Evaluation of the zonal wind stress response to SST in the CMIP5 AMIP simulations

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.

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Using Initialized Hindcasts to Assess Simulations of 1970–2009 Equatorial Pacific SST, Zonal Wind Stress, and Surface Flux Trends

Journal of Climate ◽

10.1175/jcli-d-13-00709.1 ◽

2014 ◽

Vol 27 (19) ◽

pp. 7385-7393 ◽

Cited By ~ 1

Author(s):

Amy Solomon

Keyword(s):

Time Scales ◽

Wind Stress ◽

Zonal Wind ◽

Surface Flux ◽

Warming Trend ◽

Warm Pool ◽

Equatorial Pacific ◽

External Forcing ◽

Cold Tongue ◽

Zonal Wind Stress

Abstract Initialized decadal hindcasts are used to assess simulations of 1970–2009 equatorial Pacific SST, zonal wind stress, and surface flux trends. Initialized hindcasts are useful to assess how well the models simulate observed trends, as well as how simulations of observed trends (due primarily to natural variability) differ from ensemble-mean forecasted trends (due to the response to an increase in external forcing). All models forecast a statistically significant warming trend in both the warm-pool and cold-tongue regions. However, while the warm-pool warming trend is within the observed estimates, the cold-tongue warming trend is an order of magnitude larger than an ENSO residual estimated using SST instrumental reconstructions. Multimodel ensemble means formed using forecasts 6–10 years from initialization with 40 ensemble members do not produce an unambiguous zonal SST gradient response to an increase in external forcing. Systematic biases are identified in forecasts of surface fluxes. For example, in the warm-pool region all year-1 forecasts produce SST trends similar to observations but ocean mixed layer and net surface heat flux trends with an opposite sign to air–sea datasets. In addition, year-1 forecasts produce positive shortwave feedbacks on decadal time scales, whereas 6–10-yr forecasts produce negative or statistically insignificant shortwave flux feedbacks on decadal time scales, suggesting sensitivity to circulations forced by the initialized ocean state. In the cold-tongue region initialized ensembles forecast positive net radiative flux trends even though shortwave flux trends are negative (i.e., for increasing cloudiness). This is inconsistent with air–sea datasets, which uniformly show that the net surface radiative flux feedback is a damping of the underlying SSTs.

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Causes of Strengthening and Weakening of ENSO Amplitude under Global Warming in Four CMIP5 Models*

Journal of Climate ◽

10.1175/jcli-d-14-00439.1 ◽

2015 ◽

Vol 28 (8) ◽

pp. 3250-3274 ◽

Cited By ~ 60

Author(s):

Lin Chen ◽

Tim Li ◽

Yongqiang Yu

Keyword(s):

Global Warming ◽

Wind Stress ◽

Zonal Wind ◽

Southern Oscillation ◽

Heat Budget ◽

Meridional Overturning Circulation ◽

Cmip5 Models ◽

Zonal Wind Stress ◽

Equatorial Thermocline ◽

Change In Mean

Abstract The mechanisms for El Niño–Southern Oscillation (ENSO) amplitude change under global warming are investigated through quantitative assessment of air–sea feedback processes in present-day and future climate simulations of four models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5). Two models (MPI-ESM-MR and MRI-CGCM3) project strengthened ENSO amplitude, whereas the other two models (CCSM4 and FGOALS-g2) project weakened ENSO amplitude. A mixed layer heat budget diagnosis shows that the major cause of the projected ENSO amplitude difference between the two groups is attributed to the changes of the thermocline and zonal advective feedbacks. A weaker (stronger) equatorial thermocline response to a unit anomalous zonal wind stress forcing in the Niño-4 region is found in CCSM4 and FGOALS-g2 (MPI-ESM-MR and MRI-CGCM3). The cause of the different response arises from the change in the meridional scale of ENSO. A narrower (wider) meridional width of sea surface temperature (SST) and zonal wind stress anomalies causes a strengthening (weakening) of the equatorial thermocline response and thus stronger Bjerknes and zonal advective feedbacks, as the subsurface temperature and zonal current anomalies depend on the thermocline response; consequently, the ENSO amplitude increases (decreases). The change of ENSO meridional width is caused by the change in mean meridional overturning circulation in the equatorial Pacific Ocean, which depends on change of mean wind stress and SST warming patterns under global warming.

