Examination of the mixed layer deepening process during convection using LES

Analysis of large-eddy simulation data of the ocean mixed layer under convection reveals that the contribution from wind stress decreases with time as a result of inertial oscillation in the extratropical ocean and that it leads to a rapid increase of the bulk and gradient Richardson number at the mixed layer depth. The criteria for the mixed layer deepening in the widely used mixed layer models, such as the Niiler–Kraus (NK) model, the Price–Weller–Pinkel (PWP) model, and the K-profile parameterization (KPP) model, are examined in view of these results, and its implication on the model predictability is discussed. Copyright 2010 American Meteorological Societ

Longitudinal variation of large scale vertical motion in the tropics,

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology, 1970.Bibliography: leaves 33-34.by Arthur C. Kyle.M.S

Links between central west western australian rainfall variability and large-scale climate drivers

Over the past century, and especially after the 1970s, rainfall observations show an increase (decrease) of the wet summer (winter) season rainfall over northwest (southwest) Western Australia. The rainfall in central west Western Australia (CWWA), however, has exhibited comparatively much weaker coastal trends, but a more prominent inland increase during the wet summer season. Analysis of seasonally averaged rainfall data from a group of stations, representative of both the coastal and inland regions of CWWA, revealed that rainfall trends during the 1958-2010 period in the wet months of November-April were primarily associated with El Niñ o-Southern Oscillation (ENSO), and with the southern annular mode (SAM) farther inland. During the wet months of May-October, the Indian Ocean dipole (IOD) showed the most robust relationships. Those results hold when the effects of ENSOor IOD are excluded, and were confirmed using a principal component analysis of sea surface temperature (SST) anomalies, rainfall wavelet analyses, and point-by-point correlations of rainfall with global SST anomaly fields. Although speculative, given their long-term averages, reanalysis data suggest that from 1958 to 2010 the increase inCWWAinland rainfall largely is attributable to an increasing cyclonic anomaly trend over CWWA, bringing onshore moist tropical flow to the Pilbara coast. During May-October, the flow anomaly exhibits a transition from an onshore to offshore flow regime in the 2001-10 decade, which is consistent with the observed weaker drying trend during this period. © 2013 American Meteorological Society

blocking events and the stability of the polar vortex

The present study investigates non-linear dynamics of atmospheric flow phenomena on different scales as interactions of vortices. Thereby, we apply the idealised, two-dimensional concept of point vortices considering two important issues in atmospheric dynamics. First, we propose this not widely spread concept in meteorology to explain blocked weather situations using a three-point vortex equilibrium. Here, a steady state is given if the zonal mean flow is identical to the opposed translational velocity of the vortex system. We apply this concept exemplarily to two major blocked events establishing a new pattern recognition technique based on the kinematic vorticity number to determine the circulations and positions of the interacting vortices. By using reanalysis data, we demonstrate that the velocity of the tripole in a westward direction is almost equal to the westerly flow explaining the steady state of blocked events. Second, we introduce a novel idea to transfer a stability analysis of a vortex equilibrium to the stability of the polar vortex concerning its interaction with the quasi-biennial oscillation (QBO). Here, the point vortex system is built as a polygon ring of vortices around a central vortex. On this way we confirm observations that perturbations of the polar vortex during the QBO east phase lead to instability, whereas the polar vortex remains stable in QBO west phases. Thus, by applying point vortex theory to challenging problems in atmospheric dynamics we show an alternative, discrete view of synoptic and planetary scale motion

Near-surface salinity reveals the oceanic sources of moisture for Australian precipitation through atmospheric moisture transport

Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 33(15), (2020): 6707-6730, https://doi.org/10.1175/JCLI-D-19-0579.1.The long-term trend of sea surface salinity (SSS) reveals an intensification of the global hydrological cycle due to human-induced climate change. This study demonstrates that SSS variability can also be used as a measure of terrestrial precipitation on interseasonal to interannual time scales, and to locate the source of moisture. Seasonal composites during El Niño–Southern Oscillation/Indian Ocean dipole (ENSO/IOD) events are used to understand the variations of moisture transport and precipitation over Australia, and their association with SSS variability. As ENSO/IOD events evolve, patterns of positive or negative SSS anomaly emerge in the Indo-Pacific warm pool region and are accompanied by atmospheric moisture transport anomalies toward Australia. During co-occurring La Niña and negative IOD events, salty anomalies around the Maritime Continent (north of Australia) indicate freshwater export and are associated with a significant moisture transport that converges over Australia to create anomalous wet conditions. In contrast, during co-occurring El Niño and positive IOD events, a moisture transport divergence anomaly over Australia results in anomalous dry conditions. The relationship between SSS and atmospheric moisture transport also holds for pure ENSO/IOD events but varies in magnitude and spatial pattern. The significant pattern correlation between the moisture flux divergence and SSS anomaly during the ENSO/IOD events highlights the associated ocean–atmosphere coupling. A case study of the extreme hydroclimatic events of Australia (e.g., the 2010/11 Brisbane flood) demonstrates that the changes in SSS occur before the peak of ENSO/IOD events. This raises the prospect that tracking of SSS variability could aid the prediction of Australian rainfall.This research is funded through the Earth System and Climate Change Hub of the Australian government’s National Environmental Science Programme. The assistance of computing resources from the National Computational Infrastructure supported by the Australian Government is acknowledged. CCU acknowledges support from the U.S. National Science Foundation under Grant OCE-1663704. MF was supported by the by Centre for Southern Hemisphere Oceans Research (CSHOR), which is a joint initiative between the Qingdao National Laboratory for Marine Science and Technology (QNLM), CSIRO, University of New South Wales and University of Tasmania. The authors wish to acknowledge PyFerret (https://ferret.pmel.noaa.gov/Ferret/) and the Cimate Data Operators (https://code.mpimet.mpg.de/projects/cdo/) for the data analysis and graphical representations in this paper

Plausible Drying and Wetting Scenarios for Summer in Southeastern South America

Summer rainfall trends in southeastern South America (SE-SA) have received attention in recent decades because of their importance for climate impacts. More than one driving mechanism has been identified for the trends, some of which have opposing effects. It is still not clear how much each mechanism has contributed to the observed trends or how their combined influence will affect future changes. Here, we address the second question and study how the CMIP6 summer SE-SA rainfall response to greenhouse warming can be explained by mechanisms related to large-scale extratropical circulation responses in the Southern Hemisphere to remote drivers (RDs) of regional climate change. We find that the regional uncertainty is well represented by combining the influence of four RDs: tropical upper-tropospheric amplification of surface warming, the delay in the stratospheric polar vortex breakdown date, and two RDs characterizing recognized tropical Pacific SST warming patterns. Applying a storyline framework, we identify the combination of RD responses that lead to the most extreme drying and wetting scenarios. Although most scenarios involve wetting, SE-SA drying can result if high upper-tropospheric tropical warming and early stratospheric polar vortex breakdown conditions are combined with low central and eastern Pacific warming. We also show how the definition of the SE-SA regional box can impact the results since the spatial patterns characterizing the dynamical influences are complex and the rainfall changes can be averaged out if these are not considered when aggregating. This article’s perspective and the associated methodology are applicable to other regions of the globe. Significance Statement: Summer rainfall in southeastern South America (SE-SA) affects an area where around 200 million people live. The observed trends suggest long-term wetting, and most climate models predict a wetting response to greenhouse warming. However, in this work, we find that there is a physically plausible combination of large-scale circulation changes that can promote drying, which means SE-SA drying is a possibility that cannot be ignored. We also show that the definition of the SE-SA regional box can impact regional rainfall analysis since the spatial patterns characterizing the dynamical influences are complex and the changes can be averaged out if these are not considered when aggregating. This perspective and the associated methodology are applicable to other regions of the globe

REPORT OF THE ELEVENTH SESSION

INTERNATIONA

The Dominant Patterns of Intraseasonal Rainfall Variability in May–October and November–April over the Tropical Western Pacific

The space–time structure of intraseasonal (10–90 day) rainfall variability in the western tropical Pacific is studied using daily 3B42 TRMM and ERA-Interim reanalysis data for the period 1998–2014. Empirical orthogonal function (EOF) analysis of 10–90-day filtered daily rainfall anomalies identifies two leading modes in both May–October and November–April; together these modes explain about 11%–12% of the total intraseasonal variance over the domain in both seasons and up to 60% over large areas of the western Pacific in both climatological periods. The two leading modes in May–October are linearly related to each other and both are well correlated with the Madden–Julian oscillation (MJO) indices. Although the two leading EOF modes in November–April are linearly independent of each other, both show statistically significant correlations with the MJO. The phase composites of 30–80-day filtered data show that the two leading modes are associated with strong eastward and northward propagation of rainfall anomalies in May–October, and eastward and southward propagation of rainfall anomalies in November–April. The eastward propagation of rainfall anomalies in both seasons and southeastward propagation related with EOF2 in November–April is linked to the development of low-level moisture flux convergence ahead of the active convection. Similarly, the northward propagation in May–October is also connected with low-level moisture flux convergence, but surface wind and evaporation variations are also important. The wind–evaporation–SST feedback mechanism drives the southeastward propagation of rainfall anomalies associated with EOF1 in November–April. The different mechanisms for southeastward propagation associated with two leading modes in November–April suggest dynamically different relations with the MJO.publishedVersio

