Search Results

You are looking at 1 - 10 of 15 items for

  • Author or Editor: Sultan Hameed x
  • All content x
Clear All Modify Search
Sultan Hameed and Iqbal Pittalwala

Abstract

No abstract available.

Full access
Antoni Jordi and Sultan Hameed

Abstract

The authors investigate the interannual variability of surface air temperature in the Gulf of Lion (northwestern Mediterranean Sea) during the winter season, which has been proposed as the dominant factor for the Mediterranean Intermediate Water (WIW) formation. Recent studies suggest that the variability of WIW formation in the Gulf of Lion can be partially explained by the North Atlantic Oscillation. To examine this process in more detail, the authors separate the North Atlantic Oscillation into two centers of action, the Azores high and the Icelandic low. This approach reveals that the pressure of the Icelandic low controls the surface air temperature in the Gulf of Lion, and the influence of the Azores high is insignificant. It is found that the winds over Europe are predominately northerly during winters with high pressure values of the Icelandic low. These conditions also correspond to colder air temperatures in the Gulf of Lion, which have been proposed previously to be correlated with ocean convection to intermediate depths.

Full access
Nicholas Leonardo and Sultan Hameed

Abstract

Past investigations have shown that interannual variability in winter precipitation over the western United States is related to large-scale sea level pressure fluctuations. California is adjacent to the North Pacific subtropical high, also known as the Hawaiian high. This study analyzes the relationship between interannual variations of winter precipitation in California and fluctuations in the Hawaiian high’s pressure and location. Defining objective indices to characterize the high, it is shown that precipitation in California is significantly anticorrelated with the high’s intensity. Precipitation is also shown to decrease as the high shifts southward or eastward. A linear regression model of December–March precipitation averaged over all of California with the pressure and longitude of the high as independent variables explains 43% of the precipitation variance during 1949–2012. Variation of the Hawaiian high’s pressure is the major factor impacting winter precipitation in both Northern and Southern California. Smaller contributions are made by variations of the longitudinal position of the Hawaiian high to Northern California and by ENSO to Southern California. By comparison, the Pacific–North American (PNA) pattern does not significantly impact winter precipitation over California. The interannual fluctuations of the Hawaiian high’s pressure in winter are related to diabatic heating over the tropical Pacific and the Aleutian region, and are not related to diabatic heating over the West Coast of the United States. The Hawaiian high’s pressure and its latitude and longitude positions do not show decadal trends, and their interannual variations are not correlated with air temperatures averaged over the Northern Hemisphere.

Full access
Kristina I. Kirkyla and Sultan Hameed

Abstract

Using precipitation values obtained from a version of the Oregon State University general circulation model and observational gridded data, harmonic analysis has been employed to study the seasonal variation of precipitation over the conterminous United States. Maps of the first, second and third harmonic amplitudes and phases provide a useful source of comparison between model output and observational data. Results indicate that the method of harmonic analysis allows a more analytical comparison between model predictions and data than the conventional approach of representing the annual march in the form of a curve of mean monthly rainfall amounts. The method delineates regional boundaries of the various precipitation regimes in the United States. The GCM captures a significant amount of the regional detail in precipitation climatology when its results are decomposed by harmonic analysis.

Full access
Bradford S. Barrett and Sultan Hameed

Abstract

Monthly precipitation in Chile (30°–55°S) was found to vary by intensity, latitude, and longitude of the South Pacific high (SPH). In austral winter, precipitation was higher when the SPH was weaker and when it was centered farther west. In austral spring, precipitation was higher when the SPH was weaker, similar to winter. However, spring precipitation was not found to be related to SPH longitude, and higher precipitation was found when the SPH was centered farther north. In austral summer, no relationship was found between precipitation and either SPH intensity or longitude, but positive correlations were found between precipitation and latitude of the SPH. In austral autumn, correlation patterns between precipitation and all three SPH metrics more closely resembled those seen in winter. The results of a multiple linear regression confirmed the importance of two SPH metrics (intensity and longitude) and the unimportance of a third SPH metric (latitude) in understanding variability in winter, summer, and autumn precipitation in central and southern Chile. In spring, regression results confirmed a relationship between precipitation and SPH intensity and latitude. Furthermore, the SPH intensity and longitude in winter combined to hindcast monthly precipitation with a better goodness of fit than five El Niño–Southern Oscillation metrics traditionally related to Chilean precipitation. Anomalies of lower-tropospheric circulation and vertical velocities were found to support the observed relationships between SPH and precipitation. Based on these results, a physical mechanism is proposed that employs the SPH as a metric to aid in understanding variability in precipitation in central and south-central Chile in all seasons.

Full access
Sultan Hameed, Alan Meinster, and Kenneth R. Sperber

Abstract

The Oregon State University coupled upper ocean-atmosphere GCM has been shown to qualitatively simulate the Southern Oscillation. A composite analysis of the warm and cold events simulated in this 23-year integration has been performed. During the low phase of the Southern Oscillation, when warm anomalies occur in the eastern Pacific, the model simulates for the Atlantic region during March–May 1) a deficit of precipitation over the tropical South American continent, 2) Caribbean and Gulf of Mexico sea level pressure and sea surface temperature are in phase with the eastern Pacific anomalies, while those east of the Nordeste region are out of phase, and 3) northeast trade winds are anomalously weak and southwest trade winds are anomalously strong (as inferred from surface current anomalies). The anomalies in the oceanic processes are induced by perturbations in the atmospheric circulation over the Atlantic and are coupled to changes in the Walker circulation. During the high phase of the simulated Southern Oscillation, conditions in the atmosphere and ocean are essentially the reverse of the low phase. The model produces a response in the South American region during the opposing phases of the Southern Oscillation that is in general agreement with observations.

