David P. Stepaniak

Indices of El Niño Evolution

Abstract To characterize the nature of El Nino–Southern Oscillation (ENSO), sea surface temperature (SST) anomalies in different regions of the Pacific have been used. An optimal characterization of both the distinct character and the evolution of each El Nino or La Nina event is suggested that requires at least two indices: (i) SST anomalies in the Nino-3.4 region (referred to as N3.4), and (ii) a new index termed here the Trans-Nino Index (TNI), which is given by the difference in normalized anomalies of SST between Nino-1+2 and Nino-4 regions. The first index can be thought of as the mean SST throughout the equatorial Pacific east of the date line and the second index is the gradient in SST across the same region. Consequently, they are approximately orthogonal. TNI leads N3.4 by 3 to 12 months prior to the climate shift in 1976/77 and also follows N3.4 but with opposite sign 3 to 12 months later. However, after 1976/77, the sign of the TNI leads and lags are reversed.

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.

Evolution of El Niño–Southern Oscillation and global atmospheric surface temperatures

[1]xa0The origins of the delayed increases in global surface temperature accompanying El Nino events and the implications for the role of diabatic processes in El Nino–Southern Oscillation (ENSO) are explored. The evolution of global mean surface temperatures, zonal means and fields of sea surface temperatures, land surface temperatures, precipitation, outgoing longwave radiation, vertically integrated diabatic heating and divergence of atmospheric energy transports, and ocean heat content in the Pacific is documented using correlation and regression analysis. For 1950–1998, ENSO linearly accounts for 0.06°C of global surface temperature increase. Warming events peak 3 months after SSTs in the Nino 3.4 region, somewhat less than is found in previous studies. Warming at the surface progressively extends to about ±30° latitude with lags of several months. While the development of ocean heat content anomalies resembles that of the delayed oscillator paradigm, the damping of anomalies through heat fluxes into the atmosphere introduces a substantial diabatic component to the discharge and recharge of the ocean heat content. However, most of the delayed warming outside of the tropical Pacific comes from persistent changes in atmospheric circulation forced from the tropical Pacific. A major part of the ocean heat loss to the atmosphere is through evaporation and thus is realized in the atmosphere as latent heating in precipitation, which drives teleconnections. Reduced precipitation and increased solar radiation in Australia, Southeast Asia, parts of Africa, and northern South America contribute to surface warming that peaks several months after the El Nino event. Teleconnections contribute to the extensive warming over Alaska and western Canada through a deeper Aleutian low and stronger southerly flow into these regions 0–12 months later. The 1976/1977 climate shift and the effects of two major volcanic eruptions in the past 2 decades are reflected in different evolution of ENSO events. At the surface, for 1979–1998 the warming in the central equatorial Pacific develops from the west and progresses eastward, while for 1950–1978 the anomalous warming begins along the coast of South America and spreads westward. The eastern Pacific south of the equator warms 4–8 months later for 1979–1998 but cools from 1950 to 1978.

Quality of Reanalyses in the Tropics

Broad vertical layer-averaged temperatures from the microwave sounder unit (MSU) are used as a quasiindependent validation of temperature fields from the U.S. National Centers for Environmental Prediction‐ National Center for Atmospheric Research (NCEP‐NCAR) and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses. While the MSU and NCEP‐NCAR temperatures show fairly good agreement overall, large discrepancies with ECMWF temperatures indicate that changes in the satellite observing system may have adversely affected the ECMWF reanalyses, especially in the Tropics. Two spurious discontinuities are present in tropical temperatures with jumps to warmer values throughout the Tropics below 500 mb in late 1986 and early 1989, and further spurious interannual variability is also present. These features are also reflected in the specific humidity fields. The temperature discrepancies have a complex vertical structure with height that is not fully understood, although it seems that the problems partly arise from positive reinforcement of biases in satellite radiances with those of the assimilating model first guess. Changes in the observing system provide a limit to the usefulness of the reanalyses in some climate studies.

The atmospheric energy budget and implications for surface fluxes and ocean heat transports

Abstract Comprehensive diagnostic comparisons and evaluations have been carried out with the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and European Centre for Medium Range Weather Forecasts (ECMWF) reanalyses of the vertically integrated atmospheric energy budgets. For 1979 to 1993 the focus is on the monthly means of the divergence of the atmospheric energy transports. For February 1985 to April 1989, when there are reliable top-of-the-atmosphere (TOA) radiation data from the Earth Radiation Budget Experiment (ERBE), the implied monthly mean surface fluxes are derived and compared with those from the assimilating models and from the Comprehensive Ocean Atmosphere Data Set (COADS), both locally and zonally integrated, to deduce the implied ocean meridional heat transports.While broadscale aspects and some details of both the divergence of atmospheric energy and the surface flux climatological means are reproducible, especially in the zonal means, differences are also readily apparent. Systematic differences are typically ∼20u2009W m−2. The evaluation highlights the poor results over land. Land imbalances indicate local errors in the divergence of the atmospheric energy transports for monthly means on scales of 500 km (T31) of 30 W m−2 in both reanalyses and ∼50 W m−2 in areas of high topography and over Antarctica for NCEP/NCAR. Over the oceans in the extratropics, the monthly mean anomaly time series of the vertically integrated total energy divergence from the two reanalyses correspond reasonably well, with correlations exceeding 0.7. A common monthly mean climate signal of about 40u2009W m−2 is inferred along with local errors of 25 to 30 W m−2 in most extratropical regions. Except for large scales, there is no useful common signal in the tropics, and reproducibility is especially poor in regions of active convection and where stratocumulus prevails. Although time series of monthly anomalies of surface bulk fluxes from the two models and COADS agree very well over the northern extratropical oceans, the total fields all contain large systematic biases which make them unsuitable for determining ocean heat transports. TOA biases in absorbed shortwave, outgoing longwave and net radiation from both reanalysis models are substantial (>20 W m−2 in the tropics) and indicate that clouds are a primary source of problems in the model fluxes, both at the surface and the TOA. Time series of monthly COADS surface fluxes are shown to be unreliable south of about 20∘N where there are fewer than 25 observations per 5∘u2009square per month. Only the derived surface fluxes give reasonable implied meridional ocean heat transports.

