Abstract

A global version of the GFDL modular ocean model is forced using conventional restoring boundary conditions (BCs), mixed BCs (i.e., restoring the upper-level temperature but specifying a fixed salt flux), and stochastic fluxes of both heat and freshwater.

The climatology of the model is found to drift if stochastic freshwater fluxes are applied at high latitudes under mixed BCs. The drift is global in extent: the ocean is generally warmer in the North Pacific and Weddell Sea but cooler and fresher at depths elsewhere in the Southern Ocean and in the North Atlantic. There is a slight reduction (by about 5%) in the meridional overturning of the Southern Ocean and the North Atlantic. The drift of the barotropic flow is most pronounced in the Southern Ocean and is associated with a permanent meandering of the Antarctic Circumpolar Current.

The drift occurs within a few decades, suggesting that it may be important in enhanced greenhouse scenarios for early next century that have been obtained using coupled atmosphere-ocean GCMS. It is also possible that some of the intrinsic variability identified in the same models is actually a residual drift.

The drift depends upon convective adjustment to occur but can be amplified by the surface heat flux parameterization, both locally and by an additional feedback associated with large-scale flow changes. In an extreme case, the latter leads to a total collapse of the thermohaline circulation associated with North Atlantic Deep Water Formation. A similar mechanism underlies the drift that can occur when the switch from restoring to mixed BCs is made.

The heat flux feedback represents the atmosphere-ocean coupling in the model, so this aspect of the drift can be regarded as a coupled mode that actually contributes to the mean state of the coupled system. The existence of such modes makes some climatic drift in coupled models inevitable, if the individual components are equilibrated separately prior to coupling.

The applicability of these results to more sophisticated coupled models depends, in part, upon how well the restoring BC on temperature captures the heal flux feedback they exhibit.

No abstract available.

Abstract

This paper examines changes in the strength of the Walker circulation (WC) using the pressure difference between the western and eastern equatorial Pacific. Changes in observations and in 35 climate models from the Coupled Model Intercomparison Project (CMIP) phase 5 (CMIP5) are determined. On the one hand, 78% of the models show a weakening of the WC over the twentieth century, consistent with the observations and previous studies using CMIP phase 3 (CMIP3) models. However, the observations also exhibit a strengthening in the last three decades (i.e., from 1980 to 2012) that is statistically significant at the 95% level. The models, on the other hand, show no consensus on the sign of change, and none of the models shows a statistically significant strengthening over the same period. While the reasons for the inconsistency between models and observations is not fully understood, it is shown that the ability of the models to generate trends as large as the observed from internal variability is reduced because most models have weaker than observed levels of both multidecadal variability and persistence of interannual variability in WC strength.

In the twenty-first-century future projections, the WC weakens in 25 out of 35 models, under representative concentration pathway (RCP) 8.5, 9 out of 11 models under RCP6.0, 16 out of 18 models under RCP4.5, and 12 out of 15 models under RCP2.6. The projected decrease is also consistent with results obtained previously using models from CMIP3. However, as the reasons for the inconsistency between modeled and observed trends in the last three decades are not fully understood, confidence in the model projections is reduced.

