Abraham H. Oort

Observed Interannual Variability in the Hadley Circulation and Its Connection to ENSO

Abstract Based on a 26-yr set of daily global upper-air wind data for the period January 1964–December 1989, the interannual variability in the strength of the tropical Hadley cells is investigated. Although several measures of the intensity of the zonal-mean cells are discussed, the main focus is on the maximum in the streamfunction in the northern and southern Tropics. The streamfunction was computed from observed monthly mean latitude versus pressure cross sections of the zonal-mean meridional wind component. Significant seasonal variations are found in the strength, latitude, and height of the maximum streamfunction for both Hadley cells. Significant correlations are also observed between the Hadley cells and the El Nino-Southern Oscillation phenomenon. During the extreme seasons, only one “winter” Hadley cell dominates the Tropics, with the rising branch in the summer hemisphere and the sinking branch in the winter hemisphere. Superimposed on this “normal” one-cell winter Hadley circulation in the Tr...

Global Climate Variations Connected with Sea Surface Temperature Anomalies in the Eastern Equatorial Pacific Ocean for the 1958–73 Period

Abstract The sea surface temperature anomalies in the eastern equatorial Pacific Ocean are shown to demarcate a “key region” new 130°W for observed variations in the global general circulation. Various techniques are used to describe global conditions during warm and cold sea surface conditions in the key region based on a global 15-year set of surface and upper air analyses. Earlier work is confirmed that during warm episodes 1) the westerly jets in both hemispheres are strengthened, and 2) some of the semi-permanent circulation features, such as the intertropical convergence zone, the subtropical high over the western and central Pacific, and the Aleutian low, increase in strength, and vice versa for cold episodes. The time series of monthly-mean atmospheric temperature averaged over the entire man of the Northern Hemisphere is found to be highly correlated with the sea surface temperature anomalies in the key region. The highest correlation of r = 0.65 is found when the atmosphere lags the ocean by 6 m...

ON ESTIMATES OF THE ATMOSPHERIC ENERGY CYCLE

Abstract Several estimates of the rate at which the mean and eddy forms of both kinetic energy and available potential energy are generated, converted, and dissipated in the atmosphere are compared in tabular form. From these tables a selection is made of those values which are, in the authors opinion, representative for the yearly energy cycle in the Northern Hemisphere.

Global Angular Momentum and Energy Balance Requirements from Observations

Publisher Summary This chapter presents an overview of global angular momentum and energy balance requirements deduced through observations. The structure of the basic circulation, the temperature, humidity, and kinetic energy fields, both regarding their long-term mean values and their day-to-day and seasonal variability characteristics, are discussed. Following this, the three-dimensional structure of the transports by the transient eddy, stationary eddy, and mean meridional circulations are explained. The chapter describes how the source regions of incoming solar radiation and the sink regions of outgoing terrestrial radiation are distributed over the globe, and how the atmosphere and oceans respond to these forcing factors. It also explores what happens to the solar energy once it has entered the atmosphere. The storage and transport of energy in the atmosphere is analyzed, followed by a discussion on the amount of energy that flows into the underlying surface. The storage and transport of energy in the oceans is also discussed..

The Climatology of Relative Humidity in the Atmosphere

Abstract The present paper deals with the analysis of the time-average relative humidity fields in the atmosphere. Twice-daily estimates of relative humidity are used. After some theoretical considerations on the relations between relative humidity, other moisture parameters, and temperature, a critical analysis of the various sources of data is made considering their possible limitations. Various methods of computing relative humidity are formulated and discussed. The global distribution of relative humidity at various levels shows that it is not zonally uniform with centers of various intensities at all latitudes. The global maps show maxima in the equatorial zone and minima in the dry subtropical belts around 30°N and 30°S. The land–sea contrast and variations related to the orographic relief are also apparent. The general pattern of relative humidity is similar at all levels but its magnitude decreases with altitude. The seasonal analyses show a similar pattern as the annual analyses but are slightly ...

Entropy budget of the atmosphere

The balance equations for the entropy in the atmosphere are presented and discussed. Using observed energy fluxes and atmospheric temperatures, we present estimates of the various terms in the global mean entropy budget. The largest boundary fluxes of entropy are associated with the emission of longwave radiation. The fluxes of entropy associated with turbulent and molecular diffusion are found to be much smaller. On the planetary scale the mean outgoing flux of entropy at the top of the atmosphere is found to be about 22 times larger than the mean incoming flux of entropy through solar radiation. The rates of entropy production and destruction by the various irreversible processes that occur in the atmosphere are also computed. The entropy production terms involved in the release of latent heat and the absorption of solar radiation are, by far, the largest sources of entropy for the atmosphere, whereas the production of entropy associated with the absorption of longwave radiation is an order of magnitude smaller. The destruction is mainly accomplished through cooling by outgoing longwave radiation. The contributions of the sensible heat fluxes and friction are relatively small. Regional contributions to the total entropy generation in the atmosphere are studied by considering an equatorial region bounded by two latitudinal walls at 15oN and 15oS and a polar region poleward of 70oN. The rates of entropy generation by the various diabatic processes are highest in the equatorial region; part of the generated entropy is exported to higher latitudes. Most natural phenomena occurring in the climate system are characterized by great irreversibility. For example, the turbulent motion in the planetary boundary layer does not spontaneously develop into the large-scale organized flow of the general circulation; a cloud cannot be reconstituted from the same water it lost previously through precipitation; rivers do not flow backward from the sea to their headwaters; and ocean water does not decompose spontaneously into oxygen and hydrogen. We will analyze the entropy budget of the atmosphere since it is important for the study of the thermodynamical behavior of the climatic system and, in particular, for atmospheric phenomena. The second law for an isolated system,

