Zonal-mean Jet Research Articles

Rotational/divergent flow and energy conversion of thermal tides in a Venus general circulation model

Venusian thermal tides are discussed as a three-dimensional response to solar heating, which is strongly modified by both barotropic and baroclinic energy conversions in a fully developed super-rotation. Thermal tides such as gravity waves propagate upward and downward from the cloud top (∼65 km) at low latitudes, while tidal gyres such as Rossby waves appear at high latitudes according to the general circulation model (GCM) developed at Atmosphere and Ocean Research Institute (AORI), The University of Tokyo. Around the cloud-top heating maximum, strong poleward/equatorward tidal flows are formed by both divergent and rotational flows that are in phase, and equatorial super-rotation is accelerated by the equatorward and vertical momentum fluxes of the diurnal tides and the vertical flux of the semidiurnal tides. These thermal tides are a result of solar heating as well as energy conversions from the zonal-mean jet and its related temperature gradient. The gravity wave is thermally forced in the cloud-top heating maximum where diabatic heating produces eddy available potential energy. The Rossby waves are formed in mid- and high-latitudinal regions where diurnal barotropic energy conversion occurs around the zonal-mean jet core and semidiurnal baroclinic energy conversion occurs below the cloud layer far from the solar heating region.

Revisiting the wintertime emergent constraint of the southern hemispheric midlatitude jet response to global warming

Abstract. Most climate models show a poleward shift of the southern hemispheric zonal-mean jet in response to climate change, but the inter-model spread is large. In an attempt to constrain future jet responses, past studies have identified an emergent constraint between the climatological jet latitude and the future jet shift in austral winter. However, we show that the emergent constraint only arises in the zonal mean and not in separate halves of the hemisphere, which questions the physicality of the emergent constraint. We further find that the zonal-mean jet latitude does not represent the latitude of a zonally coherent structure, due to the presence of a double-jet structure in the Pacific region during this season. The zonal asymmetry causes the previously noted large spread in the zonal-mean climatology but not in the response, which underlies the emergent constraint. We therefore argue that the emergent constraint on the zonal-mean jet cannot narrow down the spread in future wind responses, and we propose that emergent constraints on the jet response in austral winter should be based on regional rather than zonal-mean circulation features.

Assessment of trends in an integrated climate metric—analysis of 200 mbar zonal wind for the period 1958–2021

Using two reanalysis datasets on latitude bands of nearly equal area, we analyze the trend of 200 mbar zonal wind anomalies for the period 1958–2021 as well as annual-mean latitude of the zonal mean jet stream. We do not find much change of 200 mbar zonal wind in both two datasets although the increasing trend over the high latitude bands from 45 to 61°S for both datasets, and the decreasing trend in 0–5°N and 0–10°S for ERA5 are significant with small magnitudes. In addition, the polar jet core moves poleward in the Northern Hemisphere, but the movement is not statistically significant, while no poleward shift at all has occurred in the Southern Hemisphere. This work provides us a way to look for the long-term changes and variability in atmospheric circulation.

Energy Exchanges in Saturn's Polar Regions From Cassini Observations: Eddy-Zonal Flow Interactions.

Saturn's polar regions (polewards of ∼63° planetocentric latitude) are strongly dynamically active with zonal jets, polar cyclones and the intriguing north polar hexagon (NPH) wave. Here we analyze measurements of horizontal winds, previously obtained from Cassini images by Antuñano et al. (2015), https://doi.org/10.1002/2014je004709, to determine the spatial and spectral exchanges of kinetic energy (KE) between zonal mean zonal jets and nonaxisymmetric eddies in Saturn's polar regions. Eddies of most resolved scales generally feed KE into the eastward and westward zonal mean jets at rates between 4.3 × 10−5 and 1.4 × 10−4 W kg−1. In particular, the north polar jet (at 76°N) was being energized at a rate of ∼10−4 W kg−1, dominated by the contribution due to the zonal wavenumber m = 6 NPH wave itself. This implies that the hexagon was not being driven at this time through a barotropic instability of the north polar jet, but may suggest a significant role for baroclinic instabilities, convection or other internal energy sources for this feature. The south polar zonal mean jet KE was also being sustained by eddies in that latitude band across a wide range of m. In contrast, results indicate that the north polar vortex may have been weakly barotropically unstable at this time with eddies of low m gaining KE at the expense of the axisymmetric cyclone. However, the southern axisymmetric polar cyclone was gaining KE from non‐axisymmetric components at this time, including m = 2 and its harmonics, as the elliptical distortion of the vortex may have been decaying.

