West Coasts Of Continents Research Articles

Global Climatology of Low‐Level‐Jets: Occurrence, Characteristics, and Meteorological Drivers

AbstractLow‐level jets (LLJs), vertical profiles with a wind speed maxima in the lowest hundred meters of the troposphere, have multiple impacts in the Earth system, but a global present‐day climatology based on contemporary data does not exist. We use the spatially and temporally complete data set from ERA5 reanalysis to compile a global climatology of LLJs for studying the formation mechanisms, characteristics, and trends during the period of 1992–2021. In the global mean, LLJs are detected 21% of the time with more cases over land (32%) than over the ocean (15%). We classified the LLJs into three categories: non‐polar land (LLLJ), polar land (PLLJ), and coastal (CLLJ) LLJs. For LLLJ, the averaged frequency of occurrence is 20% and 75% of them are associated with a near‐surface temperature inversion as a prerequisite for an inertial oscillation. PLLJs are also associated with a temperature inversion and occur even more frequently with 59% of the time. These are also the lowest and the strongest LLJs among the three categories. CLLJs are particularly frequent in some marine hotspots, situated along the west coast of continents, with neutral to unstable stratification close to the surfaces and a stably stratified layer aloft. We found distinct regional trends in both the frequency and intensity of LLJs over the past decades, which can have implications for the emission and transport of aerosols, and the transport of atmospheric moisture. Future studies could address changes in LLJs and the associated implications in more detail, based on the here released ERA5‐based LLJ data.

Middle Permian palaeoclimatic-palaeoceanographic evolution and its controls on organic matter accumulation in the Lower Yangtze upwelling region

For more than 10 million years (∼272–260 Ma), the Middle Permian underwent periodic episodes of vigorous upwelling and massive organic matter burial along the west coasts of continents, as testified by the abundant black chert-mudstone records. However, less attention has been paid to the palaeoclimatic-palaeoceanographic evolution in this upwelling area and its relationship with the mechanisms of coeval organic matter accumulation. To better understand this scenario, multiple geochemical analyses were performed on organic-rich deposits of the Gufeng Formation of the Lower Yangtze region. Our results showed that the Middle Permian climatic conditions gradually shifted from warm-humid to hot-humid. Comprehensive geochemical analyses using multiple proxies suggested that the study area was characterised by a hybrid palaeoceanographic system influenced by both upwelling and restriction. The coupled climatic-oceanic evolution went through four stages. The Roadian stage was characterised by gradually enhanced upwelling but relatively oxic conditions and low productivity; the Wordian-early Capitanian stage was marked by the alternating development of perennial and seasonal upwelling with higher productivity and anoxic-euxinic conditions; the early-middle Capitanian stage was distinguished by an intermittent seasonal upwelling with relatively decreased productivity and weak anoxic conditions; the middle Capitanian stage immediately after the deposition of the Gufeng Formation transitioned from a transient-restricted condition to a weak upwelling setting and was dominated by extremely low productivity and suboxic conditions. Further studies on these organic-rich sediments show that the integration of biological productivity, redox conditions, detrital inputs, and hydrographic restrictions under coupled climatic-oceanic forcing could have had a complex influence on their formation. In this upwelling setting, organic matter accumulation is dominated by changes in both upwelling-driven primary productivity and oxygen minimum zone-related reducing conditions, but to a lesser extent by the dilution of climate-influenced detrital inputs.

Changes of South‐Central Pacific Large‐Scale Environment Associated With Hydrometeors‐Radiation‐Circulation Interactions in a Coupled GCM

AbstractA common deficiency in coupled atmosphere‐ocean models is the lack of stratocumulus clouds near the west coasts of continents and shallow cumulus clouds over the trade wind regions. In this study, we examine changes of large‐scale trade wind environment associated with hydrometeors‐radiation‐circulation interactions, focusing over the south‐central Pacific, by carrying out experiments with falling ice (snow) radiative effects (FIREs) on or off using the CESM1‐CAM5 coupled model. Compared to observations/reanalysis data and an experiment with FIREs on (SON), an experiment with FIREs off (NOS) tends to have too‐high (0.3–0.9 K) sea surface temperature (SST) over the eastern half of the Pacific, and weaker surface wind stress over the northeast side but stronger stress over the southwest side of the study domain. The difference between NOS and SON shows lower‐tropospheric wind fields with a cyclonic‐like wind pattern, which is similar to surface wind stress, accompanied with weaker subsidence over the northeast side. The cyclonic‐like wind pattern causes stronger moisture convergence, accompanied with horizontal warm and moist advections, and stronger effective ascending motion over the northeast side of the study domain, building large‐scale environmental conditions to facilitate middle‐ and high‐clouds instead of shallow cumulus clouds in nature. This large‐scale environment in conjunction with the higher SSTs may be also responsible for the lack of the stratocumulus clouds near the coast of America. Remaining biases in SON such as the double intertropical convergence zone and parameterization deficiencies may also prevent a more realistic simulation of trade wind clouds and their associated large‐scale environments.

