Sultan Hameed

Meteorologically driven trends in sea level rise

[1]xa0Determining the rate of global sea level rise (GSLR) during the past century is critical to understanding recent changes to the global climate system. However, this is complicated by non-tidal, short-term, local sea-level variability that is orders of magnitude greater than the trend. While the non-dimensional North Atlantic Oscillation (NAO) index can explain some of this variability in the Atlantic, significant results have been largely restricted to Europe. We show that dimensional indices of the position and intensity of the atmospheric centers of action (COAs) comprising the NAO are correlated with a major fraction of the variability and trend at 5 Atlantic Ocean tide gauges since 1900. COA fluctuations are shown to influence winds, pressure and sea-surface temperatures, thereby influencing sea level via a suite of coastal oceanographic processes. These findings reduce variability in regional sea level rise estimates and indicate a meteorological driver of sea-level trends.

The dominant influence of the Icelandic Low on the position of the Gulf Stream northwall

[1]xa0The location where the Gulf Stream separates from the American coast and turns eastward is called its northwall. The interannual fluctuations of the northwall are significantly correlated with the North Atlantic Oscillation with a lag of two years. When the Azores High and the Icelandic Low pressures are taken as independent variables, the latter dominates the relationship with the northwall, and the influence of the Azores High pressure is insignificant. This is consistent with the hypothesis that the major oceanic control of the northwall is in the southward flow of Labrador Sea Water into the Slope Sea. The alternative mechanism that the interaction of westward propagating Rossby waves with the American coast is responsible for northwall fluctuations is considered less likely because its initiation requires perturbations of the eastward winds in the mid-Atlantic region, and they are very likely dependent on the Azores High. The analysis suggests that the time lag between perturbations of the Icelandic Low and the northwall varies between one and three years.

On the variability of African dust transport across the Atlantic

[1]xa0We investigate the interannual variability of Saharan dust transport over the Atlantic by using the TOMS/Nimbus-7 and TOMS/Earth Probe daily aerosol data. We focus on the winter season, and on the area off the North-West African coast (15–30°N, 30–5°W). Previous studies have suggested that the variability of the Saharan dust can be partially explained by the North Atlantic Oscillation. In an alternative approach, we correlate the aerosol data with the positions and the surface pressure values of the two “Centers of Action” in the Atlantic, the Azores High and the Icelandic Low. This approach decouples the two semi-permanent pressure systems, and it reveals that the Icelandic Low does not play a role in the dust concentration for our region of interest. Instead, the position of the Azores High turns out to be the most important factor, with the Azores High latitude yielding the highest correlation (r = 0.48).

Saharan mineral dust transport into the Caribbean: Observed atmospheric controls and trends

[1]Each summer large amounts of mineral dust from the Sahara are transported across the Atlantic and arrive at the Caribbean with far-reaching implications for climate in this region. In this paper we analyze summer season interannual variability of North African mineral dust over the Caribbean using the Total Ozone Mapping Spectrometer (TOMS)/Nimbus 7 (1979‐1992) and TOMS/Earth Probe (1998‐2000) satellite aerosol data. We apply the ‘‘centers of action’’ approach to gain insight into the atmospheric controls on Saharan dust transport into the Caribbean and identify longitudinal displacement and pressure fluctuation of the Hawaiian High as well as longitudinal displacement of the Azores High as key players. In contrast, traditional indices such as the North Atlantic Oscillation and the Southern Oscillation are not correlated with the mineral dust variations over the Caribbean region. We utilize National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis to investigate the underlying physical mechanisms and to identify meteorological conditions that correspond to high and low dust loads. Our analysis shows that two different transport routes from distinct source regions are responsible for transporting mineral dust into the Caribbean: a northern mode in which dust mobilized from the Sahara travels westward controlled primarily by the Azores High and a southern mode in which intense dust clouds originating in the Sahel region travel over the Gulf of Guinea to reach the Caribbean. The latter is controlled primarily by teleconnections with the Hawaiian High.

On the widespread winter fog in northeastern Pakistan and India

During the last two winters widespread fog frequently occurred in northeastern India and Pakistan, in a region extending over 1500 km. A particularly severe fog episode lasted from mid-December, 1998 to early January, 1999. The fog caused extensive economic damage and disruptions in transport. We determined concentrations of SO42−, NO3−, and selected trace elements at Lahore, Pakistan during and after the fog event by collecting aerosols on Whatman 41 filters every 12 h. SO42− concentrations of up to 100 µg/m³ were observed during fog. The SO42−/Se ratios and trace element data suggest a distant source of SO42− aerosols, hundreds of kms away. Lahore was downwind of coal-burning in India during the fog. The high concentrations of SO42− observed suggest a more extensive investigation of the chemistry and transport processes in this region is necessary to delineate emission sources and develop control strategies as there are serious likely effects on human health and economy in a region populated by hundreds of millions of people, and on global climate change through direct and indirect forcing.

