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Thomas R. Knutson and Syukuro Manabe

Abstract

The time-mean response over the tropical Pacific region to a quadrupling Of CO2 is investigated using a global coupled ocean-atmosphere general circulation model. Tropical Pacific sea surface temperatures (SSTs) rise by about 4°–5°C. The zonal SST gradient along the equator decreases by about 20%, although it takes about one century (with C02 increasing at 1% per year compounded) for this change to become clearly evident in the model. Over the central equatorial Pacific, the decreased SST gradient is accompanied by similar decreases in the easterly wind stress and westward ocean surface currents and by a local maximum in precipitation increase.

Over the entire rising branch region of the Walker circulation, precipitation is enhanced by 15%, but the time-mean upward motion decreases slightly in intensity. The failure of the zonal overturning atmospheric circulation to intensify with a quadrupling of CO2 is surprising in light of the increased time-mean condensation heating over the “warm pool” region. Three aspects of the model response are important for interpreting this result. 1) The time-mean radiative cooling of the upper troposphere is enhanced, due to both the pronounced upper-tropospheric warming and to the large fractional increase of upper-tropospheric water vapor. 2) The dynamical cooling term, −ω̄∂θ̄/∂p, is enhanced due to increased time-mean static stability (−∂θ̄/∂p). This is an effect of moist convection, which keeps the lapse rate close to the moist adiabatic rate, thereby making −∂θ̄/∂p larger in a warmer climate. The enhanced radiative cooling and increased static stability allow for the enhanced time-mean heating by moist convection and condensation to be balanced without stronger time-mean upward motions. 3) The weaker surface zonal winds and wind stress in the equatorial Pacific are consistent with the reduced zonal SST gradient. The SST gradient is damped by the west-east differential in evaporative surface cooling (with greater evaporative cooling in the west than in the east). This evaporative damping increases with increasing temperature, owing to the temperature dependence of saturation mixing ratios, which leads to a reduction in the SST gradient in the warmer climate.

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Thomas R. Knutson and Jeffrey Ploshay

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Observed sea level pressure (SLP) trends for 1901–10, 1951–10, and 1981–2010 are assessed using two observed data sources (HadSLP2_lowvar and 20CRv3) compared to a CMIP5 multimodel ensemble. The CMIP5 simulations include runs with (i) no external forcing (Control runs), (ii) natural external forcing only (Natural-Forcing), or (iii) natural plus anthropogenic forcings combined (All-Forcings). We assess whether the CMIP5 All-Forcing ensemble is consistent with observations and whether there is model-based evidence for detectable anthropogenic influence for the observed SLP trends. For the 1901–2010 and 1951–2010 trends, a robustly detectable anthropogenic signal in both observational data products is a zonal band of SLP increase extending over much of the Southern Hemisphere extratropics (30°–50°S). In contrast, the HadSLP2_lowvar and 20CRv3 observed data products disagree on the sign of the century-scale trends in SLP over much of the low-latitude region 25°N–25°S. These differences will limit confident detection/attribution/consistency conclusions for lower-latitude regions, at least until the observational data product discrepancies are better reconciled. The Northern Hemisphere extratropics remains a difficult region for identifying any detectable anthropogenic influence for annual- or seasonal-mean SLP trends. Overall, our results highlight the difficulty in detecting and attributing anthropogenic signals in SLP for relatively short time scales. The observed 1981–2010 regional trends typically have a different pattern and magnitude from the simulated externally forced trends. Consequently, our results suggest that internal variability is likely the dominant driver of most observed 1981–2010 regional trend features, including the pronounced increase in SLP over the central and eastern equatorial Pacific.

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Thomas R. Knutson and Syukuro Manabe

Abstract

In this report, global coupled ocean–atmosphere models are used to explore possible mechanisms for observed decadal variability and trends in Pacific Ocean SSTs over the past century. The leading mode of internally generated decadal (>7 yr) variability in the model resembles the observed decadal variability in terms of pattern and amplitude. In the model, the pattern and time evolution of tropical winds and oceanic heat content are similar for the decadal and ENSO timescales, suggesting that the decadal variability has a similar “delayed oscillator” mechanism to that on the ENSO timescale. The westward phase propagation of the heat content anomalies, however, is slower and centered slightly farther from the equator (∼12° vs 9°N) for the decadal variability. Cool SST anomalies in the midlatitude North Pacific during the warm tropical phase of the decadal variability are induced in the model largely by oceanic advection anomalies.

