Unforced Variability Research Articles

A chrysophyte-based quantitative reconstruction of winter severity from varved lake sediments in NE Poland during the past millennium and its relationship to natural climate variability

Chrysophyte cysts are recognized as powerful proxies of cold-season temperatures. In this paper we use the relationship between chrysophyte assemblages and the number of days below 4 °C (DB4 °C) in the epilimnion of a lake in northern Poland to develop a transfer function and to reconstruct winter severity in Poland for the last millennium. DB4 °C is a climate variable related to the length of the winter. Multivariate ordination techniques were used to study the distribution of chrysophytes from sediment traps of 37 low-land lakes distributed along a variety of environmental and climatic gradients in northern Poland. Of all the environmental variables measured, stepwise variable selection and individual Redundancy analyses (RDA) identified DB4 °C as the most important variable for chrysophytes, explaining a portion of variance independent of variables related to water chemistry (conductivity, chlorides, K, sulfates), which were also important. A quantitative transfer function was created to estimate DB4 °C from sedimentary assemblages using partial least square regression (PLS). The two-component model (PLS-2) had a coefficient of determination of Rcross2 = 0.58, with root mean squared error of prediction (RMSEP, based on leave-one-out) of 3.41 days. The resulting transfer function was applied to an annually-varved sediment core from Lake Żabińskie, providing a new sub-decadal quantitative reconstruction of DB4 °C with high chronological accuracy for the period AD 1000–2010. During Medieval Times (AD 1180–1440) winters were generally shorter (warmer) except for a decade with very long and severe winters around AD 1260–1270 (following the AD 1258 volcanic eruption). The 16th and 17th centuries and the beginning of the 19th century experienced very long severe winters. Comparison with other European cold-season reconstructions and atmospheric indices for this region indicates that large parts of the winter variability (reconstructed DB4 °C) is due to the interplay between the oscillations of the zonal flow controlled by the North Atlantic Oscillation (NAO) and the influence of continental anticyclonic systems (Siberian High, East Atlantic/Western Russia pattern). Differences with other European records are attributed to geographic climatological differences between Poland and Western Europe (Low Countries, Alps). Striking correspondence between the combined volcanic and solar forcing and the DB4 °C reconstruction prior to the 20th century suggests that winter climate in Poland responds mostly to natural forced variability (volcanic and solar) and the influence of unforced variability is low.

Comparing the model-simulated global warming signal to observations using empirical estimates of unforced noise.

The comparison of observed global mean surface air temperature (GMT) change to the mean change simulated by climate models has received much public and scientific attention. For a given global warming signal produced by a climate model ensemble, there exists an envelope of GMT values representing the range of possible unforced states of the climate system (the Envelope of Unforced Noise; EUN). Typically, the EUN is derived from climate models themselves, but climate models might not accurately simulate the correct characteristics of unforced GMT variability. Here, we simulate a new, empirical, EUN that is based on instrumental and reconstructed surface temperature records. We compare the forced GMT signal produced by climate models to observations while noting the range of GMT values provided by the empirical EUN. We find that the empirical EUN is wide enough so that the interdecadal variability in the rate of global warming over the 20th century does not necessarily require corresponding variability in the rate-of-increase of the forced signal. The empirical EUN also indicates that the reduced GMT warming over the past decade or so is still consistent with a middle emission scenario's forced signal, but is likely inconsistent with the steepest emission scenario's forced signal.

Contributions of atmospheric circulation variability and data coverage bias to the warming hiatus

AbstractThe warming hiatus shows a strong seasonal and geographical asymmetry, with cooling in the Northern Hemisphere winter, especially over land, and warming elsewhere and in the other seasons. We show that the characteristics of the Northern Hemisphere winter cooling in 1998–2012 can mostly be explained by missing observations and by internal variability in the atmospheric circulation of the Northern Hemisphere extratropics. Estimates of the annual and seasonal temperature trends in 1998–2012 obtained by considering the concurrent effects of unforced natural variability and of coverage bias are much closer to the corresponding long‐term trends. Reanalyses suggest that the coverage bias was exceptionally pronounced during recent years and that an area of strong warming was missed due to the incomplete observational coverage. Coupled Model Intercomparison Project Phase 5 climate models indicate that trends in atmospheric circulation during the hiatus period did not occur as a response to anthropogenic forcing.

