Current and Thermal Variations to Westerly Wing Bursts in the Equatorial Pacific Ocean

Effect of climate on Enteric Fever incidence in Ahmedabad, India

This paper investigates the structure and dynamics of the Equatorial Undercurrent (EUC) of the Indian Ocean by analyzing in situ observations and reanalysis data and performing ocean model experiments using an ocean general circulation model and a linear continuously stratified ocean model. The results show that the EUC regularly occurs in each boreal winter and spring, particularly during February and April, consistent with existing studies. The EUC generally has a core depth near the 20°C isotherm and can be present across the equatorial basin. The EUC reappears during summer–fall of most years, with core depth located at different longitudes and depths. In the western basin, the EUC results primarily from equatorial Kelvin and Rossby waves directly forced by equatorial easterly winds. In the central and eastern basin, however, reflected Rossby waves from the eastern boundary play a crucial role. While the first two baroclinic modes make the largest contribution, intermediate modes 3–8 are also important. The summer–fall EUC tends to occur in the western basin but exhibits obvious interannual variability in the eastern basin. During positive Indian Ocean dipole (IOD) years, the eastern basin EUC results largely from Rossby waves reflected from the eastern boundary, with directly forced Kelvin and Rossby waves also having significant contributions. However, the eastern basin EUC disappears during negative IOD and normal years because westerly wind anomalies force a westward pressure gradient force and thus westward subsurface current, which cancels the eastward subsurface flow induced by eastern boundary–reflected Rossby waves. Interannual variability of zonal equatorial wind that drives the EUC variability is dominated by the zonal sea surface temperature (SST) gradients associated with IOD and is much less influenced by equatorial wind associated with Indian monsoon rainfall strength.

Using satellite-derived surface current products, we examined annual current variations in the equatorial Pacific Ocean and associated zonal displacement of the Pacific warm pool. Annual current variations, generated mainly by annual equatorial Rossby waves, displaced the warm pool eastward (westward) during boreal spring-summer (winter), with accompanying increase (decrease) in the zonal heat advection at a rate of 0.2-0.4 °C mon-1 in the warm pool region in normal years when neither El Nino nor La Nina event take place; similar seasonal increases (decreases) have also been observed in El Nino years. Annual zonal currents variations attributable to oceanic Rossby waves provide more (less) favorable background sea surface conditions for the onset of El Nino during boreal summer (winter) by weakening (strengthening) the zonal sea surface temperature gradient through eastward (westward) heat transport near the equator in the warm pool region. Annual zonal currents variations might play a key role in seasonally locking the onset of El Nino events in the warm pool region.

El Nino has distinctive oceanic and atmospheric signatures that have different influences on tropical cyclone (TC) activity over the western North Pacific (WNP). This study compares TC activity over the WNP basin among strong Central-Pacific (CP), mixed CP, and strong Eastern-Pacific (EP) El Nino. Results suggest that TC activity with strong intensity and long lifespan occurred more frequently during strong (i.e. strong EP and strong CP) El Nino years than during mixed-CP El Nino years. This is attributed primarily to a combined modulation of the amplitude and duration of warm sea-surface temperature anomalies (SSTAs) over the tropical Pacific, and variations in large-scale environmental conditions; i.e. eddy kinetic energy, omega, relative humidity, and vertical wind shear (VWS). During CP (i.e. strong CP and mixed CP) El Nino events, enhanced TC genesis is observed over a large part of the WNP. During strong-CP El Nino events, more TCs are generated east of 140°E in the southwestern WNP, whereas more TCs are generated west of 140°E during mixed-CP El Nino events. This is due to a reduced magnitude of VWS and a westward shift in warm SSTAs over the central equatorial Pacific. When an anomalous anticyclonic circulation over the Indo-China Peninsula is combined with a westward extension of the western North Pacific subtropical high, TCs heading northward are associated with strong mean winds during strong El Nino events, resulting primarily from enhanced eddy kinetic energy, omega, and relative humidity, and weak VWS. Overall, TC activity during the three types of El Nino shows distinct generation locations, evolution patterns, and intensities, and the evolving spatial patterns of SSTA play an important role in modulating TC activity.

