New estimates of freshwater discharge from continents were derived using stream-flow records from the world's largest 921 rivers, supplemented with estimates of discharge from unmonitored areas based on the ratios of runoff and drainage area between the unmonitored and monitored regions. The farthest downstream river-flow data were extrapolated to the river mouth using river transport model simulations forced by a runoff field. This new continental discharge estimate was then applied to estimate the meridional transport of freshwater within the oceans. The relatively new estimates of net water fluxes (P-E) over ocean surfaces derived from atmospheric moisture budget analyses based on the NCEP/NCAR and ECMWF reanalyses were used in the calculation of oceanic freshwater transport. Our results, which are improved in many aspects compared with previous estimates, show that global continental discharge is about 37288 km 3 yr -1 (1.2 Sv, 1 Sv = 1×10 6 m 3 s -1 ) or about 35% of terrestrial precipitation. Compared with earlier indirect estimates of oceanic freshwater transport, our new estimates derived using the 921- river based discharge and the ECMWF reanalysis based P-E show improved agreement with available direct estimates for the Atlantic, Pacific, and Indian Ocean basins. The new estimates also show increased southward transports in the Atlantic Ocean and increased northward transports in the South Pacific Ocean.
Projections of future climate change depend largely on the results of computer models. Such models are becoming increasingly sophisticated, but they do not offer the certainties that policy-makers would like.
Global Earth’s Energy Imbalance (EEI) is a fundamental metric of climate change, and the local distribution of the imbalance has implications for regional climate variations. It has been a major challenge to rein in the uncertainties and reasonably establish the EEI. Previous chapters have exploited the local energy imbalance estimates to examine resulting heat transports and anomalies, and teleconnections. The atmosphere and oceans are dynamically active, and many phenomena attempt to move heat to where it can best be either lost in some sense, such as by radiation to space, or perhaps sequestered by being buried deep in the ocean. Although there is somewhat constrained effectiveness in many phenomena, such as hurricanes or ENSO, in redistributing heat and keeping regions cooler than they otherwise would be, these aspects are often not replicated well in climate models. Hence it is vital to understand the net heat gain, and how much and where heat is distributed within the Earth system. How much heat might be readily purged and serve as a negative feedback to warming?
The IPCC (Intergovernmental Panel on Climate Change) is a primary user of studies of the ocean and how it is changing.Following an introduction to the IPCC, the role of the ocean in climate is outlined, and the main findings from the 2007 IPCC assessment are described, along with outstanding issues for ocean science.The progress in addressing these issues is briefly discussed along with some recommendations for ocean observations.
The Sun is the center of our climate system. It provides a fairly steady stream of radiant energy to Earth, and in order to keep from heating up, Earth in turn radiates energy back out into space. In between the incoming and outgoing radiation are all of the rich complex processes involved in the climate system including all of the weather systems, the entire hydrological cycle, the ocean, land and ice, and the multitude of forms of heat and energy on the planet. The internal interactive components in the climate system (Fig. 1.1) include the atmosphere, oceans, ice, and land.
No abstract is provided for this article.
No abstract is provided for this article.
No abstract is provided for this article.
The hydrological cycle fundamentally involves evaporation (E) of moisture from the land and vegetation or ocean surface into the atmosphere, and back again as precipitation (P) (Fig. 10.1). Evaporation produces evaporative cooling at the surface and moistens the air. It includes transpiration from plants in which water enters the atmosphere through the tiny stomata in leaves as photosynthesis occurs. Together these are called evapotranspiration. Precipitation and the relationships to humidity and sea surface temperatures (SSTs) were introduced in Section 5.4. Water vapor is moved around by the atmosphere, and the precipitation occurs elsewhere, often in preferred locations such as mid-latitude storm tracks or tropical monsoons and convergence zones.
An analysis of the spring–summer 1988 drought and 1993 floods over North America reveals a reversal in the sign of anomalies in several fields. Large sea surface temperature anomalies of opposite signs existed in the tropical Pacific with strong La Niña conditions in 1988 and a mature El Niño in 1993. The distribution of tropical convection in the convergence zones and associated latent heating of the atmosphere were correspondingly altered, implying a large-scale switch in the anomalous tropical heating and forcing of extratropical quasi-stationary waves in the atmosphere, influencing the subtropical jet stream over the North Pacific and across North America. In 1988 the jet stream and the closely related storm track of high-frequency disturbances in the upper troposphere were displaced into Canada well north of the normal location—the farthest north of any year from 1979 to 1993. In 1993 a broader jet stream and the storm track were displaced well south of normal to a more springlike location across the United States—the farthest south by over 200 km of any year from 1979 to 1993. High-frequency eddy activity in the Pacific-North American storm track is shown to reinforce the anomalous jet streams in both years. An analysis of the moisture budgets reveals a stronger river of atmospheric moisture flowing across the Gulf of Mexico into the central and eastern United States in 1993. Also, in the lower atmosphere, the storm track in 1993 was more active, and its lower latitude allowed the cyclonic disturbances to tap into the moisture source, transport moisture into the upper Mississippi River basin, and precipitate it out. It is deduced that local evaporation may have enhanced the precipitation and helped perpetuate and prolong the conditions. In contrast, in 1988 disturbances were weaker and displaced far enough north to avoid most of the moisture source, and the drought was perpetuated by the dry conditions. Consequently, these effects should be viewed as feedbacks that amplify and prolong the response, while from the standpoint of the atmosphere, the anomalous tropical Pacific sea surface temperatures are a notable (but not the sole) external forcing of the patterns.
