Hadley circulation. A side view depicting the familiar atmospheric overturning cell linking Earth's tropics and subtropics. In "On the dynamics of the Hadley circulation and subtropical drying," Karnauskas and Ummenhofer (2014) argue that this perspective hides some of the basic, underlying dynamics and spatial structure of the Hadley circulation.

One of the primary research themes of the Oceans and Climate Lab is to understand the dynamics of Earth's climate at global and regional scales. How and why does climate vary from year to year, decade to decade, century to century, and beyond? A particular focus is on the dynamics of the tropical ocean and atmosphere as a coupled system, such as the El Nino-Southern Oscillation (ENSO) and other intrinsic modes of climate variability. How do the tropics, extratropics, and high latitude regions interact? How will key aspects of Earth's climate respond to the radiative forcing associated with greenhouse gas emissions over the course of this century? The Oceans and Climate Lab at CU Boulder uses observations (including satellites) and numerical models to explore these questions.

Support: NOAA, NSF, US CLIVAR, Alfred P. Sloan Foundation, Singapore Ministry of Education

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Despite the importance and vulnerability of small island nations and their ecosystems, they frequently have insufficient observations to provide baseline or long–term perspectives on climate variability and change, and global model experiments rarely have the resolution to include them. Many studies in observational climatology, climate modeling, and paleoclimate thus depend to varying degrees on an approximation equating near–surface marine and terrestrial island climates, often with direct implications for island societies and resources. Here we investigate the validity of this approximation, and by extension the viability of offshore moorings to serve as proxies for island climate variability, by comparing monthly mean observations on a diverse set of 17 islands across the global tropics with those from ocean moorings proximate to each island. While some island–mooring pairs exhibit a mean offset in surface air temperature, these cannot be explained as a simple function of one parameter including distance from the mooring, station elevation, or island dimensions. Overall, the seasonal to interannual variability in near–surface climate at monthly time scales at meteorological stations on tropical islands is captured remarkably well by moorings positioned up to 1,000 km offshore.

Karnauskas, K. B., J. P. Donnelly, and K. J. Anchukaitis, 2017: An intercomparison of monthly surface air temperature on islands and proximate moorings across the tropical Indo–Pacific. arXiv, pending.

The El Niño-Southern Oscillation (ENSO) is the dominant mode of tropical Pacific climate variability at interannual timescales, with profound influences on seasonal weather and ecosystems worldwide. In particular, the physical and biological conditions along the US West Coast, an area that supports one of the most productive marine ecosystems in the world, are strongly influenced by ENSO. Specifically, during El Niño events, alongshore winds weaken and upwelling is reduced, resulting in warmer surface waters, reduced nutrient supply to the euphotic zone, and reduced biological productivity. While these conditions during El Niño events are well known, the exact mechanisms involved and the origin of event-to-event differences in ENSO impacts are not fully understood. Here, we review our current state of knowledge on ENSO and its different expressions, the mechanisms by which ENSO influences the US West Coast, and possible approaches for understanding the predictability of those impacts.

Capotondi, A., K. B. Karnauskas, A. Miller, and A. Subramanian, 2017: ENSO diversity and its implications for US West Coast marine ecosystems. US CLIVAR Variations, 15(1), 16–21.

Under a warming climate, amplification of the water cycle and changes in precipitation patterns over land are expected to occur, subsequently impacting the terrestrial water balance. On global scales, such changes in terrestrial water storage (TWS) will be reflected in the water contained in the ocean and can manifest as global sea level variations. Naturally occurring climate-driven TWS variability can temporarily obscure the long-term trend in sea level rise, in addition to modulating the impacts of sea level rise through natural periodic undulation in regional and global sea level. The internal variability of the global water cycle, therefore, confounds both the detection and attribution of sea level rise. Here, we use a suite of observations to quantify and map the contribution of TWS variability to sea level variability on decadal timescales. In particular, we find that decadal sea level variability centered in the Pacific Ocean is closely tied to low frequency variability of TWS in key areas across the globe. The unambiguous identification and clean separation of this component of variability is the missing step in uncovering the anthropogenic trend in sea level and understanding the potential for low-frequency modulation of future TWS impacts including flooding and drought.

Hamlington, B. D., J. T. Reager, M.–H. Lo, K. B. Karnauskas, and R. R. Leben, 2017: Separating decadal global water cycle variability from sea level rise. Nature Scientific Reports, 7, 995, doi: 10.1038/s41598-017-00875-5.

