scholarly journals An Enhancement of Low-Frequency Variability in the Kuroshio–Oyashio Extension in CCSM3 owing to Ocean Model Biases

2010 ◽  
Vol 23 (23) ◽  
pp. 6221-6233 ◽  
Author(s):  
Lu Anne Thompson ◽  
Young-Oh Kwon
Keyword(s):  
Ocean Circulation ◽  
Decadal Variability ◽  
Kuroshio Extension ◽  
Sea Surface ◽  
Western Boundary ◽  
Coupled Models ◽  
Climate System Model ◽  
Sst Variability ◽  
The Mean ◽  
The Kuroshio

Abstract Enhanced decadal variability in sea surface temperature (SST) centered on the Kuroshio Extension (KE) has been found in the Community Climate System Model version 3 (CCSM3) as well as in other coupled climate models. This decadal peak has higher energy than is found in nature, almost twice as large in some cases. While previous analyses have concentrated on the mechanisms for such decadal variability in coupled models, an analysis of the causes of excessive SST response to changes in wind stress has been missing. Here, a detailed comparison of the relationships between interannual changes in SST and sea surface height (SSH) as a proxy for geostrophic surface currents in the region in both CCSM3 and observations, and how these relationships depend on the mean ocean circulation, temperature, and salinity, is made. We use observationally based climatological temperature and salinity fields as well as satellite-based SSH and SST fields for comparison. The primary cause for the excessive SST variability is the coincidence of the mean KE with the region of largest SST gradients in the model. In observations, these two regions are separated by almost 500 km. In addition, the too shallow surface oceanic mixed layer in March north of the KE in the subarctic Pacific contributes to the biases. These biases are not unique to CCSM3 and suggest that mean biases in current, temperature, and salinity structures in separated western boundary current regions can exert a large influence on the size of modeled decadal SST variability.

scholarly journals Field Investigations of Coastal Sea Surface Temperature Drop after Typhoon Passages

2018 ◽  
Author(s):  
Dong-Jiing Doong ◽  
Jen-Ping Peng ◽  
Alexander V. Babanin
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Coastal Upwelling ◽  
Cold Water ◽  
Temperature Drop ◽  
Open Ocean ◽  
Sea Surface ◽  
Sst Variability ◽  
The Mean ◽  
The Kuroshio

Abstract. Sea surface temperature (SST) variability affects marine ecosystems, fisheries, ocean primary productivity, and human activities and is the primary influence on typhoon intensity. SST drops of a few degrees in the open ocean after typhoon passages have been widely documented; however, few studies have focused on coastal SST variability. The purpose of this study is to determine typhoon-induced SST drops in the near-coastal area (within 1 km of the coast) and understand the possible mechanism. The results of this study were based on extensive field data analysis. Significant SST drop phenomena were observed at the Longdong buoy in northeastern Taiwan during 43 typhoons over the past 20 years (1998~2017). The mean SST drop (∆SST) after a typhoon passage was 6.1 °C, and the maximum drop was 12.5 °C (Typhoon Fungwong in 2008). The magnitude of SST drop was larger than most of the observations in the open ocean. The mean duration of SST drop was 24 hours, and on average, 26.1 hours were required for the SST to recover to the original temperature. The coastal SST drops at Longdong were correlated with the moving tracks of typhoons. When a typhoon passes south of Longdong, the strong and persistent longshore winds induce coastal upwelling and pump cold water up to the surface, which is the dominant cause of SST drops along the coast. In this study, it was determined that cold water mainly intruded from the Kuroshio subsurface in the Okinawa Trough, which is approximately 50 km from the observation site. The magnitude of coastal SST drops depends on the area of overlap between typhoons generating strong winds and the Kuroshio. The dataset used in this study can be accessed by https://doi.pangaea.de/10.1594/PANGAEA.895002.

scholarly journals North Pacific Decadal Variability in the Community Climate System Model Version 2

2007 ◽  
Vol 20 (11) ◽  
pp. 2416-2433 ◽  
Author(s):  
Young-Oh Kwon ◽  
Clara Deser
Keyword(s):  
Heat Flux ◽  
Wind Stress ◽  
North Pacific ◽  
Decadal Variability ◽  
Kuroshio Extension ◽  
Flux Divergence ◽  
Wind Stress Curl ◽  
Climate System Model ◽  
Spectral Peaks ◽  
The Kuroshio

