Gap Winds Research Articles

Hydraulic Analysis and Cold-Air Pool Interaction of the Raco Gap Wind

Abstract Raco is a local wind occurring in central Chile where the Maipo River Canyon exits into the Santiago valley. The intensification of the easterly down-canyon flow starts at any time during some cold season nights, accompanied by increases in temperature and drops in humidity. The hypothesis of the raco being a gap wind controlled by the narrowest section in the 12-km canyon exit corridor is tested with data from two events in July 2018 and July 2019. The data are analyzed in the framework of hydraulic theory, and a subcritical-to-supercritical transition is documented to occur at the narrows of the gap where the Froude number is close to unity, confirmed by radiosondes launched in the narrows in 2019. For the raco flow, the sum of potential and kinetic energy is conserved upstream of the narrows, while the acceleration occurring farther downstream loses a large fraction of energy to frictional dissipation. The raco events occur under the influence of regional subsidence, but a differential nocturnal warming of the in-canyon air mass is responsible for a pressure gradient driving the raco. In the 2019 case, a ceilometer mounted on an instrumented pickup truck documented the structure and movement of the interface between the raco air and the cold-air pool (CAP) existing over the valley to the west. Together with a radiosonde launched near the CAP–raco surface front, the observations reveal the intense shear-driven mixing taking place at the interface and the factors supporting the establishment of a stationary front.

Comparing GlobCurrent dataset with numerical results from a high-resolution implementation of the POLCOMS-WAM coupled system under a strong gap wind over the Gulf of Tehuantepec

Comparing GlobCurrent dataset with numerical results from a high-resolution implementation of the POLCOMS-WAM coupled system under a strong gap wind over the Gulf of Tehuantepec

The Diurnal Cycle of East Pacific Convection, Moisture, and CYGNSS Wind Speed and Fluxes

AbstractThe far eastern tropical Pacific is one of the rainiest locations on Earth. This region is characterized by a robust diurnal cycle with precipitation initiating over the Andes mountains in Colombia in the late afternoon and moving offshore overnight while organizing into mesoscale convective systems (MCSs). As the MCSs move westward, they can seed tropical easterly waves that develop into tropical cyclones. Novel measurements of near‐surface wind speeds and latent heat flux (LHFLX) from the Cyclone Global Navigation Satellite System (CYGNSS) mission and ERA5 reanalysis are used to evaluate changes to regional flow patterns, moisture, and LHFLX that favor a strong westward propagating diurnal cycle of precipitation in this region. Days with strongly versus weakly westward propagating diurnal precipitation are compared during the extended boreal summer of the CYGNSS era (May–October 2018–2021). Strongly propagating days have a more moist lower troposphere with stronger vertically integrated moisture flux convergence than days with weakly propagating precipitation. The enhanced low‐level moisture is supported by increased wind‐driven LHFLX from stronger regional flows including the Choco and Caribbean Low‐Level Jets and Panama and Papagayo gap winds. Increased offshore convergence and vorticity also help sustain convection during the days with strongly propagating precipitation. Background variations in the physical environment, as opposed to diurnal variations, seem more important for supporting a robust diurnal cycle of precipitation in the region.

Random Forest Approach for Improving Nonconvective High Wind Forecasting across Southeast Wyoming

Abstract High winds are one of the key forecast challenges across southeast Wyoming. The complex mountainous terrain across the region frequently results in strong gap winds in localized areas, as well as more widespread bora and chinook winds in the winter season (October–March). The predictors and general weather patterns that result in strong winds across the region are well understood by local forecasters. However, no single predictor provides notable skill by itself in separating warning-level events from others. Random forest (RF) classifier models were developed to improve upon high wind prediction using a training dataset constructed of archived observations and model parameters from the North American Regional Reanalysis (NARR). Three locations were selected for initial RF model development, including the city of Cheyenne, Wyoming, and two gap regions along Interstate 80 (Arlington) and Interstate 25 (Bordeaux). Verification scores over two winters suggested the RF models were beneficial relative to current operational tools when predicting warning-criteria high wind events. Three case studies of high wind events provide examples of the RF models’ effectiveness to forecast operations over current forecast tools. The first case explores a classic, widespread high wind scenario, which was well anticipated by local forecasters. A more marginal scenario is explored in the second case, which presented greater forecast challenges relating to timing and intensity of the strongest winds. The final case study carefully uses Global Forecast System (GFS) data as input into the RF models, further supporting real-time implementation into forecast operations.