The effects of externally forced tropical sea surface temperature (SST) anomalies on long-term Walker circulation changes are investigated through numerical atmospheric general circulation model (AGCM) experiments. In response to the observed tropics-wide SST trend, which exhibits a prominent interbasin warming contrast (IBWC) with smaller warming in the Pacific than the Indian and Atlantic Oceans that includes a weak La Niña–like pattern in the equatorial Pacific, pronounced low-level easterly anomalies emerge over the equatorial Pacific. Through sensitivity experiments, the intensification of the Pacific trade winds (PTWs) is attributable to the IBWC, whereas the slightly enhanced zonal SST gradient within the equatorial Pacific plays a small role relative to the observed IBWC. It is further demonstrated that the greater Indian Ocean warming forces low-level easterly anomalies over the entire equatorial Pacific, while the greater tropical Atlantic warming-driven enhancement of PTWs is located over the central equatorial Pacific. In contrast to observations, a negligible IBWC emerges in the tropical SST trends of CMIP5 historical simulations due to a strong El Niño–like warming in the tropical Pacific. Lacking the observed IBWC (and the observed enhancement of the zonal SST gradient within the equatorial Pacific), the PTWs in the CMIP5 ensemble can only weaken.

Zhang, L., and K. B.  Karnauskas, 2017: The Role of Tropical Interbasin SST Gradients in Forcing Walker Circulation Trends. J. Climate, 30(2), 499–508, doi: 10.1175/JCLI-D-16-0349.1.

Water evaporating from the ocean sustains precipitation on land. This ocean-to-land moisture transport leaves an imprint on sea surface salinity (SSS). Thus, the question arises of whether variations in SSS can provide insight into terrestrial precipitation. This study provides evidence that springtime SSS in the subtropical North Atlantic ocean can be used as a predictor of terrestrial precipitation during the subsequent summer monsoon in Africa. Specifically, increased springtime SSS in the central to eastern subtropical North Atlantic tends to be followed by above-normal monsoon-season precipitation in the African Sahel. In the spring, high SSS is associated with enhanced moisture flux divergence from the subtropical oceans, which converges over the African Sahel and helps to elevate local soil moisture content. From spring to the summer monsoon season, the initial water cycling signal is preserved, amplified, and manifested in excessive precipitation. According to our analysis of currently available soil moisture data sets, this 3-month delay is attributable to a positive coupling between soil moisture, moisture flux convergence, and precipitation in the Sahel. Because of the physical connection between salinity, ocean-to-land moisture transport, and local soil moisture feedback, seasonal forecasts of Sahel precipitation can be improved by incorporating SSS into prediction models. Thus, expanded monitoring of ocean salinity should contribute to more skillful predictions of precipitation in vulnerable subtropical regions, such as the Sahel.

Li, L., R. W. Schmitt, C. C. Ummenhofer, and K. B. Karnauskas, 2016: North Atlantic salinity as a predictor of Sahel rainfall. Science Advances, doi: 10.1126/sciadv.1501588.

Moisture originating from the subtropical North Atlantic feeds precipitation throughout the Western Hemisphere. This ocean-to-land moisture transport leaves its imprint on sea surface salinity (SSS), enabling SSS over the subtropical oceans to be used as an indicator of terrestrial precipitation. This study demonstrates that springtime SSS over the northwestern portion of the subtropical North Atlantic significantly correlates with summertime precipitation over the United States (US) Midwest. The linkage between springtime SSS and the Midwest summer precipitation is established through ocean-to-land moisture transport followed by a soil moisture feedback over the southern US. In the spring, high SSS over the northwestern subtropical Atlantic coincides with a local increase in moisture flux divergence. The moisture flux is then directed toward and converges over the southern US, which experiences increased precipitation and soil moisture. The increased soil moisture influences the regional water cycle both thermodynamically and dynamically, leading to excessive summer precipitation in the Midwest. Thermodynamically, the increased soil moisture tends to moisten the lower troposphere and enhances the meridional humidity gradient north of 36°N. Thus, more moisture will be transported and converged into the Midwest by the climatological low-level wind. Dynamically, the increases in soil moisture over the Southern US enhance the west-to-east soil moisture gradient eastward of the Rocky Mountains, which can help to intensify the Great Plains Low-level jet in the summer converging more moisture into the Midwest. Due to these robust physical linkages, the springtime SSS outweighs the leading SST modes in predicting the Midwest summer precipitation and significantly improves rainfall prediction in this region.

