A linear model study of the mid-tropospheric ridge and its displacement

A linear model of the steady response of a stratified fluid to isolated heat sources is used to study the maintainence of the mean position of the mid-tropospheric ridge and its displacemerit. It is well known that the performance of the southwest Indian monsoon is intimately related to the latitudinal position of the April 500 hPa ridge along 75°E. Recent observational studies have demonstrated that the winter/spring snow cover over Eurasia are negatively related to the April 500 hPa ridge position. In this study we propose one possible physical mechanism of southward displacement of the mid-tropospheric ridge. The anomalous cooling associated with the increased snow cover in Eurasia may be considered as a heat sink north of the tropical heal sources. It is demonstrated that such a heat sink can result in significant southward displacement of the mid-tropospheric ridge.

Role and impact of Siberian High on the temporal variation of Indian northeast monsoon rainfall

The relation between the intensity of Siberian High, defined as the mean sea level pressure over the Siberian region bounded by 87.5 & 102.5° E longitudes and 47.5 & 52.5° N latitudes (PSH) and Indian northeast monsoon rainfall has been studied in antecedent and concurrent modes based on monthly/seasonal mean PSH and monthly/seasonal rainfall data of Tamil Nadu (NMR) for the 34 year period, 1971 to 2004. It has been found that a positive relationship exists between the PSH and NMR of October-November (ON) which is significant in the antecedent mode [PSH(AS/Sep)] and modest in concurrent mode. The relationship turns negative for NMR (Dec) with both PSH(Sep) (antecedent) and PSH(Dec) (concurrent). By and large, negative anomaly profile of PSH during September-November (SON) followed by a positive PSH (Dec) anomaly is associated with a deficient NMR, but, a normal to positive PSH anomaly profile in SON becoming negative in December is associated with an excess NMR. The manifestation of PSH on NMR has been shown to be by way of modulating the strength of low level easterlies over the Bay of Bengal off the southeast coast of peninsular India as well as the latitudinal positions of Sub Tropical Ridge at 200 hPa (STR) and Equatorial Trough at 850 hPa (ET) over India. An intense PSH (Sep) is associated with strengthening of easterlies over the Bay of Bengal as well as southward location of STR/ET thereby favouring a good NMR (ON). During December, a weaker than normal PSH (Dec) is associated with northward location of ET from its normal latitudinal position near the equator which becomes conducive for good NMR(Dec). That a weaker than normal PSH in December is associated with good NMR(Dec) is comprehended from an analysis of time series of PSH. It has been found that the PSH itself undergoes a phase change in December on most occasions, i.e., an intense PSH (Nov) is by and large, followed by a weaker than normal PSH (Dec) and vice versa.

Relation between southern oscillation index and Indian northeast monsoon as revealed in antecedent and concurrent modes

The relation between Southern Oscillation Index (SOI) and Indian northeast monsoon has been studied in antecedent and concurrent modes based on monthly /seasonal mean SOI and monthly/seasonal rainfall data of Tamil Nadu for the 104 year period, 1901 to 2004. It has been found that a good negative relationship exists between the SOI and Indian northeast monsoon in antecedent and concurrent modes, the former being stronger than the later. In the concurrent mode, a strong negative relationship exists during the beginning of the season which changes as the season advances and turns positive during the fag end of the season. Such a changing nature of relationship is explained through the variation of latitudinal positions of 200 hPa Sub Tropical Ridge (STR) / Equatorial Trough (ET) and the location of these with reference to the latitudinal location of the area benefited by the northeast monsoon. It has been shown that a positive (negative) SOI shifts the STR north(south)wards throughout the year. The relationship between latitudinal position of STR and the Indian northeast monsoon rainfall (NMR) is negative during the beginning of the season and turns positive during the fag end of the season, which is similar to the relationship between SOI and NMR. The relation between upper tropospheric wind/temperature anomalies and NMR also shows a similar change in relationship. Westerly wind and negative temperature anomalies in October changing to easterly wind and positive temperature anomalies in January are by and large associated with good northeast monsoon activity. The reversal in the relationship between latitudinal position of STR and NMR as the season advances has also been partly explained based on theoretical considerations by invoking the tilting term of the vorticity equation. Thus the SOI appears to manifest itself on Indian northeast monsoon rainfall by way of modulating the latitudinal positions of STR. An analysis based on Australian “Rainman” software on winter monsoon rainfall of some Sri Lankan and southeast Asian stations has substantiated the changing nature of relationship. A study of dates of onset and withdrawal of Indian northeast monsoon in relation to SOI has revealed that negative (positive) SOI in September is associated with early (late) onset. But, continuation of negative SOI throughout the season favours early and abrupt withdrawal. Positive SOI during the fag end of the season is frequently associated with extension of the monsoon into January of the next year.

