Publications

Earth's Energy Balance

No abstract is provided for this article.

The Global Monsoon as Seen through the Divergent Atmospheric Circulation

A comprehensive description is given of the global monsoon as seen through the large-scale overturning in the atmosphere that changes with the seasons, and it provides a basis for delimiting the monsoon regions of the world. The analysis focuses on the mean annual cycle of the divergent winds and associated vertical motions, as given by the monthly mean fields for 1979–93 reanalyses from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) and European Centre for Medium-Range Weather Forecasts (ECMWF), which are able to reproduce the dominant modes. A complex empirical orthogonal function analysis of the divergent circulation brings out two dominant modes with essentially the same vertical structures in all months of the year. The first mode, which depicts the global monsoon, has a simple vertical structure with a maximum in vertical motion at about 400 mb, divergence in the upper troposphere that is strongest at 150 mb and decays to zero amplitude above 70 mb, and convergence in the lower troposphere with a maximum at 925 mb (ECMWF) or 850 mb (NCEP). However, this mode has a rich three-dimensional spatial structure that evolves with the seasons. It accounts for 60% of the annual cycle variance of the divergent mass circulation and dominates the Hadley circulation as well as three overturning transverse cells. These include the Pacific Walker circulation; an Americas–Atlantic Walker circulation, both of which comprise rising motion in the west and sinking in the east; and a transverse cell over Asia, the Middle East, North Africa, and the Indian Ocean that has rising motion in the east and sinking toward the west. These exist year-round but migrate and evolve considerably with the seasons and have about a third to half of the mass flux of the peak Hadley cell. The annual cycle of the two Hadley cells reveals peak strength in early February and early August in both reanalyses. A second monsoon mode, which accounts for 20% of the variance, features relatively shallow but vigorous overturning with the maximum vertical velocities near 800 mb, outflow from 750 to 350 mb, and inflow peaking at 925 mb. It is especially strong over Africa where the shallow, mostly meridional overturning migrates back and forth across the equator with the seasons. It influences the Middle East, has a signature over Australia, and is also an important component of the overturning in the tropical eastern Pacific and Atlantic, and thus of the convergence zones in these regions. The relationship of the global monsoon to the regional monsoons is described over six zonal sectors: Africa, Australia–Asia, North America, South America, and the Pacific and Atlantic Oceans. Only the two ocean areas do not undergo a seasonal reversal required for monsoons, although they have direct overturning cells and they nevertheless participate in the global monsoon through the changes in large-scale overturning. The regional meridional cross sections highlight the importance of the shallow overturning cell in lower-troposphere monsoon activity. The steadiness of the overturning circulation is determined by comparing the signal of the seasonal mean vertical motions at 500 mb with the standard deviation of the transient daily variations. Locations where this signal exceeds 60% of the daily noise correspond closely with the regional centers of the monsoon.

An Evaluation and Intercomparison of Global Analyses from the National Meteorological Center and the European Centre for Medium Range Weather Forecasts

In order to help establish a global climate record data sets of global analyses from the U.S. National Meteorological Center (NMC) and the European Centre for Medium Range Weather Forecasts (ECMWF) have been comprehensively evaluated. A detailed chronology of the changes in the analysis-forecast system at NMC and ECMWF has been compiled and the main impacts on the analyses have been identified. Discontinuities have been found in certain characteristics of the analyses when major changes occur. The main quantities so affected are the divergent wind component and associated vertical motion fields, and the moisture fields. A detailed intercomparison of the two data sets and statistical results show fairly widespread agreement between the analyses from the two centers over the Northern Hemisphere extratropics. In general, the quality of the analyses is much lower in the tropics and Southern Hemisphere. This is reflected in much greater differences in wind fields south of 20°N, with root-mean-square differences in the north-south and east-west components often exceeding 5ms−1 above ~500 mb throughout most of this region. Much greater differences in geopotential height south of ~30°S exist. Major problems at NMC prior to May 1986 occur south of ~50°S. ECMWF has also experienced difficulties over and around Antarctica, especially prior to 1982. In the tropics, there are major disagreements between the analyses of the divergent wind field and associated vertical motions which have become more intense and more realistic with time, but still appear to be poorly known. The relative humidity field is the poorest known and has undergone major changes with time at both centers. Possible reasons for these results are discussed and some implications and recommendations are given.

