Meridional Moisture Flux Research Articles

The cooperative effects of November Arctic sea ice and Eurasian snow cover on the Eurasian surface air temperature in January–February

AbstractDue to their significant influence on large‐scale atmospheric circulation and climate anomalies, the variability of Arctic sea ice and Eurasian snow cover during late autumn and their combined effects have garnered increasing attention. This study aims to investigate the physical mechanism underlying the covariation among the Barents‐Kara Seas (BKS) sea ice concentration (SIC), Eurasian snow cover extent (SCE) and the ensuing winter Eurasian surface air temperature (SAT). The statistics results of singular value decomposition suggest a significant linkage between the decreased BKS SIC, zonal “negative–positive” dipole SCE anomalies over Eurasia in November and cold Eurasian SAT in January–February (JF). Observational diagnosis analyses about the meridional moisture, heat transport and surface heat flux demonstrate that subpolar Eurasian anticyclonic circulation plays a crucial role in connecting the predominant modes of SIC and SCE. Furthermore, the BKS SIC and Eurasian SCE anomalies can jointly excite upward‐propagating planetary waves into the stratosphere, while simultaneously reducing the subpolar meridional temperature gradient. This results in westerly wind deceleration and favours the continuous planetary wave propagation. Consequently, the stratospheric polar vortex is significantly weakened, along with negative Northern Annular Mode anomalies propagating downward from the stratosphere to troposphere. Negative‐phase Arctic Oscillation anomalies correspondingly develop during JF, resulting in widespread cold anomalies over the Eurasian continent. These results are further confirmed by numerical sensitivity experiments from the Community Atmosphere Model forced by the above mentioned SIC and SCE anomalies. The empirical hindcast model analyses further suggest that the prediction skill of JF Eurasian SAT is enhanced when both the November BKS SIC and Eurasian SCE signals are considered.

Seasonal Variations and Spatial Patterns of Arctic Cloud Changes in Association with Sea Ice Loss during 1950–2019 in ERA5

Abstract The dynamic and thermodynamic mechanisms that link retreating sea ice to increased Arctic cloud amount and cloud water content are unclear. Using the fifth generation of the ECMWF Reanalysis (ERA5), the long-term changes between years 1950–79 and 1990–2019 in Arctic clouds are estimated along with their relationship to sea ice loss. A comparison of ERA5 to CERES satellite cloud fractions reveals that ERA5 simulates the seasonal cycle, variations, and changes of cloud fraction well over water surfaces during 2001–20. This suggests that ERA5 may reliably represent the cloud response to sea ice loss because melting sea ice exposes more water surfaces in the Arctic. Increases in ERA5 Arctic cloud fraction and water content are largest during October–March from ∼950 to 700 hPa over areas with significant (≥15%) sea ice loss. Further, regions with significant sea ice loss experience higher convective available potential energy (∼2–2.75 J kg−1), planetary boundary layer height (∼120–200 m), and near-surface specific humidity (∼0.25–0.40 g kg−1) and a greater reduction of the lower-tropospheric temperature inversion (∼3°–4°C) than regions with small (<15%) sea ice loss in autumn and winter. Areas with significant sea ice loss also show strengthened upward motion between 1000 and 700 hPa, enhanced horizontal convergence (divergence) of air, and decreased (increased) relative humidity from 1000 to 950 hPa (950–700 hPa) during the cold season. Analyses of moisture divergence, evaporation minus precipitation, and meridional moisture flux fields suggest that increased local surface water fluxes, rather than atmospheric motions, provide a key source of moisture for increased Arctic clouds over newly exposed water surfaces during October–March. Significance Statement Sea ice loss has been shown to be a primary contributor to Arctic warming. Despite the evidence linking large sea ice retreat to Arctic warming, some studies have suggested that enhanced downwelling longwave radiation associated with increased clouds and water vapor is the primary reason for Arctic amplification. However, it is unclear how sea ice loss is linked to changes in clouds and water vapor in the Arctic. Here, we investigate the relationship between Arctic sea ice loss and changes in clouds using the ERA5 dataset. Improved knowledge of the relationship between Arctic sea ice loss and changes in clouds will help further our understanding of the role of the cloud feedback in Arctic warming.