In this paper we investigate the evolution of moderate El Nino events during their developing phase with the objective to understand why some of them did not evolve as extreme events despite favourable conditions for the non-linear amplification of the Bjerknes feedback (i.e. warm SST in Austral winter in the eastern equatorial Pacific). Among the moderate events, two classes are considered consisting in the Eastern Pacific (EP) El Nino events and Central Pacific (CP) events. We first show that the observed SST variability across moderate El Nino events (i.e. inter-event variability) is largest in the far eastern Pacific (east of ~ 130°W) in the Austral winter prior to their peak, which is associated to either significant warm anomaly (moderate EP El Nino) or an anomaly between weak warm and cold (moderate CP El Nino) as reveals by the EOF analysis of the SST anomaly evolution during the development phase of El Nino across the El Nino years. Singular value decomposition (SVD) analysis of SST and wind stress anomalies across the El Nino years further indicates that the inter-event SST variability is associated with an air–sea mode explaining 31% of the covariance between SST and wind stress. The associated SST pattern consists in SST anomalies developing along the coast of Ecuador in Austral fall and expanding westward as far as 130°W in Austral winter. The associated wind stress pattern features westerlies (easterlies) west of ~ 130°W along the equator peaking around June–August for EP (CP) El Nino events. This air–sea mode is interpreted as resulting from a developing seasonal Bjerknes feedback for EP El Nino events since it is shown to be associated to a Kelvin wave response at its peak phase. However equatorial easterlies east of 130°W emerge in September that counters the growing SST anomalies associated to the air–sea mode. These have been particularly active during both the 1972 and the 2015 El Nino events. It is shown that the easterlies are connected to an off-equatorial southerly wind off the coast of Peru and Ecuador. The southerly wind is a response to the coastal SST anomalies off Peru developing from Austral fall. Implications of our results for the understanding of the seasonal ENSO dynamics and diversity are discussed in the light of the analysis of two global climate models simulating realistically ENSO diversity (GFDL_CM2.1 and CESM).

Several 5 to 10 m s−1 westerly wind bursts of 10–15 days' duration occurred in the western equatorial Pacific during November 1989 to January 1990. The response to these wind bursts was characterized by a 400‐ to 600‐km‐wide eastward jet in the upper 100–150 m along the equator between 135°E and the date line. Flow in this jet accelerated to speeds of over 100 cm s−1 within 1 week after the onset of westerly winds in November 1989 in association with supertyphoon Irma. In addition, a 20 to 40 cm s−1 westward counterflow developed between 2°N and 2°S below the surface jet separating it from the eastward flow of the Equatorial Undercurrent in the thermocline. Changes in surface layer zonal volume transports in the western Pacific due to westerly wind bursts were 25–56 Sv based on comparison of three shipboard velocity transects in November and December 1989. Although fluctuations in current speeds in the thermocline were generally smaller and less directly related to local wind forcing than those in the surface layer, the Equatorial Undercurrent decelerated to less than 20 cm s−1 (i.e., less than half its speed before the onset of westerlies) by early December 1989. Westerly winds also produced a strong surface convergence in both the meridional and zonal directions west of the date line. In late November 1989, this convergence was associated with downwelling of 2–3 m d−1 along the equator, and a 20‐ to 30‐m depression of the thermocline. In addition, sea level rose by 10 dyn cm from mid‐November to mid‐December between 1°S and 3°N, 165°E. Sea surface temperature dropped over a large region by 0.2°–0.5°C during episodes of high westerly winds, and some episodes of high winds were associated with 0.2°–0.3°C sea surface temperature inversions supported by salt‐stratified buoyancy gradients in the upper 100 m. The near‐surface salinity balance was dominated by lateral advection when relatively fresh water from north of a salinity front centered near 3°N was advected southward by Ekman drift in late November 1989. Displacement of this front past a current meter mooring at 0°, 165°E, appeared as a local drop of 1 psu in surface layer salinity in 10 days. These results are discussed in the context of a swing in the climate system towards El Niño‐Southern Oscillation‐like conditions in early 1990.

