Re-emergence of North Atlantic subsurface ocean temperature anomalies in a seasonal forecast system (original) (raw)
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Quarterly Journal of The Royal Meteorological Society, 2003
Lead–lag Maximum Covariance Analysis (MCA) between National Centers for Environmental Prediction re-analysis sea surface temperature (SST) and 500 hPa geopotential-height fields shows that autumn tropical Atlantic SST anomalies are significantly linked with the following-winter North Atlantic Oscillation (NAO). The ability of the Météo-France atmospheric general circulation model ARPEGE to reproduce this relationship is tested, by forcing it with autumn tropical SST anomalies derived from lead–lag MCA analysis results. The autumn SST forcing induces a strong wave-like simultaneous response in October and November. The occurrence of the autumn weather regimes is also affected, in agreement with the significant spatial correlation of the midlatitude part of the wave response with the NAO pattern. By coupling the model with a slab ocean in midlatitudes, we show that the thermal coupling between the ocean and the atmosphere allows a better representation of the midlatitude part of the response. A negative autumn tropical SST anomaly triggers an interaction between the midlatitude SST, the low-frequency circulation and the storm-track activity, which reinforces and maintains a positive phase of the NAO until winter. Copyright © 2003 Royal Meteorological Society
Persistent atmospheric and oceanic anomalies in the North Atlantic from summer 2009 to summer 2010
2011
In this work, the authors analyze the air-sea interaction processes associated with the persistent atmospheric and oceanic anomalies in the North Atlantic Ocean during summer 2009-summer 2010 with a recordbreaking positive sea surface temperature anomaly (SSTA) in the hurricane Main Development Region (MDR) in the spring and summer of 2010. Contributions to the anomalies from the El Niñ o-Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and a long-term trend are identified. The warming in the tropical North Atlantic during summer 2009-summer 2010 represented a typical response to ENSO, preconditioned and amplified by the influence of a strong and persistent negative phase of the NAO. The longterm trends enhanced the warming in the high and low latitudes and weakened the cooling in the midlatitudes. The persistent negative phase of the NAO was associated with active thermodynamic air-sea interaction in the North Atlantic basin. Surface wind anomalies associated with the NAO altered the ocean surface heat flux and changed the SSTA, which was likely further enhanced by the positive wind speed-evaporation-SST feedback. The total heat flux was dominated by the latent and sensible heat fluxes, while the shortwave radiation contributed to the tropical SSTA to a lesser degree. Sensitivity experiments with an atmospheric general circulation model forced by observed SST in the Atlantic Ocean alone suggested that the Atlantic SSTA, which was partly forced by the NAO, had some positive contribution to the persistence of the negative phase of the NAO. Therefore, the persistent NAO condition is partly an outcome of the global climate anomalies and the ocean-atmosphere feedback within the Atlantic basin. The combination of the ENSO, NAO, and long-term trend resulted in the record-breaking positive SSTA in the MDR in the boreal spring and summer of 2010. On the basis of the statistical relationship, the SSTA pattern in the North Atlantic was reasonably well predicted by using the preceding ENSO and NAO as predictors.
Atmosphere–Ocean Interaction in the North Atlantic: Near-Surface Climate Variability
Journal of Climate, 1998
The impact of an interactive ocean on the midlatitude atmosphere is examined using a 31-yr integration of a variable depth mixed layer ocean model of the North Atlantic (between 20Њ and 60ЊN) coupled to the NCAR Community Climate model (CCM1). Coupled model results are compared with a 31-yr control simulation where the annual cycle of sea surface temperatures is prescribed. The analysis focuses on the northern fall and winter months.
The atmospheric response to North Atlantic SST anomalies in seasonal prediction experiments
Tellus A, 2003
Seasonal forecasts performed over a 26 yr period as part of the Historical Seasonal Forecasting Project (HFP) are used to analyze the influence of North Atlantic sea surface temperature (SST) anomalies on the atmospheric circulation, its seasonality, and model dependence. The signals related to the El Niño events are first removed from both the SST and the atmospheric data. The North Atlantic SST and the ensemble mean forecast are then correlated over the 26 yr to identify the model response to the SST forcing. The signal-to-noise ratio shows that in spring there is a significant forecast signal that is related to the SST anomaly in the North Atlantic. In that season the two models used in the HFP yield responses to the SST anomaly that are both similar to each other and to the observed response. For the other seasons the agreement between the responses and the observed atmospheric anomalies is poor. In winter the response is very sensitive to the model used.