A Synoptic View of the Onset of the Mid-Latitude QBO Signal

AbstractThe extratropical effect of the quasi-biennial oscillation (QBO), known as the Holton-Tan effect, is manifest as aweaker, warmer winter Arctic polar vortex during the east QBO phase. While previous studies have shown that the extratropical QBO signal is caused by the modified propagation of planetary waves in the stratosphere, the mechanism dominating the onset and seasonal development of the Holton-Tan effects remains unclear. Here, the governing wave-mean flow dynamics of the early winter extratropical QBO signal onset and its reversibility is investigated on a synoptic timescale with a finite-amplitude diagnostic using reanalysis and a chemistry-climate model. The extratropical QBO signal onset in October is found to primarily result from modulated stratospheric life-cycles of wave pulses entering the stratosphere from the troposphere, rather than from a modulation of their tropospheric wave source. A comprehensive analysis of the wave activity budget during fall, when the stratospheric winter polar vortex starts forming and waves start propagating up into the stratosphere, shows significant differences. During the east QBO phase, the deceleration of the mid-high latitude stratospheric zonal mean jet by the upward propagating wave pulses is less reversible, due to stronger dissipation processes, while during the west phase, a more reversible deceleration of the main polar vortex is found owing to the waves being dissipated at lower latitudes, accompanied by a weak but different response of the tropospheric subtropical jet. From this synoptic wave-event viewpoint, the early season onset of the Holton-Tan effect results from the cumulative effect of the QBO dependent wave-induced deceleration during the life cycle of individual upward wave pulses.

A Barotropic Mechanism for the Response of Jet Stream Variability to Arctic Amplification and Sea Ice Loss

Previous studies have found that the most consistent response of the eddy-driven jet to sea ice loss and Arctic amplification in fully coupled general circulation models (GCMs) is a broad region of anomalous easterlies on the poleward flank. In this study, a similar response is noted in a dry dynamical core GCM with imposed surface heating at the pole, and it is shown that in both a fully coupled GCM’s North Atlantic basin and the dry dynamical core, the anomalous easterlies cause an asymmetrical narrowing of the jet on the poleward flank of the climatological jet. A suite of barotropic model simulations run with polar forcing shows decreased jet positional variability consistent with a narrowing of the jet profile, and it is proposed that this narrowing decreases the distance Rossby waves can propagate away from the jet core, which drives changes in jet variability. Since Rossby wave propagation and dissipation is intrinsic to the development and maintenance of the eddy-driven jet, and is tightly coupled to a jet’s variability, this acts as a meridional constraint on waves’ ability to propagate outside of the jet core, leading to the decreased variability in zonal-mean jet position. The results from all three models demonstrates that this relationship is present across a model hierarchy.

Summertime Planetary Wave Resonance in the Northern and Southern Hemispheres

Slow-moving planetary waves of high amplitudes are often associated with persistent surface weather conditions. This persistence can lead to extreme weather events with potentially serious implications for society and nature. Quasi-resonant amplification (QRA) of planetary waves has been proposed as a mechanism to generate high-amplitude hemisphere-wide patterns of wavenumbers 6–8 in the Northern Hemisphere (NH) summer. Here this mechanism is studied in both hemispheres. Analyzing 1979–2015 reanalysis data, evidence for QRA in the Southern Hemisphere (SH) is found for wavenumbers 4 and 5. It is shown that the difference in resonating wavenumbers between hemispheres stems from the different magnitude and latitudinal shape of the respective zonal-mean zonal winds. During resonance events a strong and narrow jet is observed in both hemispheres, whereas the emergence of a second zonal mean jet at high latitudes (i.e., a “double jet”) is seen in the NH only. Strong and narrow jets can form stable, zonally oriented waveguides, which are an essential prerequisite for resonance. After investigating the waves’ preferred phase positions during QRA, a bimodal behavior is identified for wavenumbers 4 and 5 in the SH and for wavenumber 6 in the NH, whereas wavenumbers 7 and 8 exhibit a single preferred phase position in the NH. Composite plots of meridional wind and temperature anomalies during periods when the resonating wave is in the preferred phase position indicate those regions that are most likely to experience heat extremes. These regions include western North America, western Europe, and central Eurasia in the NH and Patagonia and Australia in the SH.

