519 publications from this institution
Kevin E. Trenberth emphasizes the fundamental role of energy flows in the climate system and anthropogenic climate change. The distribution of heat, or more generally, energy, is the main determinant of weather patterns in the atmosphere and their impacts. The topics addressed cover many facets of climate and the climate crisis. These include the diurnal cycle; the seasons; energy differences between the continents and the oceans, the poles and the tropics; interannual variability such as Nino; natural decadal variability; and ice ages. Human-induced climate change rides on and interacts with all of these natural phenomena, and the result is an unevenly warming planet and changing weather extremes. The book emphasizes the need to not only slow or stop climate change, but also to better prepare for it and build resilience. Students, researchers, and professionals from a wide range of backgrounds will benefit from this deeper understanding of climate change.
A pathological problem has been discovered in the NCEP reanalyses in the stratosphere. It is manifested most strongly as a two-delta vertical wave in the divergence of the wind field above steep topography especially where the wind increases with altitude in the stratosphere. It is present primarily above 50 mb at the topmost four levels in the NCEP model used for data assimilation and appears to be directly related to the use of the sigma (terrain following) coordinate system and the upper boundary condition in the assimilating model. Recommendations suggested for addressing the problem include switching to a hybrid coordinate system that transitions to a pressure coordinate in the stratosphere, and with a damping upper boundary condition. Certain climate diagnostics are greatly impacted by these pathologies.
No abstract is provided for this article.
No abstract is provided for this article.
A detailed analysis of atmospheric temperatures at the South Pole and McMurdo Sound is presented. Missing data are common, especially in the stratosphere, and the usual practice of computing monthly means as an average of all available observations produces unreliable results because the annual cycle is aliased onto the interannual variations and longer term trends. A methodology to rectify this involves computation of the smoothed mean annual cycle for each day of the year and then subsequently analyzing the anomalies. The persistence of the anomalies within each month reveals that regular observations about every three days are required to produce a reliable climate record throughout the troposphere and lower stratosphere. A comparison of station data with lower stratospheric analyzed values from the National Meteorological Center (NMC) reveals big discrepancies at times arising mainly from methods used to produce the NMC analyses. The mean annual cycle of temperature features the coreless winter at low levels at both stations. The largest amplitude annual cycle occurs at 10 mb with maximum temperatures in December. The maximum occurs progressively later at lower levels down to 200 mb, where it occurs in February. The seasonal transition occurs more rapidly in spring than in autumn. Interannual fluctuations are dominated by a quasi-biennial variation. Noticeable downward trends in temperature were found mainly near the time of greatest variability, in late spring. Temperatures decreased between 50 and 100 mb in October and November and at 100 mb in December and January; but only from 1985 to 1987 have monthly means gone outside the range of previous variations. The most extreme anomalies, of as much as −21°C, were found in November 1987. The downward trends arise from a delay in the spring warming apparently brought about by the diminished solar heating due to low ozone amounts associated with the “ozone hole.” Trends from 1979 to 1986 or 1987, which is the period of satellite data on temperatures and ozone, are not representative of the overall record which begins in 1956.