ON THE ANNUAL VARIATION OF THE MONTHLY MEAN MERIDIONAL CIRCULATION

Abstract Data for a 5-yr period from a dense network of upper air stations have been used to determine the annual cycle in the mean meridional circulation north of 15°S. Only during the transition months April, May and October, November is there some degree of symmetry with respect to the Equator. During the other months of the year, the Hadley cell of the winter hemisphere with a maximum strength of about 23 × 1013 gm sec–1 appears always to dominate the circulation. The Hadley cell of the summer hemisphere practically disappears, except possibly near the surface. Maximum meridional velocities connected with the winter Hadley cell are about 2.5 m sec–1 near 1000 mb and over 3 m sec–1 near 200 mb. Mean vertical velocities attain values of about 5 and 8 mm sec–1 in the downward and upward branches of the winter Hadley cells. A rather weak Ferrel circulation (about 4 × 1013 gm sec–1) and a very weak polar circulation (about 1 × 1013 gm sec–1) are computed in middle and high latitudes throughout the year. Wi...

Estimates of the energy cycle of the oceans

A new formulation of the general problem of the large-scale energetics for the global oceans is presented and analyzed. Using a variety of ocean surface observations, some of the terms in the energy balance equations, such as the time rates of change, the generation rates of available gravitational potential energy G(P) and kinetic energy G(K), and the conversion rate from available gravitational potential energy into kinetic energy C(P, K), are estimated for annual and seasonal mean conditions. An attempt is also made to measure the uncertainty of these terms in order to assess the reliability of the results. The interseasonal variability is analyzed showing that the winter hemisphere represents the most active region of the globe. Using the data and some reasonable assumptions, pictures of the energy cycle in the global oceans for yearly and seasonal conditions are constructed. The hemispheric and global dissipation rates of available gravitational potential energy D(P) and of kinetic energy D(K) are estimated as residuals, assuming that the contribution from the internal energy C(I, K) is only a minor factor. After analyzing the general consistency of the picture of the energy cycle for the oceans, the conclusion is reached that the G(P) and G(K) terms are about equally important terms needed to describe and understand the structure and dynamics of the global ocean circulation.

Historical trends in the surface temperature over the oceans based on the COADS

Based on a new and unique compilation of surface ship data over the world ocean, the Comprehensive Ocean-Atmosphere Data Set (COADS), the temperature records since 1870 were thoroughly analyzed. Results for the air and sea surface temperature are presented for the two hemispheres and for individual 10°-latitude wide belts. The results support most earlier results of a relatively warm period over the oceans in the late 19th century, a cool period between about 1905 and 1930, and a warm period from the 1940s onward. Of considerable interest and perhaps unique in the historical record are the diverging trends in the midlatitude sea and air temperatures of the two hemispheres during the 1960s and early 1970s with relative cooling in the Northern Hemisphere and relative heating in the Southern Hemisphere.

Correlation analyses between sea surface temperature anomalies in the eastern equatorial Pacific and the world ocean

Based on data from the Comprehensive Ocean-Atmosphere Data Set (COADS), objective analyses of the monthly mean sea surface temperature (SST) were prepared at GFDL for each month of the 110-year period 1870–1979. Time series of various indices characterizing the SST anomalies averaged over the eastern equatorial Pacific (EEP), the tropical oceans and the world ocean are presented for monthly, yearly and decadal time-averaging periods. Global correlations maps are given for each decade of the 1870–1979 period. They show the spatial connections between the monthly SST anomalies in the EEP and in other parts of the world ocean and how these connections vary for the different decades. On the intermonthly time scale the SST anomalies in the EEP and those in the tropical and world oceans are found to be highly correlated, with maximum correlations values of 0.91 at zero lag for the tropical oceans during the 1950–1959 decade and 0.81 for the world ocean during the 1970–1979 decade. Positive correlation values of r⩾0.36 persist on average from about 4 months before to about 8 months after the EEP anomalies occur. There is a clear tendency for the tropical and world ocean anomalies to lag behind the EEP anomalies. Comparing different oceans, we find the tendency for the tropical SST anomalies in the Indian and Atlantic Oceans to lag behind those in the EEP region by about 1 and 3 months, respectively. On the interannual time scale the EEP anomalies are also well correlated with those in the other regions, having an average correlation of 0.84 for the tropical oceans and of about 0.7 for the world ocean.