Covariability of Components of Poleward Atmospheric Energy Transports on Seasonal and Interannual Timescales

Abstract Vertically integrated atmospheric energy and heat budgets are presented with a focus on the zonal mean transports and divergences of dry static energy, latent energy, their sum (the moist static energy), and the total (which includes kinetic energy), as well as their partitioning into the within-month transient and quasi-stationary components. The latter includes the long-term mean and interannual variability from 1979 to 2001 and, in the Tropics, corresponds to the large-scale overturning global monsoon and the embedded Hadley and Walker circulations. In the extratropics, it includes the quasi-stationary planetary waves, which are primarily a factor in the Northern Hemisphere winter. In addition to the mean annual cycle, results are presented for the interannual variability. In the extratropics, poleward transports of both latent and dry static energy reinforce one another. However, the results highlight strong cancellations between the transports of latent and dry static energy in the Tropics a...

Seamless Poleward Atmospheric Energy Transports and Implications for the Hadley Circulation

Abstract 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 m...

Interannual variations in the atmospheric heat budget

[1]xa0Interannual variability of the atmospheric heat budget is explored via a new data set of the computed vertically integrated energy transports to examine relationships with other fields. A case study reveals very large monthly divergences of these transports regionally with El Nino-Southern Oscillation (ENSO) and the associated changes with the Pacific-North American teleconnection pattern, and with the North Atlantic Oscillation. In the tropical Pacific during large El Nino events the anomalous divergence of the atmospheric energy transports exceeds 50 W m−2 over broad regions for several months. Examination of the corresponding top-of-the-atmosphere net radiative fluxes shows that it is primarily the surface fluxes from the ocean to the atmosphere that feed the divergent atmospheric transports. A systematic investigation of the covariability of sea surface temperatures (SSTs) and the divergence of atmospheric energy transport, using singular value decomposition analysis of the temporal covariance, reveals ENSO as dominant in the first two modes, explaining 62% and 12% of the covariance in the Pacific domain and explaining 39.5% and 15.4% globally for the first and second modes, respectively. The first mode is well represented by the time series for the SST index for Nino 3.4 region (170°W–120°W, 5°N–5°S). Regression analysis allows a more complete view of how the SSTs, outgoing longwave radiation, precipitation, diabatic heating, and atmospheric circulation respond with ENSO. The second mode indicates aspects of the systematic evolution of ENSO with time, with strong lead and lag correlations. It primarily reflects differences in the evolution of ENSO across the tropical Pacific from about the dateline to coastal South America. High SSTs associated with warm ENSO events are damped through surface heat fluxes into the atmosphere, which transports the energy into higher latitudes and throughout the tropics, contributing to loss of heat by the ocean, while the cold ENSO events correspond to a recharge phase as heat enters the ocean. Diabatic processes are clearly important within ENSO evolution.

Interannual Variability of Patterns of Atmospheric Mass Distribution

Abstract Using the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) for 1958 to 2001, adjusted for bias over the southern oceans prior to 1979, an analysis is made of global patterns of monthly mean anomalies of atmospheric mass, which is approximately conserved globally. It differs from previous analyses of atmospheric circulation by effectively area weighting surface or sea level pressure that diminishes the role of high latitudes. To examine whether global patterns of behavior exist requires analysis of all seasons together (as opposite seasons occur in each hemisphere). Empirical orthogonal function (EOF) analysis, R-mode varimax-rotated EOF analysis, and cyclostationary EOF (CSEOF) analysis tools are used to explore patterns and variability on interannual and longer time scales. Clarification is given of varimax terminology and procedures that have been previously misinterpreted. The dominant global monthly variability overall is associated with the Southern Hemis...

Accuracy of atmospheric energy budgets from analyses

Abstract Issues relevant to achieving an accuracy of better than 10 W m−2 on 250-km scales for monthly means in the atmospheric energy balance are explored from the standpoint of the formulation and computational procedures using the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR, hereafter referred to as NCEP) and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses. The focus is on the vertically integrated energy components, their monthly tendencies, transports, and divergences, using the most accurate computations in model and pressure coordinates. Approximate equations have often been used previously; although relatively small compared with the moist static energy, kinetic energy transports should be taken into account, as divergences can exceed several tens of watts per square meter. Changes in energy storage terms over a month are not negligible, as they are typically over 25 W m−2 in storm track regions. Transports of energy a...