scholarly journals Seasonal Propagation of Sea Level along the Equator in the Atlantic

2009 ◽  
Vol 39 (4) ◽  
pp. 1069-1074 ◽  
Author(s):  
Lucia Bunge ◽  
Allan J. Clarke
Keyword(s):  
Wind Stress ◽  
Zonal Wind ◽  
Function Analysis ◽  
Sea Surface Height ◽  
Sea Surface ◽  
Water Volume ◽  
Kelvin Wave ◽  
Equatorial Atlantic ◽  
Wind Stress Curl ◽  
Zonal Wind Stress

Abstract In the equatorial Atlantic the sea surface height (SSH) anomaly field is dominated by an annual signal propagating eastward. This signal has been previously interpreted in terms of propagating waves. In this article it is argued that this propagating signal is not a free equatorial Kelvin wave because the phase velocity observed is too small compared to first, second, or third baroclinic mode Kelvin waves, and is not the result of an equatorial forced wave because the zonal wind stress does not show a similar propagation. Rather, it is suggested that the eastward propagation in SSH is due to the sum of two independent modes of variability: one mainly driven by the wind stress curl off the equator, and the other driven by the zonal wind stress along the equator. These two modes are uncorrelated in time and space and therefore can be conveniently separated by an empirical orthogonal function analysis of the equatorial Atlantic sea surface height. The first mode explains 74% of the variance, is one-signed in longitude, and is interpreted as the variability of the warm water volume above the thermocline. The second mode explains 24% of the variance, consists of an east–west tilt along the equator, and is driven by variations of the zonal equatorial wind stress.

scholarly journals Evaluation of CMIP3 and CMIP5 Wind Stress Climatology Using Satellite Measurements and Atmospheric Reanalysis Products

2013 ◽  
Vol 26 (16) ◽  
pp. 5810-5826 ◽  
Author(s):  
Tong Lee ◽  
Duane E. Waliser ◽  
Jui-Lin F. Li ◽  
Felix W. Landerer ◽  
Michelle M. Gierach
Keyword(s):  
Climate Variability ◽  
Wind Stress ◽  
Ocean Circulation ◽  
Zonal Wind ◽  
Global Ocean ◽  
Equatorial Pacific Ocean ◽  
Equatorial Atlantic ◽  
Wind Stress Curl ◽  
Meridional Heat Transport ◽  
Easterly Wind

Abstract Wind stress measurements from the Quick Scatterometer (QuikSCAT) satellite and two atmospheric reanalysis products are used to evaluate the annual mean and seasonal cycle of wind stress simulated by phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5). The ensemble CMIP3 and CMIP5 wind stresses are very similar to each other. Generally speaking, there is no significant improvement of CMIP5 over CMIP3. The CMIP ensemble–average zonal wind stress has eastward biases at midlatitude westerly wind regions (30°–50°N and 30°–50°S, with CMIP being too strong by as much as 55%), westward biases in subtropical–tropical easterly wind regions (15°–25°N and 15°–25°S), and westward biases at high-latitude regions (poleward of 55°S and 55°N). These biases correspond to too strong anticyclonic (cyclonic) wind stress curl over the subtropical (subpolar) ocean gyres, which would strengthen these gyres and influence oceanic meridional heat transport. In the equatorial zone, significant biases of CMIP wind exist in individual basins. In the equatorial Atlantic and Indian Oceans, CMIP ensemble zonal wind stresses are too weak and result in too small of an east–west gradient of sea level. In the equatorial Pacific Ocean, CMIP zonal wind stresses are too weak in the central and too strong in the western Pacific. These biases have important implications for the simulation of various modes of climate variability originating in the tropics. The CMIP as a whole overestimate the magnitude of seasonal variability by almost 50% when averaged over the entire global ocean. The biased wind stress climatologies in CMIP not only have implications for the simulated ocean circulation and climate variability but other air–sea fluxes as well.