A network-based detection scheme for the jet stream core

Abstract. The polar and subtropical jet streams are strong upper-level winds with a crucial influence on weather throughout the Northern Hemisphere midlatitudes. In particular, the polar jet is located between cold arctic air to the north and warmer subtropical air to the south. Strongly meandering states therefore often lead to extreme surface weather. Some algorithms exist which can detect the 2-D (latitude and longitude) jets' core around the hemisphere, but all of them use a minimal threshold to determine the subtropical and polar jet stream. This is particularly problematic for the polar jet stream, whose wind velocities can change rapidly from very weak to very high values and vice versa. We develop a network-based scheme using Dijkstra's shortest-path algorithm to detect the polar and subtropical jet stream core. This algorithm not only considers the commonly used wind strength for core detection but also takes wind direction and climatological latitudinal position into account. Furthermore, it distinguishes between polar and subtropical jet, and between separate and merged jet states. The parameter values of the detection scheme are optimized using simulated annealing and a skill function that accounts for the zonal-mean jet stream position (Rikus, 2015). After the successful optimization process, we apply our scheme to reanalysis data covering 1979–2015 and calculate seasonal-mean probabilistic maps and trends in wind strength and position of jet streams. We present longitudinally defined probability distributions of the positions for both jets for all on the Northern Hemisphere seasons. This shows that winter is characterized by two well-separated jets over Europe and Asia (ca. 20° W to 140° E). In contrast, summer normally has a single merged jet over the western hemisphere but can have both merged and separated jet states in the eastern hemisphere. With this algorithm it is possible to investigate the position of the jets' cores around the hemisphere and it is therefore very suitable to analyze jet stream patterns in observations and models, enabling more advanced model-validation.

Effects of Bulk Composition on the Atmospheric Dynamics on Close-in Exoplanets

Abstract Super Earths and mini Neptunes likely have a wide range of atmospheric compositions, ranging from low molecular mass atmospheres of H2 to higher molecular atmospheres of water, CO2, N2, or other species. Here we systematically investigate the effects of atmospheric bulk compositions on temperature and wind distributions for tidally locked sub-Jupiter-sized planets, using an idealized 3D general circulation model (GCM). The bulk composition effects are characterized in the framework of two independent variables: molecular weight and molar heat capacity. The effect of molecular weight dominates. As the molecular weight increases, the atmosphere tends to have a larger day–night temperature contrast, a smaller eastward phase shift in the thermal phase curve, and a smaller zonal wind speed. The width of the equatorial super-rotating jet also becomes narrower, and the “jet core” region, where the zonal-mean jet speed maximizes, moves to a greater pressure level. The zonal-mean zonal wind is more prone to exhibit a latitudinally alternating pattern in a higher molecular weight atmosphere. We also present analytical theories that quantitatively explain the above trends and shed light on the underlying dynamical mechanisms. Those trends might be used to indirectly determine the atmospheric compositions on tidally locked sub-Jupiter-sized planets. The effects of the molar heat capacity are generally small. But if the vertical temperature profile is close to adiabatic, molar heat capacity will play a significant role in controlling the transition from a divergent flow in the upper atmosphere to a jet-dominated flow in the lower atmosphere.

