A linear diagnosis of the coupled extratropical ocean-atmosphere system in the GFDL GCM (original) (raw)
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Atmospheric GCM Response to Extratropical SST Anomalies: Synthesis and Evaluation*
Journal of Climate, 2002
We examine the advances in our understanding of extratropical atmosphere-ocean interaction over the past decade and a half, focusing on the atmospheric response to sea surface temperature anomalies. The main goal of the paper is to assess what was learned from general circulation model (GCM) experiments over the recent two decades or so. Observational evidence regarding the nature of the interaction and dynamical theory of atmospheric anomalies forced by surface thermal anomalies are reviewed. We then proceed to examine three types of GCM experiments used to address this problem: models with fixed climatological conditions and idealized, stationary SST anomalies; models with seasonally evolving climatology forced with realistic, time-varying SST anomalies; and models coupled to an interactive ocean. From representative recent studies, we argue that the extratropical atmosphere does respond to changes in underlying SST although the response is small compared to internal (unforced) variability. Two types of interactions govern the response: One is an eddy-mediated process, in which a baroclinic response to thermal forcing induces and combines with changes in the position or strength of the storm tracks. This process can lead to an equivalent barotropic response that feeds back positively on the ocean mixed layer temperature. The other is a linear, thermodynamic interaction in which an equivalent-barotropic low-frequency atmospheric anomaly forces a change in SST and then experiences reduced surface thermal damping due to the SST adjustment. Both processes contribute to an increase in variance and persistence of low-frequency atmospheric anomalies and, in fact, may act together in the natural system.
The seasonal cycle of tropical Pacific sea surface temperature in a coupled GCM
Climate Dynamics, 2000
The mechanisms responsible for the seasonal cycle in the tropical central and eastern Paci®c sea surface temperature (SST) are investigated using a coupled general circulation model. We ®nd that the annual westward propagation of SST anomalies along the equator is explained by a two-stage process. The ®rst stage sets the phase of the variation at the eastern boundary. The strengthening of the local Hadley Circulation in boreal summer leads to a strengthening of the northward winds that blow across the equator. These stronger winds drive enhanced evaporation and entrainment cooling of the oceanic mixed layer. The resulting change in SST is greatest in the east because the mixed layer is at its shallowest there. As the east Paci®c SST cools the zonal SST gradient in the central Paci®c becomes more negative. This development signals the onset of the second stage in the seasonal variation of equatorial SST. In response to the anomalous SST gradient the local westward wind stress increases. This increase drives cooling of the oceanic mixed layer in which no single mechanism dominates: enhanced evaporation, wind-driven entrainment, and westward advection all contribute. We discuss the role that equatorial upwelling plays in modulating mixed layer depth and hence the entrainment cooling, and we highlight the importance of seasonal variations in mixed layer depth. In sum these processes act to propagate the SST anomaly westward.
Journal of Climate, 2018
The heat budget of the Pacific equatorial cold tongue (ECT) is explored using the GFDL-FLOR coupled GCM (the forecast-oriented low ocean resolution version of CM2.5) and ocean reanalyses, leveraging the two-layer framework developed in Part I. Despite FLOR's relatively weak meridional stirring by tropical instability waves (TIWs), the model maintains a reasonable SST and thermocline depth in the ECT via two compensating biases: 1) enhanced monthly-scale vertical advective cooling below the surface mixed layer (SML), due to overly cyclonic off-equatorial wind stress that acts to cool the equatorial source waters; and 2) an excessive SST contrast between the ECT and off-equator areas, which boosts the equatorward heat transport by TIWs. FLOR's strong advective cooling at the SML base is compensated by strong downward diffusion of heat out of the SML, which then allows FLOR's ECT to take up a realistic heat flux from the atmosphere. Correcting FLOR's climatological SST and wind stress biases via flux adjustment (FA) leads to weaker deep advective cooling of the ECT, which then erodes the upper-ocean thermal stratification, enhances vertical mixing, and excessively deepens the thermocline. FA does strengthen FLOR's meridional shear of the zonal currents in the east Pacific, but this does not amplify either the simulated TIWs or their equatorward heat transport, likely due to FLOR's coarse zonal ocean resolution. The analysis suggests that to advance coupled simulations of the ECT, improved winds and surface heat fluxes must go hand in hand with improved subseasonal and parameterized ocean processes. Implications for model development and the tropical Pacific observing system are discussed.
