Variability of the Tropical Winter Climate

The long-term monthly and seasonal prediction of the behavior of the winter meteorological systems (the Cold Fronts, the Extratropical Cyclones) and of the circulation that they determine in the atmosphere is of great scientific, social and economic interest, given the influence that the variables associated with them have in the development of many socio-economic activities, being able to alter their rhythm and influence catastrophically in a determined territory. It is assumed that the imposition of strong zonal variations in atmospheric flow on a synoptic scale modifies the behavior of these systems. This forcing, which can be said to determine anomalous behavior of the winter circulation, is caused by the influence of the factors that regulate climate variability, which are the circulatory patterns that come from the coupling of the low stratosphere with the tropospheric column, as they are: the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO), or the interaction between them; and / or with the event El Niño / Southern Oscillation (ENSO). Therefore, over the study area, which ranges from 10º N to 50º N and from 110º W to 50º W, the behavior of the cold fronts (CF) and the Extratropical Cyclones (EC) is analyzed under the joint forcing of these main modes of climatic variability, with the aim of determining the predictability of seasonal anomalies in the winter period of the year in the tropics. A subject that is totally useful nowadays, for the understanding, mitigation and adaptation to Climate Change.In this way, the quarters from October to December and from January to March that correspond to the winter seasons from 1950 to 2018, are classified according to the frequency of affectation of the CF and the EC, which can be normal, high or low, according to the threshold determined in this study. Using the Pearson and Spearman correlation tests, and the Principal Component Analysis (PCA), it was found which oscillations determine these behaviors, and the level of influence of each one in the presence of the atmospheric variability imposed by the others. Therefore, the factors which regulate the seasonal variability of these winter systems were obtaind. All the foregoing was done using the chronological basis of the CF and EC that covers from 1950 to the present, and the characterizations used by the Institute of Meteorology of Cuba (INSMET) for the six types of cold fronts (taking into account the change in the direction and intensity of the winds in its path) and the Extratropical Cyclones (due to its evolutionary zone). In representation of the oscillations studied, the monthly series of the indices of each of them were used: the Oceanic Niño Index (ONI), the NAO Index, and the Arctic Oscillation Index (AOi); extracted from the database of the US Climate Prediction Center.
It is noted, among the obtained results, that the combined influence of the positive phase of the ENSO and negative of the NAO in both quarters is greater on the Extratropical Cyclones in the evolutionary zone of the Gulf of Mexico (those that develop in this area, including those that are formed on the coastal line and its subsequent development occurs on the waters of the Gulf) that in the continental (its origin and evolution is on any part of the continent, also include those that enter the area from the western United States) .
On the classic cold fronts (cold fronts associated with extratropical lows, in which the wind changes counterclockwise) the influence of the NAO and the AO is greater than that of the ENSO. For the cold fronts "revesinos" (turn of the wind is in clockwise sense) and the secondary cold fronts there are no significant correlations of any of the oscillations studied. About the moderate cold fronts (they produce winds between 33 and 55 km/h), it was obtained that it is the AO the forcing factor that has a significant influence on its behavior, which always correlates significantly with the NAO. In all cases, these correlations were obtained with a high significance value.
In the particular case of Extratropical Cyclones, the synoptic tropospheric patterns associated with their affectation are determined, as well as their variation before the main sources of interannual and multiannual variability in the tropics and extratropics. From these patterns are obtained the significant elements that are directly related to a greater incidence of Extratropical Cyclones on their evolutionary zones, a condition that for the continental and Gulf of Mexico evolutionary zone, it is determined, to a greater extent, by the phase negative of the NAO.