The interannual variation of Nordeste rainfall is shown to be dominated by a few band-limited frequencies. These frequencies are found in the SST series of those regions of the Atlantic and Pacific oceans where strong correlations with Nordeste precipitation exist.

Full access
Lequan Chi, Sultan Hameed, and Christopher L. P. Wolfe

Abstract

The path of the Gulf Stream as it leaves the continental shelf near Cape Hatteras is marked by a sharp gradient in ocean temperature known as the North Wall. The latitude location of the Gulf Stream North Wall (GSNW) has previously been estimated by subjective analysis of daily maps of sea surface temperatures. Recently, presented an objective procedure by fitting an error function to the SST profile across the Gulf Stream at 81 longitude positions. The fit smooths over not only the GSNW but also the much colder waters from the Labrador Sea on the continental shelf. Watelet et al.’s procedure is therefore likely to misidentify the shelf-slope front as the Gulf Stream North Wall, leading to a systematic northward bias the in North Wall position.

Full access
Jae N. Lee, Drew T. Shindell, and Sultan Hameed

Abstract

The response of the seasonal tropical circulation to an 11-yr solar cycle forcing is studied with the Goddard Institute for Space Studies (GISS) ModelE, which includes fully interactive atmospheric chemistry. To identify characteristic solar signals in the tropical circulation, the model experiments are carried out with certain imposed conditions: a doubly amplified solar forcing and the present-day and preindustrial greenhouse gases and aerosol conditions, with the mixed layer or fully coupled dynamic ocean model. In both the model and the NCEP reanalysis, tropical humidity increases in response to enhanced solar irradiance are found to be statistically significant in both solstice seasons. Changes are also found in the vertical velocities for both the Hadley and Walker circulations in some areas of the Pacific region. With present-day greenhouse gas and aerosol conditions, the ascending branch of the Hadley cell is enhanced near the equator, and the intertropical convergence zone (ITCZ) is shifted northward in response to solar forcing during the boreal winter. Enhancement of the meridionally averaged vertical velocity over the western Pacific indicates strengthening of the Walker circulation in response to solar forcing in both solstice seasons. In present-day conditions, the tropical circulation response to an 11-yr solar forcing is generally consistent with that derived from previous observational works.

Full access
Alejandra Sanchez-Franks, Sultan Hameed, and Robert E. Wilson

Abstract

The Gulf Stream’s north wall east of Cape Hatteras marks the abrupt change in velocity and water properties between the slope sea to the north and the Gulf Stream itself. An index of the north wall position constructed by Taylor and Stephens, called Gulf Stream north wall (GSNW), is analyzed in terms of interannual changes in the Icelandic low (IL) pressure anomaly and longitudinal displacement. Sea surface temperature (SST) composites suggest that when IL pressure is anomalously low, there are lower temperatures in the Labrador Sea and south of the Grand Banks. Two years later, warm SST anomalies are seen over the Northern Recirculation Gyre and a northward shift in the GSNW occurs. Similar changes in SSTs occur during winters in which the IL is anomalously west, resulting in a northward displacement of the GSNW 3 years later. Although time lags of 2 and 3 years between the IL and the GSNW are used in the calculations, it is shown that lags with respect to each atmospheric variable are statistically significant at the 5% level over a range of years. Utilizing the appropriate time lags between the GSNW index and the IL pressure and longitude, as well as the Southern Oscillation index, a regression prediction scheme is developed for forecasting the GSNW with a lead time of 1 year. This scheme, which uses only prior information, was used to forecast the GSNW from 1994 to 2015. The correlation between the observed and forecasted values for 1994–2014 was 0.60, significant at the 1% level. The predicted value for 2015 indicates a small northward shift of the GSNW from its 2014 position.

Full access
Meredith S. Croke, Robert D. Cess, and Sultan Hameed

Abstract

Land-based observations of cloud cover, for the period 1900–87 and averaged over three geographical regions of the United States (coastal southwest, coastal northeast, and southern plains), show strong positive correlations with one estimate of global mean surface temperature, a finding consistent with prior investigations that suggest cloud cover over land has increased during global warm periods relative to cold periods. It is also found that the strengths of three permanent high/low pressure systems (North Pacific high, Icelandic low, and Azores high) are negatively correlated with global mean surface temperature, suggesting a possible connection between regional cloud cover, for certain locations, and the strengths of adjacent high/low pressure systems. Specifically, for the regions considered it is suggested that the coastal southwest cloud cover is related to changes in the strength of the subtropical North Pacific high, that for the southern plains also to the strength of the North Pacific high, and that for the coastal northeast to the strength of the Icelandic low. Thus the climate-induced change in cloud cover for certain regions appears related, at least in part, to climate-induced change in the strengths of adjacent high/low pressure systems, and plausible physical explanations for this relation are provided for the three regions that have been studied. This does not, of course, provide a direct physical cause-and-effect explanation for the changes in regional cloud cover, because the mechanisms that cause the intensities of the high/low pressure systems to change are not understood.

Full access