Jian Ling

Synoptic-Scale Controls of Persistent Low Temperature and Icy Weather over Southern China in January 2008

Abstract 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.

Structural Evolution in Heating Profiles of the MJO in Global Reanalyses and TRMM Retrievals

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 ev...

Global versus Local MJO Forecast Skill of the ECMWF Model during DYNAMO

AbstractThis study introduces a concept of global versus local forecast skill of the Madden–Julian oscillation (MJO). The global skill, measured by a commonly used MJO index [the Real-time Multivariate MJO (RMM)], evaluates the model’s capability of forecasting global patterns of the MJO, with an emphasis on the zonal wind fields. The local skill is measured by a method of tracking the eastward propagation of MJO precipitation. It provides quantitative information of the strength, propagation speed, and timing of MJO precipitation in a given region, such as the Indian Ocean. Both global and local MJO forecast skills are assessed for ECMWF forecasts of three MJO events during the 2011–12 Dynamics of the MJO (DYNAMO) field campaign. Characteristics of error growth differ substantially between global and local MJO forecast skills, and between the three MJO quantities (strength, speed, and timing) of the local skill measure. They all vary considerably among the three MJO events. Deterioration in global foreca...

MJO Signals in Latent Heating: Results from TRMM Retrievals

Abstract Four Tropical Rainfall Measuring Mission (TRMM) datasets of latent heating were diagnosed for signals in the Madden–Julian oscillation (MJO). In all four datasets, vertical structures of latent heating are dominated by two components—one deep with its peak above the melting level and one shallow with its peak below. Profiles of the two components are nearly ubiquitous in longitude, allowing a separation of the vertical and zonal/temporal variations when the latitudinal dependence is not considered. All four datasets exhibit robust MJO spectral signals in the deep component as eastward propagating spectral peaks centered at a period of 50 days and zonal wavenumber 1, well distinguished from lower- and higher-frequency power and much stronger than the corresponding westward power. The shallow component shows similar but slightly less robust MJO spectral peaks. MJO signals were further extracted from a combination of bandpass (30–90 day) filtered deep and shallow components. Largest amplitudes of bo...

Large-Scale Distinctions between MJO and Non-MJO Convective Initiation over the Tropical Indian Ocean

AbstractIn this study, the authors seek large-scale signals that may distinguish MJO from non-MJO convective events before they start over the Indian Ocean. Three such signals were found. Low-level easterly anomalies extend from the surface to the midtroposphere and move from the western to eastern Indian Ocean. Surface pressure anomalies exhibit a zonal structure of wavenumber 1 with an equatorial low-pressure surge penetrating eastward from Africa through the Indian Ocean and reaching the Maritime Continent. Negative temperature anomalies in the middle to upper troposphere start over the Indian Ocean and move eastward. All of them emerge 20 days before convective initiation of the MJO and move eastward at speeds close to that of the MJO without any direct connection to MJO convection. They are not obviously related to the extratropics in any discernible way or any preceding MJO events. They are absent in non-MJO convective events. These signals provide useful information for forecasting MJO initiation o...

Diabatic Heating Profiles in Recent Global Reanalyses

AbstractDiabatic 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 th...

Potential Vorticity of the Madden-Julian Oscillation

AbstractThis 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 th...

Evolution of the Madden–Julian Oscillation in Two Types of El Niño

AbstractEvolution characteristics of the Madden–Julian oscillation (MJO) during the eastern Pacific (EP) and central Pacific (CP) types of El Nino have been investigated. MJO activities are strengthened over the western Pacific during the predeveloping and developing phases of EP El Nino, but suppressed during the mature and decaying phases. In contrast, MJO activities do not show a clear relationship with CP El Nino before their occurrence over the western Pacific, but they increase over the central Pacific during the mature and decaying phases of CP El Nino. Lag correlation analyses further confirm that MJO activities over the western Pacific in boreal spring and early summer are closely related to EP El Nino up to 2–11 months later, but not for CP El Nino. EP El Nino 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 Nino can suppress convection and moisture flux convergence the...

Barrier Effect of the Indo-Pacific Maritime Continent on the MJO: Perspectives from Tracking MJO Precipitation

AbstractExplanations 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 ...

Relationship between SST anomalies and the intensity of intraseasonal variability

Intraseasonal variability (ISV) is a primary source for the sub-seasonal prediction that affects the livelihood of billions of people. Interannual variation of ISV intensity is important for seasonal prediction of ISV impacts on severe weathers. Existing measures of overall tropical ISV intensity, however, do not show any significant simultaneous relationship with external sea surface temperature anomalies (SSTAs). In this study, it is proposed that the ISV intensity, represented by the seasonal standard deviation of the 30–90-day filtered outgoing longwave radiation (OLR), has a good relation with the external SSTAs. With this measure, two major components of the interannual variability of global ISV intensity are detected for both boreal summer and winter: EOF1 calls its variability over the central Pacific and EOF2 is associated with the variability over the Indo-Pacific Warm Pool region. More importantly, each of these two components is significantly related to SSTAs over a specific tropical region. The central Pacific ISV intensity is strong during central Pacific warming, while the ISV intensity is strong over the Indo-Pacific Warm Pool region during eastern Pacific cooling. The eastern and central Pacific warming has very different impacts on the ISV intensity: The eastern Pacific warming largely reduces the winter ISV intensity over the Indian Ocean, while the central Pacific warming only induces neutral winter ISV intensity anomalies over the Indian Ocean. In the summer, the ISV intensity variability is confined near the equator associated with the central Pacific warming; the eastern Pacific warming, however, induces large ISV intensity variability over the western North pacific because of strong northeastward propagation of the boreal summer ISV under the easterly vertical shear.