The water cycle

The Changing Character of Precipitation

From a societal, weather, and climate perspective, precipitation intensity, duration, frequency, and phase are as much of concern as total amounts, as these factors determine the disposition of precipitation once it hits the ground and how much runs off. At the extremes of precipitation incidence are the events that give rise to floods and droughts, whose changes in occurrence and severity have an enormous impact on the environment and society. Hence, advancing understanding and the ability to model and predict the character of precipitation is vital but requires new approaches to examining data and models. Various mechanisms, storms and so forth, exist to bring about precipitation. Because the rate of precipitation, conditional on when it falls, greatly exceeds the rate of replenishment of moisture by surface evaporation, most precipitation comes from moisture already in the atmosphere at the time the storm begins, and transport of moisture by the storm-scale circulation into the storm is vital. Hence, the intensity of precipitation depends on available moisture, especially for heavy events. As climate warms, the amount of moisture in the atmosphere, which is governed by the Clausius–Clapeyron equation, is expected to rise much faster than the total precipitation amount, which is governed by the surface heat budget through evaporation. This implies that the main changes to be experienced are in the character of precipitation: increases in intensity must be offset by decreases in duration or frequency of events. The timing, duration, and intensity of precipitation can be systematically explored via the diurnal cycle, whose correct simulation in models remains an unsolved challenge of vital importance in global climate change. Typical problems include the premature initiation of convection, and precipitation events that are too light and too frequent. These challenges in observations, modeling, and understanding precipitation changes are being taken up in the NCAR “Water Cycle Across Scales” initiative, which will exploit the diurnal cycle as a test bed for a hierarchy of models to promote improvements in models.

Effects of Mount Pinatubo Volcanic Eruption on the Hydrological Cycle as an Analog of Geoengineering

[1] The problem of global warming arises from the buildup of greenhouse gases such as carbon dioxide from burning of fossil fuels and other human activities that change the composition of the atmosphere and alter outgoing longwave radiation (OLR). One geoengineering solution being proposed is to reduce the incoming sunshine by emulating a volcanic eruption. In between the incoming solar radiation and the OLR is the entire weather and climate system and the hydrological cycle. The precipitation and streamflow records from 1950 to 2004 are examined for the effects of volcanic eruptions from El Chichon in March 1982 and Pinatubo in June 1991, taking into account changes from El Nino-Southern Oscillation. Following the eruption of Mount Pinatubo in June 1991 there was a substantial decrease in precipitation over land and a record decrease in runoff and river discharge into the ocean from October 1991–September 1992. The results suggest that major adverse effects, including drought, could arise from geoengineering solutions.

Evaluation of the Atmospheric and Hydrological Cycle in the NCEP Reanalyses

No abstract is provided for this article.

Climate change and extreme weather events [presentation]

No abstract is provided for this article.

Effects of Mount Pinatubo Volcanic Eruption on the Hydrological Cycle as an Analog of Geoengineering

No abstract is provided for this article.

Intercomparison of NMC and ECMWF Global Analyses: 1980-1986

No abstract is provided for this article.

Heat budgets and poleward atmospheric energy transports [presentation]

No abstract is provided for this article.

Observed and simulated full-depth ocean heat content changes for 1970–2005

. Greenhouse-gas emissions have created a planetary energy imbalance that is primarily manifested by increasing ocean heat content (OHC). Updated observational estimates of full-depth OHC change since 1970 are presented that account for recent advancements in reducing observation errors and biases. The full-depth OHC has increased by 0.74 [0.68, 0.80] × 1022 J/yr (0.46 Wm−2) and 1.22 [1.16–1.29] ×1022 J/yr (0.75 Wm−2) for 1970-2005 and 1992-2005 respectively, with 5 % to 95 % confidence interval of the median. The CMIP5 models show large spread in OHC changes, but the ensemble median has excellent agreement with our observational estimate: 0.68 [0.54–0.82] × 1022 J/yr (0.41 Wm−2) from 1970 to 2005 and 1.25 [1.10–1.41] × 1022 J/yr (0.77 Wm−2) from 1992 to 2005. These results increase confidence in both the observational and model estimates to quantify and study changes in Earth’s energy imbalance over the historical period. We suggest that OHC be a fundamental metric for climate model validation and evaluation.

Attribution and the Hiatus

Temperatures on Earth are rising at the surface and especially for the oceans. But how is it known that this is due to human activities? The main way this question has been addressed in most places is to use climate modeling, where a climate model can be run with and without various forcings and the results compared with observations. In these models, the increase in carbon dioxide is generally assumed. But how is it known that the increased carbon dioxide comes from human activities?

CLIMATE AND CLIMATE CHANGE | Intergovernmental Panel on Climate Change

An outline is given of the IPCC and its role in helping to lay out options and consequences of actions or lack thereof related to anthropogenic climate change. A description is provided of how the IPCC is set up and functions, the scientific assessment process, who participates, and the intergovernmental linkages, all of which lead to the IPCC reports every 5–7 years. The procedures are designed to provide policy relevant but not policy prescriptive scientific advice to policy makers and the public. The main findings are briefly described along with the recent controversy involving IPCC.

Applications of an Updated Atmospheric Energetics Formulation

As observations and atmospheric reanalyses have improved, the diagnostics that can be computed with confidence also increase. Accordingly, a new formulation of the energetics of the atmosphere is laid out, with a view to advancing diagnostic studies of Earth’s energy budget and flows. It is utilized to produce assessments of the vertically integrated divergences in both the atmosphere and ocean. Careful conservation of mass is required, with special attention given to the hydrological cycle and redistribution of mass associated with precipitation and evaporation, and a new method for ensuring this is developed. It guarantees that the atmospheric divergence is associated with moisture and precipitation, unlike previous methods. A new term, identified as associated with the enthalpy of precipitation, is included in a preliminary way. It is sensitive to the formulation, and the use of temperature in degrees Celsius instead of Kelvin greatly reduces errors and produces the extra term with values up to about ±5 W m −2 . New results for 2000 to 2016 are presented for the vertical-mean and annual-mean diabatic atmospheric heating, atmospheric moistening, and total atmospheric energy divergence. Results for the atmospheric divergence are combined with top-of-atmosphere radiation observations to deduce total surface energy fluxes. Along with estimates of changes in ocean heat content, the Atlantic Ocean meridional heat transports are recomputed for March 2000 through 2013. The new results are compared with previous estimates and an assessment is made of the effects of the new mass balance, change in temperature scale, and the extra precipitation enthalpy term.

