The role of meridional density differences for a wind-driven overturning circulation

Experiments with the coupled climate model CLIMBER-3α, which contains an oceanic general circulation model, show deep upwelling in the Southern Ocean to be proportional to the surface wind stress in the latitudinal band of Drake Passage. At the same time, the distribution of the Southern Ocean upwelling onto the oceanic basins is controlled by buoyancy distribution; the inflow into each basin being proportional to the respective meridional density difference. We observe approximately the same constant of proportionality for all basins, and demonstrate that it can be directly related to the flow geometry. For increased wind stress in the Southern Ocean, the overturning increases both in the Atlantic and the Indo-Pacific basin. For strongly reduced wind stress, the circulation enters a regime where Atlantic overturning is maintained through Pacific upwelling, in order to satisfy the transports set by the density differences. Previous results on surface buoyancy and wind stress forcing, obtained with different models, are reproduced within one model in order to distill a consistent picture. We propose that both Southern Ocean upwelling and meridional density differences set up a system of conditions that determine the global meridional overturning circulation.     

Seasonal variability of oceanological conditions in the northern part of the Tatar Strait

An average long-term distribution of temperature and salinity is analyzed for different months (May–November) computed on the basis of materials accumulated at standard oceanological sections of the northern part of the Tatar Strait. The main attention is paid to the section Korsakov Cape-Cape Syurkum crossing the water area under study practically in the middle. In early spring, the cold waters with salinity of more than 33‰ are registered at the section Korsakov Cape-Cape Syurkum. The waters with smaller salinity are revealed only in late spring, in June. In the same period, the intensification of cold intermediate layer occurs, first of all, in the western part of the section. The waters in the surface layer near the Sakhalin coast are warmed more than at the continental shelf. During the summer, this difference gradually decreases and the surface layer temperature becomes even in September. On the contrary, the spatial salinity gradients increase. In the fall, under the influence of northern and northwestern winds being typical of this period, the upwelling is formed near the Sakhalin coast and the cold dense waters emerge in the narrow coastal strip. The direction of alongshore flow changes from northern to the southern one. At the section Korsakov Cape-Cape Syurkum in November, the influence of small-salinity waters associated with the Amur River runoff is significantly revealed.     

A low-level jet along the Benguela coast, an integral part of the Benguela current ecosystem

The Benguela Current Ecosystem of Southern Africa is the strongest wind-driven coastal upwelling system known. This is one of the most productive ocean areas in the world, extremely rich in fishery resources with a total catch in excess of one million tons per annum. Marine life off the coast relies heavily on the nutrient-rich upwellings of the cold Benguela current. Warming events occur along this coast in association with many El Niños. These tremendously disrupt the coastal ecosystem, reducing productivity and devastating the anchovy and sardine fisheries. This article demonstrates for the first time the existence of a low-level atmospheric jet along the Benguela Coast of the southeastern Atlantic. Blowing parallel to the coast, this jet drives the coastal upwelling system and is part of a mechanism that links Pacific El Niño events to Southern Africa. The existence of such a jet has tremendous implications for the Benguela current and its response to climatic variability and change because a positive feedback exists between the intensity of this jet and the intensity of coastal upwelling. This may enhance the response of the Benguela Current Ecosystem to climatic variability, making it particularly susceptible to the impacts of global climate change.     

Downscaling over Vietnam using the stretched-grid CCAM: verification of the mean and interannual variability of rainfall

Rainfall over Vietnam is highly variable from north to south, due to the interaction of the monsoonal winds with the terrain. There is high rainfall from April to September, and little rainfall from October to March (except along the central Vietnam coast). In order to study the ability of the Commonwealth Scientific and Industrial Research Organisation stretched-grid Conformal Cubic Atmospheric Model (CCAM) to capture the climatic and interannual variability of rainfall, downscaled simulations at approximately 20 km horizontal resolution over the region were produced for the period 1979–2001. A scale-selective digital filter was used to force the winds, temperature and sea-level pressure from the ERA-Interim reanalysis for length scales greater than about 700 km. For wind and temperature, the forcing is applied for pressure-sigma levels above about 0.9. ERA-Interim sea surface temperatures were used over the oceans. The simulations were primarily validated against the gridded Asian Precipitation Highly Resolved Observational Data Integration Toward Evaluation of the Water Resources rainfall dataset and station observations using standard statistical methods. It was found that CCAM reproduces well the amount and spatial variability of rainfall, with an area-averaged bias for the entire study domain of less than 1 mm day^?1; CCAM is also able to capture the rainfall pattern under different El Niño Southern Oscillation phases reasonably well for the dry season. For interannual variability, the simulation generally performed better for North and Central Vietnam than for South Vietnam, where rainfall variability was overestimated.     