Contrasting movement of wind based equatorial trough and equatorial cloud zone over Indian southern peninsula and adjoining Bay of Bengal during the onset phase of northeast monsoon

ABSTRACT. The northeast monsoon sets in over southern parts of peninsular India after the retreat of southwest monsoon and in association with the southward movement of equatorial trough. The INSAT satellite imageries scrutinised during the past several years revealed that the cloud bands at the time of northeast monsoon onset moved from south Bay into the southern peninsula, a feature that contrasts with the north to south movement of the equatorial trough. The paper investigates this aspect based on a dataset of lower level upper winds of the peninsula, rainfall data and INSAT OLR data for the 20 year period 1981–2000. The super epoch profiles of zonal winds, latitudinal position of equatorial trough with reference to northeast monsoon onset dates have been derived and studied. The region with OLR values less than 230 W/m2 was defined as the equatorial cloud zone and the movement of northern limit of ECZ was studied based on the normal pentad OLR data and also the superposed epoch profiles. From these analysis it has been established that at the time of northeast monsoon onset, the wind based equatorial trough moves south of Comorin whereas the cloud zone in the Bay of Bengal moves from south to north. Reasons for the occurrence of such a contrasting feature have been ascribed to features such as decreasing strength of lower level easterlies from north to south over coastal Tamil Nadu, reversal of temperature gradient between Chennai and Thiruvananthapuram at the time of onset and the dynamics of 40-day oscillation. The northeast monsoon activity over coastal Tamil Nadu has been found to be negatively correlated with the low level zonal winds over the coast, the degree of relation decreasing from north to south and also from October to December. Based on the results derived in the study and also the other known features of northeast monsoon a thematic model of northeast monsoon onset listing the events that precede and succeed the onset has been postulated.

Understanding physical drivers of the 2015/16 marine heatwaves in the Northwest Atlantic

The Northwest Atlantic, which has exhibited evidence of accelerated warming compared to the global ocean, also experienced several notable marine heatwaves (MHWs) over the last decade. We analyze spatiotemporal patterns of surface and subsurface temperature structure across the Northwest Atlantic continental shelf and slope to assess the influences of atmospheric and oceanic processes on ocean temperatures. Here we focus on MHWs from 2015/16 and examine their physical drivers using observational and reanalysis products. We find that a combination of jet stream latitudinal position and ocean advection, mainly due to warm core rings shed by the Gulf Stream, plays a role in MHW development. While both atmospheric and oceanic drivers can lead to MHWs they have different temperature signatures with each affecting the vertical structure differently and horizontal spatial patterns of a MHW. Northwest Atlantic MHWs have significant socio-economic impacts and affect commercially important species such as squid and lobster.

Influence of latitude and moisture effects on the barotropic instability of an idealized ITCZ

A large fraction of tropical cyclones (TCs) are generated near the intertropical convergence zone (ITCZ), and barotropic instability of the related wind shear has been shown to be an important generation mechanism. The latitudinal position of the ITCZ shifts seasonally and may shift poleward in response to global warming. Aquaplanet GCM simulations have shown TC-generation frequency to vary with position of the ITCZ. These results, and that moisture plays an essential role in the dynamics, motivate the present study on the growth rates of barotropic instability in ITCZ-like zonal wind profiles. Base-state zonal wind profiles are generated by applying a prescribed forcing (representing zonally-averaged latent heat release in the ITCZ) to a shallow-water model. Shifting the latitudinal position of the forcing alters these profiles, with a poleward shift leading to enhanced barotropic instability. Next, an examination of how latent release impacts the barotropic breakdown of these profiles is considered. To do this, moisture is explicitly represented using a tracer variable. Upon supersaturation, precipitation occurs and the related latent heat release is parameterized as a mass transfer out of the dynamically active layer. Whether moisture serves to enhance or reduce barotropic growth rates is found to depend on how saturation humidity is represented. In particular, taking it to be constant or a function of the layer thickness (related to temperature) leads to a reduction, whereas taking it to be a specified function of latitude leads to an enhancement. Simple arguments are given to support the idea that moisture effects should lead to a reduction in the moist shallow water model and that a poleward shift of the ITCZ should lead to an enhancement of barotropic instability.

Changes in Tibetan Plateau latitude as an important factor for understanding East Asian climate since the Eocene: A modeling study

<p><span>Previous climate modeling studies suggest that the surface uplift of the Himalaya–Tibetan plateau (TP) is a crucial parameter for the onset and intensification of the East Asian monsoon during the Cenozoic. Most of these studies have only considered the Himalaya–TP in its present location between ∼26°N and ∼40°N despite numerous recent geophysical studies that reconstruct the Himalaya–TP 10° or more of latitude to the south during the early Paleogene. We have designed a series of climate simulations to explore the sensitivity of East Asian climate to the latitude of the Himalaya–TP. Our simulations suggest that the East Asian climate strongly depends on the latitude of the Himalaya–TP. Surface uplift of a proto-Himalaya–TP in the subtropics intensifies aridity throughout inland Asia north of ∼40°N and enhances precipitation over East Asia. In contrast, the rise of a proto-Himalaya–TP in the tropics only slightly intensifies aridity in inland Asia north of ∼40°N, and slightly increases precipitation in East Asia. Importantly, this climate<br>sensitivity to the latitudinal position of the Himalaya–TP is non-linear, particularly for precipitation across East Asia.</span></p>

