suggests that inhibiting convective development over the sea could be a possible mechanism for the barrier effect of the MC. Preceding conditions for MJO-C include stronger low-level zonal moisture flux convergence and higher SST in the MC region. Possible connections between these large-scale conditions and the land versus sea distributions of MJO rainfall through the diurnal cycle are discussed.

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Jian Ling and Chidong Zhang

Abstract

Diabatic and latent heating profiles from four global reanalyses and three Tropical Rainfall Measuring Mission (TRMM) algorithms were compared: first generally for the tropics and then in the context of the Madden–Julian oscillation (MJO). Most of them exhibit three heating maxima corresponding to known convection centers over South America, Africa, and the Indian–western Pacific warm pool, but they still differ substantially in many ways. Most noticeably, a double-peak vertical structure with one peak in the upper and the other in the lower troposphere and relative weak heating over the Maritime Continent in comparison to heating over the Indian and western Pacific Oceans are clearly seen in some heating data but absent in others. Heating profiles associated with the MJO were diagnosed through composites and case studies. The composites were constructed as functions of MJO phases at three longitudes representing the Indian Ocean, Maritime Continent, and western Pacific, respectively. Four MJO events were chosen for the case studies, two over the Indian Ocean and two over the western Pacific. No consistent structural evolution of heating profiles through the life cycle of the MJO could be found either among different datasets in their composites at a given longitude and their case studies for a given individual MJO event or among different longitudes and MJO events within a given dataset. Nonetheless, the previously reported westward tilt in the heating field of the MJO, composed of low-level heating preceding deep heating in an active phase of the MJO and upper-level heating immediately following the active phase, is more likely to be observed over the western Pacific than other locations. The discrepancies among the datasets illustrate the infancy of estimating diabatic heating profiles from satellite observations and the need to improve the quality of the data assimilation products.

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