scholarly journals Near-Annual SST Variability in the Equatorial Pacific in a Coupled General Circulation Model

2005 ◽  
Vol 18 (21) ◽  
pp. 4454-4473 ◽  
Author(s):  
Renguang Wu ◽  
Ben P. Kirtman
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Equatorial Pacific ◽  
Near Surface ◽  
Boreal Spring ◽  
Eastern Equatorial Pacific ◽  
Zonal Advection ◽  
The Mean ◽  
Annual Mode ◽  
Sst Anomalies

Abstract Equatorial Pacific sea surface temperature (SST) anomalies in the Center for Ocean–Land–Atmosphere Studies (COLA) interactive ensemble coupled general circulation model show near-annual variability as well as biennial El Niño–Southern Oscillation (ENSO) variability. There are two types of near-annual modes: a westward propagating mode and a stationary mode. For the westward propagating near-annual mode, warm SST anomalies are generated in the eastern equatorial Pacific in boreal spring and propagate westward in boreal summer. Consistent westward propagation is seen in precipitation, surface wind, and ocean current. For the stationary near-annual mode, warm SST anomalies develop near the date line in boreal winter and decay locally in boreal spring. Westward propagation of warm SST anomalies also appears in the developing year of the biennial ENSO mode. However, warm SST anomalies for the westward propagating near-annual mode occur about two months earlier than those for the biennial ENSO mode and are quickly replaced by cold SST anomalies, whereas warm SST anomalies for the biennial ENSO mode only experience moderate weakening. Anomalous zonal advection contributes to the generation and westward propagation of warm SST anomalies for both the westward propagating near-annual mode and the biennial ENSO mode. However, the role of mean upwelling is markedly different. The mean upwelling term contributes to the generation of warm SST anomalies for the biennial ENSO mode, but is mainly a damping term for the westward propagating near-annual mode. The development of warm SST anomalies for the stationary near-annual mode is partially due to anomalous zonal advection and upwelling, similar to the amplification of warm SST anomalies in the equatorial central Pacific for the biennial ENSO mode. The mean upwelling term is negative in the eastern equatorial Pacific for the stationary near-annual mode, which is opposite to the ENSO mode. The development of cold SST anomalies in the aftermath of warm SST anomalies for the westward propagating near-annual mode is coupled to large easterly wind anomalies, which occur between the warm and cold SST anomalies. The easterly anomalies contribute to the cold SST anomalies through anomalous zonal, meridional, and vertical advection and surface evaporation. The cold SST anomalies, in turn, enhance the easterly anomalies through a Rossby-wave-type response. The above processes are most effective during boreal spring when the mean near-surface-layer ocean temperature gradient is the largest. It is suggested that the westward propagating near-annual mode is related to air–sea interaction processes that are limited to the near-surface layers.

scholarly journals Near-Surface Transport Properties and Lagrangian Statistics during Two Contrasting Years in the Adriatic Sea

2020 ◽  
Vol 8 (9) ◽  
pp. 681
Author(s):  
Saeed Hariri
Keyword(s):  
Transport Properties ◽  
General Circulation Model ◽  
General Circulation ◽  
Circulation Model ◽  
Mean Flow ◽  
Near Surface ◽  
Surface Transport ◽  
The Real ◽  
Lagrangian Statistics ◽  
The Mean

This paper describes the near-surface transport properties and Lagrangian statistics in the Adriatic semi-enclosed basin using synthetic drifters. Lagrangian transport models were used to simulate synthetic trajectories from the mean flow fields obtained by the Massachusetts Institute of Technology general circulation model (MITgcm), implemented in the Adriatic from October 2006 until December 2008. In particular, the surface circulation properties in two contrasting years (2007 had a mild winter and cold fall, while 2008 had a normal winter and hot summer) are compared here. In addition, the Lagrangian statistics for the entire Adriatic Basin after removing the Eulerian mean circulation for numerical particles were calculated. The results indicate that the numerical particles were slower in this simulation when compared with the real drifters. This is because of the reduced energetic flow field generated by the MIT general circulation model during the selected years. The numerical results showed that the balanced effects of the wind-driven recirculation in the northernmost area(which would be a sea response to the Bora wind field) and the Po River discharge cause the residence times to be similar during the two selected years (182 and 185 days in 2007 and 2008, respectively). Furthermore, the mean angular momentum, diffusivity, and Lagrangian velocity covariance values are smaller than in the real drifter observations, while the maximum Lagrangian integral time scale is the same.

scholarly journals Origin and Pathway of Equatorial 13°C Water in the Pacific Identified by a Simulated Passive Tracer and Its Adjoint*

