Long-Range Predictability in the Tropics. Part II: 30–60-Day Variability

2005 ◽  
Vol 18 (5) ◽  
pp. 634-650 ◽  
Author(s):  
Thomas Reichler ◽  
John O. Roads
Keyword(s):  
Boundary Conditions ◽  
Long Range ◽  
General Circulation ◽  
Initial Conditions ◽  
Southern Oscillation ◽  
Intraseasonal Variability ◽  
Intraseasonal Oscillation ◽  
Circulation Model ◽  
Lead Times ◽  
Initial And Boundary Conditions

Abstract It is suggested that the slow evolution of the tropical Madden–Julian oscillation (MJO) has the potential to improve the predictability of tropical and extratropical circulation systems at lead times beyond 2 weeks. In practice, however, the MJO phenomenon is extremely difficult to predict because of the lack of good observations, problems with ocean forecasts, and well-known model deficiencies. In this study, the potential skill in forecasting tropical intraseasonal variability is investigated by eliminating all those errors. This is accomplished by conducting five ensemble predictability experiments with a complex general circulation model and by verifying them under the perfect model assumption. The experiments are forced with different combinations of initial and boundary conditions to explore their sensitivity to uncertainties in those conditions. When “perfect” initial and boundary conditions are provided, the model produces a realistic climatology and variability as compared to reanalysis, although the spectral peak of the simulated MJO is too broad. The effect of initial conditions is noticeable out to about 40 days. The quality of the boundary conditions is crucial at all lead times. The small but positive correlations at very long lead times are related to intraseasonal variability of tropical sea surface temperatures (SSTs). When model, initial, and boundary conditions are all perfect, the useful forecast skill of intraseasonal variability is about 4 weeks. Predictability is insensitive to the El Niño–Southern Oscillation (ENSO) phenomenon, but it is enhanced during years when the intraseasonal oscillation is more active. The results provide evidence that the MJO must be understood as a coupled system. As a consequence, it is concluded that further progress in the long-range predictability effort may require the use of fully interactive ocean models.

Prediction of Indian Summer Monsoon Onset Using Dynamical Subseasonal Forecasts: Effects of Realistic Initialization of the Atmosphere

2015 ◽  
Vol 143 (3) ◽  
pp. 778-793 ◽  
Author(s):  
Andrea Alessandri ◽  
Andrea Borrelli ◽  
Annalisa Cherchi ◽  
Stefano Materia ◽  
Antonio Navarra ◽  
...  
Keyword(s):  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
General Circulation ◽  
Large Scale ◽  
Initial Conditions ◽  
Intraseasonal Oscillation ◽  
Circulation Model ◽  
Tropical Indian Ocean ◽  
Monsoon Onset ◽  
Mean State

Abstract Ensembles of retrospective 2-month dynamical forecasts initiated on 1 May are used to predict the onset of the Indian summer monsoon (ISM) for the period 1989–2005. The subseasonal predictions (SSPs) are based on a coupled general circulation model and recently they have been upgraded by the realistic initialization of the atmosphere with initial conditions taken from reanalysis. Two objective large-scale methods based on dynamical-circulation and hydrological indices are applied to detect the ISM onset. The SSPs show some skill in forecasting earlier-than-normal ISM onsets, while they have difficulty in predicting late onsets. It is shown that significant contribution to the skill in forecasting early ISM onsets comes from the newly developed initialization of the atmosphere from reanalysis. On one hand, atmospheric initialization produces a better representation of the atmospheric mean state in the initial conditions, leading to a systematically improved monsoon onset sequence. On the other hand, the initialization of the atmosphere allows some skill in forecasting the northward-propagating intraseasonal wind and precipitation anomalies over the tropical Indian Ocean. The northward-propagating intraseasonal modes trigger the monsoon in some early-onset years. The realistic phase initialization of these modes improves the forecasts of the associated earlier-than-normal monsoon onsets. The prediction of late onsets is not noticeably improved by the initialization of the atmosphere. It is suggested that late onsets of the monsoon are too far away from the start date of the forecasts to conserve enough memory of the intraseasonal oscillation (ISO) anomalies and of the improved representation of the mean state in the initial conditions.