Quarterly Journal of the Royal Meteorological Society, 1994
A series of 120-day ensemble integrations of a general circulation model, designed to assess the impact of geographically localized sea-surface-temperature (SST) anomalies in both the tropics and extratropics, are described. These experiments contribute firstly to an appraisal of the relative roles of tropical and extratropical SST anomalies on interannual variability of the large-scale circulation in the northern extratropics, and secondly to an assessment of the role of quasi-stationary diabatic-heating anomalies on model systematic error, including blocking activity. Overall it is found that SST anomalies associated with El Niiio and La Niiia have a larger and more reproducible impact on the extratropics than the chosen extratropical SST anomalies. These extratropical anomalies were localized to the northwest Pacific, and northwest Atlantic, with realistic amplitude. However, unlike earlier studies, a response to the extratropical North Pacific SST anomalies has been obtained over the North Pacific which is correlated with the sign of the imposed SST anomaly. The response to extratropical SST anomalies in the northwest Atlantic are similar to the results obtained from an earlier study. The downstream responses to the extratropical Pacific and Atlantic SST anomalies are qualitatively similar to one another. Overall it is concluded that the northern large-scale flow is influenced by such extratropical SST anomalies. The response to idealized tropical SST anomalies was also studied. In particular, a localized anomaly over Indonesia had a very substantial impact on the Hadley circulation, on zonal flow, and on blocking frequency over the North Pacific and Europe. This response was such as to reduce model systematic error: locally in the vicinity of the SST anomaly, remotely around the tropics, and remotely in the extratropics. A similar, though weaker, impact on Euro-Atlantic blocking was obtained with an idealized Caribbean SST anomaly. Further statistical and dynamical analyses suggested that the extratropical response to the Indonesian SST anomaly occurs through the creation of two distinct planetary-scale regimes, in one of which the formation of blocks is much favoured by increased ridges on the north-eastem side of the oceans.
Climate Dynamics, 1999
The mechanisms responsible for the mean state and the seasonal and interannual variations of the coupled tropical Pacific-global atmosphere system are investigated by analyzing a thirty year simulation, where the LMD global atmospheric model and the LODYC tropical Pacific model are coupled using the delocalized physics method. No flux correction is needed over the tropical region. The coupled model reaches its regime state roughly after one year of integration in spite of the fact that the ocean is initialized from rest. Departures from the mean state are characterized by oscillations with dominant periodicites at annual, biennial and quadriennial time scales. In our model, equatorial sea surface temperature and wind stress fluctuations evolved in phase. In the Central Pacific during boreal autumn, the sea surface temperature is cold, the wind stress is strong, and the Inter Tropical Convergence Zone (ITCZ) is shifted northwards. The northward shift of the ITCZ enhances atmospheric and oceanic subsidence between the equator and the latitude of organized convention. In turn, the stronger oceanic subsidence reinforces equatorward convergence of water masses at the thermocline depth which, being not balanced by equatorial upwelling, deepens the equatorial thermocline. An equivalent view is that the deepening of the thermocline proceeds from the weakening of the meridional draining of near-surface equatorial waters. The inverse picture prevails during spring, when the equatorial sea surface temperatures are warm. Thus temperature anomalies tend to appear at the thermocline level, in phase opposition to the surface conditions. These subsurface temperature fluctuations propagate from the Central Pacific eastwards along the thermocline; when reaching the surface in the Eastern Pacific, they trigger the reversal of sea surface temperature anomalies. The whole oscillation is synchronized by the apparent meridional motion of the sun, through the seasonal oscillation of the ITCZ. This possible mechanism is partly supported by the observed seasonal reversal of vorticity between the equator and the ITCZ, and by observational evidence of eastward propagating subsurface temperature anomalies at the thermocline level.
Influence of Extratropical Thermal and Wind Forcings on Equatorial Thermocline in an Ocean GCM*
Journal of Physical Oceanography, 2004
The equatorial thermocline variability in the Pacific in response to the extratropical thermal and wind forcings is investigated with an ocean general circulation model [Modular Ocean Model, version 3 (MOM3)]. Sensitivity experiments show that the extratropical wind forcing and thermal forcing contribute equally to the equatorial variability. The wind-induced response is attributed to the off-equatorial wind within 30Њ of the equator; the thermal-induced response can be traced to higher latitudes. The thermal forcing affects the equator mainly through the equatorward transport of the perturbation temperature by mean subduction flow; the wind forcing affects the equator by changing the strength of meridional overturning circulations. It is also found that the Southern Hemisphere contributes more to the equatorial variability than the Northern Hemisphere under both external forcings.