Distinct off-equatorial zonal wind stress and oceanic responses for EP and CP type ENSO events

2021 ◽  
pp. 1-47
Keyword(s):  
Spatial Structure ◽  
Wind Stress ◽  
Zonal Wind ◽  
Enso Event ◽  
Sea Surface ◽  
Sea Surface Temperatures ◽  
Wind Stress Curl ◽  
Central Pacific ◽  
Enso Events ◽  
Surface Temperatures

Abstract This study utilises observations and a series of idealised experiments to explore whether Eastern Pacific (EP) and Central Pacific (CP) type El Niño-Southern Oscillation (ENSO) events produce surface wind stress responses with distinct spatial structures. We find that the meridionally broader sea surface temperatures (SST) during CP events lead to zonal wind stresses that are also meridionally broader than those found during EP type events, leading to differences in the near-equatorial wind stress curl. These wind spatial structure differences create differences in the associated pre- and post-ENSO event WWV response. For instance, the meridionally narrow winds found during EP events have: i) weaker wind stresses along 5°N and 5°S, leading to weaker Ekman induced pre-event WWV changes; and ii) stronger near-equatorial wind stress curls that lead to a much larger post-ENSO event WWV changes than during CP events. The latter suggests that, in the framework of the recharge oscillator model, the EP events have stronger coupling between sea surface temperatures (SST) and thermocline (WWV), supporting more clearly the phase transition of ENSO events, and therefore the oscillating nature of ENSO than CP events. The results suggest that the spatial structure of the SST pattern and the related differences in the wind stress curl, are required along with equatorial wind stress to accurately model the WWV changes during EP and CP type ENSO events.

scholarly journals Predictability Loss in an Intermediate ENSO Model due to Initial Error and Atmospheric Noise*

2006 ◽  
Vol 19 (15) ◽  
pp. 3572-3588 ◽  
Author(s):  
Alicia R. Karspeck ◽  
Alexey Kaplan ◽  
Mark A. Cane
Keyword(s):  
Wind Stress ◽  
Zonal Wind ◽  
Initial Conditions ◽  
Sea Surface ◽  
Thermocline Depth ◽  
Error Growth ◽  
Initial Error ◽  
Ensemble Spread ◽  
Observational Error ◽  
Zonal Wind Stress

Abstract The seasonal and interannual predictability of ENSO variability in a version of the Zebiak–Cane coupled model is examined in a perturbation experiment. Instead of assuming that the model is “perfect,” it is assumed that a set of optimal initial conditions exists for the model. These states, obtained through a nonlinear minimization of the misfit between model trajectories and the observations, initiate model forecasts that correlate well with the observations. Realistic estimates of the observational error magnitudes and covariance structures of sea surface temperatures, zonal wind stress, and thermocline depth are used to generate ensembles of perturbations around these optimal initial states, and the error growth is examined. The error growth in response to subseasonal stochastic wind forcing is presented for comparison. In general, from 1975 to 2002, the large-scale uncertainty in initial conditions leads to larger error growth than continuous stochastic forcing of the zonal wind stress fields. Forecast ensemble spread is shown to depend most on the calendar month at the end of the forecast rather than the initialization month, with the seasons of greatest spread corresponding to the seasons of greatest anomaly variance. It is also demonstrated that during years with negative (and rapidly decaying) Niño-3 SST anomalies (such as the time period following an El Niño event), there is a suppression of error growth. In years with large warm ENSO events, the ensemble spread is no larger than in moderately warm years. As a result, periods with high ENSO variance have greater potential prediction utility. In the realistic range of observational error, the ensemble spread has more sensitivity to the initial error in the thermocline depth than to the sea surface temperature or wind stress errors. The thermocline depth uncertainty is the principal reason why initial condition uncertainties are more important than wind noise for ensemble spread.

scholarly journals Seasonal Variations of the Seychelles Dome Simulated in the CMIP3 Models

2009 ◽  
Vol 39 (2) ◽  
pp. 449-457 ◽  
Author(s):  
Takaaki Yokoi ◽  
Tomoki Tozuka ◽  
Toshio Yamagata
Keyword(s):  
Wind Stress ◽  
Zonal Wind ◽  
Seasonal Variations ◽  
Coupled Model ◽  
Wind Stress Curl ◽  
Coupled Models ◽  
Zonal Wind Stress ◽  
Semiannual Variation ◽  
Intercomparison Project ◽  
Ekman Upwelling

Abstract Using outputs from the “twentieth-century climate in coupled models” (20c3m) control run of the Coupled Model Intercomparison Project, phase 3 (CMIP3), coupled GCMs, the authors have examined how seasonal variations of the Seychelles Dome (SD) are simulated in the southwestern Indian Ocean. The observed SD shows a dominant semiannual signal due to the semiannual variation in the local Ekman upwelling resulting from a combination of two terms related to the wind stress curl and the zonal wind stress. However, all models fail to reproduce this important mechanism. In particular, the latter contribution—that determined by the seasonal variation of the zonal wind stress associated with the Indian monsoon—is not well simulated. Successful models need to reproduce the asymmetric nature of the monsoon: a shorter and stronger summer monsoon and a longer and weaker winter monsoon. Possible remedies for the model bias are also discussed.

