ALBERT

Description: The causes of persistent droughts and wet periods, or pluvials, over western North America are examined in model simulations of the period from 1856 to 2000. The simulations used either (i) global sea surface temperature data as a lower boundary condition or (ii) observed data in just the tropical Pacific and computed the surface ocean temperature elsewhere with a simple ocean model. With both arrangements, the model was able to simulate many aspects of the low-frequency (periods greater than 6 yr) variations of precipitation over the Great Plains and in the American Southwest including much of the nineteenth-century variability, the droughts of the 1930s (the “Dust Bowl”) and 1950s, and the very wet period in the 1990s. Results indicate that the persistent droughts and pluvials were ultimately forced by persistent variations of tropical Pacific surface ocean temperatures. It is argued that ocean temperature variations outside of the tropical Pacific, but forced from the tropical Pacific, act to strengthen the droughts and pluvials. The persistent precipitation variations are part of a pattern of global variations that have a strong hemispherically and zonally symmetric component, which is akin to interannual variability, and that can be explained in terms of interactions between tropical ocean temperature variations, the subtropical jets, transient eddies, and the eddy-driven mean meridional circulation. Rossby wave propagation poleward and eastward from the tropical Pacific heating anomalies disrupts the zonal symmetry, intensifying droughts and pluvials over North America. Both mechanisms of tropical driving of extratropical precipitation variations work in summer as well as winter and can explain the year-round nature of the precipitation variations. In addition, land–atmosphere interactions over North America appear important by (i) translating winter precipitation variations into summer evaporation and, hence, precipitation anomalies and (ii) shifting the northward flow of moisture around the North Atlantic subtropical anticyclone eastward from the Plains and Southwest to the eastern seaboard and western Atlantic Ocean.

Description: The possible role that tropical Pacific SSTs played in driving the megadroughts over North America during the medieval period is addressed. Fossil coral records from the Palmyra Atoll are used to derive tropical Pacific SSTs for the period from a.d. 1320 to a.d. 1462 and show overall colder conditions as well as extended multidecadal La Niña–like states. The reconstructed SSTs are used to force a 16-member ensemble of atmosphere GCM simulations, each with different initial conditions, with the atmosphere coupled to a mixed layer ocean outside of the tropical Pacific. Model results are verified against North American tree ring reconstructions of the Palmer Drought Severity Index. A singular value decomposition analysis is performed using the soil moisture anomaly simulated by another 16-member ensemble of simulations forced by global observed SSTs for 1856–2004 and tree ring reconstructions of the Palmer Drought Severity Index for the same period. This relationship is used to transfer the modeled medieval soil moisture anomaly (relative to the modern simulation) into a model-estimated Palmer Drought Severity Index. The model-estimated Palmer Drought Severity Index reproduces many aspects of both the interannual and decadal variations of the tree ring reconstructions, in addition to an overall drier climate that is drier than the tree ring records suggest. The model-estimated Palmer Drought Severity Index simulates two previously identified “megadroughts,” a.d. 1360–1400 and a.d. 1430–60, with a realistic spatial pattern and amplitude. In contrast, the model fails to produce a period of more normal conditions in the early fifteenth century that separated these two megadroughts. The dynamical link between tropical SSTs and the North American megadroughts is akin to that operating in modern droughts. The model results are used to argue that the tropical Pacific played an active role in driving the megadroughts. However, the match between simulated and reconstructed hydroclimate is such that it is likely that both the coral-reconstructed SST anomalies contain significant errors and that SST anomalies in other basins also played a role in driving hydroclimate variations over North America during the late medieval period.

