The Asymmetric Atmospheric Response to the Decadal Variability of Kuroshio Extension during Winter

Abstract One of the most commonly used metrics for both locating the Madden–Julian oscillation (MJO) geographically and defining the intensity of MJO convective activity is the real-time multivariate MJO (RMM) index. However, a climatology of the MJO, particularly with respect to the frequency of activity levels or of consecutive days at certain activity thresholds, does not yet exist. Thus, several climatological aspects of the MJO were developed in this study: 1) annual and 2) seasonal variability in MJO intensity, quantified using four defined activity categories (inactive, active, very active, and extremely active); 3) persistence in the above-defined four categories; 4) cycle length; and 5) low-frequency (decadal) variability. On an annual basis, MJO phases 1 and 2 occurred more often, and phase 8 occurred less often, than the other phases throughout the year. Notable seasonality was also found, particularly in the frequency of extremely active MJO in March–May (8% of days) compared with June–August (only 1% of days). The MJO was persistent in time and across intensity categories, and all activity categories the following day had at least an 80% chance of maintaining their amplitudes. Implications of this climatology are discussed, including length of complete MJO cycles (the shortest of which was 17 days) and correlations between MJO amplitude and atmospheric response.

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Observational Evidence of Frontal-Scale Atmospheric Responses to Kuroshio Extension Variability

Journal of Climate ◽

10.1175/jcli-d-14-00829.1 ◽

2015 ◽

Vol 28 (23) ◽

pp. 9459-9472 ◽

Cited By ~ 12

Author(s):

Yi-Hui Wang ◽

W. Timothy Liu

Keyword(s):

Kuroshio Extension ◽

Linear Regression Analysis ◽

Surface Wind ◽

Reanalysis Data ◽

Satellite Observations ◽

Climate Index ◽

Boreal Winter ◽

Atmospheric Response ◽

Free Atmosphere ◽

Low Frequencies

Abstract This study investigates the regional atmospheric response to the Kuroshio Extension (KE) using a combination of multiple satellite observations and reanalysis data from boreal winter over a period of at least a decade. The goal is to understand the relationship between KE variations and atmospheric responses at low frequencies. A climate index is used to measure the interannual to decadal KE variability, which leaves remarkable imprints on the mesoscale sea surface temperature (SST). Clear spatial coherence between the SST signals and frontal-scale atmospheric variables, including surface wind convergence, vertical velocity, precipitation, and clouds, is identified by linear regression analysis. Consistent with previous studies, the penetrating effect of the KE variability on the free atmosphere is found. The westward tilt of the atmospheric response above the KE near 500 hPa is revealed. The difference in the associations of frontal-scale air temperature and geopotential height with the KE variability between the satellite observations and the reanalysis data suggests an imperfect interpretation of frontal-scale oceanic forcing on the overlying atmosphere in the reanalysis assimilation system.

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Decadal variability of the Kuroshio Extension

10.5194/egusphere-egu2020-1759 ◽

2020 ◽

Author(s):

Guidi Zhou ◽

Xuhua Cheng

Keyword(s):

Decadal Variability ◽

Kuroshio Extension ◽

Relaxation Oscillation ◽

Mesoscale Eddy ◽

Height Anomaly ◽

Intrinsic Variability ◽

Mean Flow ◽

The Real ◽

Eddy Activity ◽

The Kuroshio

<p>The decadal variability of the Kuroshio Extension (KE) is investigated using altimeter observations (AVISO) and the output of an ocean model (OFES). It is shown that the KE decadal variability is manifested in its strength, latitudinal position, and zonal extent, as well as the associated mesoscale eddy activity. Two differences between the two datasets are identified: (a) In OFES, the eddy activity positively correlates with the KE mode index when it leads by a few years, whereas in AVISO the two are negatively and concurrently correlated. (b) In OFES, the positive KE mode is associated with large meanders of the Kuroshio south of Japan, but in AVISO they are irrelevant. These differences indicate that the generation mechanism of KE's decadal variability is different in OFES and the real ocean. The sea surface height anomaly (SSHA) is then decomposed into major components including the wind-driven Rossby waves and residual (intrinsic) variability. The relationship between the two components are virtually the same in OFES and in AVISO, showing a negative correlation when the wind-driven part leads by a few years. Further diagnostics based on OFES reveals that the residual SSHA originates from the downstream region over the Shatsky Rise, slowly propagates westward, and is driven by eddy potential energy transfer. The OFES results partly conform to the intrinsic relaxation oscillation theory put forth by idealized model analyses, but in the latter the SSHA signal originates from the upstream Kuroshio. A new mechanism is then proposed for OFES: the decadal variability of the KE is first a result of the intrinsic relaxation oscillation probably excited by wind forcing, which regulates the strength of the KE&#8217;s inflow and thus modulates the downstream topography interaction, resulting in different downstream mesoscale eddy activity that further feeds back on the mean-flow. The mechanism for the real ocean is also reassessed.</p>

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On the Decadal Variability of the Eddy Kinetic Energy in the Kuroshio Extension

Journal of Physical Oceanography ◽

10.1175/jpo-d-16-0201.1 ◽

2017 ◽

Vol 47 (5) ◽

pp. 1169-1187 ◽

Cited By ~ 20

Author(s):

Yang Yang ◽

X. San Liang ◽

Bo Qiu ◽

Shuiming Chen

Keyword(s):