Dynamics of gap winds in the Great Rift Valley, Ethiopia: emphasis on strong winds at Lake Abaya

Abstract. Lake Abaya, located in the Great Rift Valley (GRV) in Ethiopia, is affected by regularly occurring strong winds that cause water waves, which in turn affect the lake's ecology and food web. The driving forces for these winds, however, are yet unexplained. Hence, the main goal of this study is to provide a physical explanation for the formation of these strong winds in the GRV and especially at Lake Abaya. To this aim, two case studies were performed based on measurements, ERA5 reanalysis data and mesoscale numerical simulations conducted with the Weather Research and Forecasting (WRF) model. The simulations revealed that in both cases a gap flow downstream of the narrowest and highest part of the GRV (i.e. the pass) led to high wind speeds of up to 25 m s−1. Two types of gap flow were identified: a north-eastern gap flow and a south-western gap flow. The wind directions are in line with the orientation of the valley axis and depend on the air mass distribution north and south of the valley and the resulting along-valley pressure gradient. The air mass distribution was determined by the position of the Intertropical Convergence Zone relative to the GRV. The colder air mass was upstream of the GRV in both case studies. During the day, the convective boundary layer in the warmer air mass on the downstream side heated up more strongly and quickly than in the colder air mass. The most suitable variable describing the timing of the gap flow was found to be the pressure gradient at pass height, which corresponds roughly to the 800 hPa pressure level. In both cases the gap flow exhibited a strong daily cycle, which illustrates the importance of the thermal forcing due to differential heating over complex terrain in addition to the large-scale forcing due to air mass differences. The start, strength, and the duration of the gap winds within the valley depended on location. For both cases, the strongest winds occurred after sunset and in the ongoing night downstream of the gap and on the corresponding lee slope. The ERA5 reanalysis captures both events qualitatively well but with weaker wind speeds than in the mesoscale numerical simulations. Hence, ERA5 is suitable for a future climatological analysis of these gap flows.

Unified Wind-Wave Growth and Spectrum Functions for All Water Depths: Field Observations and Model Results

Abstract Wind-wave development is governed by the fetch- or duration-limited growth principle that is expressed as a pair of similarity functions relating the dimensionless elevation variance (wave energy) and spectral peak frequency to fetch or duration. Combining the pair of similarity functions, the fetch or duration variable can be removed to form a dimensionless function of elevation variance and spectral peak frequency, which is interpreted as the wave energy evolution with wave age. The relationship is initially developed for quasi-neural stability and quasi-steady wind forcing conditions. Further analyses show that the same fetch, duration, and wave-age similarity functions are applicable to unsteady wind forcing conditions, including rapidly accelerating and decelerating mountain gap wind episodes and tropical cyclone (TC) wind fields. Here it is shown that with the dimensionless frequency converted to dimensionless wavenumber using the surface wave dispersion relationship, the same similarity function is applicable in all water depths. Field data collected in shallow to deep waters and mild to TC wind conditions and synthetic data generated by spectrum model computations are assembled to illustrate the applicability. For the simulation work, the finite-depth wind-wave spectrum model and its shoaling function are formulated for variable spectral slopes. Given wind speed, wave age, and water depth, the measured and spectrum-computed significant wave heights and the associated growth parameters are in good agreement in forcing conditions from mild to TC winds and in all depths from deep ocean to shallow lake. Significance Statement This paper presents a growth function and spectrum model to describe wind-wave development in all water depths. Their applicability covers a wide range of wind forcing conditions including steady, accelerating, decelerating, and tropical cyclone events. Support for the unified spectrum model and growth function is presented with field observations and numerical computations.

