Subtropical Land Regions Research Articles

The Influences of Atlantic Sea Surface Temperature Anomalies on the ENSO-Independent Interannual Variability of East Asian Summer Monsoon Rainfall

Abstract As the most dominant tropical climate mode on the interannual time scale, El Niño–Southern Oscillation (ENSO) is suggested to significantly influence the interannual variation of East Asian summer monsoon rainfall (IEASMR). However, the leading mode of IEASMR remains almost untouched when the impacts of preceding ENSO events are linearly removed, suggesting the existence of alternative impact factors and predictability sources of IEASMR. After removing the impact of ENSO, the sea surface temperature anomalies (SSTAs) over both the tropical Atlantic and extratropical North Atlantic are found to be related to IEASMR through atmospheric teleconnections. Positive SSTA over the tropical Atlantic could induce tropical diabatic heating, which triggers an equivalent barotropic Rossby wave train emanating from the Atlantic, going across the Eurasian continent, and ending with a cyclonic anomaly over northeast Asia. The tropical diabatic heating could also induce western North Pacific anomalous anticyclone via tropical routes. The dipole SSTA pattern with cooling in the west and warming in the east over the extratropical North Atlantic induces local circulation anomalies through heat flux exchange, which could further perturb a Rossby wave train with a cyclonic anomaly over northeast Asia, thus modulating IEASMR. Numerical experiments with prescribed atmospheric heating associated with Atlantic SSTAs could realistically reproduce these teleconnections toward IEASMR. By adding the predictability sources of Atlantic SSTAs, the seasonal hindcast skills of IEASMR could be significantly improved over both the tropical western North Pacific and subtropical land regions such as central China and Japan. Significance Statement The purpose of this article is to identify the alternative impact factors of the interannual variation of East Asian summer monsoon rainfall (IEASMR), after removing the impact of ENSO, considering the limited contribution of ENSO to the variances of IEASMR. Here, we find that the Atlantic sea surface temperature anomalies (SSTAs) play a considerable role in driving IEASMR. The impact of ENSO on IEASMR is mainly confined to the tropical western North Pacific, while the Atlantic SSTAs influence rainfall over subtropical East Asia and the tropical western North Pacific through both tropical and extratropical routes. The results unravel the important roles of Atlantic SSTAs in driving ENSO-independent IEASMR, which will have a large implication for the seasonal prediction of East Asian summer climate.

Warmer Pliocene Upwelling Site SST Leads to Wetter Subtropical Coastal Areas: A Positive Feedback on SST

AbstractThe early‐to mid‐Pliocene (5.3–3 Ma), characterized by warmer temperatures and similar CO2 concentrations to present day, is considered a useful analog for future warming scenarios. Geological evidence suggests that during the Pliocene, many modern‐day desert regions received higher levels of rainfall and supported large perennial lakes and wetter vegetation types. These wetter conditions have been difficult to reconcile with model predictions of 21st century drying over most subtropical land regions. Using an atmospheric General Circulation Model, we show that underestimates of Pliocene rainfall over certain areas in models may be related to insufficient sea surface temperature (SST) warmth simulated over relatively local eastern boundary current regions. When SSTs off the coast of California are raised to more closely match some proxy reconstructions, rainfall increases over much of adjacent western North America. Over the southwestern USA, this increased rainfall is mainly due to a convergent monsoonal circulation that develops over late boreal summer. A smaller wintertime increase in precipitation also occurs due to differences in rainfall associated with midlatitude cyclones. Wetter land conditions are expected to weaken upwelling‐favorable coastal winds, so that increased rainfall caused by coastal SST warming suggests a positive feedback that could help sustain wet, Pliocene‐like conditions.