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The Effect of Wind Stress Anomalies and Location in Driving Pacific Subtropical Cells and Tropical Climate

Journal of Climate ◽

10.1175/jcli-d-18-0071.1 ◽

2019 ◽

Vol 32 (5) ◽

pp. 1641-1660 ◽

Cited By ~ 4

Author(s):

Giorgio Graffino ◽

Riccardo Farneti ◽

Fred Kucharski ◽

Franco Molteni

Keyword(s):

Wind Stress ◽

Zonal Wind ◽

Tropical Climate ◽

Global Ocean ◽

Subsurface Water ◽

Zonal Wind Stress ◽

Wind Stress Anomaly ◽

Equatorial Thermocline ◽

The Tropics ◽

Sst Anomalies

Abstract The importance of subtropical and extratropical zonal wind stress anomalies on Pacific subtropical cell (STC) strength is assessed through several idealized and realistic numerical experiments with a global ocean model. Different zonal wind stress anomalies are employed, and their intensity is strengthened or weakened with respect to the climatological value throughout a suite of simulations. Subtropical strengthened (weakened) zonal wind stress anomalies result in increased (decreased) STC meridional mass and energy transport. When upwelling of subsurface water into the tropics is intensified (reduced), a distinct cold (warm) anomaly appears in the equatorial thermocline and up to the surface, resulting in significant tropical sea surface temperature (SST) anomalies. The use of realistic wind stress anomalies also suggests a potential impact of midlatitude atmospheric modes of variability on tropical climate through STC dynamics. The remotely driven response is compared with a set of simulations where an equatorial zonal wind stress anomaly is imposed. A dynamically distinct response is achieved, whereby the equatorial thermocline adjusts to the wind stress anomaly, resulting in significant equatorial SST anomalies as in the remotely forced simulations but with no role for STCs. Significant anomalies in Indonesian Throughflow transport are generated only when equatorial wind stress anomalies are applied, leading to remarkable heat content anomalies in the Indian Ocean. Equatorial wind stress anomalies do not involve modifications of STC transport but could set up the appropriate initial conditions for a tropical–extratropical teleconnection involving Hadley cells, exciting an STC anomalous transport, which ultimately feeds back on the tropics.

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Assessing the Twenty-First-Century Shift in ENSO Variability in Terms of the Bjerknes Stability Index*

Journal of Climate ◽

10.1175/jcli-d-13-00438.1 ◽

2014 ◽

Vol 27 (7) ◽

pp. 2577-2587 ◽

Cited By ~ 69

Author(s):

Joke F. Lübbecke ◽

Michael J. McPhaden

Keyword(s):

Wind Stress ◽

Zonal Wind ◽

Stability Index ◽

Equatorial Pacific ◽

Central Pacific ◽

Western Basin ◽

Ocean Reanalysis ◽

Zonal Wind Stress ◽

Bj Index ◽

Sst Anomalies

Abstract A decadal change in the character of ENSO was observed around year 2000 toward weaker-amplitude, higher-frequency events with an increased occurrence of central Pacific El Niños. Here these changes are assessed in terms of the Bjerknes stability index (BJ index), which is a measure of the growth rate of ENSO-related SST anomalies. The individual terms of the index are calculated from ocean reanalysis products separately for the time periods 1980–99 and 2000–10. The spread between the products is large, but they show a robust weakening of the thermocline feedback due to a reduced thermocline slope response to anomalous zonal wind stress as well as a weakened wind stress response to eastern equatorial Pacific SST anomalies. These changes are consistent with changes in the background state of the tropical Pacific: cooler mean SST in the eastern and central equatorial Pacific results in reduced convection there together with a westward shift in the ascending branch of the Walker circulation. This shift leads to a weakening in the relationship between eastern Pacific SST and longitudinally averaged equatorial zonal wind stress. Also, despite a steeper mean thermocline slope in the more recent period, the thermocline slope response to wind stress anomalies weakened due to a smaller zonal wind fetch that results from ENSO-related wind anomalies being more confined to the western basin. As a result, the total BJ index is more negative, corresponding to a more strongly damped system in the past decade compared to the 1980s and 1990s.