Kinetic Energy ◽

Potential Energy ◽

Decadal Variability ◽

Kuroshio Extension ◽

Baroclinic Instability ◽

Eddy Kinetic Energy ◽

Time Varying ◽

Barotropic Instability ◽

Available Potential Energy ◽

Decadal Modulation

AbstractPrevious studies have found that the decadal variability of eddy kinetic energy (EKE) in the upstream Kuroshio Extension is negatively correlated with the jet strength, which seems counterintuitive at first glance because linear stability analysis usually suggests that a stronger jet would favor baroclinic instability and thus lead to stronger eddy activities. Using a time-varying energetics diagnostic methodology, namely, the localized multiscale energy and vorticity analysis (MS-EVA), and the MS-EVA-based nonlinear instability theory, this study investigates the physical mechanism responsible for such variations with the state estimate from the Estimating the Circulation and Climate of the Ocean (ECCO), Phase II. For the first time, it is found that the decadal modulation of EKE is mainly controlled by the barotropic instability of the background flow. During the high-EKE state, violent meanderings efficiently induce strong barotropic energy transfer from mean kinetic energy (MKE) to EKE despite the rather weak jet strength. The reverse is true in the low-EKE state. Although the enhanced meander in the high-EKE state also transfers a significant portion of energy from mean available potential energy (MAPE) to eddy available potential energy (EAPE) through baroclinic instability, the EAPE is not efficiently converted to EKE as the two processes are not well correlated at low frequencies revealed in the time-varying energetics. The decadal modulation of barotropic instability is found to be in pace with the North Pacific Gyre Oscillation but with a time lag of approximately 2 years.

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Interannual to Decadal Variability of the Upper-Ocean Heat Content in the Western North Pacific and Its Relationship to Oceanic and Atmospheric Variability

Journal of Climate ◽

10.1175/jcli-d-17-0506.1 ◽

2018 ◽

Vol 31 (13) ◽

pp. 5107-5125 ◽

Cited By ~ 4

Author(s):

Hanna Na ◽

Kwang-Yul Kim ◽

Shoshiro Minobe ◽

Yoshi N. Sasaki

Keyword(s):

North Pacific ◽

Western North Pacific ◽

Large Scale ◽

Decadal Variability ◽

Kuroshio Extension ◽

Heat Content ◽

Western Boundary ◽

Delayed Response ◽

Atmospheric Variability ◽

Wind Stress Curl

Three-dimensional oceanic thermal structures and variability in the western North Pacific (NP) are examined on the interannual to decadal time scales and their relationship to oceanic and atmospheric variability is discussed by analyzing observation and reanalysis data for 45 years (1964–2008), which is much longer than the satellite-altimetry period. It is shown that the meridional shift of the Kuroshio Extension (KE) and subarctic frontal zone (SAFZ) is associated with the overall cooling/warming over the KE and SAFZ region (KE–SAFZ mode). It appears, however, that changes in KE strength induce different signs of thermal anomalies to the south and north of the KE, not extended to the SAFZ (KE mode), possibly contributing to noncoherent variability between the KE and SAFZ. Thus, the KE and SAFZ are dependent on each other in the context of the KE–SAFZ mode, while the KE is independent of the SAFZ in terms of the KE mode. This intricate relationship is associated with different linkages to atmospheric variability; the KE–SAFZ mode exhibits a relatively fast response to the large-scale wind stress curl forcing in the NP, whereas the KE mode is related to a delayed response to the atmospheric forcing via jet-trapped baroclinic Rossby wave propagation. It is suggested that further knowledge of the underlying mechanisms of the two modes would contribute to understanding ocean–atmosphere feedback as well as potential predictability over the western boundary current region in the NP.

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Dynamical Links between the Decadal Variability of the Oyashio and Kuroshio Extensions

Journal of Climate ◽

10.1175/jcli-d-17-0397.1 ◽

2017 ◽

Vol 30 (23) ◽

pp. 9591-9605 ◽

Cited By ~ 25

Author(s):

Bo Qiu ◽

Shuiming Chen ◽

Niklas Schneider

Keyword(s):

Decadal Variability ◽

Kuroshio Extension ◽

Critical Role ◽

Mesoscale Eddy ◽

Phase Changes ◽

Intensity Change ◽

Western Boundary ◽

Boundary Current ◽

Contemporaneous Correlation ◽

Budget Analysis

Rather than a single and continuous boundary current outflow, long-term satellite observations reveal that the Oyashio Extension (OE) in the North Pacific Subarctic Gyre comprises two independent, northeast–southwest-slanted front systems. With a mean latitude along 40°N, the western OE front exists primarily west of 153°E and is a continuation of the subarctic gyre western boundary current. The eastern OE front, also appearing along 40°N, is located between 153° and 170°E, whose entity is disconnected from its western counterpart. During 1982–2016, both of the OE fronts exhibit prominent decadal fluctuations, although their signals show little contemporaneous correlation. An upper-ocean temperature budget analysis based on the Estimating the Circulation and Climate of the Ocean, phase II (ECCO2), state estimate reveals that the advective temperature flux convergence plays a critical role in determining the low-frequency temperature changes relating to the OE fronts. Specifically, the western OE front variability is controlled by the decadal mesoscale eddy modulations in the upstream Kuroshio Extension (KE). An enhanced eddy activity increases the poleward heat transport and works to strengthen the western OE front. The eastern OE front variability, on the other hand, is dictated by both the meridional shift of the KE position and the circulation intensity change immediately north of the eastern OE. Different baroclinic adjustment speeds for the KE and OE are found to cause the in-phase changes between these latter two processes. Lack of contemporaneous correlation between the decadal western and eastern OE variability is found to be related to the interaction of the meridionally migrating KE jet with the Shatsky Rise near 159°E.

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