Geographical distribution of extreme deep and intense convective storms on Earth

Vigorous convective storms can not only produce severe destructive weather events and lead to huge damages, but also play a vital role in stratosphere–troposphere exchange (STE). Based on 16-yr Tropical Rainfall Measuring Mission (TRMM) observational data, global geographical distribution of extremely deep and intense convective storms has been investigated. The results show that convective storms are mainly distributed in the tropics, with the largest storm event density located in tropical Africa, tropical Americas and the Maritime Continent. However, compared to convective storms, the most intense and deepest convective storms present a distinctly different geographical distribution pattern, which is true even if the same convective storm samples are used. Overall, tropical Africa is the only region in the world where the distributions of deep and intense convective storms overlap. Other than that, convective storms in the Maritime Continent region are characterized by large convective depth but weak convective intensity, while the opposite is true in subtropical land regions. The top 1% deep convective storms occur most frequently in boreal winter (48%) and spring (36%), and are mainly distributed in the Maritime Continent and tropical Africa. Those most intense convective storms show a weak seasonal variation in quantity while their distributions are significantly different from season to season. Further analysis reveals that convective properties of storms in different regions are distinctly different, and even different in the same region such as tropical Africa but in different seasons. This result suggests that although the formation of convective storm requires sufficient water vapor supply and instable atmospheric conditions, deep convection needs more water vapor supply to develop compared to intense storms. • The geographic distributions of deep and intense convective storms exhibit distinctly different patterns. • Convective storms are strong in convective depth in maritime continent, but strong in convective intensity in subtropical continent. • Deep convection is more likely to occur in a more humid environment than intense storms.

The Response of Local Precipitation and Sea Level Pressure to Hadley Cell Expansion

AbstractNumerous lines of observational evidence suggest that Earth's tropical belt has expanded over the past 30–40 years. It is natural to expect that this poleward displacement should be associated with drying on the poleward margins of the subtropics, but it is less clear to what degree the drying should be zonally symmetric. This study tests the degree to which poleward motion of the Hadley cell boundary is associated with changes in local precipitation or sea level pressure and the degree to which those changes are zonally symmetric. Evidence from both reanalysis data and global climate models reveals that the local changes associated with Hadley cell expansion are mostly confined to certain centers of action which lie primarily over oceans. Consequently, the tropical expansion measured by zonally averaged variables is not associated with systematic drying over subtropical land regions, as is often assumed.

Nitrous oxide emissions 1999 to 2009 from a global atmospheric inversion

Abstract. N2O surface fluxes were estimated for 1999 to 2009 using a time-dependent Bayesian inversion technique. Observations were drawn from 5 different networks, incorporating 59 surface sites and a number of ship-based measurement series. To avoid biases in the inverted fluxes, the data were adjusted to a common scale and scale offsets were included in the optimization problem. The fluxes were calculated at the same resolution as the transport model (3.75° longitude × 2.5° latitude) and at monthly time resolution. Over the 11-year period, the global total N2O source varied from 17.5 to 20.1 Tg a−1 N. Tropical and subtropical land regions were found to consistently have the highest N2O emissions, in particular in South Asia (20 ± 3% of global total), South America (13 ± 4%) and Africa (19 ± 3%), while emissions from temperate regions were smaller: Europe (6 ± 1%) and North America (7 ± 2%). A significant multi-annual trend in N2O emissions (0.045 Tg a−2 N) from South Asia was found and confirms inventory estimates of this trend. Considerable interannual variability in the global N2O source was observed (0.8 Tg a−1 N, 1 standard deviation, SD) and was largely driven by variability in tropical and subtropical soil fluxes, in particular in South America (0.3 Tg a−1 N, 1 SD) and Africa (0.3 Tg a−1 N, 1 SD). Notable variability was also found for N2O fluxes in the tropical and southern oceans (0.15 and 0.2 Tg a−1 N, 1 SD, respectively). Interannual variability in the N2O source shows some correlation with the El Niño–Southern Oscillation (ENSO), where El Niño conditions are associated with lower N2O fluxes from soils and from the ocean and vice versa for La Niña conditions.

Indian summer monsoon rainfall predictability and variability associated with Northwest Pacific circulation in a suit of coupled model hindcasts