According to the time cross-section of SST in the equatorial eastern Pacific and the historical data on typhoon actions over the western Pacific (including the South China Sea), a composite analysis of the actions of typhoon over the western Pacific in El Nino year (SST in the equatorial eastern Pacific are continuously higher than normal) and in the inverse El Nino year (there are continuative negative anomalies of SST in the equatorial eastern Pacific) is carried out. The results show that the actions of typhoon are in close relation with El Nino: The annual average number of typhoons over the western Pacific and South China Sea is less than normal in El Nino year and more in the inverse El Nino year; The annual average number of the landing typhoon on the continent of China bears the same relationship with El Nino; The anomalies of typhoon actions mainly occur during July-November and their starting are behind the anomaly of SST in the equatorial eastern Pacific.

Bolivia is located at the crossroad of the major climatic influences of Northern and Southern-South America, which turns this country into a natural laboratory to investigate the interactions between ocean-climate and fire variability. We chose two oceanic indices: MEI (multivariate ENSO Index) and AMO (Atlantic Multidecadal Oscillation) to select the three most representative years for four oceanic conditions: El Nino, La Nina, AMO, and standard years (understood as years with little ocean influences), for the period 1992–2012. We investigated how i) rainfall (dry vs wet seasons) and ii) fire responded in five Bolivian biomes (Tropical Moist Forests, Tropical Dry Forests, Tropical Grasslands, Tropical Montane, and Seasonally Flooded ecosystems) under these oceanic conditions. Bolivia showed a strong rainfall increase in El Nino years in both seasons (wet/dry), while AMO showed the strongest droughts in both seasons. La Nina showed a bipolar response with rainfall increases in the wet season and a very marked rainfall decrease in the dry season. Drought significantly increased fire numbers in AMO years, being the most significant fire condition and suggesting a larger fire influence of the Atlantic than the Pacific at the national level. Surprisingly, the amount of fire was very large under normal years (STD) and similar to fire levels under La Nina, suggesting generalized fire conditions in the country, except for El Nino years that bring rainfall excess and little fire. The most fire-affected biomes were the seasonally flooded and dry forests, followed by the grassland/savannah biome. Montane areas showed the least fire, but satellite fire omission is well known in the Andean region.

Eighty-seven seismic refraction profiles have been obtained to define the geologic structure in the upper crust associated with the midcontinent gravity high in Minnesota and Wisconsin. The seismic measurements were taken across a fixed spread of seven geophones from distances up to 13 km. A structural section was prepared for each profile by interpretation of the travel-time graph, and the individual sections were compiled onto regional cross sections. Measured seismic velocities in bedrock fall in the 2.5–7.1 km/sec range. Observed velocities can be assigned to seven categories corresponding to Paleozoic, upper, middle, and lower Upper Keweenawan strata, Middle Keweenawan volcanics, pre-Keweenawan felsic-to-intermediate intrusive rocks, and pre-Keweenawan mafic intrusive rocks. These groups display good continuity through the area and suggest the lithologic equivalence of rock bodies between geologic provinces. The St. Croix horst and its flanking basins underlie the midcontinent gravity high and its parallel gravity lows north of Minneapolis. Minimum throw along the western and eastern boundary fault zones reaches about 3.0 and 2.0 km. Sedimentary rocks in the eastern basin reach a thickness of at least 2.6 km. A complex horst-like structure also underlies the midcontinent gravity high in southern Minnesota; an uplifted basaltic body is bordered by sedimentary basins having strata about 3.0 km thick. Middle Keweenawan basalts are present locally in the eastern and western basins underlying the Upper Keweenawan strata. Rocks probably lithologically equivalent to the Oronto group are rare in the western basin, common in small basins on the St. Croix horst, and abundant in the eastern basin. Rocks probably lithologically equivalent to the Bayfield group are extensive in the western and eastern basins, but they have not been found on the St. Croix horst. The Bayfield group seems to be several kilometers thick across Douglas County north of the Douglas fault, and it does not appear to increase in thickness under the Bayfield peninsula.

