Cold Intrusion Research Articles

Relation of Eurasian Snow Cover and Indian Summer Monsoon Rainfall: Importance of the Delayed Hydrological Effect

Abstract This observationally based study demonstrates the importance of the delayed hydrological response of snow cover and snowmelt over the Eurasian region and Tibet for variability of Indian summer monsoon rainfall during the first two months after onset. Using snow cover fraction and snow water equivalent data during 1967–2003, it is demonstrated that, although the snow-albedo effect is prevalent over western Eurasia, the delayed hydrological effect is strong and persistent over the eastern part. Long soil moisture memory and strong sensitivity of surface fluxes to soil moisture variations over eastern Asia and Tibet provide a mechanism for soil moisture anomalies generated by anomalies in winter and spring snowfall to affect rainfall during the initial months in summer. Dry soil moisture anomalies over the eastern Eurasian region associated with anomalous heating at the surface and midtroposphere help in anchoring of an anomalous upper-tropospheric “blocking” ridge around 100°E and its persistence. This not only leads to prolonged weakening of the subtropical westerly jet but also shifts its position southward of 30°N, followed by penetration of anomalous troughs in the westerlies into the Indian region. Simultaneously, intrusion of cold and dry air from the midlatitudes can reduce the convective instability and hence rainfall over India after the onset. Such a southward shift of the jet can also significantly weaken the vertical easterly wind shear over the Indian region in summer and lead to decrease in rainfall. This delayed hydrological effect also has the potential to modulate the snow–atmosphere coupling strength for temperature and precipitation in operational forecast models through soil moisture–evaporation–precipitation feedbacks.

The influence of deep convection on aeration of the bottom zone in Baikal

We investigated the influence of deep forced temperature convection on aeration of the bottom water layer in the pelagic zone of Baikal. Temperature T (± 0.002°C) and oxygen O2 concentration (± 0.01 mg/L) were measured by using the SBE-25 probe with the SBE-43 oxygen sensor in May-July 2006−2007 and 2009−2013. Oxygen input into the bottom layer with cold intrusions was determined for 79 cases. The study revealed a quantitative correlation between the heat deficit and the rise in oxygen content in the bottom layer during cold intrusions. This correlation was used to reconstruct the values of intrusion aeration of the bottom layer during 1993−2005. It is ascertained that the intrusion-caused 2006−2013-average oxygen input into Baikal’s bottom layer is virtually close to the annual oxygen demand for oxidation processes in the water column and bottom sediments. In the southern and middle parts of Baikal, intrusion-caused input of oxygen corresponds to a higher rate of its demand (4.5−4.6x10-4 mg/L per day). A slight increase in oxygen concentration was detected in the bottom layer of the South and Middle Baikal and a slight decrease in the bottom layer of North Baikal during 1993−2013.

Pathways and mechanisms of offshore water intrusions on the Espírito Santo Basin shelf (18°S–22°S, Brazil)

AbstractThe pathways and physical mechanisms associated with intrusions of cold, nutrient‐rich South Atlantic Central Water (SACW) on the continental shelf of the Espírito Santo Basin (ESB), off southeast Brazil (18°S–22°S), are investigated. To this end, a set of process‐oriented, Primitive‐Equation (PE) numerical models are used, together with an independent and more complete PE model, available observations and simple theoretical ideas. SACW enters the model ESB shelf mostly through two preferential pathways along the Tubarão Bight (TB, 19.5°S–22°S). These pathways are found to be locations where an equatorward along‐isobath pressure gradient force (PGFy*) of m s−2) develops in response to steady wind forcing. This equatorward PGFy* is essentially in geostrophic balance, inducing onshore flow across the shelf edge, and most of the shelf proper. The Brazil Current (BC) imparts an additional periodic (in the along‐shelf direction) PGFy* on the shelf. The intrinsic pycnocline uplifting effect of the BC in making colder water available at the shelf edge is quantified. The BC also induces local intrusions by inertially overshooting the shelf edge, consistent with estimated Rossby numbers of 0.3–0.5. In addition, the planetary β‐effect is related to a background equatorward PGFy*. A modified Arrested Topographic Wave model is shown to be a plausible rationalization for the shelf‐wide spreading of the pressure field imparted by the BC at the shelf edge. The deep‐ocean processes examined here are found to enhance the onshore transport of SACW, while wind forcing is found to dominate it at leading order.