A numerical simulation of a strong windstorm event in the Taebaek Mountain Region in Korea during the ICE-POP 2018

During the International Collaborative Experiments for the Pyeongchang 2018 Olympic and Paralympic winter games (ICE-POP 2018), a strong windstorm occurred on the lee side of the Taebaek Mountains on 14 February 2018, when a low-pressure system passed through the northern part of the Korean Peninsula. At that time, a prevailing westerly wind with warm advection and an inversion layer at the top of the mountains provided favorable conditions for the downslope windstorm, resulting in structural damage at the Olympic Park. This led to the delay and cancellation of the ski jump and biathlon games. To investigate the generation mechanism responsible for the downslope windstorm during the ICE-POP 2018, we performed a numerical simulation using the Weather and Research Forecast model with the finest horizontal grid spacing of 333 m. The model effectively reproduced the multi-scale flows, such as the synoptic-scale low-pressure system, upstream sounding, and downslope winds, with slightly overestimated surface winds at some local sites on the lee side. Analysis of vertical cross-sections across the mountains showed a steep descent of potential temperature on the lee slope and a rapid recovery on the leeward side, showing the evidence of hydraulic jump with a Froude number of 0.9. During the windstorm event, mountain waves were generated with horizontal wavelengths that varied with time due to the change in background wind and stability along with movement of the low-pressure system. Using the dispersion relationship for internal gravity waves, the Scorer parameter for quasi-stationary mountain waves showed that the waves with horizontal wavelengths smaller than 10 km were trapped below the altitudes of 6–9 km. There were no signals of mountain wave breaking and wind reversal with height (wave-induced critical level), implying that the downslope windstorm event was generated by the mechanisms of hydraulic jump and partial reflection. Strengthened jet streams in the upstream and inversion layers at the top and gap winds in the valleys of the mountains also facilitated the downslope winds on the lee side.

Spatial-temporal characteristics of particulate matters and different formation mechanisms of four typical haze cases in a mountain city

The mechanisms by which pollution forms and acts in mountain cities are unique and remain unclear owing to terrain and meteorological conditions in the mountains. This study investigates the pollution characteristics in Chongqing, a typical mountain city, over the period from 2013 to 2017. Routine observed surface meteorological data, CALIPSO data, ERA5 reanalysis data and the backward trajectory calculation were used for analysis and comparison among four typical pollution cases. The results can be summarized as follows: (1) Pollutants in firework and unfavorable weather condition case mainly originated from local emissions. Anthropogenic aerosols that were characterized by spherical and small particles were detected, which contributed to the higher atmospheric scattering ability. Meanwhile, high humidity was beneficial to hygroscopic growth, resulting in a mixture of haze and fog forming low-visibility conditions. The strong inversion of the upper-layer and the presence of high mountains meant that local pollutants became confined within the basin region, leading to the formation and maintenance of pollution. The important roles of the weather conditions that are associated with mountains, such as cold air damming, low-level jet, valley and gap wind, were also revealed by the variable pollution conditions. (2) Airflow originating from Guizhou, Guangxi, and Hunan, where numerous fire points were detected, transported pollutants that were released from straw burning through upper-lower layer transport to the basin during the straw burning case. The aerosols detected showed low depolarization but a high color ratio, which are the typical characteristics of smoke. The inversion layer did not cover the entire basin, indicating that smoke aerosols were transported by downdraft to mix with local pollutants. (3) During the dust case, large amounts of dust aerosols were detected reaching from the surface to the troposphere, with a special double-layer structure. A lower depolarization ratio but stronger scattering ability was observed for the dust aerosols above the study region, which was attributed to mixing with other aerosols and chemical processes. Secondary circulation occurred when the inversion layer was at around 700 hPa, bringing dust aerosols from around 850 hPa to the surface. This study provides an understanding of the optical properties of the various types of aerosols and highlights the importance of weather condition in the formation and maintenance of pollution events. The results emphasize the need for intense vertical measurement in mountain cities.

