Sensitivity of January Climate Response to the Magnitude and Position of Equatorial Pacific Sea Surface Temperature Anomalies

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  • 1 Department of Meteorology, University of Utah, Salt Lake City, UT 84112
  • | 2 National Center for Atmospheric Research, Boulder, CO 80307
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Abstract

We present here the results of two categories of equatorial Pacific sea surface temperature (SST) anomaly experiments using the NCAR Community Climate Model in the perpetual-January mode. Each experiment consists of an ensemble of eight independent realizations of 90 days duration. Anomaly fields are derived by subtracting the ensemble-average fields in the control experiment from the ensemble-average fields in an experiment with an SST anomaly present.

In the first category of experiments (1W, 2W, 3W) we examine the response in the tropics and midlatitudes to a change in the amplitude of an SST anomaly of fixed shape and position. This SST anomaly produces an equatorial Pacific precipitation anomaly just east of the dateline. We show that the most significant feature of the midiatitude response is a feature resembling the teleconnection pattern known as the Pacific/North American (PNA) pattern. The amplitude of this response becomes insensitive to an increase in the tropical precipitation anomaly beyond the value in the 2W experiment.

In the second category of experiments (2W, EPAC, Far EPAC) we investigate the sensitivity of the response to positioning of the SST anomaly at longitudes progressively farther cast of the dateline. The midlatitude response seen in the ensemble-average anomaly maps is a geographically-fixed PNA pattern whose amplitude and statistical significance is smaller the farther east the SST anomaly. We also examine the midiatitude response for each 90-day realization and find that the probability of exciting the PNA pattern decreases as the SST anomaly is moved eastward.

The insensitivity of the location of the model PNA pattern to the longitude of anomalous tropical heating supports the conclusion of Simmons and collaborators that the PNA pattern is related to an unstable mode of the atmosphere deriving energy from the zonally asymmetric climatological mean flow. The decline of amplitude of the model ensemble-mean PNA pattern with eastward positioning of the SST anomaly is consistent with the findings of Simmons and collaborators and Palmer and Mansfield that the western part of the tropical Pacific is a more effective region than the eastern part for excitation of this type of midlatitude response.

Abstract

We present here the results of two categories of equatorial Pacific sea surface temperature (SST) anomaly experiments using the NCAR Community Climate Model in the perpetual-January mode. Each experiment consists of an ensemble of eight independent realizations of 90 days duration. Anomaly fields are derived by subtracting the ensemble-average fields in the control experiment from the ensemble-average fields in an experiment with an SST anomaly present.

In the first category of experiments (1W, 2W, 3W) we examine the response in the tropics and midlatitudes to a change in the amplitude of an SST anomaly of fixed shape and position. This SST anomaly produces an equatorial Pacific precipitation anomaly just east of the dateline. We show that the most significant feature of the midiatitude response is a feature resembling the teleconnection pattern known as the Pacific/North American (PNA) pattern. The amplitude of this response becomes insensitive to an increase in the tropical precipitation anomaly beyond the value in the 2W experiment.

In the second category of experiments (2W, EPAC, Far EPAC) we investigate the sensitivity of the response to positioning of the SST anomaly at longitudes progressively farther cast of the dateline. The midlatitude response seen in the ensemble-average anomaly maps is a geographically-fixed PNA pattern whose amplitude and statistical significance is smaller the farther east the SST anomaly. We also examine the midiatitude response for each 90-day realization and find that the probability of exciting the PNA pattern decreases as the SST anomaly is moved eastward.

The insensitivity of the location of the model PNA pattern to the longitude of anomalous tropical heating supports the conclusion of Simmons and collaborators that the PNA pattern is related to an unstable mode of the atmosphere deriving energy from the zonally asymmetric climatological mean flow. The decline of amplitude of the model ensemble-mean PNA pattern with eastward positioning of the SST anomaly is consistent with the findings of Simmons and collaborators and Palmer and Mansfield that the western part of the tropical Pacific is a more effective region than the eastern part for excitation of this type of midlatitude response.

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