Abstract

The Walker circulation (WC) is one of the world’s most prominent and important atmospheric systems. The WC weakened during the twentieth century, reaching record low levels in recent decades. This weakening is thought to be partly due to global warming and partly due to internally generated natural variability. There is, however, no consensus in the literature on the relative contribution of external forcing and natural variability to the observed weakening of the WC. This paper examines changes in the strength of the WC using an index called BoxΔP, which is equal to the difference in mean sea level pressure across the equatorial Pacific. Change in both the observations and in World Climate Research Programme (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3) climate models are examined. The annual average BoxΔP declines in the observations and in 15 out of 23 models during the twentieth century (results that are significant at or above the 95% level), consistent with earlier work. However, the magnitude of the multimodel ensemble mean (MMEM) 1901–99 trend (−0.10 Pa yr⁻¹) is much smaller than the magnitude of the observed trend (−0.52 Pa yr⁻¹). While a wide range of trends is evident in the models with approximately 90% of the model trends in the range (−0.25 to +0.1 Pa yr⁻¹), even this range is too narrow to encompass the magnitude of the observed trend. Twenty-first-century changes in BoxΔP under the Special Report on Emissions Scenarios (SRES) A1B and A2 are also examined. Negative trends (i.e., weaker WCs) are evident in all seasons. However, the MMEM trends for the A1B and A2 scenarios are smaller in magnitude than the magnitude of the observed trend. Given that external forcing linked to greenhouse gases is much larger in the twenty-first-century scenarios than twentieth-century forcing, this, together with the twentieth-century results mentioned above, would seem to suggest that external forcing has not been the primary driver of the observed weakening of the WC. However, 9 of the 23 models are unable to account for the observed change unless the internally generated component of the trend is very large. But indicators of observed variability linked to El Niño–Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation have modest trends, suggesting that internally variability has been modest. Furthermore, many of the nine “inconsistent” models tend to have poorer simulations of climatic features linked to ENSO. In addition, the externally forced component of the trend tends to be larger in magnitude and more closely matches the observed trend in the models that are better able to reproduce ENSO-related variability. The “best” four models, for example, have a MMEM of −0.2 Pa yr⁻¹ (i.e., approximately 40% of the observed change), suggesting a greater role for external forcing in driving the observed trend. These and other considerations outlined below lead the authors to conclude that (i) both external forcing and internally generated variability contributed to the observed weakening of the WC over the twentieth century and (ii) external forcing accounts for approximately 30%–70% of the observed weakening with internally generated climate variability making up the rest.

Abstract

The variability in the number of severe floods that occurred in coastal catchments in southeastern Australia since the mid–nineteenth century, along with the variability in both the frequency of the weather types that triggered the floods and the associated death tolls, is analyzed. Previous research has shown that all of the severe floods identified were associated with one of two major weather types: east coast lows (ECLs) and tropical interactions (TIs). El Niño–Southern Oscillation (ENSO) is shown to strongly modulate the frequency of severe coastal flooding, weather types, and the number of associated deaths. The analysis presented herein, which examines links over more than a century, provides one of very few known statistically significant links between ENSO and death tolls anywhere in the world. Over the period 1876/77–2013/14 the average numbers of coastal floods, ECLs, TIs, and deaths associated with freshwater drowning in La Niña years are 92%, 55%, 150%, and 220% higher than the corresponding averages in El Niño years. The average number of deaths per flood in La Niña years is 3.2, which is 66% higher than the average in El Niño years. Death tolls of 10 or more occurred in only 5% of El Niño years, but in 27% of La Niña years. The interdecadal Pacific oscillation also modulates the frequency of severe floods, weather types, and death tolls. The results of this study are consistent with earlier research over shorter periods and broader regions, using less-complete datasets.

Abstract

Cool-season (April to October) rainfall dominates the annual average rainfall over Victoria, Australia, and is important for agriculture and replenishing reservoirs. Rainfall during the cool season has been unusually low since the beginning of the Millennium Drought in 1997 (~12% below the twentieth-century average). In this study, 24 CMIP5 climate models are used to estimate 1) the extent to which this drying is driven by external forcing and 2) future rainfall, taking both external forcing and internal natural climate variability into account. All models have preindustrial, historical, and twenty-first-century (RCP2.6, RCP4.5, and RCP8.5) simulations. It is found that rainfall in the past two decades is below the preindustrial average in two-thirds or more of model simulations. However, the magnitude of the multimodel median externally forced drying is equivalent to only 20% of the observed drying (interquartile range of 40% to −4%), suggesting that the drying is dominated by internally generated rainfall variability. Underestimation of internal variability of rainfall by the models, however, increases the uncertainties in these estimates. According to models the anthropogenically forced drying becomes dominant from 2010 to 2029, when drying is evident in over 90% of the model simulations. For the 2018–37 period, it is found that there is only a ~12% chance that internal rainfall variability could completely offset the anthropogenically forced drying. By the late twenty-first century, the anthropogenically forced drying under RCP8.5 is so large that internal variability appears too small to be able to offset it. Confidence in the projections is lowered because models have difficulty in simulating the magnitude of the observed decline in rainfall.

Abstract

A simple model of the lower atmospheric layers and land/sea ice surface is described and analyzed. The model is able to depict with reasonable accuracy the global ocean heat fluxes. Due to the model's simplicity, insight into the mechanisms underlying particular heat flux responses is possible. Such an analysis is carried out for the regional Gulf Stream heat flux response (which is gualitatively correct in the model), and it is shown that atmospheric transient eddy heat transport is crucial to the modeled response. The perturbation response of the model to tropical SST anomalies is also analyzed, and it is demonstrated that the atmospheric transport processes incorporated in the model are responsible for a scale-dependent response. The magnitude of this response is shown to be significantly different to that obtained with formulations previously used by ocean modelers.

