Maritime Continent Region Research Articles

Uncertainties in TRMM‐Era multisatellite‐based tropical rainfall estimates over the Maritime Continent

AbstractThis study investigates the regional and seasonal rainfall rate retrieval uncertainties within nine state‐of‐the‐art satellite‐based rainfall products over the Maritime Continent (MC) region. The results show consistently larger differences in mean daily rainfall among products over land, especially over mountains and along coasts, compared to over ocean, by about 20% for low to medium rain rates and 5% for heavy rain rates. However, rainfall differences among the products do not exhibit any seasonal dependency over both surface types (land and ocean) of the MC region. The differences between products largely depends on the rain rate itself, with a factor 2 difference for light rain and 30% for intermediate and high rain rates over ocean. The rain‐rate products dominated by microwave measurements showed less spread among themselves over ocean compared to the products dominated by infrared measurements. Conversely, over land, the rain gauge‐adjusted post–real‐time products dominated by microwave measurements produced the largest spreads, due to the usage of different gauge analyses for the bias corrections. Intercomparisons of rainfall characteristics of these products revealed large discrepancies in detecting the frequency and intensity of rainfall. These satellite products are finally evaluated at subdaily, daily, monthly, intraseasonal, and seasonal temporal scales against high‐quality gridded rainfall observations in the Sarawak (Malaysia) region for the 4 year period 2000–2003. No single satellite‐based rainfall product clearly outperforms the other products at all temporal scales. General guidelines are provided for selecting a product that could be best suited for a particular application and/or temporal resolution.

Precipitation Associated with Convergence Lines

Precipitation is often organized along coherent lines of low-level convergence, which at longer time and space scales form well-known convergence zones over the world’s oceans. Here, an automated, objective method is used to identify instantaneous low-level convergence lines in reanalysis data and calculate their frequency for the period 1979–2013. Identified convergence lines are combined with precipitation observations to assess the extent to which precipitation around the globe is associated with convergence lines in the lower troposphere. It is shown that a large percentage of precipitation (between 65% and 90%) over the tropical oceans is associated with such convergence lines, with large regional variations of up to 30% throughout the year, especially in the eastern Pacific and Atlantic Oceans. Over land, the annual-mean proportion of precipitation associated with convergence lines ranges between 30% and 60%, and the lowest proportions (less than 15%) associated with convergence lines occur on the eastern flank of the subtropical highs. Overall, much greater precipitation is associated with long coherent lines (greater than 300 km in length) than with shorter fragmented lines (less than 300 km), and the majority of precipitation associated with shorter lines occurs over land. The proportion of precipitation not associated with any convergence line primarily occurs where both precipitation and frequency of convergence lines are low. The high temporal and spatial resolution of the climatology constructed also enables an examination of the diurnal cycle in the relationship between convergence lines and precipitation. Here an example is provided over the tropical Maritime Continent region.

Simulation, Modeling, and Dynamically Based Parameterization of Organized Tropical Convection for Global Climate Models

Abstract A new approach for treating organized convection in global climate models (GCMs) referred to as multiscale coherent structure parameterization (MCSP) introduces physical and dynamical effects of organized convection that are missing from contemporary parameterizations. The effects of vertical shear are approximated by a nonlinear slantwise overturning model based on Lagrangian conservation principles. Simulation of the April 2009 Madden–Julian oscillation event during the Year of Tropical Convection (YOTC) over the Indian Ocean using the Weather Research and Forecasting (WRF) Model at 1.3-km grid spacing identifies self-similar properties for squall lines, MCSs, and superclusters embedded in equatorial waves. The slantwise overturning model approximates this observed self-similarity. The large-scale effects of MCSP are examined in two categories of GCM. First, large-scale convective systems simulated in an aquaplanet model are approximated by slantwise overturning with attention to convective momentum transport. Second, MCSP is utilized in the Community Atmosphere Model, version 5.5 (CAM5.5), as tendency equations for second-baroclinic heating and convective momentum transport. The difference between MCSP and CAM5.5 is a direct measure of the global effects of organized convection. Consistent with TRMM measurements, the MCSP generates large-scale precipitation patterns in the tropical warm pool and the adjoining locale; improves precipitation in the intertropical convergence zone (ITCZ), South Pacific convergence zone (SPCZ), and Maritime Continent regions; and affects tropical wave modes. In conclusion, the treatment of organized convection by MCSP is salient for the next generation of GCMs.