Intrusive layering of the Antarctic slope front

Cascading of cold Antarctic shelf water (ASW) initiates compensatory isopycnic upwelling of the warm Circumpolar Deep Water (CDW). The baroclinic/thermoclinic Antarctic slope front (ASF) is formed, and a mesoscale intrusive structure develops on the shelf edge and slope. Mesoscale processes when the ASF peaks are periodically accompanied by local baroclinic instability, which forms a smaller-scale intrusive structure. Therefore, the ASF is naturally subdivided into two layers according to the intrusion scales (vertical δН and horizontal L) and the horizontal parameters of the front (thermoclinity (T L)ρ and baroclinity γρ). Analysis of ASF intrusive layering due to the baroclinic factor supports the following conclusion: the higher the (T L)ρ of the ASF, the greater the intrusion intensity |δθ| (temperature anomaly amplitude), while an increase in γρ of the ASF leads to a decrease in intrusion scales δН and L. Frontal intrusions can be distinguished by a development degree. Regardless of the degree of development, all warm intrusions are characterized by vertical density stratification, while cold intrusions are characterized by density quasihomogeneity. According to field data, the ASF instability process is subdivided into four stages. When theASF is baroclinically unstable, the local baroclinic deformation radius Rd L of the front is close in magnitude to the horizontal scale L of the intrusions that form, and their characteristic vertical scale δH is close to the typical vertical scale of front instability.

Spatial structure and seasonal changes of nekton assemblages in the Taiwan Strait

The Taiwan Strait is a transition zone between the East China Sea and the South China Sea with unique hydrology and a geographical environment that creates special marine community features. To analyze the spatial structure and seasonal changes of the nekton assemblages in the Taiwan Strait, seasonal experimental trawl surveys were conducted during 2006–2007. The results showed that there were two assemblages in the area with different sets of species, and the average similarity within each group varies between 39.38% and 74.20%. By using multivariate statistical analysis and analyzing the distribution of dominant species, we found that the structures of the assemblages had obvious seasonal variation. The middle region from the Putian transect to the Xiamen transect could be considered a mixing area for the two assemblages. The analysis of the relationship between species assemblages and environmental factors indicated that temperature was the most important factor affecting the community structure in cold seasons, and 22.5°C and 17°C could be considered dividing lines for spring and winter, respectively. In warm seasons, the most important factor was water depth, but the relationship with depth was not as significant, with a correlation between 0.264 and 0.399. The seasonal changes of nekton assemblages basically reflected the dynamic currents in the Taiwan Strait. The south coastal assemblage extended to nearly the entire area of the Taiwan Strait along with a strong and intense warm current that shrinks in spring and winter when the southward intrusion of the cold Zhejiang-Fujian coastal current becomes stronger. The impact of shortterm and long-term environmental changes, such as extreme weather, global warming and human activity on nekton assemblages, had been recognized but need further research. Our study on nekton assemblages could be used as a baseline for measuring future changes.

Extratropical Impacts on Atlantic Tropical Cyclone Activity

Abstract With warm sea surface temperature (SST) anomalies in the tropical Atlantic and cold SST anomalies in the east Pacific, the unusually quiet hurricane season in 2013 was a surprise to the hurricane community. The authors’ analyses suggest that the substantially suppressed Atlantic tropical cyclone (TC) activity in August 2013 can be attributed to frequent breaking of midlatitude Rossby waves, which led to the equatorward intrusion of cold and dry extratropical air. The resultant mid- to upper-tropospheric dryness and strong vertical wind shear hindered TC development. Using the empirical orthogonal function analysis, the active Rossby wave breaking in August 2013 was found to be associated with a recurrent mode of the midlatitude jet stream over the North Atlantic, which represents the variability of the intensity and zonal extent of the jet. This mode is significantly correlated with Atlantic hurricane frequency. The correlation coefficient is comparable to the correlation of Atlantic hurricane frequency with the main development region (MDR) relative SST and higher than that with the Niño-3.4 index. This study highlights the extratropical impacts on Atlantic TC activity, which may have important implications for the seasonal predictability of Atlantic TCs.