SimCloud version 1.0: a simple diagnostic cloud scheme for idealized climate models

Abstract. A simple diagnostic cloud scheme (SimCloud) for general circulation models (GCMs), which has a modest level of complexity and is transparent in describing its dependence on tunable parameters, is proposed in this study. The large-scale clouds, which form the core of the scheme, are diagnosed from relative humidity. In addition, the marine low stratus clouds, typically found off the west coast of continents over subtropical oceans, are determined largely as a function of inversion strength. A “freeze-dry” adjustment based on a simple function of specific humidity is also available to reduce an excessive cloud bias in polar regions. Other cloud properties, such as the effective radius of cloud droplet and cloud liquid water content, are specified as simple functions of temperature. All of these features are user-configurable. The cloud scheme is implemented in Isca, a modeling framework designed to enable the construction of GCMs at varying levels of complexity, but could readily be adapted to other GCMs. Simulations using the scheme with realistic continents generally capture the observed structure of cloud fraction and cloud radiative effect (CRE), as well as its seasonal variation. Specifically, the explicit low-cloud scheme improves the simulation of shortwave CREs over the eastern subtropical oceans by increasing the cloud fraction and cloud water path. The freeze-dry adjustment alleviates the longwave CRE biases in polar regions, especially in winter. However, the longwave CRE in tropical regions and shortwave CRE over the extratropics are both still too strong compared to observations. Nevertheless, this simple cloud scheme provides a suitable basis for examining the impacts of clouds on climate in idealized modeling frameworks.

Retracted: The impact of a boundary layer height formulation on the GEOS‐5 model climate

AbstractPlanetary boundary layer (PBL) processes are important for the estimation of surface‐atmosphere exchanges that impact global climate. One way of characterizing the strength of these processes is the PBL depth. In the Goddard Earth Observing System (GEOS‐5) atmospheric general circulation model, the PBL depth is also used in calculating the turbulent length scale, which, in turn, is used in estimating the turbulence and vertical mixing within the model. Therefore, changing the PBL depth definition directly affects the model climate. This study evaluates the climatological model response of two long‐term simulations using different PBL depth definitions. The first definition is based on a bulk Richardson number; the second uses a combination of the same bulk Richardson number definition over land plus a definition based on the turbulent eddy diffusion coefficient over water. The two simulations produce different spatiotemporal patterns of temperature, specific humidity, and wind speed related to the differences in turbulence. The largest differences, as expected, are present over water. Due to differences in atmospheric stability, the relationship between the two PBL depth estimates differ among the majority of the oceans and off the west coasts of continents, affecting the climatic response. Due to its optimization of the climatic response while maintaining a realistic diurnal cycle of PBL depth, the mixed PBL depth configuration is preferred.

Improved Low-Cloud Simulation from the Community Atmosphere Model with an Advanced Third-Order Turbulence Closure