We investigated the influence of dynamical in-consistency of initial conditions on the predictive skill of decadal climate predictions. The investigation builds on the fully coupled global model “Coupled GCM for Earth Simulator” (CFES). In two separate experiments, the ocean component of the coupled model is full-field initialized with two different initial fields from either the same coupled model CFES or the GECCO2 Ocean Synthesis while the atmosphere is initialized from CFES in both cases. Differences between both experiments show that higher SST forecast skill is obtained when initializing with coupled data assimilation initial conditions (CIH) instead of those from GECCO2 (GIH), with the most significant difference in skill obtained over the tropical Pacific at lead year one. High predictive skill of SST over the tropical Pacific seen in CIH reflects the good reproduction of El Nino events at lead year one. In contrast, GIH produces additional erroneous El Nino events. The tropical Pacific skill differences between both runs can be rationalized in terms of the zonal momentum balance between the wind stress and pressure gradient force, which characterizes the upper equatorial Pacific. In GIH, the differences between the oceanic and atmospheric state at initial time leads to imbalance between the zonal wind stress and pressure gradient force over the equatorial Pacific, which leads to the additional pseudo El Nino events and explains reduced predictive skill. The balance can be reestablished if anomaly initialization strategy is applied with GECCO2 initial conditions and improved predictive skill in the tropical Pacific is observed at lead year one. However, initializing the coupled model with self-consistent initial conditions leads to the highest skill of climate prediction in the tropical Pacific by preserving the momentum balance between zonal wind stress and pressure gradient force along the equatorial Pacific.

Based on surface winds at 6-h intervals for four Northern Winters and Springs (1970-73), two cases of strong westerly wind bursts were identified in the core of the equatorial western Pacific (-155°E). One case occurred in early April and another in early May 1972, both prior to the maximum sea surface temperature anomalies along the Peruvian coast during the 1972 ENSO event. During the Northern Spring, as an anomalously strong anticyclone moves rapidly from north-central China to its east coast, the surface wind fields to the southeast of the Philippines respond swiftly, turning from an easterly background to northerly. In the meantime, surface pressure in the far western equatorial Pacific tends to rise. These rapid equatorial resposes are probably due to gravity wave-like motions induced by the pressure-wind imbalance in the midlatitudes. The local pressure increase in the extreme western Pacific enhances the west-to-east pressure gradient in the equatorial trough zone and results in a strong westerly wind acceleration in the core of the equatorial western Pacific. This acceleration is also preceded by a west-to-east displacement of the pressure surge in the equatorial trough zone. The enhanced zonal pressure-gradient force and the associated eastward displacement of the equatorial pressure surge are two critical factors for initiating westerly wind bursts. Westerly wind surges detected primarily from fixed-station data compare favorably with those calculated from ship records of an independent source.

Nitrate and chlorophyll distributions in relation to thermohaline and current structures in the western tropical Pacific during 1985–1989

Measurements of a fresh surface anomaly (fresh lens) produced by rainfall during a westerly wind burst have been analyzed. The measurements were made in December 1992 as part of the Coupled Ocean–Atmosphere Response Experiment in the western equatorial Pacific (2°S, 156°E). Measurements included radar estimates of rainfall, upper-ocean temperature (T), salinity (S), horizontal velocity, and microstructure. In situ observations of the fresh lens were made 5 to 7 hours after its formation. In the 5 hours after formation, the lens deepened to a depth of 40 m as indicated by its salinity anomaly. Salinity and temperature were highly correlated within the lens, consistent with its initial formation by cold rainfall. The T–S relation exhibited curvature, which can be explained by surface cooling and upper-ocean mixing subsequent to formation of the lens. The lens exhibited a horizontal velocity anomaly in the direction of wind, which extended down to a depth of 40 m. The horizontal velocity anomaly is ...