Temperature variability caused by internal tides in the coral reef ecosystem of Hanauma bay, Hawai‘i

Hanauma Bay Nature Preserve is a shallow bay (<30m depth) on the island of O‘ahu, Hawai‘i, offshore of which tidal flow over deep ridge topography (500–1000m depth) is known to generate semidiurnal frequency internal tides. A field experiment was conducted during March to June 2009 to determine whether the deep internal tides propagate shoreward to influence variability in temperature and currents in the bay environment. Temperature observations in the bay exhibit a diurnal cycle that is strongest near the surface (upper 10m) and is associated with solar heating. In early summer (May–June), as the upper mixed layer warms and a shallow seasonal thermocline develops, temperature fluctuations in deeper bay waters (>15m depth) become dominated by large semidiurnal variations (up to 2.7°C) that are attributed to the internal tide. These temperature drops caused by the internal tide occur consistently twice a day under summer stratification at depths as shallow as 15m, while smaller temperature drops (up to 1.8°C) occur occasionally at 5m. Although semidiurnal band temperatures vary seasonally, semidiurnal band currents exhibit similar magnitudes in spring and summer. This suggests that the weak temperature fluctuations in spring are due to the bay residing entirely in the upper mixed layer at this time of year, while internal tide energy continues to influence currents. Observations made along a cross-shore/vertical transect at the center of the bay with an autonomous underwater vehicle highlight the structure of cold intrusions that fill a large portion of the bay as well as the relationship between temperature, salinity, chlorophyll, and backscatter. Near-bottom, advective heat flux estimates at the mouth of the bay indicate that the internal tide tends to advect cold water into the bay primarily on the northeast side of the bay entrance, with cold water outflow on the opposite side. The observations highlight the role of the internal tide along with seasonal changes in stratification in temperature variability in shallow ecosystems, particularly those close to generation sites.

Characterisation of water temperature variability within a harbour connected to a large lake

Abstract Many coastal embayments in the Laurentian Great Lakes have highly variable temperatures due to pronounced movements of the thermocline in the nearshore zone. As an example, we document the diverse thermal regimes in Toronto Harbour, which is also the site of some of the most extensive fish habitat restoration activities in Lake Ontario. Toronto Harbour is characterised by considerable thermal variability as a result of diurnal heat fluxes and large amplitude movements of the thermocline of Lake Ontario. During the ice-free period from April–November 2013, an array of benthic and surface temperature loggers were deployed to obtain the spatio-temporal distribution of water temperatures. Complementary measurements of stratification were made in Lake Ontario at a site 5 km offshore. The dominant periods of short-term thermal variability were 12, 17, and 24 h, reflecting both diurnal heat fluxes and inertial oscillations of Lake Ontario's thermocline. The thermocline in Lake Ontario was observed to oscillate by as much as 15 m, around a mean depth of 9 m, which is comparable to the mean depth of Toronto Harbour. Cold intrusions were found to quickly flow from the lake into the harbour and lead to rapid drops in temperature (e.g., as much as 15 °C in less than 4 h). Such “cold shock” events may be associated with a variety of negative effects on many aquatic organisms, especially warm-water fishes. We consider the potential impacts of the observed temperature variability on cool and warm-water fish species that are the target of restoration activities.