Abstract In this study, a simplified intermediately prognostic higher-order turbulence closure (IPHOC) is implemented in the Community Atmosphere Model, version 5 (CAM5), to provide a consistent treatment of subgrid-scale cloud processes, except for deep convection. The planetary boundary layer (PBL) height is prognosticated to better resolve the discontinuity of temperature and moisture above the PBL top. Single-column model tests show that fluxes of liquid water potential temperature and total water, cloud fraction, and liquid water content are improved with this approach. The simplified IPHOC package replaces the boundary layer dry and moist turbulence parameterizations, the shallow convection parameterization, and the liquid-phase part of the cloud macrophysics parameterization in CAM5. CAM5-IPHOC improves the simulation of the low-level clouds off the west coasts of continents and the storm track region in the Southern Hemisphere (SH). The transition from stratocumulus to cumulus clouds is more gradual. There are also improvements on the cloud radiative forcing, especially shortwave, in the subsidence regime. The improvements in the relationships among low cloud amount, surface relative humidity, lower tropospheric stability, and PBL depth are seen in some stratocumulus regions. CAM5-IPHOC, however, produces weaker precipitation at the South Pacific convergence zone than CAM5 because of less energy flux into the SH atmosphere. The more downward surface shortwave radiative cooling and the less top-of-the-atmosphere longwave cloud radiative heating in the SH relative to the Northern Hemisphere explains the anomalous cooling and the lesser energy flux into the SH, which is related to the underestimate of extratropical middle/high clouds in the SH.

The strength of the tropical inversion and its response to climate change in 18 CMIP5 models

We examine the tropical inversion strength, measured by the estimated inversion strength (EIS), and its response to climate change in 18 models associated with phase 5 of the coupled model intercomparison project (CMIP5). While CMIP5 models generally capture the geographic distribution of observed EIS, they systematically underestimate it off the west coasts of continents, due to a warm bias in sea surface temperature. The negative EIS bias may contribute to the low bias in tropical low-cloud cover in the same models. Idealized perturbation experiments reveal that anthropogenic forcing leads directly to EIS increases, independent of “temperature-mediated” EIS increases associated with long-term oceanic warming. This fast EIS response to anthropogenic forcing is strongly impacted by nearly instantaneous continental warming. The temperature-mediated EIS change has contributions from both uniform and non-uniform oceanic warming. The substantial EIS increases in uniform oceanic warming simulations are due to warming with height exceeding the moist adiabatic lapse rate in tropical warm pools. EIS also increases in fully-coupled ocean–atmosphere simulations where \(\hbox {CO}_{2}\) concentration is instantaneously quadrupled, due to both fast and temperature-mediated changes. The temperature-mediated EIS change varies with tropical warming in a nonlinear fashion: The EIS change per degree tropical warming is much larger in the early stage of the simulations than in the late stage, due to delayed warming in the eastern parts of the subtropical oceans. Given the importance of EIS in regulating tropical low-cloud cover, this suggests that the tropical low-cloud feedback may also be nonlinear .

A Planetary Boundary Layer Height Climatology Derived from ECMWF Reanalysis Data

Abstract A planetary boundary layer (PBL) height climatology from ECMWF reanalysis data is generated and analyzed. Different methods are first compared to derive PBL heights from atmospheric temperature, pressure, and relative humidity (RH), which mostly make use of profile gradients, for example, in RH, refractivity, and virtual or potential temperature. Three methods based on the vertical gradient of RH, virtual temperature, and potential temperature were selected for the climatology generation. The RH-based method appears to capture the inversion that caps the convective boundary layer very well as a result of its temperature and humidity dependence, while the temperature-based methods appear to capture the PBL better at high latitudes. A validation of the reanalysis fields with collocated radiosonde data shows generally good agreement in terms of mean PBL height and standard deviation for the RH-based method. The generated ECMWF-based PBL height climatology shows many of the expected climatological features, such as a fairly low PBL height near the west coast of continents where stratus clouds are found and PBL growth as the air is advected over warmer waters toward the tropics along the trade winds. Large seasonal and diurnal variations are primarily found over land. The PBL height can exceed 3 km, mostly over desert areas during the day, although large values can also be found in areas such as the ITCZ. The robustness of the statistics was analyzed by using information on the percentage of outliers. Here in particular, the sea-based PBL was found to be very stable.

Southern Hemisphere westerly wind changes during the Last Glacial Maximum: paleo-data synthesis