An index of observed SST over a broad triangular region of the tropical and subtropical Pacific indicates a warming rate of +0.41°C (100 yr)−1 since 1900, +1.2°C (100 yr)−1 since 1949, and +2.9°C (100 yr)−1 since 1971. All three warming trends are highly unusual in terms of their duration, with occurrence rates of less than 0.5% in a 2000-yr simulation of internal climate variability using a low-resolution model. The most unusual is the trend since 1900 (96-yr duration): the longest simulated duration of a trend of this magnitude is 85 yr. This suggests that the observed trends are not entirely attributable to natural (internal) variability alone, although natural variability could potentially account for much of the observed trends. To quantitatively explore the possible role of greenhouse gases and aerosols in the observed warming trends, two simulations (using different initial conditions) of twentieth-century climate change due to these two radiative forcings were analyzed. These simulate an accelerated warming trend [∼2°C (100 yr)−1] in the triangular Pacific region beginning around the 1960s and suggest that nearly all of the recent warming in the region could be attributable to such a thermal forcing. In summary, the authors’ model results indicate that the observed warming trend in the eastern tropical Pacific is not likely to be solely attributable to internal (natural) climate variability. Instead, it is likely that a sustained thermal forcing, such as the increase of greenhouse gases in the atmosphere, has been at least partly responsible for the observed warming.

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Thomas R. Knutson and Fanrong Zeng

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Precipitation trends for 1901–2010, 1951–2010, and 1981–2010 over relatively well-observed global land regions are assessed for detectable anthropogenic influences and for consistency with historical simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5). The CMIP5 historical all-forcing runs are broadly consistent with the observed trend pattern (1901–2010), but with an apparent low trend bias tendency in the simulations. Despite this bias, observed and modeled trends are statistically consistent over 59% of the analyzed area. Over 20% (9%) of the analyzed area, increased (decreased) precipitation is partly attributable to anthropogenic forcing. These inferred human-induced changes include increases over regions of the north-central United States, southern Canada, Europe, and southern South America and decreases over parts of the Mediterranean region and northern tropical Africa. Trends for the shorter periods (1951–2010 and 1981–2010) do not indicate a prominent low trend bias in the models, as found for the 1901–2010 trends. An atmosphere-only model, forced with observed sea surface temperatures and other climate forcing agents, also underpredicts the observed precipitation increase in the Northern Hemisphere extratropics since 1901. The CMIP5 all-forcing ensemble’s low bias in simulated trends since 1901 is a tentative finding that, if borne out in further studies, suggests that precipitation projections using these regions and models could overestimate future drought risk and underestimate future flooding risk, assuming all other factors equal.

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Gabriel A. Vecchi and Thomas R. Knutson

Abstract

This study assesses the impact of imperfect sampling in the presatellite era (between 1878 and 1965) on North Atlantic hurricane activity measures and on the long-term trends in those measures. The results indicate that a substantial upward adjustment of hurricane counts may be needed prior to 1965 to account for likely “missed” hurricanes due to sparse density of reporting ship traffic. After adjusting for the estimate of missed hurricanes in the basin, the long-term (1878–2008) trend in hurricane counts changes from significantly positive to no significant change (with a nominally negative trend). The adjusted hurricane count record is more strongly connected to the difference between main development region (MDR) sea surface temperature (SST) and tropical-mean SST than with MDR SST. These results do not support the hypothesis that the warming of the tropical North Atlantic due to anthropogenic greenhouse gas emissions has caused Atlantic hurricane frequency to increase.

