Atlantic air-sea interaction and model validation

C K Folland C K Folland,M J Rodwell M J Rodwell

doi:10.4401/ag-3388

Abstract

An analysis of observations from 1948-1998 suggests that the atmosphere in the North Atlantic region does respond to North Atlantic Sea-Surface Temperatures (SSTs) throughout the annual cycle. In the subtropics, high geopotential heights are seen to be a local response to warm SSTs. In winter, the North Atlantic Oscillation responds to a «tripole» pattern in North Atlantic SSTs. In summer, anticyclonicity over the U.K. is seen downstream of warm SST anomalies off Newfoundland and is possibly also related to warm subtropical SSTs. Such responses imply a degree of seasonal predictability and help quantify the strength of natural ocean-atmosphere coupled modes of variability. The average of an ensemble of 10 simulations of the HadAM3 atmospheric model forced with observed SSTs for the same period produces robust ocean-forced responses which agree well with those identifi ed in the observations and with a previous model. The agreement is encouraging as it confi rms the physical signifi cance of the observational results and suggests that the model responds with the correct patterns to SST forcing. In the subtropics, the magnitude of the ensemble mean response is comparable with the observational response. In the extratropics, the magnitude of the model response is about half that of the observations. Although atmospheric internal variability may have affected the observed atmospheric patterns and there are considerations regarding the lack of two-way air-sea interaction with an atmospheric model, it is suggested that the model?s extratropical response may be too weak. The 10 individual simulations of HadAM3 and 28 50-year periods of the ocean-atmosphere model, HadCM3, display similar results to each other with generally weaker ocean-forced links than observed. Seasonal predictability may, therefore, be too low in HadCM3 and low-frequency coupled modes under-represented. A moderate increase in the extratropics in the sensitivity of surface heat fl uxes to surface temperatures is one possibility for improving these model deficiencies.

Highlights

Observations and climate models display considerable low-frequency variability
We recognise that teleconnections from other regions will play a part in Atlantic-region climate variability, we focus here on air-sea interactions in the North Atlantic region
A lagged singular value decomposition technique has been applied to observations from 1948 to 1998, 10 simulations for the same period of an atmospheric model (HadAM3) and 28 50-year periods of an ocean-atmosphere model (HadCM3) simulation in order to validate mechanisms of low-frequency variability

Summary

Introduction

Observations and climate models display considerable low-frequency variability. A large part of this variability occurs naturally, due to air-sea interactions and ocean dynamics for example. Folland predominantly associated with atmospheric internal variability and there is little guarantee that the model is behaving well at longer (multiannual to multi-decadal) timescales when oceanatmosphere interactions become more important. By trying to isolate short-timescale air-sea interaction in the observations, it may be possible to help validate modelled low-frequency variability. Rodwell and Folland (2002), hereafter RF02, were able to demonstrate that the Hadley Centre’s ocean-atmosphere climate model (HadCM3) represents reasonably well atmospheric internal variability and aspects of atmosphere-to-ocean forcing in the North Atlantic region. RF02 considered the Hadley Centre’s atmospheric general circulation model (HadAM2b) in order to investigate directly results in Rodwell et al (1999) involving one-way forcing from SSTs, and to address subsequent comments (e.g., Bretherton and Battisti, 2000).

Data and models

The analysis method

SST-forced patterns

Predictability at longer SST lead-times

Climate model validation

Summary