Abstract

Increases in greenhouse gas emissions are expected to cause changes both in climatic variability in the Pacific linked to El Niño–Southern Oscillation (ENSO) and in long-term average climate. While mean state and variability changes have been studied separately, much less is known about their combined impact or relative importance. Additionally, studies of projected changes in ENSO have tended to focus on changes in, or adjacent to, the Pacific. Here we examine projected changes in climatic conditions during El Niño years and in ENSO-driven precipitation variability in 36 CMIP5 models. The models are forced according to the RCP8.5 scenario in which there are large, unmitigated increases in greenhouse gas concentrations during the twenty-first century. We examine changes over much of the globe, including 25 widely spread regions defined in the IPCC special report Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX). We confirm that precipitation variability associated with ENSO is projected to increase in the tropical Pacific, consistent with earlier research. We also find that the enhanced tropical Pacific variability drives ENSO-related variability increases in 19 SREX regions during DJF and in 18 during JJA. This externally forced increase in ENSO-driven precipitation variability around the world is on the order of 15%–20%. An increase of this size, although substantial, is easily masked at the regional level by internally generated multidecadal variability in individual runs. The projected changes in El Niño–driven precipitation variability are typically much smaller than projected changes in both mean state and ENSO neutral conditions in nearly all regions.

Abstract

El Niño–Southern Oscillation strongly influences the interannual variability of rainfall over the Pacific, shifting the position and orientation of the South Pacific convergence zone (SPCZ) and intertropical convergence zone (ITCZ). In 1982/83 and 1997/98, very strong El Niño events occurred, during which time the SPCZ and ITCZ merged into a single zonal convergence zone (szCZ) extending across the Pacific at approximately 5°S. The sea surface temperature anomalies (SSTAs) reached very large values and peaked farther east compared to other El Niño events. Previous work shows that tropical Pacific precipitation responds nonlinearly to changing the amplitude of the El Niño SSTA even if the structure of the SSTA remains unchanged, but large canonical El Niño SSTAs cannot reproduce the szCZ precipitation pattern. This study conducts idealized, SST-forced experiments, starting with a large-amplitude canonical El Niño SSTA and gradually adding a residual pattern until the full (1982/83) and (1997/98) mean SST is reproduced. Differences between the canonical and strong El Niño SSTA patterns are crucial in generating an szCZ event. Three elements influence the precipitation pattern: (i) the local meridional SST maxima influences the ITCZ position and western Pacific precipitation, (ii) the total zonal SST maximum influences the SPCZ position, and (iii) the equatorial Pacific SST influences the total amount of precipitation. In these experiments, the meridional SST gradient increases as the SSTAs approach szCZ conditions. Additionally, the precipitation changes evident in szCZ years are primarily driven by changes in the atmospheric circulation, rather than thermodynamic changes. The addition of a global warming SST pattern increases the precipitation along the equator and shifts the ITCZ farther equatorward.

Abstract

This study investigates the response of a stochastically forced coupled atmosphere–ocean model of the equatorial Pacific to off-equatorial wind stress anomaly forcing. The intermediate-complexity coupled ENSO model comprises a linear, first baroclinic mode, ocean shallow water model with a steady-state, two–pressure level (250 and 750 mb) atmospheric component that has been linearized about a state of rest on the β plane. Estimates of observed equatorial region stochastic forcing are calculated from NCEP–NCAR reanalysis surface winds for the period 1950–2006 using singular value decomposition. The stochastic forcing is applied to the model both with and without off-equatorial region wind stress anomalies (i.e., poleward of 12.5° latitude). It is found that the multiyear changes in the equatorial Pacific thermocline depth “background state” induced by off-equatorial forcing can affect the amplitude of modeled sea surface temperature anomalies by up to 1°C. Moreover, off-equatorial wind stress anomalies increased the modeled amplitude of the two biggest El Niño events in the twentieth century (1982/83 and 1997/98) by more than 0.5°C, resulting in a more realistic modeled response. These equatorial changes driven by off-equatorial region wind stress anomalies are highly predictable to two years in advance and may be useful as a physical basis to enhance multiyear probabilistic predictions of ENSO indices.