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Robert Fajber
,
Adam H. Monahan
, and
William J. Merryfield

Abstract

The timing of daily extreme wind speeds from 10 to 200 m is considered using 11 yr of 10-min averaged data from the 213-m tower at Cabauw, the Netherlands. This analysis is complicated by the tendency of autocorrelated time series to take their extreme values near the beginning or end of a fixed window in time, even when the series is stationary. It is demonstrated that a simple averaging procedure using different base times to define the day effectively suppresses this “edge effect” and enhances the intrinsic nonstationarity associated with diurnal variations in boundary layer processes. It is found that daily extreme wind speeds at 10 m are most likely in the early afternoon, whereas those at 200 m are most likely in between midnight and sunrise. An analysis of the joint distribution of the timing of extremes at these two altitudes indicates the presence of two regimes: one in which the timing is synchronized between these two layers, and the other in which the occurrence of extremes is asynchronous. These results are interpreted physically using an idealized mechanistic model of the surface layer momentum budget.

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Arlan Dirkson
,
William J. Merryfield
, and
Adam H. Monahan

Abstract

Seasonal forecasts of Arctic sea ice using dynamical models are inherently uncertain and so are best communicated in terms of probabilities. Here, we describe novel statistical postprocessing methodologies intended to improve ensemble-based probabilistic forecasts of local sea ice concentration (SIC). The first of these improvements is the application of the parametric zero- and one-inflated beta (BEINF) probability distribution, suitable for doubly bounded variables such as SIC, for obtaining a smoothed forecast probability distribution. The second improvement is the introduction of a novel calibration technique, termed trend-adjusted quantile mapping (TAQM), that explicitly takes into account SIC trends and is applied using the BEINF distribution. We demonstrate these methods using a set of 10-member ensemble SIC hindcasts from the Third Generation Canadian Climate Coupled Global Climate Model (CanCM3) over the period 1981–2017. Though fitting ensemble SIC hindcasts to the BEINF distribution consistently improves probabilistic hindcast skill relative to a simpler “count based” probability approach in perfect model experiments, it does not itself correct model biases that may reduce this improvement when verifying against observations. The TAQM calibration technique is effective at removing SIC biases present in CanCM3 and improving forecast reliability. Over the recent 2000–17 period, TAQM-calibrated SIC hindcasts show improved skill relative to uncalibrated hindcasts. Compared against a climatological reference forecast adjusted for the trend, TAQM-calibrated hindcasts show widespread skill, particularly in September, even at 3–4-month lead times.

Open access
R. S. Ajayamohan
,
William J. Merryfield
, and
Viatcheslav V. Kharin

Abstract

The nature of the increasing frequency of extreme rainfall events (ERE) in central India is investigated by relating their occurrence to synoptic activity. Using a long record of the paths and intensities of monsoon synoptic disturbances, a synoptic activity index (SAI) is defined whose interannual variation correlates strongly with that in the number of ERE, demonstrating a strong connection between these phenomena. SAI furthermore shows a rising trend that is statistically indistinguishable from that in ERE, indicating that the increasing frequency of ERE is likely attributable to a rising trend in synoptic activity. This synoptic activity increase results from a rising trend in relatively weak low pressure systems (LPS), and it outweighs a declining trend in stronger LPS.

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Reinel Sospedra-Alfonso
,
William J. Merryfield
, and
Viatcheslav V. Kharin

Abstract

This paper examines potential predictability (PP) and actual skill for snow water equivalent (SWE) in the Canadian Seasonal to Interannual Prediction System (CanSIPS). A significant PP is found for SWE, with potentially predictable variance over 50% of the total variance at up to a 5-month lead in mid- to high latitudes in forecasts initialized after snow onset. Much, though not all, of this PP stems from a tendency for SWE anomalies to persist through the snow season. Although the spring melt acts as a PP barrier regardless of initialization date, in some regions significant PP reemerges in the following snow season. This is due primarily to ENSO teleconnections that are modeled realistically by CanSIPS, particularly in northwestern North America. Actual skill of CanSIPS in forecasting SWE is assessed using several verification datasets. Highest skills are obtained using a blend of five such datasets, consistent with the hypothesis that skill scores are degraded by errors in the verification data as well as by forecast errors, and that observational errors can be reduced by blending multiple datasets, much as forecast errors can be reduced by averaging across different models. Actual skill for SWE is comparable to, though generally lower than, that implied by PP. This is due in part to the similar autocorrelation properties of the forecast and observed SWE anomalies, which provide skill through anomaly persistence, combined with reasonably accurate initialization of SWE by CanSIPS. Long-lead skill across snow seasons is found to be linked to ENSO, particularly in western North America, much as for PP.

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William J. Merryfield
,
Greg Holloway
, and
Ann E. Gargett

Abstract

A global ocean model is described in which parameterizations of diapycnal mixing by double-diffusive fingering and layering are added to a stability-dependent background turbulent diffusivity. Model runs with and without double-diffusive mixing are compared for annual-mean and seasonally varying surface forcing. Sensitivity to different double-diffusive mixing parameterizations is considered. In all cases, the locales and extent of salt fingering (as diagnosed from buoyancy ratio R ρ ) are grossly comparable to climatology, although fingering in the models tends to be less intense than observed. Double-diffusive mixing leads to relatively minor changes in circulation but exerts significant regional influences on temperature and salinity.

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William J. Merryfield
,
Patrick F. Cummins
, and
Greg Holloway

Abstract

Inviscid equilibria of barotropic flows over finite-amplitude topography are determined by means of statistical mechanics, extending previous quasigeostrophic theory. Imposing constraints of energy and enstrophy conservation leads to a linear relation between equilibrium mean potential vorticity and mean transport streamfunction. This relation is tested numerically and is found to hold over a wide range of topographic amplitudes. Implications for improving parameterizations of entropy generation by eddies are discussed.