A simple climatology of westerly jet streams in global reanalysis datasets part 1: mid-latitude upper tropospheric jets

A simple closed contour object identification scheme has been applied to the zonal mean monthly mean zonal wind fields from nine global reanalysis data sets for 31 years of the satellite era (1979–2009) to identify objects corresponding to westerly jet streams. The results cluster naturally into six individual jet streams but only the mid-latitude upper-tropospheric jets are considered here. The time series of the jet properties from all reanalyses are decomposed into seasonal means and anomalies, and correlations between variables are evaluated, with the aim of identifying robust features which can form the basis of evaluation metrics for climate model simulations of the twentieth century. There is substantial agreement between all the reanalyses for all jet properties although there are some systematic differences with particular data sets. Some of the results from the object identification applied to the reanalyses are used in a simple example of a model evaluation score for the zonal mean jet seasonal cycle.

Reconstructing Sea Level Pressure Variability via a Feature Tracking Approach

Abstract The aim of this paper is to quantify the contribution of synoptic transients to the full spectrum of space–time variability of sea level pressure (SLP) in middle latitudes. In previous work by the authors it was shown that tracking cyclones and anticyclones in an idealized atmospheric model allows one to reconstruct a surprisingly large fraction of the model’s variability, including not only synoptic components, but also its large-scale low-frequency component. Motivated by this result, the authors performed tracking of cyclones and anticyclones and estimated cyclone and anticyclone size and geometry characteristics in the observed SLP field using the 1948–2008 NCEP–NCAR reanalysis dataset. The reconstructed synoptic field was then produced via superimposing radially symmetrized eddies moving along their actual observed trajectories. It was found that, similar to earlier results for an idealized model, the synoptic reconstruction so obtained accounts for a major fraction of the full observed SLP variability across a wide range of time scales, from synoptic to those associated with the low-frequency variability (LFV). The synoptic reconstruction technique developed in this study helps elucidate connections between the synoptic eddies and LFV defined via more traditional spatiotemporal filtering. In particular, we found that the dominant variations in the position of the zonal-mean midlatitude jet are synonymous with random ultralow-frequency redistributions of cyclone and anticyclone trajectories and, hence, is inseparable of that in the storm-track statistics.

Kinematics of Eddy–Mean Flow Interaction in an Idealized Atmospheric Model

Abstract The authors analyze atmospheric variability simulated in a two-layer baroclinic β-channel quasigeostrophic model by combining Eulerian and feature-tracking analysis approaches. The leading mode of the model's low-frequency variability (LFV) is associated with the irregular shifts of the zonal-mean jet to the north and south of its climatological position accompanied by simultaneous intensification of the jet, while the deviations from the zonal-mean fields are dominated by propagating anomalies with wavenumbers 3–5. The model's variability is shown to stem from the life cycles of cyclones and anticyclones. In particular, synthetic streamfunction fields constructed by launching idealized composite-mean eddies along the actual full-model-simulated cyclone/anticyclone tracks reproduce nearly perfectly not only the dominant propagating waves, but also the jet-shifting LFV. The composite eddy tracks conditioned on the phase of the jet-shifting variability migrate north or south along with the zonal-mean jet. The synoptic-eddy life cycles in the states with poleward (equatorward) zonal-jet shift exhibit longer-than-climatological lifetimes; this is caused, arguably, by a barotropic feedback associated with preferred anticyclonic (cyclonic) wave breaking in these respective states. Lagged correlation and cross-spectrum analyses of zonal-mean jet position time series and the time series representing mean latitudinal location of the eddies at a given time demonstrate that jet latitude leads the storm-track latitude at low frequencies. This indicates that the LFV associated with the jet-shifting mode here is more dynamically involved than being a mere consequence of the random variations in the distribution of the synoptic systems.