Abstract North Pacific decadal oceanic and atmospheric variability is examined from a 650-yr control integration of the Community Climate System Model version 2. The dominant pattern of winter sea surface temperature (SST) variability is similar to the observed “Pacific decadal oscillation,” with maximum amplitude along the Kuroshio Extension. SST anomalies in this region exhibit significant spectral peaks at approximately 16 and 40 yr. Lateral geostrophic heat flux divergence, caused by a meridional shift of the Kuroshio Extension forced by basin-scale wind stress curl anomalies 3–5 yr earlier, is responsible for the decadal SST variability; local surface heat flux and Ekman heat flux divergence act as a damping and positive feedback, respectively. A simple linear Rossby wave model is invoked to explicitly demonstrate the link between the wind stress curl forcing and decadal variability in the Kuroshio Extension. The Rossby wave model not only successfully reproduces the two decadal spectral peaks, but also illustrates that only the low-frequency (>10-yr period) portion of the approximately white noise wind stress curl forcing is relevant. This model also demonstrates that the weak and insignificant decadal spectral peaks in the wind stress curl forcing are necessary for producing the corresponding strong and significant oceanic peaks in the Kuroshio Extension. The wind stress curl response to decadal SST anomalies in the Kuroshio Extension is similar in structure but opposite in sign and somewhat weaker than the wind stress curl forcing pattern. These results suggest that the simulated North Pacific decadal variability owes its existence to two-way ocean–atmosphere coupling.

scholarly journals A Comparative Analysis of Kuroshio Extension Indices from a Modeling Perspective

2015 ◽  
Vol 28 (14) ◽  
pp. 5873-5881 ◽  
Author(s):  
Stefano Pierini
Keyword(s):  
Decadal Variability ◽  
Kuroshio Extension ◽  
Altimeter Data ◽  
Model Studies ◽  
Robust Model ◽  
Latitudinal Position ◽  
The Mean ◽  
Baroclinic Rossby Wave ◽  
The Kuroshio ◽  
Broad Scale

Abstract Dynamical indices previously introduced to characterize the Kuroshio Extension (KE) decadal variability (DV) from altimeter data are comparatively analyzed to assess their ability to reveal the same phenomenon from model outputs. Based on this analysis, a new combined index identifying each stage of the KE DV in numerical simulations is then proposed. The analysis begins by recognizing that the numerical simulation of the nonlinear frontal KE DV is very sensitive to changes in model implementation, whereas the linear broad-scale baroclinic Rossby wave field known to trigger the KE DV is a robust model feature. This selective model sensitivity poses a subtle problem concerning the diagnosis of the KE DV from model outputs, and requires the use of indices that unequivocally identify the KE frontal structure. The capability of six indices to achieve this task is thus investigated. Two model outputs representing paradigms of a fairly realistic simulation and of an unrealistic simulation in which only the broad-scale variability is present are used. A synthesized index is well captured by both simulations: it is therefore recognized to be unsuitable for model studies. An integrated SSH index does not resolve explicitly the frontal variability. Among the remaining indices, the KE path length (modified through the application of the wavelet transform) and the mean KE latitudinal position are shown to provide, in combination, an unambiguous identification of each stage of the KE DV (recognized to exhibit chaotic hysteresis) and are therefore suggested to be used with model outputs.

scholarly journals Eddy–Mean Flow Interaction in the Kuroshio Extension Region

2011 ◽  
Vol 41 (6) ◽  
pp. 1182-1208 ◽  
Author(s):  
Stephanie Waterman ◽  
Nelson G. Hogg ◽  
Steven R. Jayne
Keyword(s):  
Kuroshio Extension ◽  
Wave Radiation ◽  
Western Boundary ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Flow Interaction ◽  
Flow Interactions ◽  
The Mean ◽  
Jet Structure ◽  
The Kuroshio

Abstract The authors use data collected by a line of tall current meter moorings deployed across the axis of the Kuroshio Extension (KE) jet at the location of maximum time-mean eddy kinetic energy to characterize the mean jet structure, the eddy variability, and the nature of eddy–mean flow interactions observed during the Kuroshio Extension System Study (KESS). A picture of the 2-yr record mean jet structure is presented in both geographical and stream coordinates, revealing important contrasts in jet strength, width, vertical structure, and flanking recirculation structure. Eddy variability observed is discussed in the context of some of its various sources: jet meandering, rings, waves, and jet instability. Finally, various scenarios for eddy–mean flow interaction consistent with the observations are explored. It is shown that the observed cross-jet distributions of Reynolds stresses at the KESS location are consistent with wave radiation away from the jet, with the sense of the eddy feedback effect on the mean consistent with eddy driving of the observed recirculations. The authors consider these results in the context of a broader description of eddy–mean flow interactions in the larger KE region using KESS data in combination with in situ measurements from past programs in the region and satellite altimetry. This demonstrates important consistencies in the along-stream development of time-mean and eddy properties in the KE with features of an idealized model of a western boundary current (WBC) jet used to understand the nature and importance of eddy–mean flow interactions in WBC jet systems.

scholarly journals Field investigations of coastal sea surface temperature drop after typhoon passages