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Ann E. Gargett
,
William J. Merryfield
, and
Greg Holloway

Abstract

The potential for differential turbulent transport of oceanic temperature (T) and salinity (𝒮) is explored using three-dimensional direct numerical simulations of decaying stratified turbulence. The simulations employ a realistic molecular diffusion coefficient for T, and one for a “salt” scalar S that is 10 times smaller. Initially, a uniformly stratified medium is disturbed by a turbulent burst whose initial energy is assigned a range of values. In each instance, transports of T integrated over the subsequent decay of the burst exceed those of S. The more energetic cases occupy parameter ranges similar to, and exhibit spectral characteristics that are essentially indistinguishable from, those of direct observations of turbulence in the stratified ocean interior. In these cases, the turbulent diffusivity of T exceeds that of S by 6%–22%. These simulations underestimate the degree of differential diffusion between T and salinity 𝒮 (which has a molecular diffusivity 100 times less than T); thus at the Reynolds numbers attained by the simulations these results constitute lower bounds for differential diffusion associated with sporadic turbulence in the ocean. The simulation results are consistent with previous laboratory and two-dimensional numerical experiments and suggest that the assumption of equal turbulent diffusivities for T and 𝒮, commonly used in circulation modeling and in interpreting oceanic mixing measurements, should be reconsidered.

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Ann E. Gargett
,
William J. Merryfield
, and
Greg Holloway

Abstract

The potential for differential turbulent transport of oceanic temperature (T) and salinity ( S ) is explored using three-dimensional direct numerical simulations of decaying stratified turbulence. The simulations employ a realistic molecular diffusion coefficient for T, and one for a “salt” scalar S that is 10 times smaller. Initially, a uniformly stratified medium is disturbed by a turbulent burst whose initial energy is assigned a range of values. In each instance, transports of T integrated over the subsequent decay of the burst exceed those of S. The more energetic cases occupy parameter ranges similar to, and exhibit spectral characteristics that are essentially indistinguishable from, those of direct observations of turbulence in the stratified ocean interior. In these cases, the turbulent diffusivity of T exceeds that of S by 6%–22%. These simulations underestimate the degree of differential diffusion between T and salinity S (which has a molecular diffusivity 100 times less than T); thus at the Reynolds numbers attained by the simulations these results constitute lower bounds for differential diffusion associated with sporadic turbulence in the ocean. The simulation results are consistent with previous laboratory and two-dimensional numerical experiments and suggest that the assumption of equal turbulent diffusivities for T and S , commonly used in circulation modeling and in interpreting oceanic mixing measurements, should be reconsidered.

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Hai Lin
,
Ryan Muncaster
,
Jacques Derome
,
William J. Merryfield
, and
Gulilat Diro

Abstract

In contrast to boreal winter when extratropical seasonal predictions benefit greatly from ENSO-related teleconnections, our understanding of forecast skill and sources of predictability in summer is limited. Based on 40 years of hindcasts of the Canadian Seasonal to Inter-annual Prediction System version 3 (CanSIPSv3), this study shows that predictions for the Northern Hemisphere summer surface air temperature are skillful more than six months in advance in several middle latitude regions, including eastern Europe–Middle East, central Siberia–Mongolia–North China, and the western United States. These midlatitude regions of statistically significant predictive skill appear to be connected to each other through an upper tropospheric circum-global wave train. Although a large part of the forecast skill for the surface air temperature and 500 hPa geopotential height is attributable to the linear trend associated with global warming, there is significant long-lead seasonal forecast skill related to interannual variability. Two additional idealized hindcast experiments are performed to help shed light on sources of the long-lead forecast skill using one of the CanSIPSv3 models and its uncoupled version. It is found that tropical ENSO related SST anomalies contribute to the forecast skill in the western United States, while land surface conditions in winter, including snow cover and soil moisture, in the Siberian and western United States regions have a delayed or long-lasting impact on the atmosphere, which leads to summer forecast skill in these regions. This implies that improving land surface initial conditions and model representation of land surface processes is crucial for further development of a seasonal forecasting system.

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Oleg A. Saenko
,
Xiaoming Zhai
,
William J. Merryfield
, and
Warren G. Lee

Abstract

Several recent studies have shown that ocean western boundaries are the primary regions of eddy energy dissipation. Globally, the eddy energy sinks have been estimated to integrate to about 0.2 TW. This is a sizable fraction of the tidal energy dissipation in the deep oceanic interior, estimated at about 1.0 TW and contributing to diapycnal mixing. The authors conduct sensitivity experiments with an ocean general circulation model assuming that the eddy energy is scattered into high-wavenumber vertical modes, resulting in energy dissipation and locally enhanced diapycnal mixing. When only the tidal energy dissipation maintains diapycnal mixing, the overturning circulation, and stratification in the deep ocean are too weak. With the addition of the eddy dissipation, the deep-ocean thermal structure becomes closer to that observed and the overturning circulation and stratification in the abyss become stronger. Furthermore, the mixing associated with the eddy dissipation can, on its own, drive a relatively strong overturning. The stratification and overturning in the deep ocean are sensitive to the vertical structure of diapycnal mixing. When most of this energy dissipates within 300 m above the bottom, the abyssal overturning and stratification are too weak. Allowing for the dissipation to penetrate higher in the water column, such as suggested by recent observations, results in stronger stratification and meridional circulation. Zonal circulation is also affected. In particular, the Drake Passage transport becomes closer to its observational estimates with the increase in the vertical scale for turbulence above topography. Consistent with some theoretical models, the Drake Passage transport increases with the increase in the mean upper-ocean diffusivity.

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