Abstract

Using reanalysis data from the Goddard Earth Observing System (GEOS) Data Assimilation System, the authors have documented the basic three-dimensional features of anomalous atmospheric hydrologic processes observed during the El Niño–Southern Oscillation (ENSO). The most dominant anomaly pattern features a pair of subtropical temperature maxima straddling the equator in the upper troposphere coupled to a corresponding pair of temperature minima in the lower stratosphere in the form of a dipole. Over the Tropics and subtropics, the water vapor content is increased in regions of large-scale ascent with maximum response in the middle troposphere, whereas substantial drying is found in the descending branches of the Walker and Hadley circulations. While the temperature and moisture patterns in the lower troposphere are thermodynamically linked to the sea surface temperature anomaly pattern, the distribution of temperature and water vapor in the upper troposphere is largely controlled by dynamics and much less by thermodynamics. The troposphere–stratosphere temperature dipole is fundamentally due to the rising of the tropopause associated with hydrostatic expansion and vertical ascent in regions of enhanced deep convection. The rising motion pushes colder upper-tropospheric air into the lower stratosphere where the climatological temperature gradient reverses. No such dipole anomaly exists in the moisture field.

Numerical experiments with the GEOS GCM show that while atmospheric dynamics are principally responsible for the generation of the basic structures of the temperature and moisture anomalies observed during ENSO, the interaction between the hydrologic cycle and radiation plays an important role in enhancing and modifying the response. The role of hydrologic cycle–radiation interaction is most important in rendering the atmosphere more unstable both columnwise and locally, through enhanced longwave heating in the middle and lower troposphere and cooling above. The enhanced instability leads to intensified Hadley and Walker circulations, which are accompanied by stronger latent heating and a more vigorous hydrologic cycle. The intensified hydrologic cycle promotes further warming and moistening of the middle and lower troposphere, and cooling and drying in the stratosphere. The radiation–dynamics feedback leads to a new equilibrium climate state in which the increased heat transport by convection into the upper troposphere and stratosphere is balanced by increased radiative cooling, which removes the local excessive heat buildup.

Abstract

A Mount Everest ice core analyzed at high resolution for major and trace elements (Sr, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, Bi, U, Tl, Al, S, Ca, Ti, V, Cr, Mn, Fe, Co) and spanning the period a.d. 1650–2002 is used to investigate the sources of and variations in atmospheric dust through time. The chemical composition of dust varies seasonally, and peak dust concentrations occur during the winter–spring months. Significant correlations between the Everest dust record and dust observations at stations suggest that the Everest record is representative of regional variations in atmospheric dust loading. Back-trajectory analysis in addition to a significant correlation of Everest dust concentrations and the Total Ozone Mapping Spectrometer (TOMS) aerosol index indicates that the dominant winter sources of dust are the Arabian Peninsula, Thar Desert, and northern Sahara. Factors that contribute to dust generation at the surface include soil moisture and temperature, and the long-range transport of dust aerosols appears to be sensitive to the strength of 500-mb zonal winds. There are periods of high dust concentration throughout the 350-yr Mount Everest dust record; however, there is an increase in these periods since the early 1800s. The record was examined for recent increases in dust emissions associated with anthropogenic activities, but no recent dust variations can be conclusively attributed to anthropogenic inputs of dust.