索马里越赤道气流对越南降水变化的影响

基于1979-2002年CMAP全球降水资料和NCEP／NCAR再分析资料，研究了越南降水的季节和年际变化特征及其受索马里越赤道气流的影响。结果表明，越南南北方降水具有不同的季节和年际变化特征，南方降水主要集中在夏季和秋季，且夏、秋季降水的变化呈反位相关系，而北方降水则集中在夏季。夏季索马里气流与同期及后期越南南方降水有很好的相关性，同期为显著的正相关，后期为显著的负相关。相对而言，索马里越赤道气流和越南北方降水的关系不显著。另外，夏季赤道印度洋西风与越南夏秋季降水的相关与索马里气流类似，表明索马里越赤道气流可能是通过改变纬向水汽输送来影响越南南方降水。     

Dynamics of oceanological parameters in the Apsheron marine area of the Caspian Sea

Presented are the results of measurements of some oceanological parameters of the Caspian Sea carried out on the territory of oil and gas production in the Apsheron marine area in 2007–2012. The results are compared with the data of analogous measurements carried out in preceding years under different regimes of sea level fluctuations.     

Long-term changes of climate and hydrological regime in the Baltic region and their causes

A complex analysis is made of the variability of climatic, hydrological, and oceanological processes and environmental factors in the Baltic Sea region in the 20th century and at the beginning of the 21st century. The cause-and-effect relations between the climate dynamics over the North Atlantic and hydrometeorological conditions in the Baltic basin are established. Different variants of the North Atlantic Oscillation (NAO) indices were used as an atmospheric circulation intensity measure.     

Wind-driven changes in Southern Ocean residual circulation, ocean carbon reservoirs and atmospheric CO2

The effect of idealized wind-driven circulation changes in the Southern Ocean on atmospheric CO₂ and the ocean carbon inventory is investigated using a suite of coarse-resolution, global coupled ocean circulation and biogeochemistry experiments with parameterized eddy activity and only modest changes in surface buoyancy forcing, each experiment integrated for 5,000 years. A positive correlation is obtained between the meridional overturning or residual circulation in the Southern Ocean and atmospheric CO₂: stronger or northward-shifted westerly winds in the Southern Hemisphere result in increased residual circulation, greater upwelling of carbon-rich deep waters and oceanic outgassing, which increases atmospheric pCO₂ by ～20 μatm; weaker or southward-shifted winds lead to the opposing result. The ocean carbon inventory in our model varies through contrasting changes in the saturated, disequilibrium and biogenic (soft-tissue and carbonate) reservoirs, each varying by O(10–100) PgC, all of which contribute to the net anomaly in atmospheric CO₂. Increased residual overturning deepens the global pycnocline, warming the upper ocean and decreasing the saturated carbon reservoir. Increased upwelling of carbon- and nutrient-rich deep waters and inefficient biological activity results in subduction of unutilized nutrients into the ocean interior, decreasing the biogenic carbon reservoir of intermediate and mode waters ventilating the Northern Hemisphere, and making the disequilibrium carbon reservoir more positive in the mode waters due to the reduced residence time at the surface. Wind-induced changes in the model carbon inventory are dominated by the response of the global pycnocline, although there is an additional abyssal response when the peak westerly winds change their latitude, altering their proximity to Drake Passage and changing the depth extent of the southward return flow of the overturning: a northward shift of the westerly winds isolates dense isopycnals, allowing biogenic carbon to accumulate in the deep ocean of the Southern Hemisphere, while a southward shift shoals dense isopycnals that outcrop in the Southern Ocean and reduces the biogenic carbon store in the deep ocean.     