Long-Range Predictability in the Tropics. Part I: Monthly Averages

2005 ◽  
Vol 18 (5) ◽  
pp. 619-633 ◽  
Author(s):  
Thomas Reichler ◽  
John O. Roads
Keyword(s):  
Boundary Conditions ◽  
Long Range ◽  
Large Scale ◽  
Initial Conditions ◽  
Southern Oscillation ◽  
Western Hemisphere ◽  
Eastern Hemisphere ◽  
Condition Effect ◽  
Temporal And Spatial Characteristics ◽  
Hemisphere Winter

Abstract The sensitivity to initial and boundary conditions of monthly mean tropical long-range forecasts (1–14 weeks) during Northern Hemisphere winter is studied with a numerical model. Five predictability experiments with different combinations of initial conditions and prescribed ocean boundary conditions are conducted in order to investigate the temporal and spatial characteristics of the perfect model forecast skill. It is shown that initial conditions dominate a tropical forecast during the first 3 weeks and that they influence a forecast for at least 8 weeks. The initial condition effect is strongest over the Eastern Hemisphere and during years when the El Niño–Southern Oscillation (ENSO) phenomenon is weak. The relatively long sensitivity to initial conditions is related to a complex combination of dynamic and thermodynamic effects, and to positive internal feedbacks of large-scale convective anomalies. At lead times of more than 3 weeks, boundary forcing is the main contributor to tropical predictability. This effect is particularly strong over the Western Hemisphere and during ENSO. Using persisted instead of observed sea surface temperatures leads to useful forecast results only over the Western Hemisphere and during ENSO.

scholarly journals Reproducibility of Seasonal Land Surface Climate

2006 ◽  
Vol 7 (1) ◽  
pp. 114-136 ◽  
Author(s):  
Thomas J. Phillips
Keyword(s):  
Land Surface ◽  
Seasonal Cycle ◽  
General Circulation ◽  
Climate Model ◽  
Initial Conditions ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Seasonal Prediction ◽  
Climate Simulation ◽  
Continental Climate

Abstract In this study, the sensitivity of the continental seasonal climate to initial conditions is estimated from an ensemble of decadal simulations of an atmospheric general circulation model with the same specifications of radiative forcings and monthly ocean boundary conditions, but with different initial states of atmosphere and land. As measures of the “reproducibility” of continental climate for different initial conditions, spatiotemporal correlations are computed across paired realizations of 11 model land surface variables in which the seasonal cycle is either included or excluded—the former case being pertinent to climate simulation and the latter to seasonal prediction. It is found that the land surface variables that include the seasonal cycle are impacted only marginally by changes in initial conditions; moreover, their seasonal climatologies exhibit high spatial reproducibility. In contrast, the reproducibility of a seasonal land surface anomaly is generally low, although it is substantially higher in the Tropics; its spatial reproducibility also markedly fluctuates in tandem with warm and cold phases of the El Niño–Southern Oscillation. However, the overall degree of reproducibility depends on the particular land surface anomaly considered. It is also shown that the predictability of a land surface anomaly implied by its reproducibility statistics is consistent with what is inferred from more conventional predictability metrics. Implications of these results for climate model intercomparison projects and for operational forecasts of seasonal continental climate also are elaborated.

scholarly journals Comparative Evaluation of Two Ensembles for Long-Range Forecasting of Monsoon Rainfall

2009 ◽  
Vol 137 (9) ◽  
pp. 2893-2907 ◽  
Author(s):  
P. Goswami ◽  
K. C. Gouda
Keyword(s):  
Long Range ◽  
Time Scales ◽  
Summer Monsoon ◽  
Indian Summer Monsoon ◽  
General Circulation ◽  
Initial Conditions ◽  
Circulation Model ◽  
Ensemble Average ◽  
Ensemble Forecasting ◽  
Initial States