The seesaw response of the Intertropical and South Pacific convergence zones to hemispherically asymmetric thermal forcing

<p>Considerations based on atmospheric energetics and aqua-planet model simulations link the latitudinal position of the global intertropical convergence zone (ITCZ) to atmospheric cross-equatorial energy transport—a greater southward transport corresponds to a more northerly position of the ITCZ. This study, rather than concentrating of the zonally-averaged ITCZ, focuses on the tropical Pacific and looks separately at precipitation in the northern and southern hemispheres. Using numerical experiments, we show that in the tropical Pacific the response of the fully coupled ocean-atmosphere system to a hemispherically asymmetric thermal forcing, modulating atmospheric cross-equatorial energy transport, involves an interplay between the ITCZ and its counterpart in the South Pacific—the Southern Pacific convergence zone (SPCZ). This interplay leads to interhemispheric seesaw changes in tropical precipitation, such that the latitudinal position of each rain band remains largely fixed, but their intensities follow a robust inverse relationship. The seesaw behavior is also evident in the past and future coupled climate simulations of the Climate Model Intercomparison Project Phase 5 (CMIP5). We further show that the tropical Pacific precipitation response to thermal forcing is qualitatively different between the aquaplanet (without ocean heat transport), slab-ocean (with climatological ocean heat transport represented by a “Q-flux”) and fully-coupled model configurations. Specifically, the induced changes in the ITCZ latitudinal position successively decrease, while the seesaw precipitation intensity response becomes more prominent, from the aqua-planet to the slab-ocean to the fully-coupled configuration. The ITCZ/SPCZ seesaw can explain a precipitation dipole pattern observed in paleoclimate without invoking a too strong climate forcing and is relevant to future projections of tropical precipitation.</p>

Atmospheric circulation and variability of meteorological elements in near-shore Black Sea region

Effect of solar radiation and atmospheric circulation on variability of air temperature, atmospheric pressure, and rainfall in near-shore Black Sea region was examined. The action of the latitudinal position of trajectory cyclonic and anticyclonic movements and their times of occurrence in seasonal variability of meteorological elements was established. Three factor action on cyclonican and anticyclonic movement latitudinal position was determined. This is air temperature (year cycle), North-Atlantic oscillation, and lunar weather tides. Seasonal times of occurrence of cyclon and anticyclone, movements of southerly and northerly trajectories, and variability of atmospheric pressure and rainfall in near-shore Black Sea region was estimated.

MINIMAL WATER RUNOFF OF THE AZOV RIVER BASIN AREA DURING SUMMER-AUTUMN AND WINTER LOW-WATER PERIODS

For the purpose of value and analysis of the statistical characteristics of low-water runoff during summer-autumn and winter low flow periods on the rivers of the Priazov’s, used the time series of the minimum runoff of the rivers by 16 WGS during the period of from beginning observation till 2015 inclusive. In order to protect the water resources in the region, it is necessary to use its rationale, especially during low water periods than are minimal water discharges. For these aims, It is necessary to estimate the values of the characteristics of the minimum runoff in the Priazov region on the modern initial data, which is relevant, both in scientific and practical terms. Before generalizing the mean runoff modules in the summer-autumn and winter periods, the influence of local factors (latitudinal position, aforestation, and swampy watersheds) on their value was investigated. No significant influence of local factors was revealed, except for a good relationship with the latitude of the catchment centers. To determine the minimum runoff in winter for unexplored rivers of the territory, a map of isolines of 30-day minimum runoff modules is proposed. The distribution over the territory is uneven and varies from 0.30 l / (s • km2) in the southwestern part of the territory to 3.25 l / (s • km2) in the northeastern parts. The isolines are drawn with a step of 0.20 l / (s • km2). The map error is ± 4.3%, which corresponds to the accuracy of the initial information and the requirements of the current regulatory document SNiP 2.10.14-83. In the summer-autumn period, the distribution over the territory of the average minimum runoff modules is similar – in the south (in the Molochnaya river basin) low values are observed from 0.080 l / (s • km2), significantly increases in the northeast direction to 2.50 l / (s • km2) in the Kripenʹka river basin. Isolines are also drawn with a step of 0.20 l / (s • km2). The error in determining the minimum runoff in the summer-autumn period according to the proposed map is slightly higher and amounts to ± 7.7%, but it also meets the requirements for the accuracy of calculating the low-water runoff. To determine the coefficients of variability of low-water runoff, the obtained regional calculation equations, the accuracy of the calculation for which is provided by significant correlation coefficients; the skewness coefficient is normalized in relation to the coefficient of variation at the 2.0 level. The proposed regional method for determining the value of the minimum runoff for the summer-autumn and winter periods makes it possible to use it without modifications in order to reliably substantiate the runoff characteristics during the dry season on the Priazov rivers.