Climate/Weather Connection
The Problem
The jet stream over the Pacific shifts suddenly. A series of powerful oceanic storms are now lined up to hit the West Coast, while a severe drought plagues the Midwest and recurrent severe storms and tornadoes threaten Florida. To what extent are the shifts in the jet stream and the unusual weather conditions related, and to what extent are they predictable based on processes observed in the tropics? Do some of these weather patterns that often occur on roughly 1-to-4 week timescales within a season produce intrinsically more (or less) predictable weather conditions than others? The answers to such questions are critical to many decision-makers, including those involved in water resource allocation, coastal zone management, air quality management, hazard mitigation, business planning, and agriculture.
"The [El Niño] winter season of 1997-1998 was one of the wettest
on record across California, with typically double the normal amount
of rainfall and about $550 million in flood and storm damage statewide.
However, the previous season [Figure 1], with only normal rainfall, had
over triple the damage ($1.8 billion). This discrepancy appears to be
best explained by timing of storms within the season and not the total
seasonal rainfall.
San Francisco State University.
 |
Figure 1. State-averaged precipitation for the winter of 1996-1997. (after -- K.C. Mo, Mon. Wea. Rev., 1999, 2759-2776) |
This Initiative will help answer these questions through focused efforts on observing, diagnosing, and modeling the connection between tropical variability on multi-week time scales, and the resulting shifts in the tracks of tropical and extratropical cyclones.
Background and Motivation
With its predictions of a major El Niño event during 1997-98, beginning in late summer 1997, the NOAA Climate Prediction Center also forecast the likelihood of an unusually wet winter in California. This long-lead seasonal forecast enabled California's emergency managers to prepare well in advance for the possibility of unusually severe winter storms. When the first half of the winter proved unusually dry, there was considerable confusion and skepticism in the public and decision-makers regarding NOAA's winter-season forecast. However, beginning in late January and through much of February, a nearly continuous series of violent storms slammed into the West Coast, confirming NOAA's new seasonal forecast capabilities. |
Figure 2. 300-hPa data (CDC) for the period February 2-6, 1998 showing the development of deep tropical convection and the subsequent intensification of the jet stream to the north. |
| Cumulative precipitation (Shasta Dam) for the winter of 1997-1998 showing enhanced precipitation in early February. |
Research subsequent to the 1997-98 El Niño event has provided compelling evidence that variations in tropical rainfall and winds over the tropical Pacific within the winter season first delayed the El Nino rains in California and then produced nearly ideal conditions for their production [Figure 2].
This research leads to the central hypothesis motivating this Initiative: that more skillful and useful weather and climate forecasts can be produced through improved capabilities to observe, understand, and model subseasonal tropical-extratropical variability, as well as the effects of other major non-ENSO climate variations affecting mid-latitudes.
However, despite the effects of tropical sub-seasonal variability on weather and climate predictions, with their concurrent impact on society and the economy, operational forecast models do not simulate these week-to-week tropical fluctuations well, if at all. It is likely that both atmospheric and oceanic conditions are important in determining the evolution of these events. Currently, forecast models typically do not allow for the two-way ocean-atmosphere feedbacks that can substantially influence key sub-seasonal modes of tropical variability such as the Madden-Julian Oscillation (which has a 30-to-60 day time scale) and shorter period forcings, and can energize mid-latitude storms.
These observing and modeling problems pose fundamental challenges to NOAA in achieving its goal of improved weather and climate forecasts on weekly to seasonal time scales. Unless NOAA now substantially expands its observational, research, and prediction efforts to understand and exploit the predictive value of the critical links between sub-seasonal tropical and extratropical weather variability, it will be unable to improve its services to the nation in these high-impact areas where it is only now beginning to establish credibility.
Approach
Intensive observational, diagnostic, and modeling studies will be performed to identify and realize the full forecast utility of key physical processes that occur with tropical-extratropical interactions, and that are not currently well-represented in operational prediction systems. These include:An intensive observational effort utilizing both in-situ and satellite measurements to
- Document the structure of variations in tropical rainfall on weekly to monthly timescales,
- Provide intensive observations of interactions with the Pacific storm track and its subsequent regional impacts across the U.S.,
- Document coupled ocean-atmosphere interactions affecting both tropical systems and mid-latitude oceanic and land-falling storms, and
An expanded diagnostic and modeling effort to:
- Understand the relationship between sub-seasonal tropical variability and changes in the frequency, location and intensity of tropical and extratropical cyclones and monsoonal flow affecting the U.S.
- Improve the simulation of key processes (such as the tropical sub-seasonal variability and air-sea feedbacks) to realize their full potential for improving local forecasts of extreme weather events.
These efforts will build on experiments such as CALJET (1997-98), which established that such data could improve flood warnings issued by the NWS, and PACJET (2001-2003) that will follow storms from the ocean into regional watersheds. In addition the program will build on the results from the central Pacific now emerging from NORPEX (1998) that provide further evidence of the linkages between tropical and extratropical variability. Follow-on experiments such as THORPEX in the 2003- 2006 time and the additional observations available from an enhanced ocean observing system and the underlying research carried out by programs such as CLIVAR can also play in critical role in these studies of subseasonal variability.