Detecting changes in Arctic methane emissions: limitations of the inter-polar difference of atmospheric mole fractions

We consider the utility of the annual inter-polar difference (IPD) as a metric for changes in Arctic emissions of methane (CH4). The IPD has been previously defined as the difference between weighted annual means of CH4 mole fraction data collected at stations from the two polar regions (defined as latitudes poleward of 53∘&thinsp;N and 53∘&thinsp;S, respectively). This subtraction approach (IPD) implicitly assumes that extra-polar CH4 emissions arrive within the same calendar year at both poles. We show using a continuous version of the IPD that the metric includes not only changes in Arctic emissions but also terms that represent atmospheric transport of air masses from lower latitudes to the polar regions. We show the importance of these atmospheric transport terms in understanding the IPD using idealized numerical experiments with the TM5 global 3-D atmospheric chemistry transport model that is run from 1980 to 2010. A northern mid-latitude pulse in January 1990, which increases prior emission distributions, arrives at the Arctic with a higher mole fraction and ≃12 months earlier than at the Antarctic. The perturbation at the poles subsequently decays with an e-folding lifetime of ≃4 years. A similarly timed pulse emitted from the tropics arrives with a higher value at the Antarctic ≃11 months earlier than at the Arctic. This perturbation decays with an e-folding lifetime of ≃7 years. These simulations demonstrate that the assumption of symmetric transport of extra-polar emissions to the poles is not realistic, resulting in considerable IPD variations due to variations in emissions and atmospheric transport. We assess how well the annual IPD can detect a constant annual growth rate of Arctic emissions for three scenarios, 0.5&thinsp;%, 1&thinsp;%, and 2&thinsp;%, superimposed on signals from lower latitudes, including random noise. We find that it can take up to 16 years to detect the smallest prescribed trend in Arctic emissions at the 95&thinsp;% confidence level. Scenarios with higher, but likely unrealistic, growth in Arctic emissions are detected in less than a decade. We argue that a more reliable measurement-driven approach would require data collected from all latitudes, emphasizing the importance of maintaining a global monitoring network to observe decadal changes in atmospheric greenhouse gases.</p

Interactions between the stratospheric polar vortex and Atlantic circulation on seasonal to multi-decadal timescales

Variations in the strength of the Northern Hemisphere winter polar stratospheric vortex can influence surface variability in the Atlantic sector. Disruptions of the vortex, known as sudden stratospheric warmings (SSWs), are associated with an equatorward shift and deceleration of the North Atlantic jet stream, negative phases of the North Atlantic Oscillation, and cold snaps over Eurasia and North America. Despite clear influences at the surface on sub-seasonal timescales, how stratospheric vortex variability interacts with ocean circulation on decadal to multi-decadal timescales is less well understood. In this study, we use a 1000 year preindustrial control simulation of the UK Earth System Model to study such interactions, using a wavelet analysis technique to examine non-stationary periodic signals in the vortex and ocean. We find that intervals which exhibit persistent anomalous vortex behaviour lead to oscillatory responses in the Atlantic Meridional Overturning Circulation (AMOC). The origin of these responses appears to be highly non-stationary, with spectral power in vortex variability at periods of 30 and 50 years. In contrast, AMOC variations on longer timescales (near 90-year periods) are found to lead to a vortex response through a pathway involving the equatorial Pacific and quasi-biennial oscillation. Using the relationship between persistent vortex behaviour and the AMOC response established in the model, we use regression analysis to estimate the potential contribution of the 8-year SSW hiatus interval in the 1990s to the recent negative trend in AMOC observations. The result suggests that approximately 30 % of the trend may have been caused by the SSW hiatus

Preindustrial control simulations with HadGEM3-GC3.1 for CMIP6

Pre‐industrial control simulations with the HadGEM3‐GC3.1 climate model are presented at two resolutions. These are N216ORCA025, which has a horizontal resolution of 60km in the atmosphere and 0.25° in the ocean, and N96ORCA1, which has a horizontal resolution of 130km in the atmosphere and 1° in the ocean. The aim of this study is to document the climate variability in these simulations, make comparisons against present‐day observations (albeit under different forcing), and discuss differences arising due to resolution. In terms of interannual variability in the leading modes of climate variability the two resolutions behave generally very similarly. Notable differences are in the westward extent of El‐Niño and the pattern of Atlantic multidecadal variability, in which N216ORCA025 compares more favourably to observations, and in the Antarctic Circumpolar Current, which is far too weak in N216ORCA025. In the North Atlantic region, N216ORCA025 has a stronger and deeper AMOC, which compares well against observations, and reduced biases in temperature and salinity in the North Atlantic subpolar gyre (NA SPG). These simulations are being provided to the sixth Coupled Model Intercomparison Project (CMIP6) and provide a baseline against which further forced experiments may be assessed