2019 ◽  
Vol 11 (1) ◽  
pp. 323-340 ◽  
Author(s):  
Dong-Jiing Doong ◽  
Jen-Ping Peng ◽  
Alexander V. Babanin
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Coastal Upwelling ◽  
Cold Water ◽  
Temperature Drop ◽  
Open Ocean ◽  
Sea Surface ◽  
Sst Variability ◽  
The Mean ◽  
The Kuroshio

Abstract. Sea surface temperature (SST) variability affects marine ecosystems, fisheries, ocean primary productivity and human activities and is the primary influence on typhoon intensity. SST drops of a few degrees in the open ocean after typhoon passages have been widely documented; however, few studies have focused on coastal SST variability. The purpose of this study is to determine typhoon-induced SST drops in the near-coastal area (within 1 km of the coast) and understand the possible mechanism. The results of this study were based on extensive field data analysis. Significant SST drop phenomena were observed at the Longdong Buoy in northeastern Taiwan during 43 typhoons over the past 20 years (1998–2017). The mean SST drop (ΔSST) after a typhoon passage was 6.1 ∘C, and the maximum drop was 12.5 ∘C (Typhoon Fungwong in 2008). The magnitude of the SST drop was larger than most of the observations in the open ocean. The mean duration of the SST drop was 24 h, and on average, 26.1 h were required for the SST to recover to the original temperature. The coastal SST drops at Longdong were correlated with the moving tracks of typhoons. When a typhoon passes south of Longdong, the strong and persistent longshore winds induce coastal upwelling and pump cold water up to the surface, which is the dominant cause of the SST drops along the coast. In this study, it was determined that cold water mainly intruded from the Kuroshio subsurface into the Okinawa Trough, which is approximately 50 km from the observation site. The magnitude of coastal SST drops depends on the area of overlap between typhoons generating strong winds and the Kuroshio. The dataset used in this study can be accessed from https://doi.org/10.1594/PANGAEA.895002.

scholarly journals On the Relationship between Synoptic Wintertime Atmospheric Variability and Path Shifts in the Gulf Stream and the Kuroshio Extension

2009 ◽  
Vol 22 (12) ◽  
pp. 3177-3192 ◽  
Author(s):  
Terrence M. Joyce ◽  
Young-Oh Kwon ◽  
Lisan Yu
Keyword(s):  
North Atlantic ◽  
North Pacific ◽  
Gulf Stream ◽  
Kuroshio Extension ◽  
Storm Track ◽  
Western Boundary ◽  
Atmospheric Variability ◽  
The North ◽  
The Kuroshio ◽  
The North Pacific

Abstract Coherent, large-scale shifts in the paths of the Gulf Stream (GS) and the Kuroshio Extension (KE) occur on interannual to decadal time scales. Attention has usually been drawn to causes for these shifts in the overlying atmosphere, with some built-in delay of up to a few years resulting from propagation of wind-forced variability within the ocean. However, these shifts in the latitudes of separated western boundary currents can cause substantial changes in SST, which may influence the synoptic atmospheric variability with little or no time delay. Various measures of wintertime atmospheric variability in the synoptic band (2–8 days) are examined using a relatively new dataset for air–sea exchange [Objectively Analyzed Air–Sea Fluxes (OAFlux)] and subsurface temperature indices of the Gulf Stream and Kuroshio path that are insulated from direct air–sea exchange, and therefore are preferable to SST. Significant changes are found in the atmospheric variability following changes in the paths of these currents, sometimes in a local fashion such as meridional shifts in measures of local storm tracks, and sometimes in nonlocal, broad regions coincident with and downstream of the oceanic forcing. Differences between the North Pacific (KE) and North Atlantic (GS) may be partly related to the more zonal orientation of the KE and the stronger SST signals of the GS, but could also be due to differences in mean storm-track characteristics over the North Pacific and North Atlantic.

Interannual Tropical Pacific Sea Surface Temperatures and Their Relation to Preceding Sea Level Pressures in the NCAR CCSM2

2006 ◽  
Vol 19 (6) ◽  
pp. 998-1012 ◽  
Author(s):  
Bruce T. Anderson ◽  
Eric Maloney
Keyword(s):  
Sea Level ◽  
Tropical Pacific ◽  
Equatorial Pacific ◽  
Sea Surface ◽  
Boreal Winter ◽  
Atmospheric Variability ◽  
Climate System Model ◽  
Sst Variability ◽  
Ocean Atmosphere ◽  
Sst Anomalies

