El Nino

• Some of the major properties of a warm event during the Northern Hemisphere winter include: 

1. Abnormally warm tropical Pacific sea surface temperatures (SSTs) extend far to the east, all the way to Peru by early to mid-winter. 

2. The East Asian jet stream, which normally weakens in the mid-Pacific, extends far eastward during El Nino, often merging with an enhanced sub-tropical jet and extending across Mexico, the Gulf of Mexico and the far southeastern U.S. by mid-winter. 

3. The climatological stationary ridge, normally situated along the western edge of North America, weakens and shifts slightly eastward, while the climatological trough in eastern Canada also weakens, by mid-winter. 

         These deviations from the climatological flow pattern cause upper level winds to be more westerly, on average, over the Nation. They also lead to a region of abnormal cyclonic vorticity over much of the southern U.S.. These conditions are associated with increased storminess across the southern U.S. and dry, relatively warm conditions in the north.

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Forecasts
 

Days 1 & 2 Forecasts

Days 3-5 Forecasts

Days 6-10 Forecasts

Days 8-14 Forecasts

Monthly Forecasts
 

• Integrated Soil Moisture Anomaly Forecasts - CPC
 An hydrologist's version of a bucket model has been integrated forward in time to obtain a physical measure of  the soil moisture (in mm) at 344 Climate Divisons in the US. The input to the soil water mass balance is the observed monthly precipitation (distributed equally over all time steps). The main loss is through evaporation, which is calculated from temperature by the Thornwhaite expression. Other sink terms are parameterized surface runoff and subsurface loss to groundwater. The five free parameters of this model were tuned to reproduce the observed runoff in some riverbasins in Oklahoma, and is applied as is everywhere else.An independent check on Illinois soil moisture measurements was encouraging. As a reference, see J. Huang ,  H. M. van den Dool and K. G. Georgakakos, 1996: Analysis of model-calculated soil moisture over the US (1931-1993) and applications to long range temperature forecasts. J Climate., 9, 1350-1362.

For background the CPC soil model was integrated starting in 1931, and is updated each month. The 1961-90 climatology is subtracted to form the anomalies shown. The current latest map is for the end of November 1997.

The forecasts for soil moisture anomlies at the end-of-April and June 1998 are the composite mean for the 10 past cases when DJF was a warm El Nino event. The units are standard deviation (X100). Clearly, the soil moisture anomaly integrates the precipitation and temperature anomalies associated with a warm event into a fairly smooth and large-scale field. The relationship between ENSO and soil moisture is not simultaneous, because soil moisture reflects past conditions. Contrary to the more noisy temperature and precip, soil moisture has reasonably high skill under cross validation through at least July of next summer. I.e., as of now we have a forecast with 7 month lead and a good guarantee for skill. Significant moisture shortage is thus to be expected at the beginnning of the growing season in the Ohio Valley. The dryness lingers and expands areally to cover much of the east by July. The SW emerges with a very wet soil from the El Nino winter, and this feature too persists into the summer. By August all memory is lost, and the soil climate resets itself. (HVD 12/23/97)

Seasonal Forecasts
 

• Official