An observable constraint on climate sensitivity, based on variations in mid-tropospheric relative humidity (RH) and their impact on clouds, is proposed. We show that the tropics and subtropics are linked by teleconnections that induce seasonal RH variations that relate strongly to albedo (via clouds), and that this covariability is mimicked in a warming climate. A present-day analog for future trends is thus identified whereby the intensity of subtropical dry zones in models associated with the boreal monsoon is strongly linked to projected cloud trends, reflected solar radiation, and model sensitivity. Many models, particularly those with low climate sensitivity, fail to adequately resolve these teleconnections and hence are identifiably biased. Improving model fidelity in matching observed variations provides a viable path forward for better predicting future climate.
The mean annual cycle in surface wind stress over the global oceans from surface wind analyses from the European Centre for Medium Range Weather Forecasts (ECMWF) for seven years (1980–86) is presented. The drag coefficient is a function of wind speed and atmospheric stability, and the density is computed for each observation. Annual and seasonal mean climatologies of wind stress, wind stress and Sverdrup transport and the first two annual harmonies of the wind stress are presented. The Northern and Southern hemispheres are contrasted as an the Pacific and Atlantic basins. The representativeness of the climatology is also assessed. The main shortcomings with the current results are in the topics. The wind stress statistics over the southern ocean are believed to be the moon reliable because of the paucity of direct wind observations. Annual mean values exceed 2 dyn cm−2 over the eastern hemisphere near 50°S and locally exceed 3 dyn cm−2 in the southern Indian Ocean; values much larger than in previous climatologies. The 12 month variations dominate the annual cycle over most of the globe and are strongest in the Arabian Sea, North Pacific and North Atlantic. But strong semiannual components occur especially over the Southern Ocean and in the North Pacific. The former are associated with semiannual increases in the strength of the southern westerlies whereas in the North Pacific, the semiannual cycle occurs locally largely because of the annual variations in intensity and meridional movement of the Aleutian low and subtropical high. The wind stress considerably from year to year. Over most of the world's ocean the mean annual cycle explains less then 45% of the monthly variance in each of the wind stress components and the curl of wind stress. In addition, mean values for the climatology differ significantly from those of previous periods. There is good reason to believe that these differences in the Northern Hemisphere an mostly real and represent climate variations on interannual and decadal time scales that have major implications for the circulation of the oceans. A related factor is that this period included two Pacific Warm Events (El Niños), but no Cold (La Niña) Events.
A detailed vertically integrated atmospheric heat and energy budget is presented along with estimated heat budgets at the surface and top-of-atmosphere for the subtropics. It is shown that the total energy transports are remarkably seamless in spite of greatly varying mechanisms. From the Tropics to about 31° latitude, the primary transport mechanisms are the Hadley and Walker overturning circulations. In the extratropics the energy transports are carried out by baroclinic eddies broadly organized into storm tracks and quasi-stationary waves that covary in a symbiotic way as the location and activity in storm tracks are determined by, and in turn help maintain through eddy transports, the quasi-stationary flow. In the upward branch of the Hadley cell, the predominant diabatic process is latent heating that results from convergence of moisture by the circulation itself. Hence large poleward transports of dry static energy are compensated by equatorward transports of latent energy, resulting in a modest poleward transport of moist static energy. The subsidence warming in the downward branch is compensated by cooling in the subtropics that mainly arises from energy transport to higher latitudes by transient baroclinic eddies that are stronger in the winter hemisphere. Effectively, the outgoing longwave radiation to space is distributed over middle and high latitudes and is not limited to the clear dry regions in the subtropics. Further, some of the radiative cooling in the subtropics is a consequence of the circulation. Hence the cooling by transient eddies in the subtropics is a fundamental driver of the observed Hadley circulation and realizes the seamless transport from Tropics to extratropics, while tropical sea surface temperatures over the oceans determine where the upward branch is located. The relatively clear skies in the subtropics further provide for ample absorption of solar radiation at the surface where it feeds strong evaporation, which exceeds precipitation, and supplies the equatorward flow of latent energy into the upward branch of the Hadley circulation as well as the poleward transports into midlatitude storm tracks. The evaporation is sufficiently strong that it is also compensated by a subsurface ocean heat transport that in turn is driven by the Hadley circulation surface winds.