Urban-Induced Changes on Local Circulation in Complex Terrain: Central Mexico Basin

Land use land cover (LULC) significantly impacts local circulation in the Mexico Basin, particularly wind field circulations such as gap winds, convergence lines, and thermally induced upslope/downslope wind. A case study with a high-pressure system over the Mexico Basin isolates the influence of large-scale synoptic forcing. Numerical simulations reveal a wind system composed of meridional circulation and a zonal component. Thermal pressure gradients between the Mexico basin and its colder surroundings create near-surface convergence lines as part of the meridional circulation. Experiments show that the intensity and organization of meridional circulations and downslope winds increase when LULC changes from natural and cultivated land to urban. Zonal circulation exhibits a typical circulation pattern with the upslope flow and descending motion in the middle of the basin. Large values of moist static energy are near the surface where air parcels pick up energy from the surface either as fluxes of enthalpy or latent heat. Surface heat fluxes and stored energy in the ground are drivers of local circulation, which is more evident in zonal circulation patterns.

Evidence of eddy-related deep-ocean current variability in the northeast tropical Pacific Ocean induced by remote gap winds

Abstract. There has been a steady increase in interest in mining of deep-sea minerals in the Clarion–Clipperton Zone (CCZ) in the eastern Pacific Ocean during the last decade. This region is known to be one of the most eddy-rich regions in the world ocean. Typically, mesoscale eddies are generated by intense wind bursts channeled through gaps in the Sierra Madre mountains in Central America. Here, we use a combination of satellite and in situ observations to evaluate the relationship between deep-sea current variability in the region of potential future mining and eddy kinetic energy (EKE) in the vicinity of gap winds. A geometry-based eddy detection algorithm has been applied to altimetry sea surface height data for a period of 24 years, from 1993 to 2016, in order to analyze the main characteristic parameters and the spatiotemporal variability of mesoscale eddies in the northeast tropical Pacific Ocean (NETP). Significant differences between the characteristics of eddies with different polarity (cyclonic vs. anticyclonic) were found. For eddies with lifetimes longer than 1 d, cyclonic polarity is more common than anticyclonic rotation. However, anticyclonic eddies are larger in size, show stronger vorticity, and survive longer in the ocean than cyclonic eddies (often 90 d or more). Besides the polarity of eddies, the location of eddy formation should be taken into consideration when investigating the impacted deep-ocean region as we found eddies originating from the Tehuantepec (TT) gap winds lasting longer in the ocean and traveling farther distances in a different direction compared to eddies produced by the Papagayo (PP) gap winds. Long-lived anticyclonic eddies generated by the TT gap winds are observed to travel distances up to 4500 km offshore, i.e., as far as west of 110∘ W. EKE anomalies observed in the surface of the central ocean at distances of ca. 2500 km from the coast correlate with the seasonal variability of EKE in the region of the TT gap winds with a time lag of 5–6 months. A significant seasonal variability of deep-ocean current velocities at water depths of 4100 m was observed in multiple-year time series data, likely reflecting the energy transfer of the surface EKE generated by the gap winds to the deep ocean. Furthermore, the influence of mesoscale eddies on deep-ocean currents is examined by analyzing the deep-ocean current measurements when an anticyclonic eddy crosses the study region. Our findings suggest that despite the significant modulation of dominant current directions driven by the bottom-reaching eddy, the current magnitude intensification was not strong enough to trigger local sediment resuspension in this region. A better insight into the annual variability of ocean surface mesoscale activity in the CCZ and its effects on deep-ocean current variability can be of great help to mitigate the impact of future potential deep-sea mining activities on the benthic ecosystem. On an interannual scale, a significant relationship between cyclonic eddy characteristics and El Niño–Southern Oscillation (ENSO) was found, whereas a weaker correlation was detected for anticyclonic eddies.