Prediction of the South China Sea Dipole Using SSA-MEM

ABSTRACT

A dipole structure appears in the sea surface height off the central coast of Vietnam during boreal summer in the South China Sea. This dipole, which possesses a chlorophyll signature associated with higher phytoplankton concentrations arising from nutrient upwelling, is important for the productivity of local fisheries. Multi-satellite sea level anomalies are used to investigate the life cycle of the dipole structure. By applying empirical orthogonal function (EOF) analysis, the third EOF mode (EOF 3) is found to represent the major variations of the dipole structure. By removing the temporal noise of EOF 3, a South China Sea dipole index is defined. This index captures the life cycle of the dipole including its generation, mature strength, and final termination. Both one-dimensional and two-dimensional forecasts are generated using a statistical forecasting method that combines singular-spectrum analysis and the maximum entropy method. The appearance of the dipole structure can be predicted with an accuracy of 78% at one-month lead times and an accuracy of 61% at one-year lead times.     

The Southern Westerlies during the last glacial maximum in PMIP2 simulations

The Southern Hemisphere westerly winds are an important component of the climate system at hemispheric and global scales. Variations in their intensity and latitudinal position through an ice-age cycle have been proposed as important drivers of global climate change due to their influence on deep-ocean circulation and changes in atmospheric CO₂. The position, intensity, and associated climatology of the southern westerlies during the last glacial maximum (LGM), however, is still poorly understood from empirical and modelling standpoints. Here we analyse the behaviour of the southern westerlies during the LGM using four coupled ocean-atmosphere simulations carried out by the Palaeoclimate Modelling Intercomparison Project Phase 2 (PMIP2). We analysed the atmospheric circulation by direct inspection of the winds and by using a cyclone tracking software to indicate storm tracks. The models suggest that changes were most significant during winter and over the Pacific ocean. For this season and region, three out four models indicate decreased wind intensities at the near surface as well as in the upper troposphere. Although the LGM atmosphere is colder and the equator to pole surface temperature gradient generally increases, the tropospheric temperature gradients actually decrease, explaining the weaker circulation. We evaluated the atmospheric influence on the Southern Ocean by examining the effect of wind stress on the Ekman pumping. Again, three of the models indicate decreased upwelling in a latitudinal band over the Southern Ocean. All models indicate a drier LGM than at present with a clear decrease in precipitation south of 40°S over the oceans. We identify important differences in precipitation anomalies over the land masses at regional scale, including a drier climate over New Zealand and wetter over NW Patagonia.     

On the response of the oceanic wind-driven circulation to atmospheric CO2 increase

A global, flux-corrected climate model is employed to predict the surface wind stress and associated wind-driven oceanic circulation for climate states corresponding to a doubling and quadrupling of the atmospheric CO₂ concentration in a simple 1% per year CO₂ increase scenario. The model indicates that in response to CO₂ increase, the position of zero wind stress curl in the mid-latitudes of the Southern Hemisphere shifts poleward. In addition, the wind stress intensifies significantly in the mid-latitudes of the Southern Hemisphere. As a result, the rate of water circulation in the subpolar meridional overturning cell in the Southern Ocean increases by about 6 Sv (1 Sv=10⁶ m³ s⁻¹) for doubled CO₂ and by 12 Sv for quadrupled CO₂, implying an increase of deep water upwelling south of the circumpolar flow and an increase of Ekman pumping north of it. In addition, the changes in the wind stress and wind stress curl translate into changes in the horizontal mass transport, leading to a poleward expansion of the subtropical gyres in both hemispheres, and to strengthening of the Antarctic Circumpolar Current. Finally, the intensified near-surface winds over the Southern Ocean result in a substantial increase of mechanical energy supply to the ocean general circulation.     

Variation in upwelling intensity along the NorthWest Iberian Peninsula (Galicia)

Thermohaline variables were simultaneously measured, July 17, 2000, at four Galician rias under favourable upwelling conditions to analyse upwelling intensity. The intensity of coastal upwelling is shown to be strongly dependent on the wind pattern. The maximum amplitudes of wind stress were observed at the western coast (0.30 Nm^-2) with southward direction (favourable to upwelling) and these are shown to be related to low temperature (12°C) and high salinity (35.8) in the estuaries mouth. From Cape Finisterre to Cape Estaca de Bares the wind stress showed a lower magnitude (0.05-0.10 Nm^-2) with a direction oblique to the coast. The temperature showed an increase (14°C) and the salinity a decrease (35.4) because the lower upwelling intensity at this part of the coast. The observed decrease in upwelling from south to north is not an isolated event, since the measured wind conditions are persistent during summer months, at least in the period with available QuikSCAT data (1999-2004).     