The Effect of Tropospheric Jet Latitude on Coupling between the Stratospheric Polar Vortex and the Troposphere

Abstract A dry general circulation model is used to investigate how coupling between the stratospheric polar vortex and the extratropical tropospheric circulation depends on the latitude of the tropospheric jet. The tropospheric response to an identical stratospheric vortex configuration is shown to be strongest for a jet centered near 40° and weaker for jets near either 30° or 50° by more than a factor of 3. Stratosphere-focused mechanisms based on stratospheric potential vorticity inversion, eddy phase speed, and planetary wave reflection, as well as arguments based on tropospheric eddy heat flux and zonal length scale, appear to be incapable of explaining the differences in the magnitude of the jet shift. In contrast, arguments based purely on tropospheric variability involving the strength of eddy–zonal mean flow feedbacks and jet persistence, and related changes in the synoptic eddy momentum flux, appear to explain this effect. The dependence of coupling between the stratospheric polar vortex and the troposphere on tropospheric jet latitude found here is consistent with 1) the observed variability in the North Atlantic and the North Pacific and 2) the trend in the Southern Hemisphere as projected by comprehensive models.

Assessment of surface winds over the Atlantic, Indian, and Pacific Ocean sectors of the Southern Ocean in CMIP5 models: historical bias, forcing response, and state dependence

An assessment of the fifth Coupled Models Intercomparison Project (CMIP5) models' simulation of the near‐surface westerly wind jet position and strength over the Atlantic, Indian and Pacific sectors of the Southern Ocean is presented. Compared with reanalysis climatologies there is an equatorward bias of 3.3° (inter‐model standard deviation of ± 1.9°) in the ensemble mean position of the zonal mean jet. The ensemble mean strength is biased slightly too weak, with the largest biases over the Pacific sector (−1.4 ± 1.2 m/s, −19%). An analysis of atmosphere‐only (AMIP) experiments indicates that 28% of the zonal mean position bias comes from coupling of the ocean/ice models to the atmosphere. The response to future emissions scenarios (RCP4.5 and RCP8.5) is characterized by two phases: (i) the period of most rapid ozone recovery (2000–2049) during which there is insignificant change in summer; and (ii) the period 2050–2098 during which RCP4.5 simulations show no significant change but RCP8.5 simulations show poleward shifts (0.33, 0.18 and 0.27°/decade over the Atlantic, Indian and Pacific sectors, respectively), and increases in strength (0.07, 0.08 and 0.15 m/s/decade, respectively). The models with larger equatorward position biases generally show larger poleward shifts (i.e. state dependence). This inter‐model relationship is strongest over the Pacific sector (r = −0.91) and weakest over the Atlantic sector (r = −0.39). An assessment of jet structure shows that over the Atlantic sector jet shift is not clearly linked to indices of jet structure whereas over the Pacific sector the distance between the sub‐polar and sub‐tropical westerly jets appears to be important.

Has the East Asian westerly jet experienced a poleward displacement in recent decades?

Previous studies have suggested a poleward shift of the zonally averaged jet stream due to rapid warming over continents. However, the regional characteristics of the change in the jet stream are not yet understood. Here, we present evidence suggesting that the East Asian westerly jet did not shift poleward in past decades (1980–2004 relative to 1958–1979), both in winter and summer. Rather, the jet axis has moved southward in summer, but its meridional position is steady in winter. The main change of the jet stream in winter is the enhancement of its intensity. These changes in both summer and winter are consistent with the corresponding changes in the large meridional tropospheric temperature-gradient zone. Based on these results, we suggest that the changes of the jet stream over East Asia are unique and are different from the zonal mean jet stream over the Northern Hemisphere and over the North Atlantic region.