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Wind Stress Variations and Interannual Sea Surface Temperature Anomalies in the Eastern Equatorial Pacific

Journal of Climate ◽

10.1175/jcli3618.1 ◽

2006 ◽

Vol 19 (2) ◽

pp. 226-241 ◽

Cited By ~ 36

Author(s):

Xuebin Zhang ◽

Michael J. McPhaden

Keyword(s):

Wind Stress ◽

Zonal Wind ◽

El Niño ◽

El Nino ◽

Equatorial Pacific ◽

Eastern Pacific ◽

Zonal Wind Stress ◽

Eastern Equatorial Pacific ◽

Equatorial Wave ◽

Stress Variations

Abstract Vertical advection of temperature is the primary mechanism by which El Niño–Southern Oscillation (ENSO) time-scale sea surface temperature (SST) anomalies are generated in the eastern equatorial Pacific. Variations in vertical advection are mediated primarily by remote wind-forced thermocline displacements, which control the temperature of water upwelled to the surface. However, during some ENSO events, large wind stress variations occur in the eastern Pacific that in principle should affect local upwelling rates, the depth of the thermocline, and SST. In this study, the impact of these wind stress variations on the eastern equatorial Pacific is addressed using multiple linear regression analysis and a linear equatorial wave model. The regression analysis indicates that a zonal wind stress anomaly of 0.01 N m−2 leads to approximately a 1°C SST anomaly over the Niño-3 region (5°N–5°S, 90°–150°W) due to changes in local upwelling rates. Wind stress variations of this magnitude occurred in the eastern Pacific during the 1982/83 and 1997/98 El Niños, accounting for about 1/3 of the maximum SST anomaly during these events. The linear equatorial wave model also indicates that depending on the period in question, zonal wind stress variations in the eastern Pacific can work either with or against remote wind stress forcing from the central and western Pacific to determine the thermocline depth in the eastern Pacific. Thus, zonal wind stress variations in the eastern Pacific contribute to the generation of interannual SST anomalies through both changes in local upwelling rates and changes in thermocline depth. Positive feedbacks between the ocean and atmosphere in the eastern Pacific are shown to influence the evolution of the surface wind field, especially during strong El Niño events, emphasizing the coupled nature of variability in the region. Implications of these results for understanding the character of event-to-event differences in El Niño and La Niña are discussed.

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An Analysis of ENSO Prediction Skill in the CFS Retrospective Forecasts

Journal of Climate ◽

10.1175/2008jcli2565.1 ◽

2009 ◽

Vol 22 (7) ◽

pp. 1801-1818 ◽

Cited By ~ 22

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.

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Climate Change over the Equatorial Indo-Pacific in Global Warming*

Journal of Climate ◽

10.1175/2008jcli2581.1 ◽

2009 ◽

Vol 22 (10) ◽

pp. 2678-2693 ◽

Cited By ~ 14

Author(s):

Chie Ihara ◽

Yochanan Kushnir ◽

Mark A. Cane ◽

Victor H. de la Peña

Keyword(s):

Climate Change ◽

Global Warming ◽

Indian Ocean ◽

Wind Stress ◽

Zonal Wind ◽

Climate Models ◽

Emission Scenario ◽

Equatorial Indian Ocean ◽

Zonal Wind Stress ◽

Sst Gradient

Abstract The response of the equatorial Indian Ocean climate to global warming is investigated using model outputs submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report. In all of the analyzed climate models, the SSTs in the western equatorial Indian Ocean warm more than the SSTs in the eastern equatorial Indian Ocean under global warming; the mean SST gradient across the equatorial Indian Ocean is anomalously positive to the west in a warmer twenty-first-century climate compared to the twentieth-century climate, and it is dynamically consistent with the anomalous westward zonal wind stress and anomalous positive zonal sea level pressure (SLP) gradient to the east at the equator. This change in the zonal SST gradient in the equatorial Indian Ocean is detected even in the lowest-emission scenario, and the size of the change is not necessarily larger in the higher-emission scenario. With respect to the change over the equatorial Pacific in climate projections, the subsurface central Pacific displays the strongest cooling or weakest warming around the thermocline depth compared to that above and below in all of the climate models, whereas changes in the zonal SST gradient and zonal wind stress around the equator are model dependent and not straightforward.