scholarly journals Coupled Decadal Variability in the North Pacific: An Observationally Constrained Idealized Model*

2007 ◽  
Vol 20 (14) ◽  
pp. 3602-3620 ◽  
Author(s):  
Bo Qiu ◽  
Niklas Schneider ◽  
Shuiming Chen
Keyword(s):  
Wind Stress ◽  
North Pacific ◽  
Time Scale ◽  
Large Scale ◽  
Decadal Variability ◽  
Kuroshio Extension ◽  
Sea Surface Height ◽  
Sea Surface ◽  
Lateral Migration ◽  
Wind Stress Curl

Abstract Air–sea coupled variability is investigated in this study by focusing on the observed sea surface temperature signals in the Kuroshio Extension (KE) region of 32°–38°N and 142°E–180°. In this region, both the oceanic circulation variability and the heat exchange variability across the air–sea interface are the largest in the midlatitude North Pacific. SST variability in the KE region has a dominant time scale of ∼10 yr and this decadal variation is caused largely by the regional, wind-induced sea surface height changes that represent the lateral migration and strengthening/weakening of the KE jet. The importance of the air–sea coupling in influencing KE jet is explored 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 time-scale 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 sea surface height (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 preexisting 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 ∼10 yr, slightly longer than the basin-crossing time of the baroclinic Rossby waves along the KE latitude.

Contributions of Wind Forcing and Surface Heating to Interannual Sea Level Variations in the Atlantic Ocean

2006 ◽  
Vol 36 (9) ◽  
pp. 1739-1750 ◽  
Author(s):  
Cécile Cabanes ◽  
Thierry Huck ◽  
Alain Colin de Verdière
Keyword(s):  
Atlantic Ocean ◽  
Sea Level ◽  
Wind Stress ◽  
Large Scale ◽  
Sea Surface Height ◽  
Sea Surface ◽  
Altimeter Data ◽  
Local Response ◽  
Sea Level Variability ◽  
Sea Level Variations

Abstract Interannual sea surface height variations in the Atlantic Ocean are examined from 10 years of high-precision altimeter data in light of simple mechanisms that describe the ocean response to atmospheric forcing: 1) local steric changes due to surface buoyancy forcing and a local response to wind stress via Ekman pumping and 2) baroclinic and barotropic oceanic adjustment via propagating Rossby waves and quasi-steady Sverdrup balance, respectively. The relevance of these simple mechanisms in explaining interannual sea level variability in the whole Atlantic Ocean is investigated. It is shown that, in various regions, a large part of the interannual sea level variability is related to local response to heat flux changes (more than 50% in the eastern North Atlantic). Except in a few places, a local response to wind stress forcing is less successful in explaining sea surface height observations. In this case, it is necessary to consider large-scale oceanic adjustments: the first baroclinic mode forced by wind stress explains about 70% of interannual sea level variations in the latitude band 18°–20°N. A quasi-steady barotropic Sverdrup response is observed between 40° and 50°N.

scholarly journals On the fast response of the Southern Ocean to changes in the zonal wind

Ocean Science ◽  
2007 ◽  
Vol 3 (3) ◽  
pp. 417-427 ◽  
Author(s):  
D. J. Webb ◽  
B. A. de Cuevas
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Zonal Wind ◽  
Surface Wind ◽  
Drake Passage ◽  
Fast Response ◽  
Momentum Balance ◽  
Zonal Wind Stress ◽  
Model Studies ◽  
Barotropic Flow

Abstract. Model studies of the Southern Ocean, reported here, show that the Antarctic Circumpolar Current responds within two days to changes in the zonal wind stress at the latitudes of Drake Passage. Further investigation shows that the response is primarily barotropic and that, as one might expect, it is controlled by topography. Analysis of the results show that the changes in the barotropic flow are sufficient to transfer the changed surface wind stress to the underlying topography and that during this initial phase baroclinic processes are not involved. The model results also show that the Deacon Cell responds to changes in the wind stress on the same rapid time scale. It is shown that the changes in the Deacon Cell can also be explained by the change in the barotropic velocity field, an increase in the zonal wind stress producing an increased northward flow in shallow regions and southward flow where the ocean is deep. This new explanation is unexpected as previously the Deacon Cell has been thought of as a baroclinic feature of the ocean. The results imply that where baroclinic processes do appear to be involved in either the zonal momentum balance of the Southern Ocean or the formation of the Deacon Cell, they are part of the long term baroclinic response of the ocean's density field to the changes in the barotropic flow.