Evolution of meteorological drought characteristics in Vietnam during the 1961–2007 period

The drought conditions over the seven sub-climatological regions in Vietnam are examined using three meteorological drought indices: de Martonne J, PED, and Standardized Precipitation Index (SPI). According to the seasonal probabilities of drought occurrence estimated by the de Martonne index, droughts mainly occur between November and March in all the sub-regions. The PED index and the SPI index generally show high probabilities of drought occurrence from April to August and from May to October, respectively. In the southern sub-regions of Vietnam, droughts more frequently occur in El Niño years and wet conditions are more frequently observed in La Niña years. However, such El Niño–Southern Oscillation influences are not clearly observed in the northern sub-regions. During 1961–2007, droughts significantly increased in the northern part of Vietnam. In the southern regions, PED shows increasing drought conditions while J and SPI show decreasing drought trends for almost all the stations.     

Numerical simulation of the summer wake of Rottnest Island,Western Australia

During the summer, a northward, wind-driven current dominates the Rottnest Island region in southwestern Australia. Field studies have shown that the interaction between Rottnest Island and the northward current generates upwelling at the western end of the island, which is advected downstream, resulting in isotherm doming in the wake region. Flow curvature-induced secondary circulation has been proposed as the dominant mechanism responsible for this upwelling. Here, a three-dimensional numerical model, together with field observations, was used to undertake a detailed investigation of the three-dimensional flow structure in the wake region. Comparison of the observed upwelling pattern and the simulated flows revealed the island's dominant role in generating upwelling. This result was confirmed with the use of idealized numerical experiments. The modeling results confirmed the presence of secondary circulation, generated as a result of flow curvature at the western end of the island, which caused strong upwelling and extended downstream.     

Spectral analysis of the AVHRR sea surface temperature variability off the west coast of Vancouver Island

Abstract

From 16 AVHRR infrared satellite images of the west coast of Vancouver Island, British Columbia, collected during the five summers of 1984–1988, 4 alongshore temperature transects were sampled. Upon Fourier transforming the transect data, we found that the energy spectra of the temperature variance in alongshore wavenumber space in general followed a –2.1 power law, which agreed with previous observations from other parts of the world.

Summer images may be divided into 2 types: upwelling dominated and non‐upwelling dominated. When a strong upwelling‐induced alongshore cold front was observed, the regimes shoreward and seaward of the front had distinctly different spectra. Cross‐spectral analysis of transect data between images taken a day apart in the presence of strong upwelling events revealed significant coherence at the low wavenumber regime (wavelength 300 km and above, corresponding to the large eddies) and often at the high wavenumber regime (wavelength 30 km or below, corresponding to the fine structures of the eddies). The coherence dropped for images taken 2 or more days apart, suggesting a decorrelation time‐scale of about 2 days. In the absence of strong upwelling and associated eddies, summer transect temperature data from different years often showed a similar alongshore linear trend in addition to possible large differences in the mean temperature.     

Barrier layer in the South China Sea during summer 2000

Using temperature–salinity profiles obtained from a cruise in summer 2000, the structure and formation of the barrier layer (BL) in the South China Sea (SCS) are investigated. Fresh water flux, ocean circulation, and wind stirring are important for BL formation, depending on regions. In the eastern SCS, Philippine mountains induce heavy rainfall, resulting in a fresh water cap at the surface and the formation of a thick wide spread BL. In the northwestern basin on the lee of the Annam Cordillera range, by contrast, a rain shadow reduces fresh water flux, which along with wind-induced upwelling, prevents the BL forming. Southeast of Vietnam, a thick BL forms as the Mekong River plume is advected by the northeastward western boundary current and its offshore extension. In the southeastern basin, the surface water is mixed deeply under the strong southwesterly monsoon, unfavorable for the BL formation despite heavy rainfall. In the Luzon Strait, the east/southeastward surface Ekman drift carries fresh SCS surface water, riding on the intruding Kuroshio meander that carries well-mixed, warm and saline water. The vertical overlapping of these two water masses gives rise to a thick BL.     