Cross-shelf penetrating fronts: A response of buoyant coastal water to ambient pycnocline undulation

Offshore-penetrating tongues of coastal water have been frequently observed during the downwelling-favorable monsoon season at specific locations in waters off the Min-Zhe Coast, a region influenced by a buoyant coastal current originating from the Changjiang River. This process plays an important role in cross-shelf material exchange in the East China Sea (ECS), but the underlying mechanisms are not fully understood. This study suggests that the penetrating fronts are the response of buoyant coastal water to along-isobath undulation of the ambient pycnocline that is controlled by the temperature stratification in seawater. When the ambient pycnocline descends sharply in the downshelf direction, coastal water is transported offshore due to the joint effect of baroclinicity and relief (JEBAR), and thus generates a penetrating front. Along-isobath pycnocline undulation in the ECS can arise from nonuniform tidal mixing due to tidal wave divergence off the Min-Zhe Coast. Onshelf intrusion of cold and dense Kuroshio subsurface water prevents thorough mixing of the pycnocline. Different from the common cross-shelf transport phenomena induced by winds or frontal instabilities, such a tidal mechanism should produce penetrating fronts at specific locations, in agreement with observations.

A new dynamical index for classification of cold surge types over East Asia

The cold surges over East Asia can be classified into wave-train type and blocking type according to their dynamic origins. In the present study, two dynamic indices are proposed to objectively identify cold surge types using potential temperature (θ) on the dynamic tropopause at 2-potential vorticity units (2-PVU) surface. The two indices are designed to represent primary characteristics of the two types of cold surge. The wave-train index (WI) is defined as a difference of anomalous θ on the 2-PVU surface between the western North Pacific and northeast China, which captures a southward (northward) intrusion of cold (warm) air mass related to the trough-ridge pattern. The blocking index (BI) is defined as a difference of anomalous θ between the subarctic region and northeast China, which indicates air mass overturning related to a reversal of the usual meridional θ gradient commonly observed in the occurrence of blocking type cold surge. Composite analyses based on the distribution of the WI and BI clearly demonstrate the dynamic evolutions of corresponding cold surge types. The wave-train cold surge is associated with a southeastward expansion of the Siberian High and northerly wind near surface, which is caused by growing baroclinic waves. During the blocking cold surge, a geopotential height dipole indicating the subarctic blocking and deepening of East Asian coastal trough induces a southward expansion of the Siberian High and northeasterly wind. Compared to the wave-train type, the blocking cold surge exhibits a longer duration and stronger intensity. In the new framework of these dynamic indices, we can detect a third type of cold surge when both the wave-train and the blocking occur together. In addition, we can exclude the events that do not have the essential features of the upper tropospheric disturbances or the subarctic anticyclonic circulation, which are responsible for cold surge occurrence, using the new indices.

Revisiting wintertime cold air intrusions at the east of the Andes: propagating features from subtropical Argentina to Peruvian Amazon and relationship with large-scale circulation patterns

This study investigates the spatial and temporal characteristics of cold surges that propagates northward along the eastern flank of the Andes from subtropical to tropical South America analysing wintertime in situ daily minimum temperature observations from Argentina, Bolivia and Peru and ERA-40 reanalysis over the 1975–2001 period. Cold surges usually last 2 or 3 days but are generally less persistent in the southern La Plata basin compared to tropical regions. On average, three to four cold surges are reported each year. Our analysis reveals that 52 % of cold episodes registered in the south of La Plata basin propagate northward to the northern Peruvian Amazon at a speed of around 20 m s−1. In comparison to cold surges that do not reach the tropical region, we demonstrate that these cold surges are characterized, before they reach the tropical region, by a higher occurrence of a specific circulation pattern associated to southern low-level winds progression toward low latitudes combined with subsidence and dry condition in the middle and low troposphere that reinforce the cold episode through a radiative effect. Finally, the relationship between cold surges and atmosphere dynamics is illustrated for the two most severe cold intrusions that reached the Peruvian and Bolivian Amazon in the last 20 years.