Seasonally distinct contributions of greenhouse gases and anthropogenic aerosols to historical changes in Arctic moisture budget

Arctic hydrological cycle has important global implications through modifying deep ocean circulation via fresh water fluxes. However, attribution of the observed changes in Arctic moisture budget remains challenging due to the lack of reliable observations. Here, as a first step, past changes in hydrological cycle over the Arctic were evaluated using CMIP6 historical simulations. To examine possible influences of individual external forcings, CMIP6 multi-model simulations performed under natural-plus-anthropogenic (ALL), greenhouse gas (GHG), natural (NAT), and aerosol (AER) forcings were compared. Results indicate that Arctic precipitation and evaporation increase in ALL and GHG but decrease in AER. In ALL and GHG, Arctic precipitation increases in summer mainly due to enhanced poleward moisture transport whereas Arctic moistening during winter is affected more by increased surface evaporation over sea-ice retreat areas. In AER, Arctic precipitation tends to decrease due to reduced evaporation over sea-ice advance areas during cold months. Poleward meridional moisture flux (MMF) across 70°N is the strongest in summer due to the highest moisture. GHG has stronger MMF than other forcing simulations due to larger increase in moisture in line with stronger warming. The MMF response is found to be largely determined by variations in transient eddies with distinct seasonal and regional contributions.

Late Quaternary aeolian environments, luminescence chronology and climate change for the Monahans dune field, Winkler County, West Texas, USA

• Thermal transfer luminescence dating indicates Monahans aeolian record spans 550 ka. • Chronostratigraphic analyses identified at least six aeolian depositional periods (ADP). • Prominent ADP occur at 545 to 475, 300 to 260, 70 to 45, and post 16 ka. • A Playa lake deposit dated to 235 to 195 ka identified adjacent to Monahans dune field. • ADPs occur preferential during stadials, when the Laurentide ice is <80 % maximum volume. Dune fields on the Southern High Plains such as the Monahans in West Texas are archives of Quaternary environmental variability. Stratigraphic analyses and sixty-one optically stimulated luminescence (OSL) ages from seven Geoprobe cores and one section from the Monahans reveal a ∼ 550 ka old aeolian sedimentary record with seven carbonate/argillic paleosols and a playa-lake margin deposit. OSL ages on quartz-grains from aeolian sediments by two protocols, single aliquot regeneration (SAR) and thermal transfer (TT), yield congruent ages between 50 and 250 ka, and the oldest ages of ca. 550 ka, potentially minima. This chronostratigraphic analysis and finite-mixture modeling of the OSL-age distribution identify-four aeolian depositional periods (ADP) at 545 to 475, 300 to 260, 70 to 45, and post 16 ka and possibly-two additional ADPs 460 to 420 ka and 350 to 320 ka. Playa lake deposits identified west of the Monahans and correlative to carbonate-rich paleosols indicate that wetter conditions prevailed during interglacial MIS 7, 235 to 195 ka. Another wetter period, 25 to 16 ka, with the formation of Lake King in the adjacent Rio Grande Valley is correlative with a pedogenically-modified <2 m-thick aeolian sand. This study underscores that there may be multiple climatic states, during glacials and interglacials, associated with wetter conditions. In turn, the thickest, preserved aeolian deposits are associated with transitional climate periods, penecontemporaneous with stadials, when the Laurentide ice sheet was <80 % of the last glacial maximum volume, with precipitation-bearing zonal circulation shifted northward and weakened meridional moisture flux.