Description: Both naturally occurring La Niña events and model-projected anthropogenic-driven global warming are associated with widespread drying in the subtropics to midlatitudes. Models suggest anthropogenic drying should already be underway but climate variability on interannual to multidecadal time scales can easily obscure any emerging trend, making it hard to assess the validity of the simulated forced change. Here, the authors address this problem by using model simulations and the twentieth-century reanalysis to distinguish between natural variability of, and radiatively forced change in, hydroclimate on the basis of the mechanisms of variations in the three-dimensional moisture budget that drive variations in precipitation minus evaporation (P − E). Natural variability of P − E is dominated by the El Niño–Southern Oscillation (ENSO) cycle and is “dynamics dominated” in that the associated global P − E anomalies are primarily driven by changes in circulation. This is quite well reproduced in the multimodel mean of 15 models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4)/Coupled Model Intercomparison Project 3 (CMIP3). In contrast, radiatively forced P − E change is “thermodynamics mediated” in that the rise in specific humidity leads to intensified patterns of moisture transport and P − E. But, as for ENSO, the poleward shift of the storm tracks and mean meridional circulation cells also contribute to changes in P − E. However, La Niña and radiatively forced changes in the zonal mean flow are distinct in the tropics. These distinctions are applied to the post-1979 record of P − E in the twentieth-century reanalysis. ENSO-related variations strongly influence the observed P − E trend since 1979, but removal of this influence leaves an emerging pattern of P − E change consistent with the predictions of the IPCC AR4/CMIP3 models over this period together with, to some extent, consistent contributions from dynamical and thermodynamical mechanisms and consistent changes in the zonal mean circulation. The forced trends are currently weak compared to those caused by internal variability.

Description: Observations, atmosphere models forced by historical SSTs, and idealized simulations are used to determine the causes and mechanisms of interannual to multidecadal precipitation anomalies over southeast South America (SESA) since 1901. About 40% of SESA precipitation variability over this period can be accounted for by global SST forcing. Both the tropical Pacific and Atlantic Oceans share the driving of SESA precipitation, with the latter contributing the most on multidecadal time scales and explaining a wetting trend from the early midcentury until the end of the last century. Cold tropical Atlantic SST anomalies are shown to drive wet conditions in SESA. The dynamics that link SESA precipitation to tropical Atlantic SST anomalies are explored. Cold tropical Atlantic SST anomalies force equatorward-flowing upper-tropospheric flow to the southeast of the tropical heating anomaly, and the vorticity advection by this flow is balanced by vortex stretching and ascent, which drives the increased precipitation. The 1930s Pampas Dust Bowl drought occurred, via this mechanism, in response to warm tropical Atlantic SST anomalies. The atmospheric response to cold tropical Pacific SSTs also contributed. The tropical Atlantic SST anomalies linked to SESA precipitation are the tropical components of the Atlantic multidecadal oscillation. There is little evidence that the large trends over past decades are related to anthropogenic radiative forcing, although models project that this will cause a modest wetting of the climate of SESA. As such, and if the Atlantic multidecadal oscillation has shifted toward a warm phase, it should not be assumed that the long-term wetting trend in SESA will continue. Any reversal to a drier climate more typical of earlier decades would have clear consequences for regional agriculture and water resources.

Description: This study aims to understand the dynamical mechanisms driving the changes in the general circulation of the atmosphere due to increased carbon dioxide (CO2) by looking into the transient step-by-step adjustment of the circulation. The transient atmospheric adjustment is examined using the National Center for Atmospheric Research Community Atmosphere Model, version 3, coupled to a slab ocean model, and the CO2 concentration in the atmosphere is uniformly and instantaneously doubled. The thermal structure and circulation response is well established after one year of integration, with the magnitudes gradually increasing afterward toward quasi equilibrium. Tropical upper-tropospheric warming occurs in the first month. The expansion of the warming in the mid- and upper troposphere to the subtropics occurs later and is found to be primarily dynamically driven due to the intensification of transient eddy momentum flux convergence and resulting anomalous descending motion in this region. The poleward displacement of the midlatitude tropospheric jet streams occurs together with the change in eddy momentum flux convergence, but only after the intensification of the subpolar westerlies in the stratosphere. The results demonstrate the importance of the tropospheric eddies in setting up the extratropical tropospheric response to global warming.