Peculiarities of the flow front structure in the Buor-Khaya Bay (the Laptev Sea) in winter

Hydrological flow front whose location and structure depend on the value of the Lena River water flow forms in winter under the ice in the Buor-Khaya Bay. The flow front is characterized by the baroclinic and thermocline components. Under the lens of desalinated waters, the layers of increased temperature and salinity alternate with the layers of decreased salinity and temperature. On the periphery of the front, the thermocline stratification results in the formation of the cold isopycnic intrusion at intermediate levels. The cross section of the cold intrusion in the bay amounted to ∼50–85 km and its thickness, from 3–5 to 15 m. The waters of cold intrusions are rich with oxygen and favor the ventilation of the bay waters. The winter flow increase results in the significant widening of the river impact zone, warming of the coastal zone waters, and stratification intensification.

Effect of magnetic field on near-wall coherent structures and heat transfer in magnetohydrodynamic turbulent channel flow of low Prandtl number fluids

A numerical study is carried out of the magnetic field effects on the coherent structures and the associated heat transfer in a turbulent channel flow with constant temperature at the bottom (cold) and top (hot) walls. Results from direct numerical simulations are conditionally sampled in order to extract the dominant coherent structures in the near-wall region for flows with and without a uniform external magnetic field in the wall-normal direction. The Reynolds number based on the bulk velocity and the wall distance is 5600, while only a representative small Stuart number of 0.01 is explored. Two fluids with Prandtl numbers of 0.01 and 0.71 are studied. It is shown that the conditionally averaged quasi-streamwise vortices are modified by the magnetic field with their size being increased and their strength decreased. The underlying organized fluid motions are damped by the Lorentz force and the turbulent heat transfer related to the action of quasi-streamwise vortices is decreased by the magnetic field. For the higher Prandtl number fluid, a similarity between the coherent temperature and the coherent streamwise velocity fluctuations is observed for both types of flow. This is diminished for the lower Prandtl number fluid, especially in the magnetohydrodynamic flow, inhibiting the intrusion of cold (hot) fluid from the cold (hot) wall towards the central region.

Specific features of cold bottom intrusion in Baikal according to observations in 1993–2009

The specific features of cold bottom intrusion in Baikal are considered based on the results of observations of 1993–2009. It is shown that the intrusion-related renewal of bottom water takes place not regularly and not simultaneously in different hollows. Its maximums were recorded in Southern Baikal in 1997 and 2007, in Middle Baikal in 1995, 2006, and 2009, and in Northern Baikal in 1997 and 2009. In these years, the volume of cold bottom layer in some hollows reached 200–470 km3 and its total cooling reached −20 to −60 MJ 109. Cold intrusions were more frequent and had greater effect on the bottom layer in Southern Baikal than in other parts of the lake. The intrusions, especially in the years of their active development, are shown to cause water cooling in both the bottom layer and the major part of the deep-water zone of all hollows.

Observation and numerical prediction of torrential rainfall over Korea caused by Typhoon Rusa (2002)

On landfall in 2002, Typhoon Rusa caused record‐breaking rainfall (870.5 mm/d) in Gangneung at the foot of the Taebaek Mountain range, Korea. In this study, the predictability of the torrential rainfall associated with the typhoon and the detailed mesoscale precipitation distribution were investigated through numerical simulation. The weather research and forecasting (WRF) model successfully simulates the mesoscale rainfall distibution and timing. With a 10 km (3.3 km) horizontal grid, the model predicted 830.8 mm (1307.9 mm) of rain with some uncertainty in the observed local maximum in coverage. This study shows that the complexity of forecasting is associated with the interactions between environmental flows, typhoon flow, and topography. In bogus and 1‐day‐earlier initial‐time experiments, the typhoon track and intensity are substantially improved, although this has little impact on successful rainfall simulation. The interaction between the large‐scale features and the proper vertical structure of the typhoon is key. The simulated rainfall at the cloud‐resolving spacing of 3.3 km is greater than that at 10 km because of the increased terrain height and not the greater resolution. Analysis of observed and simulated data shows that the torrential rainfall had two different causes with two peaks in the hourly rainfall at Gangneung. The first peak links to a mesoscale frontal structure, characterized by strong moisture and thermal gradients formed by the intrusion of cold, dry northerly air from a midlatitude trough and moist southerly air from the typhoon. The second peak results from the direct effect of the typhoon and the lifting of the moisture‐laden typhoon winds.