The Weather Hazards Associated with the US-Bangla Aircraft Accident at the Tribhuvan International Airport, Nepal

AbstractSafe flights over the Tribhuvan International Airport (TIA), Kathmandu, Nepal, remain a considerable challenge. Since the airport opened, there have been 13 aircraft accidents during landings and takeoffs that have claimed 392 lives. A detailed understanding and dependable forecast of atmospheric conditions that may develop over the complex terrain of the midhills of central Nepal Himalaya are yet to be achieved. The present study discusses the near-surface atmospheric conditions possibly associated with the most recent fatal crash at TIA on 12 March 2018 as revealed by the Advanced Research version of the Weather Research and Forecasting (WRF-ARW) Model routine forecast. At the time of the accident, two prominent gap winds were converging in the valley, thereby, forming a crosswind and a mix of strong up- and downdrafts over the airfield. As a result, the near-surface atmosphere was significantly turbulent. Unexpected encounters with such turbulent winds are a likely contributor to the fatal crash. This indicates that the knowledge of near-surface atmospheric conditions, critically needed by pilots in advance, for safe operations over the airfield may be generated with WRF-ARW forecasts.

A Numerical Study of Windstorms in the Lee of the Taebaek Mountains, South Korea: Characteristics and Generation Mechanisms

The Yeongdong region, located east of the Taebaek Mountains, South Korea, often experiences severe windstorms in spring, causing a lot of damages, especially when forest fires spread out rapidly by strong winds. Here, the characteristics and generation mechanisms of the windstorms in the Yeongdong region on 8 April 2012 are examined through a high-resolution Weather Research and Forecasting (WRF) model simulation. In the Yangyang area, the steep descent of the isentropes on the lee slope of the mountain and their recovery farther leeward are seen. Inversion layers and incoming flow in hydraulic jump regime suggest that the hydraulic jump is responsible for the downslope windstorm. In the Jangjeon area, the plume-shaped wind pattern extending seaward from the gap exit is seen when the sea-level pressure difference between the gap inside and the gap exit, being responsible for the gap winds, is large. In the Uljin area, downslope windstorms pass over the region with weak wind, low Richardson number, and deep planetary boundary layer (PBL), making banded pattern in the wind and PBL height fields. This study demonstrates that the characteristics of the windstorms in the lee of the Taebaek Mountains and their generation mechanisms differ depending on local topographic features.

Dipole-wind interactions under gap wind jet conditions in the Gulf of Tehuantepec, Mexico: A surface drifter and satellite database analysis.

Gap wind jets (Tehuano winds) trigger supersquirts of colder water and mesoscale asymmetric dipoles in the Gulf of Tehuantepec (GT). However, the effects of successive gap wind jets on dipoles and their effects inside eddies have not yet been studied. Based on the wind fields, geostrophic currents, and surface drifter dispersion, this research documented three dipoles triggered and modified by Tehuano winds. Once a dipole develops, successive gap wind jets strengthen the vortices, and the anticyclonic eddy migrates southwestward while the cyclonic eddy is maintained on the east side of the GT. During the wind relaxation stage, the cyclonic eddy may propagate westward, but due to the subsequent re-intensification of the Tehuano winds, the vortex could break down, as was suggested by surface drifter dispersion pattern and geostrophic field data. The effect of the Tehuano winds was evaluating via eddy-Ekman pumping. Under Tehuano wind conditions, Ekman downwelling (upwelling) inside the anticyclonic (cyclonic) eddies may reach ~ -2.0 (0.5) m d-1 and decrease as the wind weakens. In the absence of Tehuano winds, Ekman downwelling inside the anticyclonic eddy was ~ 0.1 (-0.1) m d-1. The asymmetry of downwelling and upwelling inside eddies during Tehuano wind events may be associated with Tehuano wind forcing.