Seasonal prediction of the Caribbean rainfall cycle

Rainfall in the Caribbean is an important resource for numerous stakeholders in the region. Based on previous work, which identified several variables that could provide predictive skill of rainfall in the region, canonical correlation analysis is applied to assess forecast skill for station-averaged sub-regional frequency and intensity of the Early-Rainy Season (ERS) and Late-Rainy Season (LRS) wet days, and magnitude of the Mid-Summer Drought (MSD). Predictor fields are explored from the ERA-Interim and North American Multi-Model Ensemble (NMME). The use of sea-level pressure (SLP), 850 hPa zonal winds (u850), vertically integrated zonal (UQ), and meridional (VQ) moisture fluxes show comparable, if not better, forecast skill than sea-surface temperatures (SSTs), which is generally the commonly-used predictor in a given region’s seasonal climate forecasts. Generally, the highest predictive skill is found for the frequency of wet days. Rainfall characteristics in the Central and Eastern Caribbean have significant predictive skill. Forecast skill of rainfall characteristics in the Northwestern and Western Caribbean are lower and less consistent. The sub-regional differences and consistently significant skill across lead times up to at least two months can be attributed to persistent SST/SLP anomalies during the ERS that resemble the North Atlantic Oscillation, and that resembles the summer-time onset of the El Niño-Southern Oscillation during the LRS. The anomalous spatial patterns during the MSD bear resemblance to both the ERS and LRS signals. The results provide additional variables that can be used to forecast rainfall characteristics in the Caribbean and a way to tailor seasonal forecasts for each sub-region.

Atmospheric circulation patterns that trigger heavy rainfall in West Africa

AbstractClassification of atmospheric circulation patterns (CP) is a common tool for downscaling rainfall, but it is rarely used for West Africa. In this study, a two‐step classification procedure is proposed for this region, which is applied from 1989 to 2010 for the Sudan‐Sahel zone (Central Burkina Faso) with a focus on heavy rainfall. The approach is based on a classification of large‐scale atmospheric CPs (e.g., Saharan Heat Low) of the West African Monsoon using a fuzzy rule‐based method to describe the seasonal rainfall variability. The wettest CPs are further classified using meso‐scale monsoon patterns to better describe the daily rainfall variability during the monsoon period. A comprehensive predictor screening for the seasonal classification indicates that the best performing predictor variables (e.g., surface pressure, meridional moisture fluxes) are closely related to the main processes of the West African Monsoon. In the second classification step, the stream function at 700 hPa for identifying troughs and ridges of tropical waves shows the highest performance, providing an added value to the overall performance of the classification. Thus, the new approach can better distinguish between dry and wet CPs during the rainy season. Moreover, CPs are identified that are of high relevance for daily heavy rainfall in the study area. The two wettest CPs caused roughly half of the extremes on about 6.5% of days. Both wettest patterns are characterized by an intensified Saharan Heat Low and a cyclonic rotation near the study area, indicating a tropical wave trough. Since the classification can be used to condition other statistical approaches used in climate sciences and other disciplines, the presented classification approach opens many different applications for the West African Monsoon region.

Evaluation of extreme precipitation over Asia in CMIP6 models

Based on four reanalyses or gridded data sets (ERA5, 20CR, APHRODITE and REGEN), we provide an overview of 23 Historical and 7 HighResMIP experiments’ performance from the Coupled Model Intercomparison Project Phase 6 (CMIP6) (for short, 6-Hist, HighRes) in simulating seven extreme precipitation indices over Asia defined by the Expert Team on Climate Change Detection and Indices (ETCCDI). We compare them with 28 Historical experiments in CMIP5 (5-Hist). CMIP5 and CMIP6 models are generally able to reproduce extreme precipitation’s spatial distribution and their trend patterns in comparison to the benchmark data set (APHRODITE). The overall performance of individual model is summarized by a “portrait” diagram based on four statistics for each index. We divide all 58 models into three groups (A, the top 20%; B, the median 60% and C group, the last 20%) according to MR rankings (the comprehensive ranking measure). Based on the “portrait” diagram and MR rankings, models that perform relatively well for all seven extreme precipitation indices include HadCM3, HadGEM2-AO, HadGEM2-CC and HadGEM2-ES from 5-Hist, EC-Earth3, EC-Earth3-Veg from 6-Hist and ECMWF-IFS-HR, ECMWF-IFS-LR, ECMWF-IFS-MR from HighRes. The simulated performance of CMIP6 is polarized, for the top four and the last five ranking models are both from CMIP6. Compared with the counterpart models in CMIP6 and CMIP5, the improvement of PCC (pattern correlation coefficient) is more obvious. Furthermore, the dry biases of CMIP6 (both 6-Hist and HighRes) in Southern China and India and the wet biases of CMIP6 in Tibet are reduced compared to CMIP5. This may benefit from the improvement that CMIP6 models can capture the characteristics of meridional moisture flux convergence, and improve the overestimation or underestimation of meridional and zonal specific humidity eddies compared to CMIP5.

