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Low-pressure Systems (low-pressure + system)

Distribution by Scientific Domains

Earth and Environmental Science	50%

Selected Abstracts

160 Copepodology for the Phycologist with Apologies to G. E. Hutchenson

JOURNAL OF PHYCOLOGY, Issue 2003
P. A. Tester
Heterocapsa triquetra is one of the most common bloom forming dinoflagellates found in estuaries and near shore regions around the world. In order to bloom, H. triquetra optimizes a suite of factors including low grazing pressure, increased nutrient inputs, alternative nutrient sources, and favorable salinity and hydrodynamic conditions, as well as the negative factors of temperature-limited growth, short day lengths, and periods of transient light limitation. The prevailing environmental conditions associated its wintertime blooms are largely the result of atmospheric forcing. Low-pressure systems moved through coastal area at frequent intervals and are accompanied by low air temperatures and rainfall. Runoff following the rainfall events supplies nutrients critical for bloom initiation and development. Heterocapsa triquetra blooms can reach chl a levels >100 mg L,1 and cell densities between 1 to 6×106 L,1. As the blooms develop, nutrient inputs from the river became insufficient to meet growth demand and H. triquetra feeds mixotrophically, reducing competition from co-occurring phytoplankton. Cloud cover associated with the low-pressure systems light limit H. triquetra growth as do low temperatures. More importantly though, low temperatures limit micro and macrozooplankton populations to such an extent that grazing losses are minimal. [source]

The extratropical transition of hurricane Irene (1999): A potential-vorticity perspective

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 598 2004
A. Agusti-panareda
Abstract Extratropical transition (ET) of tropical cyclones is common in all ocean basins where tropical cyclones recurve polewards. After the tropical cyclone experiences ET, a rapid deepening can take place resulting in the development of a very large and deep extratropical cyclone. The ET of hurricane Irene (1999) was an example of such an ,explosive' ET. Irene formed in the Caribbean and experienced ET as it moved poleward, resulting in a low-pressure system which deepened 39 hPa in 24 hours (according to the Met Office analyses). The extent to which the hurricane was responsible for the explosive extratropical development has been determined by performing Met Office Unified Model forecasts from initial states with and without the hurricane. The circulation and temperature anomalies associated with the hurricane were removed from the initial state using potential-vorticity inversion. The moisture anomaly co-located with the hurricane core was also removed. The results show that an extratropical cyclogenesis event takes place regardless of the presence of the hurricane in the initial conditions. However, the hurricane makes a significant difference to the track and central mean-sea-level pressure evolution of the resulting extratropical cyclone. When Irene was present the track of the extratropical cyclone was more zonal and the cyclone deepening rate was twice as fast as when Irene was not present. These effects appear to be particularly associated with a negative potential-vorticity anomaly and enhanced divergent flow in the region of the upper-level outflow of the transforming hurricane rather than with the hurricane vortex. Results also show that the presence of the hurricane resulted in a significant downstream surface-low development in the eastern Atlantic. Copyright © 2004 Royal Meteorological Society. [source]

METEOROLOGICAL TSUNAMIS IN SOUTHERN BRITAIN: AN HISTORICAL REVIEW,

GEOGRAPHICAL REVIEW, Issue 2 2009
SIMON K. HASLETT
ABSTRACT. Meteorological tsunamis, or meteo-tsunamis, are long-period waves that possess tsunami characteristics but are meteorological in origin, although they are not storm surges. In this article we investigate the coast of southern Britain-the English Channel, the Bristol Channel, and the Severn Estuary-for the occurrence of tsunami-like waves that, in the absence of associated seismic activity, we recognize as meteo-tsunamis. The passage of squall lines over the sea apparently generated three of these events, and two seem to have been far-traveled, long-period waves from mid-North Atlantic atmospheric low-pressure systems. The remaining three wave events appear to have been associated with storms that, among possible explanations, may have induced large-amplitude standing waves-such as seiches-or created long-period waves through the opposition of onshore gale-force winds and swells with high ebb tidal current velocities. This coastal hazard has resulted in damage and loss of life and should be considered in future coastal defense strategies and in beachuser risk assessments. [source]

Weather regimes and their connection to the winter rainfall in Portugal

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 1 2005
J.A. Santos
Abstract Wintertime rainfall over Portugal is strongly coupled with the large-scale atmospheric flow in the Euro-Atlantic sector. A K -means cluster analysis, on the space spanned by a subset of the empirical orthogonal functions of the daily mean sea-level pressure fields, is performed aiming to isolate the weather regimes responsible for the interannual variability of the winter precipitation. Each daily circulation pattern is keyed to a set of five weather regimes (C, W, NAO,, NAO+ and E). The dynamical structure of each regime substantiates the statistical properties of the respective rainfall distribution and validates the clustering technique. The C regime is related to low-pressure systems over the North Atlantic that induce southwesterly and westerly moist winds over the country. The W regime is characterized by westerly disturbed weather associated with low-pressure systems mainly located over northern Europe. The NAO, regime is manifested by weak low-pressure systems near Portugal. The NAO+ regime corresponds to a well-developed Azores high with generally settled and dry weather conditions. Finally, the E regime is related to anomalous strong easterly winds and rather dry conditions. Although the variability in the frequencies of occurrence of the C and NAO, regimes is largely dominant in the interannual variability of the winter rainfall throughout Portugal, the C regime is particularly meaningful over northern Portugal and the NAO, regime acquires higher relevance over southern Portugal. The inclusion of the W regime improves the description of the variability over northern and central Portugal. Dry weather conditions prevail in both the NAO+ and E regimes, with hardly any exceptions. The occurrence of the NAO+ and the NAO, regimes is also strongly coupled with the North Atlantic oscillation. Copyright © 2005 Royal Meteorological Society [source]