Ice‐edge upwelling off the Newfoundland coast during LIMEX

Abstract

Analysis of CTD, Batfish and drifter data collected in the southern Labrador marginal ice zone during LIMEX (Labrador Ice Margin Experiment) in 1987 shows strong evidence of upwelling at the ice edge. The most clear indication of the upwelling is the rise of isopycnals and the increase of surface density near the ice edge. The upwelling zone has a width of 6 km extending from the ice edge, and an upwelling depth of at least 100 m. The existence of the upwelling zone along the ice edge is shown to be related to the character of the ice edge. Upwelling is more likely to occur at sharp and compacted ice edges. A two‐dimensional coupled ice‐ocean dynamical model of a continuously stratified ocean with a coastal boundary on a sloping bottom is used to study the dynamics of ice‐edge upwelling. The model results are in qualitative agreement with observations. A sensitivity analysis of the model is presented.     

Mixed layer heat budget of the El Niño in NCEP climate forecast system

The mechanisms controlling the El Niño have been studied by analyzing mixed layer heat budget of daily outputs from a free coupled simulation with the Climate Forecast System (CFS). The CFS is operational at National Centers for Environmental Prediction, and is used by Climate Prediction Center for seasonal-to-interannual prediction, particularly for the prediction of the El Niño and Southern Oscillation (ENSO) in the tropical Pacific. Our analysis shows that the development and decay of El Niño can be attributed to ocean advection in which all three components contribute. Temperature advection associated with anomalous zonal current and mean vertical upwelling contributes to the El Niño during its entire evolutionary cycle in accordance with many observational, theoretical, and modeling studies. The impact of anomalous vertical current is found to be comparable to that of mean upwelling. Temperature advection associated with mean (anomalous) meridional current in the CFS also contributes to the El Niño cycle due to strong meridional gradient of anomalous (mean) temperature. The surface heat flux, non-linearity of temperature advection, and eddies associated with tropical instabilities waves (TIW) have the tendency to damp the El Niño. Possible degradation in the analysis and closure of the heat budget based on the monthly mean (instead of daily) data is also quantified.     

Wind effects on the upwelling variability in the coastal zone of Primorye (the northwest of the Sea of Japan)

The variability of upwelling events in the coastal zone of Primorye in the northwestern part of the Sea of Japan is studied using the SeaWinds/QuikSCAT scatterometer wind data for the period of 1999-2009. The intensity of upwelling is defined by the wind-induced offshore Ekman transport (the upwelling index). It was found that along the southern coast of Primorye upwelling events occur from September to March (April). The winter monsoon period is the most favorable for the upwelling development. In the eastern part of the coastal zone of Primorye upwelling is observed in transitional seasons between winter and summer monsoon (February-April and September-October). On the northeastern coast of Primorye, the upwelling season is from August to October (November). The common feature of the coastal zone of Primorye is a wind-driven upwelling in autumn (September-October). The interannual variability of winter upwelling along the southern coast of Primorye is related to the East Asia high pressure center (the Siberian High). The upwelling intensifies in the years with positive air pressure anomalies in the Siberian High and weakens in the years with negative anomalies.     

Asymmetry in the tropical Indian Ocean subsurface temperature variability

The empirical orthogonal function (EOF) analysis of subsurface temperature shows a dominant north-south mode of interannual variability in the Tropical Indian Ocean (TIO) at around 100 m depth (thermocline). This subsurface mode (SSM) of variability evolves in September-November (SON) as a response to Indian Ocean Dipole and intensifies during December-February (DJF) reinforced by El Niño and Southern Oscillation (ENSO) forcing. The asymmetry in the evolution of positive and negative phases of SSM and its impacts on the modulation of surface features are studied. The asymmetry in the representation of anomalous surface winds along the equator and off-equatorial wind stress curl anomalies are primarily responsible for maintaining the asymmetry in the subsurface temperature through positive and negative phases of the SSM. During the positive phase of SSM, downwelling Rossby waves generated by anticyclonic wind stress curl propagate towards the southwestern TIO (SWTIO), the thermocline ridge region of mean upwelling. The warmer subsurface water associated with the downwelling Rossby waves upwells in the region of mean upwelling and warms the surface resulting in strong subsurface-surface coupling. Such interaction processes are however weak during the negative phase of SSM. The asymmetry in the subsurface-surface interaction during the two phases of SSM and its impact on the modulation of surface features of TIO are also reported. In addition to the ENSO forcing, self-maintenance of SSM during DJF season is evident in the positive SSM (PSSM) years through modulation of subsurface surface coupling and air-sea coupling. This positive feedback during PSSM years is maintained by the deepening thermocline, warm SSTs and convection. The asymmetry in the thermocline evolution is more evident in the SWTIO and southern TIO.