Role of the South China Sea in Southern China rainfall: meridional moisture flux transport

The South China Sea (SCS) serves as the main source of moisture for rainfall in Southern China (SC) and the meridional moisture transport to SC is dominated by wind changes during the first rainy season (April–June). El Niño-Southern Oscillation (ENSO) and Tropical Northwestern Pacific (TNWP) variability modulate the SC rainfall through anomalous anticyclonic circulation over the western North Pacific by strengthening the SCS meridional moisture transport to SC. However, our study indicates that the SCS is not only the intermediary in which ENSO or the TNWP affects the SC rainfall but also plays an independent role in the modulation of the SC rainfall. Notably, the SCS meridional moisture transport has a lower impact on the SC rainfall during the second rainy season (July–September), especially in July. At that time, the main cause of the SC rainfall is the southward moisture flux anomaly across its northern boundary with the anomalous cyclone over SC. This cyclone suppresses the moisture flux out of SC and leads to moisture convergence in SC. Moreover, we present a new concept by analyzing internal differences of moisture circulation during the second rainy season. Either strengthening the meridional moisture flux into SC across its southern boundary or suppressing the moisture flux out of SC across its northern boundary is important depending on whether or not the moisture from the SCS can converge in SC, which is mainly determined by the amplitude of moisture transport fluxes in SC.

Pathways of meridional atmospheric moisture transport in the central Arctic

Atmospheric moisture transport plays an important role in latent heat release and hydrologic interactions in the Arctic. In recent years, with the rapid decline in sea ice, this transport has changed. Here, we calculated the vertically integrated atmospheric moisture meridional transport (AMTv) from two global reanalysis datasets, from 1979–2015, and found moisture pathways into the central Arctic. Four stable pathways showed an occurrence frequency greater than 70%, and these pathways exhibited a perennial seasonal pattern in the atmosphere above the Laptev Sea Pathway (LSP), Canadian Arctic Archipelago Pathway (CAAP), both sides of the Greenland plateau. Another seasonal pathway appeared above the east of the Chukchi Sea (CSP) during the melting/freezing months (March to September). Through these pathways, AMTv contributed a total moisture exchange of 60%–80%—averaged over a 75°N circle—and focused on the low troposphere. Transports across the LSP, CSP and CAAP pathways likely create an enclosed moisture route. Meridional moisture fluxes are intensified in the Pacific sector of Arctic (PSA), especially during melting/freezing months. AMTv interannual variabilities are illustrated mainly in the Laptev Sea and the east Greenland pathway. Results indicate that accompanying a tendency for a stronger Beaufort Sea High in this sea level pressure field, AMTv through PSA pathways, switched from output to input, and approximately 960 km3 of equivalent liquid water was transferred into the central Arctic during each decade. The detrended AMTv increment is highly correlated with the rapid decline of old ice areas (correlation coefficient is −0.78) for their synchronous fluctuations in the 1980s and the last decade.