Abstract

The Madden–Julian oscillation (MJO) exhibits pronounced seasonality, with one of the key unanswered questions being the following: what controls the maximum in MJO precipitation variance in the Southern Hemisphere during boreal winter? In this study, we examine a set of global climate model simulations in which the eccentricity and precession of Earth’s orbit are altered to change the boreal winter mean state in an attempt to reveal the processes that are responsible for the MJO’s amplitude in the boreal winter. In response to the forced insolation changes, the north–south asymmetry in sea surface temperature is amplified in boreal fall, which intensifies the Hadley circulation in boreal winter. The stronger Hadley circulation yields higher mean precipitation and stronger mean lower-tropospheric westerlies in the southern part of the Indo-Pacific warm pool. The MJO precipitation variability increases significantly where the mean precipitation and lower-tropospheric westerlies strengthen. In the column-integrated moisture budget of the simulated MJO, only surface latent heat flux feedback shows a trend that is consistent with the MJO’s amplitude, suggesting an important role for the surface latent heat flux feedback in the MJO’s amplitude during the boreal winter. An analysis of the moisture–precipitation relationship in the simulations shows that the increase in the mean precipitation lowers the convective moisture adjustment time scale, leading to the increase in precipitation variance. Our results suggest that the mean-state precipitation plays a critical role in the maintenance mechanism of the MJO.

Abstract

The role of the North Pacific as a regulator of boreal summer climate over Eurasia and North America is investigated using observational data. Two summertime interannual climate modes associated with sea surface temperature (SST) variability in the North Pacific are identified. The first mode shows an elongated zone of warm (cold) SST anomalies in the central North Pacific along 40°N, with temporal variability significantly correlated with El Niño during the preceding spring, but its subsequent evolution is quite different from El Niño. The second mode exhibits a seesaw SST variation between the northern and southern North Pacific and is independent of El Niño. Both modes are linked to coherent SST anomalies over the North Atlantic, suggesting the presence of an “atmospheric bridge” linking the two extratropical oceans.

Using the principal component of the most dominant mode as the North Pacific index (NPI), composite analyses show that the positive (negative) phase of NPI features a warm (cold) North Pacific associated with the formation of contemporaneous low-level stationary anticyclones (cyclones) over the North Pacific and North Atlantic, respectively. The anticyclones (cyclones) are linked by quasi-zonally symmetric circulation anomalies in the middle to upper troposphere spanning Eurasia and North America, accompanied by a poleward (equatorward) shift of the subtropical jet and storm tracks. Associated with the positive (negative) phase of NPI, are hot/dry (cool/wet) summers over Japan, Korea, and eastern-central China, which are linked to hot/dry (cool/wet) conditions in the Pacific Northwest, western Canada, the U.S. northern Great Plains, and the Midwest. Cumulative probability computed from pentad temperature and rainfall data show that the odds of occurrence of extreme events are impacted consistently with the mean climate shift during opposite phases of the NPI. The possible roles of air–sea interaction and transient-mean flow interaction in exciting and sustaining the climate modes are discussed.

Abstract

Equatorial Atlantic variability is dominated by the Atlantic Niño peaking during the boreal summer. Studies have shown robust links of the Atlantic Niño to fluctuations of the St. Helena subtropical anticyclone and Benguela Niño events. Furthermore, the occurrence of opposite sea surface temperature (SST) anomalies in the eastern equatorial and southwestern extratropical South Atlantic Ocean (SAO), also peaking in boreal summer, has recently been identified and termed the SAO dipole (SAOD). However, the extent to which and how the Atlantic Niño and SAOD are related remain unclear. Here, an analysis of historical observations reveals the Atlantic Niño as a possible intrinsic equatorial arm of the SAOD. Specifically, the observed sporadic equatorial warming characteristic of the Atlantic Niño (~0.4 K) is consistently linked to southwestern cooling (~−0.4 K) of the Atlantic Ocean during the boreal summer. Heat budget calculations show that the SAOD is largely driven by the surface net heat flux anomalies while ocean dynamics may be of secondary importance. Perturbations of the St. Helena anticyclone appear to be the dominant mechanism triggering the surface heat flux anomalies. A weakening of the anticyclone will tend to weaken the prevailing northeasterlies and enhance evaporative cooling over the southwestern Atlantic Ocean. In the equatorial region, the southeast trade winds weaken, thereby suppressing evaporation and leading to net surface warming. Thus, it is hypothesized that the wind–evaporation–SST feedback may be responsible for the growth of the SAOD events linking southern extratropics and equatorial Atlantic variability via surface net heat flux anomalies.