Li, L., R. W. Schmitt, C. C. Ummenhofer, and K. B. Karnauskas, 2016: Implications of North Atlantic Sea Surface Salinity for Summer Precipitation over the US Midwest: Mechanisms and Predictive Value. J. Climate, doi: 0.1175/JCLI-D-15-0520.1.

Ice shelves modulate Antarctic contributions to sea-level rise1 and thereby represent a critical, climate-sensitive interface between the Antarctic ice sheet and the global ocean. Following rapid atmospheric warming over the past decades2,3 , Antarctic Peninsula ice shelves have progressively retreated4, at times catastrophically5. This decay supports hypotheses of thermal limits of viability for ice shelves via surface melt forcing3,5,6. Here we use a polar-adapted regional climate model7 and satellite observations8 to quantify the nonlinear relationship between surface melting and summer air tempera- ture. Combining observations and multimodel simulations, we examine melt evolution and intensification before observed ice shelf collapse on the Antarctic Peninsula. We then assess the twenty-first-century evolution of surface melt across Antarctica under intermediate and high emissions climate scenarios. Our projections reveal a scenario-independent doubling of Antarctic-wide melt by 2050. Between 2050 and 2100, however, significant divergence in melt occurs between the two climate scenarios. Under the high emissions pathway by 2100, melt on several ice shelves approaches or surpasses intensities that have historically been associated with ice shelf collapse, at least on the northeast Antarctic Peninsula.

Trusel, L. D., K. E. Frey, S. B. Das, K. B. Karnauskas, P. K. Munneke, E. van Meijgaard, and M. R. van den Broeke, 2015: Divergent trajectories of Antarctic surface melt under two twenty–first–century climate scenarios. Nature Geoscience, 8, 927–932, doi: 10.1038/ngeo2563.

Carilli et al. [2014] present new geochemical proxy records of sea surface temperature (SST) and salinity spanning 1959–2010 from coral cores collected from Butaritari Atoll, Gilbert Islands, Republic of Kiribati (3.07°N, 172.75°E). The continuous measurement of SST by in situ and satellite platforms generally began in the early 1980s (e.g., the Tropical Atmosphere Ocean [TAO] array of moorings [McPhaden et al. 1998] and the Advanced Very High Resolution Radiometer [AVHRR] infrared satellite [Reynolds et al. 2002]), and instrumental reconstructions extending back to the mid–nineteenth century (e.g., Smith et al. 2008) rely on sparse observational sampling both in space in time. The Carilli et al. [2014] records thus represent a potentially valuable addition to estimates of past climate variability in the important western tropical Pacific region, confirming the dominant role of El Niño–Southern Oscillation (ENSO). However, the conclusion that the Walker Circulation has exhibited a long–term weakening trend over the last several decades is based on an interpretation of the proxy record that does not consider well–documented natural interdecadal variability, and is based on trends in a proxy record that are opposite from trends in the observational record over the overlapping portion of the satellite era. In addition, Carilli et al. [2014] misinterpret the relationship between their proxy record of SST and trends in ocean circulation due to incorrect assumptions about the physical oceanographic context of Butaritari. These two issues are addressed below.

Karnauskas, K. B., A. L. Cohen, and E. J. Drenkard, 2015: Comment on “Equatorial Pacific coral geochemical records show recent weakening of the Walker Circulation” by J. Carilli et al. Paleoceanography, 30(5), 570–574, doi: 10.1002/2014PA002753.

The El Niño Southern Oscillation (ENSO) is a naturally occurring mode of tropical Pacific variability, with global impacts on society and natural ecosystems. While it has long been known that El Niño events display a diverse range of amplitudes, triggers, spatial patterns, and life cycles, the realization that ENSO's impacts can be highly sensitive to this event-to-event diversity is driving a renewed interest in the subject. This paper surveys our current state of knowledge of ENSO diversity, identifies key gaps in understanding, and outlines some promising future research directions.

Capotondi, A., A. T. Wittenberg, M. Newman, E. Di Lorenzo, J.–Y. Yu, P. Braconnot, J. Cole, B. Dewitte, B. Giese, E. Guilyardi, F.–F. Jin, K. Karnauskas, B. Kirtman, T. Lee, N. Schneider, Y. Xue, and S.–W. Yeh, 2015: Understanding ENSO diversity. Bull. Amer. Meteor. Soc., 96(6), 921–938, doi: 10.1175/BAMS-D-13-00117.1.