A more complete and new formulation of the orographic forcing and new thermal forcings are included in a steady state model of the Northern Hemisphere planetary waves. When both forcings are included, the simulation produces excellent results which are compared in detail with observations. In particular, the Siberian high, the tropospheric East Asian trough and subtropical tropospheric East Asian jet stream maxima are well reproduced even though the forcing is primarily extratropical in origin. The modes uses a lower boundary condition in which the orographic forcing is determined by the effects of the total flow, not just the zonal mean basic state. Consequently, the net orographic forcing changes when thermal forcing is added and the tow solution is not equal to the linear sum of the solutions with orographic and thermal forcings separately. The thermally induced orographic forcing is found to be very significant and, in the troposphere, there is strong interaction between the two forcings with both of roughly equal importance. However, the Iowa-latitude vertically propagating waves am deflected by the subtropical jet and absorbed in the low-latitude easterlies. Thus only the mid-high latitude planetary waves are important in the stratosphere which seems to be dominated by the thermally forced component. The model is forced with new estimates of diabetic heating from several FGGE analyses. The sensitivity of the results to different heatings and their assumed vertical profile is examined. The amplitude of the lower-troposphere response is very sensitive to the vertical profile but there are much smaller changes at upper levels which are dominated by the remote response. Large differences in the response to the different diabatic heatings are found at high latitudes and over the Pacific Ocean. However, when orographic forcing is also included, these differences diminish indicating a smaller sensitivity to uncertainties in heating, and thus the orographic forcing is acting to constrain the total response. This is in marked contrast to the model results when the traditional lower-boundary condition (in which the waves are decoupled) is used since then the total response is entirely linear. Wave 1 is too weak in the model and this is most likely mainly due to deficiencies in the thermal forcing. The model eddy fluxes of heat and momentum show excellent agreement with observations in location although the poleward heat flux is somewhat weak. These reveal noticeable improvements over the wave-decoupled model which produces too large a response with the lower-tropospheric heat flux too far south at 35°N in association with a degraded simulation of the Siberian high and East Asian trough, in particular.
No abstract is provided for this article.
While hurricanes occur naturally, human‐caused climate change is supercharging them and exacerbating the risk of major damage. Here using ocean and atmosphere observations, we demonstrate links between increased upper ocean heat content due to global warming with the extreme rainfalls from recent hurricanes. Hurricane Harvey provides an excellent case study as it was isolated in space and time. We show that prior to the beginning of northern summer of 2017, ocean heat content was the highest on record both globally and in the Gulf of Mexico, but the latter sharply decreased with hurricane Harvey via ocean evaporative cooling. The lost ocean heat was realized in the atmosphere as moisture, and then as latent heat in record‐breaking heavy rainfalls. Accordingly, record high ocean heat values not only increased the fuel available to sustain and intensify Harvey but also increased its flooding rains on land. Harvey could not have produced so much rain without human‐induced climate change. Results have implications for the role of hurricanes in climate. Proactive planning for the consequences of human‐caused climate change is not happening in many vulnerable areas, making the disasters much worse.
It has been traditional in meteorology to divide the velocity field up into rotational and divergent components, but not the geopotential field. Yet any balance condition, such as the geostrophic relation or linear balance equation, is a diagnostic relation which states that not only can the balanced velocity field be computed from the geopotential field, but also that the geopotential can be derived from the velocity field. If the latter approach is adopted, then the difference between the observed and computed geopotential is the quantity we refer to as the divergent, or in some cases, ageostrophic, geopotential. In fact, for any balance set of equations, it is essential to partition the geopotential in such a way in order to derive an equivalent set of momentum equations. The momentum equations equivalent to the linear balance set of equations are given. The linear balance equation is integrated to give a diagnostic relation between the rotational wind components and the gradient of the rotational geopotential plus an extra term which involves the gradient of the planetary vorticity advection potential (PVAP). The partitioning of the velocity field into rotational and divergent parts v = vr + vd does not depend on any other field. Given vr and the associated streamfunction, we have computed Φ, from the linear balance equation. The difference Φd = Φ − Φr is of order Rossby number times Φ, has global scale dominated by wave 1, and tends to be a maximum near the equator. This partitioning depends upon the balance equation used. The different components of Φ have implications for how the observed Φ should and should not be used in diagnostic studies and provide a new interpretation as to what the ageostrophic component of the flow is. In particular, the ageostrophic wind is partitioned into the divergent wind plus contributions arising from the gradient of the PVAP and the gradient of the divergent geopotential rotated 90°. The rotational and divergent geopotential fields and the PVAP have been computed from climatological mean January and July conditions. In addition for January, the tendencies due to the Coriolis form associated with the ageostrophic velocity are given and are shown to be related to the acceleration of jets in entrance regions and deceleration in exit regions of in excess of 30 m s−1/day for the Northern Hemisphere. However, while all three terms contributing to the ageostrophic velocity are of roughly equal importance overall, the divergent wind is less important in jet entrance and exit regions and the gradients of the PVAP and Φd terms are dominant. This illustrates the kinematic nature of these acceleration terms and shows the balance that exists in the momentum budget, but provides little insight into the cause of the existence of the mean jets.