The effect of aerosols and sea surface temperature on China's climate in the late twentieth century from ensembles of global climate simulations

AbstractOver the late twentieth century, China has seen a strong increase in aerosol emissions, whose quantitative role for observed changes in surface solar radiation (SSR), surface air temperature (SAT), and precipitation remains debated. We use ensembles of transient sensitivity experiments with the global climate model ECHAM5 from the Max Planck Institute for Meteorology, Hamburg, Germany, combined with the Hamburg Aerosol Module to examine the effect of aerosols and prescribed, observation‐based sea surface temperatures (SSTs) on the above variables. Observations and control experiments agree reasonably well in eastern China in terms of SSR dimming (−6 ± 2 W/m2/decade, 1960–2000; stronger than in models of the Coupled Model Intercomparison Project Phase 5, CMIP5), statistically nonsignificant summer SAT trend (1950–2005), and drying in summer from 1950 to 1990 (−2.5% to −3.5% per decade, essentially via reduction of convective precipitation). Other observed features are not reproduced by the model, e.g., precipitation increase in the 1990s in the Yangtze River valley or, from the 1960s onward, the strong winter warming in northern China and Mongolia and SSR dimming in western China. Aerosol effects are stronger for sulfur dioxide than for black and organic carbon and are more pronounced at lower model resolution. Transient SSTs are crucial for decadal‐scale SAT variability over land, especially the strong warming in the 1990s, and, via SST forced reduction of cloud cover, for the ceasing of SSR dimming around the year 2000. Unforced cloud variability leads to relevant scatter (up to ±2 W/m2/decade) of modeled SSR trends at individual observation sites.

Analysis of the regional pattern of sea level change due to ocean dynamics and density change for 1993–2099 in observations and CMIP5 AOGCMs

Predictions of twenty-first century sea level change show strong regional variation. Regional sea level change observed by satellite altimetry since 1993 is also not spatially homogenous. By comparison with historical and pre-industrial control simulations using the atmosphere–ocean general circulation models (AOGCMs) of the CMIP5 project, we conclude that the observed pattern is generally dominated by unforced (internal generated) variability, although some regions, especially in the Southern Ocean, may already show an externally forced response. Simulated unforced variability cannot explain the observed trends in the tropical Pacific, but we suggest that this is due to inadequate simulation of variability by CMIP5 AOGCMs, rather than evidence of anthropogenic change. We apply the method of pattern scaling to projections of sea level change and show that it gives accurate estimates of future local sea level change in response to anthropogenic forcing as simulated by the AOGCMs under RCP scenarios, implying that the pattern will remain stable in future decades. We note, however, that use of a single integration to evaluate the performance of the pattern-scaling method tends to exaggerate its accuracy. We find that ocean volume mean temperature is generally a better predictor than global mean surface temperature of the magnitude of sea level change, and that the pattern is very similar under the different RCPs for a given model. We determine that the forced signal will be detectable above the noise of unforced internal variability within the next decade globally and may already be detectable in the tropical Atlantic.

Inferring changes in ENSO amplitude from the variance of proxy records

AbstractOne common approach to investigating past changes in El Niño–Southern Oscillation (ENSO) amplitude is through quantifying the variance of ENSO‐influenced proxy records. However, a component of the variance of all such proxies will reflect influences that are unrelated to the instrumental climatic indices from which modern ENSO amplitudes are defined. The unrelated component of proxy variance introduces a fundamental source of uncertainty to all such constraints on past ENSO amplitudes. Based on a simple parametric approach to modeling this uncertainty, we present guidelines for the magnitudes of proxy variance change required to robustly infer the following: (i) any change at all in ENSO amplitude and (ii) a change in ENSO amplitude that exceeds the plausible range of unforced variability. It is noted that more extreme changes in proxy variance are required to robustly infer decreases, as opposed to increases, in past ENSO amplitude from modern levels.