Influence of the Icelandic Low latitude on the frequency of Greenland tip jet events: Implications for Irminger Sea convection

[1] The occurrence of Greenland tip jet events has been reported as the dominant factor controlling the formation of intermediate water in the Irminger Sea. It has been suggested that the frequency of these events is correlated with the North Atlantic Oscillation. To examine this process in more detail, we separate the North Atlantic Oscillation into Icelandic Low and Azores High components and carry out a regression fit of the frequency of tip jet events between 1961 and 2005. Our findings suggest that the frequency of Greenland tip jet events is highly dependent on the latitude of the Icelandic Low and the 2-year time-lagged February Icelandic Low latitude, with R 2 = 0.48. We find that the winds near the southern tip of Greenland are predominately westerly during years when the Iceland Low is located above 63N latitude. These conditions also correspond to colder air temperatures in the Labrador and Irminger Seas, implying larger oceanic heat losses due to the Greenland tip jet events and hence stronger convective overturning in the Irminger Sea.

Precursors of Copepod Abundance in the Gulf of Maine in Atmospheric Centers of Action and Sea Surface Temperature

Interannual variations of total copepod abundance in the Gulf of Maine (1961-1991) are analyzed and related to the semi-permanent atmospheric pressure systems: Icelandic Low (IL) and the Azores High (AH). These centers of action dominate atmospheric and oceanic circulation in the North Atlantic. Cross-correlation analysis of zooplankton, sea surface temperature (SST) and the atmospheric characteristics have revealed different (from one to three years) time lags between the above characteristics. A multiple stepwise regression analysis gave a correlation value of 0.7 between observed and predicted interannual changes of total copepod abundance. A discussion is included of possible mechanisms that may contribute to the dynamic links that transfer atmospheric variation into observed changes in zooplankton abundance.

A mechanism for sun‐climate connection

[1]xa0Mechanisms by which small changes in the suns energy output during the solar cycle can cause changes in weather and climate have been a puzzle and the subject of intense research in recent decades. Here we report that differences in surface circulation conditions during solar maximum and minimum periods are caused by differences in the frequencies with which circulation perturbations in the stratosphere reach the surface. A much greater fraction of stratospheric perturbations penetrate to the surface during solar maximum conditions than during minimum conditions. This difference is more striking when the zonal wind direction in the tropics is from the west: no stratospheric signals reach the surface when equatorial 50 hPa winds are from the west under solar minimum conditions, and over 50 percent reach the surface under solar maximum conditions. It has been previously shown that stratospheric circulation perturbations reaching the surface change weather patterns by imposing atmospheric pressure anomalies characteristic of the Arctic oscillation.

Relationship of Sahel Precipitation and Atmospheric Centers of Action

The dynamics associated with drought in the Sahel have attracted considerable attention in the recent literature. This paper evaluates Sahel precipitation using the paradigm of the atmospheric centers of action, that is, the extended semipermanent highs and lows that dominate regional circulations and are evident in sea level pressure patterns. We find that Sahel precipitation is significantly influenced by changes in the Azores High and the South Asia Low. Specifically, about 50 percent of the variance of July to September rainfall over the Sahel is explained by changes in the Azores High Longitude position and South Asia Low pressure. In contrast, the contribution of the Southern Oscillation to Sahel precipitation is smaller in comparison. Results presented in this paper suggest that a key test for a climate model in simulating variability of Sahel rainfall is the accuracy with which the model simulates the dynamics of South Asia Low and the Azores High.

Evidence of a weakly absorbing intermediate mode of aerosols in AERONET data from Saharan and Sahelian sites

[1]xa0Accurate retrievals of aerosol size distribution are necessary to estimate aerosols impact on climate and human health. The inversions of the Aerosol Robotic Network (AERONET) usually retrieve bimodal distributions. However, when the inversion is applied to Saharan and Sahelian dust, an additional mode of intermediate size between the coarse and fine modes is sometimes seen. This mode explains peculiarities in the behavior of the Angstrom exponent, along with the fine mode fraction retrieved using the spectral deconvolution algorithm, observed in a March 2006 dust storm. For this study, 15 AERONET sites in northern Africa and on the Atlantic are examined to determine the frequency and properties of the intermediate mode. The mode is observed most frequently at Ilorin in Nigeria. It is also observed at Capo Verde and multiple sites located within the Sahel but much less frequently at sites in the northern Sahara and the Canary Islands. The presence of the intermediate mode coincides with increases in Angstrom exponent, fine mode fraction, single-scattering albedo, and to a lesser extent percent sphericity. The Angstrom exponent decreases with increasing optical depth at most sites when the intermediate mode is present, but the fine mode fraction does not. Single-scattering albedo does not steadily decrease with fine mode fraction when the intermediate mode is present, as it does in typical mixtures of dust and biomass-burning aerosols. Continued investigation is needed to further define the intermediate modes properties, determine why it differs from most Saharan dust, and identify its climate and health effects.