Bibliography - Ants Leetma

1. Lau, Ngar-Cheung, Ants Leetma, and Mary Jo Nath, February 2008: Interactions between the responses of North American climate to El Niño–La Niña and to the secular warming trend in the Indian–Western Pacific Oceans. Journal of Climate, 21(3), 476-494.
   [ Abstract PDF ]

   The modulation of El Niño and La Niña responses by the long-term sea surface temperature (SST) warming trend in the Indian–Western Pacific (IWP) Oceans has been investigated using a large suite of sensitivity integrations with an atmospheric general circulation model. These model runs entail the prescription of anomalous SST conditions corresponding to composite El Niño or La Niña episodes, to SST increases associated with secular warming in IWP, and to combinations of IWP warming and El Niño/La Niña. These SST forcings are derived from the output of coupled model experiments for climate settings of the 1951–2000 and 2001–50 epochs. Emphasis is placed on the wintertime responses in 200-mb height and various indicators of surface climate in the North American sector. The model responses to El Niño and La Niña forcings are in agreement with the observed interannual anomalies associated with warm and cold episodes. The wintertime model responses in North America to IWP warming bear a distinct positive (negative) spatial correlation with the corresponding responses to La Niña (El Niño). Hence, the amplitude of the combined responses to IWP warming and La Niña is notably higher than that to IWP warming and El Niño. The model projections indicate that, as the SST continues to rise in the IWP sector during the twenty-first century, the strength of various meteorological anomalies accompanying La Niña (El Niño) will increase (decrease) with time. The response of the North American climate and the zonal mean circulation to the combined effects of IWP forcing and La Niña (El Niño) is approximately equal to the linear sum of the separate effects of IWP warming and La Niña (El Niño). The summertime responses to IWP warming bear some similarity to the meteorological anomalies accompanying extended droughts and heat waves over the continental United States.

2. Seager, R, Mingfang Ting, Isaac Held, Y Kushnir, Jian Lu, G A Vecchi, H-P Huang, N Harnik, Ants Leetma, Ngar-Cheung Lau, C Li, Jennifer Velez, and N Naik, 2007: Model projections of an imminent transition to a more arid climate in southwestern North America. Science, 316(5828), doi:10.1126/science.1139601.
   [ Abstract ]

   How anthropogenic climate change will affect hydroclimate in the arid regions of southwestern North America has implications for the allocation of water resources and the course of regional development. Here we show that there is a broad consensus among climate models that this region will dry in the 21st century and that the transition to a more arid climate should already be under way. If these models are correct, the levels of aridity of the recent multiyear drought or the Dust Bowl and the 1950s droughts will become the new climatology of the American Southwest within a time frame of years to decades.

3. Lau, Ngar-Cheung, Ants Leetma, and Mary Jo Nath, 2006: Attribution of Atmospheric Variations in the 1997–2003 Period to SST Anomalies in the Pacific and Indian Ocean Basins. Journal of Climate, 19(15), 3507-3628.
   [ Abstract PDF ]

   The individual impacts of sea surface temperature (SST) anomalies in the deep tropical eastern–central Pacific (DTEP) and Indo-western–central Pacific (IWP) on the evolution of the observed global atmospheric circulation during the 1997–2003 period have been investigated using a new general circulation model. Ensemble integrations were conducted with monthly varying SST conditions being prescribed separately in the DTEP sector, the IWP sector, and throughout the World Ocean. During the 1998–2002 subperiod, when prolonged La Niña conditions occurred in DTEP and the SST in IWP was above normal, the simulated midlatitude atmospheric responses to SST forcing in the DTEP and IWP sectors reinforced each other. The anomalous geopotential height ridges at 200 mb in the extratropics of both hemispheres exhibited a distinct zonal symmetry. This circulation change was accompanied by extensive dry and warm anomalies in many regions, including North America. During the 1997–98 and 2002–03 El Niño events, the SST conditions in both DTEP and IWP were above normal, and considerable cancellations were simulated between the midlatitude responses to the oceanic forcing from these two sectors. The above findings are contrasted with those for the 1953–58 and 1972–77 periods, which were characterized by analogous SST developments in DTEP, but by cold conditions in IWP. It is concluded that a warm anomaly in IWP and a cold anomaly in DTEP constitute the optimal SST configuration for generating zonally elongated ridges in the midlatitudes. Local diagnoses indicate that the imposed SST anomaly alters the strength of the zonal flow in certain longitudinal sectors, which influences the behavior of synoptic-scale transient eddies farther downstream. The modified eddy momentum transports in the regions of eddy activity in turn feed back on the local mean flow, thus contributing to its zonal elongation. These results are consistent with the inferences drawn from zonal mean analyses, which accentuate the role of the eddy-induced circulation on the meridional plane.