Improving Seasonal Precipitation Forecasts for Agriculture in the Orinoquía Region of Colombia

Abstract Canonical correlation analysis (CCA) is used to improve the skill of seasonal forecasts in the Orinoquía region, where over 40% of Colombian rice is produced. Seasonal precipitation and frequency of wet days are predicted, as rice yields simulated by a calibrated crop model are better correlated with wet-day frequency than with precipitation amounts in June–August (JJA). Prediction of the frequency of wet days, using as predictors variables from the NCEP Climate Forecast System, version 2 (CFSv2), results in a forecast with higher skill than models predicting seasonal precipitation amounts. Using wet-day frequency as an alternative climate variable reveals that the distribution of daily rainfall is both more relevant for rice yield variability and more skillfully predicted than seasonal precipitation amounts. Forecast skill can also be improved by using the Climate Hazards Infrared Precipitation with Stations (CHIRPS) merged satellite–station JJA precipitation as the predictand in a CCA model, especially if the predictor is CFSv2 vertically integrated meridional moisture flux (VQ). The probabilistic hindcast derived from the CCA model using CHIRPS as the predictand can successfully discriminate above-normal, normal, and below-normal terciles of over 80% of the stations in the region. This is particularly relevant for stations that, due to discontinuity in their time series, are not included in station-only CCA models but are still in need of probabilistic seasonal forecasts. Finally, CFSv2 VQ performs better than precipitation as the predictor in CCA, which we attribute to CFSv2 being more internally consistent in regards to sea surface temperature (SST)-forced VQ variability than to SST-forced precipitation variability in the Orinoquía region.

Estimated Impacts of Climate Change on Eddy Meridional Moisture Transport in the Atmosphere

Research findings suggest that water (hydrological) cycle of the earth intensifies in response to climate change, since the amount of water that evaporates from the ocean and land to the atmosphere and the total water content in the air will increase with temperature. In addition, climate change affects the large-scale atmospheric circulation by, for example, altering the characteristics of extratropical transient eddies (cyclones), which play a dominant role in the meridional transport of heat, moisture, and momentum from tropical to polar latitudes. Thus, climate change also affects the planetary hydrological cycle by redistributing atmospheric moisture around the globe. Baroclinic instability, a specific type of dynamical instability of the zonal atmospheric flow, is the principal mechanism by which extratropical cyclones form and evolve. It is expected that, due to global warming, the two most fundamental dynamical quantities that control the development of baroclinic instability and the overall global atmospheric dynamics—the parameter of static stability and the meridional temperature gradient (MTG)—will undergo certain changes. As a result, climate change can affect the formation and evolution of transient extratropical eddies and, therefore, macro-exchange of heat and moisture between low and high latitudes and the global water cycle as a whole. In this paper, we explore the effect of changes in the static stability parameter and MTG caused by climate change on the annual-mean eddy meridional moisture flux (AMEMF), using the two classical atmospheric models: the mid-latitude f-plane model and the two-layer β-plane model. These models are represented in two versions: “dry,” which considers the static stability of dry air alone, and “moist,” in which effective static stability is considered as a combination of stability of dry and moist air together. Sensitivity functions were derived for these models that enable estimating the influence of infinitesimal perturbations in the parameter of static stability and MTG on the AMEMF and on large-scale eddy dynamics characterized by the growth rate of unstable baroclinic waves of various wavelengths. For the base climate change scenario, in which the surface temperature increases by 1 °C and warming of the upper troposphere outpaces warming of the lower troposphere by 2 °C (this scenario corresponds to the observed warming trend), the response of the mass-weighted vertically averaged annual mean MTG is -0.2 ℃ per 1000 km. The dry static stability increases insignificantly relative to the reference climate state, while on the other hand, the effective static stability decreases by more than 5.4%. Assuming that static stability of the atmosphere and the MTG are independent of each other (using One-factor-at-a-time approach), we estimate that the increase in AMEMF caused by change in MTG is about 4%. Change in dry static stability has little effect on AMEMF, while change in effective static stability leads to an increase in AMEMF of about 5%. Thus, neglecting atmospheric moisture in calculations of the atmospheric static stability leads to tangible differences between the results obtained using the dry and moist models. Moist models predict ~9% increase in AMEMF due to global warming. Dry models predict ~4% increase in AMEMF solely because of the change in MTG. For the base climate change scenario, the average temperature of the lower troposphere (up to ~4 km), in which the atmospheric moisture is concentrated, increases by ~1.5 ℃. This leads to an increase in specific humidity of about 10.5%. Thus, since both AMEMF and atmospheric water vapor content increase due to the influence of climate change, a rather noticeable restructuring of the global water cycle is expected.

