519 publications from this institution
The incoming energy to the Earth system is in the form of solar radiation and roughly corresponds to that of a black body at the temperature of the Sun of about 6000 K. The Sun’s emissions peak at a wavelength of about 0.6 μm and much of this energy is in the visible part of the electromagnetic spectrum although some extends beyond the red into the infrared and some extends beyond the violet into the ultraviolet (Fig. 4.5). As noted earlier, because of the very nearly spherical shape of Earth, at any time half of Earth is in night (Fig. 1.2) and the average amount of energy incident on a level surface outside the atmosphere is one-quarter of the total solar irradiance, or 340 W m−2. About 30% of this energy is scattered or reflected back to space by air molecules, tiny airborne particles (known as aerosols), clouds in the atmosphere, or by Earth’s surface, which leaves about 240 W m−2 on average to warm Earth’s surface and atmosphere (Fig. 3.1).
No abstract is provided for this article.
Over the past decade, reanalyses of multi‐decadal series of past observations have become established as an important and widely utilized resource for the study of atmospheric and oceanic processes and predictability Reanalyses, which are produced using modern versions of the data assimilation systems developed for numerical weather prediction (NWP),also are being applied increasingly in many other fields that require a record of the state of either the atmosphere or its underlying land and ocean surfaces. Estimation of renewable energy resources, calculation of microwave telecommunication signal losses, and study of bird migration are just three examples. High‐resolution operational NWP systems provide good quality analyses for timely study of recent conditions, including climatic extremes. However, changes made to improve the operational systems introduce inhomogeneities in time series of operational analyses that limit their utility for studies of longer‐term climate variability and change. Lower‐resolution reanalyses produced using an up‐to‐date data assimilation system provide products for all but the last few years that are generally superior to those available from the archives of past operational products. The reanalysis products are, by design, more suitable than their operational counterparts for use in studies of longer‐term variability in climate, although they remain susceptible to changes in the observing system that can make accurate depiction of long‐term trends problematic.
The development of a nine-layer, quasi-geostrophic, highly truncated spectral model of the atmosphere is described. The model is global, extends to 0.05 mb (71 km) with roughly 10-km resolution in the stratosphere, and includes an annual heating cycle. Preliminary integrations without eddies reveal the seasonal variation of a thermally driven circulation. A model integration was made to simulate the months of December and January without nonzonal forcing, thus being more representative of a Southern Hemisphere winter. The overall features of the atmosphere were well simulated. A midlatitude temperature maximum was produced in the winter mesosphere of the model, which was driven in the manner of the lower stratosphere. With the inclusion of the annual heating cycle, the model successfully reproduced a more intense circulation in January than existed in December. This caused the maximum tropospheric meridional temperature gradient in the winter hemisphere to occur weeks prior to the maximum in the external heating field. A seasonally coupled index cycle in the very long waves was of significance in producing transient upward energy propagation and, as such, may be the source of sudden stratospheric warming events.
Indices of the southern hemisphere circulation are derived for the Australasian area using sea level pressure data. As well as considering indices of zonal and meridional flow, indices are derived based on the empirical orthogonal function analysis of Trenberth (1975). The first two patterns, P1 and P2, of this analysis can be approximately represented by only a few station values, and in this way their time series can be extended. ‘Regimes’ of weather are defined based on the short term mean behaviour of P1 and P2. These regimes are also characterized by large changes in the periodicities at which most of the variance occurs. The quasibiennial oscillation shows up as a fairly stable feature of the meridional indices and P2. Trends in both P1 and P2 since the 1940s are related to the rising temperature trend in New Zealand. There has been a trend for less westerly especially between 25 and 45°S, some increase in westerly south of 45°S and less southwesterly or more northeasterly component to the flow across the whole of New Zealand as a result of the increased tendency for anticyclones to persist to the east of New Zealand rather than over Australia and the Tasman Sea, and their preference for higher latitude blocking.
The consistency of analyzed changes in surface wind stress, sea level pressures and surface temperatures between 1980–86 and previous periods indicates the reality of statistically significant and substantial climate changes in the Northern Hemisphere, especially over the North Pacific, on decadal time scales. Cooling in North Pacific sea surface temperatures and warming along the west coast of North America and Alaska are ascribed mainly to the changes in thermal advection associated with a deeper and more extensive Aleutian Low.
The utility of a simple index for monitoring the Southern Oscillation signal is explored in detail. Based upon sea level pressure data at the two stations Tahiti (T) and Darwin (D), an optimal index, in the sense that it combines the Southern Oscillation variance into one series is the combination [Tn + Dn] where the subscript n denotes normalization by the overall standard deviation of each series. A direct measure of the noise due to small-scale or transient phenomena that are not a part of the large-scale coherent Southern Oscillation fluctuations is the index [Tn + Dn]. It is recommended that this index of noise also should be monitored in order to determine the representativeness of the Southern Oscillation index. The signal-to-noise ratio is shown to depend upon the cross correlation between Darwin and Tahiti, and can be increased by applying weighted moving average low-pass filters to the data. Monthly data exhibit a signal-to-noise ratio, defined as the ratio of the standard deviations, of 1.44 and this increases to 1.97 for seasonal data. An 11-term low-pass filter is designed that increases the signal-to-noise ratio to 2.70 without adversely reducing the variance in frequencies that are important in the Southern Oscillation. Resulting time series plots are presented.