2009 ◽  
Vol 39 (8) ◽  
pp. 1836-1853 ◽  
Author(s):  
Tangdong Qu ◽  
Shan Gao ◽  
Ichiro Fukumori ◽  
Rana A. Fine ◽  
Eric J. Lindstrom
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
South Pacific ◽  
Equatorial Region ◽  
Equatorial Pacific ◽  
Western Boundary ◽  
Passive Tracer ◽  
The South ◽  
Mode Water ◽  
Eastern Equatorial Pacific

Abstract The origin and pathway of the thermostad water in the eastern equatorial Pacific Ocean, often referred to as the equatorial 13°C Water, are investigated using a simulated passive tracer and its adjoint, based on circulation estimates of a global general circulation model. Results demonstrate that the source region of the 13°C Water lies well outside the tropics. In the South Pacific, some 13°C Water is formed northeast of New Zealand, confirming an earlier hypothesis on the water’s origin. The South Pacific origin of the 13°C Water is also related to the formation of the Eastern Subtropical Mode Water (ESTMW) and the Sub-Antarctic Mode Water (SAMW). The portion of the ESTMW and SAMW that eventually enters the density range of the 13°C Water (25.8 < σθ < 26.6 kg m−3) does so largely by mixing. Water formed in the subtropics enters the equatorial region predominantly through the western boundary, while its interior transport is relatively small. The fresher North Pacific ESTMW and Central Mode Water (CMW) are also important sources of the 13°C Water. The ratio of the southern versus the northern origins of the water mass is about 2 to 1 and tends to increase with time elapsed from its origin. Of the total volume of initially tracer-tagged water in the eastern equatorial Pacific, approximately 47.5% originates from depths above σθ = 25.8 kg m−3 and 34.6% from depths below σθ = 26.6 kg m−3, indicative of a dramatic impact of mixing on the route of subtropical water to becoming the 13°C Water. Still only a small portion of the water formed in the subtropics reaches the equatorial region, because most of the water is trapped and recirculates in the subtropical gyre.

scholarly journals Inter-annual tropical Pacific climate variability in an isotope-enabled CGCM: implications for interpreting coral stable oxygen isotope records of ENSO

2013 ◽  
Vol 9 (1) ◽  
pp. 741-773 ◽  
Author(s):  
T. Russon ◽  
A. W. Tudhope ◽  
G. C. Hegerl ◽  
M. Collins ◽  
J. Tindall
Keyword(s):  
Oxygen Isotope ◽  
General Circulation ◽  
Climate Models ◽  
Circulation Model ◽  
Equatorial Pacific ◽  
Stable Oxygen Isotope ◽  
Ocean Climate ◽  
Eastern Equatorial Pacific ◽  
Stable Oxygen ◽  
Pacific Climate Variability

Abstract. Water isotope-enabled coupled atmosphere/ocean climate models allow for exploration of the relative contributions to coral stable oxygen isotope (δ18Ocoral) variability arising from Sea Surface Temperature (SST) and the isotopic composition of seawater (δ18Osw). The unforced behaviour of the isotope-enabled HadCM3 Coupled General Circulation Model affirms that the extent to which inter-annual δ18Osw variability contributes to that in model δ18Ocoral is strongly spatially dependent, ranging from being negligible in the eastern equatorial Pacific to accounting for 50% of δ18Ocoral variance in parts of the western Pacific. In these latter cases, a significant component of the inter-annual δ18Osw variability is correlated to that in SST, meaning that local calibrations of the effective local δ18Ocoral–SST relationships are likely to be essential. Furthermore, the relationship between δ18Osw and SST in the central and western equatorial Pacific is non-linear, such that the interpretation of model δ18Ocoral in the context of a linear dependence on SST alone may lead to overestimation (by up to 20%) of the SST anomalies associated with large El-Niño events. Intra-model evaluation of a salinity-based pseudo-coral approach shows that such an approach captures the first-order features of the model δ18Osw behaviour. However, the utility of the pseudo-corals is limited by the extent of spatial variability seen within the modelled slopes of the temporal salinity–δ18Osw relationship.

scholarly journals Inter-annual tropical Pacific climate variability in an isotope-enabled CGCM: implications for interpreting coral stable oxygen isotope records of ENSO

2013 ◽  
Vol 9 (4) ◽  
pp. 1543-1557 ◽  
Author(s):  
T. Russon ◽  
A. W. Tudhope ◽  
G. C. Hegerl ◽  
M. Collins ◽  
J. Tindall
Keyword(s):  
Oxygen Isotope ◽  
General Circulation ◽  
Climate Models ◽  
Circulation Model ◽  
Equatorial Pacific ◽  
Stable Oxygen Isotope ◽  
Model Interpretation ◽  
Eastern Equatorial Pacific ◽  
Stable Oxygen ◽  
Pacific Climate Variability