Abstract This paper describes aspects of tropical interannual ocean/atmosphere variability in the NCAR Community Climate System Model Version 2.0 (CCSM2). The CCSM2 tropical Pacific Ocean/atmosphere system exhibits much stronger biennial variability than is observed. However, a canonical correlation analysis technique decomposes the simulated boreal winter tropical Pacific sea surface temperature (SST) variability into two modes, both of which are related to atmospheric variability during the preceding boreal winter. The first mode of ocean/atmosphere variability is related to the strong biennial oscillation in which La Niña–related sea level pressure (SLP) conditions precede El Niño–like SST conditions the following winter. The second mode of variability indicates that boreal winter tropical Pacific SST anomalies can also be initiated by SLP anomalies over the subtropical central and eastern North Pacific 12 months earlier. The evolution of both modes is characterized by recharge/discharge within the equatorial subsurface temperature field. For the first mode of variability, this recharge/discharge produces a lag between the basin-average equatorial Pacific isotherm depth anomalies and the isotherm–slope anomalies, equatorial SSTs, and wind stress fields. Significant anomalies are present up to a year before the boreal winter SLP variations and two years prior to the boreal winter ENSO-like events. For the second canonical factor pattern, the recharge/discharge mechanism is induced concurrent with the boreal winter SLP pattern approximately one year prior to the ENSO-like events, when isotherms initially deepen and change their slope across the basin. A rapid deepening of the isotherms in the eastern equatorial Pacific and a warming of the overlying SST anomalies then occurs during the subsequent 12 months.

scholarly journals Processes Shaping the Frontal-Scale Time-Mean Surface Wind Convergence Patterns around the Kuroshio Extension in Winter

2020 ◽  
Vol 33 (1) ◽  
pp. 3-25
Author(s):  
Ryusuke Masunaga ◽  
Hisashi Nakamura ◽  
Bunmei Taguchi ◽  
Takafumi Miyasaka
Keyword(s):  
Kuroshio Extension ◽  
Surface Wind ◽  
Convective Precipitation ◽  
Western Boundary ◽  
Boundary Currents ◽  
Sst Front ◽  
Atmospheric Structure ◽  
Mean Convergence ◽  
Daily Scale ◽  
The Kuroshio

AbstractHigh-resolution satellite observations and numerical simulations have revealed that climatological-mean surface wind convergence and precipitation are enhanced locally around the midlatitude warm western boundary currents (WBCs) with divergence slightly to their poleward side. While steep sea surface temperature (SST) fronts along the WBCs have been believed to play an important role in shaping those frontal-scale atmospheric structures, the mechanisms and processes involved are still under debate. The present study explores specific daily scale atmospheric processes that are essential for shaping the frontal-scale atmospheric structure around the Kuroshio Extension (KE) in winter, taking advantage of a new product of global atmospheric reanalysis. Cluster analysis and case studies reveal that a zonally extending narrow band of surface wind convergence frequently emerges along the KE, which is typically observed under the surface northerlies after the passage of a developed synoptic-scale cyclone. Unlike its counterpart around the cyclone center and associated cold front, the surface convergence tends to be in moderate strength and more persistent, contributing dominantly to the distinct time-mean convergence/divergence contrast across the SST front. Accompanying ascent and convective precipitation, the band of convergence is a manifestation of a weak stationary atmospheric front anchored along the SST front or generation of a weak meso-α-scale cyclone. By reinforcing the ascent and convergence, latent heating through convective processes induced by surface convergence plays an important role in shaping the frontal-scale atmospheric structure around the KE.

scholarly journals Upper Ocean Thermal Responses to Sea Spray Mediated Turbulent Fluxes during Typhoon Passage

2015 ◽  
Vol 2015 ◽  
pp. 1-15 ◽  
Author(s):  
Lianxin Zhang ◽  
Changlong Guan ◽  
Chunjian Sun ◽  
Siyu Gao ◽  
Shaomei Yu
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Kuroshio Extension ◽  
Turbulent Fluxes ◽  
Sea Surface ◽  
Sea Spray ◽  
Upper Ocean ◽  
Ocean Temperature ◽  
Turbulent Model ◽  
The Kuroshio

A one-dimensional turbulent model is used to investigate the effect of sea spray mediated turbulent fluxes on upper ocean temperature during the passage of typhoon Yagi over the Kuroshio Extension area in 2006. Both a macroscopical sea spray momentum flux algorithm and a microphysical heat and moisture flux algorithm are included in this turbulent model. Numerical results show that the model can well reproduce the upper ocean temperature, which is consistent with the data from the Kuroshio Extension Observatory. Besides, the sea surface temperature is decreased by about 0.5°C during the typhoon passage, which also agrees with the sea surface temperature dataset derived from Advanced Microwave Scanning Radiometer for the Earth Observing and Reynolds. Diagnostic analysis indicates that sea spray acts as an additional source of the air-sea turbulent fluxes and plays a key role in increasing the turbulent kinetic energy in the upper ocean, which enhances the temperature diffusion there. Therefore, sea spray is also an important factor in determining the upper mixed layer depth during the typhoon passage.

Decadal variability of the Kuroshio Extension

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’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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