New Perspectives on Observed and Simulated Antarctic Sea Ice Extent Trends Using Optimal Fingerprinting Techniques*

Abstract Using optimal fingerprinting techniques, a detection analysis is performed to determine whether observed trends in Southern Ocean sea ice extent since 1979 are outside the expected range of natural variability. Consistent with previous studies, it is found that for the seasons of maximum sea ice cover (i.e., winter and early spring), the observed trends are not outside the range of natural variability and in some West Antarctic sectors they may be partially due to tropical variability. However, when information about the spatial pattern of trends is included in the analysis, the summer and autumn trends fall outside the range of internal variability. The detectable signal is dominated by strong and opposing trends in the Ross Sea and the Amundsen–Bellingshausen Sea regions. In contrast to the observed pattern, an ensemble of 20 CMIP5 coupled climate models shows that a decrease in Ross Sea ice cover would be expected in response to external forcings. The simulated decreases in the Ross, Bellingshausen, and Amundsen Seas for the autumn season are significantly different from unforced internal variability at the 95% confidence level. Unlike earlier work, the authors formally show that the simulated sea ice response to external forcing is different from both the observed trends and simulated internal variability and conclude that in general the CMIP5 models do not adequately represent the forced response of the Antarctic climate system.

A 600 years warm-season temperature record from varved sediments of Lago Plomo, Northern Patagonia, Chile (47°S)

High-resolution records of calibrated proxy data for the past millennium are fundamental to place current changes into the context of pre-industrial natural forced and unforced variability. Although the need for regional spatially-explicit comprehensive reconstructions is widely recognized, the proxy data sources are still scarce, particularly for the Southern Hemisphere and especially for South America.We present a 600-year long warm season temperature record from varved sediments of Lago Plomo, a proglacial lake of the Northern Patagonian Ice field in Southern Chile (46°59′S, 72°52′W, 203 m a.s.l.). The thickness of the bright summer sediment layer relative to the dark winter layer (measured as total brightness; % reflectance 400–730 nm) is calibrated against warm season SONDJF temperature (1900–2009; r = 0.58, p(aut) = 0.056, RE = 0.52; CE = 0.15, RMSEP = 0.28 °C; five-year triangular filtered data). In Lago Plomo, warm summer temperatures lead to enhanced glacier melt and suspended sediment transport, which results in thicker light summer layers and to brighter sediments. Although Patagonia shows pronounced regional differences in decadal temperature trends and variability, the 600 years temperature reconstruction from Lago Plomo compares favourably with other regional/continental temperature records, but also emphasizes significant regional differences for which no data and information existed so far. These regional differences seem to be real as they are also reflected in modern climate data sets (1900–2010). The reconstruction shows pronounced subdecadal – multidecadal variability with cold phases during parts of the Little Ice Age (16th and 18th centuries) and in the beginning of the 20th century. The most prominent warm phase is the 19th century which is as warm as the second half of the 20th century. The exceptional summer warmth AD 1780–1810 is also found in other archives of Northern Patagonia and Central Chile. Our record shows the delayed 20th century warming in the Southern Hemisphere.The comparison between winter precipitation and summer temperature (inter-seasonal coupling) from Lago Plomo reveals alternating phases with parallel and contrasting decadal trends of winter precipitation and summer temperature (positive and negative running correlations Rwinter PP; summer TT). This observation from the sediment proxy data is also confirmed by two sets of reanalysis data for the 20th century. Reanalysis data show that phases with negative correlations between winter precipitation and summer temperature (e.g., dry winters and warm summers) at Lago Plomo are characteristic for periods when circumpolar Westerly flow is displaced southward and enhanced around 60°S.

Regions of significant influence on unforced global mean surface air temperature variability in climate models

AbstractWe document the geographic regions where local variability is most associated with unforced global mean surface air temperature (GMT) variability in Coupled Model Intercomparison Project Phase 5 coupled global climate models (GCMs) at both the subdecadal and interdecadal timescales. For this purpose, Regions of Significant Influence on GMT are defined as locations that have a statistically significant correlation between local surface air temperature (SAT) and GMT (with a regression slope greater than 1), and where local SAT variation leads GMT variation in time. In both GCMs and observations, subdecadal timescale GMT variability is most associated with SAT variation over the eastern equatorial Pacific. At the interdecadal timescale, GMT variability is also linked with SAT variation over the Pacific in many GCMs, but the particular spatial patterns are GCM dependent, and several GCMs indicate a primary association between GMT and SAT over the Southern Ocean. We find that it is difficult to validate GCM behavior at the interdecadal timescale because the pattern derived from observations is highly depended on the method used to remove the forced variability from the record. The magnitude of observed GMT variability is near the ensemble median at the subdecadal timescale but well above the median at the interdecadal timescale. GCMs with a stronger subdecadal relationship between GMT and SAT over the Pacific tend to have more variable subdecadal GMT while GCMs with a stronger interdecadal relationship between GMT and SAT over parts of the Southern Ocean tend to have more variable GMT.