Cloud heights measured by MISR from 2000 to 2015

AbstractDavies and Molloy (2012) reported a decrease in the global effective cloud height over the first 10 years of Multiangle Imaging Spectroradiometer (MISR) measurements on the Terra satellite. We have reexamined their time series for possible artefacts that might especially affect the initial portion of the record when the heights appeared anomalously high. While variations in sampling were shown to be inconsequential, an artefact due to the change in equator crossing time that affected the first 2 years was discovered, and this has now been corrected. That correction, together with the extension of the time series by five more years, yields no significant overall trend in global heights during the first 15 years of Terra operation. The time series is dominated by large interannual fluctuations associated with La Niña events that mask any overall trend on a global scale. On a regional basis, the cloud heights showed significant interannual variations of much larger amplitude, sometimes with fairly direct cancellation between regions. There were unexplained differences between the two hemispheres in the timing of height anomalies. These differences persisted over a large range of extratropical latitudes, suggestive of teleconnections. Within the tropics, there were very strong changes associated with the Central Pacific and Indonesian Maritime Continent regions that oscillated out of phase with each other, with interannual amplitudes that exceeded 1 km.

Why does the MJO detour the Maritime Continent during austral summer?

AbstractEastward propagation of the Madden‐Julian Oscillation (MJO) detours the Maritime Continent (MC) region southward during austral summer, exhibiting enhanced convective activity preferentially in the southern part of the MC area with much weaker anomalies in the central MC area. Column‐integrated moist static energy budget of the MJO is analyzed to understand the processes responsible for the MJO detouring. Results show that zonal and meridional moisture advection is the essential process to the MJO detouring, causing the difference between the southern and central MC regions in the moisture recharge before and during the MJO onset. Further analysis reveals that moisture advection by MJO perturbation winds acting upon the background moisture gradient has the dominant contribution to the regional contrast between the central and southern MC areas. The zonal moisture advection is greater in the southern MC region because the zonal gradient of the background moisture field is much steeper in the southern MC area than in the central MC area. The onset of the Australian monsoon in austral summer contributes to the establishment of the sharp zonal moisture gradient in the southern MC region. The meridional moisture advection is weaker in the central MC area because meridional wind anomalies associated with the MJO vary regionally, presumably through interactions with the topography and land‐sea contrast.

Possible impacts of spring sea surface temperature anomalies over South Indian Ocean on summer rainfall in Guangdong-Guangxi region of China

Based on observational and reanalysis data for 1979–2015, the possible impacts of spring sea surface temperature anomalies (SSTA) over the South Indian Ocean on the inter-annual variations of summer rainfall in Guangdong and Guangxi Provinces (i.e., the Guangdong-Guangxi area, GG) were analysed in this study. The physical mechanism behind these impacts was explored. Two geographic regions over [65°E–95°E, 35°S–25°S] and [90°E–110°E, 20°S–5°S] were defined as the western pole region and the eastern pole region, respectively, for the GG summer precipitation (PGG)-related South Indian Ocean dipole SSTA pattern (R-SIODP). The difference between springtime SST anomalies averaged over the western pole region and that averaged over the eastern pole region was defined as the R-SIODP index. The correlation between the spring R-SIODP index and GG summer precipitation can reach up to 0.52. In the spring of positive R-SIODP anomaly, southerly winds over the western pole of the R-SIODP weaken, whereas the southeast trade winds over the eastern pole strengthen. By means of the wind-evaporation-SST feedback mechanism, the enhanced southeast trade winds can weaken the evaporation over the western pole of the R-SIODP and enhance the evaporation over the eastern pole. This results in a sustained positive SSTA in the western pole of the R-SIODP and a sustained negative SSTA in the eastern pole, whereby the distribution of the SSTAs maintains until summer. The SST dipole abnormally enhances the cross-equatorial airflow near 105°E, which intensifies the anomalous anti-cyclonic circulation over South China Sea at 850 hPa and simultaneously results in abnormal enhancement of water vapour transport to GG. Additionally, the SST dipole promotes abnormal divergence in the lower troposphere and abnormal convergence in the upper troposphere over the maritime continent (MC) region. Moreover, the low-level convergence in GG is enhanced, which results in abnormal enhancement of ascending motion in the GG that is conducive to positive summer rainfall anomaly in this region. In this study, the spring R-SIODP index, the SST to the east of Australia and to the east of southern Africa, and the North Atlantic oscillation (NAO) index were used to construct a statistical prediction model for the inter-annual variability of the GG summer rainfall anomaly. This model can well predict the accuracy of the inter-annual variation of GG summer rainfall.

