Seasonal Prediction of North Atlantic Accumulated Cyclone Energy and Major Hurricane Activity

Building upon our previous seasonal hurricane prediction model, here we develop two statistical models to predict the number of major hurricanes (MHs) and accumulated cyclone energy (ACE) in the North Atlantic basin using monthly data from March to May for an early June forecast. The input data include zonal pseudo–wind stress to the 3/2 power, sea surface temperature in the North Atlantic, and, depending on the magnitude of the Atlantic multidecadal oscillation index, the multivariate ENSO index. From 1968 to 2017, these models have a mean absolute error of 0.96 storms for MHs and 30 units for ACE. When tested over an independent period from 1958 to 1967, the models show a 22% improvement for MHs and 16% for ACE over a no-skill metric based on a 5-yr running average. Both the MH and ACE results show consistent improvements over those produced by three other centers using statistical–dynamical hybrid models and a 5-yr running average prediction over the period 2000–17 for MHs (2003–17 for ACE) in a simulated real-time prediction. These improvements vary from 25% to 37% for MHs and from 15% to 37% for ACE. While most forecasting centers called for a slightly above-average hurricane season in May/June 2017, our models predicted in June 2017 a very active season, in much better agreement with observations.

Nearly Synchronous Multidecadal Oscillations of Surface Air Temperature in Punta Arenas and the Atlantic Multidecadal Oscillation Index

The Atlantic multidecadal oscillation (AMO) signature in southern South America (SA) is examined using the surface air temperature (T-air) of Punta Arenas, Chile (53.0°S, 70.85°W), during the 1888–2016 period. The T-air shows multidecadal oscillations with a significant positive correlation of 0.77 to the AMO index. The relations of the Punta Arenas T-air time series with the AMO-related global sea surface temperature (SST) and regional circulation anomaly patterns are discussed. During the warm (cold) AMO phase, a cold (warm) center in southwestern Atlantic waters induces low-level anticyclonic (cyclonic) anomalies in the region, which together with the cyclonic (anticyclonic) anomalies in the southeastern Pacific channel the northerly (southerly) flow over southern SA. This meridional flow transports warm (cold) air from lower (higher) latitudes into the Punta Arenas region. Therefore, the temperature horizontal advection at the low level is the main thermodynamic process that alters the Punta Arenas T-air in a multidecadal time scale. The use of a relation between a long T-air surface sensor series in southern SA with the AMO presents a novel approach in climate monitoring and modeling.

Relationship between the Atlantic Multidecadal Oscillation index and variability of mean annual flows for catchments in the St. Lawrence watershed (Quebec, Canada) during the past century

This paper, based on an analysis of the mean annual flow (MAF) of 16 natural rivers over the period 1934–2000, has the following goals: (1) to determine the different temporal variability modes of the MAF in the St. Lawrence watershed, (2) to describe the temporal variability of the streamflow in each mode and (3) to analyze the influence of the AMO (Atlantic Multidecadal Oscillation) on the interannual and interdecadal variability of mean annual flows in this Quebec watershed. This paper shows that the interannual variability of mean annual flow was not synchronous on both sides of the river. During the period analyzed, MAF variability was characterized by a tendency to decrease on the south shore (right bank) but to increase on the north shore (left bank). A correlation analysis reveals that the influence of the AMO was limited exclusively to the north shore of the river, which is characterized by a continental climate. This correlation is negative. On the south shore, streamflow is correlated negatively to the AO (Arctic Oscillation).