# Presentation: 228027

## Long-lasting locally coupled anomalies

- M. Peña1, E. Kalnay1 and M. Cai2
- 1University of Maryland; 2 Florida State University

- EMC Seminar Thurs. 29 Jan 2004

## Content

- Phase relationship of anomalies (e.g., “cyclonic over warm”) and forcing direction.
- Preferential phase relationship of long-lasting Ocean-Atmos and Land-Atmos coupled anomalies.
- Consequences of assuming “ocean-driving” (AMIP) scenarios to the simulation and prediction of coupled anomalies.

## Definitions

- _Anomaly_: departure from the annual cycle that exceeds a threshold. The threshold used is one fourth of the local standard deviation.
- _Duration_ of an anomaly: time interval that the variable continuously exceeded the threshold.
- _Locally Coupled_ anomalies: coincide spatially and temporally in the two media.

## Anomalies over the ocean

- Zonally averaged number of anomalies

**colors denote the number of cases**

**Anomalies over the ocean**

## Coupled anomalies

**Coupled anomalies**

**colors denote the number of cases**

## Simplified Mo and Kalnay’ Dynamical Rule

**Diagnosing the driving direction**

## Driving direction

**Driving direction**

**NCEP Reanalysis**

**%**

**%**

- Atmosphere-driving anomaly cases

- Ocean-driving anomaly cases

**Anomalies lasting at least 15 days**

**Atmosphere-driving cases dominate the extratropics!**

## Driving direction

**Driving direction**

**ECMWF Reanalysis (14 years)**

**%**

- Atmosphere-driving anomaly cases

- Ocean-driving anomaly cases

**%**

**Anomalies lasting at least 15 days**

## Correlation SST and 850 hPa CV

**Extratropics:**
**Atmos leads Ocean**

## Leading mode of covariability

- _[Slide 10](slide10.xml)_

- North Pacific

- 500 hPa
- height

- SST

**Deser and Timlin 1997**

**40****o****N**

**Leading mode of covariability**

## North Pacific Basin-wide modes of covariability

**North Pacific Basin-wide modes of covariability**

**Atmos-driving**
**Phase relationship**

## Diagnostic rule

**Diagnostic rule**

- The geographic distribution of the forcing direction obtained from the diagnostic rule is:
- Independent on the reanalysis data set used
- Very similar in the daily, five-day average, and monthly data (no shown).
- Consistent with Lag-lead correlation technique
- Consistent with basin-wide modes of variability
**Knowing the simultaneous phase relationship in a given time we can estimate the local forcing direction.**

## Normal and Abnormal ocean-atmos coupling

- In the extratropics anomalies with “cyclonic over cold” and “anticyclonic over warm”  relationship tend to last longer.
- In the tropics “cyclonic over warm” and “anticyclonic over cold” tend to last longer.

**“****Abnormally coupled” **anomalies have shorter duration

**“****Normal Coupling”**

## AMIP: one-way interaction scheme

- Atmospheric GCM run with prescribed observed SST (AMIP runs) are usually assumed to be the upper limit for potential predictive skill (“perfect SST”).
- However, they assume (incorrectly) that the ocean *always forces* the atmosphere.

## AMIP simulation of anomalies

**Fewer long-lasting simulated anomalies than observed in the extratropics**

**More long-lasting simulated anomalies in the tropics**

**Number of cases of AMIP simulated minus Reanalysis**

**Ignoring the atmosphere’s feedback leads to:**

## AMIP simulation of anomalies

**Reanalysis minus AMIP cases**

**AMIP ignores**
**these cases in**
**the extratropics**

## Normal coupling and AMIP run

- Atmosphere-driving in the extratropics and ocean-driving in the tropics constitute the “normal coupling”.
- There are “abnormal coupled anomalies” (atmosphere-driving in the tropics and ocean-driving in the extratropics), but they are invariably short-lived.
- As a result, in AMIP runs (where the ocean is _always_ forcing the atmosphere) there are too many long lasting anomalies in the tropics, and too few in the extratropics

## Analysis of Medium Range Forecasts

- Skill of prediction of the 500hPa height.
- CDC’s Reforecast data:
- Daily 21-forecast runs with a frozen version of the NCEP MRF.
- Initialized with the Reanalysis data.
**SST anomalies at the initial time are kept constant through the run.**
- Anomaly Correlation Forecast (ACF): measures how close the forecast pattern verifies with the observed pattern in a given region.

## Measuring Skill and Forcing direction

- Fraction area of “Ocean-driving” phase relationship diagnosed (in the Reanalysis) prior to the Initial Time.

**Measuring Skill and Forcing direction**

## MRF and Forcing direction

**Northern Pacific**

## 500 hPa composites

**~ + PNA pattern**

**Atmos-driving**

**~ - PNA Pattern**

**Ocean-driving**

**L**

**L**

**L**

**H**

**H**

**H**

## Skill of AMIP simulation

- Measure the error of simulating high amplitude anomalies wrt reanalysis in the monthly data.
- Coupling index, CI:
- 	CI= -1 if all the grid-points in a given area 		experience “**atmosphere-driving**”
- 	CI=+1 if all the grid-points experience 			“**ocean-driving**”

## Skill of AMIP simulation

**In the extratropics, errors in AMIP are larger in “atmosphere-driving” situations; In the tropics, errors are larger in “ocean-driving”**

**AMIP error**

## Skill of AMIP simulation

**Cross-correlation CI and simulation error**

**Regions where “atmosphere-driving” situations hurt the skill of simulation**

## Skill and forcing direction in Northern Pacific

- When “atmosphere-driving” situations dominate
- 		 decrease in skill of simulation/prediction of AMIP-like models.
- 		 increase in skill of persistence forecast
- The opposite happens when the “ocean-driving” situations dominate.
- Predominant “atmosphere-driving” or “ocean-driving” situations resemble a negative or positive PNA pattern, suggesting local extratropical coupling is an important root of the PNA.

**Notes:**
21-day long re-forecasts made from the Reanalysis support this observation:
, because this corresponds to “normal coupling”
(which are always ocean-driving),

## Land-Atmosphere coupling

- Distribution of 15-days or longer-lasting anomalies

**Long lasting anomalies have a preferential local phase relationship between vorticity and surface temperature**

## Local phase relationship and Soil Moisture

**Relationship**
**Low-level circulation-Skin Temperature**

**Soil Moisture**

**Local phase relationship and Soil Moisture**

## Cross-correlation 850hPa CV and ST

**Cross-correlation 850hPa CV and ST**

**Daily**

**Monthly**

## Energy- and moisture-limited regions

**Considering the**** ****ST-Precipitation feedback:**
**The vegetative and wet region of the SE US requires large amounts of energy to induce significant changes in ST.**
**The very dry region of the SW US requires large amounts of moisture to produce changes in the precipitation regime.**

## Regional differences over the U.S.

**Regional differences over the U.S.**

## Land-Atmosphere coupling

- There is a preferential phase relationship of long-lasting anomalies over the continents.
- Over the very wet and very dry regions, the normal coupling is characterized by “cyclonic over cold” “anticyclonic over warm”.
- Over regions to the east of high range mountains, such as the Rockies, the normal coupling is “cyclonic over warm” and “anticyclonic over cold”.

## Thanks !