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.
In January 2008, central and southern China experienced persistent low temperatures, freezing rain, and snow. The large-scale conditions associated with the occurrence and development of these snowstorms are examined in order to identify the key synoptic controls leading to this event. Three main factors are identified: 1) the persistent blocking high over Siberia, which remained quasi-stationary around 65°E for 3 weeks, led to advection of dry and cold Siberian air down to central and southern China; 2) a strong persistent southwesterly flow associated with the western Pacific subtropical high led to enhanced moisture advection from the Bay of Bengal into central and southern China; and 3) the deep inversion layer in the lower troposphere associated with the extended snow cover over most of central and southern China. The combination of these three factors is likely responsible for the unusual severity of the event, and hence a long return period.
Diabatic heating profiles are extremely important to the atmospheric circulation in the tropics and therefore to the earth's energy and hydrological cycles. However, their global structures are poorly known because of limited information from in situ observations. Some modern global reanalyses provide the temperature tendency from the physical processes. Their proper applications require an assessment of their accuracy and uncertainties. In this study, diabatic heating profiles from three recent global reanalyses [ECMWF Interim Re-Analysis (ERA-Interim), Climate Forecast System Reanalysis (CFSR), and Modern Era Retrospective Analysis for Research and Applications (MERRA)] are compared to those derived from currently available sounding observations in the tropics and to each other in the absence of the observations. Diabatic heating profiles produced by the reanalyses match well with those based on sounding observations only at some locations. The three reanalyses agree with each other better in the extratropics, where large-scale condensation dominates the precipitation process in data assimilation models, than in the tropics, where cumulus parameterization dominates. In the tropics, they only agree with each other in gross features, such as the contrast between the ITCZs over different oceans. Their largest disagreement is the number and level of heating peaks in the tropics. They may produce a single, double, or triple heating peak at a given location. It is argued that cumulus parameterization cannot be the sole source of the disagreement. Implications of such disagreement are discussed.
Possible effects of the diurnal cycle in land convection on propagation of the Madden–Julian Oscillation over the Indo‐Pacific Maritime Continent (MC) were investigated using satellite observations. Four features distinguishable from their respective climatology are uniquely associated with MJO events that cross the MC: strong precipitation over land as their convection centers approach the MC, subsequent increased soil moisture, reduced diurnal amplitude of land convection, and the dominance of precipitation over water by nondiurnal convection as their convection centers move over the MC. These results provide observational evidence for a proposed MAritime Continent Convective diurnal Cycle mechanism in which the diurnal cycle in land convection acts as an intrinsic barrier effect on MJO propagation over the MC.
Evolution characteristics of the Madden–Julian oscillation (MJO) during the eastern Pacific (EP) and central Pacific (CP) types of El Niño have been investigated. MJO activities are strengthened over the western Pacific during the predeveloping and developing phases of EP El Niño, but suppressed during the mature and decaying phases. In contrast, MJO activities do not show a clear relationship with CP El Niño before their occurrence over the western Pacific, but they increase over the central Pacific during the mature and decaying phases of CP El Niño. Lag correlation analyses further confirm that MJO activities over the western Pacific in boreal spring and early summer are closely related to EP El Niño up to 2–11 months later, but not for CP El Niño. EP El Niño tends to weaken the MJO and lead to a much shorter range of its eastward propagation. Anomalous descending motions over the Maritime Continent and western Pacific related to El Niño can suppress convection and moisture flux convergence there and weaken MJO activities over these regions during the mature phase of both types of El Niño. MJO activities over the western Pacific are much weaker in EP El Niño due to the stronger anomalous descending motions. Furthermore, the MJO propagates more continuously and farther eastward during CP El Niño because of robust moisture convergence over the central Pacific, which provides adequate moisture for the development of MJO convection.
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 offequatorial 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 MJOconstrained 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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