Recent work on the influence of the Galápagos Archipelago on the ocean circulation of the tropical Pacific and global climate variability is reviewed from the standpoint of numerical models, high–resolution satellite measurements, and in situ observations, including updates with new model results and recent observations. For climate variations over years, decades, centuries, and well beyond, the Galápagos is geographically positioned in the heart of some of the largest and most important changes. Models and observations provide compelling evidence that the Galápagos cannot be neglected in regional and basin–scale ocean dynamics and biogeochemistry, warranting further measurements and hypothesis-testing. Differences between various model experiments and their potential implications for the physical and biogeochemical state of the tropical Pacific Ocean make a clear case that much depends on the nature of the island–current interactions and processes involved. Implications for climate model improvement and global change are discussed.

Karnauskas, K. B., R. Murtugudde, and W. B. Owens, 2014: Climate and the Global Reach of the Galápagos Archipelago: State of the Knowledge, in The Galápagos: A Natural Laboratory for the Earth Sciences (eds K. S. Harpp, E. Mittelstaedt, N. d'Ozouville and D. W. Graham), John Wiley & Sons, Inc, Hoboken, New Jersey, doi: 10.1002/9781118852538.ch11.

Changes in subtropical precipitation and the Hadley circulation (HC) are inextricably linked. The original Halley–Hadley model cannot explain certain aspects of the Earth’s meridional circulation in the tropics and is of limited use in understanding regional changes in precipitation. Here, we expand on previous work on the regional and seasonal aspects of the HC, in particular how land–sea temperature contrasts contribute to the strength and width of the HC. We show that the Earth’s HC is well described by three regionally distinct cells along the eastern edges of the major ocean basins with opposite circulations elsewhere. Moreover, comparable summertime hemisphere cells emerge in each region. While it has been recognized that continents modify the meridional pressure gradient, we demonstrate that a substantial part of the Earth’s HC is driven by zonal pressure gradients (ZPGs) that only exist due to continental heating and air–sea interaction. Projected changes in land–sea temperature contrasts in a warming climate due to the relatively low thermal capacity of land will also affect ZPGs and thus HC strength and width, with implications for extremes in hydroclimate and freshwater resources across the increasingly vulnerable subtropics.

Karnauskas, K. B., and C. C. Ummenhofer, 2014: On the dynamics of the Hadley circulation and subtropical drying. Clim. Dyn., 42(9–10), 2259–2269, doi: 10.1007/s00382-014-2129-1.

In part III of a three-part study on North American climate in phase 5 of the Coupled Model Intercomparison Project (CMIP5) models, the authors examine projections of twenty-first-century climate in the representative concentration pathway 8.5 (RCP8.5) emission experiments. This paper summarizes and synthesizes results from several coordinated studies by the authors. Aspects of North American climate change that are examined include changes in continental-scale temperature and the hydrologic cycle, extremes events, and storm tracks, as well as regional manifestations of these climate variables. The authors also examine changes in the eastern North Pacific and North Atlantic tropical cyclone activity and North American intraseasonal to decadal variability, including changes in teleconnections to other regions of the globe. Projected changes are generally consistent with those previously published for CMIP3, although CMIP5 model projections differ importantly from those of CMIP3 in some aspects, including CMIP5 model agreement on increased central California precipitation. The paper also highlights uncertainties and limitations based on current results as priorities for further research. Although many projected changes in North American climate are consistent across CMIP5 models, substantial intermodel disagreement exists in other aspects. Areas of disagreement include projections of changes in snow water equivalent on a regional basis, summer Arctic sea ice extent, the magnitude and sign of regional precipitation changes, extreme heat events across the northern United States, and Atlantic and east Pacific tropical cyclone activity.

Maloney, E., and Coauthors, 2014: North American climate in CMIP5 experiments: Part III: Assessment of Twenty–First Century projections. J. Climate, 27(6), 2230–2270, doi: 10.1175/JCLI-D-13-00273.1.