The Northwest Pacific (NWP) circulation (subtropical high) is an important component of the East Asian summer monsoon system. During summer (June–August), anomalous lower tropospheric anticyclonic (cyclonic) circulation appears over NWP in some years, which is an indicative of stronger (weaker) than normal subtropical high. The anomalous NWP cyclonic (anticyclonic) circulation years are associated with negative (positive) precipitation anomalies over most of Indian summer monsoon rainfall (ISMR) region. This indicates concurrent relationship between NWP circulation and convection over the ISMR region. Dry wind advection from subtropical land regions and moisture divergence over the southern peninsular India during the NWP cyclonic circulation years are mainly responsible for the negative rainfall anomalies over the ISMR region. In contrast, during anticyclonic years, warm north Indian Ocean and moisture divergence over the head Bay of Bengal-Gangetic Plain region support moisture instability and convergence in the southern flank of ridge region, which favors positive rainfall over most of the ISMR region. The interaction between NWP circulation (anticyclonic or cyclonic) and ISMR and their predictability during these anomalous years are examined in the present study. Seven coupled ocean–atmosphere general circulation models from the Asia-Pacific Economic Cooperation Climate Center and their multimodel ensemble mean skills in predicting the seasonal rainfall and circulation anomalies over the ISMR region and NWP for the period 1982–2004 are assessed. Analysis reveals that three (two) out of seven models are unable to predict negative (positive) precipitation anomalies over the Indian subcontinent during the NWP cyclonic (anticyclonic) circulation years at 1-month lead (model is initialized on 1 May). The limited westward extension of the NWP circulation and misrepresentation of SST anomalies over the north Indian Ocean are found to be the main reasons for the poor skill (of some models) in rainfall prediction over the Indian subcontinent. This study demonstrates the importance of the NWP circulation variability in predicting summer monsoon precipitation over South Asia. Considering the predictability of the NWP circulation, the current study provides an insight into the predictability of ISMR. Long lead prediction of the ISMR associated with anomalous NWP circulation is also discussed.

Mid-Pliocene global climate simulation with MRI-CGCM2.3: set-up and initial results of PlioMIP Experiments 1 and 2

Abstract. The mid-Pliocene (3.3 to 3.0 million yr ago), a globally warm period before the Quaternary, is recently attracting attention as a new target for paleoclimate modelling and data-model synthesis. This paper reports set-ups and results of experiments proposed in Pliocene Model Intercomparison Project (PlioMIP) using a global climate model, MRI-CGCM2.3. We conducted pre-industrial and mid-Pliocene runs by using the coupled atmosphere-ocean general circulation model (AOGCM) and its atmospheric component (AGCM) for the PlioMIP Experiments 2 and 1, respectively. In addition, we conducted two types of integrations in AOGCM simulation, with and without flux adjustments on sea surface. General characteristics of differences in the simulated mid-Pliocene climate relative to the pre-industrial in the three integrations are compared. In addition, patterns of predicted mid-Pliocene biomes resulting from the three climate simulations are compared in this study. Generally, difference of simulated surface climate between AGCM and AOGCM is larger than that between the two AOGCM runs, with and without flux adjustments. The simulated climate shows different pattern between AGCM and AOGCM particularly over low latitude oceans, subtropical land regions and high latitude oceans. The AOGCM simulations do not reproduce wetter environment in the subtropics relative to the present-day, which is suggested by terrestrial proxy data. The differences between the two types of AOGCM runs are small over the land, but evident over the ocean particularly in the North Atlantic and polar regions.

Intercomparison of Deep Convection over the Tibetan Plateau–Asian Monsoon Region and Subtropical North America in Boreal Summer Using CloudSat/CALIPSO Data

AbstractDeep convection in the Tibetan Plateau–southern Asian monsoon region (TP–SAMR) is analyzed using CloudSat and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data for the boreal summer season (June–August) from 2006 to 2009. Three subregions are defined—the TP, the southern slope of the plateau (PSS), and the SAMR—and deep convection properties (such as occurrence frequency, internal vertical structure, system size, and local environment) are compared among these subregions. To cast them in a broader context, four additional regions that bear some similarity to the TP–SAMR are also discussed: East Asia (EA), tropical northwestern Pacific (NWP), and western and eastern North America (WNA and ENA, respectively).The principal findings are as follows: 1) Compared to the other two subregions of the TP–SAMR, deep convection over the TP is shallower, less frequent, and embedded in smaller-size convection systems, but the cloud tops are more densely packed. These characteristics of deep convection over the TP are closely related to the unique local environment, namely, a significantly lower level of neutral buoyancy (LNB) and much drier atmosphere. 2) In a broader context in which all seven regions are brought together, deep convection in the two tropical regions (NWP and SAMR; mostly over ocean) is similar in many regards. A similar conclusion can be drawn among the four subtropical continental regions (TP, EA, WNA, and ENA). However, tropical oceanic and subtropical land regions present some significant contrasts: deep convection in the latter region occurs less frequently, has lower cloud tops but comparable or slightly higher tops of large radar echo (e.g., 0 and 10 dBZ), and is embedded in smaller systems. The cloud tops of the subtropical land regions are generally more densely packed. Hence, the difference between the TP and SAMR is more of a general contrast between subtropical land regions and tropical oceanic regions during the boreal summer. 3) Deep convection over the PSS possesses some uniqueness of its own because of the distinctive terrain (slopes) and moist low-level monsoon flow. 4) Results from a comparison between the daytime (1:30 p.m.) and nighttime (1:30 a.m.) overpasses are largely consistent with researchers’ general understanding of the diurnal variation of tropical and subtropical deep convection.