4. Vecchi, G A., Brian J Soden, Andrew T Wittenberg, Isaac Held, Ants Leetma, and Matthew J Harrison, 2006: Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing. Nature, 441(7089), 73-76.
   [ Abstract PDF ]

   Since the mid-nineteenth century the Earth's surface has warmed1, 2, 3, and models indicate that human activities have caused part of the warming by altering the radiative balance of the atmosphere1, 3. Simple theories suggest that global warming will reduce the strength of the mean tropical atmospheric circulation4, 5. An important aspect of this tropical circulation is a large-scale zonal (east–west) overturning of air across the equatorial Pacific Ocean—driven by convection to the west and subsidence to the east—known as the Walker circulation6. Here we explore changes in tropical Pacific circulation since the mid-nineteenth century using observations and a suite of global climate model experiments. Observed Indo-Pacific sea level pressure reveals a weakening of the Walker circulation. The size of this trend is consistent with theoretical predictions, is accurately reproduced by climate model simulations and, within the climate models, is largely due to anthropogenic forcing. The climate model indicates that the weakened surface winds have altered the thermal structure and circulation of the tropical Pacific Ocean. These results support model projections of further weakening of tropical atmospheric circulation during the twenty-first century4, 5, 7.

5. Karl, T R., J Lawrimore, and Ants Leetma, 2005: Observational and modeling evidence of climate change. EM, 11-17.
   [ Abstract PDF ]

   Scientific understanding of past changes and variability in the Earth's climate comes from the analysis of numerous sources of instrumental and proxy data, as well as computer model simulations of the Sun/Earth/atmosphere processes causing the changes. Subject to certain limitations and uncertainties, instrumental data are available to assess climate change and variability from the late 1800s to present. The study of climate in the preceding centuries is possible primarily through paleoclimate reconstructions using proxy sources such as tree rings, marine and lake sediments, ice cores, corals, and borehole records. Improvements in monitoring systems have been made with technological advances such as radar and satellite instruments, but climate research continues to be affected by a number of limitations, including inadequate spatial coverage of many variables and systems with inadequate instrument calibration methods. Model simulations of the Earth's changing climate over many centuries are possible by numerical solutions of time-dependent equations on supercomputers. The following are brief summaries of observed changes in the Earth's climate resulting from studies of instrumental and proxy records, as well as simulated climate from global climate models. We also include a discussion of inherent problems in observing systems and future solutions that will further enhance the scientific understanding of the Earth's climate.

6. Lau, Ngar-Cheung, Ants Leetma, Mary Jo Nath, and H L Wang, 2005: Influences of ENSO-induced Indo-Western Pacific SST anomalies on extratropical atmospheric variability during the Boreal summer. Journal of Climate, 18(15), doi:10.1175/JCLI3445.1.
   [ Abstract ]