The evaluation of the North Atlantic climate system in UKESM1 historical simulations for CMIP6

Earth System models enable a broad range of climate interactions that physical climate models are unable to simulate. However, the extent to which adding Earth System components changes or improves the simulation of the physical climate is not well understood. Here we present a broad multi-variate evaluation of the North Atlantic climate system in historical simulations of the UK Earth System Model (UKESM1) performed for CMIP6. In particular, we focus on the mean state and the decadal timescale evolution of important variables that span the North Atlantic Climate system. In general, UKESM1 performs well and realistically simulates many aspects of the North Atlantic climate system. Like the physical version of the model, we find that changes in external forcing, and particularly aerosol forcing, are an important driver of multi-decadal change in UKESM1, especially for Atlantic Multi-decadal Variability and the Atlantic Meridional Overturning Circulation. However, many of the shortcomings identified are similar to common biases found in physical climate models, including the physical climate model that underpins UKESM1. For example, the summer jet is too weak and too far poleward; decadal variability in the winter jet is underestimated; intra-seasonal stratospheric polar vortex variability is poorly represented; and Arctic sea ice is too thick. Forced shortwave changes may be also too strong in UKESM1, which, given the important role of historical aerosol forcing in shaping the evolution of the North Atlantic in UKESM1, motivates further investigation. Therefore, physical model development, alongside Earth System development, remains crucial in order to improve climate simulations

Variability in the polar vortex and its interaction with the climate system

Variability in the strength of the Northern Hemisphere stratospheric polar vortex is an important climate feature. Strong vortex conditions, as well as disruptions of the vortex, known as sudden stratospheric warmings (SSW), are associated with significant tropospheric and surface variations. This makes them an important feature for improving the skill of seasonal weather forecasts. This thesis addresses some ongoing research questions regarding the nature of variations in the polar vortex and their interactions with the wider climate system. First, multi-decadal variability of SSW events is examined in a 1000-yr pre-industrial simulation of a coupled global climate model. A wavelet spectral decomposition method shows that hiatus events (intervals of 5 years or more with no SSWs) and consecutive SSW events (extended intervals with at least one SSW in each year) vary on multi-decadal timescales with the most persistent spectral feature appearing at a period of 90 years. This long-term SSW variability is found to be associated with similar variations in amplitude and vertical coherence of the stratospheric quasi-biennial oscillation (QBO), likely via the well-known Holton-Tan link where the QBO has an in-season influence over the vortex. Interactions between multi-decadal variability in vortex strength and modes of surface and ocean variability are subsequently examined. Intervals that exhibit persistent anomalous vortex behaviour are found to lead to oscillatory responses in the Atlantic Meridional Overturning Circulation (AMOC). These AMOC responses peak in magnitude at lags of 2-3 years and ~17 years following the vortex anomalies. The vortex-AMOC interaction is characterised by non-stationary variations at periods of 30 and 50 years and involves a corresponding modulation of the North Atlantic Oscillation (NAO). Using the relationship between persistent vortex behaviour and the AMOC response established in the model, the contribution of the observed 8-year SSW hiatus interval in the 1990s to the recent negative trend in AMOC observations is estimated. This analysis suggests that approximately 30\% of the trend may have been caused by the SSW hiatus. The AMOC is also found to vary on longer timescales, similar to the 90-year SSW variability, but this periodicity is not present in the NAO signal. An additional mechanism is proposed where long-term variations in the AMOC influence the vortex via a pathway involving the equatorial Pacific and QBO. Finally, the role that vertical coherence in the QBO plays in its teleconnections is investigated using a set of climate model experiments that impose a perpetually deep and shallow QBO. While the magnitude of the vortex and surface responses to the QBO is significantly larger in the deep QBO experiment compared to the shallow experiment, they are of the opposite sign to those shown in previous work. The origin of these unexpected responses is explored, and the perpetual deep QBO is shown to induce unrealistically large anomalies in the equatorial semi-annual oscillation (SAO). The discrepancy is consistent with a modulation of the vortex and surface response from the SAO region, but showing this explicitly is challenging. Further experiments with imposed SAO conditions, as well as the QBO, are proposed to help establish the true nature of these teleconnections