Two Types of Baroclinic Life Cycles during the Southern Hemisphere Summer

AbstractBaroclinic eddy life cycles of the Southern Hemisphere (SH) summer are investigated with NCEP–NCAR reanalysis data. A composite analysis is performed for the years 1980 through 2004. Individual life cycles are identified by local maxima in synoptic-scale eddy energy. Two types of baroclinic life cycles are examined, each defined by the strength of the barotropic energy conversion 2 days prior to the maximum baroclinic growth. For one life cycle, the barotropic conversion is anomalously weak before the maximum baroclinic growth; for the other, the barotropic conversion is anomalously strong. These two life cycles are referred to as the weak barotropic (WB) and strong barotropic (SB) life cycles.The analyses for the WB life cycle find that a poleward anomalous wave activity flux is observed within the SH tropics and subtropics just before the initial growth of the synoptic-scale eddies. In contrast, the SB life cycle exhibits an equatorward anomalous wave activity flux prior to the initial wave development. For the WB life cycle, these changes in the wave activity flux are shown to induce a mean meridional circulation that weakens and broadens the midlatitude zonal mean jet and reduces the baroclinicity in the midlatitude lower troposphere. Opposite characteristics are observed for the SB life cycle. Since the eddy growth rate is found to be greater in the WB life cycle, these results suggest that the influences of the barotropic governor mechanism (a reduction in horizontal shear coinciding with more rapidly growing baroclinic eddies) and the midlatitude baroclinicity oppose each other at the beginning of the life cycle, with the former being dominant.Both the WB and SB life cycles coincide with anomalous tropical convection. For the WB life cycle, there is a strengthening of the convection over the Maritime Continent, and for the SB life cycle there is a weakening in the convection over the same region. These results suggest that the two types of baroclinic life cycles are ultimately triggered by convection in the tropics.

The Effect of Lower Stratospheric Shear on Baroclinic Instability

Abstract Using a hierarchy of models, and observations, the effect of vertical shear in the lower stratosphere on baroclinic instability in the tropospheric midlatitude jet is examined. It is found that increasing stratospheric shear increases the phase speed of growing baroclinic waves, increases the growth rate of modes with low synoptic wavenumbers, and decreases the growth rate of modes with higher wavenumbers. The meridional structure of the linear modes, and their acceleration of the zonal mean jet, changes with increasing stratospheric shear, but in a way that apparently contradicts the observed stratosphere–troposphere northern annular mode (NAM) connection. This contradiction is resolved at finite amplitude. In nonlinear life cycle experiments it is found that increasing stratospheric shear, without changing the jet structure in the troposphere, produces a transition from anticyclonic (LC1) to cyclonic (LC2) behavior at wavenumber 7. All life cycles with wavenumbers lower than 7 are LC1, and all with wavenumber greater than 7 are LC2. For the LC1 life cycles, the effect of increasing stratospheric shear is to increase the poleward displacement of the zonal mean jet by the eddies, which is consistent with the observed stratosphere–troposphere NAM connection. Finally, it is found that the connection between high stratospheric shear and high-tropospheric NAM is present by NCEP–NCAR reanalysis data.

Atmospheric Circulation and Its Effect on Arctic Sea Ice in CCSM3 Simulations at Medium and High Resolution*

Abstract The simulation of Arctic sea ice and surface winds changes significantly when Community Climate System Model version 3 (CCSM3) resolution is increased from T42 (∼2.8°) to T85 (∼1.4°). At T42 resolution, Arctic sea ice is too thick off the Siberian coast and too thin along the Canadian coast. Both of these biases are reduced at T85 resolution. The most prominent surface wind difference is the erroneous North Polar summer anticyclone, present at T42 but absent at T85. An offline sea ice model is used to study the effect of the surface winds on sea ice thickness. In this model, the surface wind stress is prescribed alternately from reanalysis and the T42 and T85 simulations. In the offline model, CCSM3 surface wind biases have a dramatic effect on sea ice distribution: with reanalysis surface winds annual-mean ice thickness is greatest along the Canadian coast, but with CCSM3 winds thickness is greater on the Siberian side. A significant difference between the two CCSM3-forced offline simulations is the thickness of the ice along the Canadian archipelago, where the T85 winds produce thicker ice than their T42 counterparts. Seasonal forcing experiments, with CCSM3 winds during spring and summer and reanalysis winds in fall and winter, relate the Canadian thickness difference to spring and summer surface wind differences. These experiments also show that the ice buildup on the Siberian coast at both resolutions is related to the fall and winter surface winds. The Arctic atmospheric circulation is examined further through comparisons of the winter sea level pressure (SLP) and eddy geopotential height. At both resolutions the simulated Beaufort high is quite weak, weaker at higher resolution. Eddy heights show that the wintertime Beaufort high in reanalysis has a barotropic vertical structure. In contrast, high CCSM3 SLP in Arctic winter is found in association with cold lower-tropospheric temperatures and a baroclinic vertical structure. In reanalysis, the summertime Arctic surface circulation is dominated by a polar cyclone, which is accompanied by surface inflow and a deep Ferrel cell north of the traditional polar cell. The Arctic Ferrel cell is accompanied by a northward flux of zonal momentum and a polar lobe of the zonal-mean jet. These features do not appear in the CCSM3 simulations at either resolution.