An approximate formulation of how much moisture that precipitates out comes from local evaporation versus horizontal transport, referred to as "recycling," has allowed new estimates of recycling to be mapped globally as a function of length scale. The recycling is formulated in terms of the "intensity of the hydrological cycle" I, which is alternatively referred to as a "precipitation efficiency" as it denotes the fraction of moisture flowing through a region that is precipitated out, and a "moistening efficiency," M, which is defined as the fraction of moisture evaporated from a region to that flowing through. While datasets of the pertinent quantities have improved, they still contain uncertainties. Results show that often the intensity is not greatest at times of greatest precipitation because moisture transport into the region is also a maximum, especially in the monsoonal regions. The annual cycle variations of I are fairly small over North America and Europe while large seasonal variations in M occur in most places. Seasonal mean maps of precipitation, evaporation (E), and atmospheric moisture transport are presented and discussed along with the seasonal and annual means of derived precipitation and moisture efficiencies and the recycling fraction. The recycling results depend greatly on the scale of the domain under consideration and global maps of the recycling for seasonal and annual means are produced for 500- and 1000-km scales that therefore allow the heterogeneity of the fields across river basins to be captured. Global annual mean recycling for 500-km scales is 9.6%, consisting of 8.9% over land and 9.9% over the oceans. Even for 1000-km scales, less than 20% of the annual precipitation typically comes from evaporation within that domain. Over the Amazon, strong advection of moisture dominates the supply of atmospheric moisture over much of the river basin but local evaporation is much more prominent over the southern parts, and, for the annual cycle as a whole, about 34% of the moisture is recycled. Over the Mississippi Basin, the recycling is about 21%. The smaller number mostly reflects the smaller domain size. Relatively high annual values of recycling (>20%) occur in the subtropical highs, where E is high and the advective moisture flux is small, and in convergence zones where, again, the advective moisture flux is small. Low annual values occur over the southern oceans, the North Pacific, and the eastern equatorial Pacific, where the moisture flux is at a maximum.
Computations of the surface wind stress and pseudostress over the global oceans have been made using surface winds from the European Centre for Medium Range Weather Forecasts for 7 years. The drag coefficient is a function of wind speed and atmospheric stability, and the air density is computed for each observation. Assuming a constant density, the effective drag coefficient required to convert the pseudostress into a stress has been computed for each month of the year using several methods. Because the drag coefficient varies from day-to-day and with the seasons, the effective drag coefficient cannot be uniquely defined and is a useful concept if only the very gross characteristics of the field are of interest and errors of the order of 10% are tolerable. Even then, the spatial and seasonal variations in CD must be taken into amount, and occasionally the wind stress may be greatly in error.
No abstract is provided for this article.
In this study, we investigate trends in total column water vapour (TCWV) retrieved from measurements of the Ozone Monitoring Instrument (OMI) for the time range between January 2005 to December 2020. The trend analysis reveals on global average an annual increase in the TCWV amount of approximately +0.056 kg m−2 y−1 or +0.24 % y−1. After the application of a Z-test (to the significance level of 5 %) and a false discovery rate test to the results of the trend analysis, mainly positive trends remain, in particular over the Northern subtropics in the East Pacific. Combining the relative TCWV trends with trends in air temperature, we also analyze trends in relative humidity (RH) on local scale. This analysis reveals that the assumption of temporally invariant RH is not always fulfilled: we obtain increasing and decreasing RH trends over large areas of the ocean and land surface and also observe that these trends are not limited to arid and humid regions, respectively. For instance, we find decreasing RH trends over the (humid) tropical Pacific ocean in the region of the intertropical convergence zone. Interestingly, these decreasing RH trends in the tropical Pacific ocean coincide well to decreasing trends in precipitation. Additional investigations of the global response of TCWV to changes in (surface) air temperature show that the relative TCWV trends do not follow a Clausius-Clapeyron response (i.e. 6–7 % K−1) and are about 2 to 3 times higher even for the case of global averages. Moreover, by combining the trends of TCWV, surface temperature, and precipitation we derive trends for the global water vapour turnover time (TUT) of approximately +0.02 d y−1. Also, we obtain a TUT rate of change of around 11 % K−1 which is 2 to 4 times higher than the values obtained in previous studies.
No abstract is provided for this article.
No abstract is provided for this article.
An analysis has been made of the mean height and wind fields throughout the troposphere of the Southern Hemisphere for May 1972–January 1978 based on daily synoptic hemispheric analyses of geopotential height produced by the Australian World Meteorological Center. While the means resemble those previously obtained in a series of atlases based primarily on the period 1957–66, significant differences do exist. The reality of these trends is investigated using station data. Some of the changes in the means are due to different analysis techniques, inhomogeneities in the data used by the atlases, and changes in the station network and data availability. Consequently, spurious trends are evident in the South Pacific near Rapa Island where the data network has improved, and near Easter Island and southern South America where the Australian analyses at 0000 GMT are less reliable than elsewhere. The geopotential height increases over Antarctica and height decreases over the subtropical continents appear to be mostly real, whereas the increased intensity of the oceanic subtropical highs cannot be confirmed although this probably reflects a real change in the circulation. Nevertheless, there is a distinctive overall pattern to the trends. Between the periods 1957–66 and 1972–78 the relatively warm atmosphere over the subtropical continents has become cooler, particularly in the western regions, while over the oceans the atmosphere has warmed slightly. In high latitudes, atmospheric temperature contrasts in the warm and cool sectors of Antarctica have become further enhanced. The close association between the trends and the distribution of continents is perhaps a hint that the trends may be related to changes in continentality.