ACCESS-S1 The new Bureau of Meteorology multi-week to seasonal prediction system

ACCESS-S1 will be the next version of the Australian Bureau of Meteorology's seasonal prediction system, due to become operational in early 2018. The multiweek and seasonal performance of ACCESS-S1 has been evaluated based on a 23-year hindcast set and compared to the current operational system, POAMA. The system has considerable enhancements compared to POAMA, including higher vertical and horizontal resolution of the component models and state-ofthe-art physics parameterisation schemes. ACCESS-S1 is based on the UK Met Office GloSea5-GC2 seasonal prediction system, but has enhancements to the ensemble generation strategy to make it appropriate for multi-week forecasting, and a larger ensemble size.ACCESS-S1 has markedly reduced biases in the mean state of the climate, both globally and over Australia, compared to POAMA. ACCESS-S1 also better predicts the early stages of the development of the El Niño Southern Oscillation (through the predictability barrier) and the Indian Ocean Dipole, as well as multi-week variations of the Southern Annular Mode and the Madden-Julian Oscillation — all important drivers of Australian climate variability. There is an overall improvement in the skill of the forecasts of rainfall, maximum temperature (Tmax) and minimum temperature (Tmin) over Australia on multi-week timescales compared to POAMA. On seasonal timescales the differences between the two systems are generally less marked. ACCESS-S1 has improved seasonal forecasts over Australia for the austral spring season compared to POAMA, with particularly good forecast reliability for rainfall and Tmax. However, forecasts of seasonal mean Tmax are noticeably less skilful over eastern Australia for forecasts of late autumn and winter compared to POAMA.The study has identified scope for improvement of ACCESS-S in the future, particularly 1) reducing rainfall errors in the Indian Ocean and Maritime Continent regions, and 2) initialising the land surface with realistic soil moisture rather than climatology. The latter impacts negatively on the skill of the temperature forecasts over eastern Australia and is being addressed in the next version of the system, ACCESS-S2.

ACCESS-S1 The new Bureau of Meteorology multi-week to seasonal prediction system

ACCESS-S1 will be the next version of the Australian Bureau of Meteorology's seasonal prediction system, due to become operational in early 2018. The multiweek and seasonal performance of ACCESS-S1 has been evaluated based on a 23-year hindcast set and compared to the current operational system, POAMA. The system has considerable enhancements compared to POAMA, including higher vertical and horizontal resolution of the component models and state-ofthe-art physics parameterisation schemes. ACCESS-S1 is based on the UK Met Office GloSea5-GC2 seasonal prediction system, but has enhancements to the ensemble generation strategy to make it appropriate for multi-week forecasting, and a larger ensemble size. ACCESS-S1 has markedly reduced biases in the mean state of the climate, both globally and over Australia, compared to POAMA. ACCESS-S1 also better predicts the early stages of the development of the El Nino Southern Oscillation (through the predictability barrier) and the Indian Ocean Dipole, as well as multi-week variations of the Southern Annular Mode and the Madden-Julian Oscillation — all important drivers of Australian climate variability. There is an overall improvement in the skill of the forecasts of rainfall, maximum temperature (Tmax) and minimum temperature (Tmin) over Australia on multi-week timescales compared to POAMA. On seasonal timescales the differences between the two systems are generally less marked. ACCESS-S1 has improved seasonal forecasts over Australia for the austral spring season compared to POAMA, with particularly good forecast reliability for rainfall and Tmax. However, forecasts of seasonal mean Tmax are noticeably less skilful over eastern Australia for forecasts of late autumn and winter compared to POAMA. The study has identified scope for improvement of ACCESS-S in the future, particularly 1) reducing rainfall errors in the Indian Ocean and Maritime Continent regions, and 2) initialising the land surface with realistic soil moisture rather than climatology. The latter impacts negatively on the skill of the temperature forecasts over eastern Australia and is being addressed in the next version of the system, ACCESS-S2.