Downslope windstorms in the Isthmus of Tehuantepec during Tehuantepecer events: a numerical study with WRF high-resolution simulations

Abstract. Tehuantepecers or Tehuanos are extreme winds produced in the Isthmus of Tehuantepec, blowing south through Chivela Pass, the mountain gap across the isthmus, from the Gulf of Mexico into the Pacific Ocean. They are the result of the complex interaction between large-scale meteorological conditions and local orographic forcings around Chivela Pass, and occur mainly in winter months due to cold air damming in the wake of cold fronts that reach as far south as the Bay of Campeche. Even though the name refers mostly to the intense mountain gap outflow, Tehuantepecer episodes can also generate other localized extreme wind conditions across the region, such as downslope windstorms and hydraulic jumps, which are strong turbulent flows that have a direct effect on the Pacific side of the isthmus and the Gulf of Tehuantepec. This study focuses on investigating these phenomena using high horizontal and vertical resolution WRF (Weather Research and Forecasting) model simulations. In particular, we employ a 4-nested grid configuration with up to 444 m horizontal spacing in the innermost domain and 70 hybrid-sigma vertical levels, 8 of which lie within the first 200 m above ground. We select one 36 h period in December 2013, when favorable conditions for a strong gap wind situation were observed. The high-resolution WRF experiment reveals a significant fine-scale structure in the strong Tehuano wind flow, beyond the well known gap jet. Depending on the Froude number upstream of the topographic barrier, different downslope windstorm conditions and hydraulic jumps with rotor circulations develop simultaneously at different locations east of Chivela Pass with varied crest height. A comparison with observations suggests that the model accurately represents the spatially heterogeneous intense downslope windstorm and the formation of mountain wave clouds for several hours, with low errors in wind speed, wind direction and temperature.

A Numerical Study of “Hijikawa-Arashi”: A Thermally Driven Gap Wind Visualized by Nocturnal Fog

AbstractA renowned local wind in Japan, “Hijikawa-Arashi,” is a thermally driven nocturnal gap wind accompanied by fog. The wind is visually identified by the fog along the valley of the Hijikawa River between the Ozu basin and the Seto Inland Sea during the early morning in autumn and winter. A fine-resolution numerical model is employed to reproduce the main observed features of Hijikawa-Arashi. A vertical resolution of 10 m or less at the lowest level is required to express the nocturnal radiative cooling of the land that is required for fog formation in the basin, and fine horizontal resolution is necessary to express a realistic valley through which the fog is advected to the sea. Multiple hydraulic jumps accompanied by supercritical flow occur because of the complex topography. Both moisture transport by the sea breeze during the daytime and evaporation from the land surface are important for accumulating moisture to produce the fog.

Wind velocity and wind curl variability over the Black Sea from QuikScat and ASCAT satellite measurements

Measurements of QuikScat for 2000–2009 and ASCAT scatterometers for 2009–2016 are used to describe the variability of the Black Sea wind velocity and wind curl. High-resolution (12.5 km) scatterometry data provide a possibility to investigate several small-scale features (“hot points”) of the wind field in the basin related to the topographic effects. They include: the gap winds near the Bosphorus, the Kerch Strait, and Tuapse; tip jets at the south and west of the Crimean peninsula, several capes of the Anatolian coast; the Kolkheti valley winds; wind shadow zones at the eastern Turkey coast and the western Crimean coast. Driving factors and directional variability of wind in these “hot points” is discussed.QuikScat and ASCAT data are used to describe seasonal and interannual variability of wind characteristics in 2000–2016. Comparison with in-situ data shows that QuikScat noticeably overestimates wind magnitude during low winds conditions (<2 m/s). ASCAT winds are generally lower than QuikScat, but the ASCAT wind curl is higher. The amount of low winds values (<2 m/s) in the ASCAT data is 6% of the total data, and it is 1% in the QuikScat data. Overestimation of low winds decreases wind gradients near the wind shadows, which is the most possible reason of the underestimation of the wind curl in the QuikScat dataset.Wind curl has a different sign and a completely different interannual variability in the western and eastern parts of the sea. Near the eastern coast of the basin, high-resolution satellite measurements reveal three powerful small-scales maximums of cyclonic vorticity. These maximums are related to the wind jets flowing around topographic obstacles in the vicinity of the Kerch Strait, in the mountain gap near Tuapse, and from the Kolkheti valley. They are observed throughout the whole year, but are largest in winter. The contribution of these maximums to the overall cyclonic vorticity is essential (30% in the QuikScat data and 50% in the ASCAT data). In the western part of the basin, the wind curl is negative (anticyclonic) on average, with the highest anticyclonic curl observed in summer. Winds are decreasing in 2000–2016 in agreement with the data on the 20 century. At the same time the basin-average wind curl rises due to its increase in the eastern Black Sea. We speculate that these trends are associated with the displacement of the Siberian Anticyclone to the west. The related rise of pressure over the eastern Europe correlates significantly with the wind curl variability on interannual time scales.

