North Pacific Western Boundary Current Research

by Bo Qiu

One of Bo's research interests in the past decade has focused on the large-scale variability in the Kuroshio Extension (KE) region. Specifically, he has been interested in (1) describing the variabilityof the KE system through analyses of satellite and in-situ data, (2) examining its dynamic causes, and (3) assessing its impact upon the regional eddy heat transport, water mass property variations, and the ocean's roles in potentially influencing the overlying atmosphere.

Variability in the KE system:

Satellite altimetry measurements of the past 14 years reveal that the KE jet oscillated between two distinct dynamic states (see Fig.1).
In the stable states (10/1992-06/1995 and 01/2002-12/2004), the KE jet has an intensified eastward surface transport, a well-defined
strong RG, and a northerly zonal mean flow path. In the unstable states (07/2005-12/2001 and 01/2005-present), the KE jet has a reduced surface transport, a weak RG, and a more southerly flow path. Causes for this bimodal oscillation can be sought in the basin-scale wind stress curl forcing in the eastern North Pacific related to the Pacific decadal oscillations (PDOs). Specifically, when the PDO index is positive
(right panel in Fig.2), the Aleutian Low intensifies and shifts southward, generating negative sea surface height (SSH) anomalies in the eastern
North Pacific through Ekman divergence. As these wind-induced SSH anomalies propagate to the west as baroclinic Rossby waves (mid panel
in Fig.2), they weaken the zonal KE jet, shifting its path southward and resulting in an unstable dynamic state (left panel in Fig.2; see
Qiu and Chen 2005 for a more detailed dynamic explanation). When the PDO index changes to negative, the enhanced Ekman pumping generates positive SSH anomalies that enhance the KE jet as they propagate to the west. It is worth emphasizing that this decadal, bimodal variation of the KE jet detected from the altimetry measurements is in contrast to many of the previous modeling studies, which have often emphasized self-sustained bimodal oscillations in a WBC extension system.

Figure 1. Yearly paths of the Kuroshio and Kuroshio Extension defined by the 170-cm contours in the weekly SSH fields. Here paths of every 14 days are plotted. Notice the relatively stable paths in 1993-1994 and 2002-2004, and the highly variable paths in other years.		Figure 2. (a) Time series of eddy kinetic energy (EKE) in the upstream KE region of 141-153E and 32-38N. Here, EKE is calculated using the weekly SSH anomaly data by assuming geostrophy. For comparison with the PDO index, the time series is plotted as the deviation from the regional mean EKE value. (b) Sea surface height anomalies along thezonal band of 32-34N. The KE recirculation gyre occupies the western segment of 140-160E. (c) PDO index from http://jisao.washington.edu/pdo/PDO.latest.

Oceanic impact:

Changes in the KE's dynamic state have a profound impact on the regional SST signals. As the KE strengthens and its mean path shifts northward, lateral advection tends to generate persistent, positive SST anomalies. The opposite is true when the KE transitions to an unstable state. Another important effect of the KE's changes in its dynamic state is its influence on the regional water mass properties. For example, when the KE is in an unstable state, it mixes high potential vorticity (PV) subpolar-origin surface water to the southern RG. This changes the
local stratification and results in a pre-conditioning state that is unfavorable for the formation of subtropical mode water (STMW). By
combining the altimetry data with the historical hydrographic and Argo data, we found that the pre-conditioning effect related to the decadal
changes in the KE's dynamic state contributed more to the time-varying STMW formation rate than does the regional atmospheric net surface heat flux forcing (Qiu and Chen 2006).

KE's role in the air-sea coupled system:

The importance of the air-sea coupling in influencing KE jet is explored in Qiu et al. (2007) by dividing the large-scale wind forcing into those
associated with the intrinsic atmospheric variability and those induced by the SST changes in the KE region. The latter signals are extracted
from the NCEP-NCAR reanalysis data using the lagged correlation analysis. In the absence of the SST feedback, the intrinsic atmospheric forcing enhances the decadal and longer timescale SST variance through oceanic advection, but fails to capture the observed decadal spectral peak. When the SST feedback is present, a warm (cold) KE SST anomaly works to generate a positive (negative) wind stress curl in the eastern North Pacific basin, resulting in negative (positive) local SSH anomalies through Ekman divergence (convergence). As these wind-forced SSH anomalies propagate into the KE region in the west, they shift the KE jet and alter the sign of the pre-existing SST anomalies. Given the spatial pattern of the SST-induced wind stress curl forcing, the optimal coupling in the midlatitude North Pacific occurs at the period of ~10yr, slightly longer than the basin crossing time of the baroclinic Rossby waves along the KE latitude.

References:

Qiu, B., and S. Chen, 2005: Variability of the Kuroshio Extension jet, recirculation gyre and mesoscale eddies on decadal timescales.
J. Phys. Oceanogr., 35, 2090-2103.

Qiu, B., and S. Chen, 2006: Decadal variability in the formation of the North Pacific Subtropical Mode Water: Oceanic versus atmospheric
control. J. Phys. Oceanogr., 36, 1365-1380.

Qiu, B., N. Schneider, and S. Chen, 2007: Coupled decadal variability in the North Pacific: An observationally-constrained idealized model.
J. Climate, in press (preprint: http://www.soest.hawaii.edu/oceanography/bo/QSC06.pdf).