Extreme Intertropical Convergence Zone shifts over Southern Maritime Continent

AbstractThis study investigated the Intertropical Convergence Zone (ITCZ) extreme shifts in the Southern Maritime Continent (SMC) region in austral spring during the period 1979–2015. The results have shown that the ITCZ Southern Boundary (SB) presents great correlations with a meridional Sea Surface Temperature (SST) gradient and Indian Ocean Dipole (IOD) events. However, besides these local forcing, the extreme ITCZ shifts are also influenced by El Niño‐Southern Oscillation (ENSO) events and an associated zonal SST gradient. The extreme shifts are due mainly to the combination of changes in Walker Circulation (WC) in the Pacific and Indian Ocean (IO) and local SST anomalies. Therefore, during ITCZ southward extreme shifts, the WC is stronger leading to the strengthening of the northwest equatorial winds in the IO, which pumps moist air and boosts the diabatic heating in the SMC due to the local positive SST anomalies present in this region. Otherwise, during northward extreme shifts, the WC is weaker, not allowing the performance of the northwest equatorial winds in the IO, which associated with the local negative SST anomalies, leads to drier conditions in the SMC region. Another interesting result found here is that a strong meridional or zonal index not necessarily implies a larger shift. The combination of very strong ENSO and IOD events can lead to greater ITCZ SB shifts. Besides, the ITCZ SB shifts can act as a bridge linking IOD and ENSO.

On the Presence of Tropical Vortices over the Southeast Asian Sea–Maritime Continent Region

Abstract Reanalysis and observation data from 1979 to 2010 are used to study the climatological behavior of regional vortices over the Southeast Asian sea–Maritime Continent region (SEAMC). After eliminating tropical cyclones from the International Best Tracks Archive for Climate Stewardship (IBTrACS), significant numbers of vortices remained over the region. The results also show that the vortices, rather than being relatively stationary near the Borneo coast in the winter, were consistently present over the SEAMC throughout the year, migrating from the area of the coast of Vietnam, the Philippines, and the Southeast Asian sea in summer to the island of Borneo region in winter. These vortices can produce significant amounts of rainfall in Vietnam, especially in central coastal regions during the postsummer monsoon (autumn) period.

Sea-Breeze Dynamics and Convection Initiation: The Influence of Convective Parameterization in Weather and Climate Model Biases

Abstract There are some long-established biases in atmospheric models that originate from the representation of tropical convection. Previously, it has been difficult to separate cause and effect because errors are often the result of a number of interacting biases. Recently, researchers have gained the ability to run multiyear global climate model simulations with grid spacings small enough to switch the convective parameterization off, which permits the convection to develop explicitly. There are clear improvements to the initiation of convective storms and the diurnal cycle of rainfall in the convection-permitting simulations, which enables a new process-study approach to model bias identification. In this study, multiyear global atmosphere-only climate simulations with and without convective parameterization are undertaken with the Met Office Unified Model and are analyzed over the Maritime Continent region, where convergence from sea-breeze circulations is key for convection initiation. The analysis shows that, although the simulation with parameterized convection is able to reproduce the key rain-forming sea-breeze circulation, the parameterization is not able to respond realistically to the circulation. A feedback of errors also occurs: the convective parameterization causes rain to fall in the early morning, which cools and wets the boundary layer, reducing the land–sea temperature contrast and weakening the sea breeze. This is, however, an effect of the convective bias, rather than a cause of it. Improvements to how and when convection schemes trigger convection will improve both the timing and location of tropical rainfall and representation of sea-breeze circulations.