Pacific interdecadal variability driven by tropical–extratropical interactions
Climate Dynamics, 2013
Interactions between the tropical and subtropical northern Pacific at decadal time scales are examined using uncoupled oceanic and atmospheric simulations. An atmospheric model is forced with observed Pacific sea surface temperatures (SST) decadal anomalies, computed as the difference between the 2000-2009 and the 1990-1999 period. The resulting pattern has negative SST anomalies at the equator, with a global pattern reminiscent of the Pacific decadal oscillation. The tropical SST anomalies are responsible for driving a weakening of the Hadley cell and atmospheric meridional heat transport. The atmosphere is then shown to produce a significant response in the subtropics, with wind-stress-curl anomalies having the opposite sign from the climatological mean, consistent with a weakening of the oceanic subtropical gyre (STG). A global ocean model is then forced with the decadal anomalies from the atmospheric model. In the North Pacific, the shallow subtropical cell (STC) spins down and the meridional heat transport is reduced, resulting in positive tropical SST anomalies. The final tropical response is reached after the first 10 years of the experiment, consistent with the Rossby-wave adjustment time for both the STG and the STC. The STC provides the connection between subtropical wind stress anomalies and tropical SSTs. In fact, targeted simulations show the importance of off-equatorial wind stress anomalies in driving the oceanic
Modeling North Pacific SST anomalies as a response to anomalous atmospheric forcing
J. Mar. Syst., 1: 155-168, 1990
"Large-scale sea surface temperature anomalies (SSTA) in the North Pacific ocean are often persistent for several months during wintertime. There is observational evidence that these patterns are forced by anomalous atmospheric circulation. Since the latter is in part related to the tropical El Nino/Southern Oscillation (ENSO) phenomenon it is hypothesized that part of the North Pacific SSTA's may be interpreted as remote oceanic response to anomalous equatorial Pacific SSTA's. Two experiments with a multi-level primitive equation model of the North Pacific have been conducted to study the influence of such anomalous atmospheric circulation on the SST. In both experiments anomalous wind stress as derived from the 1950-1979 COADS subset is specified as anomalous forcing. In experiment 1 no anomalous heat flux is introduced whereas in experiment 2 anomalous heat fluxes are estimated from anomalous surface winds and a simple advective atmosphere. In both experiments the GCM SSTA response are able to reproduce the main features of the time series of observed SSTA, in particular in winter. In experiment 1, however, the magnitudes are systematically too low. The addition of anomalous heat fluxes in experiment 2 significantly improves the simulation. The ENSO signal is clearly present in both simulations."
The Annular Response to Tropical Pacific SST Forcing
Journal of Climate, 2006
The leading pattern of Northern Hemisphere winter height variability exhibits an annular structure, one related to tropical west Pacific heating. To explore whether this pattern can be excited by tropical Pacific SST variations, an atmospheric general circulation model coupled to a slab mixed layer ocean is employed. Ensemble experiments with an idealized SST anomaly centered at different longitudes on the equator are conducted. The results reveal two different response patterns—a hemispheric pattern projecting on the annular mode and a meridionally arched pattern confined to the Pacific–North American sector, induced by the SST anomaly in the west and the east Pacific, respectively. Extratropical air–sea coupling enhances the annular component of response to the tropical west Pacific SST anomalies. A diagnosis based on linear dynamical models suggests that the two responses are primarily maintained by transient eddy forcing. In both cases, the model transient eddy forcing response ...
Climate Dyn. 4, 157-174, 1990
Analyses indicate that the Atlantic Ocean sea surface temperature (SST) was considerably colder at the beginning than in the middle of the century. In parallel a systematic change in the North Atlantic sealeve pressure (SLP) pattern was observed. To find ou whether the SST and SLP changes analyzed are consistent, which would indicate that the SST change was real and not an instrumental artifact, a response experiment with a low-resolution (T21) atmospheric GCM was performed. Two perpetual January simulations were conducted which differ solely in the Atlantic Ocean (40 ° S-60 ° N) SST: the "cold" simulation utilizes the SSTs for the period 1904-1913; the "warm" simulation uses the SSTs for the period 1951-1960. Also, a "control" run with the model's standard SST somewhat betwee the "cold" and "warm" SST was made. For th response analysis, a rigorous statistical approach wa taken. First, the null hypothesis of identical horizontal distributions was subjected to a multivariate significance test. Second, the level of recurrence was estimated. The multivariate statistical approaches are based on hierarchies of test models. We examined thre different hierarchies: a scale-dependent hierarchy based on spherical harmonics (S), and two physically motivated ones, one based on the barotropic normal modes of the mean 300 hPa flow (B) and one based on the eigenmodes of the advection diffusion operator at 1000 hPa (A). The intercomparison of the "cold" and "warm" experiments indicates a signal in the geostrophic stream function that in the S-hierarchy is significantly nonzero and highly recurrent. In the A-hierarchy the low level temperature field is identified as being significantly and recurrently affected by the altered SST distribution. The SLP signal is reasonably similar to the SLP change observed. Unexpectedly, the upper level streamfunction signal does not appear to be significantly nonzero in the B-hierarchy. If, however, the pairs of experiments "warm versus control" and "cold versus control" are examined in the B-hierarchy, a highly significant and recurrent signal emerges. We conclude that the "cold versus warm" response is not a "small disturbance" that would allow the signal to b described by eigenmodes of the linear system. An analysis of the three-dimensional structure of the signal leads to the hypothesis that two different mechanisms are acting to modify the model's mean state. At low levels, local heating and advection are dominant, but at upper levels the extratropical signal is a remote response to modifications of the tropical convection.