Winter-to-winter recurrence of sea surface temperature, salinity and mixed layer depth anomalies
Progress in Oceanography, 2001
The mean seasonal cycle of mixed layer depth (MLD) in the extratropical oceans has the potential to influence temperature, salinity and mixed layer depth anomalies from one winter to the next. Temperature and salinity anomalies that form at the surface and spread throughout the deep winter mixed layer are sequestered beneath the mixed layer when it shoals in spring and are then re-entrained into the surface layer in the subsequent fall and winter. Here we document this "reemergence mechanism" in the North Pacific Ocean using observed SSTs, subsurface temperature fields from a data assimilation system, and coupled atmosphere-ocean model simulations. Observations indicate that the dominant large-scale SST anomaly pattern that forms in the North Pacific during winter recurs in the following winter. The model simulation with mixed layer ocean physics reproduced the winter-to-winter recurrence, while model simulations with observed SSTs specified in the tropical Pacific and a 50 m slab in the North Pacific did not. This difference between the model results indicate that the winter-to-winter SST correlations are due to the reemergence mechanism and not by similar atmospheric forcing of the ocean in consecutive winters and that SST anomalies in the tropical Pacific associated with El Niño are not essential for reemergence to occur.
Understanding the Persistence of Sea Surface Temperature Anomalies in Midlatitudes
Journal of Climate, 2003
An extension of the simple stochastic climate model of Frankignoul and Hasselman that includes the effects of seasonal variations in upper-ocean mixed layer depth upon the persistence of winter sea surface temperature (SST) anomalies is proposed. Seasonal variations in mixed layer depth allow for the ''reemergence mechanism,'' whereby thermal anomalies stored in the deep winter mixed layer persist at depth through summer and become partially reentrained into the mixed layer during the following winter. In this way, SST anomalies can recur from winter to winter without persisting through the intervening summer. Reformulating the simple stochastic climate model in terms of an effective ocean thermal capacity given by the depth of the winter mixed layer, thereby implicitly taking into account reemergence, is shown to provide a favorable fit to the observed winterto-winter SST autocorrelations in the North Atlantic and Pacific, and represents a considerable improvement over the original model. The extended model also compares favorably with results from an entraining bulk ocean mixed layer model coupled to an atmospheric general circulation model. The authors propose that the extended model be adopted as the new ''null hypothesis'' for interannual SST variability in middle and high latitudes.
Sea-Surface Temperature Anomaly Generation in Relation to Atmospheric Storms
Bulletin of the American Meteorological Society, 1978
An extended period of reduced surface heat and momentum fluxes due to the absence of atmospheric storms may result in upper-ocean temperature anomalies that persist for months. The predominance of either anomalously high or low temperatures is related to the ocean thermal structure that is established on the transition date between the winter and summer regimes.
Summer interactions between weather regimes and surface ocean in the North-Atlantic region
2010
This study aims at understanding the summer ocean-atmosphere interactions in the North Atlantic European region on intraseasonal timescales. The CNRMOM1d ocean model is forced with ERA40 (ECMWF Re-Analysis) surface fluxes with a 1-h frequency in solar heat flux (6 h for the other forcing fields) over the 1959-2001 period. The model has 124 vertical levels with a vertical resolution of 1 m near the surface and 500 m at the bottom. This ocean forced experiment is used to assess the impact of the North Atlantic weather regimes on the surface ocean. Composites of sea surface temperature (SST) anomalies associated with each weather regime are computed and the mechanisms explaining these anomalies are investigated. Then, the SST anomalies related to each weather regime in the oceanforced experiment are prescribed to the ARPEGE Atmosphere General Circulation Model. We show that the interaction with the surface ocean induces a positive feedback on the persistence of the Blocking regime, a negative feedback on the persistence of the NAO-regime and favours the transition from the Atlantic Ridge regime to the NAO-regime and from the Atlantic Low regime toward the Blocking regime.