On the Moisture Origins of Tornadic Thunderstorms

AbstractTornadic thunderstorms rely on the availability of sufficient low-level moisture, but the source regions of that moisture have not been explicitly demarcated. Using the NOAA Air Resources Laboratory HYSPLIT model and a Lagrangian-based diagnostic, moisture attribution was conducted to identify the moisture source regions of tornadic convection. This study reveals a seasonal cycle in the origins and advection patterns of water vapor contributing to winter and spring tornado-producing storms (1981–2017). The Gulf of Mexico is shown to be the predominant source of moisture during both winter and spring, making up more than 50% of all contributions. During winter, substantial moisture contributions for tornadic convection also emanate from the western Caribbean Sea (&gt;19%) and North Atlantic Ocean (&gt;12%). During late spring, land areas (e.g., soil and vegetation) of the contiguous United States (CONUS) play a more influential role (&gt;24%). Moisture attribution was also conducted for nontornadic cases and tornado outbreaks. Findings show that moisture sources of nontornadic events are more proximal to the CONUS than moisture sources of tornado outbreaks. Oceanic influences on the water vapor content of air parcels were also explored to determine if they can increase the likelihood of an air mass attaining moisture that will eventually contribute to severe thunderstorms. Warmer sea surface temperatures were generally found to enhance evaporative fluxes of overlying air parcels. The influence of atmospheric features on synoptic-scale moisture advection was also analyzed; stronger extratropical cyclones and Great Plains low-level jet occurrences lead to increased meridional moisture flux.

Which Extratropical Cyclones Contribute Most to the Transport of Moisture in the Southern Hemisphere?

AbstractPredicted changes in Southern Hemisphere (SH) precipitation and Antarctic ice mass correspond to variations in the meridional moisture flux (MMF). Thirty‐five years of ERA‐Interim reanalysis data are combined with an extratropical cyclone (ETC) identification and tracking algorithm to investigate factors controlling SH MMF variability in the midlatitudes and near Antarctica. ETC characteristics which exert the strongest control on ETC MMF are determined thus identifying which ETCs contribute most to SH moisture transport. ETC poleward propagation speed exerts the strongest control on the ETC MMF across the Antarctic coastline. In SH winter, ETCs with the largest poleward propagation speeds transport 2.5 times more moisture than an average ETC. In the midlatitudes, ETC genesis latitude and poleward propagation speed have a similar influence on ETC MMF. Surprisingly, ETC maximum vorticity has little control on ETC MMF. Cyclone compositing is used to determine the reasons for these statistical relationships. ETCs generally exhibit a dipole of poleward and equatorward MMF downstream and upstream of the cyclone center, respectively. However, ETCs with the largest poleward propagation speeds resemble open frontal waves with strong poleward moisture transport downstream of the cyclone center only and thus result in the largest MMF. These results suggest that inhomogeneous trends and predicted changes in precipitation over Antarctica may be due to changes in cyclone track orientation, associated with changes to the large‐scale background flow, in addition to changes in cyclone number or intensity.

The Signature of Southern Hemisphere Atmospheric Circulation Patterns in Antarctic Precipitation.