Subseasonal Variability of Precipitation in China during Boreal Winter

Abstract Using pentad data of the Northern Hemisphere extended winter (November–March) from 1979 to 2012 derived from the daily rainfall of the National Meteorological Information Center of China, subseasonal variability of precipitation in China is analyzed. The two dominant modes of subseasonal variability are identified with an empirical orthogonal function (EOF) analysis. The first EOF mode (EOF1) is characterized by a monopole in South China, whereas the second EOF mode (EOF2) has a meridional dipole structure with opposite precipitation anomalies over the Yangtze River basin and the coastal area of South China. These two modes tend to have a phase shift to each other in both space and time, indicating that part of their variability represents a southward-propagating pattern. The subseasonal variability is decomposed into two components: one related to the Madden–Julian oscillation (MJO) and the other independent of MJO. It is found that the MJO contributes to about 10% of the precipitation variability in South China. EOF1 is associated with MJO phase 3, corresponding to enhanced equatorial convection in the Indian Ocean and depressed convection in the western Pacific, while EOF2 is related to MJO phase 5 when the enhanced tropical convection moves to the Maritime Continent region. Subseasonal precipitation variability in China that is independent of the MJO is especially affected by processes including tropical convection variability and the “cold surge” phenomenon or the development of a Siberian high and cold-air outbreak in East Asia associated with a wave train from the North Atlantic.

Seasonal climate summary southern hemisphere (spring 2014): El Niño continues to try to break through, and Australia has its warmest spring on record (again!)

Southern hemisphere circulation patterns and associated anomalies for the aus-tral spring 2014 are reviewed, with emphasis given to the Pacific Basin climate indicators and Australian rainfall and temperature patterns. Equatorial sea-surface temperatures in the Pacific warmed through the season, and the South-ern Oscillation just exceeded El Nino thresholds. However, although trade wind strength weakened at times, particularly late in the season, tropical cloudiness near the Date Line and trade wind strength were more often close to average, suggesting the atmosphere was still not firmly linked with the warmer ocean be-low. The negative Indian Ocean Dipole values of winter flipped to neutral or weakly positive values through spring. Pressures in the Australian and Maritime Continent regions were high. Around Antarctica, sea ice reached a record extent in September, with expansion most prominent in the Indian Ocean sector and north of the Ross Sea. Winds to the north of these regions were strong. The Antarctic ozone hole was of moderate severity, and of similar size to ozone holes in the early 1990s. Over Australia, it was the warmest spring on record, surpassing the record warm spring of 2013. A strong anomaly in the 200 hPa geopotential height persisted over the south-east of the country through October and November. Australia was particularly dry in the east through spring, but Western Australia received good rainfall. Column ozone levels in the Australian region were slightly above average and consistent with increased ozone transport to mid-latitudes associat-ed with the phase of the Quasi-Biennial Oscillation

Asian summer monsoon onset in simulations and CMIP5 projections using four Chinese climate models

The reproducibility and future changes of the onset of the Asian summer monsoon were analyzed based on the simulations and projections under the Representative Concentration Pathways (RCP) scenario in which anthropogenic emissions continue to rise throughout the 21st century (i.e. RCP8.5) by all realizations from four Chinese models that participated in the Coupled Model Intercomparison Project Phase 5 (CMIP5). Delayed onset of the monsoon over the Arabian Sea was evident in all simulations for present-day climate, which was associated with a too weak simulation of the low-level Somali jet in May. A consistent advanced onset of the monsoon was found only over the Arabian Sea in the projections, where the advanced onset of the monsoon was accompanied by an increase of rainfall and an anomalous anticyclone over the northern Indian Ocean. In all the models except FGOALS-g2, the enhanced low-level Somali jet transported more water vapor to the Arabian Sea, whereas in FGOALS-g2 the enhanced rainfall was determined more by the increased wind convergence. Furthermore, and again in all models except FGOALS-g2, the equatorial SST warming, with maximum increase over the eastern Pacific, enhanced convection in the central West Pacific and reduced convection over the eastern Indian Ocean and Maritime Continent region, which drove the anomalous anticyclonic circulation over the western Indian Ocean. In contrast, in FGOALS-g2, there was minimal (near-zero) warming of projected SST in the central equatorial Pacific, with decreased convection in the central West Pacific and enhanced convection over the Maritime Continent. The broader-scale differences among the models across the Pacific were related to both the differences in the projected SST pattern and in the present-day simulations.