Primary production values in Lake Ontario were low in winter, reached a maximum in midspring, declined during summer, and slightly increased in fall. Rate of increase of production for inshore waters (< 20 m depth) was greater especially in spring and early summer with a greater maximum reached earlier than in offshore waters. Assimilation numbers, mgC fixed/mg chlorophyll a per hour, in Lake Ontario were fairly constant over the lake with a yearly range of 1.2–1.6. Primary production showed a linear relationship to chlorophyll a concentration, as also occurred in Lake Erie.Lake Erie primary production varied in its three basins. Seasonally, in the Eastern Basin, production was highest in spring with a midsummer decline, and small peaks in fall. The Western Basin had a maximum in midsummer whereas the Central Basin had peaks in late summer and early fall. Assimilation numbers were highest in the Western Basin (up to 3.5 mgC/mg chlorophyll a per hour) and lowest in the mid-Central Basin and Eastern Basin with values of approximately 1.4 mgC/mg chlorophyll a per hour. A definite westerly increase of assimilation number was observed.Up to early summer, the two lakes were fairly equal in surface production but integral photosynthesis, mgC/m2 per hour was higher in Lake Ontario than in Lake Erie. The same was valid in November and December. In summer, Lake Erie was higher in production on both a mgC/m3 per hour and mgC/m2 per hour basis. For the period, April–December, Lake Ontario's total estimated yield was 170 gC/m2, whereas for Lake Erie values of 160, 210, and 310 gC/m2 were found for the Eastern, Central, and Western basins, respectively.

Sakskoe Lake is the most thoroughly studied water body in Crimea. Its therapeutic mud and brine are exploited by balneological treatment methods. During the two centuries, when the lake was used by humans, it was subdivided into seven water bodies by dams, with two of these bodies (Eastern and Western) being now utilized for medical purposes, and the other five being protective. The Eastern (currently exploited) basin is now used to produce therapeutic mud and brine. The bottoms of the saline basins are covered by sediments: an upper layer of black silt, which is underlain by gray, steel-gray, occasionally bluish silt, which is used as therapeutic mud. The lake is fed by surface and groundwaters. Retaining dams and flood embankments erected in nearby ravines preclude desalination of the lake with flood and rain waters. Seawater is pumped to the lake through channels to preclude its drying. A quay is now actively constructed around the lake and will likely notably affect its hydrological and ecological conditions. The aim of the present study was the comparative analysis of the chemical and physical characteristics of the Eastern and Western basins of the Sakskoe Lake at the period of intensive building activity on their coasts at the summer period of 2019. Brine samples were collected at two tested sites in each of the lakes. Water salinity was measured by a PAL-06S LTA GO (Japan) refractometer and was expressed in ‰. The pH and Eh of the waters and oxygen dissolved in them were determined in the laboratory by an Expert-001 (Econix-Expert Moexa CoLtd, Russia) analyzer, with the use of appropriate Volta (Russia) selective electrodes. Ions concentration was determined spectrophotometrically and by atomic absorption method. Various living stages of Artemia populations in two water bodies were studied according the standard microscopic methods. The obtained results demonstrated, that the total content of the cations in the brine of the Western basin was in 1,5-fold higher as compared with the data of the brine in the Eastern basin (108,4 and 71,2 g/l respectively). The values were directly correlated with the high salinity of the Western basin brine. The identical trend was shown for the anions concentration, which was significantly greatly in the Western basin related to Eastern one (193,14 and 125,41 g/l correspondingly), which was connected with the different hydrogeological conditions of the formation of the ecosystems of the tested water bodies. At the other hand, pH, salinity and Eh of the brine in the Eastern basin were lower than in the brine of the Western basin, while the concentration of dissolved oxygen was higher. In the brine of the Eastern basin of the lake all living stages of Artemia, including cysts, nauplia and adults were found, while in the Western basin only cysts were shown. Therefore, according the obtained results we could suggest, that anthropogenic processes which were taken place on the coasts of the Sakskoe Lake accompanied with the intensive building activity did not influence on the lake ecosystem. The further monitoring studies including the testing of the ions concentrations in the brine of two basins, Artemia populations state are required for the understanding the changes of the both ecosystems and the possible reasons of their changes. It is important for development of the optimization of the management of Sakskoe lake and its resources exploitation.