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Thomas R. Knutson, Syukuro Manabe, and Daifang Gu

Abstract

An analysis is presented of simulated ENSO phenomena occurring in three 1000-yr experiments with a low-resolution (R15) global coupled ocean–atmosphere GCM. Although the model ENSO is much weaker than the observed one, the model ENSO’s life cycle is qualitatively similar to the “delayed oscillator” ENSO life cycle simulated using much higher resolution ocean models. Thus, the R15 coupled model appears to capture the essential physical mechanism of ENSO despite its coarse ocean model resolution. Several observational studies have shown that the amplitude of ENSO has varied substantially between different multidecadal periods during the past century. A wavelet analysis of a multicentury record of eastern tropical Pacific SST inferred from δ 18O measurements suggests that a similar multidecadal amplitude modulation of ENSO has occurred for at least the past three centuries. A similar multidecadal amplitude modulation occurs for the model ENSO (2–7-yr band), which suggests that much of the past amplitude modulation of the observed ENSO could be attributable to internal variability of the coupled ocean–atmosphere system. In two 1000-yr CO2 sensitivity experiments, the amplitude of the model ENSO decreases slightly relative to the control run in response to either a doubling or quadrupling of CO2. This decreased variability is due in part to CO2-induced changes in the model’s time-mean basic state, including a reduced time-mean zonal SST gradient. In contrast to the weaker overall amplitude, the multidecadal amplitude modulations become more pronounced with increased CO2. The frequency of ENSO in the model does not appear to be strongly influenced by increased CO2. Since the multidecadal fluctuations in the model ENSO’s amplitude are comparable in magnitude to the reduction in variability due to a quadrupling of CO2, the results suggest that the impact of increased CO2 on ENSO is unlikely to be clearly distinguishable from the climate system “noise” in the near future—unless ENSO is substantially more sensitive to increased CO2 than indicated in the present study.

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Thomas R. Knutson and Robert E. Tuleya

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Previous studies have found that idealized hurricanes, simulated under warmer, high-CO2 conditions, are more intense and have higher precipitation rates than under present-day conditions. The present study explores the sensitivity of this result to the choice of climate model used to define the CO2-warmed environment and to the choice of convective parameterization used in the nested regional model that simulates the hurricanes. Approximately 1300 five-day idealized simulations are performed using a higher-resolution version of the GFDL hurricane prediction system (grid spacing as fine as 9 km, with 42 levels). All storms were embedded in a uniform 5 m s−1 easterly background flow. The large-scale thermodynamic boundary conditions for the experiments— atmospheric temperature and moisture profiles and SSTs—are derived from nine different Coupled Model Intercomparison Project (CMIP2+) climate models. The CO2-induced SST changes from the global climate models, based on 80-yr linear trends from +1% yr−1 CO2 increase experiments, range from about +0.8° to +2.4°C in the three tropical storm basins studied. Four different moist convection parameterizations are tested in the hurricane model, including the use of no convective parameterization in the highest resolution inner grid. Nearly all combinations of climate model boundary conditions and hurricane model convection schemes show a CO2-induced increase in both storm intensity and near-storm precipitation rates. The aggregate results, averaged across all experiments, indicate a 14% increase in central pressure fall, a 6% increase in maximum surface wind speed, and an 18% increase in average precipitation rate within 100 km of the storm center. The fractional change in precipitation is more sensitive to the choice of convective parameterization than is the fractional change of intensity. Current hurricane potential intensity theories, applied to the climate model environments, yield an average increase of intensity (pressure fall) of 8% (Emanuel) to 16% (Holland) for the high-CO2 environments. Convective available potential energy (CAPE) is 21% higher on average in the high-CO2 environments. One implication of the results is that if the frequency of tropical cyclones remains the same over the coming century, a greenhouse gas–induced warming may lead to a gradually increasing risk in the occurrence of highly destructive category-5 storms.

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Thomas R. Knutson and Robert E. Tuleya

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A response is made to the comments of Michaels et al. concerning a recent study by the authors. Even after considering Michaels et al.’s comments, the authors stand behind the conclusions of the original study. In contrast to Michaels et al., who exclusively emphasize uncertainties that lead to smaller future changes, uncertainties are noted that could lead to either smaller or larger changes in future intensities of hurricanes than those summarized in the original study, with accompanying smaller or larger societal impacts.