Uncertainty in Twenty-First-Century CMIP5 Sea Level Projections

Abstract The representative concentration pathway (RCP) simulations included in phase 5 of the Coupled Model Intercomparison Project (CMIP5) quantify the response of the climate system to different natural and anthropogenic forcing scenarios. These simulations differ because of 1) forcing, 2) the representation of the climate system in atmosphere–ocean general circulation models (AOGCMs), and 3) the presence of unforced (internal) variability. Global and local sea level rise projections derived from these simulations, and the emergence of distinct responses to the four RCPs depend on the relative magnitude of these sources of uncertainty at different lead times. Here, the uncertainty in CMIP5 projections of sea level is partitioned at global and local scales, using a 164-member ensemble of twenty-first-century simulations. Local projections at New York City (NYSL) are highlighted. The partition between model uncertainty, scenario uncertainty, and internal variability in global mean sea level (GMSL) is qualitatively consistent with that of surface air temperature, with model uncertainty dominant for most of the twenty-first century. Locally, model uncertainty is dominant through 2100, with maxima in the North Atlantic and the Arctic Ocean. The model spread is driven largely by 4 of the 16 AOGCMs in the ensemble; these models exhibit outlying behavior in all RCPs and in both GMSL and NYSL. The magnitude of internal variability varies widely by location and across models, leading to differences of several decades in the local emergence of RCPs. The AOGCM spread, and its sensitivity to model exclusion and/or weighting, has important implications for sea level assessments, especially if a local risk management approach is utilized.

Natural variability of CO2 and O2 fluxes: What can we learn from centuries‐long climate models simulations?

AbstractOcean carbon uptake and oxygen content estimates over the past decades suggest that the anthropogenic carbon sink has changed and that the oxygen concentration in the ocean interior has decreased. Although these detected changes appear consistent with those expected from anthropogenic forced climate change, large uncertainties remain in the contribution of natural variability. Using century‐long simulations (500–1000 years) of unforced natural variability from six Earth System Models (ESMs), we examine the internally driven natural variability of carbon and oxygen fluxes from interannual to multidecadal time scales. The intensity of natural variability differs between the ESMs, in particular, decadal variability locally accounts for 10–50% of the total variance. Although the variability is higher in all regions with strong climate modes (North Atlantic, North Pacific, etc.), we find that only the Southern Ocean and the tropical Pacific significantly modulate the global fluxes. On (multi)decadal time scales, deep convective events along the Antarctic shelf drive the global fluxes variability by transporting deep carbon‐rich/oxygen‐depleted waters to the surface and by reducing the sea‐ice coverage. On interannual time scales, the global flux is modulated by (1) variations of the upwelling of circumpolar deep waters associated with the southern annular mode in the subpolar Southern Ocean and (2) variations of the equatorial/costal upwelling combined with changes in the solubility‐driven fluxes in response to El Niño Southern Oscillation (ENSO) in the tropical Pacific. We discuss the challenges of measuring and detecting long‐term trends from a few decade‐long records influenced by internal variability.

Rapid grounding line migration induced by internal ice stream variability

AbstractObservations indicate that the grounding line position of West Antarctica is sensitive to both forced and unforced ice stream variabilities. This study endeavors to characterize and understand unforced ice stream variability and associated grounding line migration. We use a flowline ice stream model with an undrained plastic bed, lateral shear stresses, and a stretched grid refined in the grounding zone. This model exhibits parameter space structure and hysteresis behavior similar to simpler ice stream models. Low prescribed temperature at the ice surface or weak geothermal heating produces thermal oscillations between active and stagnant phases. As in previous spatially resolved ice flow models, thermal activation propagates as an “activation wave.” This model's fine resolution of the grounding zone allows for accurate simulations of transient, unforced grounding line migration. Upstream of the grounding zone, horizontal grid spacing of 1 km is required to accurately resolve activation waves. Activation waves induce the grounding line to migrate over 100 km at a rate that can exceed 1 km/yr. This is followed during the active phase by retreat, which then continues for the duration of the stagnant phase. Grounding line retreat is the result of a negative mass balance near the grounding line but is not necessarily associated with negative mass balance for the entire ice stream in our simulations of internal variability. The novel approach and experiments described in this study show that there can be large excursions in grounding line position in the absence of either external forcing or retrograde slopes.