Abstract. Water isotope-enabled coupled atmosphere–ocean climate models allow for exploration of the relative contributions to coral stable oxygen isotope (δ18Ocoral) variability arising from sea surface temperature (SST) and the isotopic composition of seawater (δ18Osw). The unforced behaviour of the isotope-enabled HadCM3 coupled general circulation model suggests that the extent to which inter-annual δ18Osw variability contributes to that in model δ18Ocoral is strongly spatially dependent, ranging from being negligible in the eastern equatorial Pacific to accounting for 50% of δ18Ocoral variance in parts of the western Pacific. In these latter cases, a significant component of the inter-annual δ18Osw variability is correlated to that in SST, meaning that local calibrations of the effective local δ18Ocoral–SST relationships are likely to be essential. Furthermore, the relationship between δ18Osw and SST can be non-linear, such that the model interpretation of central and western equatorial Pacific δ18Ocoral in the context of a linear dependence on SST alone leads to overestimation (by up to 20%) of the SST anomalies associated with large El Niño events. Intra-model evaluation of a salinity-based pseudo-coral approach shows that such an approach captures the first-order features of the model δ18Osw behaviour. However, the utility of the pseudo-corals is limited by the extent of spatial variability seen within the modelled slopes of the temporal salinity–δ18Osw relationship.

The Equatorial Pacific Cold Tongue Bias in a Coupled Climate Model

2008 ◽  
Vol 21 (22) ◽  
pp. 5852-5869 ◽  
Author(s):  
Vasubandhu Misra ◽  
L. Marx ◽  
M. Brunke ◽  
X. Zeng
Keyword(s):  
General Circulation ◽  
Low Frequency ◽  
Coupled Model ◽  
Circulation Model ◽  
Equatorial Pacific ◽  
Cold Tongue ◽  
Equatorial Pacific Ocean ◽  
Time Step ◽  
Coupling Interval ◽  
The Mean

Abstract A set of multidecadal coupled ocean–atmosphere model integrations are conducted with different time steps for coupling between the atmosphere and the ocean. It is shown that the mean state of the equatorial Pacific does not change in a statistically significant manner when the coupling interval between the atmospheric general circulation model (AGCM) and the ocean general circulation model (OGCM) is changed from 1 day to 2 or even 3 days. It is argued that because the coarse resolution of the AGCM precludes resolving realistic “weather” events, changing the coupling interval from 1 day to 2 or 3 days has very little impact on the mean coupled climate. On the other hand, reducing the coupling interval to 3 h had a much stronger impact on the mean state of the equatorial Pacific and the concomitant general circulation. A novel experiment that incorporates a (pseudo) interaction of the atmosphere with SST at every time step of the AGCM was also conducted. In this unique coupled model experiment, the AGCM at every time step mutually interacts with the skin SST. This skin SST is anchored to the bulk SST, which is updated from the OGCM once a day. Both of these experiments reduced the cold tongue bias moderately over the equatorial Pacific Ocean with a corresponding reduction in the easterly wind stress bias relative to the control integration. It is stressed from the results of these model experiments that the impact of high-frequency air–sea coupling is significant on the cold tongue bias. The interannual variation of the equatorial Pacific was less sensitive to the coupling time step between the AGCM and the OGCM. Increasing (reducing) the coupling interval of the air–sea interaction had the effect of weakening (marginally strengthening) the interannual variations of the equatorial Pacific Ocean. It is argued that the low-frequency response of the upper ocean, including the cold tongue bias, is modulated by the atmospheric stochastic forcing on the coupled ocean–atmosphere system. This effect of the atmospheric stochastic forcing is affected by the frequency of the air–sea coupling and is found to be stronger than the rectification effect of the diurnal variations of the air–sea interaction on the low frequency. This may be a result of a limitation in the coupled model used in this study in which the OGCM has an inadequate vertical resolution in the mixed layer to sustain diurnal variations in the upper ocean.

An Analysis of ENSO Prediction Skill in the CFS Retrospective Forecasts

2009 ◽  
Vol 22 (7) ◽  
pp. 1801-1818 ◽  
Author(s):  
Renguang Wu ◽  
Ben P. Kirtman ◽  
Huug van den Dool
Keyword(s):  
Wind Stress ◽  
Zonal Wind ◽  
Equatorial Pacific ◽  
Prediction Skill ◽  
Thermocline Depth ◽  
Zonal Wind Stress ◽  
Boreal Spring ◽  
Eastern Equatorial Pacific ◽  
Western Equatorial Pacific ◽  
Sst Anomalies

