Detecting decadal climate phase reversal in near past: implications of recent North Pacific climate variability

Shoshiro Minobe (Graduate School of Hokkaido University, Sapporo, Japan)

It is difficult to predict Pacific Decadal Variability, then how about nowcast?

Mechanisms of PDV• Two different views

– Stochastically forced variations, with short damping time (e.g., Newman 2003; 2007).

– Oscillatory phenomena due to coupled air-sea mode or extraterrestrial forcing, suggested by several models (e.g., Zhong et al 2009; Tanaka et al. 2012; Meehl et al. 2009).

Air-sea coupled multidecadal mode Zhong et al. (2009)

• Salinity signal propagating in the subarctic region reaches the northwestern North Pacific, influence on Kuroshio-Oyashio extensions, from which feedback to the atmosphere making a oscillation.

Lag correlation of salinity(contour) & dynamic height(color) onto KEO SST. 0-500 m average, along 50N, 25-80 year band-pass filter.

Rossby wave due to salinity!

18.6-yr tidal modulation causes oscillation in the ocean & atmosphere. Tanaka et al. (2012)

• 430-yr AOGCM integration with & without 18.6-yr tide modulation.

Spectra of Aleutian low strength (NPI)

SST: clim (contour), composite (color)

Tidal modulation no uniform only Kuril

SLP: clim (contour), composite (color)

11-yr solar cycle

Meehl et al. (2009 Science)

Composite of four 11 solar peak years. for precipitation Stippling indicates significance at the 5% level, and dashed lines indicate position of climatological precipitationmaxima.

Most of CMIP5 models underperform persistency prediction

• Predictability of PDO by CMIP5 models are generally low. Only MIROC5 outperform persistency prediction.

Decadal prediction is difficult, then a decadal nowcast is possible? Kim et al. 2012 GRL

Good new for Atlantic researchers.

IPCC-class models can be useful for predictions with 3-6 year lead time. But many of them do not update their results operationally (only for AR5 and AR6). Kim et al. 2012 GRL

1998/99shift? “Pacific Ocean Showing Signs of Major

Shifts in the Climate” JPL Bill Patzert

20, Jan, 1999.

Chavez et al. (2003 Science)

3-yr running mean

A more recent change?

Bromirski, Miller et. al 2011 (JGR-O)

3-yr running mean

Motivation, cont.• The decadal nowcast is not easy, because a basic

method is to extract decadal variability is low-pass filtering, which needs not only past data but also future information for a data point to be filtered.

• Thus, decadal nowcast cannot avoid uncertainty from future.

• It should be useful to know decadal variability including explicit estimation of the uncertainty, using a method as possible as simple.

• This can gives a measure how extraordinary or just ordinary phenomena are going on.

Approach • To do so, we generate 1,000 future data of climate

indices (NPI and PDO index) using AR-1 model, and each time series, consist of observed past data and AR-1 future estimation, is filtered.

• The resultant 1,000 filtered data allow us to estimate uncertainty of decadal variability in near past.

• Filtering: decadal filter & bidecadal filter (10 & 30-yr half power point)

• This method is tentatively called End-Effect Estimate Filter (EEE-Filter).

AR-1 (first-order autoregressive) model

'dy y n sdt

er oi

( 1) ( )y n y nr noise

This is equivalent to Manu’s process oriented model

Data are seasonally sampled (one for year) and lag is one year.

EEE-Filter for 1st Atmos & Ocean modes

Phase reversal probability: NPI: 100% PDO: 100%

Consistent with Bromirski, Miller, et al. (2011)

SLP Epoch difference

• Pattern of SLP diff. of the recent change is similar to that of the 70s shift.

color: SLP diff., contour: confidence limit (95% solid, 90% dashed)

EEE-Filter, end yr 2008-201150%, 5 & 95%, 5-yr running

The phase reversal was detected in 2009.

EEE-Filter, end yr 1990-1993

5-year running average detected phase reversal for 1998/99 minor shift at the end year 1991-1993, but EEE-Filter shows no significant phase reversal.

50%, 5 & 95%, 5-yr running

A decadal prediction speculation

In 1999 it was suggested that the next phase-reversal of bidecadal variability may occur from 2000 to 2007.

Roughly consistent!

(Minobe 1999 GRL)

EEE-Bidecadal Filter

Hypothesis of Yasuda (2005, 2009) for Tidal mixing influence on climate.

Bidecadal-filter (10 & 30-yr half power point)

50%, 5 & 95%, Tide+lag4 yr

Chhak et al. (2009)

NPGO (Annual mean)

EEE-Filter for 2nd Ocean & Atmos modes

SLP

Conclusions• End-Effect Estimate Filter (EEE-Filter) is

proposed. • EEE-Filter detects decadal phase reversal of

Aleutian Low/PDO around 2006/07 in 2009, – consistent with a decade-ago speculation by

Minobe (1999 GRL). • NPGO may be going to change its phase

soon.• Any suggestions for improvements are

welcomed!