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The Effect of the Galápagos Islands on ENSO in Forced Ocean and Hybrid Coupled Models

Journal of Physical Oceanography ◽

10.1175/2008jpo3848.1 ◽

2008 ◽

Vol 38 (11) ◽

pp. 2519-2534 ◽

Cited By ~ 13

Author(s):

Kristopher B. Karnauskas ◽

Raghu Murtugudde ◽

Antonio J. Busalacchi

Keyword(s):

Wind Stress ◽

Zonal Wind ◽

El Niño ◽

Time Scale ◽

El Nino ◽

Galapagos Islands ◽

Galápagos Islands ◽

Coupled Models ◽

Zonal Wind Stress ◽

Sst Anomaly

Abstract An ocean general circulation model (OGCM) of the tropical Pacific Ocean is used to examine the effects of the Galápagos Islands on the El Niño–Southern Oscillation (ENSO). First, a series of experiments is conducted using the OGCM in a forced context, whereby an idealized El Niño event may be examined in cases with and without the Galápagos Islands. In this setup, the sensitivity of the sea surface temperature (SST) anomaly response to the presence of the Galápagos Islands is examined. Second, with the OGCM coupled to the atmosphere via zonal wind stress, experiments are conducted with and without the Galápagos Islands to determine how the Galápagos Islands influence the time scale of ENSO. In the forced setup, the Galápagos Islands lead to a damped SST anomaly given an identical zonal wind stress perturbation. Mixed layer heat budget calculations implicate the entrainment mixing term, which confirms that the difference is due to the Galápagos Islands changing the background mean state, that is, the equatorial thermocline as diagnosed in a previous paper. In the hybrid coupled experiments, there is a clear shift in the power spectrum of SST anomalies in the eastern equatorial Pacific. Specifically, the Galápagos Islands lead to a shift in the ENSO time scale from a biennial to a quasi-quadrennial period. Mechanisms for the shift in ENSO time scale due to the Galápagos Islands are discussed in the context of well-known paradigms for the oscillatory nature of ENSO.

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Seasonal Variation of the Seychelles Dome

Journal of Climate ◽

10.1175/2008jcli1957.1 ◽

2008 ◽

Vol 21 (15) ◽

pp. 3740-3754 ◽

Cited By ~ 69

Author(s):

Takaaki Yokoi ◽

Tomoki Tozuka ◽

Toshio Yamagata

Keyword(s):

Seasonal Variation ◽

Indian Ocean ◽

Wind Stress ◽

Zonal Wind ◽

General Circulation ◽

Indian Monsoon ◽

Circulation Model ◽

The Other ◽

Boreal Summer ◽

Zonal Wind Stress

Abstract Using an ocean general circulation model (OGCM), seasonal variation of the Seychelles Dome (SD) is investigated for the first time. The SD is an oceanic thermal dome located in the southwestern Indian Ocean, and its influence on sea surface temperature is known to play an important role in the Indian monsoon system. Its seasonal variation is dominated by a remarkable semiannual cycle resulting from local Ekman upwelling. This semiannual nature is explained by different contributions of the following two components of the Ekman pumping: one term that is proportional to the planetary beta and the zonal wind stress and the other term that is proportional to the wind stress curl. The former is determined by the seasonal change in the zonal component of the wind stress vector above the SD; it is associated with the Indian monsoon and causes downwelling (upwelling) during boreal summer (boreal winter). The latter, whose major contribution comes from the meridional gradient of the zonal wind stress, also shows a clear annual cycle with strong upwelling during boreal summer and fall. However, it remains almost constant for 5 months from June to October, even though the zonal wind stress itself varies significantly during this period. The above overall feature is due to the unique location of the SD; it is located between the following two regions: one is dominated by the seasonal variation in wind stress resulting from the Indian monsoon, and the other is dominated by the southeasterly trade winds that prevail throughout a year. The above uniqueness provides a novel mechanism that causes the strong semiannual cycle in the tropical Indian Ocean.

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