The climate of West Antarctica is strongly influenced by remote forcing from the tropical Pacific. For example, recent surface warming over West Antarctica reflects atmospheric circulation changes over the Amundsen Sea, driven by an atmospheric Rossby wave response to tropical sea surface temperature (SST) anomalies. Here, it is demonstrated that tropical Pacific SST anomalies also influence the source and transport of marine-derived aerosols to the West Antarctic Ice Sheet. Using records from four firn cores collected along the Amundsen coast of West Antarctica, the relationship between sea ice–modulated chemical species and large-scale atmospheric variability in the tropical Pacific from 1979 to 2010 is investigated. Significant correlations are found between marine biogenic aerosols and sea salts, and SST and sea level pressure in the tropical Pacific. In particular, La Niña–like conditions generate an atmospheric Rossby wave response that influences atmospheric circulation over Pine Island Bay. Seasonal regression of atmospheric fields on methanesulfonic acid (MSA) reveals a reduction in onshore wind velocities in summer at Pine Island Bay, consistent with enhanced katabatic flow, polynya opening, and coastal dimethyl sulfide production. Seasonal regression of atmospheric fields on chloride (Cl−) reveals an intensification in onshore wind velocities in winter, consistent with sea salt transport from offshore source regions. Both the source and transport of marine aerosols to West Antarctica are found to be modulated by similar atmospheric dynamics in response to remote forcing. Finally, the regional ice-core array suggests that there is both a temporally and a spatially varying response to remote tropical forcing.

Criscitiello, A. S., S. B. Das, K. B. Karnauskas, M. J. Evans, K. E. Frey, I. Joughin, E. J. Steig, J. R. McConnell, and B. Medley, 2014: Tropical Pacific influence on source and transport of marine aerosols to West Antarctica. J. Climate, 27(3), 1343–1363, doi: 10.1175/JCLI-D-13-00148.1.

The hypothesis that northern high-latitude atmospheric variability influences decadal variability in the tropical Pacific Ocean by modulating the wind jet blowing over the Gulf of Tehuantepec (GT) is examined using the high-resolution configuration of the MIROC 3.2 global coupled model. The model is shown to have acceptable skill in replicating the spatial pattern, strength, seasonality, and time scale of observed GT wind events. The decadal variability of the simulated GT winds in a 100-year control integration is driven by the Arctic Oscillation (AO). The regional impacts of the GT winds include strong sea surface cooling, increased salinity, and the generation of westward-propagating anticyclonic eddies, also consistent with observations. However, significant nonlocal effects also emerge in concert with the low-frequency variability of the GT winds, including anomalously low upper ocean heat content (OHC) in the central tropical Pacific Ocean. It is suggested that the mesoscale eddies generated by the wind stress curl signature of the GT winds, which propagate several thousand kilometers toward the central Pacific, contribute to this anomaly by strengthening the meridional overturning associated with the northern subtropical cell. A parallel mechanism for the decadal OHC variability is considered by examining the Ekman and Sverdrup transports inferred from the atmospheric circulation anomalies in the northern midlatitude Pacific directly associated with the AO.

Karnauskas, K. B., 2014: Arctic forcing of decadal variability in the tropical Pacific Ocean in a high–resolution global coupled GCM. Clim. Dyn., 42(11–12), 3375–3388, doi: 10.1007/s00382-013-1836-3.

This is the second part of a three-part paper on North American climate in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that evaluates the twentieth-century simulations of intraseasonal to multidecadal variability and teleconnections with North American climate. Overall, the multimodel ensemble does reasonably well at reproducing observed variability in several aspects, but it does less well at capturing observed teleconnections, with implications for future projections examined in part three of this paper. In terms of intraseasonal variability, almost half of the models examined can reproduce observed variability in the eastern Pacific and most models capture the midsummer drought over Central America. The multimodel mean replicates the density of traveling tropical synoptic-scale disturbances but with large spread among the models. On the other hand, the coarse resolution of the models means that tropical cyclone frequencies are underpredicted in the Atlantic and eastern North Pacific. The frequency and mean amplitude of ENSO are generally well reproduced, although teleconnections with North American climate are widely varying among models and only a few models can reproduce the east and central Pacific types of ENSO and connections with U.S. winter temperatures. The models capture the spatial pattern of Pacific decadal oscillation (PDO) variability and its influence on continental temperature and West Coast precipitation but less well for the wintertime precipitation. The spatial representation of the Atlantic multidecadal oscillation (AMO) is reasonable, but the magnitude of SST anomalies and teleconnections are poorly reproduced. Multidecadal trends such as the warming hole over the central–southeastern United States and precipitation increases are not replicated by the models, suggesting that observed changes are linked to natural variability.