We provide the first comprehensive analysis of the relationships between large‐scale patterns of Southern Hemisphere climate variability and the detailed structure of Antarctic precipitation. We examine linkages between the high spatial resolution precipitation from a regional atmospheric model and four patterns of large‐scale Southern Hemisphere climate variability: the southern baroclinic annular mode, the southern annular mode, and the two Pacific‐South American teleconnection patterns. Variations in all four patterns influence the spatial configuration of precipitation over Antarctica, consistent with their signatures in high‐latitude meridional moisture fluxes. They impact not only the mean but also the incidence of extreme precipitation events. Current coupled‐climate models are able to reproduce all four patterns of atmospheric variability but struggle to correctly replicate their regional impacts on Antarctic climate. Thus, linking these patterns directly to Antarctic precipitation variability may allow a better estimate of future changes in precipitation than using model output alone.

Analysis of Low-Level Atmospheric Moisture Transport Associated with the West African Monsoon

Abstract The major objective of this study is to re-evaluate the ocean–land transport of moisture for rainfall in West Africa using 1979–2008 NCEP–NCAR reanalysis data. The vertically integrated atmospheric water vapor flux for the surface–850 hPa is calculated to account for total low-level moisture flux contribution to rainfall over West Africa. Analysis of mean monthly total vapor fluxes shows a progressive penetration of the flux into West Africa from the south and west. During spring (April–June), the northward flux forms a “moisture river” transporting moisture current into the Gulf of Guinea coast. In the peak monsoon season (July–September), the southerly transport weakens, but westerly transport is enhanced and extends to 20°N owing to the strengthening West African jet off the west coast. Mean seasonal values of total water vapor flux components across boundaries indicate that the zonal component is the largest contributor to mean moisture transport into the Sahel, while the meridional transport contributes the most over the Guinea coast. For the wet years of the Sahel rainy season (July–September), active anomalies are displaced farther north compared to the long-term average. This includes the latitude of the intertropical front (ITF), the extent of moisture flux, and the zone of strong moisture flux convergence, with an enhanced westerly flow. For the dry Sahel years, the opposite patterns are observed. Statistically significant positive correlations between the zonal moisture fluxes and Sudan–Sahel rainfall totals are most pronounced when the zonal fluxes lead by 1–4 pentads. However, although weak, they still are statistically significant at lags 3 and 4 for meridional moisture fluxes.

Assessing trends in lower tropospheric heat content in the central United States using equivalent temperature

ABSTRACTIsobaric equivalent temperature (TE) is the temperature that an air parcel would have if all associated water vapour were condensed and the resulting latent heat used to increase the temperature of the parcel. It is therefore an ideal metric for assessing changes in (1) total near‐surface heat content associated with both temperature and moisture content and (2) the joint behaviour of temperature and humidity, which is relevant to both lower atmospheric stability and human heat stress during extreme temperature events. We present results from an analysis of 50 years (1961–2010) of daily TE and its temperature and moisture components at seven stations in the central United States. The annual means of daily TEmax and TEmin increased at all stations during the period of analysis with the largest changes occurring in TEmin, largely as a result of increasing minimum air temperature. At western locations significant increases in the annual mean TEmax were also observed, resulting from a combination of increases in Tmax and humidity. Despite small summer (JJA) trends in maximum air temperature, summer TE trends were generally larger than their annual counterparts. The timing of the observed variations and the resulting spatial pattern are consistent with observed changes in meridional moisture flux associated with the Great Plains low‐level jet. Heat waves in the region were found to be characterized by increasing TEmin, primarily resulting from increases in minimum air temperature. At western stations, heat waves were also characterized by increasing TEmax as a result of positive trends in humidity. In most cases, equivalent temperature provides a perspective on local environmental change that differs from what is provided by consideration of temperature alone.

