Linear Statistical Short-Term Climate Predictive Skill in the Northern Hemisphere

Abstract

In this study, the sources and strengths of statistical short-term climate predictability for local surface climate (temperature and precipitation) and 700-mb geopotential height in the Northern Hemisphere are explored at all times of the year at lead times of up to one year. Canonical correlation analysis is the linear statistical methodology employed. Predictor and predictand averaging periods of 1 and 3 months are used, with four consecutive predictor periods, followed by a lead time and then a single predictand period. Predictor fields are quasi-global sea surface temperature (SST), Northern Hemisphere 700-mb height, and prior values of the predictand field itself. Cross-validation is used to obtain, to first order, uninflated skill estimates. Results reveal mainly modest statistical predictive skill except for certain fields, locations, and times of the year when predictability is far above chance expectation and good enough to be beneficial to appropriate users. The time of year when skills are generally highest is January through April. Global SST is the most skill-producing predictor field, perhaps because 1) the lower boundary condition is a more consistent influence on climate on timescales of 1 to 3 months than the atmosphere's internal dynamics, or 2) SST is the only field in this study that provides tropical information directly. Prediction is generally more skillful on the 3-month than 1-month timesale. The skill of the forecasts is often insensitive to the forecast lead time; that is, inserting 3, or sometimes 6 or more, months between the predictor and predictand periods causes little skill decrease from that of 1 month or less. This has favorable implications for long-lead forecasting. Much of the higher skill occurs in association with fluctuations of the El Niño/Southern Oscillation (ENSO) and is found in midwinter through midspring in specific pockets of the Pacific and North American regions. Predictive skill for precipitation is also found in the same context but is lower than that for 700-mb height or temperature. Warm season predictability, slightly lower than that of winter-spring and not clearly documented in earlier work, is related to episodes of like-signed SST anomalies in the tropical oceans throughout the world in the preceding months. There is an interdecadal component in the variability of these global SST conditions. Generalized positive (negative) 700-mb and surface temperature anomalies in middle to late summer (but fall in southern Europe), generally at subtropical latitudes throughout much of the Northern Hemisphere (but with some midlatitude continental protrusions), occur following episodes of uniformly positive (negative) SST anomalies in the tropical oceans throughout the world in the preceding winter through late spring. The occurrence of a mature warm (cold) ENSO extreme the previous winter may contribute to such a worldwide SST condition in the intervening spring season. In the United States, the effect is a general (monopole) anomalous warmth (coolness) from mid-July through August across much of the country.