Importance and origin of halosteric contribution to sea level change in the southeast Indian Ocean during 2005–2013

AbstractSteric sea level change has been identified as one of the major contributors to the regional variability of sea level trends observed by satellite altimetry for the past two decades. This contribution varies in space and time. The temperature (thermosteric) contribution to sea level has generally been found to be more important than the salinity (halosteric) effect. Based on sea level measurements from satellite altimetry and temperature and salinity data from Argo floats during 2005–2013, we found that the southeast Indian Ocean experiences a large halosteric contribution to sea level change. The conspicuously large halosteric contribution is associated with a freshening in the upper 300 m. Neither local atmospheric forcing such as Ekman pumping and E − P nor halosteric signal transmitted from the western tropical Pacific can explain this freshening. An enhanced precipitation in the Maritime Continent region and the observed strengthening of the Indonesian throughflow are the likely causes.

Improvement of CGCM prediction for wet season precipitation over Maritime Continent using a bias correction method

ABSTRACTA new model output statistics method – Ensemble Selective Simple Linear Regression (E‐SSLR) – is developed based on SLR in order to increase the seasonal prediction skill of a Coupled General Circulation Model (CGCM) over the Maritime Continent (MC), a region with large model simulation errors. E‐SSLR is applied to Pusan National University (PNU) CGCM hindcast over the MC region for the period of 1981–2010 to reduce the systematic model bias in boreal winter (DJF) seasonal mean precipitation and outgoing long‐wave radiation (OLR) anomalies. Three oceanic indices (Nino 3.4, El Nino Modoki and Indian Ocean Dipole (IOD) Mode indices) and one atmospheric index (Southern Oscillation Index, SOI) produced from PNU CGCM hindcast are used as SLR predictor. E‐SSLR consists of three steps: Selection, SLR and Ensemble. The selection and ensemble steps are added to the conventional SLR step to overcome the weakness of the linear regression method. In the selection step, the grids with a temporal correlation coefficient between predictor and predictand exceeding the threshold are selected. These grids (grid‐selected) are corrected by SLR in the second step. For the grids that are grid‐not‐selected, the original CGCM results are used without further correction. This prevents insignificant statistical correction due to the application of low correlated predictors to the SLR. The correction effect of E‐SSLR is analysed in terms of deterministic and categorical analyses. The result shows that the seasonal predictability of DJF seasonal precipitation and OLR in the MC region is increased by using E‐SSLR, and this increment is statistically significant. The correction effect is larger when indices with high predictability that are closely correlated with the predictand are used as predictors.

Global-to-Regional Nested Grid Climate Simulations in the GFDL High Resolution Atmospheric Model

A two-way nested grid version of the Geophysical Fluid Dynamics Laboratory High Resolution Atmosphere Model (HiRAM) has been developed that uses simple methods for providing nested grid boundary conditions and mass-conserving nested-to-global communication. Nested grid simulations over the Maritime Continent and over North America were performed, each at two different resolutions: a 110-km mean grid cell width refined by a factor of 3, and a 50-km mean grid cell width refined by a factor of 2. Nested grid simulations were compared against uniform-resolution simulations, and against reanalyses, to determine the effect of grid nesting on both the modeled global climate and the simulation of small-scale features. Orographically forced precipitation was robustly found to be simulated with more detail and greater realism in a nested grid simulation compared with when only the coarse grids were simulated alone. Tropical precipitation biases were reduced in the Maritime Continent region when a nested grid was introduced. Both results were robust to changes in the nested grid parameterization tunings. In North America, cold-season orographic precipitation was improved by nesting, but precipitation biases in the central and eastern United States were little changed. Improving the resolution through nesting also allowed for more intense rainfall events, greater Kelvin wave activity, and stronger tropical cyclones. Nested grid boundary artifacts were more pronounced when a one-way, noninteractive nested grid was used.