Abstract It is now well known that changes in initial conditions can give rise to substantial changes in the forecasts even at the long range. Ensemble averaging of forecasts from different initial conditions provides an efficient way of assessing and reducing uncertainties in the forecasts due to inherent uncertainties in the initial conditions. However, the procedure for generating the ensemble of forecasts has to be based on careful consideration. Although there now exist several well-tested frameworks for ensemble forecasting at the short range, the procedure for and impact of ensemble forecasting on long-range forecasting of monsoons remain relatively less explored. In particular, the procedure for the choice of the ensemble for long-range forecasting of monsoons needs special considerations. The Indian summer monsoon is characterized by a number of intraseasonal oscillations (ISOs) whose phases and amplitudes can significantly affect the monsoon forecast and that can be adequately sampled only using initial states spread over time scales comparable to the characteristic time scales of these ISOs. It is shown that use of initial states spread over a longer period (such as 1 April–1 May) results in a better ensemble average for long-range forecasting of Indian summer monsoon than that from an ensemble of closely packed states with a shorter lead. An optimized configuration for long-range forecasting of monsoons using a variable resolution general circulation model is adopted. The climatological monthly mean SST field is used to assess realizable skill, as use of observed SST provides only potential skill. Then five-member wide-lead (1 April–1 May) ensemble average forecasts are compared with five-member compact-lead (27 April–1 May) ensemble average forecasts for 24 (1980–2003) hindcasts; it is shown that the skill of the wide-lead ensemble average is superior to that of the compact-lead ensemble at different spatial and temporal scales in spite of the longer lead of the former.

scholarly journals The role of boundary and initial conditions for dynamical seasonal predictability

2003 ◽  
Vol 10 (3) ◽  
pp. 211-232 ◽  
Author(s):  
T. J. Reichler ◽  
J. O. Roads
Keyword(s):  
Boundary Conditions ◽  
Time Scales ◽  
General Circulation ◽  
Initial Conditions ◽  
Spatial Scales ◽  
Circulation Model ◽  
Forecast Skill ◽  
Initial State ◽  
Tropical Forcing ◽  
The Impact

Abstract. The importance of initial state and boundary forcing for atmospheric predictability is explored on global to regional spatial scales and on daily to seasonal time scales. A general circulation model is used to conduct predictability experiments with different combinations of initial and boundary conditions. The experiments are verified under perfect model assumptions as well as against observational data. From initial conditions alone, there is significant instantaneous forecast skill out to 2 months. Different initial conditions show different predictability using the same kind of boundary forcing. Even on seasonal time scales, using observed atmospheric initial conditions leads to a substantial increase in overall skill, especially during periods with weak tropical forcing. The impact of boundary forcing on predictability is detectable after 10 days and leads to measurable instantaneous forecast skill at very long lead times. Over the Northern Hemisphere, it takes roughly 4 weeks for boundary conditions to reach the same effect on predictability as initial conditions. During events with strong tropical forcing, these time scales are somewhat shorter. Over the Southern Hemisphere, there is a strongly enhanced influence of initial conditions during summer. We conclude that the long term memory of initial conditions is important for seasonal forecasting.

scholarly journals Sensitivity of the tropical climate to an interhemispheric thermal gradient: the role of tropical ocean dynamics

2018 ◽  
Vol 9 (1) ◽  
pp. 285-297 ◽  
Author(s):  
Stefanie Talento ◽  
Marcelo Barreiro
Keyword(s):  
Seasonal Cycle ◽  
General Circulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Ocean Dynamics ◽  
Thermocline Depth ◽  
Thermal Forcing ◽  
Tropical Ocean ◽  
Slab Ocean

Abstract. This study aims to determine the role of the tropical ocean dynamics in the response of the climate to extratropical thermal forcing. We analyse and compare the outcomes of coupling an atmospheric general circulation model (AGCM) with two ocean models of different complexity. In the first configuration the AGCM is coupled with a slab ocean model while in the second a reduced gravity ocean (RGO) model is additionally coupled in the tropical region. We find that the imposition of extratropical thermal forcing (warming in the Northern Hemisphere and cooling in the Southern Hemisphere with zero global mean) produces, in terms of annual means, a weaker response when the RGO is coupled, thus indicating that the tropical ocean dynamics oppose the incoming remote signal. On the other hand, while the slab ocean coupling does not produce significant changes to the equatorial Pacific sea surface temperature (SST) seasonal cycle, the RGO configuration generates strong warming in the central-eastern basin from April to August balanced by cooling during the rest of the year, strengthening the seasonal cycle in the eastern portion of the basin. We hypothesize that such changes are possible via the dynamical effect that zonal wind stress has on the thermocline depth. We also find that the imposed extratropical pattern affects El Niño–Southern Oscillation, weakening its amplitude and low-frequency behaviour.