Subduction on the Northern and Southern Flanks of the Gulf Stream

Abstract Sections of temperature, salinity, dissolved oxygen, and velocity were made crossing the Gulf Stream in late January 2006 to investigate the role of frontal processes in the formation of Eighteen Degree Water (EDW), the Subtropical Mode Water of the North Atlantic. The sections were nominally perpendicular to the stream and measured in a Lagrangian frame by following a floating spar buoy drifting in the Gulf Stream’s warm core. During the survey, EDW was isolated from the mixed layer by the stratified seasonal pycnocline, suggesting that EDW was not yet actively being formed at this time in the season and at the longitudes over which the survey was conducted (64°–70°W). However, in two of the sections, the seasonal pycnocline in the core of the Gulf Stream was broken by an intrusion of cold, fresh, weakly stratified water, nearly saturated in oxygen, that appears to have been subducted from the surface mixed layer north of the stream. The intrusion was identified in three of the sections in profiles with a nearly identical temperature–salinity relation. From the western-to-easternmost sections, where the intrusion was observed, the depth of the intrusion’s salinity minimum descended by ∼90 m in the 71 h it took to complete this part of the survey. This apparent subduction occurred primarily on the upstream side of a meander trough, where the cross-stream velocity was confluent and frontogenetic. Using a variant of the omega equation, the vertical velocity driven by the confluent flow was inferred and yielded downwelling in the vicinity of the intrusion spanning 10–40 m day−1, a range of values consistent with the intrusion’s observed descent, suggesting that frontal subduction was responsible for the formation of the intrusion. In the easternmost section located downstream of the meander trough, the flow was diffluent, driving an inferred vertical circulation that was of the opposite sense to that in the section upstream of the trough. In transiting the two sides of the trough, the intrusion was observed to move toward the center of the stream between the downwelling branches of the opposing vertical circulations, resulting in a downward Lagrangian mean vertical velocity and net subduction. Hydrographic evidence of the subduction of weakly stratified surface waters was seen in the southern flank of the Gulf Stream as well. The solution of the omega equation suggests that this subduction was associated with a relatively shallow vertical circulation confined to the upper 200 m of the water column in the proximity of the front marking the southern edge of the warm core.

Circulation and variability of the North Atlantic Current in the vicinity of the Mid-Atlantic Ridge

Physical observations have been used to describe the circulation and water masses during a cruise whose primary objective was to study the effect of the Mid-Atlantic Ridge on benthic ecosystems at four sites along and across the Ridge between 49°N and 54°N. The cruise observations have been expanded using Argo floats and 16 years of satellite altimeter data to examine temporal variability over months to years. Water masses and transports are consistent with prior observations, showing subtropical water with large mesoscale eddy variability but weak mean eastward flow at the southern sites and cooler, subpolar water with low eddy variability and weak mean westward flow at the northern sites north of the Charlie Gibbs Fracture Zone. An interesting feature was high salinity bottom water at the southern sites, indicating a contribution of Iceland–Scotland Overflow Water mixed southwards through the central rift of the Mid-Atlantic Ridge. Another feature was that 27 Sv (1 Sv=10 6 m 3 s −1) eastward transport was mostly in the top 500 m of the water column except at the latitudes of the Charlie Gibbs Fracture Zone where two-thirds of the transport was deeper than 500 m. An intrusion of cold water extended 300 km southwards along a CTD section that paralleled the Mid-Atlantic Ridge, resulting in strong currents over the southern sites. The cold intrusion was flanked by a similar intrusion of warm water extending northwards. Each intrusion was present along the line of the section for about 6 months, controlling the pathway of 10 Sv of transport. Altimeter data reveal many similar intrusions, consisting of or spinning off mesoscale eddies which are slow moving and long lasting, often remaining identifiable near one vicinity for a year or more. Descriptions of the North Atlantic Current as a current with several branches which shift from time to time are a consequence of these long lasting, slowly changing eddies. Thus the North Atlantic Current is best described as a train of near-stationary eddies and meanders with the transport pathways and branching controlled by the slowly varying eddies.