Sheffield, J., and Coauthors, 2013: North American climate in CMIP5 experiments. Part II: Evaluation of historical simulations of intraseasonal to decadal variability. J. Climate, 26(23), 9247–9290, doi: 10.1175/JCLI-D-12-00593.1.

Following the recent discovery of the "Modoki" El Niño, a proliferation of studies and debates has ensued concerning whether Modoki is dynamically distinct from "Canonical" El Niño, how Modoki impacts and teleconnections differ, and whether Modoki events have been increasing in frequency or amplitude. Three decades of reliable, high temporal-resolution observations of coupled ocean-atmosphere variability in the equatorial Pacific reveal a rich diversity of El Niños. Although central and eastern Pacific sea surface temperature (SST) anomalies appear mechanistically separable in terms of local and remote forcing, their frequent overlap precludes robust classifications. All observed El Niños appear to be a mixture of locally (central Pacific) and remotely forced (eastern Pacific) SST anomalies. Submonthly resolution appears essential for this insight and for the proper dynamical diagnosis of El Niño evolution; thus, the use of long-term monthly reconstructions for classification and trend analysis is strongly cautioned against.

Karnauskas, K. B., 2013: Can we distinguish canonical El Niño from Modoki? Geophys. Res. Lett., 40(19), 5246–5251, doi: 10.1002/grl.51007.

The global distribution, seasonal evolution, and underlying mechanisms for the climatological midsummer drought (MSD) are investigated using a suite of relatively high spatial and temporal resolution station observations and reanalysis data with particular focus on the Pacific coast of Central America and southern Mexico. Although the MSD of Central America stands out in terms of spatial scale and coherence, it is neither unique to the Greater Caribbean Region (GCR) nor necessarily the strongest MSD on Earth based on an objective analysis of several global precipitation data sets. A mechanism for the MSD is proposed that relates the latitudinal dependence of the two climatological precipitation maxima to the biannual crossing of the solar declination (SD), driving two peaks in convective instability and hence rainfall. In addition to this underlying local mechanism, a number of remote processes tend to peak during the apex of the MSD, including the North American monsoon, the Caribbean low-level jet, and the North Atlantic subtropical high, which may also act to suppress rainfall along the Pacific coast of Central America and generate interannual variability in the strength or timing of the MSD. However, our findings challenge the existing paradigm that the MSD owes its existence to a precipitation-suppressing mechanism. Rather, aided by the analysis of higher-temporal resolution precipitation records and considering variations in latitude, we suggest the MSD is essentially the result of one precipitation-enhancing mechanism occurring twice.

Karnauskas, K. B., A. Giannini, R. Seager, and A. J. Busalacchi, 2013: A simple mechanism for the climatological midsummer drought along the Pacific coast of Central America. Atmósfera, 26(2), 261–281.

Internal climate variability at the centennial time scale is investigated using long control integrations from three state-of-the-art global coupled general circulation models. In the absence of external forcing, all three models produce centennial variability in the mean zonal sea surface temperature (SST) and sea level pressure (SLP) gradients in the equatorial Pacific with counterparts in the extratropics. The centennial pattern in the tropical Pacific is dissimilar to that of the interannual El Niño–Southern Oscillation (ENSO), in that the most prominent expression in temperature is found beneath the surface of the western Pacific warm pool. Some global repercussions nevertheless are analogous, such as a hemispherically symmetric atmospheric wave pattern of alternating highs and lows. Centennial variability in western equatorial Pacific SST is a result of the strong asymmetry of interannual ocean heat content anomalies, while the eastern equatorial Pacific exhibits a lagged, Bjerknes-like response to temperature and convection in the west. The extratropical counterpart is shown to be a flux-driven response to the hemispherically symmetric circulation anomalies emanating from the tropical Pacific. Significant centennial-length trends in the zonal SST and SLP gradients rivaling those estimated from observations and model simulations forced with increasing CO2 appear to be inherent features of the internal climate dynamics simulated by all three models. Unforced variability and trends on the centennial time scale therefore need to be addressed in estimated uncertainties, beyond more traditional signal-to-noise estimates that do not account for natural variability on the centennial time scale.