The emerging anthropogenic signal in land–atmosphere carbon-cycle coupling

Earth system models do not currently account for unforced variability in land–atmosphere CO2 flux when simulating the responses of the terrestrial carbon cycle to anthropogenically forced changes in climate and atmosphere. Now, research shows that this unforced variability is larger than the forced response in many areas of the world, precluding detection of the forced carbon-cycle change for decades.

Top‐of‐atmosphere radiative contribution to unforced decadal global temperature variability in climate models

AbstractMuch recent work has focused on unforced global mean surface air temperature (T) variability associated with the efficiency of heat transport into the deep ocean. Here the relationship between unforced variability in T and the Earth's top‐of‐atmosphere (TOA) energy balance is explored in preindustrial control runs of the Coupled Model Intercomparison Project Phase 5 multimodel ensemble. It is found that large decadal scale variations in T tend to be significantly enhanced by the net energy flux at the TOA. This indicates that unforced decadal variability in T is not only caused by a redistribution of heat within the climate system but can also be associated with unforced changes in the total amount of heat in the climate system. It is found that the net TOA radiation imbalances result mostly from changes in albedo associated with the Interdecadal Pacific Oscillation that temporarily counteracts the climate system's outgoing longwave (i.e., Stefan‐Boltzmann) response to T change.

Tropical forcing of the recent rapid Arctic warming in northeastern Canada and Greenland.

Rapid Arctic warming and sea-ice reduction in the Arctic Ocean are widely attributed to anthropogenic climate change. The Arctic warming exceeds the global average warming because of feedbacks that include sea-ice reduction and other dynamical and radiative feedbacks. We find that the most prominent annual mean surface and tropospheric warming in the Arctic since 1979 has occurred in northeastern Canada and Greenland. In this region, much of the year-to-year temperature variability is associated with the leading mode of large-scale circulation variability in the North Atlantic, namely, the North Atlantic Oscillation. Here we show that the recent warming in this region is strongly associated with a negative trend in the North Atlantic Oscillation, which is a response to anomalous Rossby wave-train activity originating in the tropical Pacific. Atmospheric model experiments forced by prescribed tropical sea surface temperatures simulate the observed circulation changes and associated tropospheric and surface warming over northeastern Canada and Greenland. Experiments from the Coupled Model Intercomparison Project Phase 5 (ref. 16) models with prescribed anthropogenic forcing show no similar circulation changes related to the North Atlantic Oscillation or associated tropospheric warming. This suggests that a substantial portion of recent warming in the northeastern Canada and Greenland sector of the Arctic arises from unforced natural variability.

Recent global temperature “plateau” in the context of a new proxy reconstruction

AbstractStable global temperatures of the last 10–15 years have been a topic of considerable discussion. A new proxy extension of the global temperature record enables better placement of this feature in a longer historical perspective. The fixed‐grid composite covers the interval 1801–1984, with an extension to 1782, and anchors the global temperature record in the last major cold interval of the Little Ice Age, when carbon dioxide concentration was at preanthropogenic levels. Except for greater and longer cooling (approximately twice the length of Pinatubo) associated with the Tambora eruption, the proxy agrees with the most widely cited previous assessment of global temperature over this interval, lending more confidence to a centennial extension of the global temperature record. The proxy correlation is as high as 0.83 for the interval 1907–1984 (df = 8, p = 0.001), with the 21st century 1.0°C ± 0.2°C warmer than the nonvolcanic base state. This remarkable linearity requires a clear theoretical understanding as to how an exceedingly complex system can, on the global average, behave in such a simple way. Removal of the linear radiatively forced component from the global temperature record yields an estimate of natural variability for the last 230 years and indicates no unusual natural variability during the recent 10–15 years. Based on the estimate of unforced variability over the last 170 years, there is about a 40% chance of continued “natural cooling” over the next few years, with about a 10% chance of cooling persisting into the next decade.

Trajectory sensitivity of the transient climate response to cumulative carbon emissions

The robustness of Transient Climate Response to cumulative Emissions (TCRE) is tested using an Earth System Model (Geophysical Fluid Dynamics Laboratory-ESM2G) forced with seven different constant rates of carbon emissions (2 GtC/yr to 25 GtC/yr), including low emission rates that have been largely unexplored in previous studies. We find the range of TCRE resulting from varying emission pathways to be 0.76 to 1.04°C/TtC. This range, however, is small compared to the uncertainty resulting from varying model physics across the Fifth Coupled Model Intercomparison Project ensemble. TCRE has a complex relationship with emission rates; TCRE is largest for both low (2 GtC/yr) and high (25 GtC/yr) emissions and smallest for present-day emissions (5–10 GtC/yr). Unforced climate variability hinders precise estimates of TCRE for periods shorter than 50 years for emission rates near or smaller than present day values. Even if carbon emissions would stop, the prior emissions pathways will affect the future climate responses.