Western disturbances seen with AMSU-B and infrared sensors

Western disturbances (WD) of winter and pre-monsoon seasons are the important sources of rainfall in the Indo-Gangetic plains. WDs are troughs or circulations in the westerly winds modified by the Himalayas. Operationally, WDs are monitored using infrared (IR) and water vapour (WV) images. Advanced Microwave Sounding Unit-B (AMSU-B), flying onboard the NOAA satellites, also allows WDs to be monitored in five microwave frequencies. Two are in water vapour window (89, 150 GHz) and three are absorption channels (centred at 183.31 GHz). Unlike the top of cloud view in IR or WV, AMSU-B radiances show the effect of moisture and hydrometeors in different layers.Two cases of WD (17 April 2001 and 18–19 February 2003) are discussed using the microwave data from AMSU-B and the IR and WV data from Meteosat-5. The aim here is to demonstrate the skill of AMSU-B in delineating structure of WDs. In particular, the cold intrusion and the moist conveyor belts are examined. It was found that the multi-channel view of the AMSU-B permits a better understanding of the moist structures seen in the conveyor belts.The à trous wavelet transform is used to clearly bring out mesoscale features in WDs. AMSU-B brings out intense convection as a large depression of BTs (>50K) at 150/176 GHz, cirrus and moist bands at 180/182 GHz. Mesoscale convection lines within WDs that last short time are shown here for the first time only in the AMSU-B images. Large-scale cirrus features are separated using the à trous wavelet transform.Lastly, it is shown that there is a good likeness in the rain contours in the 3-h rain 3B42 (computed from TRMM and other data) to AMSU-B depressions in BT. Overall, AMSU-B shows better skill in delineating the structure of clouds and rain in WDs.

A numerical study of the Southwestern Atlantic Shelf circulation: Stratified ocean response to local and offshore forcing

This article discusses the results of a suite of numerical simulations of the oceanic circulation in the Southwestern Atlantic Shelf region that are aimed to characterize its mean circulation and seasonal variability and to determine the dynamical mechanisms controlling them. Our experiments indicate that south of 40°S the mean circulation is dominated by a general northeastward flow in the southern portion of the shelf, which is controlled by the discharges from the Magellan Straits, tidal mixing, wind forcing, and the offshore influence of the Malvinas Current farther north. The region from 40°S to 33°S presents the highest seasonal variability, with intrusions of cold sub‐Antarctic waters and the northward expansion of mixtures of the Río de la Plata waters in late fall and a slower retraction of the plume during spring‐summer. Wind stress variability seems to be the primarily forcing mechanism for the plume dynamics. These model results are in reasonable agreement with observations and previous model results. The present solutions also reveal important additional features of the shelf response. The along‐shelf circulation, for example, is largely driven by the western boundary currents in the middle and outer shelf, with induced transports that are 3 times larger than in experiments forced by winds and tides. The analysis also indicates that the upstream influence of the Malvinas Current is felt well beyond its retroflection point in the form of a northward middle‐shelf current and that the interaction of the Brazil Current with the Brazilian shelf topography is primarily responsible for inducing steady shelf break upwelling.

“Warm” dichothermal layer in the Sea of Okhotsk

The field data show that the ventilation of waters in the Sea of Okhotsk occurs the year round as a cyclic process and consists of several phases. During the initial phase, the shelf waters enter the offshore subsurface layers. Later, they appear at intermediate depths and spread southward as cold intrusions mixing with surrounding waters on their way. During the last phase of the cycle, the shelf waters do not penetrate the deep-sea layers, and in the vertical structure of temperature field near the northeast coast of Sakhalin, a local phenomenon of “warm” dichothermal layer is observed.