Karnauskas, K. B., J. E. Smerdon, R. Seager, and J. F. Gonzalez–Rouco, 2012: A Pacific centennial oscillation predicted by coupled GCMs. J. Climate, 25(17), 5943–5961, doi: 10.1175/JCLI-D-11-00421.1.

Decadal variations of very small amplitude [∼0.3°C in sea surface temperature (SST)] in the tropical Pacific Ocean, the genesis region of the interannual El Niño–Southern Oscillation (ENSO) phenomenon, have been shown to have powerful impacts on global climate. Future projections from different climate models do not agree on how this critical feature will change under the influence of anthropogenic forcing. A number of attempts have been made to resolve this issue by examining observed trends from the 1880s to the present, a period of rising atmospheric concentrations of greenhouse gases. A recent attempt concluded that the three major datasets disagreed on the trend in the equatorial gradient of SST. Using a corrected version of one of these datasets, and extending the analysis to the seasonal cycle, it is shown here that all agree that the equatorial Pacific zonal SST gradient has strengthened from 1880 to 2005 during the boreal fall when this gradient is normally strongest. This result appears to favor a theory for future changes based on ocean dynamics over one based on atmospheric energy considerations. Both theories incorporate the expectation, based on ENSO theory, that the zonal sea level pressure (SLP) gradient in the tropical Pacific is coupled to SST and should therefore strengthen along with the SST gradient. While the SLP gradient has not strengthened, it is found that it appears to have weakened only during boreal spring, consistent with the SST seasonal trends. Most of the coupled models included in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report underestimate the strengthening SST gradient in boreal fall, and show almost no change in the SLP gradient in any season. The observational analyses herein suggest that both theories are at work but with relative strengths that vary seasonally, and that the two theories need not be inconsistent with each other.

Karnauskas, K. B., R. Seager, A. Kaplan, Y. Kushnir, and M. A. Cane, 2009: Observed strengthening of the zonal sea surface temperature gradient across the equatorial Pacific Ocean. J. Climate, 22(16), 4316–4321, doi: 10.1175/2009JCLI2936.1.

Satellite- and gauge-based precipitation and sea surface temperature (SST) are analyzed to understand the role of SST in the east Pacific warm pool (EPWP) in the interannual variability of Central American rainfall. It is shown that, during the rainy season following the mature phase of an El Niño event, an anomalously warm EPWP can cause a rapid enhancement of the eastern Pacific intertropical convergence zone (EP ITCZ), which directly leads to a positive rainfall anomaly over Central America. Moreover, the timing and amplitude of the SST-enhanced EP ITCZ depends on the persistence of the El Niño event. The longer the equatorial SST anomaly persists, the longer the EPWP is subject to anomalous shortwave heating, and thus the stronger (and later) the subsequent SST enhancement of the EP ITCZ. The implications for regional climate monitoring and predictability are explored; potential predictability of seasonal rainfall is demonstrated 4 months in advance using an SST-based index designed to capture the essence of the above-mentioned mechanism.

Karnauskas, K. B., and A. J. Busalacchi, 2009: The role of SST in the east Pacific warm pool in the interannual variability of Central American rainfall. J. Climate, 22(10), 2605–2623, doi: 10.1175/2008JCLI2468.1.

In comparison with the western and equatorial Pacific Ocean, relatively little is known about the east Pacific warm pool (EPWP). Observations indicate that the interannual variability of sea surface temperature (SST) in the EPWP is highly correlated (0.95) with the El Niño–Southern Oscillation (ENSO). In this paper, an ocean general circulation model (OGCM) of the tropical Pacific Ocean and various atmospheric and oceanic observations are used to diagnose the physical processes governing the interannual variability of SST in the EPWP. Atmospheric forcings for the OGCM are derived purely from satellite observations between 1988 and 2004. Shortwave heating is identified as playing a dominant role in the interannual SST tendency of the EPWP. The high correlation between SST in the EPWP and eastern equatorial Pacific is therefore explained not by ocean processes, but by an atmospheric link. ENSO-driven equatorial SST anomalies modify the distribution of the overlying atmospheric vertical motions and therefore cloud cover and ultimately shortwave heating. During an El Niño event, for example, the ITCZ is equatorward displaced from its normal position over the EPWP, resulting in anomalously large shortwave heating over the EPWP. Analysis of poleward ocean heat transport and coastal Kelvin waves confirms that oceanic processes are not sufficient to explain the interannual variability of the EPWP.