Current climate models suggest that global warming will result in more frequent extreme hydrological events (floods and droughts). These results, however, must be tempered with the fact that current climate models do not realistically represent many of the processes important to the formation of clouds and precipitation at various time and space scales. For instance, the diurnal cycle of precipitation is poorly represented in most climate models. The proper representation of precipitation is a major challenge to global climate models, which typically only resolve processes at 200- to 400-km scales, and is a focus of current scientific research. This chapter addresses the current understanding of the likely climate impact on precipitation, as well as some of the key challenges facing climate modelers with regard to improving future projections of precipitation.
The largest increases in surface temperatures over the Northern Hemisphere in the decade prior to 1988 were in Alaska, while substantial decreases occurred in the North Pacific Ocean. This illustrates the considerable geographic spatial structure to interdecadal temperature variations associated with changes in the atmospheric circulation. In particular, from 1977 to 1988, there was a deeper and eastward-shifted Aleutian low-pressure system in the winter half year, which advected warmer and moister air into Alaska and colder air over the North Pacific. Associated changes in surface-wind stress and wind-stress curl altered the North Pacific Ocean currents, as revealed by the Sverdrup transport. The North Pacific changes appear to be linked through teleconnections to tropical atmosphere–ocean interactions and the frequency of El Niño versus La Niña events. Consequently, the question of why it was so warm in Alaska becomes changed to one of why there were three tropical Pacific Warm Events, but no Cold Events, from 1977 to 1988, and whether changes in El Niño frequency will be altered during climate change. At the very least, these linkages and the resulting strongly regional structure of surface-temperature variations complicate any search for a greenhouse effect and global warming.
To explore the role of hurricanes in the climate system, a detailed analysis is made of the bulk atmospheric moisture budget of Ivan in September 2004 and Katrina in August 2005 from simulations with the Weather and Research Forecasting (WRF) model at 4 km resolution without parameterized convection. Heavy precipitation exceeding 20 mm h −1 in the storms greatly exceeds the surface flux of moisture through evaporation, and vertically integrated convergence of moisture in the lowest 1 km of the atmosphere from distances up to 1600 km is the dominant term in the moisture budget, highlighting the importance of the larger‐scale environment. Simulations are also run for the Katrina case with sea surface temperatures (SSTs) increased by +1°C and decreased by −1°C as sensitivity studies. For hours 42 to 54 after the start of the simulation, maximum surface winds increased about 4.5 m s −1 (9%), and sea level pressure fell 11.5 hPa per 1°C increase in tropical SSTs. Overall, the hurricane expands in size as SSTs increase, the environmental atmospheric moisture increases at close to the Clausius‐Clapeyron equation value of about 6% K −1 and the surface moisture flux also increases mainly from Clausius‐Clapeyron effects and the changes in intensity of the storm. The environmental changes related to human influences on climate since 1970 have increased SSTs and water vapor, and the results suggest how this may have altered hurricanes and increased associated storm rainfalls, with the latter quantified to date to be of order 6 to 8%.
The climate varies on multiple timescales, but now humans are the main agents of change and are likely to remain so for the next few centuries. It is generally understood that human-induced climate change causes global warming, but what is not adequately appreciated are the direct influences on heavy rainfalls, drought and storms, at great cost to society and the environment. Although the climate change effects are modest, perhaps five to 15 per cent for these events, once thresholds are crossed, things break and damage increases non-linearly. These aspects are not properly factored into costs of climate change, and preparation for expected effects is woefully inadequate, exacerbating damage.