scholarly journals Links between Tropical Pacific SST and Cholera Incidence in Bangladesh: Role of the Eastern and Central Tropical Pacific

2008 ◽  
Vol 21 (18) ◽  
pp. 4647-4663 ◽  
Author(s):  
Benjamin A. Cash ◽  
Xavier Rodó ◽  
James L. Kinter
Keyword(s):  
El Niño ◽  
General Circulation ◽  
Physical Mechanism ◽  
El Nino ◽  
Southern Oscillation ◽  
Regional Climate ◽  
Circulation Model ◽  
Tropical Pacific ◽  
Bacterial Disease ◽  
Monsoon Circulation

Abstract Recent studies arising from both statistical analysis and dynamical disease models indicate that there is a link between incidence of cholera, a paradigmatic waterborne bacterial disease (WBD) endemic to Bangladesh, and the El Niño–Southern Oscillation (ENSO). However, a physical mechanism explaining this relationship has not yet been established. A regionally coupled, or “pacemaker,” configuration of the Center for Ocean–Land–Atmosphere Studies atmospheric general circulation model is used to investigate links between sea surface temperature in the central and eastern tropical Pacific and the regional climate of Bangladesh. It is found that enhanced precipitation tends to follow winter El Niño events in both the model and observations, providing a plausible physical mechanism by which ENSO could influence cholera in Bangladesh. The enhanced precipitation in the model arises from a modification of the summer monsoon circulation over India and Bangladesh. Westerly wind anomalies over land to the west of Bangladesh lead to increased convergence in the zonal wind field and hence increased moisture convergence and rainfall. This change in circulation results from the tropics-wide warming in the model following a winter El Niño event. These results suggest that improved forecasting of cholera incidence may be possible through the use of climate predictions.

scholarly journals Sensitivity of the tropical climate to an interhemispheric thermal gradient: the role of tropical ocean dynamics

2017 ◽  
Author(s):  
Stefanie Talento ◽  
Marcelo Barreiro
Keyword(s):  
Seasonal Cycle ◽  
General Circulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Ocean Dynamics ◽  
Thermocline Depth ◽  
Thermal Forcing ◽  
Tropical Ocean ◽  
Slab Ocean

Abstract. This study aims to determine the role of the tropical ocean dynamics in the response of the climate to an extratropical thermal forcing. We analyse and compare the outcomes of coupling an atmospheric general circulation model (AGCM) with two ocean models of different complexity. In the first configuration the AGCM is coupled with a slab ocean model while in the second a Reduced Gravity Ocean (RGO) model is additionally coupled in the tropical region. We find that the imposition of an extratropical thermal forcing (warming in the Northern Hemisphere and cooling in the Southern Hemisphere with zero global mean) produces, in terms of annual means, a weaker response when the RGO is coupled, thus indicating that the tropical ocean dynamics opposes the incoming remote signal. On the other hand, while the slab ocean coupling does not produce significant changes to the equatorial Pacific sea surface temperature (SST) seasonal cycle, the RGO configuration generates a strong warming in the centre-east of the basin from April to August balanced by a cooling during the rest of the year, strengthening the seasonal cycle in the eastern portion of the basin. We hypothesize that such changes are possible via the dynamical effect that zonal wind stress has on the thermocline depth. We also find that the imposed extratropical pattern affects El Niño Southern Oscillation, weakening its amplitude and low-frequency behaviour.

scholarly journals On solution of one linear problem with initial and boundary conditions

2021 ◽  
pp. 121
Author(s):  
S.S. Kritskaia
Keyword(s):  
Boundary Conditions ◽  
Boundary Problem ◽  
Fourth Order ◽  
Linear Problem ◽  
Initial Conditions ◽  
Initial And Boundary Conditions

We solve one boundary problem of fourth order with initial conditions, that appears, for example, when one solves the problem about lateral oscillations of elastic-viscous-relaxating rod of variable profile with variable momentum of inertia with freely supported ends.

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