Changes in the strength and position of Southern Hemisphere westerly winds during the Last Glacial cycle have been invoked to explain both millennial and glacial–interglacial climate fluctuations. However, neither paleo models nor paleodata agree on the magnitude, or even the sign, of the change in wind strength and latitude during the most studied glacial period, the Last Glacial Maximum (LGM), compared to the recent past. This paper synthesizes paleo-environmental data that have been used to infer changes in LGM winds. Data compilations are provided for changes in terrestrial moisture, dust deposition, sea surface temperatures and ocean fronts, and ocean productivity, and existing data on Southern Hemisphere ocean circulation changes during the LGM are summarized. We find that any hypothesis of LGM wind and climate change needs to provide a plausible explanation for increased moisture on the west coast of continents, cooler temperatures and higher productivity in the Subantarctic Zone, and reductions in Agulhas leakage around southern Africa. Our comparison suggests that an overall strengthening, an equatorward displacement, or no change at all in winds could all be interpreted as consistent with observations. If a single cause related to the southern westerlies is sought for all the evidence presented, then an equatorward displacement or strengthening of the winds would be consistent with the largest proportion of the observations. However, other processes, such as weakening or poleward shifts in winds, a weakened hydrological cycle, extended sea-ice cover, and changed buoyancy fluxes, cannot be ruled out as potential explanations of observed changes in moisture, surface temperature, and productivity. We contend that resolving the position and strength of westerly winds during the LGM remains elusive based on data reconstructions alone. However, we believe that these data reconstructions of environmental conditions can be used in conjunction with model simulations to identify which processes best represent westerly wind conditions during the LGM.

Mechanisms for Diurnal Variability of Global Tropical Rainfall Observed from TRMM

AbstractThe behavior and various controls of diurnal variability in tropical–subtropical rainfall are investigated using Tropical Rainfall Measuring Mission (TRMM) precipitation measurements retrieved from the three level-2 TRMM standard profile algorithms for the 1998 annual cycle. Results show that diurnal variability characteristics of precipitation are consistent for all three algorithms, providing assurance that TRMM retrievals are producing consistent estimates of rainfall variability. As anticipated, most ocean areas exhibit more rainfall at night, while over most land areas, rainfall peaks during daytime; however, important exceptions are noted.The dominant feature of the oceanic diurnal cycle is a rainfall maximum in late-evening–early-morning (LE–EM) hours, while over land the dominant maximum occurs in the mid- to late afternoon (MLA). In conjunction with these maxima are pronounced seasonal variations of the diurnal amplitudes. Amplitude analysis shows that the diurnal pattern and its seasonal evolution are closely related to the rainfall accumulation pattern and its seasonal evolution. In addition, the horizontal distribution of diurnal variability indicates that for oceanic rainfall, there is a secondary MLA maximum coexisting with the LE–EM maximum at latitudes dominated by large-scale convergence and deep convection. Analogously, there is a preponderancy for an LE–EM maximum over land coexisting with the stronger MLA maximum, although it is not evident that this secondary continental feature is closely associated with the large-scale circulation. Neither of the secondary maxima exhibit phase behavior that can be considered semidiurnal in nature. Diurnal rainfall variability over the ocean associated with large-scale convection is clearly an integral component of the general circulation.Phase analysis reveals differences in regional and seasonal features of the diurnal cycle, indicating that underlying forcing mechanisms differ from place to place. This is underscored by the appearance of secondary ocean maxima in the presence of large-scale convection, along with other important features. Among these, there are clear-cut differences between the diurnal variability of seasonal rainfall over the mid-Pacific and Indian Ocean Basins. The mid-Pacific exhibits double maxima in spring and winter but only LE–EM maxima in summer and autumn, while the Indian Ocean exhibits double maxima in spring and summer and only an LE–EM maximum in autumn and winter. There are also evident daytime maxima within the major large-scale marine stratocumulus regions off the west coasts of continents. The study concludes with a discussion concerning how the observational evidence either supports or repudiates possible forcing mechanisms that have been suggested to explain diurnal rainfall variability.

A Simple Parameterization Scheme for Subtropical Marine Stratocumulus

A simple scheme to represent the marine stratocumulus clouds off the west coast of continents is implemented in the Global Spectral Model (GSM) at Japan Meteorological Agency (JMA). The parameterization is based on diagnostic cloud schemes where cloud fraction is diagnosed mainly as a function of inversion strength considering other parameters. The global distribution of marine stratocumulus clouds off the west coast of continents is improved remarkably with this new scheme. Low-level cloud amount shows reasonable agreement with the International Satellite Cloud Climatology (ISCCP). With the improved cloud amount, the radiation fields are also improved in comparison with the Earth Radiation Budget Experiment (ERBE). Seasonal and diurnal variations of marine stratocumulus cloud amount off the west coast of continents also show reasonable agreement with surface-based cloud amount data from Klein and Hartmann (1993) and other observations.