Relating Passive 37-GHz Scattering to Radar Profiles in Strong Convection

Abstract Tropical Rainfall Measuring Mission (TRMM) Microwave Imager and precipitation radar measurements are examined for strong convective systems. Storms having similar values of minimum 37-GHz polarization-corrected temperature (PCT) are grouped together, and their vertical profiles of maximum radar reflectivity are composited. Lower 37-GHz PCT corresponds to stronger radar profiles (high reflectivity through a deep layer), but characteristic profiles for a given 37-GHz PCT are different for deep tropical ocean, deep tropical land, subtropical ocean, and subtropical land regions. Tropical oceanic storms have a sharper decrease of reflectivity just above the freezing level than storms from other regions with the same brightness temperature. Storms from subtropical land regions have the slowest decrease of reflectivity with height and the greatest mixed-phase-layer ice water content (IWC). Linear fits of 37-GHz PCT versus IWC for each region are used to scale the brightness temperatures. Counts of storms with these scaled brightness temperatures below certain thresholds suggest that not as many of the strongest storms occur in central Africa as in subtropical parts of South America, the United States, and central Asia.

Monsoon and general circulation system

The instrumental period of meteorological record is too short to capture the full variability of the climate system and, in particular to visualize the type of climate that is predicted to occur over the next decades and centuries. It is therefore important to reconstruct past climates and understand past climatic variations. Among them, those related to the monsoon appear to be more and more important for Society. The economy, culture and rhythms of life of a large part of humanity are critically influenced by the evolution and variability of the monsoon. In particular, the Asian monsoon is one of the most spectacular occurrences in the climate system, strongly influenced by the combination of the thermal contrast between the Eurasian continent and the Indo-Pacific Ocean and the presence the Tibetan Plateau. Climatologically, the monsoon regions as the most convectively active areas, are situated in the Intertropical Convergence Zone and account for the majority of global atmospheric heat and moisture transport [1] . The need to better understand the monsoon leads inevitably to the close inspection of its worldwide activity during the geological times to provide a long-term perspective from which any future change may be more effectively assessed. It is in that framework that the substantial paper by Wang Pinxian in this issue [2] is more than welcome. It is an authoritative review, and probably also the first in which the monsoon issues are reviewed in a global scale through a so long geological history. Wang’s main points are fundamental, and hence important. First, it is to emphasize the role of the tropics as an extremely important source of changes in the global climate system. Second, it is to regard the monsoons as a global feature through which the tropical forcing exerts its climatic role worldwide. I totally agree with these arguments, and share Wang’s concern that the climate and paleoclimate community has, since too long, viewed the monsoons as local phenomena, without trying enough to relate their behavior together. But the problem being extremely complex, let me take the opportunity to put it in its broad context. The reader will find an interesting brief introduction to monsoon, its origin, history and localization in http://en.wikipedia.org/wiki/Monsoon. Etymologically, monsoon comes from the Arabic word mawsim, which means season, explaining why the original definition includes only major wind systems that change direction seasonally. The most spectacular occurrence of such seasonally varying circulation is undoubtedly in Southern and Eastern Asia. It is the associated heavy rainfall there, which has allowed by extension the other regions of the world to qualify as monsoon regions. With time, the term monsoon has been broadened to include almost all of the phenomena associated with the seasonal weather cycle within tropical and subtropical land regions of the Earth, showing immediately the relevance of Wang’s argumentation to deal with monsoon as a global tropical system. Giving stronger emphasis to the tropical forcing does not mean that we can isolate the tropical climate from the rest of the world, as it is part of the Earth’s climate system. The response of the climate system to the energy that we receive from the Sun (both at the seasonal and geological time scales) involves all latitudes and seasons and depends upon the feedback mechanisms inherent in the climate system itself. Among them, the water vapor