Impact of Cross-Equatorial Meridional Transport on the Performance of the Southwest Monsoon over India

Water vapour transport over the Indian Ocean has been computed for the 30-year period (1979-2009). The monthly evolution of meridional moisture fluxes across different sections is presented. March and April clearly indicate the north to south flow of moisture across the equatorial region. During May there is intensification in the northward cross-equatorial moisture transport, which may act as a precursor of the rainy season. During the monsoon season maximum transport occurs in June with values of 1.24 ×10 11 and 5.58 ×10 12 tonnes/day for moisture and air flux across the equator respectively, which occurs in the lower atmospheric level between 1000 and 650 hPa. Our finding clearly shows that during the monsoon season across the equatorial cross-section major transport occurs between 42°E and 60°E. Analysis of moisture transport over two regions, i.e. (i) 6°S-6°N and 42- 60°E and (ii) 1.5°S-1.5°N and 42-60°E for two good (1988, 2008) and two bad (1987, 2009) monsoon years shows that during 1987, which was a drought year, the amount of moisture crossing the equator was less by about one order of magnitude compared to 1988. While during 2008, which was a normal/good monsoon year, the amount of moisture transported was almost three times compared to 2009. This clearly indicates that the moisture transport in May can be used as a predictor of monsoon performance.

The relationship between daily European precipitation and measures of atmospheric water vapour transport

ABSTRACTIn this study we investigate the relationship between daily European precipitation totals and different measures of atmospheric water vapour transport during both the winter and summer seasons. Using gridded gauge‐based precipitation estimates across Europe and the Modern‐Era Retrospective Analysis for Research and Applications (MERRA) reanalysis as the source of the water vapour transport, we examine what combination of predictor variables (related to wind and humidity at different levels and total column water) explains the most precipitation variability. Results show that stronger relationships occur in the winter than in the summer, and that the best linear models for explaining the daily rainfall variability contain the zonal and meridional water vapour transport components as predictors [e.g. winter precipitation in the European Alps (western Norway) has R2 values of about 0.5 (0.45)]. Precipitation regions most related to zonal and meridional moisture fluxes are also discussed. We also show that models requiring just three fields, namely the integrated water vapour, and the 850‐hPa zonal and meridional winds, are able to explain the precipitation variability in Europe well. Given the computational and initial storage requirements in calculating water vapour transport integrated over the atmospheric column (in particular for climate change projections), it would be possible to use these three fields in assessments of future water vapour fluxes over Europe, in turn allowing inferences to be made on future rainfall and flood events.

Indian Ocean Dipole and southern high latitude precipitation: possible links

Southern high latitude precipitation during austral spring in relation to the Indian Ocean Dipole (IOD) and ENSO is investigated in the present study. Both the IOD and ENSO generate Rossby waves trains that create positive and negative pressure anomalies. These anomalous pressure centres generate meridional moisture fluxes that impact the precipitation. Influence of the IOD is detected mainly in the Ross sea region, where the southward moisture transport induced by the low pressure cell enhances precipitation. During strong IOD years, east Antarctica near 100°E, is also characterised by enhanced precipitation induced by the southward moisture transport by a high pressure cell located south of Australia. In the Dronning Maud Land, precipitation is linked to the moisture advection through the Atlantic during ENSO years and not during the IOD years.

Atmospheric Meridional Moisture Flux over the Southern Ocean: A Story of the Amundsen Sea

AbstractThe Antarctic ice sheet constitutes the largest reservoir of freshwater on earth, representing tens of meters of sea level rise if it were to melt completely. However, because of the remote location of the continent and the concomitant sparse data coverage, much remains unknown regarding the climate variability in Antarctica and the surrounding Southern Ocean. This study uses the high-resolution ECMWF Interim Re-Analysis (ERA-Interim) data during 1979–2010 to calculate the meridional moisture transport associated with the mean circulation, planetary waves, and synoptic-scale systems. The resulting moisture flux, which is dominated by the synoptic scales, is largely consistent with results from theoretical assumptions and previous studies. Here, high interannual and regional variability in the total meridional moisture flux is found, with no significant trend over the last 30 years. Further, the variability of the meridional moisture flux cannot be explained by the southern annular mode or El Niño–Southern Oscillation, even in the Pacific sector. In addition, the Amundsen Sea sector experiences the highest variability in meridional moisture transport and reveals a statistically significant decrease in the moisture flux at synoptic scales along the coastal zone. These results suggest that the Amundsen Sea provides a window on the complex nature of atmospheric moisture transport in the high southern latitudes.