   The causes for the observed occurrence of anomalous zonally symmetric upper-level pressure ridges in the midlatitude belts of both hemispheres during the year after warm El Niño-Southern Oscillation (ENSO) events have been investigated. Sea surface temperature (SST) anomalies in the Indo-western Pacific (IWP) sector were simulated by allowing an oceanic mixed layer model for that region to interact with local atmospheric changes forced remotely by observed ENSO episodes in the eastern/central tropical Pacific. The spatiotemporal evolution of these SST conditions through a composite ENSO cycle was then inserted as lower boundary conditions within the IWP domain in an ensemble of atmospheric general circulation model (GCM) integrations. This experimental setup is seen to reproduce zonally symmetric geopotential height anomalies with maximum amplitudes being attained over the extratropics in the boreal summer after the peak phase of ENSO. The model evidence hence supports the notion that these global-scale atmospheric changes are primarily responses to SST perturbations in IWP, which are in turn linked to ENSO variability in the equatorial Pacific by the "atmospheric bridge" mechanism. Experimentation with a stationary wave model indicates that the Eastern Hemisphere portion of the aforementioned atmospheric signals are attributable to forcing by tropical heat sources and sinks associated with precipitation anomalies in the IWP region, which are closely related to the underlying SST changes. Diagnosis of the output from the GCM integrations reveals that these circulation changes due to diabatic heating are accompanied by alterations of the propagation path and intensity of the high-frequency eddies at locations farther downstream. The geopotential tendencies associated with the latter disturbances bear some resemblance to the anomalous height pattern in the Western Hemisphere. Such local eddy–mean flow feedbacks hence contribute to the zonal symmetry of the atmospheric response pattern to forcing in the IWP region. Analysis of zonally averaged circulation statistics indicates that the mean meridional circulation induced by divergence of anomalous transient eddy momentum fluxes in ENSO events could also generate zonally symmetric perturbations in midlatitudes. The model-simulated precipitation and surface temperature anomalies in the North American sector in response to SST changes in IWP suggest an increased frequency of droughts and heat waves in that region during the summer season after warm ENSO events.

7. Leetma, Ants, 2003: Seasonal forecasting: Innovations in practice and institutions. Bulletin of the American Meteorological Society, 84(12), 1686-1691.
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8. Higgins, R W., Ants Leetma, and V E Kousky, 2002: Relationships between climate variability and winter temperature extremes in the United States. Journal of Climate, 15(13), 1555-1572.
   [ Abstract PDF ]

   Time series representing two of the climate systems leading patterns of variability, namely El Niño–Southern Oscillation (ENSO) and the Arctic Oscillation (AO), are used together with 50 yr of daily mean surface air temperature data over the conterminous United States to diagnose relationships between winter temperature extremes and interannual climate variability. The aim is to focus attention on some of the physical phenomena that climate models must be able to simulate in order to be deemed credible for use in weather and climate forecasts and assessments. Since the 1950s there has been considerable decadal variability in winter surface air temperature extremes. At most locations in the United States the number of daily extremes is reduced during El Niño, and increased during La Niña and ENSO-neutral years. These changes are qualitatively consistent with a decrease in the daily mean surface air temperature variance during El Niño relative to La Niña and ENSO neutral. Changes in the number of warm extremes during a particular AO phase are largely compensated for by changes in the number of cold extremes, so that the net change in the numbers of surface air temperature extremes is close to zero. However, the AO is associated with larger changes in mean temperature than ENSO.

9. Wang, W, M Ji, A Kumar, and Ants Leetma, 2002: Dynamical forecasts of tropical Pacific SST. Experimental Long-Lead Forecast Bulletin, 11(5), 17-20.
10. Kumar, A, W-Q Wang, M P Hoerling, Ants Leetma, and M Ji, 2001: The sustained North American warming of 1997 and 1998. Journal of Climate, 14(3), 345-353.
    [ Abstract PDF ]

    North America experienced sustained and strong surface warming during 1997 and 1998. This period coincided with a dramatic swing of the El Niño–Southern Oscillation (ENSO), with El Niño in 1997 rapidly replaced by La Niña in 1998. An additional aspect of the sea surface temperatures (SSTs) was the warmth of the world oceans as a whole for the entire period, with unprecedented amplitudes within the recent instrumental record. Using a suite of dynamical and empirical model simulations, this study examines the causes for the North American warming, focusing on the role of the sea surface boundary conditions. Two sets of atmospheric general circulation model experiments, one forced with the observed global SSTs and the other with the tropical east Pacific portion only, produce similar North American wide-warming during fall and winter of 1997. The GCM results match empirical estimates of the canonical temperature response related to a strong El Niño and confirm that east equatorial Pacific SST forcing was a major factor in the continental warming of 1997. Perpetuation of that warming from spring through fall of 1998 is shown to be unrelated to equatorial east Pacific SSTs and thus cannot be attributed to the ENSO cycle directly. Yet, simulations using the observed global SSTs are shown to reproduce realistically the continuation of North American warming throughout 1998. The continental warmth occurs in tandem with a warming of the troposphere that, initially confined to tropical latitudes during El Niño's peak in 1997, spreads poleward and covers the entire globe in 1998. This evolutionary aspect of the global circulation anomalies during 1997 and 1998 is found to be a response to global SSTs and not linked directly to ENSO's evolution. Results presented here demonstrate that a significant fraction of the North American warming in 1997 and 1998 is explainable as the forced response to sea surface boundary conditions. The hand-over in the impact of those SSTs, with a classic ENSO driven signal in 1997 but an outwardly independent signal in 1998 related to the disposition of global SSTs outside the ENSO region is emphasized. The high potential predictability of North American climate during this 2-yr period raises new questions on the role of global SSTs in climate variability and the ability to predict them skillfully.