Frequency-Wavenumber Spectra of Sea Surface Temperature and Wind-Stress Curl in the Eastern North Pacific

1981 ◽  
Vol 11 (8) ◽  
pp. 1059-1077 ◽  
Author(s):  
Artemio Gallegos-Garcia ◽  
William J. Emery ◽  
Robert O. Reid ◽  
Lorenz Magaard
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Wind Stress ◽  
North Pacific ◽  
Sea Surface ◽  
Wind Stress Curl

scholarly journals Sea Surface Temperature and Wind Stress Curl Variability near a Cape

2012 ◽  
Vol 42 (11) ◽  
pp. 2073-2087 ◽  
Author(s):  
Renato M. Castelao
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Wind Stress ◽  
Current System ◽  
Empirical Orthogonal Functions ◽  
Sea Surface ◽  
Western Boundary ◽  
Surface Cooling ◽  
Wind Stress Curl ◽  
California Current System

Abstract The coupling between sea surface temperature (SST), SST gradients, and wind stress curl variability near a cape off Brazil is investigated using satellite observations and several different SST high-resolution analyses. The cape is characterized by strong SST fronts year-round, associated with upwelling and advection of warm water offshore in a western boundary current. Observations reveal a strong coupling between crosswind SST gradients and wind stress curl variability, with the predominantly negative crosswind gradients leading to negative, upwelling favorable wind stress curl anomalies. The spatial correlation between empirical orthogonal functions (EOF) of those variables is ~0.6, while the correlation between the EOF amplitude time series of the wind stress curl and crosswind SST gradients is larger than 0.7. The coupling occurs during summer and winter and is strongly modulated by variations in the wind stress directional steadiness. The intensity of the coupling is weaker than around capes on the California Current system, presumably because of higher variability in wind direction off Brazil. During periods of high wind stress directional steadiness off Cape Frio, the coupling is intensified by up to 40%–75%. Wind stress curl is also correlated with SST itself, especially in the vicinity of the cape, although not as strongly as with crosswind SST gradients. The analyses suggest that the observed wind stress curl anomalies can lead to surface cooling of as much as 1°C. If the enhanced upwelling leads to further strengthening of the upwelling front, negative wind stress curl anomalies may be intensified in a positive feedback mechanism.

scholarly journals U.S. West Coast Surface Heat Fluxes, Wind Stress, and Wind Stress Curl from a Mesoscale Model

2005 ◽  
Vol 133 (11) ◽  
pp. 3202-3216 ◽  
Author(s):  
T. Haack ◽  
S. D. Burk ◽  
R. M. Hodur
Keyword(s):  
Relative Humidity ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Wind Stress ◽  
Mesoscale Model ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Sea Surface ◽  
Wind Stress Curl ◽  
Low Level

Abstract Monthly averages of numerical model fields are beneficial for depicting patterns in surface forcing such as sensible and latent heat fluxes, wind stress, and wind stress curl over data-sparse ocean regions. Grid resolutions less than 10 km provide the necessary mesoscale detail to characterize the impact of a complex coastline and coastal topography. In the present study a high-resolution mesoscale model is employed to reveal patterns in low-level winds, temperature, relative humidity, sea surface temperature as well as surface fluxes, over the eastern Pacific and along the U.S. west coast. Hourly output from successive 12-h forecasts are averaged to obtain monthly mean patterns from each season of 1999. The averages yield information on interactions between the ocean and the overlying atmosphere and on the influence of coastal terrain forcing in addition to their month-to-month variability. The spring to summer transition is characterized by a dramatic shift in near-surface winds, temperature, and relative humidity as offshore regions of large upward surface fluxes diminish and an alongshore coastal flux gradient forms. Embedded within this gradient, and the imprint of strong summertime topographic forcing, are small-scale fluctuations that vary in concert with local changes in sea surface temperature. Potential feedbacks between the low-level wind, sea surface temperature, and the wind stress curl are explored in the coastal regime and offshore waters. In all seasons, offshore extensions of colder coastal waters impose a marked influence on low-level conditions by locally enhancing stability and reducing the wind speed, while buoy measurements along the coast indicate that sea surface temperatures and wind speeds tend to be negatively correlated.

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