Sensitivity of Arctic Climate Variability to Mean State: Insights from the Cretaceous

Abstract This study investigates Arctic climate variability during a period of extreme warmth using the Community Climate System Model, version 3 (CCSM3) coupled ocean–atmosphere general circulation model. Four mid-Cretaceous simulations were completed with different CO2 levels (1, 10, and 16 times preindustrial levels with dynamic vegetation) and vegetation treatments (10 times with specified uniform bare ground). The magnitude and frequency of Arctic temperature variability is highly sensitive to the mean state and high-latitude upper-ocean static stability. As stability increases with a rise in CO2 levels from 1 to 10 times preindustrial levels, the frequency of temperature variability increases from decades (1x) to centuries (10x with bare ground) and longer (10x) and the peak-to-peak magnitude increases from ~1° (for 1x) to ~2°C (for 10x). In the 16x simulation with a highly stratified ocean, Arctic temperature variability is low with peak-to-peak magnitudes <0.5°C. Under low CO2, Arctic climate variability is tied to sensible heat release from the ocean during movement of the sea ice margin. In absence of substantial sea ice, variability is driven by mass transport and upper-ocean salinity advection into the Arctic. In both cases, destruction of low-level clouds acts as an important feedback on low-level warming. The authors also report a link between unforced Arctic climate variability and North Pacific meridional overturning with warming events leading intensification. These results suggest that the nature of Arctic climate variability was likely much different in past climates and is likely to be so in the future.

Effects of Spectral Nudging in WRF on Arctic Temperature and Precipitation Simulations

Abstract Spectral (interior) nudging is a way of constraining a model to be more consistent with observed behavior. However, such control over model behavior raises concerns over how much nudging may affect unforced variability and extremes. Strong nudging may reduce or filter out extreme events since nudging pushes the model toward a relatively smooth, large-scale state. The question then becomes: what is the minimum spectral nudging needed to correct biases while not limiting the simulation of extreme events? To determine this, case studies were performed using a six-member ensemble of the Pan-Arctic Weather Research and Forecasting model (WRF) with varying spectral nudging strength, using WRF’s standard nudging as a reference point. Two periods were simulated, one in a cold season (January 2007) and one in a warm season (July 2007). Precipitation and 2-m temperature were analyzed to determine how changing spectral nudging strength impacts temperature and precipitation extremes and selected percentiles. Results suggest that there is a marked lack of sensitivity to varying degrees of nudging. Moreover, given that nudging is an artificial forcing applied in the model, an outcome of this work is that nudging strength can be considerably smaller than the WRF standard strength and still produce climate simulations that are much better than using no nudging.

Variability in the width of the tropics and the annular modes

The correlation between unforced variability in the latitude of the edge of the Hadley cell (ΦHC) and latitude of the surface westerlies (ΦEDJ) is examined using a simplified moist general circulation model (GCM) and a suite of state of the art GCMs. The correlation can be determined by the time‐mean separation of the two features. When the separation is small, there is a positive correlation, and as the separation between them increases, the correlation reduces. In the simplified model, a weak negative correlation emerges at large separations.The location of the anomalous meridional mass flux associated with variations in the latitude of ΦEDJ, relative to the climatological Hadley cell position, determines the extent to which ΦHC is influenced by changes in ΦEDJ. Changes in the latitude of ΦEDJ are driven by anomalous eddy momentum flux convergence, and these are approximately balanced by the Coriolis torque on the meridional flow, as expected under quasi‐geostrophic scaling. Under changing time‐mean climates, the anomalous flow associated with ΦEDJ variability translates location so that it is approximately fixed relative to the time‐mean ΦEDJ. This means that the influence of ΦEDJ variability on ΦHC varies as a function of the time‐mean separation of the features.Initial indications are that the same causal relationship holds in a suite of state of the art GCMs and that this explains the seasonal variation in the correlation between ΦHC and ΦEDJ.