scholarly journals Impact of Systematic GCM Errors on Prediction Skill as Estimated by Linear Inverse Modeling

2020 ◽  
Vol 33 (23) ◽  
pp. 10073-10095
Author(s):  
Ingo Richter ◽  
Ping Chang ◽  
Xue Liu
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Equatorial Pacific ◽  
Prediction Skill ◽  
Statistical Prediction ◽  
Equatorial Atlantic ◽  
Sst Variability ◽  
Inverse Models ◽  
Tropical Sea ◽  
Mean State

Statistical prediction of tropical sea surface temperatures (SSTs) is performed using linear inverse models (LIMs) that are constructed from both observations and general circulation model (GCM) output of SST. The goals are to establish a baseline for tropical SST predictions, to examine the extent to which the skill of a GCM-derived LIM is indicative of that GCM’s skill in forecast mode, and to examine the linkages between mean state bias and prediction skill. The observation-derived LIM is more skillful than a simple persistence forecasts in most regions. Its skill also compares well with some GCM forecasts except in the equatorial Pacific, where the GCMs are superior. The observation-derived LIM is matched or even outperformed by the GCM-derived LIMs, which may be related to the longer data record available for GCMs. The GCM-derived LIMs provide a fairly good measure for the skill achieved by their parent GCMs in forecast mode. In some cases, the skill of the LIM is actually superior to that of its parent GCM, indicating that the GCM predictions may suffer from initialization problems. A weak-to-moderate relation exists between model mean state error and prediction skill in some regions. An example is the eastern equatorial Atlantic, where an erroneously deep thermocline reduces SST variability, which in turn affects prediction skill. Another example is the equatorial Pacific, where skill appears to be linked to cold SST biases in the western tropical Pacific, which may reduce the strength of air–sea coupling.

scholarly journals The Role of Tropical Interbasin SST Gradients in Forcing Walker Circulation Trends

2017 ◽  
Vol 30 (2) ◽  
pp. 499-508 ◽  
Author(s):  
Lei Zhang ◽  
Kristopher B. Karnauskas
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Walker Circulation ◽  
Equatorial Pacific ◽  
Trade Winds ◽  
Low Level ◽  
The Pacific ◽  
Indian Ocean Warming ◽  
Tropical Sea ◽  
Sst Gradient

The effects of externally forced tropical sea surface temperature (SST) anomalies on long-term Walker circulation changes are investigated through numerical atmospheric general circulation model (AGCM) experiments. In response to the observed tropics-wide SST trend, which exhibits a prominent interbasin warming contrast (IBWC) with smaller warming in the Pacific than the Indian and Atlantic Oceans that includes a weak La Niña–like pattern in the equatorial Pacific, pronounced low-level easterly anomalies emerge over the equatorial Pacific. Through sensitivity experiments, the intensification of the Pacific trade winds (PTWs) is attributable to the IBWC, whereas the slightly enhanced zonal SST gradient within the equatorial Pacific plays a small role relative to the observed IBWC. It is further demonstrated that the greater Indian Ocean warming forces low-level easterly anomalies over the entire equatorial Pacific, while the greater tropical Atlantic warming-driven enhancement of PTWs is located over the central equatorial Pacific. In contrast to observations, a negligible IBWC emerges in the tropical SST trends of CMIP5 historical simulations due to a strong El Niño–like warming in the tropical Pacific. Lacking the observed IBWC (and the observed enhancement of the zonal SST gradient within the equatorial Pacific), the PTWs in the CMIP5 ensemble can only weaken.

scholarly journals Ocean Circulation and Tropical Variability in the Coupled Model ECHAM5/MPI-OM

2006 ◽  
Vol 19 (16) ◽  
pp. 3952-3972 ◽  
Author(s):  
J. H. Jungclaus ◽  
N. Keenlyside ◽  
M. Botzet ◽  
H. Haak ◽  
J.-J. Luo ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
General Circulation ◽  
Southern Oscillation ◽  
Surface Wind ◽  
Coupled Model ◽  
Circulation Model ◽  
Tropical Variability ◽  
Sst Variability ◽  
The Mean ◽  
Mean State