The Modeling of the North Equatorial Countercurrent in the Community Earth System Model and its Oceanic Component

AbstractThe North Equatorial Countercurrent (NECC) simulated by a coupled ocean‐atmosphere model and its oceanic component have been investigated and compared against oceanographic observations. Coupled model simulations using the Community Earth System Model version 2 are compared against ocean‐ice simulations forced by the second phase of the Coordinated Ocean‐ice Reference Experiments (CORE) data set. The modeled circulation biases behave differently to the west of and to the east of 120°W: the CORE‐forced ocean model largely underestimates the NECC transport to the west and the coupled model underestimates it to the east. Further analysis suggests that the surface wind stress and its curl is the most important forcing term for correctly simulating the NECC in both models. West of 120°W, the NECC biases in the ocean model are attributed to the southward movement of the maximum easterly trade winds in the Northern Hemisphere and the associated wind stress curl (WSC) pattern; east of 120°W, the NECC biases in the coupled model are attributed to the weak northward cross‐equatorial winds and southwestward gap winds, which lead to a weak WSC gradient at the latitude of NECC. Further analysis confirms that the WSC biases comes mainly from the zonal wind bias, which may in turn relate to the protocol of CORE‐II of adjusting reanalysis winds toward satellite data, which include the relative wind effect.

Comparison of Spatial Pattern and Mechanism between Convexity and Gap Winds

Comparison of Spatial Pattern and Mechanism between Convexity and Gap Winds

Mixed layer depth variability in the Red Sea

Abstract. For the first time, a monthly climatology of mixed layer depth (MLD) in the Red Sea has been derived based on temperature profiles. The general pattern of MLD variability is clearly visible in the Red Sea, with deep MLDs during winter and shallow MLDs during summer. Transitional MLDs have been found during the spring and fall. The northern end of the Red Sea experienced deeper mixing and a higher MLD associated with the winter cooling of the high-saline surface waters. Further, the region north of 19° N experienced deep mixed layers, regardless of the season. Wind stress plays a major role in the MLD variability of the southern Red Sea, while net heat flux and evaporation are the dominating factors in the central and northern Red Sea regions. Ocean eddies and Tokar Gap winds significantly alter the MLD structure in the Red Sea. The dynamics associated with the Tokar Gap winds leads to a difference of more than 20 m in the average MLD between the north and south of the Tokar axis.

A Global Climatology of Tropospheric Inertial Instability

Abstract A climatology of tropospheric inertial instability is constructed using the European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim) at 250, 500, and 850 hPa. For each level, two criteria are used. The first criterion is the traditional criterion of absolute vorticity that is opposite in sign to the local Coriolis parameter. The second criterion, referred to as the gradient criterion, is the traditional criterion with an added term incorporating flow curvature. Both criteria show that instability, on all pressure levels, occurs most frequently in the tropics and decreases toward the poles. Compared to the traditional criterion, the gradient criterion diagnoses instability much more frequently outside the tropics and less frequently near the equator. The global distribution of inertial instability also shows many local maxima in the occurrence of instability. A sample of these local maxima is investigated further by constructing composites of the synoptic-scale flow associated with instability. The composites show that instability occurs in association with cross-equatorial flow in the North Atlantic Ocean, the Somali jet, tip jets off northern Madagascar, the western Pacific subtropical high, gap winds across Central America, upper-level ridging over western North America, and the North Atlantic polar jet. Furthermore, relatively long-lived synoptic-scale regions of instability are found within the midlatitude jet streams.