Kuroshio Extension Variability and Forcing of the Pacific Decadal Oscillations: Responses and Potential Feedback

Abstract A forcing mechanism is sought for the large-scale circulation changes in the Kuroshio Extension region of the western North Pacific Ocean as inferred by TOPEX/Poseidon sea surface height (SSH) data. The low-frequency signal of the Kuroshio Extension over the last decade was characterized by a modulation in its zonal mean flow intensity: the mean Kuroshio Extension jet weakened progressively from 1993 to 1996 and this trend reversed after 1997. The ability to simulate the major trends in the observed SSH signals with linear vorticity dynamics leads the authors to conclude that the modulation in the zonal mean jet was remotely forced by wind stress curl anomalies in the eastern North Pacific Ocean related to the Pacific decadal oscillations (PDOs). To be specific, the weakening (strengthening) trend in 1993–96 (1997–2001) was caused by westward expansions of negative (positive) SSH anomalies south of the Kuroshio Extension and positive (negative) SSH anomalies north of the Kuroshio Extension. Emergence of oppositely signed SSH anomalies on the two sides of the Kuroshio Extension jet is due to the different propagating speeds of the baroclinic Rossby waves, which carry the wind-induced SSH anomalies generated in the eastern North Pacific at different phases of the PDOs. Hindcasting the Kuroshio Extension jet strength over the last 45 years reveals that the jet modulation has a dominant timescale of ∼12 yr. Given the location of the Kuroshio Extension jet relative to the maximum atmospheric forcing, it is found that this dominant timescale is consistent with the preferred timescale under a stochastic white-noise atmospheric forcing. It is hypothesized that this connection between the Kuroshio Extension strength and the latitudinally dependent baroclinic adjustment contributes to an increase in variance and persistence of the North Pacific midlatitude coupled system on the decadal timescale.

Kuroshio Extension Variability and Forcing of the Pacific Decadal Oscillations: Responses and Potential Feedback

A forcing mechanism is sought for the large-scale circulation changes in the Kuroshio Extension region of the western North Pacific Ocean as inferred by TOPEX/Poseidon sea surface height (SSH) data. The low-frequency signal of the Kuroshio Extension over the last decade was characterized by a modulation in its zonal mean flow intensity: the mean Kuroshio Extension jet weakened progressively from 1993 to 1996 and this trend reversed after 1997. The ability to simulate the major trends in the observed SSH signals with linear vorticity dynamics leads the authors to conclude that the modulation in the zonal mean jet was remotely forced by wind stress curl anomalies in the eastern North Pacific Ocean related to the Pacific decadal oscillations (PDOs). To be specific, the weakening (strengthening) trend in 1993–96 (1997–2001) was caused by westward expansions of negative (positive) SSH anomalies south of the Kuroshio Extension and positive (negative) SSH anomalies north of the Kuroshio Extension. Emergence of oppositely signed SSH anomalies on the two sides of the Kuroshio Extension jet is due to the different propagating speeds of the baroclinic Rossby waves, which carry the wind-induced SSH anomalies generated in the eastern North Pacific at different phases of the PDOs. Hindcasting the Kuroshio Extension jet strength over the last 45 years reveals that the jet modulation has a dominant timescale of ∼12 yr. Given the location of the Kuroshio Extension jet relative to the maximum atmospheric forcing, it is found that this dominant timescale is consistent with the preferred timescale under a stochastic white-noise atmospheric forcing. It is hypothesized that this connection between the Kuroshio Extension strength and the latitudinally dependent baroclinic adjustment contributes to an increase in variance and persistence of the North Pacific midlatitude coupled system on the decadal timescale.