Abstract This paper describes the mean ocean circulation and the tropical variability simulated by the Max Planck Institute for Meteorology (MPI-M) coupled atmosphere–ocean general circulation model (AOGCM). Results are presented from a version of the coupled model that served as a prototype for the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) simulations. The model does not require flux adjustment to maintain a stable climate. A control simulation with present-day greenhouse gases is analyzed, and the simulation of key oceanic features, such as sea surface temperatures (SSTs), large-scale circulation, meridional heat and freshwater transports, and sea ice are compared with observations. A parameterization that accounts for the effect of ocean currents on surface wind stress is implemented in the model. The largest impact of this parameterization is in the tropical Pacific, where the mean state is significantly improved: the strength of the trade winds and the associated equatorial upwelling weaken, and there is a reduction of the model’s equatorial cold SST bias by more than 1 K. Equatorial SST variability also becomes more realistic. The strength of the variability is reduced by about 30% in the eastern equatorial Pacific and the extension of SST variability into the warm pool is significantly reduced. The dominant El Niño–Southern Oscillation (ENSO) period shifts from 3 to 4 yr. Without the parameterization an unrealistically strong westward propagation of SST anomalies is simulated. The reasons for the changes in variability are linked to changes in both the mean state and to a reduction in atmospheric sensitivity to SST changes and oceanic sensitivity to wind anomalies.

Sensitivity of Global Tropical Climate to Land Surface Processes: Mean State and Interannual Variability

2013 ◽  
Vol 26 (5) ◽  
pp. 1818-1837 ◽  
Author(s):  
Hsi-Yen Ma ◽  
Heng Xiao ◽  
C. Roberto Mechoso ◽  
Yongkang Xue
Keyword(s):  
Soil Moisture ◽  
Indirect Effect ◽  
Land Surface ◽  
Seasonal Cycle ◽  
General Circulation ◽  
Global Climate ◽  
Circulation Model ◽  
Equatorial Pacific ◽  
Surface Processes ◽  
Mean State

Abstract This study examines the sensitivity of the global climate to land surface processes (LSP) using an atmospheric general circulation model both uncoupled (with prescribed SSTs) and coupled to an oceanic general circulation model. The emphasis is on the interactive soil moisture and vegetation biophysical processes, which have first-order influence on the surface energy and water budgets. The sensitivity to those processes is represented by the differences between model simulations, in which two land surface schemes are considered: 1) a simple land scheme that specifies surface albedo and soil moisture availability and 2) the Simplified Simple Biosphere Model (SSiB), which allows for consideration of interactive soil moisture and vegetation biophysical process. Observational datasets are also employed to assess the extent to which results are realistic. The mean state sensitivity to different LSP is stronger in the coupled mode, especially in the tropical Pacific. Furthermore, the seasonal cycle of SSTs in the equatorial Pacific, as well as the ENSO frequency, amplitude, and locking to the seasonal cycle of SSTs, is significantly modified and more realistic with SSiB. This outstanding sensitivity of the atmosphere–ocean system develops through changes in the intensity of equatorial Pacific trades modified by convection over land. The results further demonstrate that the direct impact of land–atmosphere interactions on the tropical climate is modified by feedbacks associated with perturbed oceanic conditions (“indirect effect” of LSP). The magnitude of such an indirect effect is strong enough to suggest that comprehensive studies on the importance of LSP on the global climate have to be made in a system that allows for atmosphere–ocean interactions.

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.