Regional Climate Modeling over the Maritime Continent. Part II: New Parameterization for Autoconversion of Convective Rainfall

Abstract This paper describes a new method for parameterizing the conversion of convective cloud liquid water to rainfall (“autoconversion”) that can be used within large-scale climate models, and evaluates the new method using the Regional Climate Model, version 3 (RegCM3), coupled to the land surface scheme Integrated Biosphere Simulator (IBIS). The new method is derived from observed distributions of cloud water content and is constrained by observations of cloud droplet characteristics and climatological rainfall intensity. This new method explicitly accounts for subgrid variability with respect to cloud water density and is independent of model resolution, making it generally applicable for large-scale climate models. This work builds on the development of a new parameterization method for convective cloud fraction, which was described in Part I. Simulations over the Maritime Continent using the Emanuel convection scheme show significant improvement in model performance, not only with respect to convective rainfall but also in shortwave radiation, net radiation, and turbulent surface fluxes of latent and sensible heat, without any additional modifications made to the simulation of those variables. Model improvements are demonstrated over a 19-yr validation period as well as a shorter 4-yr evaluation. Model performance with the Grell convection scheme is not similarly improved and reasons for this outcome are discussed. This work illustrates the importance of representing observed subgrid-scale variability in diurnally varying convective processes for simulations of the Maritime Continent region.

The sensitivity of the tropical circulation and Maritime Continent precipitation to climate model resolution

The dependence of the annual mean tropical precipitation on horizontal resolution is investigated in the atmospheric version of the Hadley Centre General Environment Model. Reducing the grid spacing from about 350 km to about 110 km improves the precipitation distribution in most of the tropics. In particular, characteristic dry biases over South and Southeast Asia including the Maritime Continent as well as wet biases over the western tropical oceans are reduced. The annual-mean precipitation bias is reduced by about one third over the Maritime Continent and the neighbouring ocean basins associated with it via the Walker circulation. Sensitivity experiments show that much of the improvement with resolution in the Maritime Continent region is due to the specification of better resolved surface boundary conditions (land fraction, soil and vegetation parameters) at the higher resolution. It is shown that in particular the formulation of the coastal tiling scheme may cause resolution sensitivity of the mean simulated climate. The improvement in the tropical mean precipitation in this region is not primarily associated with the better representation of orography at the higher resolution, nor with changes in the eddy transport of moisture. Sizeable sensitivity to changes in the surface fields may be one of the reasons for the large variation of the mean tropical precipitation distribution seen across climate models.

The Extreme Drought Event during Winter–Spring of 2011 in East China: Combined Influences of Teleconnection in Midhigh Latitudes and Thermal Forcing in Maritime Continent Region

Abstract The middle and lower reaches of the Yangtze River (MLRYR) in China experienced an extremely severe and persistent drought event from January to May of 2011. Using both the observational data and NCEP–NCAR reanalysis, features of the drought event and the related circulation anomalies were investigated. It is found that the precipitation during the investigated period of 2011 was deficient mostly along the Yangtze River. The water vapor diverged from MLRYR southward into the Bay of Bengal, South China Sea, and the Philippines. There were two factors facilitating the drought event. One was the quasi-stationary Rossby wave–related teleconnection, which propagated eastward at midhigh latitudes from the North Atlantic to East Asia, reinforcing the Siberian high and the East Asian trough, henceforth resulting in the divergence anomalies in MLRYR in the lower troposphere. This quasi-stationary wave train, though originating from the North Atlantic region, was not essentially related to the North Atlantic Oscillation. Another factor for the drought event was the persistent anomalous thermal forcing over the Maritime Continent, which induced the anomalous divergence in the upper troposphere in this region, building up an anomalous Hadley circulation with its ascent branch over the Maritime Continent and descent branch over MLRYR. This thermal forcing was possibly, but not necessarily, related to the La Niña event. The persistence of the drought event over MLRYR was due to the maintenance of the quasi-stationary waves at midhigh latitudes and the persistent anomalous thermal forcing in the Maritime Continent.