Abstract The present study documents the so-called spring prediction and persistence barriers in association with El Niño–Southern Oscillation (ENSO) in the National Centers for Environmental Prediction (NCEP) Climate Forecast System (CFS) retrospective forecasts. It is found that the spring prediction and persistence barriers in the eastern equatorial Pacific sea surface temperature (SST) are preceded by a boreal winter barrier in the western equatorial Pacific zonal wind stress. The time of the persistence barrier is closely related to the time of the ENSO phase transition, but may differ from the time of the lowest variance. The seasonal change of the signal-to-noise ratio cannot explain the persistence barrier. While the noise may lead to a drop of skill around boreal spring in the western equatorial Pacific zonal wind stress, its impacts on the skill of eastern equatorial Pacific SST is small. The equatorial Pacific zonal winds display an excessive response to ENSO-related SST anomalies, which leads to a longer persistence in the equatorial Pacific thermocline depth anomalies and a delayed transition of the eastern equatorial Pacific SST anomalies. This provides an interpretation for the prediction skill drop in boreal spring in the eastern equatorial Pacific SST. The results suggest that improving the atmospheric model wind response to SST anomalies may reduce the spring prediction barrier.

scholarly journals The Effect of Orbital Forcing on the Mean Climate and Variability of the Tropical Pacific

2007 ◽  
Vol 20 (16) ◽  
pp. 4147-4159 ◽  
Author(s):  
A. Timmermann ◽  
S. J. Lorenz ◽  
S-I. An ◽  
A. Clement ◽  
S-P. Xie
Keyword(s):  
Annual Cycle ◽  
General Circulation ◽  
Model Simulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Orbital Forcing ◽  
Frequency Entrainment ◽  
Eastern Equatorial Pacific ◽  
The Mean ◽  
Mean Climate

Abstract Using a coupled general circulation model, the responses of the climate mean state, the annual cycle, and the El Niño–Southern Oscillation (ENSO) phenomenon to orbital changes are studied. The authors analyze a 1650-yr-long simulation with accelerated orbital forcing, representing the period from 142 000 yr b.p. (before present) to 22 900 yr a.p. (after present). The model simulation does not include the time-varying boundary conditions due to ice sheet and greenhouse gas forcing. Owing to the mean seasonal cycle of cloudiness in the off-equatorial regions, an annual mean precessional signal of temperatures is generated outside the equator. The resulting meridional SST gradient in the eastern equatorial Pacific modulates the annual mean meridional asymmetry and hence the strength of the equatorial annual cycle. In turn, changes of the equatorial annual cycle trigger abrupt changes of ENSO variability via frequency entrainment, resulting in an anticorrelation between annual cycle strength and ENSO amplitude on precessional time scales.

scholarly journals ENSO Feedbacks and Associated Time Scales of Variability in a Multimodel Ensemble

2010 ◽  
Vol 23 (12) ◽  
pp. 3181-3204 ◽  
Author(s):  
Ali Belmadani ◽  
Boris Dewitte ◽  
Soon-Il An
Keyword(s):  
General Circulation ◽  
Relative Weight ◽  
Circulation Model ◽  
Ocean Model ◽  
Equatorial Pacific ◽  
Enso Cycle ◽  
Central Pacific ◽  
Surface Circulation ◽  
The Mean ◽  
Mean State

Abstract The background state of the equatorial Pacific determines the prevalence of a “slow” recharge oscillator-type ENSO over a “fast” quasi-biennial surface-driven ENSO. The first is controlled to a large extent by the thermocline feedback, whereas the latter is related to enhanced zonal advective feedback. In this study, dynamical diagnostics are used to investigate the relative importance of these two feedbacks in the Coupled Model Intercomparison Project and its relation with the differences in ENSO-like variability among the models. The focus is on the role of the mean oceanic surface circulation in controlling the relative weight of the two feedbacks. By the means of an intermediate-type ocean model of the tropical Pacific “tuned” from the coupled general circulation model (CGCM) outputs, the contribution of the advection terms (vertical versus zonal) to the rate of SST change is estimated. A new finding is that biases in the advection terms are to a large extent related to the biases in the mean surface circulation. The latter are used to infer the dominant ENSO feedback for each CGCM. This allows for the classification of the CGCMs into three groups that account for the dominant feedback process of the ENSO cycle: horizontal advection (mainly in the western Pacific), vertical advection (mainly in the eastern Pacific), and the combination of both mechanisms. Based on such classification, the analysis also reveals that the models exhibit distinctive behavior with respect to the characteristics of ENSO: for most models, an enhanced (diminished) contribution of the zonal advective feedback is associated with faster (slower) ENSO and a tendency toward a cooler (warmer) mean state in the western-to-central Pacific Ocean. The results support the interpretation that biases in the mean state are sustained/maintained by the privileged mode of variability associated with the dominant feedback mechanism in the models. In particular, the models having a dominant zonal advective feedback exhibit significant cold SST asymmetry (or negative skewness) in the western equatorial Pacific.

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