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Gabriel A. Vecchi and Thomas R. Knutson

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In this study, an estimate of the expected number of Atlantic tropical cyclones (TCs) that were missed by the observing system in the presatellite era (between 1878 and 1965) is developed. The significance of trends in both number and duration since 1878 is assessed and these results are related to estimated changes in sea surface temperature (SST) over the “main development region” (“MDR”). The sensitivity of the estimate of missed TCs to underlying assumptions is examined. According to the base case adjustment used in this study, the annual number of TCs has exhibited multidecadal variability that has strongly covaried with multidecadal variations in MDR SST, as has been noted previously. However, the linear trend in TC counts (1878–2006) is notably smaller than the linear trend in MDR SST, when both time series are normalized to have the same variance in their 5-yr running mean series. Using the base case adjustment for missed TCs leads to an 1878–2006 trend in the number of TCs that is weakly positive, though not statistically significant, with p ∼ 0.2. The estimated trend for 1900–2006 is highly significant (+∼4.2 storms century−1) according to the results of this study. The 1900–2006 trend is strongly influenced by a minimum in 1910–30, perhaps artificially enhancing significance, whereas the 1878–2006 trend depends critically on high values in the late 1800s, where uncertainties are larger than during the 1900s. The trend in average TC duration (1878–2006) is negative and highly significant. Thus, the evidence for a significant increase in Atlantic storm activity over the most recent 125 yr is mixed, even though MDR SST has warmed significantly. The decreasing duration result is unexpected and merits additional exploration; duration statistics are more uncertain than those of storm counts. As TC formation, development, and track depend on a number of environmental factors, of which regional SST is only one, much work remains to be done to clarify the relationship between anthropogenic climate warming, the large-scale tropical environment, and Atlantic TC activity.

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Thomas R. Knutson and Klaus M. Weickmann

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Life cycles of the 30–60 day atmospheric oscillation were examined by compositing 30–60 day filtered NMC global wind analyses (250 mb and 850 mb) and outgoing longwave radiation (OLR) for the years 1979–84. Separate composite life cycles were constructed for the May–October and November–April seasons using empirical orthogonal function analysis of the large-scale divergent wind field (250 mb velocity potential) to define the oscillation's phase. Monte Carlo simulations were used to assess the statistical significance of the composite OLR and vector wind fields.

Large-scale (wavenumber one) tropical divergent wind features propagate eastward around the globe throughout the seasonal cycle. The spatial relationships between these propagating circulation features and OLR are shown using sequences of composite maps. Good agreement exists between areas of upper-air divergence and areas of convection inferred from the OLR satellite data. Convection anomalies are smaller over tropical Africa and South America than over the Indian and western Pacific oceans. Anomalies of OLR are nearly negligible over cooler tropical sea surfaces. Fluctuations in summer monsoon region convection are influenced by the global-scale eastward-moving wave.

The oscillation's vertical structure varies with latitude. In the tropics, upper-level and lower-level tropospheric wind anomalies are about 180° out of phase. Poleward of about 20°, there is no pronounced phase shift between levels. In tropical and subtropical latitudes, analysis of the nondivergent circulation composites at 250 mb (ψ250) reveals cyclones to the east of the convection and anticyclones alongside or west of the convection. While convection anomalies are most pronounced in the summer hemisphere tropics, the tropical and subtropical ψ250 features are most prominent in the winter hemisphere. There is some evidence of symmetry of cyclonic and anticyclonic circulations about the equator.

A subset of the composite extratropical vector wind fields were statistically significant (95% level) at 850 and 250 mb in the winter hemisphere (25°–85° latitude), based upon a Monte Carlo simulation. During the November-April season, the East Asian jet is retracted toward Asia when positive 30–60 day convection anomalies are occurring over the equatorial Indian Ocean. The eastward shift of convection into the western and central Pacific is accompanied by a series of circulation features over northern Asia and an eastward extension of the East Asian jet. During the May-October season, the shift of large-scale tropical convection anomalies from the Indian Ocean and Indian monsoon regions to the tropical western Pacific is followed (10–15 days later) by the occurrence of strengthened westerlies over southern Australia. In contrast, the extratropical “response” in the summer hemisphere for both the May–October and November–April seasons was not statistically significant.

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