“Modes of Variability” and Climate Change

2009 ◽  
Vol 22 (10) ◽  
pp. 2639-2658 ◽  
Author(s):  
Grant Branstator ◽  
Frank Selten
Keyword(s):  
Climate Change ◽  
Stochastic Model ◽  
General Circulation ◽  
Climate Changes ◽  
Circulation Model ◽  
Meridional Wind ◽  
Model Atmosphere ◽  
Linear Pattern ◽  
Modes Of Variability ◽  
Mean State

Abstract A 62-member ensemble of coupled general circulation model (GCM) simulations of the years 1940–2080, including the effects of projected greenhouse gas increases, is examined. The focus is on the interplay between the trend in the Northern Hemisphere December–February (DJF) mean state and the intrinsic modes of variability of the model atmosphere as given by the upper-tropospheric meridional wind. The structure of the leading modes and the trend are similar. Two commonly proposed explanations for this similarity are considered. Several results suggest that this similarity in most respects is consistent with an explanation involving patterns that result from the model dynamics being well approximated by a linear system. Specifically, the leading intrinsic modes are similar to the leading modes of a stochastic model linearized about the mean state of the GCM atmosphere, trends in GCM tropical precipitation appear to excite the leading linear pattern, and the probability density functions (PDFs) of prominent circulation patterns are quasi-Gaussian. There are, on the other hand, some subtle indications that an explanation for the similarity involving preferred states (which necessarily result from nonlinear influences) has some relevance. For example, though unimodal, PDFs of prominent patterns have departures from Gaussianity that are suggestive of a mixture of two Gaussian components. And there is some evidence of a shift in probability between the two components as the climate changes. Interestingly, contrary to the most prominent theory of the influence of nonlinearly produced preferred states on climate change, the centroids of the components also change as the climate changes. This modification of the system’s preferred states corresponds to a change in the structure of its dominant patterns. The change in pattern structure is reproduced by the linear stochastic model when its basic state is modified to correspond to the trend in the general circulation model’s mean atmospheric state. Thus, there is a two-way interaction between the trend and the modes of variability.

scholarly journals Decadal SST Variability in the Southeast Indian Ocean and Its Impact on Regional Climate

2019 ◽  
Vol 32 (19) ◽  
pp. 6299-6318 ◽  
Author(s):  
Yuanlong Li ◽  
Weiqing Han ◽  
Lei Zhang ◽  
Fan Wang
Keyword(s):  
Heat Flux ◽  
Wind Speed ◽  
Indian Ocean ◽  
General Circulation Model ◽  
General Circulation ◽  
Circulation Model ◽  
Turbulent Heat ◽  
Central Pacific ◽  
Offshore Area ◽  
Sst Variability

Abstract The southeast Indian Ocean (SEIO) exhibits decadal variability in sea surface temperature (SST) with amplitudes of ~0.2–0.3 K and covaries with the central Pacific (r = −0.63 with Niño-4 index for 1975–2010). In this study, the generation mechanisms of decadal SST variability are explored using an ocean general circulation model (OGCM), and its impact on atmosphere is evaluated using an atmospheric general circulation model (AGCM). OGCM experiments reveal that Pacific forcing through the Indonesian Throughflow explains <20% of the total SST variability, and the contribution of local wind stress is also small. These wind-forced anomalies mainly occur near the Western Australian coast. The majority of SST variability is attributed to surface heat fluxes. The reduced upward turbulent heat flux (QT; latent plus sensible heat flux), owing to decreased wind speed and anomalous warm, moist air advection, is essential for the growth of warm SST anomalies (SSTAs). The warming causes reduction of low cloud cover that increases surface shortwave radiation (SWR) and further promotes the warming. However, the resultant high SST, along with the increased wind speed in the offshore area, enhances the upward QT and begins to cool the ocean. Warm SSTAs co-occur with cyclonic low-level wind anomalies in the SEIO and enhanced rainfall over Indonesia and northwest Australia. AGCM experiments suggest that although the tropical Pacific SST has strong effects on the SEIO region through atmospheric teleconnection, the cyclonic winds and increased rainfall are mainly caused by the SEIO warming through local air–sea interactions.

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.

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