Evolution of Ocean Heat Content Related to ENSO

As the strongest interannual perturbation to the climate system, El Niño–Southern Oscillation (ENSO) dominates the year-to-year variability of the ocean energy budget. Here we combine ocean observations, reanalyses, and surface flux data with Earth system model simulations to obtain estimates of the different terms affecting the redistribution of energy in the Earth system during ENSO events, including exchanges between ocean and atmosphere and among different ocean basins, and lateral and vertical rearrangements. This comprehensive inventory allows better understanding of the regional and global evolution of ocean heat related to ENSO and provides observational metrics to benchmark performance of climate models. Results confirm that there is a strong negative ocean heat content tendency (OHCT) in the tropical Pacific Ocean during El Niño, mainly through enhanced air–sea heat fluxes Q into the atmosphere driven by high sea surface temperatures. In addition to this diabatic component, there is an adiabatic redistribution of heat both laterally and vertically (0–100 and 100–300 m) in the tropical Pacific and Indian oceans that dominates the local OHCT. Heat is also transported and discharged from 20°S–5°N into off-equatorial regions within 5°–20°N during and after El Niño. OHCT and Q changes outside the tropical Pacific Ocean indicate the ENSO-driven atmospheric teleconnections and changes of ocean heat transport (i.e., Indonesian Throughflow). The tropical Atlantic and Indian Oceans warm during El Niño, partly offsetting the tropical Pacific cooling for the tropical oceans as a whole. While there are distinct regional OHCT changes, many compensate each other, resulting in a weak but robust net global ocean cooling during and after El Niño.

How Often Does It Really Rain?

The perception about whether a place is a nice place to live often depends on how often it rains (or snows). The frequency relates to how dreary the weather appears, and it is the duration much more than the amount that clouds perceptions. Yet, information about the frequency of rainfall, or precipitation in general, is spotty at best. Here, we analyze a new near-global (60°N–60°S) dataset at hourly time scales and 0.25° resolution. The dataset, the newly calibrated Climate Prediction Center morphing technique (CMORPH), enables comparison of results with 3-hourly and daily data, which is what has previously been available, and seasonal aspects are also examined. The results are quite sensitive to both the spatial scales of the data and their temporal resolutions, and it is important to get down to hourly values to gain a proper appreciation of the true frequency. At 1° resolution, values are 35% higher than for 0.25°. At 3-hourly resolution, they are about 25% higher than hourly, and at daily resolution, they are about 150% higher than hourly on average. Overall, near-global (60°N–60°S) precipitation occurs 11.0% ± 1.1% (1 sigma) of the time or, alternatively, 89.0% of the time it is not precipitating. But outside of the intertropical and South Pacific convergence zones, where values exceed 30%, and the arid and desert regions, where values are below 4%, the rates are more like 10% or so, and over land where most people live, values are closer to about 8%.

Seasonal variations in global sea level pressure and the total mass of the atmosphere

The annual cycles of sea level and surface pressures and the atmospheric pressure owing to water vapor have been analyzed in detail. Global sea level pressures undergo an annual cycle of 0.5 mbar range with a maximum in the northern winter. Global surface pressures, which represent the total mass of the atmosphere, also undergo an annual cycle of 0.5 mbar range but with the maximum in the southern winter. The changes in water vapor concentrations are responsible for the latter—water vapor has a maximum in the southern winter. The constancy of the mass of dry air is used as a check on the accuracy of computations. The total mass of the atmosphere is 513.7 × 10 16 kg with a standard error of 0.02 × 10 16 kg and an annual cycle of amplitude 0.1 × 10 16 kg. The corresponding global mean surface pressure of the atmosphere is 984.68 mbar. The mean total mass of water vapor is 1.3 × 10 16 kg which corresponds to 2.53 cm of precipitable water or 12906 km 3 of water at 0°C. The distribution of pressure and mass as a function of latitude are also presented. A substantial annual exchange of mass occurs between the hemispheres amounting to 0.7 × 10 16 kg of dry air (2.7 mbar range in hemispheric averaged pressure). Maximum pressures occur in winter, and the summer loss of mass is partially compensated for by increased water vapor amounts. The maximum net meridional flow occurs across 5°N in the transition seasons. In the northern hemisphere, water vapor undergoes an annual cycle with a range of 1.5 mbar (area averaged) and a summer maximum, and the sea level pressure owing to dry air undergoes an annual cycle of range 5 mbar and a winter maximum. Of this, 54% is due to exchanges with the southern hemisphere and 46% is due to changes in the artificial atmospheric mass used to correct from the surface to sea level.