On the Nonlinearity of Winter Northern Hemisphere Atmospheric Variability

Abstract

Nonlinearity in the Northern Hemisphere’s wintertime atmospheric flow is investigated from both an intermediate-complexity model of the extratropics and reanalyses. A long simulation is obtained using a three-level quasigeostrophic model on the sphere. Kernel empirical orthogonal functions (EOFs), which help delineate complex structures, are used along with the local flow tendencies. Two fixed points are obtained, which are associated with strong bimodality in two-dimensional kernel principal component (PC) space, consistent with conceptual low-order dynamics. The regimes reflect zonal and blocked flows. The analysis is then extended to ERA-40 and JRA-55 using daily sea level pressure (SLP) and geopotential heights in the stratosphere (20 hPa) and troposphere (500 hPa). In the stratosphere, trimodality is obtained, representing disturbed, displaced, and undisturbed states of the winter polar vortex. In the troposphere, the probability density functions (PDFs), for both fields, within the two-dimensional (2D) kernel EOF space are strongly bimodal. The modes correspond broadly to opposite phases of the Arctic Oscillation with a signature of the negative North Atlantic Oscillation (NAO). Over the North Atlantic–European sector, a trimodal PDF is also obtained with two strong and one weak modes. The strong modes are associated, respectively, with the north (or +NAO) and south (or −NAO) positions of the eddy-driven jet stream. The third weak mode is interpreted as a transition path between the two positions. A climate change signal is also observed in the troposphere of the winter hemisphere, resulting in an increase (a decrease) in the frequency of the polar high (low), consistent with an increase of zonal flow frequency.

Change and attribution of frost days and frost‐free periods in China

“…The polar high pressure then recedes southwards and overlaps with the long‐wave ridge developing on the westerly wind belt, providing the conditions for the cold air outbreak. In the mid‐troposphere, the central North Pacific anticyclone enhances the southwards push of cold air and plays a blocking role, facilitating the influx of cold polar air from the westerly path into China (Beerli & Grams, 2019; Graf & Zanchettin, 2012; Hannachi & Iqbal, 2019). For Mode 3 of FDs and FFPs, the area of the Indian Ocean warm pool and the location of the northern boundary of the PSH has been identified as crucial factors.…”

An Atmospheric Signal Lowering the Spring Predictability Barrier in Statistical ENSO Forecasts

“…Thus, we can conclude that the HI relates to the SST footprinting mechanism and actually captures the linear part of the interaction between the winter-spring NPO pattern and ENSO. Nonlinear data analysis methods, such as those of (Gavrilov et al, 2016;Hannachi & Iqbal, 2019;Kramer, 1991;Mukhin et al, 2015Mukhin et al, , 2018, could help extract a better ENSO predictor by capturing the asymmetry of the NPO pattern; nevertheless, we leave this complex task for future works and restrict our consideration to a linear analysis.…”

The Summertime Pacific‐North American Weather Regimes and Their Predictability

“…As a result, the high (low) forecast skill for a flow pattern might be due to its high (low) intrinsic predictability (Faranda, Messori, & Yiou, 2017; Ferranti et al., 2015; Lorenz, 1996; Sheshadri et al., 2021). Low forecast skill can also be due to model errors (e.g., incorrect representation of physical processes, inadequate numerical resolution), which can disproportionally affect some flow patterns more (Dawson et al., 2012; Hannachi & Iqbal, 2019; Strommen & Palmer, 2019; Weisheimer et al., 2019). In recent years, there has been a growing interest in studying the flow‐dependent predictability of the midlatitude circulation using a few distinct patterns obtained from clustering fields such as daily geopotential height at 500 hPa (Z500 hereafter).…”