Karnauskas, K. B., and A. J. Busalacchi, 2009: Mechanisms for the interannual variability of SST in the east Pacific warm pool. J. Climate, 22(6), 1375–1392, doi: 10.1175/2008JCLI2467.1.

An ocean general circulation model (OGCM) of the tropical Pacific Ocean is used to examine the effects of the Galápagos Islands on the El Niño–Southern Oscillation (ENSO). First, a series of experiments is conducted using the OGCM in a forced context, whereby an idealized El Niño event may be examined in cases with and without the Galápagos Islands. In this setup, the sensitivity of the sea surface temperature (SST) anomaly response to the presence of the Galápagos Islands is examined. Second, with the OGCM coupled to the atmosphere via zonal wind stress, experiments are conducted with and without the Galápagos Islands to determine how the Galápagos Islands influence the time scale of ENSO. In the forced setup, the Galápagos Islands lead to a damped SST anomaly given an identical zonal wind stress perturbation. Mixed layer heat budget calculations implicate the entrainment mixing term, which confirms that the difference is due to the Galápagos Islands changing the background mean state, that is, the equatorial thermocline as diagnosed in a previous paper. In the hybrid coupled experiments, there is a clear shift in the power spectrum of SST anomalies in the eastern equatorial Pacific. Specifically, the Galápagos Islands lead to a shift in the ENSO time scale from a biennial to a quasi-quadrennial period. Mechanisms for the shift in ENSO time scale due to the Galápagos Islands are discussed in the context of well-known paradigms for the oscillatory nature of ENSO.

Karnauskas, K. B., R. Murtugudde, and A. J. Busalacchi, 2008: The effect of the Galápagos Islands on ENSO in forced ocean and hybrid coupled models. J. Phys. Oceanogr., 38(11), 2519–2534, doi: 10.1175/2008JPO3848.1.

The low-frequency variability of gap winds at the Isthmuses of Tehuantepec and Papagayo is investigated using a 17-yr wind stress dataset merging the remotely sensed observations of Special Sensor Microwave Imager (SSM/I) and Quick Scatterometer (QuikSCAT) satellite sensors. A decadal signal is identified in the Tehuantepec gap winds, which is shown to be related to the Atlantic tripole pattern (ATP). Using linear regression and spectral analysis, it is demonstrated that the low-frequency variability of the Tehuantepec gap winds is remotely forced by the ATP, and the Papagayo gap winds are primarily governed by El Niño–Southern Oscillation (ENSO) with the ATP being of secondary importance. The Tehuantepec (Papagayo) time series of wind stress anomalies can be better reconstructed when the local cross-isthmus pressure difference and large-scale climate information such as the ATP (ENSO) are included, suggesting that there is important information in the large-scale flow that is not transmitted directly through the background sea level pressure gradient. The geostrophic modulation of the easterly trades in the western Caribbean also serve as a remote driver of the Papagayo gap winds, which is itself not fully independent from ENSO. Finally, it is suggested that precipitation variability in the Inter-Americas region is closely related to the same remote forcing as that of the Tehuantepec gap winds, being the ATP and associated large-scale atmospheric circulation.

Karnauskas, K. B., A. J. Busalacchi, and R. Murtugudde, 2008: Low–frequency variability and remote forcing of gap winds over the east Pacific warm pool. J. Climate, 21(19), 4901–4918, doi: 10.1175/2008JCLI1771.1.

The Palmer drought severity index (PDSI) monitors meteorological and surface hydrological parameters to represent the severity of drought conditions. PDSI datasets are developed for the NCEP North American Regional Reanalysis (NARR) and the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) to facilitate North American drought research with these datasets. The drought index calculation, in particular, allows diagnostic assessment of the relative contributions of various surface water balance terms in generation of drought conditions by selectively holding these terms to their climatological value in PDSI computations. The length of the diagnosed PDSI permits analysis of subdecadal time-scale variability, such as ENSO, whose influence on North American drought evolution is investigated. ENSO’s considerable drought impact is potentially predictable, especially in the southern half of the United States.

Karnauskas, K. B., A. Ruiz–Barradas, S. Nigam, and A. J. Busalacchi, 2008: North American droughts in ERA–40 global and NCEP North American regional reanalyses: A Palmer Drought Severity Index perspective. J. Climate, 21(10), 2102–2123, doi: 10.1175/2007JCLI1837.1.