11. Leetma, Ants, W Higgins, D Anderson, P Delecluse, and M Latif, 2001: Application of seasonal to interannual predictions: a Northern Hemisphere perspective In Observing the Oceans in the 21st Century, Melbourne, Australia, Uniprint Pty. Ltd., 39-47.
    [ Abstract ]

    Climate variability in many parts of the world can impact a range of social and economic sectors. The dominant source of interannual global climate variability is the El Niño-Southern Oscillation phenomenon (ENSO). Over the past few decades scientists have developed a capability to forecast aspects of ENSO and its impacts several seasons in advance using statistical and dynamical tests. This capability was used to mitigate some of the impacts of the major 1997-98 El Niño in many countries. However, ENSO accounts for only part of the observed variability. Other sources of climate variability are also important on seasonal, interannual, and decadal timescales. These fall into two broad classes. The first, of which ENSO is the best-known member, is related to global tropical rainfall variation on intraseasonal, seasonal, and decadal timescales. A better understanding of all the sources of rainfall variability—on all timescales and throughout the global tropics—will likely lead to improved forecast skill, since this builds on existing understanding and forecast systems developed for ENSO. The second class consists of changes in zonal flows in the midlatitudes; examples of this class are the North Atlantic Oscillation and the Arctic Oscillation. More research is required in order to develop an understanding of the origins of these zonal flow changes and to convert this into skillful forecast methodologies. Research into all of these and other yet unexplained phenomena and the subsequent development of improved seasonal forecasts will require expansions of the existing ocean observing system.

12. Wang, W, M Ji, A Kumar, and Ants Leetma, 2001: Dynamical forecasts of tropical Pacific SST. Experimental Long-Lead Forecast Bulletin, 10(3), 11-14.
13. Latif, M, D Anderson, T P Barnett, M Cane, R Kleeman, Ants Leetma, J O'Brien, Anthony Rosati, and E K Schneider, 1998: A review of the predictability and prediction of ENSO. Journal of Geophysical Research, 103(C7), 14,375-14,393.
    [ Abstract PDF ]

    A hierarchy of El Niño-Southern Oscillation (ENSO) prediction schemes has been developed during the Tropical Ocean-Global Atmosphere (TOGA) program which includes statistical schemes and physical models. The statistical models are, in general, based on linear statistical techniques and can be classified into models which use atmospheric (sea level pressure or surface wind) or oceanic (sea surface temperature or a measure of upper ocean heat content) quantities or a combination of oceanic and atmospheric quantities as predictors. The physical models consist of coupled ocean-atmosphere models of varying degrees of complexity, ranging from simplified coupled models of the "shallow water" type to coupled general circulation models. All models, statistical and physical, perform considerably better than the persistence forecast in predicting typical indices of ENSO on lead times of 6 to 12 months. The TOGA program can be regarded as a success from this perspective. However, despite the demonstrated predictability, little is known about ENSO predictability limits and the predictability of phenomena outside the tropical Pacific. Furthermore, the predictability of anomalous features known to be associated with ENSO (e.g., Indian monsoon and Sahel rainfall, southern African drought, and off-equatorial sea surface temperature) needs to be addressed in more detail. As well, the relative importance of different physical mechanisms (in the ocean or atmosphere) has yet to be established. A seasonal dependence in predictability is seen in many models, but the processes responsible for it are not fully understood, and its meaning is still a matter of scientific discussion. Likewise, a marked decadal variation in skill is observed, and the reasons for this are still under investigation. Finally, the different prediction models yield similar skills, although they are initialized quite differently. The reasons for these differences are also unclear.

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