Description: Could the Dust Bowl drought of the 1930s have been predicted in advance if the SST anomalies of the 1930s had been foreknown? Ensembles of model simulations forced with historical observed SSTs in the global ocean, and also separately in the tropical Pacific and Atlantic Oceans, are compared with an ensemble begun in January 1929 with modeled atmosphere and land initial conditions and integrated through the 1930s with climatological SSTs. The ensemble with climatological SSTs produces values for the precipitation averaged over 1932–39 that are not statistically different from model climatology. In contrast, the ensembles with global SST forcing produce a drought centered in the central plains and southwestern North America that is clearly separated from the model climatology. Both the tropical Pacific and northern tropical Atlantic SST anomalies produce a statistically significant model drought in this region. The modeled drought has a spatial pattern that is different from the observed drought, which was instead centered in the central and northern plains and also impacted the northern Rocky Mountain states but not northeastern Mexico. The model error in extending the Dust Bowl drought too far south is attributed to an incorrect response of the model to warm subtropical North Atlantic SST anomalies. The model error in the northern states cannot be attributed to an incorrect response to tropical SST anomalies. The model also fails to reproduce the strong surface air warming across most of the continent during the 1930s. In contrast, the modeled patterns of precipitation reduction and surface air temperature warming during the 1950s drought are more realistic. Tree-ring records show that the Dust Bowl pattern of drought has occurred before, suggesting that while the extensive human-induced land surface degradation and dust aerosol loading of the 1930s drought may have played an important role in generating the observed drought pattern, natural processes, possibly including land interactions, are capable of generating droughts centered to the north of the main ENSO teleconnection region. Despite this caveat, advance knowledge of tropical SSTs alone would have allowed a high-confidence prediction of a multiyear and severe drought, but one centered too far south and without strong cross-continental warming.

Description: The dynamical mechanisms associated with the impact of year-to-year variability in tropical North Atlantic (TNA) sea surface temperatures (SSTs) on North American precipitation, during the cold and warm halves of the hydrological year (October–September) are examined. Observations indicate that during both seasons warmer-than-normal TNA SSTs are associated with a reduction of precipitation over North America, mainly west of ∼90°W, and that the effect can be up to 30% of the year-to-year seasonal precipitation RMS variability. This finding confirms earlier studies with observations and models. During the cold season (October–March) the North American precipitation variability associated with TNA fluctuations is considerably weaker than its association with ENSO. During the warm season (April–September), however, the Atlantic influence, per one standard deviation of SST anomalies, is larger than that of ENSO. The observed association between TNA SST anomalies and global and North American precipitation and sea level pressure variability is compared with that found in the output of an atmospheric general circulation model (AGCM) forced with observed SST variability, both globally and in the tropical Atlantic alone. The similarity between model output and observations suggests that TNA SST variability is causal. The mechanisms of the “upstream” influence of the Atlantic on North American precipitation are seasonally dependent. In the warm season, warmer-than-normal TNA SSTs induce a local increase in atmospheric convection. This leads to a weakening of the North Atlantic subtropical anticyclone and a reduction in precipitation over the United States and northern Mexico, associated with the anomalous southward flow there. In the cold season, a response similar to the warm season over the subtropical Atlantic is identified, but there is also a concomitant suppression of convection over the equatorial Pacific, which leads to a weakening of the Aleutian low and subsidence over western North America, similar to the impact of La Niña although weaker in amplitude. The impact of TNA SST on tropical convection and the extratropical circulation is examined by a set of idealized experiments with a linear general circulation model forced with the tropical heating field derived from the full AGCM.

Description: The mechanisms of changes in the large-scale hydrological cycle projected by 15 models participating in the Coupled Model Intercomparison Project phase 3 and used for the Intergovernmental Panel on Climate Change’s Fourth Assessment Report are analyzed by computing differences between 2046 and 2065 and 1961 and 2000. The contributions to changes in precipitation minus evaporation, P − E, caused thermodynamically by changes in specific humidity, dynamically by changes in circulation, and by changes in moisture transports by transient eddies are evaluated. The thermodynamic and dynamic contributions are further separated into advective and divergent components. The nonthermodynamic contributions are then related to changes in the mean and transient circulation. The projected change in P − E involves an intensification of the existing pattern of P − E with wet areas [the intertropical convergence zone (ITCZ) and mid- to high latitudes] getting wetter and arid and semiarid regions of the subtropics getting drier. In addition, the subtropical dry zones expand poleward. The accentuation of the twentieth-century pattern of P − E is in part explained by increases in specific humidity via both advection and divergence terms. Weakening of the tropical divergent circulation partially opposes the thermodynamic contribution by creating a tendency to decreased P − E in the ITCZ and to increased P − E in the descending branches of the Walker and Hadley cells. The changing mean circulation also causes decreased P − E on the poleward flanks of the subtropics because the descending branch of the Hadley Cell expands and the midlatitude meridional circulation cell shifts poleward. Subtropical drying and poleward moistening are also contributed to by an increase in poleward moisture transport by transient eddies. The thermodynamic contribution to changing P − E, arising from increased specific humidity, is almost entirely accounted for by atmospheric warming under fixed relative humidity.