Abstract

The ability of eight climate models to simulate the Madden–Julian oscillation (MJO) is examined using diagnostics developed by the U.S. Climate Variability and Predictability (CLIVAR) MJO Working Group. Although the MJO signal has been extracted throughout the annual cycle, this study focuses on the boreal winter (November–April) behavior. Initially, maps of the mean state and variance and equatorial space–time spectra of 850-hPa zonal wind and precipitation are compared with observations. Models best represent the intraseasonal space–time spectral peak in the zonal wind compared to that of precipitation. Using the phase–space representation of the multivariate principal components (PCs), the life cycle properties of the simulated MJOs are extracted, including the ability to represent how the MJO evolves from a given subphase and the associated decay time scales. On average, the MJO decay (e-folding) time scale for all models is shorter (∼20–29 days) than observations (∼31 days). All models are able to produce a leading pair of multivariate principal components that represents eastward propagation of intraseasonal wind and precipitation anomalies, although the fraction of the variance is smaller than observed for all models. In some cases, the dominant time scale of these PCs is outside of the 30–80-day band.

Several key variables associated with the model’s MJO are investigated, including the surface latent heat flux, boundary layer (925 hPa) moisture convergence, and the vertical structure of moisture. Low-level moisture convergence ahead (east) of convection is associated with eastward propagation in most of the models. A few models are also able to simulate the gradual moistening of the lower troposphere that precedes observed MJO convection, as well as the observed geographical difference in the vertical structure of moisture associated with the MJO. The dependence of rainfall on lower tropospheric relative humidity and the fraction of rainfall that is stratiform are also discussed, including implications these diagnostics have for MJO simulation. Based on having the most realistic intraseasonal multivariate empirical orthogonal functions, principal component power spectra, equatorial eastward propagating outgoing longwave radiation (OLR), latent heat flux, low-level moisture convergence signals, and vertical structure of moisture over the Eastern Hemisphere, the superparameterized Community Atmosphere Model (SPCAM) and the ECHAM4/Ocean Isopycnal Model (OPYC) show the best skill at representing the MJO.

Abstract

The atmospheric anomalies for the 1997/98 El Niño–Southern Oscillation (ENSO) period have been analyzed and intercompared using the data simulated by the atmospheric general circulation models (GCMs) of 11 groups participating in the Monsoon GCM Intercomparison Project initiated by the Climate Variability and Prediction Program (CLIVAR)/Asian–Australian Monsoon Panel. Each participating GCM group performed a set of 10 ensemble simulations for 1 September 1996–31 August 1998 using the same sea surface temperature (SST) conditions but with different initial conditions. The present study presents an overview of the intercomparison project and the results of an intercomparison of the global atmospheric anomalies during the 1997/98 El Niño period. Particularly, the focus is on the tropical precipitation anomalies over the monsoon–ENSO region and the upper-tropospheric circulation anomalies in the Pacific–North American (PNA) region.

The simulated precipitation anomalies show that all of the models simulate the spatial pattern of the observed anomalies reasonably well in the tropical central Pacific, although there are large differences in the amplitudes. However, most of the models have difficulty in simulating the negative anomalies over the Maritime Continent during El Niño. The 200-hPa geopotential anomalies over the PNA region are reasonably well reproduced by most of the models. But, the models generally underestimate the amplitude of the PNA pattern. These weak amplitudes are related to the weak precipitation anomalies in the tropical Pacific. The tropical precipitation anomalies are found to be closely related to the SST anomalies not only during the El Niño seasons but also during the normal seasons that are typified by weak SST anomalies in the tropical Pacific. In particular, the pattern correlation values of the 11-model composite of the precipitation anomalies with the observed counterparts for the normal seasons are near 0.5 for the tropical region between 30°S and 30°N.