Fog and low clouds in a coastally trapped disturbance

One of the most distinctive features of coastally trapped disturbances (CTDs) is the tongue of fog and low clouds propagating poleward along the west coast of continents against the typical coast‐parallel equatorward flow. In this study, characteristics of the fog and low clouds observed in conjunction with a CTD event on 15–16 June 2000 are investigated. In particular, we examine the relative importance of radiation, cloud microphysical processes, advection, entrainment, and surface fluxes to the fog and cloud layer associated with the CTD. A simulation of the event is produced using the Naval Research Laboratory's nonhydrostatic Coupled Ocean/Atmosphere Mesoscale Prediction System. Propagation of the wind shift and cloud tongue and the extent of fog and low clouds in the model results are compared with coastal buoy data and satellite imagery. A critical value of the Bowen ratio delineates the extent of fog with lower values to the south in the low cloud area. Because the Bowen ratio provides limited information concerning the relative roles of sensible and latent heat fluxes in the cloud layer, we examine the ratio β of integrated buoyancy generation due to liquid water flux to the integrated positive buoyancy flux. The timing difference between arrival of the CTDs' wind shift and the arrival of the fog is found to be central to the evolution of the cloud liquid water field. Several sensitivity studies are also carried out in order to further understand the roles of radiative cooling and surface latent and sensible heat fluxes.

A Comparison of Satellite Observations and Model Simulations of Column-Integrated Moisture and Upper-Tropospheric Humidity

Abstract Water vapor distributions obtained from the fourth generation ECHAM general circulation model are compared with satellite observations of total precipitable water (TPW) from the Special Sensor Microwave/Imager (SSM/1) and upper-tropospheric relative humidity (UTH) from TIROS-N Operational Vertical Sounder (TOVS). In general, the model simulators agree well with satellite observations of the climatological mean, seasonal variation, and interannual variation of moisture. There are, however, biases in the details. Underestimates in TPW and UTH are found off the west coast of continents, especially in the boreal summer over the eastern subtropical Pacific. These biases are related to both enhanced dry advection due to an excessively strong subtropical high and greater large-scale subsidence. A more intense tropical circulation in ECHAM4 is evidenced by the broadening of the high TPW and UTH zone that coincides with the equatorial convective regions. Additionally, interannual anomalies in equatorial U...

Vegetation Structure in the Mediterranean Scrub Communities of California and Chile

One successful approach towards refining the form-environment interaction in natural communities has been to examine the extent of convergence in structural and functional attributes of phylogenetically distinct biotas of climatically similar areas. This approach has been used in tropical areas (Richards 1952; Sarmiento 1972), in semideserts (Riley & Young 1966) and in the areas of Mediterranean climate (Naveh 1967; Specht 1969). The latter have long been cited as examples of evolutionary convergence (Grisebach 1872). Characterized by hot dry summers and cool moist winters, they are commonly located at middle latitudes on the west coasts of continents (specifically the west coasts of California, central Chile, South Africa, southern Australia and the Mediterranean Basin itself). These areas of similar climate display distinct biotas. The coastal mountains of southern California and central Chile were chosen as

Carbon Dioxide in the Surface Water of the Ice-covered Bering Sea

DETERMINATION of the equilibrium concentration of CO2 in the ocean surface with respect to air is necessary in order to understand the interaction of CO2 between the atmosphere and the ocean. The concentration of CO2 in water depends on its physical, chemical and biological properties. The oceans, therefore, act as both sources and sinks for CO2 in the air. Keeling1 reported that high concentrations of CO2 in the ocean appeared near the equator and were lower in the sub-tropics of all oceans. Areas along the west coasts of continents where upwelling occurs have high CO2 concentrations. In the polar regions, patterns are more complicated primarily because of a lack of seasonal data. Measurements of the equilibrium concentration of CO2 with respect to air in the surface waters of the Kara and Barents Seas during the late summer showed that the surface waters were supersaturated with CO2 near the mouths of the Ob and Yenisey Rivers and undersaturated in the Barents Sea2. The present study was undertaken to measure the actual distribution of CO2 between the atmosphere and the sea over open leads and polynyi in the ice-covered Bering Sea. Previous measurements of CO2 in ice-covered seas were made by Kelley3 in the water under the Arctic Ocean pack ice near Ice Island T-3.