West African precipitation and related atmospheric circulation

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Risk assessment with regard to the occurrence of malaria in Africa under the influence of observed and projected climate change

Malaria is one of the most serious health problems in the world. The projected climate change will probably alter the range and transmission potential of malaria in Africa. In this study, potential changes in the malaria transmission are assessed by forcing three malaria models with bias-corrected data from ensemble scenario runs of a state-of-the-art regional climate model. The Liverpool Malaria Model (LMM) from the Geography Department of the University of Liverpool is utilised. The LMM simulates the spread of malaria at a daily resolution using daily mean temperature and 10-day accumulated precipitation. The simulation of some key processes has been modified in the model, in order to reflect a more physical relationship. An extensive literature survey with regard to entomological and parasitological malaria variables enables the calibration and validation of a new LMM version. Comparison of this version with the original model exhibits marked improvements. The new version demonstrates a realistic simulation of entomological variables and of the malaria season, as well as correctly reproduces the epidemic poten tial at fringes of endemic malaria areas. Various sensitivity experiments reveal that the LMM is fairly sensitive to values of its required parameters. Effects of climatic changes on the malaria season are additionally verified by the MARA Seasonality Model (MSM). The Garki model finally enables the completion of the malaria picture in terms of the immune status and the infectiousness of different population groups, as well as relative to the age-dependent prevalence structure. In every case three ensemble runs were performed on a 0.5° grid. The LMM was driven for the present-day climate (1960-2000) by bias-corrected data from the REgional MOdel (REMO), with a land use and land cover specified by the Food and Agriculture Organization (FAO). Malaria projections were carried out for 2001-2050 according to the climate scenarios A1B and B1 as well as FAO land use and land cover changes. Garki model runs were subsequently forced by the Entomological Inoculation Rate (EIR) from the LMM. Finally, additional results relative to the malaria season were produced by MSM. For the present-day climate (1960-2000), the highest biting rates are simulated for Equatorial Africa. The malaria runs show a decrease in the malaria spread from Central Africa towards the Sahel. The length of the malaria season is closely related to monsoon rainfall. The model simulations show a marked influence of mountainous areas causing a complex pattern of the spread of malaria in East Africa. The malaria infected population reveals the expected peak in children below an age of about five years. Regions of epidemic malaria occurrence, as defined by the coefficient of variation of the annual parasite prevalence maximum, are found along a band in the northern Sahel. Farther south, malaria occurs more regularly and is therefore characterised as endemic. Epidemic-prone areas are additionally identified at various highland territories, as well as in arid and semi-arid zones of the Greater Horn of Africa. No adequate immune protection of the population was found for these areas. Largely due to land surface degradation, REMO simulates a prominent surface warming and a significant reduction in the annual rainfall amount over most of tropical Africa in either climate change scenario. Assuming no future human-imposed constraints on malaria transmission, changes in temperature and precipitation will alter the future geographic distribution of malaria. In the northern part of sub-Saharan Africa, the precipitation decline will force significant decreases of the malaria transmission in the Sahel. In addition to the withdrawal of malaria transmission along the fringe of the Sahara, the frequency of malaria occurrence will be reduced for several grid boxes of the Sahel. As a result, epidemics in these more densely populated areas will become more likely, in particularly as adults lose their immunity. The level of malaria prevalence farther south will remain stable for most areas. However, the start of the malaria season will be delayed and the transmission is expected to cease earlier. Most pronounced changes in Africa are found for East Africa. Significantly higher temperatures and slightly higher rainfall cause a substantial increase in the season length and parasite prevalence in formerly epidemic-prone areas. Territories formerly unsuitable for malaria will become suitable under the warmer future climate. The simulations indicate changes in the highland epidemic risk. At most grid boxes malaria transmission will stabilise below about 2000 m. At these altitudes the population will improve their immune status. In contrast, malaria will climb to formerly malaria-free zones above these levels enforcing the probability of malaria epidemics